Phase I Lecture

Question 1

Adrian Monk?

Answer 1

Xenobiotics from plants, bacteria, animals, pills

• The role of metabolism and excretion are to get rid of these substances
• Most are modified to facilitate excretion and some are just excreted

Question 2

Drug metabolism is?

Answer 2

Intended to be a detoxification mechanism for the body

• To inactivate or activate drugs
• Each drug metabolite must be considered as a new drug entity to the body with potential actions and toxicity

Question 3

What does the study of drug metabolism serve to explainn?

Answer 3

Function and fate of the drug in the body
Ways to manipulate the metabolic process to develop better drugs and avoid drug-drug metabolic interactions
A fuller understanding of genetic differences that predispose certain individuals to have undesired effects or toxicity

Question 4

What are the key drug metabolizing organs?

Answer 4

Liver and GI tract
Kidney, lung, nasal epithelia, or choroid plexus in the brain can also be depending on the substance and sometimes even blood plasma

Question 5

Where are phase I oxidative enzymes predominantly localized?

Answer 5

The ER

Question 6

Where are phase II oxidative enzymes located?

Answer 6

Cytosol

Question 7

What happens in phase I?

Answer 7

Introduction of a polar functional group
Slightly increases water solubility
Highly lipophilic molecules will go through phase I
– P450 enzymes put the OH on it and flag it for phase II
- Usually involves oxygen and makes metabolites retain activity or more active
- Usually adds a reactive O or N group

Question 8

What happens in phase II?

Answer 8

Attachment of highly polar groups (glucuronide, glutathione)

• Majorly increases water solubility
• Generally leads to inactive metabolites that are highly water soluble and excreted from the body

Question 9

Glutathione forms?

Answer 9

Oxided: inactive
Reduced: active form

Question 10

How does Glutathione become active?

Answer 10

PPP

• G6PDH mediates the first step and it is the first step that generate the first NADPH which is used to create the reduced form of glutathione
• – The electron comes from the NADPH to keep the glutathione levels high

Question 11

Cyclophosphamide metabolism?

Answer 11

Converts it to an active species that alkylate DNA

– induces apoptosis in cancer cells with rapid turnover (cancer chemotherapy)

Question 12

What do methotrexate and 5FU do?

Answer 12

Anticancer agents

• Block use of dTMP by the rapidly growing malignant cells
• Methotrexate is a folic acid analog that can be taken up into cells and polyglutamated to form active species

Question 13

Rate and extent of metabolism of a drug depends on?

Answer 13

Factors related to the enzyme (affinity, specificity) and cofactors/cosubstrates
- Influence the plasma drug concentration and duration of action of a drug

Question 14

What are phase I enzymes?

Answer 14

Oxidases

• Families 1, 2, and 3 are involved with drugs
• Flavin monooxygenase, alcohol DH, Aldehyde oxidase, MAO

Question 15

Cytochrome P450 Enzymes?

Answer 15

In a reduced state (Fe2+) this enzyme can bind CO

NADPH reduces Fe3+ to Fe2+ (so oxygen can bind) and then a second electron reduces oxygen

Question 16

Where is glucuronyltransferase located and what is it important for?

Answer 16

In close proximity to P450 in the smooth ER

– Allows rapid conversion between phase I and then straight to GT for phase II

Question 17

How does the iron in heme work?

Answer 17

It changes between ferrous and ferric states

– In Hb it stays as ferrous (2+)

Question 18

What are CYP P450s major areas of concern in drug therapy?

Answer 18

Inhibition of drug metabolism
Induction of drug metabolism
Genetic polymorphisms in population

Question 19

What is the concern with inhibition of drug metabolism?

Answer 19

Drugs, dietary or environmental factors compete for the same drug metabolism enzyme

• May show saturation if inhibitor is a substrate
• Usually competitive inhibition
• May be allosteric in some cases
• Some isoforms accept multiple substrates in active sites
• Slow parent drug metabolism and can raise plasma concentration

Question 20

What is the concern with induction of drug metabolism?

Answer 20

Some drugs, dietary substances and environmental factors can stimulate expression of genes and create additional drug metabolizing enzymes

• Binding to a nuclear receptor –> increase translation or transcription –> more P450 so more of activation or effect
• Cause more parent drug to be metabolized and lower the active drug concentration –> effect can be compromised

Question 21

What is Rifampin?

Answer 21

Antibacterial
Not used alone bc bacteria develope resistance to it quickly but occasionally is used alone in TB
- Notorious for causing induction of P450 –> causes change over time –> make more P450

Question 22

What happens when drugs can induce their own metabolism or metabolism of other substances?

Answer 22

Half-life changes over time

- Amount of drug can become less and less effective even though dose may not change

Question 23

Can inhibition and induction occur at the same time?

Answer 23

Yes

• Process of inhibition of P450: binding to the molecule’s active site and inhibit the reaction that comes normally
• Process of induction of P450: doesn’t have anything to do with the active site so it does not interfere with inhibition; involves increasing transcription and translation (complexes with the nuclear receptor)

Question 24

What is the concern with genetic polymorphisms in the population?

Answer 24

Variation in P450 expression can cause important implications for drug evaluation and testing in animals

Question 25

What causes down regulation of intestinal CYP?

Answer 25

Grapefruit juice because it increases felodipine oral bioavailability in humans by inhibition of intestinal CYP3A4 activity

Question 26

Acetaminophen hepatotxicity?

Answer 26

Due to saturation of usual detoxification pathways

• toxic metabolite = NAPQI (quinoneimine) which can react with sulphydryl groups in critical cellular proteins
• Causes a loss of intracellular calcium regulation disrupts mitochondrial function and leads to necrotic cell death