Metabolism plays an important role in drug discovery by providing information on metabolic stability, metabolic profiles, drug-drug interactions, and pharmacokinetics. Advances in fields like pharmacogenetics, pharmacogenomics, and drug transporters have expanded understanding of metabolic pathways and genetic influences. In vitro studies using liver microsomes and recombinant enzymes help identify metabolic enzymes and potential for drug interactions. Drug metabolism occurs in two phases: phase I introduces functional groups through reactions like oxidation, while phase II involves conjugating the drug with endogenous compounds like glucuronic acid to make it more soluble for excretion. Understanding metabolism aids in predicting toxicity, efficacy of drug candidates, and optimizing properties like stability.

2. Role of metabolism in drug discovery:
Hajra Naseer
Imrana shaheen
Erum Noreen
Rabia Sadaqat
Ayesha Naseer
Attia Arif
Instructor:
Dr. Ifzan Arshad
Fareeda Shahid
Presenters:

3. Introduction:
• History:
• Drug metabolism traditionally focused on aspect of:
• Absorption, distribution, metabolism, excretion (ADME)
• properties of compound
• area of metabolite identification
• identification of drug metabolic enzyme(DME)
• associated metabolic pathway& reaction mechanism.
• Limitation:
• It has limited scope of understanding and unavailability of invitro and invivo
tools with which to evaluate more complexes, properties and processes.

4. Advancements:
• Advances over the past decades separate pharmacogenetics,
pharmacogenomic and drug transporters have shifted metabolic paradigm.
• Knowledge of genetic and genomic of DME allows us to better understand
and predict enzyme regulation and its effects on pharmacokinetics as well as
on pharmacology.
• Advances in transporte area have provide unprecedented insight into the role
of transporter protein in ADME of drug and their consequences with respect to
drug drug interaction toxicity etc.

5. Why it is important?
• Invitro studies can give us information about:
•Metabolic stability
•Metabolic identification and profile
•Drug-drug interaction
• Metabolic stability:
• It refers to the susceptibility of compound to biotransformation of in the
context of selecting and designing drug with favourable pharmacokinetic
properties.

6. Metabolic identification and profiling:
• Metabolic identification and profiling have become a routine exercise during lead
optimization and subsequent development process.
• Therapeutic efficiency of drug is dependent on its exposure
• Drug metabolism may lead to the formation of metabolites that can either be
pharmacological active or elicit adverse effect.
• Drug-drug interaction:
• A drug-drug interaction is when a reaction takes place between two or more
prescribing drugs.
• Example : Interaction between warfarin anticoagulant (blood thinner) Fluconazole
antifungal medicine leads to potentially dangerous increase in bleeding.

7. Pharmacokinetics:
• Pharmacokinetics is 'what the body does to the
drug' .These interactions occur when one drug
alters the concentration of another drug (the
object) with clinical consequences.
• Pharmacokinetic interactions occur when the
absorption, distribution, metabolism or
elimination process of the object drug is altered by
the precipitant drug and hence such interactions
are also called as ADME interactions.
• The resultant effect is altered plasma
concentration of the object drug.

8. Classification of pharmacokinetics drug interactions
• Absorption interactions
• Distribution interactions
• Metabolism interactions
• Excretion interactions

9. Drug absorption interaction:
• Absorption interactions are those where the absorption of the object drug is altered.
• Since the oral route is the one, most frequently used to administer drugs, interactions
influencing absorption are more likely to occur within the gastrointestinal tract.
• The net effect of such an interaction is:
• Faster or slower drug absorption.
• More or, less drug absorption.
• Most clinically significant interactions occur due to the following factors ;
• Changes in gastrointestinal pH
• Changes induced by chelation
• Changes in gastrointestinal motility

10. Drug distribution interaction:
• Drug distribution Interactions are those where the distribution pattern of the object
drug is altered.
• The major mechanism for distribution interaction is alteration in protein-drug binding.
• Many drugs interact by displacement of each others binding to plasma proteins.
• Acidic drugs are known to have an affinity to bind to plasma proteins, hence when two
or more are given concomitantly, competitive binding for the same site or receptor may
displace one drug from the protein binding site increasing the amount of the displaced
free drug in plasma and various tissues setting up an interaction leading to an enhanced
potential for toxicity.

11. Metabolic interaction:
• Stimulation of metabolism
• Certain drugs stimulate the activity of hepatic microsomal enzymes. This
effect is referred as enzyme induction.
• The increased activity is due to enhanced enzyme synthesis results in
increased amounts of drug metabolizing enzyme.
• Enzyme induction will result in increased metabolism and excretion and
reduced effect of agent which is metabolized by the hepatic enzymes.
• E.g. : Warfarin and phenobarbital
• Phenobarbital increases the rate of metabolism of warfarin resulting in
decrease anticoagulant activity.

12. Drug elimination reaction:
Drug elimination reactions are those where the excretion pattern of the object drug is
altered.
• The major routes for elimination of drugs remain the kidney and bile, but there are
no significant drug - drug interactions through bile elimination, but only drug-disease
ones.
• Some drugs are excreted from the body unchanged in the active form, usually in the
urine or via the biliary tract in the faeces.
• Drugs that are chiefly excreted by the kidneys can get involved in drug interactions
by different mechanisms such as competition at active transport sites, or alterations in
glomerular Filtration, passive renal tubular reabsorption or active secretion and unnary
pH.

13. Continue….
• Changes in renal drug clearance may occur due to effects on renal tubular function or
urine pH.
• For example, probenecid reduces the renal clearance of anionic drugs such as
methotrexate and penicillin.
• Major mechanisms of excretion interactions are:
• Alteration in renal blood flow
• Alteration of urine pH v/ Competition for active secretions
• Forced diuresis
• Alteration in renal blood flow
• Alteration of urine pH
• Competition for active secretions

14. Role of metabolism in drug discovery
The reaction catalysis can be divided into
two phases.
Phase I
Metabolic reaction are functional
metabolism. It introduce the polar functional
group to the parent atom.
Phase II
They are mostly conjugated reactions.
Conjugate a polar moiety to its parent
compound.

15. Phase I
Phase I metabolism involve oxidation,
reduction and hydrolysis.
 Hydrolysis and oxidation are mainly involve
in the drug discovery.
Hydrolysis:
Biotransformation of ester, amide, and
epoxide.
Oxidation:
Hydroxylation of aromatic and aliphatic
carbons and heteroatoms.

16. Activate enzymes in Phase I metabolism
Enzymes involved in the Oxidation:
1. Esterases: abundant in human plasma
2. Microsomal Epoxide Hydrolases (mEHs):
abundant in human liver
Enzymes involved in these oxidations are mainly:
1. Aldehyde oxidase (AO),
2. Xanthine oxidase (XO),
3. Monoamine oxidases (MAOs),
4. flavin-containing monooxygenases
5. (FMOs) and cytochrome P450s

17. Role of cytochrome P450 in drug metabolism and discovery
• In vitro experiments are conducted using the human liver
• It cellular fraction have cytochrome P450 enzyme along with cofactor NADPH.
• Its role can be checked by following methods:
(1) Inhibition by isoform specific inhibitors.
(2) Formation of metabolites by cDNA-expressed CYP isoforms
(3) Metabolic correlation to known CYP activities in a panel of human liver
microsomes

18. Phase II:
• Phase ll or conjugation reactions involve combination of the functional group
of drug with an endogenous substance such as glucuronic acid, sulfate, acetate
or amino acid, to form a highly polar product, which can be effeciently excreted
from the body.
• In biotransformation of drugs, such products or metabolites have two parts:
• Exocon: the portion derived from exogenous compounds or xenobiotics
• Endocon: the portion derived from endogenous compound.
• Conjugation reactions have high energy requirements and often utilize suitable
enzymes for the reactions.

19. • The endogenous substances (endocons) for conjugation reactions are derived from
carbohydrates or amino acids and possess large molecular size.
• The molecular weight of metabolite is important for determining its path of
excretion.
• They are strongly polar or ionic in nature in order to render the water soluble
substrate.
• High molecular weight metabolites are excreted predominantly in bile, e.g.
glutathione exclusively and glucuronide mainly.
• While low molecular weight conjugates are excreted predominantly in urine.
• As the availability of endogenous conjugating substance is limited, saturation of this
process is possible and the conjugated drug may precipitate toxicity.

20. Conjugation with Glucuronic acid
• Conjugation with glucuronic acid is a common and most important phase ll reaction
in vertebrates except cats and fish.
• The biochemical donor or cofactor of glucuronic acid is uridine diphosphate-D-
glucuronic acid (UDPGA) and this reaction is carried out by enzyme uridine
diphosphate –glucuronyl transferase.
• The most active site of glucuronide synthesis is liver and it is also present in
microsomes of tissues.

21. • Glucuronidation can take place in most body tissues due to its abundance in
body, unlike donors involved in other phase ll reactions.
• In cats, the activity of glucuronyl transferase is reduced, while in fishes there
is deficiency of endogenous glucuronic acid donor.
• The limited capacity of this metabolic pathway in cats results in increase of
action, pharmacological response and potential of toxicity of several lipid
soluble drugs such as aspirin .
• A large number of drugs undergoes glucuronidation including morphine,
paracetamol and desipramine. some endogenous substances like steroides,
bilirubinand thyroxine also form glucuronides.

22. Deconjugation process
• Some glucuronide conjugates that are excreted in bile
undergo deconjugation process in the intestine mainly carried
by enzyme B-glucuronidase.
• This releases free and active drug in intestine , which may be
reabsorbed and undergo enteo-hepatic cycling.
• Deconjugation is an important process because it prolongs the
pharmacological effects of drug or produces toxic effects.

24. Drug-drug interaction:
• Drug-drug interaction also play the important role in the discovery of the
drug.
• Sometimes treatment of the disease also involved in the combination of
different drugs.
• For example, anti-HIV medication and drugs for opportunistic infections are
medicine required for treatment of HIV
• As both drugs share the same pathway catalysed by the same cytochrome.
• They can effect each others metabolism and in result affect the
pharmacokinetics of each other.
• It may lead to adverse interaction or therapeutic failure, unintended reactions,
toxic side effects, or a lack of clinical efficacy.

25. Continued….
• CYP enzymes have relatively specific substrates,
inhibitors and inducers.
• Use during drug discovery and development to study
the potential of lead molecules and candidates drugs.
• Also to check that if the DDI is beneficial or harmful
for the treatment of the respective disease.
i. Enzyme inhibition: The perpetrator drug inhibit the
enzyme which catalyse the reaction of the victim
drug. Cytochrome are the important reaction
catalysing enzyme.
ii. Enzyme induction: It creates the opposite affect as
inhibition. The catalysing enzyme produce in large
amount. It decrease the victim drug exposure and
leads to the therapeutic failure.

26. Drug-drug interaction studies
• There are different type of drug-drug interaction studies
according to the developmental stages of the drug.
i. Evaluating metabolism-based drug interactions:
a) Used to study the metabolism based interaction.
b) It including recombinant CYP enzymes, subcellular liver
microsomes, and human liver tissue.
c) The resultant data help to analyse the potential of the
drug.
d) Also provide information either it is inhibitor or inducer.