CHIRAL DRUGS - Medicinal Chemistry

CHIRAL DRUGS

Contents

• Chiral drugs

• Chiral inversion

Intended learning outcomes

At the end of this lecture, student will be able to:

Describe the importance of stereoisomerism in drug activity and drug design.

CURRENT STATUS OF CHIRAL DRUGS

1987

• 57% marketed drugs were chiral.

• 2% single enantiomers.

2006

• 80% drugs approved by the FDA were chiral.

− FDA now requires information about the structure and activity of each isomer present in a racemic mixture of a new medication.

− In 2000, 40% of drugs on sale in the US were single enantiomer-based.

− In 2004, about 80% of drugs entering market are single enantiomer variants.

What sparked the change????

• 1987 – change in FDA regulations.

• Inclusion of information on the enantiomer composition of chiral compounds in new-drug applications.

THALIDOMIDE

• One asymmetric carbon atom, exists as 2 enantiomers

• S-Enantiomer → sedative

• R-Enantiomer → Teratogen

S thalidomide

â

Sedative

R thalidomide

â

Teratogen

Chiral Inversion

• Unique metabolic pathway.

• Enzymatic or non-enzymatic.

• Involves unidirectional conversion of one enantiomeric form to another.

R →   S

CHIRAL SWITCH

• Development of a single enantiomer from a previously marketed racemate.

• Resulted in a number of agents being re-marketed as chiral drugs.

• Same or similar therapeutic indications. 

β2 ADreNErgic recePTOR AGONIST

Salbutamol

• Salbutamol →Mixture of (R)-salbutamol and (S)-salbutamol

• Levosalbutamol is the (R)-enantiomer → active bronchodilator.

Racemic and (S)-Salbutamol

• Induce airway hyper responsiveness.

• Increase sensitivity to allergen challenge.

• Inhalation of levosalbutamol → greater bronchodilatation than the

CALCIUM CHANNEL BLOCKER

Verapamil

• (S)-verapamil: vasodilating, cardiac antidepressant properties.

• (R)-verapamil: a vasodilating drug.

• Verapamil blocks the P170 glycoprotein.

• R-verapamil might be expected to be more beneficial.

S-AMLODIPINE

• S-Amlodipine → active calcium channel blocker.

• R-Amlodipine → inactive as calcium channel blocker.

• Mainly responsible for peripheral edema.

• Treatment of hypertension.

• S-Amlodipine is effective at half the dose of racemate.

• Incidence of peripheral oedema is negligible.

• Treatment of normotensive angina patients.

• S-Amlodipine is effective at half the dose of racemate.

LOCAL ANAESTHETIC

Levobupivacaine

Cardio toxicity of the drug: R-enantiomer.

• Levobupivacaine - ‘S’ enantiomer of Bupivacaine.

• Moderately more potent.

Levobupivacaine is less cardiotoxic.

SELECTIVE SEROTONIN REUPTAKE INHIBITOR

Escitalopram

• S-enantiomer of Citalopram.                                                         

• Doses that show improvement in depressed patients.

10 mg / day – ‘S’ enantiomer

40 mg / day – racemate

• Other advantages of Escitalopram.

• Faster onset of action.

• Reduction in side effects.

• Improved tolerability profile.

NON-STEROIDAL ANTI-INFLAMMATORY DRUG

Dexibuprofen

• Inhibition of cyclooxygenase activity – ‘S’ enantiomer.

• 60% of R enantiomer undergoes chiral inversion to the active S-enantiomer.

• chiral Dexibuprofen (1200 mg daily) was better than racemate (2400 mg daily).

• Highly effective NSAID.

• Low adverse effect profile.

ADVANTAGES OF CHIRAL DRUGS

• Chiral drug is a single agent instead of a mixture of two distinct drugs.

• Simplifies the interpretation of the Basic Pharmacology.

• Greater selectivity for their biological targets improved therapeutic indices and reduced adverse effects.

• Longer or shorter duration of action - more appropriate dosing frequency.

• Decreased inter individual variability.

SOME DRUGS ARE BETTER AS RACEMATES

β BLOCKERS

• β blockers currently used contain at least one chiral center.

• Most are marketed as racemates.

SUMMARY

• FDA now requires information about the structure and activity of each isomer present in a racemic mixture of a new medication.

• In 2000, 40% of drugs on sale in the US were single enantiomer-based.

• In 2004, about 80% of drugs entering market are single enantiomer variants

• Chiral inversion of drug molecules and its effect is studied.

Post a Comment

0 Comments