Biotransformation and Pharmacogenomics (Kruse) Flashcards
What is drug biotransformation?
it is the enzymatically driven process whereby a substance is changed from one chemical to another (to become more polar, possibly larger) as these are more easily excreted
can be anabolic or catabolic
Most metabolic products are less pharmacodynamically active than the parent drug
What are the three main consequences of biotransformation?
inactivation
active compound to active compound
activation
What is an example of drug inactivation?
What is an example of drug active compound to active compound?
What is an example of drug activation?
Acetylsalicylic acid to acetic acid and salicylate
Diazepam to oxazepam
L-dopa to dopamine (prodrug to active drug)
Where does biotransformation occur?
Mostly occurs in the liver at some point between absorption into the general circulation and renal elimination
What is the first pass effect?
the process by which oral drugs undergo extensive biotransformation after absorption prior to entering circulation
Note: Drugs administered parenterally do not undergo first-pass biotransformation
How does the first pass effect change bioavailability?
the first pass effect greatly limits bioavailability of some drugs such that alternative routes of administration must be explored
ex: morphine, with oral bioavailability of 25% so that parenteral administration is preferred
What are the results of Phase I reactions?
What are the results of Phase II reactions?
results in the biological inactivation of the drug
produce a metabolite with improved water solubility and increased molecular weight (enhances elimination)
What sort of reactions occurs in Phase I?
enzymes that convert the parent drug to a more polar metabolite by unmasking or introducing a functional group
Most common: Oxidation, Reduction, Hydrolysis
These reactions are catabolic and their products can be more reactive/toxic than parent drug
What sort of reactions occurs in Phase II reactions?
Conjugation occurs with endogenous substrates (glucoronic acid, sulfuric acid, acetic acid, amino acid) to improve water solubility and increase molecular weight
These reactions are anabolic
typically occur after Phase I (except in Isoniazid for TB tx)
Phase I reactions are carried out by MFOs-what are some examples?
Cytochrome P450s
Flavin-containing monooxygenases
Epoxide hydrolases
These enzymes are located in lipophilic ER membranes of the liver and other tissues
What is the main key enzyme in phase I reactions?
What are the main key enzymes in phase II reactions?
CYP3A4 (and other CYPs)
UGT=UDP-glucuronosyltransferase
GST-glutathione S transferase
NAT-N-acetyltransferase
TPMT-thiopurine methyltransferase
SULT-sulfotransferase
What are some individual factors that effect biotransformation?
differences in the rate of metabolism and elimination
genetic factors like polymorphisms in CYPs and pharmacogenetic differences in enzyme expression levels
non-genetic factors like drug interactions, age/sex, circadian rhythm, body temp, liver size and function, nutritional and env. factors
Genetic differences can effect the biotransformation of these two compounds
Succinylcholine
slow acetylator phenotype for N-acetyltranferase enzyme
What are some CYP inducers?
phenobarbital
chronic ethanol use
aromatic hydrocarbons (benzo-a-pyrene) in tobacco smoke
rifamplin
St. John’s wort
What are some CYP inhibitors?
grapefruit juice
inhibitors can be reversible and irreversible
How can age affect biotransformation?
How do disease states affect biotransformation?
liver function and blood flow decrease with increasing age
liver disease and cardiac disease can disrupt drug biotransformation
Discuss acetaminophen-induced hepatotoxicity
for an adult dose of 1.2g/d, 95% will undergo glucuronidation and sulfation and 5% will undergo P450-dependent GSH conjugation
when acetaminophen intake exceeds therapeutic doses glucuronidation and sulfation pathways are saturated and P450 pathway becomes more important. Hepatic GSH gets depleted faster than is regenerated
Toxic metabolites accumulate resulting in hepatotoxicity
How does ETOH consumption effect acetaminophen biotransformation?
I *think* alcohol and tylenol together use up the glucuronidation and sulfation enzymes faster such that GSH has to be used and can cause hepatic damage more quickly.
Can occur at smaller doses of tylenol/ETOH (doesn’t nec. need to be a tylenol overdose or a ton of booze)
Basic Vocab (It’s an LO, so I’m including it)
Pharmacogenetics
Pharmacogenomics
Allele
Polymorphism
SNPs
Pharmacogenetics: study of different drug response to allelic variation in genes affecting drug metabolism, efficacy and toxicity at genomic level
Pharmacogenomics: study of entire genome to assess multigenetic determinants of drug response
Allele: one of two or more alternative forms of a gene that arise by mutation and are found at same locus (CYP2D6*3 is variant for CYP2D6)
Polymorphism: variation in DNA sequence that is present at an allele frequency of 1% or greater in pop.
SNP: base pair substitution that occur in genome
Genetics can effect dose-response in what two ways?
by variations in pharmacokinetics and pharmacodynamics
Polymorphisms in CYP450s can cause what?
absent, decreased or increased enzyme activity
poor/slow metabolizers are at risk for toxic accumulation
Ultrafast metabolizers are at risk for being undertreated
What are some common polymorphisms in Phase II reaction enzymes?
UDP glycosyltransferase and campothetcin
N-acetyltransferase and isoniazid
butyryl cholinesterase and succhinylcholine
How does glucose-6-phosphate dehydrogenase deficiency affect the pharmacodynamic response?
most common disease-producing enxyme defect in humans
normally, G6PD produces NADPH which regenerates glutathione to protect cells from oxidative damage
inG6PD def. oxidative damage leads to hemolytic anemia in the presence of oxidants
How do ryanodine receptor mutations cause malignant hyperthermia?
inhalation anesthetics, succinylcholine given during surgery cause an elevation in CA in the sarcoplasm of muscle leading to muscle rigidity, elevation of body temp and rhabdomyolysis
What are some examples of genetic variations affecting pharmacokinetics and pharmacodynamics?
polygenic effects (variations at multiple gene loci)
polymorphisms in biotransforming enzymes or polymorphisms in drug targets can affect Warfarin biotransformation
What specific variations effect Warfarin metabolism?
CYP2C9 is a phase I drug metabolizing enzyme that primarly acts on acidic drugs including Warfarin, phenytoin and NSAIDs
Vit. K epoxide reductase complex subunit 1 is the target of anticoagulant warfarin and polymorphisms in it can lead to increased sensitivity to warfarin
this means there can be a >10x difference in dose due to polymorphisms
What are the goals of animal testing in drug development?
ID potential human toxicites without human testing
design tests to further define the toxic mechanisms
predict the specific and most relevant toxicities to be monitored in clinical trials
What are some limitations to animal testing?
some toxicity and therapeutic values in animals do not translate to humans
rare/adverse events that occur in humans unlikely detected in preclinical animal testing
What is a lead compound?
a chemical compounds that has pharmacoligical or biological activity and whose chemical strucutre is used as a starting point for chemical modifications in order to improve potency, selectivity or pharmacokinetic parameters
What is a No-effect dose?
What is a minimum lethal dose?
What is the median lethal dose?
the max dose at which a specified toxic effect is not seen
The smallest dose that is observed to kill any experimental animal under a defined set of conditions
the dose that kills approx. 50% of the animals
Clinical trial terminology (is LO, so is included here)
Control:
Randomized:
double blind:
single blind:
open label:
parallel trial:
crossover trial:
endpoint:
surrogate endpoint:
Control: established therapy or placebo againsts which the efficacy of a new agent can be compared
Randomized: pt’s entering the trial have an equal prob. of receiving the test or control
Double blind: no one knows who is getting experiment or control
Single blind: health prof. knows who is getting treatment/control but pt does not
Open lab: everyone knows who is getting tx/control
Parallel trial: at least regimens are tested simultaneously but pt’s are assigned to only one therapy
Crossover trial: pt’s receive each therapy in sequence and therefore serve as their own controls
Endpoint: measured to assess a drug’s effect
Surrogate endpoint: an outcome of therapy that predicts the real goal of therapy without being that goal
Define investigational new drug (IND)
application process before beginning clinical trials; submission contains preclinical data from animal pharm and tox studies, manufacturing information and clinical protocols
Define institutional review board (IRB)
the purpose of IRB review is to assure that appropriate steps are taken to protect the rights, safety and welfare of humans participating as subjects in the research; has the authority to approve, require modifications and to disapprove reeasrch; can be internal to each university/college or independent/commercial
Define new drug application (NDA)
an application submitted by manufacturer of a drug to the FDA after clinical trials have been completed for a liscense to market the drug for a specified indication