synthetic antibacterial agents

Synthetic Antibacterial Agents

PHRM 412

Antibacterial Agents

• Antibiotics– natural substances produced by microorganisms: penicillins, tetracyclines.

• Semi-synthetic antibiotics– chemically modified natural products: amoxicillin, doxycycline.

• Synthetic antibacterial agents– chemically synthesized, unrelated to natural substances: sulfisoxazole, ciprofloxacin.

Synthetic Antibacterial Agents

• Synthetic antibacterial compounds are divided into two major classes: topical agents and systemic agents.

• The topical agents are termed disinfectants, antiseptics, and preservatives, depending on how they are used.

Systemic agents

Some chemical classes of synthetic antibacterial agents include-

• Sulfonamides• Quinolones• Certain nitro-heterocyclic compounds,

(e.g. the nitrofurans and metronidazole)

UT antiinfectives

• Nitrofurantoin (a nitrofuran), nalidixic acid (a quinolone) and methenamine

• Failed to achieve plasma or tissue concentration for treating systemic infections

• Concentrated in the urine where they can eradicate urinary tract infections (UTI)

QUINOLONESSynthetic Antibacterial Agents

Quinolones

• The first quinolone to be marketed (in 1965) was nalidixic acid.

• The quinolone antimicrobials possess common an N-1-alkylated 3-carboxypyrid-4-one ring fused to another aromatic ring, which itself carries other substituents.

Nalidixic acid

Quinolones

• Nalidixic acid and cinoxacin are classified as first -generation quinolones

• They are considered to be minor urinary tract disinfectants

• Thus, the quinolones were of little clinical significance

Cinoxacin

Quinolones

• Addition of a fluoro group to the 6-position of the basic nucleus greatly increased the biological activity.

• Brought to the market in 1986, norfloxacin, the first of the second-generation quinolone.

Norfloxacin- R: ethyl X: CH

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Quinolones

• Following its introduction, intense competition ensued, more than a thousand second-, third-, and fourth generation analogues have now been made.

• More recent quinolones also are referred to as the fluoroquinolones

• Ciprofloxacin and levofloxacin dominate the worldwide fluoroquinolone market

Mechanism of action*

• The quinolones are rapidly bactericidal, largely as a consequence of inhibit ion of DNA gyrase and topoisomerase IV, key bacterial enzymes that dictate the conformation of DNA.

• Topoisomerase IV seems to be more important to some Gram-positive organisms and DNA gyrase to some Gram-negative organisms.

Mechanism of action

• Humans shape their DNA with a topoisomerase II, an analogous enzyme that does not bind quinolones at normally achievable doses, so the quinolones of commerce do not kill host cells.

Mechanism of action

• The Escherichia coli chromosome is a single, circular molecule of approximately 1 mm in length, whereas the cell is only 1 to 3 m long.

• Thus, the DNA molecule must be dramatically compacted in a conformationally stable way so that it can fit.

Mechanism of action

• Using the energy generated by adenosine triphosphate (ATP) hydrolysis, the molecule is progressively wound about itself in a positive super coil.

• It also must be partially unwound so that the cell has access to the genetic information that it contains.

Mechanism of action

• DNA gyrase (topoisomerase II) is an enzyme responsible for introducing negative super coils into circular duplex DNA.

• Negative super coiling relieve the torsional stress of helical DNA, facilitates unwinding, and thereby, allows transcription and replication to occur.

Mechanism of action

• DNA topoisomerase IV decatenates (unties) enchained daughter DNA molecules produced through replication of circular DNA.

• Topoisomerase IV's second function in the cell is to relax positive super coils. It shares this role with DNA gyrase, which is also able to relax positive super coils.

Structure–activity relationship

• The essential pharmacophore for the activity of quinolones is the carboxy-4-pyridone nucleus

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Structure–activity relationship

• Apparently, the carboxylic acid and the ketone are involved in binding to the DNA/DNA-gyrase enzyme system.

• Reduction of the 2,3-double bond or the 4-keto group inactivates the molecule, and substitution at C-2 interferes with enzyme–substrate complexation.

Structure–activity relationship

• Fluoro substitution at the C-6 position greatly improves antimicrobial activity by increasing the lipophilicity of the molecule, which in turn improves the drugs penetration through the bacterial cell wall.

• Additionally, C-6 fluoro increases the DNA gyrase inhibitory action.

Structure–activity relationship

• An additional fluoro group at C-8 further improves drug absorption and half- life but also may increase drug-induced photosensitivity.

Structure–activity relationship

• Heterocyclic substitution at C-7 improves the spectrum of activity especially against Gram-negative organisms.

• The piperazinyl (as in ciprofloxacin) and pyrrolidinyl (as in moxifloxacin) represent the most significant antimicrobial improvement .

Ciprofloxacin R: Cylopropyl; X: CH Moxifloxacin

Structure–activity relationship

• Unfortunately, the piperazinyl group at C-7 also increases binding to central nervous system (CNS) γ-aminobutyric acid (GABA) receptors, which accounts for CNS side effects.

• Alkyl substitution on the piperazine (lomefloxacin and ofloxacin) is reported to decrease binding to GABA, as does the addition of bulky groups at the N-1 position (sparfloxacin).

Structure–activity relationship

Lomefloxacin (phototoxicity) Ofloxacin

Sparfloxacin

*

Structure–activity relationship

• The cyclopropyl substitution (Moxifloxacin, Sparfloxacin, Gatifloxacin) at N-1 appears to broaden activity of the quinolones to include activity against atypical bacteria, including Mycoplasma, Chlamydia, and Legionella species.

Moxifloxacin Sparfloxacin

Structure–activity relationship

• The introduction of a third ring to the nucleus of the quinolones gives rise to ofloxacin.

• Additionally, ofloxaxin has an asymmetric carbon at the C-3' position.

Ofloxacin

*

Structure–activity relationship

• The S- (–) -isomer (levofloxacin) is twice as active as ofloxacin and 8- to 128-fold more potent than the R-(+)-isomer resulting from increased binding to the DNA-gyrase.

Structure–activity relationship

• Several of the quinolones produce mild to severe photosensitivity. A C-8 halogen appears to produce the highest incidence of photosensitivity via singlet oxygen and radical induction.

• Lomefloxacin has been reported to have the highest potential for producing phototoxicity.

Structure–activity relationship

• Substitution of a methoxy group at C-8 has been reported to reduce the photosensitivi ty (gatifloxacin).

Gatifloxacin

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Structure–activity relationship

• A chemical incompatibility common to all the quinolones involves the ability of these drugs to chelate polyvalent metal ions (Ca2+, Mg2+, Zn2+, Fe2+, and Al3+), resulting in decreased solubility and reduced drug absorption.

Structure–activity relationship

• Chelation occurs between the metal and the 3-carboxylic acid and 4-keto groups. Agents containing polyvalent metals should be administered at least 4 hours before or 2 hours after the quinolones.

Chelation of quinolones

Resistance

• Bacterial DNA gyrase is a tetrameric enzyme consisting of two A and two B subunits, encoded by the gyrA and gyrB genes.

• Bacterial strains resistant to the quinolones have been identified with decreased binding affinity to the enzyme because of amino acid substitution in either A or B subunits resulting from mutations in either gyrA and gyrB genes.

Resistance

• The highly polar quinolones are believed to enter the bacterial cell through densely charged porin channels in the outer bacterial membrane.

• Mutation leading to altered porin protein can lead in decreased uptake of quinolones and cause resistance.

Resistance

• In contrast to nalidixic acid, resistance to the fluoroquinolones is slow to appear but when it does appear, it is mainly due to efflux mechanisms which pump the drug back out of the cell.

Resistance

• Less common resistance mechanisms include mutations to the topoisomerase enzymes, which reduce their affinity to the agents, and alteration of porins in the outer membrane of Gram-negative organisms to limit access.

NITROHETEROAROMATIC COMPOUNDS

Synthetic Antibacterial Agents

Metronidazole

• Metronidazole was introduced in 1959 as an anti-protozoal agent, but began to be used as an antibacterial agent in the 1970s.

Metronidazole R: -OHNitroimidazole structure

Metronidazole

• After drug entering the bacterial cell the nitro group is reduced. This lowers the concentration of metronidazole within the cell which create a concentration gradient. As a result more drug can enter inside the cell.

Metronidazole

• The reduction mechanism proves toxic to the cell since free radicals (reactive oxygen species) are formed which act on DNA.

• Thus it inhibiting bacterial nucleic acid synthesis and resulting in bacterial cell death.

Figure: Structure of metronidazole and its mechanism of action

Ferrodoxin:Electron transport protein

Metronidazole

• Metronidazole also is a component of a multidrug cocktail used to treat Helicobacter pylori infections associated with gastric ulcers.

Tinidazole

• Tinidazole, another nitroimidazole, has been introduced as a competitor of metronidazole

• Compared to metornidazole it has longer duration of action

Tinidazole R: -SO2C2H5

Nitrofurantoin

• Nitrofurantoin, a widely used oral antibacterial nitrofuran, has been available since World War II.

• It is used for prophylaxis or treatment of acute urinary tract infections (UTI)

Nitrofurantoin

• Nitrofurantoin inhibits DNA and RNA functions through mechanisms that are not well understood, although bioreductive activation is suspected to be an important component of this. (undergoes reduction within bacterial cells to form radical species which act on DNA)

Methenamine

• Methenamine is a stable, inactive compound when the pH is more than 5.

• At a more acidic pH, the compound spontaneously degrades to generate formaldehyde, which has antibacterial properties.

Methenamine

• This is useful in the treatment of urinary tract infections. The normal pH of blood is slightly alkaline (7.4) and so methenamine passes round the body unchanged.

Methenamine (hexamethylenetetramine)

Methenamine

• However, once it is excreted into the infected urinary tract, it encounters urine which is acidic as a result of certain bacterial infections. Consequently, methenamine degrades to generate formaldehyde just where it is needed.

Methenamine

• Formaldehyde is an extremely reactive chemical and its antimicrobial action appears to be due to its ability to inactivate cell constituents such as proteins and nucleic acids.

• Vegetative cells are killed more quickly by formaldehyde than are spores.

• Formalin: 37-40% (w/v) formaldehyde

Methenamine

• Methenamine, a cyclic hydrocarbon, is hydrolyzed in an acid medium into ammonia and formaldehyde according to the following reaction:

N4(CH2)6 + 6H2O → 4 NH3 + 6HCHOH+

Methenamine Formaldehyde

Figure: Formation of formaldehyde from methenamine at acid pH

Methenamine

• Several characteristics of methenamine make it highly suitable for treating infections of the lower urinary tract:

(i) Bacterial resistance to formaldehyde has not been shown to develop

(ii) Significant levels of formaldehyde are not generated in the gut or in body tissues

Methenamine

(iii) Because of the first two, bacterial flora is not likely to be altered by administration of methenamine

(iv) Methenamine is relatively nontoxic and (v) Even in the form of an enteric tablet which

may be necessary to bypass gastric acidity, this drug is relatively inexpensive.

Methenamine

• Methenamine in ordinarily used doses is not likely to be effective in treating urinary infections when the urine pH ≥ 5.85.

• Under these circumstances, two alternative approaches could be considered:

(i) Increasing the dosage of methenamine; or

Methenamine

(ii) Acidifying the urine by withholding fluids or using pharmacologic doses of systemic urinary acidifiers such as ammonium chloride and ascorbic acid.

• Methenamine is frequently formulated with a weak acid, such as mandelic acid or hippuric acid.

Methenamine

• Despite the popularity of combinations of methenamine with mandelic or hippuric acid, there is no convincing evidence that in the doses used these organic acids significantly lower urine pH.

Mandelic acid Hippuric acid

Urinary analgesics

• Phenazopyridine, which is 2,6-diamino-3-(phenylazo)pyridine, is a urinary tract analgesic, commercially available as tablet.

Urinary analgesics

• Phenazopyridine hydrochloride is an azo dye with local analgesic and anaesthetic effects on the urinary tract.

• Phenazopyridine is a chemical which, when excreted into the urine (65% of an oral dose ), has a local analgesic effect.

Urinary analgesics• It is typically used in conjunction with an

antibiotic when treating a urinary tract infection.

• In this combination, phenazopyridine is taken for only a short time, typically two days, while the antibiotic is continued for longer.

• After two days, there is little evidence of any benefit from continued administration of phenazopyridine versus administration of an antibiotic only.

Urinary analgesics