Strategies in Selecting an Agent for Clinical Trials RICHARD W. PECK'

Clinical Pharmacology Glaxo Wellcome Research & Development Ltd.

Greenford Road Greenford, Middlesex

UB6 OHE, England

INTRODUCTION

Drug development is a long and expensive process. Despite the great advances in understanding of disease pathology and in scientific technology, development times are increasing. Pharmaceutical companies, who are responsible for the development of most new drugs are keen to reduce the level of risk associated with drug development. They are attempting to rationalize the process of discovery and early development in order to reduce the risk of a drug proving unsafe or ineffective late in development. In this article, I will first describe the present and historical forces that shape the current approach to discovery of potential new molecular entities, both chemical and biological. I will then discuss how preclinical and exploratory human studies can assist in picking compounds most likely to be safe and effective.

DRUG DISCOVERY

Historical Perspectives

Drug development is very ancient. The early use of pharmacologically active agents would have been as poisons to assist in the search for food as well as for medicinal and recreational purposes. These drugs were chance discoveries, and, in many ways, serendipity and luck remained the cornerstone of drug discovery up to very recent times. Only in the last few decades has the search for new drugs become a more rational process; yet, even now, serendipity has a large part to play in the discovery of new medicines.

Prior to the 1960s most drug discovery was the product of astute observation of nature and a great deal of luck. However, it would be wrong to ascribe all of the process to luck; the scientific method was present in many discoveries, and valuable lessons can be learned here. Withering, with a knowledge of medicine and botany, was able to deduce that the purple foxglove was the active ingredient in the herbal concoction used by an old woman in Shropshire. More importantly he was prepared to believe the evidence of patients being cured, rather than to dismiss the evidence

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because the explanation was unknown and contrary to accepted, scientific thought. He retained an open mind and looked for an explanation for the data rather than fit the data to a preconception.

The bark of the white willow (Sulix ulbu), from which salicylic acid was purified, had been used as an analgesic and antipyretic since antiquity. However, it was highly irritating, which limited its utility. However, Felix Hoffman, an organic chemist with Bayer, modified it to acetylsalicylic acid in an attempt to reduce its adverse effects and gave it to his father who had rheumatism. Heinrich Dreser, the director of research at Bayer, recognizing that a short name was essential for marketing purposes, used his native German to translate salicylic acid to Spirsaure and hence acetylsalicylic acid to Aspirsaure, and hence Aspirin. Thus aspirin provides useful lessons in the benefits of optimizing lead compounds and in marketing strategy.

The recognition of the value of curare, an Amazonian arrow poison, as an adjunct to surgery relied upon the observation that a man poisoned by curare could be kept alive by blowing air into his lungs, with confirmation of the observations in experiments on donkeys.’ After development of a suitably pure form, d-tubocurarine was introduced as a treatment for seizures in patients undergoing convulsive therapy. Anesthetists rapidly recognized its utility in controlling muscle contraction during surgery. The native people of the Amazon were either very lucky or highly skilled in pharmacology, as they were using a poison that was not orally absorbed and so would not harm them when eating the poisoned animal. Mr. Kirk, a member of one of David Livingstone’s expeditions was almost less fortunate. He heard that the natives kept their teeth in good order by rubbing them with the powdered seeds of Strophanthus hispidus, which they also used to poison their arrows. After trying some himself, without any dose-range finding work, Kirk developed a marked bradycardia. Making up for his earlier, “gung ho” approach to clinical testing, he decided that the risk-benefit ratio was not in his favor and took no more powder. However, he wrote down his findings, which were later read by Sir Thomas Fraser whose knowledge of pharmacology lead him to recognize the digitalis-like action of the powder and the discovery of ouabain. One can only speculate on the incidence of abdominal symptoms in the natives eating the animals shot with poisoned arrows.

The importance of publication of results in the process of drug discovery is also illustrated by the French physiologist, Gley, who carried out experiments similar to those of Banting and Best but sixteen years earlier. Failing to recognize the importance of what he saw, he did not publish but deposited a sealed letter with the Sociktd de Biologie de Paris. The letter was only opened after Banting and Best had published their findings. Zulzer also extracted insulin in 1908, but he injected too much into his dogs and they died from convulsions. Failure to recognize that the same drug could be toxic in overdose but have efficacy at low doses meant Zulzer failed to discover insulin.

The Late Twentieth Century

Drug discovery has gradually become more rational, with improvements in under- standing of chemistry and pharmacology and with increased understanding of the patbopbysiology of disease. In the 1960s, the main technological advances were in

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organic chemistry, with modifications of natural products like opiates and penicillins. During the 1970s, receptor pharmacology was increasingly understood, leading to the development of H2 antagonists, p blockers, and p2 agonists. In the 1980s and 199Os, molecular biology entered the drug discovery arena, providing agents like erythropoietin and the bone marrow stem-cell colony-stimulating factors.

As well as changes in drug discovery due to advances in technology and pharma- cology, the increased understanding of disease processes enabled the rational develop- ment of drugs like 1-dopa, HMGCoA-reductase inhibitors, and angiotensin-converting enzyme (ACE) inhibitors. Increasingly physicians can modify the mechanism of disease rather then being restricted to symptomatic therapy. However, ACE inhibitors demonstrate that newer discovery strategies do not replace the older ones, but add to them. Their benefits as antihypertensives and in the management of cardiac failure cannot be fully explained by the expected decrease in activation of the renin angioten- sin system. They also have serendipitous, beneficial effects on remodeling in vascular and cardiac tissue, which may be equally, or even more, important in producing their marked reduction in mortality from cardiac failure and diabetic nephropathy.’

Further anecdotal evidence of the role and value of serendipity in modem drug discovery comes from the antiepileptic, lam~trigine.~ First synthesized in a research program designed to develop folate antagonists, as this was thought to be an important determinant of antiepileptic activity, it was highly effective in animal models of epilepsy. Efficacy was confirmed in clinical trials, but by the time lamotrigine was first licensed in 1990, it was thought to be acting as a glutamate antagonist, and folate antagonism was recognized as unrelated to antiepileptic activity. Ultimately, lamotrigine has been determined to be a use-dependent sodium channel blocker. It remains an effective antiepileptic, albeit discovered for completely the wrong reasons.

DEVELOPMENT STRATEGY

Between 1964 and 1989, only 17.2% of new molecular entities for which an investigational new drug application (IND) was filed were subsequently approved for marketing! Of those that were terminated, 46% were for lack of efficacy and 27% for lack of safety. Over the same period, median development times from IND filing to early cessation of development or new drug application increased from 2.6 years in 1964- 1969 to 4.2 years in 1985- 1989.4 Furthermore, the cost of development of a new molecular entity increased from $80 million in 1970 to $350 million in 1988; and an estimated $500 million in 1995. The later in development that a drug fails, the more expensive it is. Consequently the pharmaceutical industry is continually looking for ways to improve the selection of candidates for efficacy trials by improving drug discovery still further and by improving the predictive value of preclinical and early human development.

The Present State

A model for the current process is proposed in FIGURE 1. Having decided on a presence in a given therapeuticldisease area, the initial step is to identify what is known of the disease pathogenesis with a view to identifying a suitable target. Target

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I Therapeutic Area I Commercial / Clinical

Physiology Pathology Genetics

Disease Models

Computer modelling Molecular libraries

Combinatorial chemistry High throughput screens

t t

t t

Pathogenesis

Define target

Identify lead

Drug Candidate

Academia / Clinical A

I , Pharmaceutical Industry

I FIGURE 1. A model for the process of drug discovery up to identification of a suitable lead compound to develop further. On the left-hand side of the model are listed some of the activities that contribute to each step; on the right is an indication of who generally performs those activities.

identification comes from an understanding of physiology, and of the pathology and genetics of the disease of interest. In general, the target is a protein, either a receptor or enzyme, and the aim is to decrease its activity. It is interesting to note that most diseases result from a loss of function, and searching for agonist drugs may be more rational than the more usual antagonisk6 Much of the basic research required to understand disease pathogenesis occurs outside the pharmaceutical industry, whereas development of drugs usually occurs within it. Close collaboration between these two groups is therefore essential for the best results that will benefit patients, research- ers, and the pharmaceutical industry.

Having identified a potential target, in vitro, and sometimes in vivo, screens are developed as surrogate markers to enable identification of a suitable lead compound. The choice of compounds to screen may be logical, based on knowledge of their other activities or their three-dimensional structure compared to that of the target. However, the increased rationality of target selection contrasts with a move to a more empirical method of choosing chemical leads. Compound libraries and combinatonal chemistry techniques produce vast numbers of different molecules whose pharmaco- logical properties need to be determined, requiring high throughput screens. Thus multiple chemical structures are synthesized and screened as opposed to the older method of synthesizing fewer structures, each of which had a greater chance of success. With high throughput screens, the modem methods find lead compounds faster than the old methods.

Preclinical Developmental Strategies

Identification of a suitable lead compound is followed by chemical modification to optimize its pharmacological properties and to improve its pharmacokinetics,

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Optimise lead 5- I Confirm activity I

I Predict human efficacykafety I

I Predict human kinetics I

FIGURE 2. A model for preclinical drug development.

pharmaceutical profile, and toxicology. It is then necessary to predict its likely human safety and efficacy (FIG. 2). Predictions of safety are based on the results of screens of pharmacological activity in a wide range of in vitro and in vivo systems, toxicology, and mutagenicity. Predictions of efficacy follow from the methodology used to first determine its activity. Animal models or in vitro screens are surrogate markers of efficacy in humans. It is assumed that they are predictive for the human disease state in humans. However, this assumption may be of uncertain validity, especially if the compound is the first of a new class; these issues are discussed in more detail below.

It is important to predict the pharmacokinetics of the compound in humans; regardless of its activity, a compound is of no use unless it gets to the site of action, and estimates of expected plasma concentrations are of value in choosing starting doses for the first administration to humans. Often the hepatic clearance of drugs can be established from in vitro experiments determining V,, and K,,, for metabolic enzymes in microsomal preparations.’ It is important to use human tissue, because predictions from animal in vitro data are less reliable. Such information allows an estimate of metabolic clearance in humans and may also suggest extensive first-pass metabolism. The use of whole hepatocytes or liver slices may extend the information gained from in vitro experiments. Furthermore, it is expected that the exact cytochrome P450 isozyme, or other enzymes involved in metabolism will have been identified prior to first human administration. This will enable understanding of potential drug interactions and reduce the requirement for human pharmacokinetic studies to investi- gate interactions with likely concomitant medications. Unexpected findings in human studies can also be investigated in vitro. After oral administration of the antiviral, netivudine, the principal metabolite was 5-propynyluracil, but it was not detected in plasma or urine for several hours after dosing.8 After intravenous netivudine, 5- propynyluracil was undetectable, suggesting that it was formed by the action of intestinal flora on unabsorbed netivudine after oral dosing. This was subsequently confirmed by the demonstration that human liver preparations did not metabolize netivudine but that it was slowly converted to 5-propynyluracil by incubation with human feces.

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Man loo 1

0.01 0.1 1 10 100 Body weight (kg)

FIGURE 3. An example of allometric scaling. Based on the log-log relationship between unadjusted clearance and body weight in a variety of mammals, clearance in man (humans) can be estimated by extrapolation.

Although metabolic predictions based on animal tissue experiments may be incor- rect, pharmacokinetic parameters in animals are extrapolated to humans by the tech- nique of allometric scaling. Mammalian species have similarities in their anatomical and physiological features that can serve as a basis for scaling between specie^.^ In general, clearance of drugs, expressed per unit of body weight, increases as body weight decreases,'O but there is a linear relationship between body weight and non- weight-adjusted clearance on a log-log plot (FIG. 3). Consequently, determining the clearance of a drug in two or more mammalian species of different body weights allows prediction of the clearance in other species, including humans. Similar predictions can be made for the volume of distribution. Knowledge of clearance and volume of distribution aIlows the calculation of likely drug concentrations. The predictions are best if clearance is mostly renal, but they can be valid for metabolized drugs too. They are most useful for intravenous dosing, as there is no need to consider interspecies differences in bioavailability.

Absorption is predicted by measuring the permeability coefficient of the drug through a layer of CaCo-2 cell monolayers, a model of the intestinal epithelium. This technique has been shown to correlate well with the percentage absorption of a wide range of different drugs in vivo."

Exploratory Clinical Development

The objectives of exploratory clinical development are to predict safety and efficacy in patients usually based on results in healthy volunteers. Some drugs, for example, chemotherapy, are too toxic for administration to volunteers, and early human data come from trials in patients. Prior to initiating clinical trials in patients, it is important to further increase confidence that the drug will be useful.

Its pharmacokinetics must be investigated and the animal and in vitro predictions confirmed. Unsuitable kinetics, for example, a very short or very long half-life, low bioavailability, or large between-subject variability in concentrations may lead to

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TABLE 1. Features of Surrogate Markers

Drug effects occur over shorter time than the clinical end point. Easier to measure than clinical end point. Simple. Safe. Predictive of the clinical effect. Allow investigation of efficacy in healthy volunteers. Measure enzyme or receptor activity or the resultant pharmacological effect. Help establish dosekoncentratiodeffect.

TABLE 2. Concepts of Validity and Reliability for Surrogate Markers

Validity The accuracy of portrayal of the real situation

Content Construct Criterion related

Concurrent Predictive

Prescriptive

Are all aspects of the disease measured? Is the theoretical basis of the marker the same as that of the disease? Does the marker correlate with standard indicators of the disease? Supporting evidence obtained at the same time as the marker. Supporting evidence gained at a later time (i.e., the marker predicts subsequent events). Marker justified on the successful outcome of a treatment choice it determined. Does the marker reflect the clinical symptomatology? Face

Reliability The reproducibility of a measurement or surrogate marker.

termination of development. Its pharmacodynamic effects and therapeutic index must be estimated, requiring an estimate of the likely therapeutic dose and the maximum tolerated dose. It is not necessary to continue dosing until significant adverse events occur; if the drug is well tolerated at several multiples of the expected therapeutic dose, that is sufficient.

In order to estimate the therapeutic dose in volunteers, it is necessary to use a surrogate marker; surrogate markers may also be very useful in early patient studies if the course of the disease is prolonged. A surrogate marker (TABLE 1) is defined as a pharmacodynamic measure that can be used as a predictor of therapeutic or adverse effects of a drug in the target patient population. To be useful, surrogate markers require validation (TABLE 2). The key issue is that they have predictive validity, in that changes in the marker predict the required clinical effect in patients. A surrogate should have both positive and negative predictive validity. However, in the case of drugs with novel mechanisms of action, there will not be a surrogate with predictive validity. In this instance the best that can be achieved is a surrogate with face validity, a symptomatology or outward appearance resembling the disease state, andor construct validity, a theoretical basis similar to that of the disease. In the development of lamotrigine, it was recognized that early clinical trials would

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still require at least 44 weeks for each individual patient? Consequently, surrogate markers were used to increase confidence in the likelihood of success and in the doses chosen for study in the longer trials. Single doses were found to significantly reduce interictal EEG spike frequency in epileptic patients’’ and to suppress the photoconvulsive response in the EEG.I3 Both surrogates had construct but not pre- dictive validity, although subsequent trials confirmed clinical efficacy at these doses. In the development of further antiepileptics with a similar pharmacology to lamotrig- ine, these models could now be considered to have positive predictive value. Their negative predictive value, or value for potential antiepileptics with different pharma- cology, would still be unknown.

Investigating the relationship between dose, plasma concentrations, and a dynamic effect is a powerful tool for predicting the dynamic effects of drugs beyond the range of doses investigated in early studies. For the topoisomerase inhibitor, GG21 I , it was possible to determine the relationship between plasma concentrations and the reduction in neutrophil and platelet counts after dosing in only 16 patients. From this data, an upper dose limit was established that would not be associated with excessive reductions in blood counts, findings subsequently confirmed in a larger group of patients.14 In the development of the short-acting anesthetic agent, remifen- tanil, early volunteer studies using subtherapeutic doses of the drug and investigating effects on the EEG spectral edge as a surrogate marker allowed accurate prediction of the onset and offset times of anesthesia. In this case the predictive validity of the surrogate marker was demonstrated by comparison with the pharmacologically related drug, alfentanil, whose clinical effects were already known.”

Developing a Novel Indication for a Licensed Drug

In some disease states, of which systemic lupus erythematosus may be one, potentially useful drugs may be developed initially for other indications. Only once on the market are additional indications considered. In this situation much is already known about the drug’s safety, pharmacokinetics, and efficacy in other, possibly related diseases. However, the likelihood of efficacy in the new indication is uncertain. The only way to be sure of efficacy is a suitably designed clinical trial with clinically relevant end points. However, in a disease such as systemic lupus erythematosus, the trials will be long and are likely to need many patients in order to have adequate statistical power. Simpler, shorter studies of the effects on validated surrogate markers would be very useful in determining the best drugs and doses to study in definitive clinical trials. Although much is already known about a licensed drug, the principles of development remain the same as for the new molecular entity.

CONCLUSIONS

The choice of drug discovery target is becoming increasingly rational as under- standing of disease mechanisms increases. Advances in organic chemistry and in screening for pharmacological activity now enable the rapid identification of poten- tially effective new molecular entities. These continue along the development track, having to clear a series of hurdles of increasing relevance to their use in patients in

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order to filter out ineffective compounds. This process combines the steady, directed approach of Aesop's tortoise with a requirement for the speed of the hare. It must also combine the talents and abilities of academic research with that of the pharmaceutical industry. In general the processes of selecting and developing the best molecular entity as a medicine occur within the pharmaceutical industry. However, much of the knowledge of what to look for comes from academic research.

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