1. Drug Metabolism and Prodrugs
Drug metabolism is the transformation of foreign compounds ( xenobiotics) into water soluble derivatives which can easily eliminate via renal route.

2. Xenobiotics ( non essential exogenous, foreign compounds )
This applies particularly to chemical compounds which enter the organism and don't serve as nutrient or other essential factors.
Xenobiotics can not be allowed to concentrate beyond limits in a living system, but must be eliminate by excretion. Xenobiotics metabolism
appears as a logical evolutionary acquisition.

3. Drug metabolism may be regarded as detoxification processes in some cases.
Not all metabolites are non toxic

4. Example

5. As a rule, the metabolism of xenobiotics takes place in two steps known as phase I & phase II reactions

6. Phase I ( functionalization reaction )
New polar groups such COOH, OH, SO3H or NH2 are introduced or unveiled from pre-existing function through oxidative, reductive
or hydrolytic reactions.

7. Phase II ( conjugation reactions )
Link an endogenous solubilizing moiety either to the original drug (if polar function are already present) or to the phase I metabolite.
Common solubilizing groups are glucuronic acid, various amino acids or sulphate grops.
The conjugate molecule, being more polar and water-soluble, is usually excreted via the renal route

8. Effect of metabolism on the therapeutic activity of drugs

10. Conclusion
Drug metabolism or biotransformation are the chemical reactions that are responsible for the conversion of drugs into other products within the body before and after they have reached their sites of action.
Almost all of these reactions are enzyme catalyzed.
A knowledge of the concepts of drug metabolism is useful in both the design of new drugs and the improvement of existing drugs.

11. Factors affecting drug metabolism
Genetic factors
Physiological factors
Pharmaceutical factors
Drug interactions

12. Biological half –life (t1/2) of various drugs
Genetic factors
Biological half –life (t1/2) of various drugs

13. Physiological factors

14. Pharmacodynamic factors
The dose, the route and the frequency of administration of drugs can affect their metabolic profiles.
Drugs given too frequently may overload the metabolic system available to it, leading to elevated blood and tissue levels of the drugs. The effect of protein binding also influences the metabolism

15. Drug interactions
Phenobarbital stimulate the metabolism of Diphenylhydantoin.
The half life of Chloropropamide, Diphenylhydantoin & Cyclophosphamide is increase in presence of Chloramphenicol.
Plasma Concentration of anticoagulants such as Warfarin are reduced by simultaneous application of barbiturate

16. Phase I (Functionalization reactions
Oxidations (electron removal, dehydrogenation and hydroxylation)
Reduction ( electron donation, hydrogenation
and removal of oxygen )
Hydrolytic reactions

17. Oxidation Reactions
The main enzymes involved in the oxidation of xenobiotics called mixed – function oxidases or monooxygenases, found mainly in the liver but also occur to less extent in other tissues.
Cytochrom P-450 ( CYP – 450 )

20. Major reactions of oxygenation catalyzed by cytochrome P450

21. Major reactions of oxygenation catalyzed by cytochrome P450

22. Oxidation reactions Carbon oxidation reaction
a - Hydroxylation of saturated aliphatic carbon atoms
b - Hydroxylation at activated SP3 carbon atoms
c - Oxidation attack on unsaturated aliphatic systems
d - Hydroxylation of aromatic rings
N-oxygenation reactions
S-oxidation reactions

23. Hydroxylation of saturated aliphatic carbon atoms
Saturated aliphatic C-H bonds are normally metabolised by hydroxylation ( ω & ω-1 )

24. Enzymatic introduction of a hydroxyl group into cyclohexane ring generally occurs at C-3 or C-4
In human the trans-4-hydroxycyclohexyl product has been reported as a major metabolite of acetohexamide ( hypoglycemic agent )

25. Terodiline
Aromatic p-hydroxylation predominate with R-isomer where as benzylic hydroxylation is preferred with S-isomer.

26. Hydroxylation at activated SP3 carbon atoms

28. Oxidation involving carbo-heteroatom system

29. N-dealkylation

32. Halogenated aliphatic derivatives

33. Oxidation attack on unsaturated aliphatic systems