1. DRUG METABOLISM

2. The Fate of a Drug

4. BIOTRANSFORMATION
Chemical modification of drugs or foreign compounds (xenobiotics) in the body.
The apparent function of drug or xenobiotics metabolism is their transformation into water-soluble derivative which can be easily eliminated via renal route.

5. Implications For Drug Metabolism
1. Termination of drug action
2. Activation of prodrug
3. Bioactivation and toxication
4. Carcinogenesis
Objective of studying drug metabolism
- medicinal chemists aware of the chemical processes involved in the biotransformation of drugs
- so design of new more safe drugs.

6. BIOTRANSFORMATION CAN LEAD TO THE FOLLOWING Inactivation
Eg: drugs like paracetomol, ibubrufen, chloromphenical etc and their metabolites are rendered inactive or less active.
Active metabolite from active drug
Activation of inactive drug
Certain drugs are inactive and need activation in the body.
Prodrug gives more bioavailability and less toxic effects.
ACTIVE DRUG
ACTIVE METABOLITE
allopurinol
alloxanthine
Digitoxin
Digoxin
Morphine
Morphine 6 glucoronide
Cholrol hydrate
trichloroethanol
Prodrug
Active metabolite
Levodopa
Dopamine
Sulindac
Sulfide metabolite
Predinosone
Prednisolone

7. Effect of drug metabolism
Active drug
Inactive Drug
Metabolic activation
Inactive prodrug
Active drug
Inactive metabolite
 Lipid Solubility
Chemical hook
R-H
R-OH

8. Two categories of xenobiotics are not or are hardly subject to metabolic transformations :
1- Hydrophilic compounds (e.g. saccharine, strong acids or bases)
2- Highly lipophilic polyhalogenated xenobiotics such as some insecticides e.g. DDT.

9. Lethal synthesis: a classic example of lethal synthesis is provided by the metabolic conversion of the nontoxic insecticide parathion into its oxygenated isostere paraoxon, a potent acetylcholinesterase inhibitor

10. Medium lung, kidney, intestine Low skin, testes, placenta, adrenals
Major Sites of Metabolism
Drug metabolism can occur in every tissue (e.g. gut, lung, kidney)..
The LIVER is the chief organ for drug metabolism because:
The blood flow through the liver is high
Hepatocytes contain numerous metabolic enzymes
High liver
Medium lung, kidney, intestine
Low skin, testes, placenta, adrenals
Very low nervous system

11. Extrahepatic microsomal enzymes
(oxidation, conjugation)
Hepatic microsomal enzymes
(oxidation, conjugation)
Hepatic non-microsomal enzymes
(acetylation, sulfation,GSH,
alcohol/aldehyde dehydrogenase,
hydrolysis, ox/red)

12. Biotransformation Nonsynthetic/ phase I/ functionalization rxn’s
Synthetic/ phase II/ conjugation

13. Non-synthetic/ phase I/ functionalization rxn’s
Oxidation
reduction
hydrolysis
cyclization
Decyclization

14. Oxidation Reduction Hydrolysis hydroxylations
aromatic, aliphatic, nitrogen
dealkylations(N-, S-, P)
deaminations
N-, S-, P- oxidations
S-replacements
epoxidations
others
Oxidation
Reduction
Hydrolysis
oxidoreductases
oxidases
monoamine oxidases
mixed function oxidases
azo reduction
nitro reduction
disulfide reduction
others
oxidoreductases
reductases
esterases
amidases
peptidases
lipases
esters
amides

15. PHASE I Oxidation Involves addition of oxygen or removal of hydrogen.
Reactions mostly occur in liver by a group of
mono-oxygenases,
cyt P450 reductase
oxygen.
Various oxidation reactions are
Hydroxylation
Oxygenation at C,N or S atoms
N or O dealkylation
Oxidative deamination
Eg: barbiturates, phenothiazines, steroids, paracetomol.
60% of drugs are metabolized primarily by CYPs.

16. 1. Oxidation reactions
The great majority of these oxidations are carried out by the haemoprotein cytochrome P-450 which is embedded within the phospholipid environment of the microsomes derived from the endoplasmic reticulum of living cells.
Cytochrome P450 (CYPs)
Cytochrome P450 is a family of enzymes located in the endoplasmic reticulum of liver.
When liver is homogenized and biochemically fractionated these enzymes are found in the microsomal fraction (small closed ER membrane fragments). Thus these enzymes are called microsomal enzymes.

17. 60% of drugs are metabolized primarily by CYPs.
Carbon monoxide binds to the reduced Fe(II) heme and absorbs at 450 nm (origin of enzyme family name).
Cytochrome P450 enzymes perform many of the most important biotransformation reactions.
These enzymes oxidize a wide variety of compounds foreign to the body.
There are at least 18 different forms of cytochrome P450 identified in humans, produced by different genes.
60% of drugs are metabolized primarily by CYPs.

18. Important cytochrome P450 enzymes: 1. CYP3A4. 2. CYP1A2.
3. CYP2C CYP2D6.
Individual enzymes differ in their substrate specificity and regulatory properties.

19. The reaction sequence illustrated by CyP Rrdox Cycle :

20. 2. Reduction reactions
Reduction
Involves removal of oxygen or addition of hydrogen.
Reactions mostly occur in liver by a group of
cyt P450 reductase
Eg: Alcohols, Warfarin, Chloramphenicol etc.
It is a major route of metabolism for aromatic nitro and azo compound as well as for a wide variety of aliphatic and aromatic N-oxides which are reduced to tertiary amines.

21. Hydrolysis
Cleavage of drug by addition of water.
Ester + water acid + alcohol in presence of enz esterase.
Occurs in liver, intestine , plasma.
Similarly, amides and polypeptides are hydrolysed by amidases and peptidase enz.
Eg: Aspirin, procaine, oxytocin etc.

22. Cyclization
Involves formation of ring structure from straight compound.
Eg: Proguanil
De- Cyclization
Involves formation of open structure from ring compound.
Eg: barbiturates, phenytoin etc.
Is a minor pathway.

24. Synthetic/ phase II/ conjugation Glutathione conjugation
Glucoronide conjugation
Sulphate conjugation
Ribonucleotide/ nucleotide conjugation
Acteylation
Methylation
Glycine conjugation
Glutathione conjugation

25. Phase II or Conjugation Reactions
Conjugation reactions link an endogenous moiety (an endocon) to the original drug (if polar functions are already present) or to the Phase I metabolite.
They are catalyzed by enzymes known as transeferases.
They involve a cofactor which binds to the enzyme in the close proximity of the substrate and carries the endogenous molecule or moiety to be transferred.
Forms highly ionized organic acid which is excreted in urine or bile.
Generally conjugation reactions have high energy requirements.

26. Glucoronide conjugation
Most important reaction carried out by UDP glucoronsyl transferases (UGT).
Drugs with hydroxyl and carboxylic acid are easily conjugated with glucoronic acid.
Glucoronidation increases the mol. Wt of the drug and thus favors excretion in bile.
Drug excreted in gut by bile is hydrolyzed bacteria and is reabsorped back and undergo same process, so this enterohepatic circulation prolongs the action of drugs.
Eg: Aspirin , paracetamol, morphine etc.

27. Beside xenobiotics, a number of endogenous substrates, notably bilirubin and steroids are eliminated as glucuronide conjugates.
In neonates and children, glucuronidation processes often are not fully developed. In such subjects, drugs and endogenous compounds (e.g. bilirubin) that are normally metabolized by glucuronidation may accumulate and cause serious toxicity.
For example, neonatal hyperbilirubinemia and gray baby syndrome, result from accumulation of toxic levels of the free chloramphenicol.

28. Acetylation
Drugs having amino or hydrazine are conjugated by acetyl co-enzyme.
Multiple genes control the N-Acetyl transferases and rate of acetylation shows genetic polymorphism
Eg: Sulphonamides, hydralazine, clonazepam etc.
Methylation
Methionine and cysteine are methyl donors.
Amines and phenols are methylated.
Eg: Adrenaline, histamine, nicotinic acid etc.
Suphate conjugation
Phenolic compounds and steroids are sulphated by sulfotransferases.
Eg: Cholramphenicol, Adrenal, Methyldopa etc.

29. Glycine Conjugation
Salicylates and other drugs with carboxylic acid are conjugated with glycine.
Glutathione Conjugation
is a minor pathway.
It also serves to inactivate highly reactive quinone or epoxide intermediate.
Presence of large amount of adducts results in tissue damage.
E.g. Paracetomol
Ribonucleoside / nucleotide synthesis
is a important pathway for activation of drugs in chemotherapy.
Mainly purine and pyramidine drugs are used.

30. Conjugation with glutathione (GSH)
Glutathione (GSH, -glutamylcysteinylglycine) is a thiol-containing tripeptide.
GSH conjugation is an important pathway by which chemical reactive electrophilic compounds are detoxified.
Many serious drug toxicities may be explainable also in terms of covalent interaction of metabolically generated electrophilic intermediates with cellular nucleophiles.
GSH protects vital cellular constituents against chemically reactive species by virtue of its nucleophilic sulfhydryl (SH) group.

31. Unlike other conjugative phase II reactions, GSH conjugation does not require the initial formation of an activated enzyme or substrate.
Two general mechanisms are known for the reaction of compounds with GSH:
1. Nucleophilic displacement at an electron-deficient carbon or heteroatom.
2- Nucleophilic addition to an electron-double bond.

32. Drugs and its metabolites are excreted in
Excretion
Drugs and its metabolites are excreted in
Urine
Faeces E.g. erythromycin, ampicillin, rifampin etc.
Exhaled air e.g. anesthetics, alcohol etc.
Saliva & sweat e.g. lithium, rifampin etc
Milk
Renal excretion
Renal excretion is responsible for excreting almost all drugs.
Net renal excretion (glomerular filtration + tubular secretion)
(or) the amount = - tubular reabsorption
of drug in urine

33. Glomerular filtration
Glomerular capillaries have pores larger and thus non- protein bound drug is filtered in the glomerulus, thus gfr filtration drug depends on plasma binding protein and renal blood flow (irrespective of lipid soluble and lipid insoluble drugs).
Glomerular filtration rate (g f r) ~ 120ml/min and declines progressively after the age of 50 and is low in case of renal failure.
Tubular reabsorption
Occurs by passive diffusion
Depends on lipid solubility and ionization of drug at urinary pH.
Non - lipid drugs are completely excreted out
The rate of drug = glomerular filtration rate
Change in pH of urine is also used in eliminating drug during drug poisoning

35. Tubular secretion
This is active transfer of organic acids and bases by two separate class of transporters OCT &OAT.
In addition, efflux transporter P-GP &MRP2 are located in luminal membrane of proximal tubules.
Active transport across tubules reduces concentration of its free form in the tubular vessels and promotes dissociation of protein bound drug which again is secreted.
Renal excretion
Renal excretion is responsible for excreting almost all drugs.
Net renal excretion (glomerular filtration + tubular secretion)
(or) the amount = − tubular reabsorption
of drug in urine

36. Kinetics of Elimination
Bioavailability (F), Vol. of distribution (V) and clearance (CL).
Clearance = Rate of elimination / conc. of drug in plasma.
Drug is eliminated by zero and first order reaction.
Half - life
The half -life of a drug is the time taken for its plasma concentration to be reduced to half its original value .
T1/2 = * vd/ CL
Where vd – vol. of distribution
CL – clearance

38. Factors affecting half – life
As patients age the skeletal muscle mass decreases which could decrease the vol. of distribution.
In obese person, due to increased adipose tissue higher dose of drug to be given to reach the target organ.
Some of drug get into the pathologic fluid space like ascites, pleural membrane causing long term toxicity.
Factors affecting half – life
Most common effect on half-life
Effects on volume of distribution
Aging
decreased
Obesity
Increased
Pathologic fluid
increased
Effects on clearance
Cyt P 450 induction (increased metabolism)
Cyt P 450 inhbition( decreased metabolism)
Cardiac failure (decreased clearance)
Hepatic failure (decreased clearance)
Renal failure (decreased clearance)

39. ENZYME INDUCTION
Certain drug substrates of CYP, upon repeated administration, “induce” or elevate the amounts of specific forms of the enzyme.
Induction results in accelerated metabolism of the drug inducer and any other drugs metabolized by the induced enzyme.
Classic Enzyme Inducers
Chronic ethanol
Phenobarbital
Tobacco (benzo[a]pyrene)
PCBs (Poly chlorinated biphenyls), dioxin (environmental)
Glucocorticoids

40. Some drugs are capable of inhibiting metabolizing enzymes.
ENZYME INHIBITION
Some drugs are capable of inhibiting metabolizing enzymes.
This is usually only important if a patient on a stable drug regimen starts taking a second drug metabolized by the same enzyme.
Examples: cimetidine, ethanol

41. Drugs that Inhibit Drug Metabolism by Forming Complexes with CYPs

42. Non-nitrogenous Substances that Effect Drug Metabolism by Forming Complexes with CYPs
Grapefruit juice - CYP 3A4 inhibitor; highly variable effects; unknown constituents
Isosafrole, safrole - CYP1A1, CYP1A2 inhibitor; found in root pear, perfume
Piperonyl butoxide & alcohol -CYP1A1, CYP1A2 inducer; insecticide constituent

43. Effect of Grapefruit Juice on Felodipine Plasma Concentration
5mg tablet
with juice
without

44. STATE OF HEALTH
Liver disease
Kidney disease
Endocrine diseases
Reduced blood flow

45. Optimising Pharmacokinetic Properties

46. Designing Prodrugs and Bioprecursors
Prodrug is any compound that undergoes biotransformation prior to exhibiting its pharmacological effects.
Prodrugs can be divided into two classes:
The Carrier-prodrugs :
Result from a temporary linkage of the active molecule with a transport moiety that is frequently of lipophilic nature.

48. Prodrugs to mask toxicity and side effects
Mask groups responsible for toxicity/side effects
Used when groups are important for activity
Example:
Aspirin for salicylic acid
Salicylic acid
Analgesic, but causes stomach
ulcers due to phenol group
Aspirin
Phenol masked by ester
Hydrolysed in body

50. Non-toxic & rapidly excreted
Barrier to
delivery of
drug
Drug
Pro-moiety
Barrier to
delivery of
drug
Pro-moiety
Drug
Drug
Biotransformation
Pro-moiety +
Drug
Non-toxic & rapidly excreted
Efficacy

51. Improvement of patient acceptance Increased site-specificity
Practical applications of carrier-prodrug design To
Improve of the lipophilicity
Enhancement of water solubility
mask toxicity and side effects
Increase the duration of Pharmacological effects
Mask polar groups
target drugs
Improvement of patient acceptance
Increased site-specificity

52. Prodrugs to increase water solubility
Often used for i.v. drugs
Allows higher concentration and smaller dose volume
May decrease pain at site of injection
Example:
Succinate ester of chloramphenicol (antibiotic)
Succinate ester
Esterase
Chloramphenicol

53. Prodrugs to increase water solubility
Example:
Phosphate ester of clindamycin (antibacterial)
Less painful on injection

54. Prodrugs to increase water solubility
Example:
Lysine ester of oestrone
Lysine ester of oestrone is better absorbed orally than oestrone
Increased water solubility prevents formation of fat globules in gut
Better interaction with the gut wall
Hydrolysis in blood releases oestrone and a non toxic amino acid

55. Add hydrophobic groups
Increase the duration of Pharmacological effects

56. Example:
Hydrophobic esters of fluphenazine (antipsychotic)
Given by intramuscular injection
Concentrated in fatty tissue
Slowly released into the blood supply
Rapidly hydrolysed in the blood supply

57. Reduces rate of excretion
Mask polar groups
Reduces rate of excretion
Example:
Azathioprine for 6-mercaptopurine
6-Mercaptopurine
(suppresses immune response)
Short lifetime - eliminated too quickly
Azathioprine
Slow conversion to 6-mercaptopurine
Longer lifetime

58. Improvement of patient acceptance
Used to reduce solubility of foul tasting orally active drugs
Less soluble on tongue
Less revolting taste
Example:
Palmitate ester of chloramphenicol (antibiotic)
Palmitate ester
Esterase
Chloramphenicol

59. Prodrugs used to target drugs
Example:
Hexamine
Stable and inactive at pH>5
Stable at blood pH
Used for urinary infections where pH<5
Degrades at pH<5 to form formaldehyde (antibacterial agent)

60. Increased site-specificity.
Two targeting possibilities can be considered:
Site-directed drug delivery
Site-specific drug release
Chemical Delivery Systems: Pharmacologically inactive molecules that require several steps of chemical and/or enzymatic conversion to the active drug and enhance drug delivery to particular organs or sites. Example: Brain-specific chemical delivery system

62.            The Bioprecursors :
Result from a molecular modification of the active principle itself. This modification generates a new compound, able to be a substrate for the metabolizing enzymes (often, oxidation and reduction). The metabolite being the expected active principle.
Example for bioprecursor design based on oxidative bioactivation is the cytotoxic agent cyclophosphamid.

63. Cyclophosphoramide for phosphoramide mustard (anticancer agent)
Phosphoramidase
(liver)
Cyclophosphoramide
Non toxic
Orally active
Phosphoramide mustard
Alkylating agent

64. Sulindac (NSAID) is a representative example for bioprecursor bioactivated by reduction.

65. Soft Drugs
Biologically active, therapeutically useful chemical molecules (drugs) characterized by a predictable and controllable in vivo deactivation, after achieving the therapeutic objective, to nontoxic, inactive compounds.

66. Soft Drugs Examples:

68. Hard drugs are "non-metabolizable drugs" or drugs which are metabolized to biologically active metabolites.
An example of a hard compound is the insecticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane)