Introduction to Biotransformation, Pharmacogenomics, and Clinical Drug Trials Lecture (Dr. Kruse) Flashcards
Drug Biotransformation
- The enzymatically‐driven process whereby a substance is CHANGED from one chemical to another
- Biotransformation reactions can be ANABOLIC as well as CATABOLIC (biotransformation vs. metabolism)
- The general Strategy for ELIMINATING compounds: biotransformation into more POLAR, and sometimes LARGER, derivatives
- POLAR and WATER SOLUBLE products are more readily EXCRETED by the KIDNEYS
- Most METABOLIC PRODUCTS are LESS Pharmacodynamically ACTIVE than the Parent Drug
Consequences of Biotransformation
1) INACTIVATION Acetylsalicylic acid (aspirin) → ace c acid + salicylate
2) ACTIVE COMPOING → active compound Diazepam → Oxazepam
3) ACTIVATION
- L‐dopa → dopamine
a) PRODRUG: an INACTIVE drug that undergoes biotransformation to BECOME an ACTIVE drug
** Mostly occurs in the LIVER at some point BETWEEN Absorption into the General Circulation and RENAL Elimination
First Pass Effect
- All XENOBIOTICS absorbed from the Small Intestine are TRANSPORTED to the LIVER via the Portal system
- The FIRST-PASS EFFECT is the process by which ORAL DRUGS undergo extensive Biotransformation after absorption prior to entering circulation
- Drugs administered Parenterally DO NOT UNDERGO FIRST-PASS BIOTRANSFORMATION
- The FIRST-PASS EFFECT greatly LIMITS the BIOAVAILABILITY of some drugs such that alternative routes of administration must be explored
a) Example: MORPHINE
b) ORAL BIOAVAILABILITY is roughly 25%
c) PARENTERAL Administration is preferred
The Phases of Biotransformation
As a general rule:
1) PHASE I reactions result in the BIOLOGICAL INACTIVATION of the DRUG
2) PHASE II reactions produce a metabolite with improved WATER SOLUBILITY and INCREASED Molecular Weight (enhances elimination)
The Phases of Drug Metabolism
1) PHASE I reactions consist of enzymes that convert the parent drug to a MORE POLAR METABOLITE by introducing or unmasking a functional group (‐OH, ‐NH2, ‐SH, ‐COOH, ‐O)
a) Oxidation, Reduction, and Hydrolysis reactions are most common
b) Phase I reactions are CATABOLIC!!!!!
c) Phase I products can be more Reactive and sometimes MORE TOXIC than the parent drug
2) PHASE II reactions consist of enzymes that form a CONJUGATE of the substrate (phase I product)
a) Conjugation with endogenous substrates such (e.g., GLUCURONIC ACID, SULFURIC ACID, ACETIC ACID, or AMINO ACID) to Improve WATER SOLUBILITY and Increase molecular weight
b) Phase II reactions are ANABOLIC!!!!!!!
3) Typical ORDER is Phase I followed by Phase II reactions (but not always – isoniazid)
Phase I Reactions
- Oxidations, Reductions, and Hydrolysis reactions
- Products are generally more reactive and may be more TOXIC than parent drug
•Phase I reactions are carried out by Mixed Function Oxidases (MFOs) or monooxegenases
a) CYTOCHROME P450s (P450 or CYP)
b) FLAVIN-CONTAINING MONOOXYGENASES (FMO)
c) EPOXIDE HYDROLASES (mEH, sEH)
•Phase I enzymes are LOCATED in LIPOPHILIC ER membranes of the LIVER (and other tissues)
Key Enzymes in Biotransformation
1) CYTOCHROME P450 (CYP450) – PHASE I !!!!!!!!!!!
a) Superfamily of enzymes that carry out phase I reactions
b) Of the more than 50 P450s identified in humans, the most important are CYP1A2, CYP2A6, CYP2D6, CYP2E1, and CYP3A4*!!!!!!!!!!!!!!!
2) PHASE II ENZYMES
a) UGT – UDP Glucuronosyltransferase
b) GST – Glutathione‐S‐Transferase
c) NAT – N‐Acetyltransferase
d) TPMT – Thiopurine Methyltransferase
e) SULT ‐ Sulfotransferase
Clinical Relevance of Biotransformation
- Individuals differ in drug distribution and in their rates of drug metabolism and elimination
- Differences in individual biotransformation rates and efficiency are some of the many factors that may affect drug dose and frequency of administration among individuals
• GENETIC FACTORS:
a) POLYMORPHISMS in Xenobiotic‐metabolizing ENZYMES
b) PHARMACOGENETIC differences in ENZYME EXPRESSION levels
• NON-GENETIC FACTORS:
- Drug‐drug interactions
- Age and sex
- Circadian rhythm
- Body temperature
- Liver size and function
- Nutritional and Environmental factors
Examples of Genetic Differences that affect Biotransformation
1) Succinylcholine (Pseudocholinesterase Degrades it)
2) Slow acetylator phenotype for N‐acetyltransferase enzyme
Examples of Non-Genetic Differences (Drug-Drug Interactions, DDIs)
1) ENZYME INDUCTION:
• Well characterized Inducers: PHENOBARBITAL, CHRONIC ETHANOL, AROMATIC HYDROCARBONS such
as [a]pyrene (Tobacco Smoke), RIFAMPIN, St. John’s WORT
2) ENZYME INHIBITION:
• Reversible and Irreversible Inhibition
• GRAPEFRUIT JUICE EFFECT!!!!!!!!!
3) Biotransformation as we age • Hepatic blood flow?!!!!
4) Disease STATES and Biotransformation
• Liver disease, Cardiac disease
Metabolism to Toxic Products
1) Acetaminophen‐induced HEPATOTOXICITY
- For an adult dose of 1.2 g/d:
a) 95% Glucuronidation and Sulfation
b) 5% P450‐dependent GSH conjugation!!!!! - When ACETAMINOPHEN intake exceeds therapeutic doses:
a) Glucuronidation and Sulfation pathways are SATURATED and P450 pathways becomes INCREASINGLY IMPORTANT
b) Over time, HEPATIC GSH is DEPLETED FASTER than is regenerated
c) TOXIC metabolites ACCUMULATE resulting in hepatotoxicity!!!!
2) What about Alcohol consumption and Acetaminophen?
• See table 4‐2 in Katzung (p. 62)
Pharmacogenetics
- The study of DIFFERENCES in Drug RESPONSE due to allelic VARIATION in Genes affecting drug Metabolism, Efficacy, and Toxicity at the Genomic level
Pharmacogenomics
- The study of the ENTIRE GENOME to assess Multigenetic DETERMINANTS of Drug Response
Polymorphism
- A variation in the DNA sequence that is present at an ALLELE FREQUENCY of 1% or GREATER in a Population
Drug Response Variability is Genetically Determined (in part)
VARIATIONS OCCUR IN:
• PHARMACOKINETICS – variation in the rate at which the body Absorbs, Transports, Metabolized, or
Excretes a drug/metabolites
• PHARMACODYNAMICS – allelic variation in a drug’s downstream targets, such as Receptors, Enzymes,
or Metabolic pathways
Variations in Phase I Reactions (PK)
- POLYMORPHISMS in CYP450s can result in Absent, Decreased, or Increased enzyme activity
- POOR METABOLIZERS at risk for accumulation of toxic drug levels
- ULTRAFAST METABOLIZERS are at risk for being undertreated with inadequate doses
Variations in Phase II Reactions (PK)
- Polymorphism in UDP‐GLYCOSYLTRANSFERASE (glucuronidation) and CAMPTOTHECIN
- Polymorphism in N‐ACETYLTRANSFERASE and ISONIAZID
- Polymorphism in (BUTYRIC) CHOLINESTERASE and SUCCINYLCHOLINE
Variations in Pharmacodynamic Response
1) GLUCOSE‐6‐PHOSPHATE DEHYDROGENASE deficiency
- Most common disease‐producing enzyme defect of humans (400 million worldwide)
- More than 400 variants described
- G6PD produces NADPH, which regenerates reduced glutathione from its oxidized
form - Reduced glutathione protects cells against oxidative damage
- G6PD deficiencies cause oxidative damage that leads to hemolytic anemia in the
presence of oxidants
2) RYANODINE RECEPTOR Mutations and Malignant hyperthermia
- INHALATIONAL Anesthetics, Succinylcholine
- Elevation of CALCIUM in the sarcoplasm of muscle leads to MUSCLE RIGIDITY, elevation of body temperature, RHABDOMYOLYSIS
Genetic Variations in BOTH Pharmacokinetics and Pharmacodynamics
• Variations at Multiple GENE LOCI (polygenic effects)
• WARFARIN (Coumadin)
- Polymorphisms in Biotransforming ENZYMES
- Polymorphisms in DRUG TARGET
Drug Development and Clinical Trials
- The process of bringing a drug to market can take up to 10 years or longer and cost upwards of $1 billion or more
- The PURPOSE of the clinical trial process is answer SPECIFIC QUESTIONS of SAFETY and EFFICACY regarding new therapies or devices in humans
- Clinical trials include FOUR PHASES (I‐IV) and are most often designed to compare one form of therapy to another experimental form
• Not all clinical trials test new therapies to cure or fight disease
a) Treatment trials to test NEW TREATMENTS
b) Prevention trials EVALUATE THERAPIES aimed at Preventative Care, Screening Trials to test new
screening methods
c) Quality of life trials explore ways to IMPROVE COMFORT and quality of life
Drug Development and Clinical Trials
Phase 1: Is it safe, Phamacokinets?
Phase 2: Does it works in patients?
Phase 3: Does it work, “Double Blind”
- 100 to 600 Patients
Phase 4: POSTMARKETING Surveillance
No- Effect Dose
- The MAXIMUM DOSE at which a specified toxic effect is not seen
Minimum Lethal Dose (LDmin)
- The SMALLEST DOSE that is observed to KILL any Experimental Animal under a
defined set of conditions
Median Lethal Dose (LD50)
- The dose that KILLS approximately 50% of the animals
Institutional Review Board (IRB)
- Also known as an INDEPENDENT ETHICS COMMITTEE (IEC) or ETHICAL REVIEW BOARD (ERB)
- The purpose of IRB review is to assure that appropriate steps are taken to protect the RIGHT, SAFETY, and WELFARE of humans participating as subjects in the research
- An IRB has the authority to APPROVE, REQUIRE MODIFICATIONS IN (to secure approval), or DISAPPROVE RESEARCH
- Can be INTERNAL to each university/college or independent/commercial
Caveats in Human Testing
1) Variable Natural history and PROGRESSION of Disease
- CROSSOVER DESIGN, large population of patients, time
2) Presence of OTHER DISEASE and RISK FACTORS
- Drug Use
3) Subject and Observer Bias
- Placebo, Single Blind, and Double Blind
Clinical Trial Endpoints
ENDPOINT: Measured to assess a Drug’s Effect (e.g., Blood Pressure is the endpoint for testing an antihypertensive agent)
Simvastatin and Ezetimibe (Vytorin)
1) How effective is Vytorin? What are the endpoints of the clinical studies?
• ENHANCE study:
- 2‐yr study comparing Vytorin to Simvastatin ALONE in patients with FAMILIAL HYPERCHOLESTEROLEMIA
- SURROGATE ENDPOINT is REDUCTION in LDL levels
- VYTORIN – 58% REDUCTION in LDL
- SIMVASTATIN ALONE – 41% REDUCTION in LDL
2) What about reduction in Major CARDIOVASCULAR EVENTS??
• SEAS study:
- Vytorin vs. placebo
***** No REDUCTION in MAJOR CARDIOVASCULAR EVENTS!!!!!
