Lecture 5: Intro to Biotransformation, Pharmacogenomics, and Clinical Trials

Question 1

What is Biotransformation?

Answer 1

chemical modification of lipophilic, unionized, or large compounds to terminate their actions and facilitate elimination

• polar, small molecular volume xenobiotics eliminated through renal excretion

Question 2

What is a Prodrug?

Answer 2

an inactive drug that undergoes biotransformation to become an active drug

• may have a protective functional group that gets removed, allowing the drug to become active

Question 3

Besides the liver, what are 4 other locations in the body that have considerable biotransforming activity?

Answer 3

GI tract, lungs, skin, and kidneys

• express smaller quantities of the the enzymes required than the liver

Question 4

What is the First-Pass Effect?

Answer 4

• oral drugs are absorbed in the small intestine and transported to the liver via hepatic portal system for metabolism
• parenteral routes do not undergo first-pass biotransformation

Question 5

What happens to estrogen and morphine when undergoing First-Pass metabolism?

Answer 5

• GI flora increases the bioavailability of estrogen by increasing enterohepatic cycling
• only 25% of oral morphine doses are bioavailable, making parenteral administration much more desirable

Question 6

What happens during Phase I of biotransformation? (What does it do, what causes it, and where?)

Answer 6

• catabolic rxn that unmasks a functional group on the drug (oxidation, reduction, hydrolysis)
• rxns carried out by mixed function oxidases (MFOs)
  
  • CYP, FMO, mEH/sEH
• rxns occur in lipophilic ER membranes in liver

Question 7

What happens during Phase II of biotransformation? (What does it do, what causes it, and where?)

Answer 7

• anabolic rxns that form a conjugate of the phase I product (polar w/high molecular weight)
• dependent on glucuronic, sulfuric, acetic, and amino acids
• takes place in the liver and is a faster than Phase I rxns

Question 8

What is the most abundant cytochrome P450 enzyme?

Answer 8

CYP3A4

• involved in metabolism of 50% of clinically used drugs
• uses oxygen and hydrogen from NADPH to carry out oxidation of substrates

Question 9

What are the 4 other major cytochrome P450 enzymes?

Answer 9

CYP1A2, CYP2A6, CYP2D6, CYP2E1

Question 10

What is Succinylcholine and what enzyme is used to metabolize it?

Answer 10

• depolarizing neuromuscular blocking drug

- genetic defect in pseudocholinesterase = metabolize at 50% the normal rate

Question 11

What is the Slow Acetylator phenotype and who does it most commonly affect?

Answer 11

• autosomal recessive = dec. N-acetyltransferase lvls
• isoniazid, hyralazine, caffeine, other amines metabolized at slower rates = hepatotoxicity
• 50% of US and 83% of French populations

Question 12

What are 5 common P450 inducers? (PEBRP)

Answer 12

phenytoin, ethanol (chronic), benzo[a]pyrene, rifampin, phenobarbitol

Question 13

What is the effect of Grapefruit juice on P450?

Answer 13

• irreversibly inhibits CYP3A4 and alters the bioavailability of drugs taken orally

Question 14

What is the effect of allopurinol and mercaptopurine on xanthine oxidase?

Answer 14

• allopurinol treats excess uric acid and inhibits xanthine oxidase
• xanthine oxidase metabolizes mercaptopurine (cancer treatment drug), so coadministration with allopurinol increases toxic effects

Question 15

How can acetaminophen become hepatotoxic?

Answer 15

• normally 95% undergoes glucuronidation while 5% is biotransformed by P450s
• when excess intake occurs, hepatic GSH is depleted faster than can be regenerated, so more toxic metabolites accumulate = hepatotoxicity

Question 16

Genetic variation of CYP2D6 and Codeine/Morphine

Answer 16

• codeine (prodrug) is converted to active morphine, which binds to mu-receptors in CNS and is used to manage mild-moderately severe pain
• people with polymorphisms in CYP2D6 may experience insufficient pain relief OR side effects (drowsiness and respiratory depression)

Question 17

Genetic variation of CYP2C19 and Clopidogrel

Answer 17

• clopidogrel is acidic antiplatelet prodrug that is converted to active thiol metabolite (only 15% of dose) that causes antiplatelet activity
• people with polymorphisms in CYP2C19 (reduced function) are at an inc. risk for adverse cardiovascular effects (acute coronary syndrome)

Question 18

Genetic variation of UGT1A1 and Irinotecan

Answer 18

• irinotecan is a topoisomerase I inhibitor prodrug (first line chemotherapy for colon/rectum carcioma)
• metabolized to SN-38 metabolite (TOXIC)
• UGT1A16 and UGT1A128 polymorphisms are at inc. risk of neutropenia and diarrhea due to SN-38 buildup

Question 19

Genetic variation of Thiopurine S-Methyltransferase (TPMT) and azathioprine, 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG)

Answer 19

`- normal enzyme attaches methyl group to aromatic/heterocyclic sulfhydrl compounds (deactivates)

• 6-MP/6-TG are activated by HGRPTase to form TGNs (thioguanine nucleotides) that are anticancer agents and have bone marrow toxicity
• TPMT and xanthine oxidase normally inactivate 6-MP and 6-TG, but polymorphisms can lead to a buildup of toxic products

Question 20

Genetic variation of G6PD and Rasburicase

Answer 20

• G6PD exclusive source of NADPH and glutathione in RBCs
• Rasburicase manages high uric acid lvls in cancer pts on chemotherapy; converts uric acid to allantoin
• hydrogen peroxide is a biproduct that can be reduced by glutathione, so use is NOT permitted in pts. with a G6PD deficiency (African/Mediterranean)

Question 21

Genetic variation of OATP1B1 and simvastatins

Answer 21

• transporter responsible for uptake of weak acid drugs (statins) and bilirubin
• polymorphism rs4149056 in SLCO1B1 genes can cause low lvls of OATP1B1
• individuals with low lvls are at increased risk of toxicity to skeletal muscle if taking simvastatins to reduce serum lipids (prevent cardiac events)

Question 22

CYP2C9 alleles and S-warfarin

Answer 22

• CYP2C92 and CYP2C93 are alleles that lead to reduced metabolism of S-warfarin and are more common in European populations
• CYP2C95/6/8/11 alleles are more prevalent in African populations

Question 23

VKORC1 allele and warfarin

Answer 23

• target of anticoagulant warfarin and a key enzyme in vitamin K recycling process
• the most important polymorphism leads to inc. sensitivity to warfarin and occurs most frequently in Asians and least frequently in Africans (VKORC1-1639G>A polymorphism)

Question 24

What is a Lead Compound?

Answer 24

• chemical compound that has pharmacological/biological activity and a structure thats used as a starting point for chemical modifications to improve potency, selectivity, pharmacokinetic parameters

Question 25

What is a Leading Candidate?

Answer 25

• lead compound that has undergone modification after animal and cell screenings
• will undergo preclinical and toxicity testing before human evaluation begins

Question 26

Why is preclinical testing preformed?

Answer 26

to evaluate a leading compounds safety and toxicity

• identifies human toxicities, designs tests to further study toxic mechanisms, and predicts the most relevant ones to be monitored in clinical trials
• want to test compound in a cellular system that best represents the disease state it will be used on

Question 27

What is the No-Effect dose, the Minimum Lethal dose (LDmin), and the Median Lethal dose (LD50)?

Answer 27

NE = max dose at which toxic effect is NOT seen
- lower than threshold of harmful effect

Min Leth = smallest dose that kills an animal

Med Leth = does that kills 50% of the animals

Question 28

What is Phase 0 of clinical testing?

Answer 28

• “microdosing” or subpharmaceutical doses given to human volunteers
• can offer supportive/alternative data that allows selection of suitable drug candidates when animal and in vitro testing isn’t reliable
• financially advantageous

Question 29

What is Phase I of clinical testing?

Answer 29

• determines if humans and animals show different responses to investigational drug and determine limits of safe dosage range
• 25-50 healthy volunteers, though in cases of significant toxicity, subjects with disease state may be included (AIDS/chemo)
• performed in inpatient clinic

Question 30

What data is reported from Phase I testing? (H/A/M)

Answer 30

absorption, half-life, and metabolism

Question 31

What is Phase II of clinical testing?

Answer 31

• 100-200 pts with the target disease are used
• usually single-blind design; performed in hospitals
• drug failure typically occurs during this phase

Question 32

What data is reported from Phase II testing? (E/D/T)

Answer 32

efficacy, dosing requirements, and toxicities

Question 33

What is Phase III of clinical testing?

Answer 33

• 300-3000 or more pts with target disease are used
• usually cross-over or double-blind design
• determine overall benefit-risk relationship of drug
• most expensive phase of clinical testing

Question 34

What phase would the FDA grant approval of a drug being used to treat serious diseases and life-threatening diseases?

Answer 34

• granted approval during Phase III trials if used to treat serious diseases
• granted during Phase II trials in a controlled-market setting if used to treat life-threatening diseases

Question 35

What is Phase IV of clinical testing?

Answer 35

• only begins after approval to market of new drug
• continues to monitor safety of the new drug
• feedback essential for drugs that have side effects occuring in every 1 of 10,000 patients
• no fixed duration