sidhu thesis

SOSA: THE NEW LEAD FROM OLD DRUGS

1. INTRODUCTION OF DRUG DISCOVERY

Medicinal chemists have efficient methods for optimizing the

potency and the profile of a given active substance. These methods

may consist of more or less intuitive approaches such as the

synthesis of analogues and isomers and isosteres, or the

modification of ring systems, etc. They may also rest on computer-

assisted design, such as identifying pharmacophores by molecular

modelling or optimizing activity by means of quantitative structure–

activity relationships. Finally, structural biology studies yield more

and more X-rays or high-field NMR descriptions of drug–receptor

interactions. In each case, at the start of the process whether one is

to identify a new chemical structure or a new mechanism of action,

the medicinal chemist is responsible for developing as rapidly as

possible more active molecules that are also more selective and less

toxic.

The real challenge is the absolute requirement to discover or

identify an original research track. Indeed, for this major step, no

codified receipt exists and the creativity of a laboratory cannot be

planned. As a result, the discovery of a new lead substance

represents the most uncertain stage in a drug development

programme. Until the 1970s, the discovery of lead compounds

depended essentially upon randomly occurring parameters such as

accidental observations, fortuitous findings, hearsay or laborious

screening of a large number of molecules. Since then, more rational

approaches have become available, based on the knowledge of

structures of the endogenous metabolites, the enzymes and the

receptors, or on the nature of the biochemical disorder implied in

the disease. Today, the decryption of the human genome represents

a mine of potentially useful targets for which ligands have to be

found.

DEPARTMENT OF PHARMACEUTICAL CHEMISTRY 1


A retrospective analysis of the ways leading to the discovery of new

drugs allows one to distinguish essentially four types of strategies

giving rise to new lead compounds. The first strategy is based on

the modification and improvement of already existing active

molecules. The second one consists of the systematic screening of

sets of arbitrarily chosen compounds on selected biological assays.

The third approach resides in the retroactive exploitation of various

pieces of biological information which result sometimes from new

discoveries made in biology and medicine, and sometimes are just

the fruits of more or less fortuitous observations. Finally, the fourth

route to new active compounds is a rational design based on the

knowledge of the molecular cause of the pathological dysfunction.1



2. STRATEGIES IN THE SEARCH FOR NEW LEAD COMPOUNDS

2.1. IMPROVEMENT OF EXISTING DRUGS

The First strategy is based on the modification and improvement of existing active

molecules. The objective is to start with known active principles and, by various

chemical transformations, prepare new molecules (sometimes referred to as “me-too

compounds”) for which an increase in potency, a better specific activity profile,

improved safety, and a formulation that is easier to handle by physicians and nurses or

more acceptable to the patient are claimed. A typical illustration of this approach is

found in the series of lovastatin analogues (lovastatin, simvastatin, pravastatin,

fluvastatin, atorvastatin, rosuvastatin, etc.). In the pharmaceutical industry,

motivations for this kind of research are often driven by competitive and economic

factors. Indeed, if the sales of a given medicine are high and if a company is in a

monopolistic situation protected by patents and trademarks, other companies will

want to produce similar medicines, if possible with some therapeutic improvements.

They will therefore use the already commercialized drug as a lead compound and

search for ways to modify its structure and some of its physical and chemical

properties while retaining or improving its therapeutic properties.



Figure 1: Cptopril and “ me too” drugs

Other example are Antihypertensive drug captopril was used as lead compound by

various companies to produce their own antihypertensive agents2. (Figure.1)

Although often disparaged as ‘mee to ‘drugs, they can offer improvement s over the

original drugs. For example, modern penicillin is more selective, more potent, and

more stable than original penicillin2.

2.2. SYSTEMATIC SCREENING

This method consists in screening new molecules, whether they are synthetic or of

natural origin, on animals or in any biological test without regard to hypotheses on its

pharmacological or therapeutic potential. It rests on the systematic use of selective

batteries of experimental models destined to mimic closely the pathological events.

The trend is to undertake in vitro rather than in vivo tests: binding assays, enzyme

inhibition measurements, activity on isolated organs or cell cultures, and so on. In

practice, systematic screening can be achieved in two different manners. The first one

is to apply to a small number of chemically sophisticated and original molecules, a

very exhaustive pharmacological investigation: this is called ‘extensive screening’.

The second method, in contrast, strives to find, among a great number of molecules

(several hundreds or thousands), one that could be active in a given indication: this is

‘random screening’.

2.2.1. Extensive screening

Extensive screening is generally applied to totally new chemical

entities coming from an original effort of chemical research or from

a laborious extraction from a natural source. For such molecules,

the high investment in synthetic or extractive chemistry justifies an



extensive pharmacological study (central nervous, cardiovascular,

pulmonary and digestive systems, antiviral, antibacterial or

chemotherapeutic properties, etc.) to detect whether there exists an

interesting potential linked to these new structures. In summary, a

limited number of molecules are studied in a thorough manner

(vertical screening). It is by such an approach that the

antihistaminic, and later on the neuroleptic properties of the amines

derived from phenothiazine, were identified. Initially these

compounds had been submitted, with negative results, to a limited

screening study only directed towards possible chemotherapeutic,

ant malarial, trypanocidal and anthelmintic activities.

More recent examples are seen by the discovery, thanks to

systematic screening programme, of the cyclopyrrolones, e.g.

zopiclone (Figure 2) , as ligands for the central benzodiazepine

receptor,3,4 or of taxol as an original and potent anticancer drug (For

a review see suffiness5)

:Figure 2 : Drugs discovered by random screening.

2.2.2. Random screening

In this case the therapeutic objective is fixed in advance and, in

contrast to the preceding case, a great number (several thousands)

of molecules is tested, but on a limited number of experimental

models only. With this method one practices so-called random

screening. This method has been used for the discovery of new

antibiotics. By submitting samples of earth collected in countries

from all over the world to a selective antibacterial and antifungal



screening, the rich arsenal of anti-infectious drugs which are

presently at the disposal of clinicians, was developed. During the

Second World War, an avaian model in chickens infected with

Plasmodium gallinaceum was used for the massive screening of

thousands of potential antimalarials. The objective was to solve, by

finding a synthetic antimalarial, the problem of the shortage of

quinine. Unfortunately, no satisfactory drug was found. Massive

screening was implemented in Europe and the United States to

discover new anticancer6 and antiepileptic drugs. Here again the

problem is to select some predictive, but cheap cellular or animal

model. A common criticism of these methods is that they constitute,

by the absence of a rational lead, a sort of fishing. Besides, the

results are very variable: nil for the discovery of new antimalarials,

rather weak for the anticancer drugs but excellent, in their time, for

the discovery of antibiotics.

Among the recent successes of this approach the discovery of

lovastatin, also called mevinolin, should be mentioned (Figure 3), 7, 8

which was the basis of a new generation of hypocholesterolemic

agents, acting by inhibition of hydroxymethyl-glutaryl-CoA

reductase (HMG-CoA reductase).

Figure 3: The natural compounds compactin (mevastatin) and lovastatin block the cholesterol biosynthesis in inhibiting the enzyme hydroxymethyl- Glutaryl-CoA reductase (HMG-CoA reductase). The later developed Compounds, simvastatin and pravastatin, are semi synthetic analogues. The open -ring derivative pravastatin is less lipophilic and therefore Presents less central side effects. For all these compounds the ring-opened Form is the actual active form in vivo.



2.2.3. High-throughput screening

The second strategy consists of systematic screening of sets of compounds arbitrarily

chosen for their diversity, by selected biological assays. This approach was useful in

the past for the discovery of new antibiotics such as streptomycin and for the

identification of compactin as an HMG-CoA reductase inhibitor. Presently, as high

throughput screening (HTS), it is applied in a very general manner to synthetic as well

as to natural compounds.

Since the 1980s, with the arrival of robotics and with the

miniaturization of in vitro testing methods, it has become possible to

combine the two preceding approaches; in other words, to screen

thousands of compounds on a large number of biological targets.

This high-throughput screening is usually applied to the

displacement of radioligands and to the inhibition of enzymes. The

present trend is to replace radioligand-based assays by

fluorescence-based measurements. As it is now possible for a

pharmaceutical company to screen several thousand molecules

simultaneously in 30 to 50 different biochemical tests, the problem

becomes one of feeding the robots with interesting molecules.

Experience gathered has confirmed that high-throughput screening allows for the

rapid identification of numerous hits, and the literature is full of success stories

obtained with that approach. Among them, one could mention the discovery of insulin

mimetics,9 of ORL1 receptor agonists,10 of protein tyrosine phosphatase-1B

inhibitors,11 of selective neuropeptide Y5 receptor antagonists,12 of selective COX-2

inhibitors,13 of corticotrophin releasing factor (CRF) receptor modulators,14 and of

CXCR2 receptor antagonists.15

Yet the HTS strategy for drug discovery has several limitations. It suffers from

inadequate diversity, has low hit rates, Low-quality hits: cost and often leads to

compounds with poor bioavailability or toxicity profiles.



In conclusion, high-throughput screening of massive libraries is

expensive, time-consuming, diversity is inadequate, discovery is

often limited by monotony, yield is low, and there is a risk of low-

quality hits. Not surprisingly, the present trend is to use smaller

libraries, and, especially, libraries with increased ‘drug-likeness’.

2.2.5. Case study

2.2.5.1. Screening of synthesis intermediates

As synthesis intermediates are chemically connected to final

products, and as they often present some common groupings with

them, it is not inconceivable that they share some pharmacological

properties. For this reason, it is always prudent also to submit these

compounds to a pharmacological evaluation. Among drugs

discovered in this way are the tuberculostatic semicarbazones: they

were initially used in the synthesis of antibacterial sulfathiazoles.

Subsequent testing of isonicotinic acid hydrazide, destined for the

synthesis of a particular thiosemicarbazone, revealed the powerful

tuberculostatic activity of the precursor which has since become a

major antitubercular drug (isoniazide). Inhibitors of the enzyme

dihydrofolate-reductase such as methotrexate (Figure 4) are used in

the treatment of leukaemia. During the search for methotrexate

analogues a very simple intermediate, mercaptopurine, was also

submitted to testing. It proved to be active but relatively toxic.

Subsequent optimization led to azathioprine, a prodrug releasing

mercaptopurine in vivo. Azathioprine was found to be more potent

as an immunosuppressive agent than previously used corticoids and

was systematically used in all organ transplantations until the

advent of cyclosporine. Another intermediate in this series,

allopurinol, inhibits xanthine-oxidase and is therefore used in the

treatment of gout.19



Figure 4 :

2.3. EXPLOITATION OF BIOLOGICAL INFORMATION

A major contribution to the discovery of new active principles comes

from the exploitation of biological information. By this is meant

information that relates to a given biological effect (fortuitous or

voluntary) provoked by some substances in man, in animals or even

in plants or bacteria. When such information becomes accessible to

the medicinal chemist, it can serve to initiate a specific line of

therapeutic research. Originally, the observed biological effect can

simply be noticed without any rational knowledge on how it works.

2.3.1. Exploitation of observations made in man

The activity of exogenous chemical substances on the human

organism can be observed under various circumstances:

ethnopharmacology, popular medicines, clinical observation of

secondary effects or adverse events, fortuitous observation of

activities of industrial chemical products, and so on. Since in all



cases the information harvested is observed directly in man, this

approach represents a notable advantage.

2.3.1.1. Study of indigenous medicines (ethnopharmacology)

Natural substances were for a long time the unique source of

medicines. At present, they constitute 30% of the active principles

used and probably more (approximately 50%) if one considers the

number of prescriptions that utilize them, particularly since use of

antibiotics plays a major role.20 Behind most of these substances

one finds indigenous medicines. As a consequence,

ethnopharmacology represents a useful source of lead compounds.

Historically, we are indebted to this approach for the identification

of the cardiotonic digitalis glucosides of the digital, the opiates and

the cinchona alkaloids.

Despite its extremely useful contributions to the modern

pharmacopoeia such as artemisin, and huperzine, folk medicine is a

rather unreliable guide in the search for new medicines. When

ethnopharmacology and the natural substance chemistry end in the

discovery of a new active substance, this latter is first reproduced

by total synthesis. It is then the object of systematic modifications

and simplifications that aim to recognize by trial and error the

minimal requirements that are responsible for the biological activity.

2.3.1.2. Clinical observation of side-effects of medicines The clinical observation of entirely unexpected side effects

constitutes a quasi in exhaustible source of tracks in the search

for lead compounds. Indeed, besides the

desired therapeutic action, most drugs possess side-effects. These

are accepted either from the beginning as a necessary evil, or

recognized only after some years of use. When side effects present

a medical interest in themselves, a planned objective can be the

dissociation of the primary from the side-effect activities: enhance



the activity originally considered as secondary and diminish or

cancel the activity that was initially dominant. Promethazine, for

example, an antihistaminic derivative of phenothiazine, has

important sedative effects. Like Laborit et al., 21 a clinician might

promote the utilization of this side-effect and direct research

towards better-profiled analogues.

This impulse was the origin of chlorpromazine, the prototype of a

new therapeutic series, the neuroleptics, whose existence was

previously unsuspected and which has revolutionized the practice of

psychiatry.10, 22 Innumerable other examples can be found in the

literature, such as the hypoglycaemic effect of some antibacterial

sulfonamide, the uricosuric effect of the Coronary-dilating drug

benziodarone, the antidepressant effect of isoniazid, an

antitubercular drug, and the hypotensive effect of β-blocking

agents.

Figure 5: The clinical observation of the hypotensive Activity of the ‘open’ (and therefore flexible) β-blocking of β-blocking activity, but retaining the antihypertensive activity.23

This last example is beautifully illustrated by the discovery of the

potassium channel activator cromakalin.23 Cromakalin is the first

antihypertensive agent shown to act exclusively through

potassium channel activation.24 This novel mechanism of action

involves an increase in the outward movement of potassium ions

through channels in the membranes of vascular smooth muscle

cells, leading to relaxation of the smooth muscle. The discovery of

this compound can be summarized as follows: β-Adrenergic



Receptor blocking drugs were not thought to have antihypertensive

effects when they were first investigated. However, pronethalol, a

drug that was never marketed, was found to reduce arterial

blood pressure in hypertensive patients with angina pectoris.

This antihypertensive effect was subsequently demonstrated for

propranolol and all otherβ-adrenergic antagonists.25 Later there

were some doubts that blockade of the adrenergic receptors was

responsible for the hypotensive activity and attempts were made, in

the classical β-blocking molecules, to dissociate the β-blockade from

the antihypertensive activity.

Among the various conceivable molecular variations which are

possible for the flexible β-blockers, it was found that conformational

restriction obtained in cyclizing the carbon atom bearing the

terminal amino group on to the aromatic ring yielded derivatives

devoid of b-blocking activity, but retaining the antihypertensive

activity (Figure 5). One of the first compounds prepared (compound

1, Figure 5) was indeed found to lower blood pressure in

hypertensive rats by a direct peripheral vasodilator mechanism; no

β-blocking activity was observed. Optimization of the activity led to

the 6-cyano-4 pyrrolidinyl- benzopyran (compound 2), which was

more than 100-fold more potent than the nitro derivative. The

replacement of the pyrrolidine by a pyrrolidinone (which is the

active metabolite) produced a three-fold increase in activity and the

optical resolution led to the (-)-3R, 4S enantiomer of cromakalim

(BRL 38227), which concentrates almost exclusively the hypotensive

activity. 23, 26, 27

2.3.1.3. New uses for old drugs

In some cases a new clinical activity observed for an old drug is

sufficiently potent and interesting to justify the immediate use of

the drug in the new indication.



Amiodarone, for example (Figure 6), was introduced as a coronary

dilator for angina. Concern about corneal deposits, discoloration of

skin exposed to sunlight and thyroid disorders led to the withdrawal

of the drug in 1967. However, in 1974 it was discovered that

amiodarone was highly effective in the treatment of a rare type of

arrhythmia known as the Wolff-Parkinson-White syndrome.

Accordingly, amiodarone was reintroduced specifically for that

purpose.28

Figure 6:

Benziodarone, initially used in Europe as a coronary dilator, proved

later to be a useful uricosuric agent. At the present time it is

withdrawn from the market due to several cases of jaundice

associated with its use.28 the corresponding brominated analogue,

benzbromarone, was specifically marketed for its uricosuric

properties.

Thalidomide, was initially launched as a sedative/hypnotic drug

(Figure 7), but withdrawn because of its extreme teratogenicity.

Under restricted conditions (no administration during pregnancy or

to any woman of childbearing age), it found a new use as an

immunomodulator. Particularly, it seems efficacious for the

treatment of erythema nodosum leprosum, a possible complication

of the chemotherapy of leprosy.29

. Figure 7: Structure of thalidomide. The marketed compound is the racemate.



A more recent example is provided by the discovery of the use of

sildenafil (Viagra, Fidure 8), a phosphodiesterase type 5 (PDE5)

inhibitor, as an efficacious, orally active agent for the treatment of

male erectile dysfunction.30, 31 Initially this compound was brought to

the clinic as a hypotensive and cardiotonic substance and its

usefulness in male erectile dysfunction resulted from clinical

observations.

Figure 8: Structure of the phosphodiesterase type 5 (PDE5) inhibitor sildenafil.30,12

In many therapeutic families each generation of compounds induces

the birth of the following one. This happened in the past for the

sulfamides, penicillins, steroids, prostaglandins and tricyclic

psychotropic families, and real genealogical trees representing the

progeny of the discoveries can be drawn. More recent examples are

found in the domain of ACE inhibitors and in the family of

histaminergic H2 antagonists.

Research programmes based on the exploitation of side effects are

of great interest in the discovery of new tracks in so far as they

depend on information about activities observed directly in man and

not in animals. On the other hand, they allow detection of new

therapeutic activities even when no pharmacological models in

animals exist.

2.3.2. Exploitation of observations made in animals

Here, we find all the research done by physiologists which has been

the basis of the discovery of vitamins, hormones and

neurotransmitters and the fall-out of various pharmacological



studies, when they were performed in vivo. Other observations

made on animals, often in a more or less fortuitous manner, have

led to useful discoveries. An example is provided by the dicoumarol-

derived anticoagulants.

The discovery of the anticancer properties of the alkaloids of Vinca

rosea constitutes a particularly beautiful example of

pharmacological feedback. Preparations from this plant had the

reputation in some popular medicines of possessing anti diabetic

virtues. During a controlled pharmacological test, these extracts

were proved to be devoid of hypoglycaemic activity. On the other

hand, it was frequently observed that the treated rats died from

acute septicaemia. A study of this phenomenon showed that it was

due to massive leukopenia. Taking the leukocyte count as the

activity end-point criterion, it became possible to isolate the main

alkaloid, vinblastine.32 at the same time, in another laboratory,

routine anticancer screening had revealed the activity of the crude

extract on murine leukaemia.33 subsequently, and the antileukaemic

acvity became a screening tool. Out of 30 alkaloids isolated from

various periwinkles, four (vinblastine, vinleurosine, vincristine and

vinrosidine) were found active in human leukaemias.34

3. DRUG DISCOVERY FROM SOME SIDE SIDE EFFECT OR NEW LEADS FROM OLD DRUGS

3.1. INRTODUCTION

Many important therapeutic discoveries have resulted from serendipitous observation.

Side effect of drug candidate in the clinical has paved the way to new application of a

drug or to a development of chemically modified analogs. Unexpected

pharmacological effect against physiologically related or other, more diverse, targets

have resulted in drug candidates with different modes of action. In the past decades,

more systemic approaches have been followed: chemo genomics, systemic



investigation of biological effects of certain target families, and the selective

optimization of drug side effects (SOSA)52.

Such side effect may result from:

A physiological reaction of the body to the action of the drug (e.g., the reflex

tachycardia resulting from the antihypertensive activity of dihydropyridine

calcium channel blocker).

Overdose of drug with narrow therapeutic range and/or unfavorable

pharmacokinetics (e.g., phenprocoumon or warfarin, which exert their action

in delayed and indirect manner by inhibition of vitamin k biosynthesis).

Action of different target by same mechanism (e.g., gastrointestinal bleeding

after cyclooxygenase inhibition by acetylsalicylic acid, bradykinin-mediated

cough as a side effect of angiotensin –converting enzyme inhibitors).

Action on organs other than the target organ (peripheral tachycardic and

hypertensive effects of dopamine after systemic application of the anti-

Parkinson drug L-dopa, sedative e side effects of lipophillic histamine H1

antagonist).

Lack of selectivity, i.e., inhibition, agonism, antagonism at several different

targets, a most common reason for drug side effect (e.g., respiratory

depression by morphine, cardio tonicity of certain drugs mediated by hERG

channel inhibitions).

Inhibition of cytochrome P450 isoenzymes (e.g., nonlinear pharmacokinetic

propafenone, producing an exponential increase of plasma level due to

inhibition of its metabolism by CYP2D6, after application of higher dose).

Drug-drug interaction resulting from P450 inhibition or induction, a very

common reason for adverse drug effects (e.g., terfenadine which exerts fatal

cardio toxicity by hERG channel inhibition in the presence of a CYP3A4

inhibitor, where its active metabolite fexofenadine is not a hERG channel

inhibitor).

Genetic deposition, either by interaction of drug with mutant target or by the

lack of certain (or mutant) metabolic enzyme (e.g., inability of about 1-3%

Caucasian population to metabolite S-warfarin, due to aCYP2C9 deficiency).



There must always be significant advantage of the achievable therapeutic benefits, as

compared to the risk of drug-related side effects. Several side effects can only be

tolerated in treatment of chronic degenerative or life-threatening disease like arthritis,

cancer, or AIDS. However, adverse drug side effect are frequently observed after

medication; their high incidence, even as a common cause of death, is only gradually

being recognized.

However, a closer inspection history of drug discovery show that many new drug

application resulted from clinical observation of side effects or from the optimization

of such unexpected side effects into new therapeutic areas Only the prominent drugs

that resulted from serendipitous observation of clinical side effects are discussed in

following sections. However, even few examples show the importance of this source

of new leads in drug research.

In addition to clinical observation of drug side effects, the optimization of side

activities that are discovered by in vitro investigation play important role in drug

research. Recently wermuth proposed using this approach as a general strategy for the

“selective optimization of side activities” (the SOSA approach)52 .



3.2. A HISTORICAL PERSPECTIVE: THE GREAT TIME OF

SERENDIPIOUS OBSERVATION

The early history of drug discovery is characterized by many serendipitous

drug discoveries. After the preparation of nitrous oxide by Humphry Davy in

the early 19th century, fun patient with this gas, and also with ether became

popular, people liked the euphoric after inhaling the chemicals. The anesthetic

property of nitrous oxide and the were discovered in the 1840s just by chance,

because participants of such events did not expiernced any pain after being

hurt52.

A more or less systemic search for new drug started in the last two decades of

the 19th century. Although acetylsalicylic acid 1 (ASS, aspirine, bayer, figure

10) was originally designed as a ‘better’ derivative of salicylic acid, it is much

more than just prodrug. ASS is much more active than its parent drug is

indeed better tolerated, but it causes gastrointestinal bleeding as a prominent

side effect. In the 1970s it became clear that both its activity and its side

effects are mediated by the same target. ASS inhibits cyclooxygenase, which

converts arachidonic acid into prostacyclin, which is further converted into

prostaglandins and thromboxane. Whereas inhibition of thromboxane

biosynthesis is responsible for the increased bleeding tendency. Since

thromocyte have no nucleus and therefore no protein biosynthesis, platelet

cyclooxygenase remains inhibited over the whole thrombocyte lifetime of

about 120 days; correspondingly thrombosis protection by aggregation

inhibition can be achieved by application of only 100 mg or even less of ASS

per day. Low-dose ASS is now standard therapy for the preparation of stroke,

heart attack and thrombosis.

Figure 10: ASS 1 is much more than prodrug of salicylic acid. It’s major Contributions to biological activity come from unique mechanism of action: The activated acetyl group is transformed to serine hydroxyl group in the binding site of cyclooxigenase.



The diuretic organomercurials are most probably the very first example of

discovery of a class of therapeutically useful drug by a clinical side effect of one

of their mechanisams.In1988, mercury salicylate was introduced for the treatment

of syphilis , followed by mersanyl in 1906 and arsphenamine (E. 606),discovered

by Paul Ehrlich in 1909. On October 7, 1919, a pale and weak 21-years-old

female, Johanna M., was brought to first medical university Clinic I Vienna, in am

insane status , with clear symptoms of severe neurosyphillis. Alfred Vogel, a 3 rd -

year medical student, was ordered to apply mercury salicylate, in a desparate

attempt to help. No knowing about properties of this compound, he asked for 10%

aqueous solution for intramuscular injection. After few days when he has not

received the solution, he was told that the compound was too insoluble. A

colleague proposed trying recently developed analog, merbaphen (Novasurol,

bayer, figure 11), and a water insoluble salt of organomercurial compound with

barbitone.

2

Figure 11 merbaphen

Mebaphen 2 was the first example of an organomercurial diuretic: some drug with

less side effect were therapeutic standard from about 1920 to 1950.

After approval by his superviser, he applied it to suffering patient. To this great

surprise, the daily urine production increased from 200-500 mL. Application to other

patients produced up to 10 L urine within 24 hours – a diuretic effect that had not

been observed before !Merbaphen was too toxic for therapeutic application, but

follow-on product held their place as diuretic till the 11950s, when another

observation of clinical side effect led to the discovery of much safer sulphonamide

diuretic52.



Figure 14

The aniline-imidazoline clonidine 17 (Catapresan, boehringer ingelheim;figure

14 )

was designed by she mist Helmut Staehle as nasal deconjestant.When the secreatory

of the colleague caught a nasty cold, she was ready to test the new drug. Telling

them I will take anything if I can just get rid of this stifles! Shortly after taking the

drug she became tired and fell asleep, After she was brought home, she continue

sleeping for about 20 hours. A controlled self-experiment by her boss, the physician

Martin Wolf ,had same outcome, with a heart rate reduction to about 40-48 beats s-

1 and blood pressure decrease to 90 vs. 60 mm Hg. Clearly, the compound was

potent antihypertensive drug, which was confirmed by further pharmacological and

clinical investigations52.

Figutr 15

Iproniazide 18 (figure 15).an alkyl analog of the antituberculous drug isoniazide

19(figure 15),surprisingly shows mood improving activity in several depressed

tuberculosis patients, which turned out to result from monoamine oxidase (MAO)

inhibitory activity. Since the compound was already registered as antituberculosis

drug and since its constituted the very first effective treatment of depression, more

than 400 000 patient received it within only one year after the first announcement

of its antidepressant activity52.Later it was withdrawn from therapy ,due to

hepatotoxic side effect.



Figure 16

D-Penicillamine 20 (figure 16) has for long time been used for the treatment of

Wilson’s disease, a metabolic disorder in which absorbed copper is deposited

mainly in the liver and in the brain. Long term application of this compound leads to

suppression of rheumatoid arthritis, which now is its main therapeutic use 52.

Figure 17



Sildenafil (Viagra, Pfizer), the first drug effective in male erectile dysfunction

(MED), has very interesting history. More than 30 years ago, the company May &

Baker started research on antiallergic xanthine derivatives. Their first leads 21

and 22 (figure 17) , being between 40 times and 100 times more active than

cromoglycate, the standard drug at this time , were structurally closely related to

sildenafil. Zaprinast 21, was clinically tested orally active ‘mast cell stabilizer’

against histamine- and exercised-induced asthmas. In addition to this activity,

zaprinast has vasodilatory and antihypertensive side effects. In the mid 1980s , Nick

Terret and his teamm at Pfizer were searching for new antihypertensive principle .

They followed the approach of enhancing biological activity of the atrial

natriuretic peptide (ANP) by prolonging the action of the second messenger of the

corresponding receptor response. For this purpose, they were looking for a

compound they would prevent the degradation of cyclic guanisone monophosphate

(cGMP) by phosphodiesterse. As zaprinast 21 was one of very few cGMP PDE

inhibitors known in 1986, they started from this lead to improve its activity and

selectivity .In 1989, the result of extensive structure modification was the PDE5-

selective inhibitor sildenafil 23 (UK-92,480;figure 8), later clinically tested as

antianginal drug .The drug turned out to be safe and well tolerated but its clinical

activity was disappointing. However, early in 1992, a 10-day toleration study in

healthy volunteers led to observation of a strange side effect. Among other effects ,

the the patient reported some penile erections after the 4th or 5th day. Although it was

not an obvious choice to test the new drug in male erectile dysfunction,its further

clinical profiling went into this direction. After convincing clinical results, Viagra

was introduced into therapy in March 198852.



4. SELECTIVE OPTIMIZATION OF SIDE ACTIVITY: THE “SOSA” APPROACH

Selective optimization of side activities of drug molecules (the SOSA

approach) is an intelligent and potentially more efficient strategy

than HTS for the generation of new biological activities. Only a

limited number of highly diverse drug molecules are screened, for

which bioavailability and toxicity studies have already been

performed and efficacy in humans has been confirmed. Once the

screening has generated a hit it will be used as the starting point for

a drug discovery program. Using traditional medicinal chemistry as

well as parallel synthesis, the initial ‘side activity’ is transformed

into the ‘main activity’ and, conversely, the initial ‘main activity’ is

significantly reduced or abolished. This strategy has a high

probability of yielding safe, bioavailable, original and patentable

analogues.53

4.1. DEFINATION, PRINCIPLES AND METHODOLOGY.

The SOSA (selective optimization of side activities) approach represents a validated

alternative to HTS (56-60). It consists of testing “old” drugs on new pharmacological

targets. The aim is to subject to pharmacological screening a limited number of drug

molecules that are structurally and therapeutically very diverse and that have known

safety and bioavailability in humans and thereby shorten the time and the cost needed

for a hit identification.

The SOSA approach proceeds in two steps.

(1) Start the screening with a limited set of carefully chosen, structurally diverse drug

molecules (a smart library of about 1000 compounds). Since bioavailability and

toxicity studies have already been performed for those drugs and since they have

proven their usefulness in human therapy, all hits will be “drug like!”

(2) Optimize hits (by means of traditional, parallel, or combinatorial chemistry) in

order to increase the affinity for the new target and decrease the affinity for the other



targets. The objective is to prepare analogues of the hit molecule in order to transform

the observed “side activity” into the main effect and to strongly reduce or abolish the

initial pharmacological activity.

As mentioned above, a differentiating peculiarity of this type of library is that it is

constituted of compounds that have already been safely given to humans. Thus, if a

compound were to “hit” with sufficient potency on an orphan target, there is a high

chance that it could rapidly be tested in patients for proof of principle. Alternatively,

if one or more compounds hit but with insufficient potency, optimized analogues can

be synthesized and the chances that these analogues will be good candidate drugs for

further development are much higher than if the initial lead is toxic or not

bioavailable.

One of these “new types” of chemical library has recently become available61. It

contains 880 biologically active compounds with high chemical and pharmacological

diversity as well as known bioavailability and safety in humans. Over 85% of the

compounds are well-established drugs, and 15% are bioactive alkaloids. For scientists

interested in drug likeness, such a library certainly fulfills in the most convincing way

the quest for “drug like” leads! Other libraries containing various amounts of drug

molecules are also available62, 63.

4.3. RATIONAL OF THE “SOSA” APPROACH.

The rationale for using chemical libraries composed of marketed drugs is that most

drugs used in humans interact with more than one target/receptor. Binding to one

target mediates efficacy, whereas binding to other targets is often the source of side

effects.

There are many published examples of drugs that interact with several receptors.

The anxiolytic diazepam not only binds to the benzodiazepine receptor but

also inhibits phosphodiesterases64.

The GABA-A receptor antagonist gabazine is a potent inhibitor of

monoamine-oxidase type A65.



Compounds as different as benzyl penicillin or D-tubocurarine unexpectedly

shows micro molar affinity for GABA-A receptors66.

N-α-Nitroarginine, a competitive antagonist of nitric oxide synthesis, was also

shown to be a muscarinic receptor antagonist67.

The dopamine receptor antagonist spiperone shows a strong affinity for

serotonin 5-HT2a receptors68.

Other antipsychotic such as clozapine and olanzapine have been shown to bind

To at least 14 different receptors such as D1, D2, D3, 5-HT2a, 5-HT2c, 5-HT3,

M1, M2, M3, M4, M5, R1, R2, and H1 receptors67. This seems to be a

Rather common feature as shown in Table 1 (results from Schaus’ and

By master’s review69), which compares some affinities for a series of eight

antipsychotic agents.

Table 1 : Affinities of Some Antipsychotic for Various Neuronal Receptors70

The above observations justify the strategy of testing well-known drugs on newly

discovered targets. When an “old” drug binds to a new target, the objective is to

synthesize analogues with increased affinity for the new target and decreased affinity

for the old target. Many examples of such activity profile reversals have been

published and are usually the result of traditional, well established medicinal

chemistry approaches.

4.4. APPLICATION OF SOSA APPROACH : Successful Examples of

SOSA Switches

4.4.1. Sulfonamides as Lead Compounds.



Two examples of SOSA switches show that extremely potent and selective

antagonists of G-protein-coupled receptors and myocardiac sodium/hydrogen

exchange (NHE) inhibitors could be derived from traditional drugs such as

sulfathiazole and amiloride.

4.4.1.1. From Sulfathiazole to Endotheline ET-A Receptor Antagonists.

A typical illustration of the SOSA approach is given by the

development of selective antagonists for the endothelin ETA

receptors by scientists from Bristol-Myers Squibb (BMS)71. Starting

from an in-house library, the antibacterial compound sulfathiazole 1

(Figure 21) was an initial, but weak, hit (ETA IC50 = 69 µM). Testing of

related sulfonamides identified the more potent sulfisoxazole 2 (ETA

IC50 = 0.78 µM). Systematic variations finally led to the potent and

selective ligand 3 (BMS-182874). This compound was orally active

in vivo and produced a long-lasting hypotensive effect.



Figure 21. A successful SOSA approach identified the Antibacterial sulfonamide Sulfathiazole as a ligand of the Endothelin ETA receptor and its optimization to the Selective and potent compounds BMS-182874, BMS-193884, and BMS-207940.71,72

Further optimization guided by pharmacokinetic considerations led

the BMS scientists to replace the naphthalene ring by a biphenyl

system72. Among the prepared compounds, 4 (BMS-193884, ETA Ki

= 1.4 nM; ETB Ki = 18,700 nM) showed promising hemodynamic

effects in a Phase II clinical trial for congestive heart failure.

Morerecent studies led to the extremely potent antagonist 5 (BMS-

207940, ETA Ki = 10 pM) representing an 80,000-fold selectivity for

ETA over ETB. The oral bioavailability of 5 is 100% in rats and it has

been shown to possess activity at a dose of 3 µg/kg by mouth [per

os (p.o.)]72.

4.4.1.2. Diuretic Amiloride as a Lead to Myocardiac Sodium/Hydrogen Exchange

(NHE) Inhibitors.

Figure 22. Amiloride-derived cardio protective sodium/hydrogen exchange (NHE) inhibitors.

Currently, five isoforms of sodium/hydrogen exchanger have been found in the

plasma membrane of mammalian cells and a sixth has been found in the

mitochondria73. The predominant isoform in the heart is type 1 (NHE-1). One of the

first papers to suggest a cardioprotective role of inhibiting NHE was published by



Karmazyn in 198874 where it was shown that amiloride 6 (Figure 22), a potassium-

sparing diuretic with NHE inhibitory activity, produced an enhanced recovery of

contractile function in isolated rat hearts subjected to global ischemia and reperfusion.

Subsequently, several investigators used amiloride and its 5-amino substituted

pyrazinoyl guanidine derivatives to demonstrate the cardioprotective potential of

inhibiting NHE in the ischemic myocardium.73,75 However, it was found later that

these amiloride derivatives interacted with other cation transporters and shared cardio

depressive activities independent of their NHE blocking activity.

Investigators at Hoechst75 were the first to synthesize a new class of more selective

NHE-1 inhibitors, the benzoylguanidine derivatives 7 and 8 (Figure 22). The first

compound showing superior efficacy and selectivity over amiloride derivatives was 7

(Hoe-694). This compound showed marked anti arrhythmic and anti ischemic activity

in several animals models and had a low toxicity profile. To synthesize a compound

superior to 7, investigators from Hoechst made 8 (Hoe-642, or cariporide mesilate) by

substituting an isopropyl for a piperidine group. This change enhanced water

solubility, activity in vitro, and NHE-1 selectivity over 7. Subsequently, other

companies, for example, Merck KGaA and Boehringer, also synthesized

benzoylguanidine derivatives such as 9 (EMD-96785, or eniporide mesilate) and 10

(BIIB-513)76. All these compounds have been shown to be cardioprotective in a

number of ischemic animal and human models.

4.4.2. Dihydropyridines as Leads.

Retrospective analyses of various drug structures led the medicinal chemists to

identify some molecular motifs that are associated with high biological activity more

frequently than other structures. Such molecular motifs were called “privileged

structures” by Evans et al77. to mean substructures that confer activity on two or more

different receptors. The implication was that the privileged structure provides the

scaffold and that the substitutions on it provide the specificity to a particular receptor.

Two monographs deal with the privileged structureconcept78,79.

The Ca2+ channel blockers containing the dihydropyridine motif certainly belong to

the class of privileged stuctures80. We will discuss how they served as a starting point



for the synthesis of α1A-adrenergic antagonists and of multi drug-resistance

modulators.

4.4.2.1. Calcium Channel Blocker Niguldipine as a Source of α1A-Adrenergic

The typical symptoms of prostatism are obstructive (poor urine stream, dribbling, large residual urine volume) and irritative (hesitancy, increased frequency of urination, nocturia) in nature and can significantly compromise the quality of life of patients. While surgical procedures or the use of 5 α –reductase inhibitors such as finasteride are used to reduce the prostatic mass, α1-adrenergic receptor antagonists such as terazozin, doxazocin, and tamsulosin relax the smooth muscles in the prostate and in the lower urinary tract and facilitate the urine flow. Antagonists81.

FFigure 23. Passage from the calcium channel antagonist niguldipine to the potent and selective α1A-Adrenergic antagonist 14 (SNAP-6383)81.

However, nonselective α1-adrenergic receptor antagonists present cardiovascular side

effects (tachycardia and orthostatic hypotension). Selective blockers of the α1A-

subtype of adrenergic receptors are assumed to alleviate the symptoms associated

with benign prostatic hyperplasia (BPH) with minimal cardiovascular side effects.

A screening program identified the calcium channel blocker niguldipine 11 (Ki = 4.6

nM for rat L-type calcium channel) as a potent ligand (Ki = 0.16 nM) of the

recombinant human α1A-adrenoceptor (Figure 23). Moreover, niguldipine presents

considerable α1A -selectivity (>300-fold over α1B- and α1D-receptors. Niguldipine was

developed as a racemate; however, the α1-adrenergic receptor antagonist properties

are mainly concentrated in the (S)-(+)-enantiomer. During mutation studies of a

series of R1A-adrenergic ligands, it appeared that mutation of either Phe-308 or Phe-

312 in the transmembrane domain 7 of the α1A-receptor results in significant losses of

affinity (4- to 1200-fold) for the antagonists prazosin, WB4101, BMY7378, (S)- (+)-



niguldipine, and 5-methyluradipil. No affinity changes were observed for the phenyl

ethylamine type of agonists82.

Progressive optimization of niguldipine yielded compounds such as 12 (SNAP-5089

(-)), 13 (SNAP-5399), and 14 (SNAP-6383)81. These compounds display nanomolar

affinities for the α1A-receptor subtype, which correlates well with the potency to

inhibit the phenylephrine induced contraction of dog prostate. Compound 14 binds to

the human recombinant α1A adrenergic receptor with a Ki of 0.36 nM and exhibited a

1000-fold selectivity improvement over other subtypes83. It proved to be efficacious in

clinical trials but was finally discarded for its cytochrome P450 3A4 isozyme-

mediated metabolism and the corresponding risk of drug-drug interaction81.

Figure 24. Monatepil maleate (15) combines calcium channel blocking activity, α1-adrenoceptor antagonism, and inhibition Of lipid hydroperoxidation.84,86

A similar finding associating calcium channel blocking activity with α1-adrenoceptor

antagonism is found in the drug monatepil maleate (15, Figure 24).84-86

In addition to the above-mentioned properties, monatepil maleate was shown to

potently inhibit copper-induced lipid hydroperoxidation of human LDL in vitro86.

4.4.2.2. Dihydropyridine-Type Calcium Channel Blockers as a Source of Multi drug- Resistance Modulators 87-89.

Figure25. Dexniguldipine 16, the (R)-(-)-enantiomer of



Niguldipine, is less active as a calcium channel blocker but Potent as a reverser of multidrug resistance.

One type of resistance of neoplastic cells to cytotoxic agents is multidrug resistance,

which may occur spontaneously or develop as a response to exposure to several

different drugs, including anthracyclines, actinomycin D, epipodophyllotoxins,

taxanes, and vinca alkaloids. The transmembrane glycoprotein, 170 kDa P-

glycoprotein90, actively extrudes susceptible drugs by pumping them out of the cell by

an ATP requiring process. P-glycoprotein is encoded by the multidrug resistance-1

gene (MDR1), which is on the long arm of chromosome 7. Multidrug resistance is due

to over expression of P-glycoprotein and perhaps other factors.

Some compounds, such as the calcium channel blocker verapamil, by binding to P-

glycoprotein, increase the intracellular accumulation of the drugs that are actively

extruded. The less active (R)-(-)-enantiomer of niguldipine, dexniguldipine (Figure

25), is a dihydropyridine derivative, which weakly blocks calcium channels86 and

shows promise as a reverser of multidrug resistance86. It only has 1/40 the affinity for

the L-type calcium channel as its enantiomer, niguldipine91. It is also much less active

at blocking calcium channels than verapamil and has less cardiovascular effects than

verapamil. P-glycoprotein has an intracellular drug acceptor with which

dexniguldipine combines. Dexniguldipine binds on receptor site 2 of P-glycoprotein,

whereas verapamil, cyclosporine A, etoposide, and vinblastine all bind at receptor site

192. Dexniguldipine inhibits protein kinase C93, is a calmodulin antagonist94, has

antitumor activity94-95, and inhibits DNA synthesis in experimental tumors96.

Dexniguldipine is about 10 times as potent as verapamil at reversing multidrug

resistance in many in vitro systems.97

4.4.3. Cyclic Analogues of β -Blockers.

Conventional β-blockers possess a number of pharmacological properties, e.g., β-

blocking, quinidine-like, local anesthetic, and hypotensive effects. With the hope of

achieving some specificity, Basil et al98. Considered the possibility of synthesizing

ring-closed analogues (closure mode 1; Figure 26). One of the prepared compounds,

3,4-dihydro- 3-hydroxy-6-methyl-1,5-benzoxazocine, was a potent β-blocker. This

activity is unlikely to be due to hydrolysis to the open-chain derivative because the

corresponding primary amine, formed by hydrolysis of the benzoxazocine ring, has



less than 0.25 the activity of the latter. Yet it is difficult to reconcile the

benzoxazocine configuration with the structural requirements associated with the

occupation of β-receptors.

Figure26. Cyclized analogues of β-blocking phenylpropanolamines.98,99,102

Attempts to exploit the sedative and anticonvulsant effects observed for propranolol

in pharmacological experiments prompted Greenwood et al99. to examine the closure

mode 2 (Figure 26). Their study led to the norepinephrine reuptake inhibitor

viloxazine, which was the first representative of a new class of antidepressant.

Later, Evans et al100-101. Envisaged the closure mode 3 (Figure 26) for the synthesis of

cyclized analogues of the phenylpropanolamine type of β-blockers. The authors

hoped that by restricting the conformation, β-blocking activity would be lost but

antihypertensive activity might be retained. This turned out to be true in animal tests

and in double-blind clinical studies and justified the development of the potassium

channel activator cromakalim102. This compound itself was further developed to yield

IKs channel blockers as potential antiarrhytmic agents.

4.4.3.1. From β-Blockers to the Potassium Channel Blocker Cromakalim.

A near-textbook illustration of the SOSA concept is given by the development of the

hypotensive drug levocromakalim starting from β-blockers such as atenolol102. β-

Blockers were introduced in the early 1970s for the treatment of angina pectoris and

hypertension. However, there was some doubt that β-blockade was responsible for

their antihypertensive activity and it was suggested that analogues with reduced

flexibility of the side chain may be devoid of β-blocking activity but would retain the

antihypertensive activity. This was the initial lead to cyclized analogues (Figure 27).



Figure 27. Passage from “open- β-blockers to the corresponding cyclized analogues102.

One of the first compounds prepared was compound 17, which for chemical reactivity

reasons bore gemdimethyl group at C-2. This compound was indeed found to lower

blood pressure in hypertensive rats by a direct peripheral vasodilator mechanism; no

β-blocking activity was observed. Optimization of the activity led to compound 18,

which was more than a 100-fold more potent than the nitro derivative. The

replacement of the pyrrolidine by a pyrrolidinone (which is the active metabolite)

produced a 3-fold increase in activity.

Finally, the optical resolution led to the (-)-(3S,4R)- enantiomer of cromakalim 19

(levocromakalim, BRL 38227) that concentrates almost exclusively the hypotensive

activity and acts exclusively as a potassium channel opener; β-blocking activity is no

longer observed.

4.4.3.2. IKs Channel Blockers as Potential Anti arrhythmic Agents103.

The IKs channel blocking ability of compound 21 (293B, Figure 28) was found to be a

side activity in another research program dealing precisely with cromakalim-related

chromanols such as compound 20 (HOE-234). Initially it was assumed that compound

21 acts indirectly on the Cl- transport by blocking an associated cAMP-regulated

potassium channel104.



Figure 28. Cromakalim-derived IKs channel blockers103.

Subsequent studies on cloned potassium channels from the guinea pig demonstrated

that the chromanol 21 specifically blocks IKs channels expressed in Xenopus oocytes

with an IC50 value of 6.2 µM105. The (3R,4S)- enantiomer was found to be more potent

than the (3S,4R)-enantiomer (IC50 = 5 and 39 µM, respectively). Further optimization

led to compound 22 (HMR-1556; IC50 120 nM) characterized by inverted

stereocenters and by the replacement of the cyano function by a trifluorobutoxy side

chain106.

4.4.4. Aminopyridazine Minaprine as Lead Substance.

Aminopyridazines and, more precisely, 3-amino-6-arylpyridazines represent another

group of privileged structures107. They present generally favorable ADME and

toxicological profiles and allow many chemical variations.

4.4.4.1. Transforming the Antidepressant Minaprine into a Muscarinic M1 Receptor Ligand.

In the field of pyridazine chemistry we could, starting from the antidepressant

minaprine 23 (Figure 29), derive various SOSA switches. Minaprine itself, in addition

to reinforcing serotonergic and dopaminergic transmission, also possesses weak

affinity for muscarinic M1 receptors (Ki =17 µM).



Figure 29. SOSA switch from the antidepressant minaprine To a nanomolar partial agonist for muscarinicM1 receptors108,109.

Three simple chemical variations (Figure 29) (shift of the methyl group from the 3- to

the 4-position (23→24), replacement of the morpholine by a tropane (24→25), and

introduction of an OH in the ortho position of the phenyl ring (25→26)) abolished the

dopaminergic and serotoninergic activities and boosted the partial agonistic

cholinergic activity of compound 26 to nanomolar concentrations108,109. The

remarkable result was that the initial activity of minaprine on the dopaminergic and

serotoninergic transmission was totally abolished in the final compound 26.

4.4.4.2. Minaprine as a Source of Reversible Acetyl cholinesterase Inhibitors

Figure 30. IC50 values for acetyl cholinesterase inhibition (electric eel enzyme).110,111

Starting from the same minaprine lead, we imagined that this molecule, being

recognized by the acetylcholine receptors, should also be recognized by the



acetylcholine enzyme. It turned out that minaprine had only a very weak affinity for

acetyl cholinesterase (600 µM on electric eel enzyme). However, relatively simple

modifications (creation of a lipophlic cationic head, increase in side chain length, and

bridging of the phenyl and the pyridazinyl rings) allowed us to reach nanomolar

affinities (Figure 30)110,111.

4.4.4.3. From Minaprine to CRF Antagonists.

Figure 31. Switch from the antidepressant molecule minaprine to the potent CRF receptor antagonist 34.112,113

Another interesting switch consisted of the progressive passage from

desmethylminaprine 31 to the bioisosteric thiadiazole 32 (Figure 31) and then to the

bioisosteric thiazoles. Trisubstitution on the phenyl ring and replacement of the

aliphatic morpholine by a pyridine led to compound 33, which exhibited some affinity

for the receptor of the 41 amino acid neuropeptide corticotrophin releasing factor

(CRF). Further optimization led to nanomolar CRF antagonists such as 34.112,113

4.4.5. Neuroleptic Benzamides as Leads.

The following two examples illustrate a somewhat more restrictive aspect of the

SOSA approach insofar as the starting drug molecules, sulpiride and clebopride, did

not actually serve for the design of new and different activities. The objective here

was to transform the initial, nonselective dopaminergic antagonists in subtype-

selective D3 and D4 ligands.

4.4.5.1. Transforming the D2/D3 Nonselective Neuroleptic Sulpiride into a D3- Selective Partial Agonist.



Starting from the D2/D3 nonselective neuroleptic sulpiride, we were able to end up

with a selective and potent D3 receptor ligand114,115. One of the important findings was

that the benzamide present in the sulpiride derivative 35 could be advantageously

replaced by a naphthamide and that additional lipophilicity on the pyrrolidine nitrogen

increased the potency.

Figure 32. Switch from the D2/D3 nonselective dopamine antagonist N-methylsulpiride 35 to the D3-selective partial agonist BP 897 39. The numbers in parentheses indicate the D2/D3 affinity ratio.

The first interesting compound resulting from these variations was the D3 antagonist

nafadotride 36 in which the cyano group replaced the N-methylsulfonamido group116.

Nafadotride presents an excellent affinity for the D3 receptor (Ki =0.11 nM) and a

D2/D3 selectivity of 9.6 (Figure 32). However, nafadotride showed very poor

bioavailability in vivo and it could not be retained for clinical development. Further

modifications brought us to introduce various piperazine side chains. An is given by

the o-methoxyphenylpiperazine 38, characterized by the deletion of the electron-

attracting group in the Meta position to the carboxamido function. Surprisingly the

additional deletion of the o-methoxy group, as in compound 39, led to a potent

dopaminergic ligand (Ki =1.2 nM for D3) with a 56:1 preferential affinity for the D3

receptor114. This compound, named Do 897 and later BP 897, behaves as a partial

dopaminergic agonist and presently undergoes phase II clinical investigations.

Potential clinical applications are the selective inhibition of cocaine-seeking behavior

by drug addicts and the possible use as neuroleptic and as a means to suppress L-

Dopa-induced dyskinesia in the treatment of Parkinson patients.

4.4.5.2. Clebopride and Nemonapride as Leads for D4-Selective Dopaminergic Antagonists117.



Figure 33. Dopaminergic D4 receptor selective Benz amides derived from clebopride and nemonapride.

In parallel to the search for D3 subtype ligands, studies aiming to create D4 subtype-

selective agents were undertaken, also starting from benzamide drug molecules.

Ohmori and co-workers118 reported on the results of modification of their potent

D2/D3/D4 antagonist nemonapride (41, YM-09151-2), an analogue of clebopride (40),

to generate the new benzamide (42, YM-43611) (Figure 33). Compound 42 has

affinity for both D4 and D3 receptors (Ki = 2.1 and 21 nM, respectively) but with 110-

fold selectivity for D4 versus D2. Affinity for α1-adrenergic, β-adrenergic,

serotonergic, muscarinic, or histaminic receptors was weak or negligible. Interestingly

this compound shows in vivo activity in the inhibition of apomorphine-induced

climbing in mice, with an ED50 of 0.32 mg/kg sc.

During further investigations of the benzamide series, Hidaka and co-workers119

prepared 43 (YM-50001). This compound showed affinity for human D4 receptors

(Ki = 5.62 nM) versus hD2, hD3, and other receptors.

4.4.6. Thalidomide, Diclofenac, and Captopril as Leads.

The three following SOSA applications describe examples in which only slight chemical changes were needed to transform the starting drug molecule into an active compound with a different profile.



4.4.6.1. Thalidomide: A Potent Inhibitor of TNF.120

Thalidomide (44, Figure 34), first synthesized as an antihistaminic in 1954, was

introduced as a sedative/ hypnotic drug in 1956 but withdrawn because of its

catastrophic teratogenicity121.

In the early 1960s, a new use was found for thalidomide as a sedative in patients

suffering from lepromatous leprosy (erythema nodosum leprosum, ENL). A rapid and

noticeable improvement of the painful neuritis experienced by these patients was

observed and published in 1965122. Particularly it appeared to be efficacious for the

treatment of erythema nodosum leprosum, a possible complication of the

chemotherapy of leprosy123. This activity was attributed to a blockade of the TNF

production, and under restricted conditions (no administration during pregnancy or to

any woman of childbearing age), thalidomide found a new use as immunomodulator.

Figure 34; Thalidomide and (S)- and (R)-R-methyl thalidomide

Efforts have been made to develop derivatives of thalidomide that would specifically

maintain the desired actions of the drug without its side effects. One approach was to

separate the effects of the (R)-isomer from the effects of the (S)-isomer. However, this

approach was not effective because in vivo racemization of thalidomide is very fast.

Stable nonracemizable analogues of thalidomide. The (R)-isomer (Figure 34) was

effectively shown to be a potent inhibitor of TNF production in certain cell lines.

Further research of selective and potent thalidomide analogues seems promising120.

4.4.6.2. From the No steroidal Anti-inflammatory Drug Diclofenac to an Inhibitor of the Fibrin Transthyretine Amyloidal Formation.



Transthyretine (TTR) is a tetrameric protein made up of four identical subunits. In

human plasma, it is the secondary carrier of thyroxin (thyroid binding globulin being

the primary carrier) and the sole transporter of the retinol-binding protein-vitamin A

complex. Under acidic conditions, such as found in the lyzosomes, TTR dissociates to

an alternatively folded, monomeric intermediate that self assembles into amyloid

fibrils. Deposition of wild-type TTR has been implicated to cause the disease senile

systemic amyloidosis (SSA), whereas mutants such as V30M and L55P are connected

with familial amyloid cardiomyopathy (FAC) and familial amyloid polyneuropathy

(FAP). A limited screening identified the no steroidal anti-inflammatory drug

diclofenac (47) as a potent inhibitor of TTR amyloid formation124. Optimization of

diclofenac (47), with the aim of preparing compounds with high inhibition capacities

but also with preferential binding to TTR with regard to the other plasma proteins,

yielded the 3,5-disubstituted positional isomer 48 and the substituted anthranilic acid

49125 (Figure 35).

Figure 35. Increasing the TTR amyloid inhibiting activity of the NSAID diclofenac as a result of the synthesis of positional isomers.

4.4.6.3. Captopril Yields Inhibitors of Serum Amyloid Component P (“SAP”).



Figure 36. Captopril epimer as lead for the design of serum Amyloid component P inhibitors126.

A high-throughput assay for inhibitors of SAP binding to Alzheimer’s disease

amyloid-β (Aβ) was performed by scientists from Roche on amyloid fibrils

immobilized in micro titer plates and was applied to screen the in-house compound

library126. Two hits were identified. The first one (50; IC50 = 100 µM) was the S-3

epimer of captopril, and the second one (51; IC50 = 5 µM) was the corresponding

dimmer (Figure 36). Optimization simplified the central spacer group in removing the

sensitive disulfide bond as well as the two methyl groups, thus eliminating two chiral

centers. The obtained compound 52 (Ro 63-8695) shows a 900 nM affinity for its

target.

4.4.7. Herbicides and Laundry Brighteners as Lead Substances.

The last two examples of this perspective represent “exotic” versions of the SOSA

approach. Effectively, they no longer deal with the optimization of side activities of

drug molecules but with the optimization of the biological activities of nondrug

molecules contained in libraries of various origins. One of the leads was an herbicide,

and the other one was a laundry brightener.

4.4.7.1. Orally Active Nonpeptidic Endothelin-A Receptor Antagonist from Herbicides.

The two initial lead structures 53 (Lu 110896) and 54 (Lu 110897) (Figure 37),

initially designed as herbicides, were discovered by screening the chemical library of

BASF for compounds that bind to the recombinant human ETA receptor127.

Figure 37. Optimization of the two herbicide leads 53 and 54 to the potent andSelective ETA antagonist 55127.



Compounds 53 and 54 bind to the ETA receptor with Ki values of 250 and 160 nM,

respectively. The binding to the ETB receptor is much weaker (Ki = 3000 and 4700

nM). With the objective of enhancing the potency while simplifying the structure and,

particularly, avoiding the presence of one of the two stereocenters, compound 55 was

prepared127 It demonstrated high potency and selectivity (Ki(ETA) = 6 nM; Ki(ETB) =

1000 nM), was orally active in vivo, 30 mg/kg po), and showed a long duration of

action127

4.4.7.2. Laundry Brightener as Starting Lead for Antiviral Compounds128

Human respiratory syncytial virus (RSV) is a major cause of respiratory tract

infections in premature babies and infants up to 6 month of age. Widespread

outbreaks occur in the winter months in the northern hemisphere each year and

frequently reach epidemic proportions. At least 50% of children are infected during

their first exposure, and almost all have been infected by 2 years of age. During a

high-throughput screen of a 20 000- compound library, a whole virus cell-based assay

identified stilbene (56, Figure 38) as a potent RSV fusion inhibitor.

This original lead has an interesting history. The compound was synthesized some 40

years earlier at American Cyanamid’s Organic Chemicals Division as part of a

program to synthesize new laundry brighteners. Its antiviral activity (IC50 = 0.15 µM)

led to a synthetic optimizing effort that yielded the biphenyl analogue 57 (IC50 =

0.05 µM).128,129

Figure 38. Laundry brightener as starting lead for antiviral compounds.



4.4.8. From diuretic chlorthiazide to antihtpertensive drug diazoxide

In some cases, an expirenced medicinal chemist knows that what functional group

will elicit a particular effect. Chlorthiazide is antihypertensive agent that has strong

diuretic effect as well.IT was known from sulphanamide side chain can give diuretic

(increased urine excretion) activity. Consequently, diozoxide was prepared as

antihypertensive drug without diuretic activity2.

58 Chlorthiazide 59 Diazoxide

Figure 39. From diuretic lead chlorthiazide to antihypertensive diazoxide

4.5. DISCUSSION

The SOSA approach appears to be an efficient strategy for drug discovery,

particularly because it is based on the screening of drug molecules, and it thus

automatically yields drug like hits. Before starting a costly HTS campaign, it can

represent an appealing alternative. Once the initial screening has provided a hit, it will

be used as the starting point for a drug discovery program. By use of traditional

medicinal chemistry as well as parallel synthesis, the initial “side activity” is

transformed into the main activity, and conversely, the initial main activity is strongly

reduced or abolished. This strategy leads with a high probability to safe, bioavailable,

original, and patentable analogues.

4.5.1. Safety and Bioavailability.

During years of practicing SOSA approaches observed that starting with a drug

molecule as lead substance in performing analogue synthesis increased notably the

probability of obtaining safe, new chemical entities. In addition, most of them satisfy



obtaining safe, new chemical entities. In addition, most f them satisfy

Lipinski’s130,veber’s131, Bergstom’s132 and and Wenlocks133 observation in term of

solubility. Oral bioavailability and drug likeness.

4.5.2. Patentability.

Figure 40.Phosphodiesterase inhibitor. The phosphodiesterase inhibitor 18 derived from the tranquillizer diazepam 17, is sufficiently chemically

different to 17 and does not interfere with earlier patents.

When a well-known drug hits a new target, there is a risk that several hundred or

several thousand analogues of this molecule are already synthesized by the initial

inventors and their early competitors. These molecules are usually protected by

patents, or they belong already to the public domain. At first glance, a high risk of

interference thus appears probable. In fact, in optimizing another therapeutic profile

than the initial one, the medicinal chemist will rapidly prepare analogues with

chemical structures very different from that of the original hit. As an example, a

medicinal chemist interested in phosphodiesterases and using diazepam as lead will

rapidly prepare compounds that are out of scope of the original patents precisely

because they exhibit dominantly PDE inhibiting properties and almost no more

affinity for the benzodiazepine receptor.

4.5.3. Originality.

The screening of a library of several hundred therapeutically diverse drug molecules

sometimes ends up with very surprising results. A nice example of unexpected

findings resulting from a systematic screening is found in the tetra cyclic compound

(1, BMS-192548) extracted from 2, Aspergillus’s Niger WB2346 (Figure 19).



1 2 BMS-192548

Figure 41.Unexpected CNS activity of the tetracycline analogue (1, BMS-192548)131.

For any medicinal chemist or pharmacologist, the similarity of this compound to the

antibiotic tetracycline (1) is striking. However, none of them would a priori forecast

that BMS-192548 exhibits central nervous system (CNS) activities. Actually the

compound turns out to be a ligand for the neuropeptide Y receptor preparations134

3 4

Figure 42. Striking analogy between the vasodilator drug flosequinan (3)and the quinolone antibiotic norfloxacin (4)132.

It seems probable that a similar emergence of a new activity occurred with

flosequinan (3, Figure 20), which is a sulfoxide bioisostere of the quinolone

antibiotics. This compound turned out to be a vasodilator and a cardiotonic drug that

totally lost any antibiotic activity132.



4.5.4. Orphan Diseases.

As mentioned above, a differentiating peculiarity of this type of library is that it is

constituted of compounds that have already been safely given to humans. Thus, if a

compound were to “hit” with sufficient potency on an orphan target, there is a high

chance that it could rapidly be tested in patients for proof of principle. This possibility

represents another advantage of the SOSA approach.



5. SUMMARY

The above discussed examples provide convincing evidence that, in addition to

many drugs that were serendipitous drug discoveries, many others have resulted

from the observed side effects, in the laboratory.

In the clinics, or during their therapeutic application. Today, we possibly focus

too much on single targets that are investigated invitro.

Hidden treasure may be discovered by testing ‘old chemisry’against new targets,

by systemically optimizing some side effect of known drugs, and by reducing

drugs that failed because of problems in their metabolisms or hERG channel

inhibition.

Thus, it might well be that known drugs are much better source of lead structures

for new projects than we anticipated so far, As a consequence, we will experience

a successful comeback of traditional medicinal chemistry 52.

The SOSA approach appears to be an efficient strategy for drug discovery,

particularly because it is based on screening drug molecules and, thus,

automatically yields drug-like hits.

Before starting a costly HTS campaign, it can represent an attractive alternative.

Once the initial screening has provided a hit, that molecule will be used as the

starting point for a drug discovery program.

Using traditional medicinal chemistry, as well as parallel synthesis, the initial

‘side activity’ is transformed into the ‘main activity’ and, conversely, the initial

‘main activity’ is strongly reduced or abolished.

This strategy has a high probability of yielding safe, bioavailability, original and

patentable analogues.

The SOSA approach can be compared with other approaches, such as hit or lead

generation from known drug metabolites or the design of drug analogues, because

it makes use of old drugs to generate new hits or leads.

As a rule, the activities of metabolites are similar or close to the activity of the

corresponding active molecule. Contrary to this, the SOSA approach is based on

optimization of side activities that are totally different to the original activity of

molecule53

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