Lecture 5: Intro to Biotransformation, Pharmacogenomics, and Clinical Trials Flashcards
What is Biotransformation?
chemical modification of lipophilic, unionized, or large compounds to terminate their actions and facilitate elimination
- polar, small molecular volume xenobiotics eliminated through renal excretion
What is a Prodrug?
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
Besides the liver, what are 4 other locations in the body that have considerable biotransforming activity?
GI tract, lungs, skin, and kidneys
- express smaller quantities of the the enzymes required than the liver
What is the First-Pass Effect?
- 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
What happens to estrogen and morphine when undergoing First-Pass metabolism?
- 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
What happens during Phase I of biotransformation? (What does it do, what causes it, and where?)
- 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
What happens during Phase II of biotransformation? (What does it do, what causes it, and where?)
- 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
What is the most abundant cytochrome P450 enzyme?
CYP3A4
- involved in metabolism of 50% of clinically used drugs
- uses oxygen and hydrogen from NADPH to carry out oxidation of substrates
What are the 4 other major cytochrome P450 enzymes?
CYP1A2, CYP2A6, CYP2D6, CYP2E1
What is Succinylcholine and what enzyme is used to metabolize it?
- depolarizing neuromuscular blocking drug
- genetic defect in pseudocholinesterase = metabolize at 50% the normal rate
What is the Slow Acetylator phenotype and who does it most commonly affect?
- autosomal recessive = dec. N-acetyltransferase lvls
- isoniazid, hyralazine, caffeine, other amines metabolized at slower rates = hepatotoxicity
- 50% of US and 83% of French populations
What are 5 common P450 inducers? (PEBRP)
phenytoin, ethanol (chronic), benzo[a]pyrene, rifampin, phenobarbitol
What is the effect of Grapefruit juice on P450?
- irreversibly inhibits CYP3A4 and alters the bioavailability of drugs taken orally
What is the effect of allopurinol and mercaptopurine on xanthine oxidase?
- allopurinol treats excess uric acid and inhibits xanthine oxidase
- xanthine oxidase metabolizes mercaptopurine (cancer treatment drug), so coadministration with allopurinol increases toxic effects
How can acetaminophen become hepatotoxic?
- 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
Genetic variation of CYP2D6 and Codeine/Morphine
- 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)
Genetic variation of CYP2C19 and Clopidogrel
- 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)
Genetic variation of UGT1A1 and Irinotecan
- 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
Genetic variation of Thiopurine S-Methyltransferase (TPMT) and azathioprine, 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG)
`- 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
Genetic variation of G6PD and Rasburicase
- 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)
Genetic variation of OATP1B1 and simvastatins
- 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)
CYP2C9 alleles and S-warfarin
- 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
VKORC1 allele and warfarin
- 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)
What is a Lead Compound?
- 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
What is a Leading Candidate?
- lead compound that has undergone modification after animal and cell screenings
- will undergo preclinical and toxicity testing before human evaluation begins
Why is preclinical testing preformed?
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
What is the No-Effect dose, the Minimum Lethal dose (LDmin), and the Median Lethal dose (LD50)?
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
What is Phase 0 of clinical testing?
- “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
What is Phase I of clinical testing?
- 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
What data is reported from Phase I testing? (H/A/M)
absorption, half-life, and metabolism
What is Phase II of clinical testing?
- 100-200 pts with the target disease are used
- usually single-blind design; performed in hospitals
- drug failure typically occurs during this phase
What data is reported from Phase II testing? (E/D/T)
efficacy, dosing requirements, and toxicities
What is Phase III of clinical testing?
- 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
What phase would the FDA grant approval of a drug being used to treat serious diseases and life-threatening diseases?
- 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
What is Phase IV of clinical testing?
- 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
