📘 Lacture -5 Factors Affecting Drug Metabolism Including Stereochemical Aspects (Medicinal Pharmaceutical Chemistry – Semester IV)

 

📘 Factors Affecting Drug Metabolism

Including Stereochemical Aspects

(Medicinal Pharmaceutical Chemistry – Semester IV)

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🌟 Introduction

Have you ever wondered why the same drug behaves differently in different patients?

Or why one enantiomer of a drug is more active than the other?

The answer lies in drug metabolism and its influencing factors, including an important and often asked topic — stereochemical aspects of drug metabolism.

This topic is high-yield for exams and very important for understanding real clinical practice.

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🔬 What is Drug Metabolism?

Drug metabolism is the biochemical modification of drugs by enzymes, mainly in the liver, to make them:

✔ More water-soluble

✔ Easier to excrete

✔ Pharmacologically less active (in most cases)

📌 Exam definition:

Drug metabolism is the enzymatic conversion of lipophilic drugs into more polar metabolites.

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🧠 Factors Affecting Drug Metabolism

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1️⃣ Age

👶 Neonates & Infants:

• Immature liver enzymes

• Slower metabolism

• Higher risk of toxicity

Example:

👉 Chloramphenicol → Gray baby syndrome

👴 Elderly:

• Reduced hepatic blood flow

• Decreased enzyme activity

📌 Dose adjustment is required.

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2️⃣ Genetic Factors (Pharmacogenetics)

Genetic variation causes differences in enzyme activity.

Types:

• Fast metabolizers

• Slow metabolizers

Example:

👉 Isoniazid

• Slow acetylators → Toxicity

• Fast acetylators → Reduced therapeutic effect

📌 Very common viva + theory question

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3️⃣ Diet & Environmental Factors

• Smoking → Enzyme induction

• Alcohol → Alters metabolism

• Food components → Enzyme inhibition or induction

Example:

👉 Grapefruit juice inhibits CYP3A4 enzymes.

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4️⃣ Drug–Drug Interactions

🔹 Enzyme Inducers:

• Rifampicin

• Phenobarbital

• Phenytoin

➡ Decrease plasma drug concentration

🔹 Enzyme Inhibitors:

• Cimetidine

• Ketoconazole

• Erythromycin

➡ Increase toxicity risk

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5️⃣ Liver Function

Liver is the major site of metabolism.

• Hepatitis

• Cirrhosis

• Fatty liver disease

📌 Metabolism is reduced → Dose reduction needed.

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6️⃣ Route of Administration

• Oral drugs → First-pass metabolism

• IV drugs → Bypass liver initially

Example:

👉 Nitroglycerin is ineffective orally due to extensive first-pass metabolism.

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7️⃣ Dose & Frequency

• High dose → Enzyme saturation

• Repeated dosing → Enzyme induction

📌 Important in chronic therapy.

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🔄 Stereochemical Aspects of Drug Metabolism

⭐ Most important & scoring section

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🧪 What is Stereochemistry in Drug Metabolism?

Many drugs exist as enantiomers (mirror-image forms).

📌 The body does not metabolize both enantiomers equally.

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🔬 Enantioselective Metabolism

• Drug-metabolizing enzymes are chiral

• They show stereoselectivity

➡ One enantiomer may be:

✔ Metabolized faster

✔ More active

✔ More toxic

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📌 Important Examples

🔹 Propranolol

• S-enantiomer → More pharmacologically active

• R-enantiomer → Metabolized differently

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🔹 Warfarin

• S-warfarin → More potent anticoagulant

• Metabolized faster than R-enantiomer

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🔹 Thalidomide

• One enantiomer → Therapeutic

• Other enantiomer → Teratogenic

📌 Classic example in exams.

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🧠 Why Stereochemical Aspects Matter?

✔ Explains variation in drug response

✔ Helps in safer drug design

✔ Basis of chiral switching in modern drugs

✔ Improves therapeutic efficacy

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✍️ Exam-Ready Answer (Write This!)

Drug metabolism is affected by age, genetic factors, diet, drug interactions, liver function, route of administration, and dose. Additionally, stereochemical aspects play a crucial role, as drug-metabolizing enzymes are chiral and show enantioselective metabolism, leading to differences in activity, toxicity, and clearance of drug enantiomers.

⭐ Full marks guaranteed

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🧩 MCQ Section 

Q. Which factor is responsible for enantioselective drug metabolism?

A) Dose

B) Route of administration

C) Chiral enzymes ✔

D) Liver blood flow

👉 Answer in comments!

📌 Question for You:

Why is S-warfarin more potent than R-warfarin?

👇 Comment your answer below.

🔗 Internal Links 

• Drug Metabolism: Phase I

• Drug Metabolism: Phase II

• First-Pass Metabolism

• Pharmacogenetics (Coming Next)