Lecture 3: Intro to Biotransformation, Pharmacogenomics, and Clinical Drug Trials Flashcards
What is the general strategy for eliminating compounds from the body?
Biotransformation of xenobiotics into more polar (and sometimes larger) and water soluble derivatives
Biotransformation may convert an inactive drug (pro-drug) to an active one, may lead to a product that is still active, or may inactivate a compound.
Provide an example of each.
-
Activation:
1) L-dopa (pro-drug) –> dopamine -
Active compound –> Active compound:
2) Diazepam –> Oxazepam -
Inactivation:
3) Acetylsalicylic acid (aspirin) –> acetic acid + salicylate
T/F drugs that are given via parenteral routes of administration undergo first-pass biotransformation?
False
What is the classic example of a drug that undergoes extensive first-pass biotransformation?
- Morphine
- Bioavailability of oral preparations is roughly 25%; parenteral administration is preferred
What happens in the phase I reactions vs. phase II reactions?
Which phase is anabolic vs. catabolic?
Where does each phase occur and which is fastest?
Phase I: makes it inactive (oxidation, reduction, hydrolysis); catabolic; products are generally more reactive and toxic; enzymes located in the lipophilic ER membranes of liver
Phase II: conjugation making the substrate more water soluble and increases MW (anabolic); occurs faster than phase I rxns; takes place in liver
Phase I reactions are carried out by what 3 types of enzymes?
Mixed function oxidases (MFOs) or monooxygenases
1) CYP450s *primary phase I enzymes
2) Flavin-containing monooxygenases (FMO)
3) Epoxide hydrolases (mEH, sEH)
What is unique about the metabolism of isoniazid?
Phase II reaction occurs first (conjugation)
What are examples of phase II enzymes?
- UGT, GST, NAT (n-acetyltransferase)
- TPMT (thiopurine methyltransferase)
- SULT (sulfotransferase)
Which CYP is the most abundantly expressed and involved in the metabolism of about 50% of clinically used drugs?
CYP3A4
People with genetic defects in which enzyme can metabolize succinylcholine at 50% the rate of normal individuals?
Pseudocholinesterase
What is the slow acetylator phenotype?
Causes the slow metabolism of which compounds?
- AR trait w/ decrease in N-acetyltransferase levels in the liver
- Isoniazid, hydralazine, caffeine and other amines
Consumption of grapefruit juice with drugs taken orally can irreversibly inhibit what?
Intestinal CYP3A4
Which drug given with Mercaptopurine prolongs the duration of Mercaptopurines action and enhances its chemotherapeutic and toxic effects?
Why?
- Allopurinol by inhibiting xanthine oxidase
- Xanthine oxidase is a key enzyme in the biotransformation pathway of mercaptopurine
What is the most important factor accounting for decreased drug metabolism in elderly patients?
Liver and kidney disease
How does acetaminophen intake exceeding therapeutic dose lead to biotransformation into more toxic products and acetaminophen-induced hepatotoxicity?
- Hepatic GSH is depleted faster than regenerated
- Toxic metabolites accumulate resulting in hepatotoxicity
Levels of which drugs with flow-limited biotransformation may rise if given to patients with cardiac disease?
- Atenolol and propranolol (beta blockers)
- Isoniazid
- Lidocaine
- Morphine
- Verapamil
In vitro studies with a lead compound are used to study what?
Which type of cellular system is used?
- Factors involved in the drug’s actions (i.e., proteins the drug may bind to and/or the pathways it may inhibit or regulate)
- Helps establish the direct drug target or effects (pharmacologic profile) at the molecular and cellular levels
- Ideally tested in a cellular system that best represents the disease state the therapy is targeting
What is the No-effect dose?
- Maximum dose at which a specified toxic effect is not seen
- Lower than the threshold of harmful effect and is oftn estimated while establishing the threshold of harmful effect
Minimum lethal dose (LDmin) vs. Median lethal dose (LD50)?
- LDmin: the smallest dose that is observed to kill any experimental animal under a defined set of conditions
- LD50: the dose that kills approximately 50% of the animals
What are 3 limitations to preclinical testing?
1) Time and cost: 2-6 yrs may be required to collect and analyze animal toxicity profiles and estimate the therapeutic index
2) Number of animals used: cost associated w/ maintenance of a single mouse can be upwards of $400/year/cage
3) Extrapolation of values: toxicity and therapeutic values in animals often translate to humans, but many do not
What is a crossover design in human clinical trials?
- Patients receive each therapy in sequence so that the patients serve as their own controls
- They will receive both the drug and placebo at different times during the study
What is a single-blind vs. double-blind design in clinical trials?
- Single-blind: only the health professionals known the identity of the tx
- Double-blind: neither the pt nor the health professional knows the identity of the therapy
What is an Investigational new drug (IND)?
- Federal law requires that a drug be the subject of an approved marketing application before it is transported or distributed across state lines
- Submission of the IND is the means by which investigators technically obtain permission from the FDA to proceed w/ drug distribution
What is Phase 0 of a clinical trial and its advantages?
- Also termed microdosing; provide early pharmacokinetic data in humans and only require minimal preclinical toxicology safety testing
- Useful in situations where in vitro and animal models prove to be unreliable
- Also financially advantageous
What is Phase I of clinical trials, how many subjects involved, and what information is obtained?
- Determines whether humans and animals show significantly different response to the drug and establishing probably limits of the safe cllinial dosage range
- 25-50 volunteers
- Absorption, half-life, and metabolism are typical data reported
What is Phase II of clinical trials, how many subjects involved, and what information is obtained?
- Typically single-blind, involving 100-200 patients
- Include an inert placebo, established active drug (positive control), and active investigational agent
- Efficacy, dosing, and toxicities are detected and recorded
*Drug failure typically occurs during this phase when drug is discovered to be toxic or not efficacious at appropriate doses
During which phase of clinical trials does drug failure typically occur?
Phase II
What is Phase III of clinical trials, how many subjects involved, and what information is obtained?
- Further established drug safety and efficacy, using large group of patients (300-3000+)
- Crossover and double-blind methods are often used to minimize errors and other bias
- Intent is to gather additional info to evaluate overall benefit-risk relationship of drug and provide adequate basis for physician labeling
- The most expensive phase due to large number of patients and data
What is Phase IV of clinical trials?
- Only after approval to market a new drug does this phase begin
- Purpose is to continue to monitor the safety of the new drug
- Allows for the identification of rare drug-induced effects or toxicities
How are individuals defined in terms of their metabolizing ability based on their allelic activity scores?
0 = Poor Metabolizer
0.5 = Intermediate Metabolizer
1-2 = Extensive Metabolizer
>2 = Ultra-rapid metabolizer
Which CYP2D6 alleles are nonfunctional, reduced function, and fully functional?
*1 and 2 are fully functional
*3-6 are nonfunctional
*10, 17, and 41 have reduced function
What is the most common nonfunctional CYP2D6 allele?
CYP2D6*4
What is the active metabolite of Codeine and which enzyme is responsible for this conversion?
- Morphine
- CYP2D6
What does CYP2C19 metabolize?
Acidic drugs (PPIs, antidepressants, antiepileptics, and antiplatelet drugs)
Which CYP2C19 alleles are fully functional, nonfunctional, and which have increased function?
*1 is fully functional
*2-3 are non-functional
*17 has increased function
How does the presence of an CYP2C19*17 allele affect a patient with another nonfunctional CYP2C19 allele?
What type of phenotype will this patient have?
- Although *17 has increased function, it is unable to fully compensate for nonfunctional alleles
- Would still be considered an IM phenotype
What is the most common nonfunctional allele of CYP2C19 and who is it seen most prevalent in?
- CYP2C19*2
- Asians
Carriers of the nonfunctional CYP2C19*2 are at an increased risk of cardiovascular events, particularly acute coronary syndrome, when taking which drug?
Clopidogrel
What is Dihydropyrimidine dehydrogenase (DPD)?
First and rate-limiting step in pyrimidine catabolism, as well as major elimination route for fluoropyrimidine chemotherapy agents
What is the most commonly observed nonfunctional allele of DPYD?
DPYD*2A
The presence of nonfunctional DPYD gene must be considered when administering which drug?
How is this drug typically given?
- Fluoropyrimidine drugs (5-FU (active compound), capecitabine, and tegafur)
- Converted to active, cytotoxic metabolites 5-FUMP and 5-FdUMP, which target cancer cells, but can build up if patients has a nonfunctional DPYD
- 5-FU must be administered IV
What is the most common nonfunctional allele of UGT1A1?
UGT1A1*28
Patients with nonfunctional UGT1A1 (*28 and *6) are at increased risk for severe life-threatening toxicities when taking which drug?
Due to decreased clearance of which metabolite?
- Irinotecan (topoisomerase I inhibitor)
- SN-38 metabolite
What is the significance of Glucose 6-phosphate dehydrogenase (G6PD)?
- First and rate-limiting step in the pentose phosphate pathway and supplies a significant amound of reduced NADPH in the body
- In RBCs, is the exclusive source of NADPH and reduced glutathione, necessary for prevention of oxidative damage
What are the 5 classifications for G6PD deficiency, normal, and enhanced activity?
Amount of enzyme activity?
Associated pathologies?
I: severe deficiency; <10%; chronic (non-spherocytic) hemolytic anemia
II: severe deficiency; <10%; acute hemolytic anemia; intermittent hemolysis
III: moderate deficiency; 10-60%; acute hemolytic anemia; hemolysis w/ stressors
IV: no deficiency; 60-150%; normal
V: no deficiency; >150%; enhanced activity
The majority of G6PD-deficient genotypes are associated with which categoery set forth by the WHO?
Class II (severe) and class III (moderate)
Patients with G6PD deficiency receiving which drug are at greatly increased risk for severe hemolytic anemia and methemoglobinemia?
Rasburicase; used in management of high uric acid levels in cancer patients undergoing chemo; produces high levels of hydrogen peroxide
Patients with reduced OATP1B1 function may experience adverse side effects when taking which drug?
Simvastatin
What are the well known enzyme (CYP) inducers?
- Phenobarbital
- Chronic ethanol
- Armoatic hydrocarbons (tobacco smoke)
- Rifampin
- St. Johns wort
The OATP1B1 transporter (encoded by the SLCO1B1 gene) located on th sinusoidal membrane of hepatocytes is resonsible for the hepatic uptake of?
Mainly weakly acidic drugs and endogenous compounds i.e., statins, methotrexate, and bilirubin
A reduced function variant in ABCG2, which encodes BCRP has been associated with pharmacokinetic changes in response to which drugs?
- Allopurinol
- Rosuvastatin
- Toxicity to various anticancer drugs
Which HLA polymorphism is associated with hypersensitivity reactions, particularly SJS, in patients receiving HIV treatment with Abacavir?
HLA-B* 57:01
Genetic variants found near which gene were found to be the most significantly associated w/ HCV tx response to PEG-IFN-α in combo w/ ribavirin?
IFNL3 gene encoding IFN-λ3 (aka IL-28B)
What is the drug target for Warfarin (pharmacodynamics)?
Vitaming K epoxide reductase complex subunit 1 (VKORC1)
Pharmacokinetic polymorphisms in which enzyme may lead to an increased risk of bleeding in patients on Warfarin therapy?
CYP2C9 (specifically CYP2C9*3 and *2)
Pharmacodynamic polymorphisms in which gene may lead to bleeding disorders or increased drug sensitivity in a patient on Warfarin?
VKORC1
What is a surrogate endpoint?
An outcome of therapy that predicts the real goal of therapy without being that goal (i.e., reduction in tumor size as a surrogate for survival)
Define the term Open Label in regards to research?
Both health professionals and patients know whether the drug is the experimental or control agent
Define Parallel trial
At least 2 regimens are tested simultaneously, but patients are assigned to only one therapy
What is an Endpoint in a clinical trial?
Measured to assess a drug’s effect (i.e., blood pressure is the endpoint for testing an antihypertensive agent)
What receptor mutation may cause malignant hyperthermia; what is the MOA?
- Ryanodine receptor mutation
- Inhalation of succinylcholine activates receptor –> increased Ca2+ in sarcoplasm of muscle leads to muscle rigidity, elevated body temp, and rhabdomyolysis
What is the acute toxicity test; how many species and routes?
Determines what?
- Usually 2 species, 2 routes
- Determines the no-effect dose and the maximum tolerated dose
When is the carcinogenic potential test required; how long and how many species used?
Determines what?
- Two years, two species
- Required when drug is intended to be used in humans for prolonged periods
- Determines gross and histologic pathology
