biotransformation of drug Biotransformation/Xenobiotic metabolism/ drug metabolism/detoxification. -Xenobiotics: a wide variety of foreign compounds to which humans get exposed in day to day life. -It includes unknown compounds, drugs, environmental pollutants, toxins. -Many xenobiotics can evoke biological responses. DEFINITION The biochemical alteration of drug or xenobiotic in the presence of various enzymes that acts as a catalyst which themselves not consumed in the reaction and there by may activate or deactivate the drug is called biotransformation. Why Biotransformation is necessary?: To easily eliminate the drug To terminate drug action by inactivating it Consequences of Biotransformation Active to Inactive: Phenobarbitone---- Hydroxyphenobarbitone Inactive (prodrug) to Active : L-Dopa ---- Dopamine Parathion -- Paraoxon Talampicillin -- Ampicillin Active to equally active: Diazepam -- Oxazepam Amitriptyline -- Nortriptyline Imipramine -- Des-imipramine Codeine -- Morphine Sites of biotransformation In the body: Liver, small and large intestines, lungs, skin, kidney, nasal mucosa & brain. Liver is considered “metabolite clearing house” for both endogenous substances and xenobiotics. Intestines are considered “initial site of drug metabolism”. FIRST PASS METABOLISM: First pass metabolism or presystemic metabolism or ‘first pass effect’ After oral administeration many drugs are absorbed from the small intestine - transported first via portal system to the liver, where they undergo extensive metabolism before reaching systemic circulation. fundamental concepts in drug biotransformation Lipid soluble drugs are poorly excreted in the urine. They tend to store in fat and/or circulate until they are converted (phase I biotransformation) to more water soluble metabolites or metabolites that conjugate (phase II biotransformation) with water soluble substances. Water soluble drugs are more readily excreted in the urine. They may be metabolized, but generally not by the CYP enzyme systems. Enzymes catalyzing phase I biotransformation reactions Enzymes catalyzing phase I biotransformation reactions include: cytochrome P-450 aldehyde and alcohol dehydrogenase deaminases esterases amidases epoxide hydratases Addition of water Cleavage of R-O or R-N bond accompanied by addition of H2O CYTOCHROME P450 The cytochrome P-450 families are referred to using an arabic numeral, e.g., CYP1, CYP2, etc. Each family has a number of subfamilies denoted by an upper case letter, e.g., CYP2A, CYP2B, etc. The individual enzymes within each subfamily are denoted by another arabic numeral, e.g., CYP3A1, CYP3A2, etc.

1. 1
NIHAL.P
FIRST YEAR M.PHARM
NATIONAL COLLEGE OF PHARMACY

2. 2
-Biotransformation/Xenobiotic
metabolism/ drug
metabolism/detoxification.
-Xenobiotics: a wide variety of foreign
compounds to which humans get exposed
in day to day life.
-It includes unknown compounds, drugs,
environmental pollutants, toxins.
-Many xenobiotics can evoke biological
responses.

3. DEFINITION
• The biochemical alteration of drug or
xenobiotic in the presence of various enzymes
that acts as a catalyst which themselves not
consumed in the reaction and there by may
activate or deactivate the drug is called
biotransformation.
3

4. Why Biotransformation is
necessary?:
• To easily eliminate the drug
• To terminate drug action by inactivating it
By changing its physicochemical properties
from:
4
Active /inactive
Lipophilic
Unionised
Nonpolar
Plasma protein
bound
Inactive /active
Hydrophilic
Ionised
Polar
Free

5. Consequences of
Biotransformation
• Active to Inactive:
Phenobarbitone----
Hydroxyphenobarbitone
• Inactive (prodrug) to Active :
L-Dopa ---- Dopamine
Parathion -- Paraoxon
Talampicillin -- Ampicillin
5

6. • Active to equally active:
Diazepam -- Oxazepam
Amitriptyline -- Nortriptyline
Imipramine -- Des-imipramine
Codeine -- Morphine
6

7. Sites of
biotransformation
• In the body: Liver, small and large intestines,
lungs, skin, kidney, nasal mucosa & brain.
• Liver is considered “metabolite clearing house”
for both endogenous substances and
xenobiotics.
• Intestines are considered “initial site of drug
metabolism”.
7

8. FIRST PASS METABOLISM:
• First pass metabolism or presystemic
metabolism or ‘first pass effect’
• After oral administeration many drugs are
absorbed from the small intestine -
transported first via portal system to the
liver, where they undergo extensive
metabolism before reaching systemic
circulation.
8

9. LIVER
INTESTINALMUCOSA
BRONCHIALMUCOSA
• NICOTINE
• ISOPRINALINE
9

10. fundamental concepts in drug
biotransformation
Lipid soluble drugs are poorly excreted in the urine.
They tend to store in fat and/or circulate until they
are converted (phase I biotransformation) to more
water soluble metabolites or metabolites that
conjugate (phase II biotransformation) with water
soluble substances.
Water soluble drugs are more readily excreted in the
urine. They may be metabolized, but generally not
by the CYP enzyme systems.
10

11. 11

12. 12

13. Drug metabolism reactions :
13

14. Enzymes catalyzing phase I biotransformation
reactions
Enzymes catalyzing phase I biotransformation reactions
include:
cytochrome P-450
aldehyde and alcohol dehydrogenase
deaminases
esterases
amidases
epoxide hydratases
14

15. 15

16. PHASES OF DRUG METABOLISM
16

17. DRUG METABOLIZING ENZYMES
17

18. 18

19. PHASE-I REACTIONS
19

20. PHASE-I REACTIONS
20

21. PHASE-I REACTIONS
21

22. Hydrolysis
• Addition of water
– Cleavage of R-O or R-N bond accompanied by addition of
H2O
22
Ether
Amide
R
O
R'
+ H2O R OH HO R'+
R
N
R'
+ H2O R N HO R'+
H
H
H

23. Reduction
23
• Nitro to amino group
• Chromium VI to Chromium III
NO NOHNO2 NHH H

24. Flavin Monooxygenases
24

25. EPOXIDE HYDROLASE
25
•Highly reactive electrophile
•Binds to DNA,RNA.PROTEINS
•Cell toxicity

26. 26
CYTOCHROME P450
The cytochrome P-450 families are referred to using an arabic numeral, e.g., CYP1,
CYP2, etc.
Each family has a number of subfamilies denoted by an upper case letter, e.g., CYP2A,
CYP2B, etc.
The individual enzymes within each subfamily are denoted by another arabic
numeral, e.g., CYP3A1, CYP3A2, etc.

27. Electron flow in microsomal drug oxidizing
system
27

28. • When phase I reactions are not producing sufficiently hydrophilic (water
soluble) or inactive metabolites to be eliminated from the body, the
drugs or metabolites formed from phase I reaction undergoes phase II
reactions.
• Generally phase I reactions provide a functional groups or handle in the
molecule that can undergo phase II reactions. Thus, phase II reactions
are those in which the functional groups of the original drug (or
metabolite formed in a phase I reaction) are masked by a conjugation
reaction.
• Phase II conjugation reactions are capable of converting these
metabolites to more polar and water soluble products.
• Many conjugative enzymes accomplish this objective by attaching small,
polar, and ionizable endogenous molecules such as glucuronic acid ,
sulfate, glycine, glutamine and glutathione to the phase I metabolite or
parent drug. The resulting conjugated products are very polar (water
soluble), resulting in rapid drug elimination from the body. 28
PHASE 2 REACTIONS

29. • These reactions require both a high-energy molecule and an
enzyme.
• The high-energy molecule consists of a coenzyme which is bound
to the endogenous substrate and the parent drug or the drug’s
metabolite resulted from phase I reaction.
• The enzymes that catalyzed conjugation reactions are called
transferases, found mainly in the liver and to a lesser extent in the
intestines and other tissues.
• Most conjugates are biologically inactive and nontoxic because
they are highly polar and unable to cross cell membrane.
• Exceptions to this are acetylated and methylated conjugates
because these phase II reactions (methylation and acetylation)
does not generally increase water solubility but serve mainly to
terminate or reduce pharmacological activity (they are usually
pharmacologically inactive).
29

30. CONJUGATION MOLECULES:
• 1- Glucuronic acid conjugation:
• It forms O-glucuronides with phenols Ar-OH, alcohols R-
OH, hydroxylamines H2N-OH,and carboxylic acid RCOOH.
• It can form N-glucuronides with sulfonamides, amines,
amides, and S-glucuronides with thiols.
• 2-Sulfate conjugation:
• It is less common.
• It is restricted to phenols, alcohols, arylamines, and N-
hydroxyl compounds.
• But primary alcohols and aromatic hydroxylamines can
form unstable sulfate conjugates which can be toxic.
30

31. • 3-Amino acid conjugation:
• By the formation of peptide link. With glycine
or glutamine.
• 4- Glutathione conjugation:
• It reacts with epoxides, alkylhalides,
sulfonates, disulfides, radical species.
• These conjugates are converted to
mercapturic acid and mostly are excreted in
bile. It is important in detoxifying potentially
dangerous environmental toxins.
31

32. • 5,6- Methylation and acetylation reactions:
• These decrease the polarity of the drugs except tertiary
amines which are converted to polar quaternary salts.
• The groups susceptible for these reactions are phenols,
amines, and thiols.
• O-methylation of meta-phenolic OH in catecholamines
• does not generally increase water solubility but serve
mainly to terminate or reduce pharmacological activity
(they are usually pharmacologically inactive).
• 7- Cholesterol conjugation:
• For carboxylic acids by ester link formation or for drug
with ester group by trans esterification.
• 8- Fatty acid conjugation:
• For drugs with alcohol functional groups by ester link
32

33. There are six conjugation pathways:
O
HO
HO
HOOC
H
O UDPOH
R X+
O
HO
HO
HOOC
H
XOH R
X = OH, NR2, CO2H, SH, acidic carbon atomGlucuroinc acid UDP
O
Adenine
P
O
OS
O
HO
O OH
HOH2O3PO
PAPS
+ R X S X
O
HO
RO
X = OH, arylamine, NHOH
Glucuronyl Transferease catalyses this conjugation reaction
Sulfotransferease catalyses this conjugation reaction
33

34. 34
There are six conjugation pathways:
C
S
CoA
O
R
+
H2N CO2H
Y
H
N CO2H
Y
H
C
R
O
H
Y = H or CH2CH2CO2H
Acyl coenzyme A
N-acyltransferase catalyses the conjugation reaction
C
S
CoA
O
R
+
H2N CO2H
Y
H
N CO2H
Y
H
C
R
O
H
Y = H or CH2CH2CO2H
Acyl coenzyme A
Glutathione S-transferase catalyses this conjugation
reaction

35. 5)-Methylation
N-acyltransferase catalyses the conjugation reaction
Methyltransferase catalyses this conjugation reaction
O
Adenine
S
+
NH2
HO2C
CH3
HOH2O3PO
R X+ R X CH3
SAM
X = OH, NH2, SH
Aceyl CoA
Y =NH2, NHNH2, SO2NH2, CONH2
R
X
C
R
O
+C
S
CoA
O
H3C
R X
35

36. Non-Enzymatic Biotransformation
• Skeletal muscle relaxants like ATRACURIUM
are metabolised in the plasma spontaneously
through molecular rearrangement without
involvement of any enzyme action.
36

37. FACTORS AFFECTING BIOTRANSFORMATION OF
DRUGS
1. Physicochemical properties of the drug
2. Chemical factors
a. Induction of drug metabolising enzymes
b. Inhibition of drug metabolising enzymes
c. Environmental chemicals
3. Biological factors
a. Species differences
b. Strains differences
c. Sex differences
d. Age
e. Diet
f. Altered physiological factors: (pregnancy, hormonal
imbalance, disease states)
g. Temporal factors: ( circadian rhythm, circannual rhythm)
37

38. Induction of Drug Metabolism
• Enzyme induction is the process by which exposure to certain
substrates (e.g., drugs, environmental pollutants) results in
accelerated biotransformation with a corresponding reduction
in unmetabolized drug.
• Many currently used drugs are well known to induce their
own metabolism or the metabolism of other drugs. Some
examples are the anticonvulsant medications phenobarbital
and carbamazepine
• Cigarette smoking can cause increased elimination of
theophylline and other compounds.
38

39. Consequences of Induction
• Increased rate of metabolism
• Decrease in drug plasma concentration
• Enhanced oral first pass metabolism
• Reduced bioavailability
• If metabolite is active or reactive, increased drug effects or
toxicity
39
Therapeutic Implications of Induction
-Most drugs can exhibit decreased efficacy due to rapid
metabolism
but drugs with active metabolites can display increased
drug effect and/or toxicity due to enzyme induction
-Dosing rates may need to be increased to maintain
effective plasma concentrations

40. Inhibition of Drug Metabolism
• Drug metabolism is an enzymatic process can
be subjected to inhibition.
• Drugs and other substances can inhibit the
metabolism of other drugs.
40

41. Some types of inhibition
• Competition between substrates for enzyme active site
Concentration of substrates
Affinity for binding site (drug with hi affinity for an
enzyme will slow the metabolism of any low affinity
drug)
• Irreversible inactivation of enzyme
Complex with heme iron of CYP450 (cimetidine,
ketoconazole)
Destruction of heme group (secobarbital)
• Depletion of cofactors such as NADH2 for phase II
enzymes
41

42. Consequences of Inhibition
• Increase in the plasma concentration of
parent drug
• Reduction in metabolite concentration
• Exaggerated and prolonged
pharmacological effects
• Increased liklihood of drug-induced
toxicity
42

43. Therapeutic Implications of Inhibition
• May occur rapidly with no warning
• Particularly effects drug prescribing for
patients on multidrug regimens
• Knowledge of the CYP450 metabolic pathway
provides basis for predicting and
understanding inhibition
43

44. 44
Reference
a. Milo gibaldi, Biopharmacetics and clinical
pharmacokinetics, 4th edition, page No:213-230
b. Leon shargel,susanna wu-pong,Andrew.B.C.YU,
Applied biopharmaceutics & pharmacokinetics,5th
edition, page No:303-347
c.D.M.Brahmankar,Sunil.B.Jaiswal, Biopharmaceutics
& pharmacokinetics A Treatise, 2nd edition, page No:
139-192
d. CVS Subrahmanyam, Text book of biopharmaceutics
& pharmacokinetics-concepts & applications,1st edition
2010, page No:339-364

45. 45