factors affecting drug response

phase I metabolism

converts drugs to a more polar metabolite, by introducing or unmasking a functional group (OH, NH2, SH)

phase II metabolism

mainly involves conjugation to metabolites

liver

main site for phase II metabolism enzymes

T/F: Phase II reactions may also precede phase I reactions

TRUE

renal excretion

mainly for small drugs or metabolites with MW < 300; drugs are filtered from the blood in the renal glomerulus, secreted into the proximal tubule, and then excreted in the urine or reabsorbed from the distal tubule into systemic circulation

distal tubule fluid

generally acidic; changing urine pH can affect the ionization of drugs

T/F: ionized compounds have slower urine excretion

FALSE: ionized compounds have enhanced urine excretion

bile excretion

mainly for larger hydrophilic compounds with MW > 500

glucuronide

conjugated added in liver in phase II reactions that adds ~ 176 MW

Compounds of MW between 300-500 excretion

excreted in both urine and bile

non-polar compound excretion

MW < 300 are not excreted in the bile

enterohepatic recycling

liver actively secretes drugs and metabolites into the bile, which is then released into the small intestine; drugs and metabolites in bile can be re-absorbed from the intestine through the portal vein back to the liver

T/F: enterohepatic recycling may affect drug excretion and bioavailability

TRUE

Gut microorganisms (flora)

can mediate biotransformations of drugs and metabolites involving decarboxylation, dehydroxylation, dealkylation, deamination, and de-glucuronidation (removing conjugates)

T/F: Some de-conjugated drugs/metabolites can be reabsorbed into the portal system to go through enterohepatic recycling

TRUE

digoxin excretion

gut flora metabolize digoxin to more polar metabolite to facilitation its fecal excretion, leading to less enterohepatic recycling

oral contraceptive bioavailability

becomes glucuronidated metabolite in the liver, gut flora de-glucuronidate metabolite to parent drug; parent drug is then recycled (via enterohepatic recycling)

Activity in the _____ influences metabolites coming from the ____; making something more polar and hydrophilic promotes excretion; if you undo metabolite steps and remove conjugates it is becoming less hydrophilic and promotes recycling

intestine; liver

biomarker

a defined characteristic that is measured as an indicator of normal biological processes, pathogenic processes, or responses to an exposure or intervention

diagnostic biomarker

a biomarker that detects or confirms the presence of a disease or condition of interest, or identifies an individual with a subtype of the disease

prognostic biomarker

a biomarker used to identify the likelihood of a clinical event, disease recurrence, or disease progression in patients with a disease or medical condition of interest

predictive biomarker

a biomarker used to identify individuals who are more likely than similar individuals without the biomarker to experience a favorable or unfavorable effect from exposure to a medical product or an environmental agent; the response could be a symptomatic benefit, improved survival, or an adverse effect

pharmacogenomics

study of genetic factors that underlie variation in drug response; more than one genetic variant may contribute to variation in drug response

pharmacogenetics

the study of variability in single genes that affect drug response

genotype

the specific genetic makeup of a person at a particular site in the DNA

phenotype

the observable characteristic or trait

allele

one of two or more alternative forms of a gene that arise by mutation and are found at the same genetic locus

polymorphism

a genetic variant that has a difference in DNA sequence compared to a reference sequence; DNA variants that are found in more than 1% of a given population

common types of genetic polymorphisms

SNPs, Indels, and CNVs

Single Nucleotide Polymorphism (SNP)

most common type of genetic variant; includes coding nonsynonymous SNPs, coding synonymous

noncoding SNPs

may be in promoters, introns, or other regulatory regions that may affect transcription factor binding, enhancers, transcript stability, or splicing

indels

type of genetic polymorphism that is insertion or deletion

copy number variations

type of genetic polymorphism (CNV); number of copies of a specific DNA segment vary from person to person

CYP3A5*1

considered a normal allele (functional is *1)

CYP3A5*3

considered abnormal allele (non-functional)

synonymous SNP

A78A (no change)

nonsense SNP

A78X (stop codon; typically always bad)

missense SNP

A78G (new amino acid; can be fine, but can also be bad)

genetic variations of enzymes

Phase I enzymes include CYP2D6 and phase II enzymes include UGT1A1

genetic variations of transporters

plasma membrane transporters are located on epithelial cells of many tissues; they mediate uptake and efflux of endogenous and xenobiotic compounds through re-absorption by distal tubule in renal nephron and hepatic secretion into bile

T/F: Genetic Variations of Transporter can Change Drugs' Effectiveness and/or Toxicity

TRUE

T/F: Genetic polymorphisms can produce changes in protein function

TRUE

T/F: genetic polymorphisms affect pharmacokinetics and pharmacodynamics

TRUE

Drug Pharmacokinetics and Genetic Polymorphisms

genetic polymorphisms exist in enzymes involved in phase I and II metabolism and drug transporters and drug efflux pumps

Drug Pharmacodynamics and Genetic Polymorphisms

genetic polymorphisms are in drug targets, such as receptors, enzymes, and others

genesight psychotropic test

DNA test to identify an individual's genotypes for specific genes of interest; this test analyzes how your genes may affect your outcomes with medications commonly prescribed to treat depression, anxiety, ADHD, and other mental health conditions (analyzing CYP enzymes)

genesight psychotropic test benefits

Could prove useful to determine which medications may require dose adjustments, which may be less likely to work, and which may have an increased risk of side effects

phase I metabolizing enzymes

function by modifying functional groups on compounds; involved in biotransformation of more than 75% of prescription drugs

CYP Phase I example

CYP2D6 is genetically polymorphic and has many different alleles identified in humans to date, including 1, 1 x N, 2, 9/10/41, and *4/5/6

CYP2D6 allele *1

normal function

CYP2D6 allele *1 x N

increased expression and function due to gene multiplication

CYP2D6 allele *2

normal function

CYP2D6 allele *2 x N

increased expression and function due to gene multiplication

CYP2D6 alleles 9, 10, or *41

reduced functional alleles

CYP2D6 alleles 4, 5, or *6

non-functional alleles

halotype

genotype on a single set of chromosomes, such as *1

diplotype

genotypes on two sets of chromosomes, such as 1 / 1 or 4 / 5

CYP2D6 diplotype 1/1 x N or 1/2 x N

has two functional alleles with multiplications, making it an ultrarapid metabolizer (UM > 2.0; highest rates of metabolism)

CYP2D6 diplotype 1/1, 1/2, 2/2, 1/41, 2/5

has two functional alleles, one with normal function and one with reduced function; considered an extensive metabolizer (EM = 1.0; second highest rates of metabolism)

CYP2D6 diplotype 4/10, 5/41

one allele of reduced functionality makes it an intermediate metabolizer (IM = 0.5; third highest rates of metabolism)

CYP2D6 diplotype 3/4, 4/4, 4/5, 5/5, 4/6

two non-functional alleles, making it a poor metabolizer (PM = 0.0; lowest rates of metabolism)

CYP2D6 substrates

tolterodine, atomoxetine, propranolol, carvedilol, nebivolol, timolol, codeine, dextromethorphan, hydrocodone, oxycodone, and fluoxetine

CYP2D6 inducers

unknown

CYP2D6 inhibitors

chloroquine, fluoxetine, haloperidol, and imatinib

CYP2C9 substrates

drugs that are metabolized by CYP2C9 enzyme; carvedilol, celecoxib, glipizide, ibuprofen, irbesartan, losartan, cannabinoids

CYP2C9 inducers

drugs that increase the activity of CYP2C9 enzyme; carbamazepine, phenobarbital, phenytoin, rifampin

CYP2C9 inhibitors

drugs that block the activity of CYP2C9 enzyme; examples include amiodarone, fluconazole, fluoxetine, metronidazole, ritonavir, trimethoprim/sulfamethoxazole

codeine

prodrug that is a weak activator of opioid receptors; converted by CYP2D6 to morphine via demethylation, which is a full agonist at opioid receptors with potent pain-reducing effects

Patient Case 1: after given the same dose of codeine in patients with a phenotype of ultra rapid CYP2D6 metabolism, the concentration of O-demethylated morphine can be as much as 45 times higher than in those with poor metabolism (those with a nonfunctional variant of CYP2D6). Why?

Answer: Ultra rapid metabolizers carry two functional CYP2D6 alleles with multiplication, resulting in much higher CYP2D6 from gene expression

Codeine Dosing for 2.0 (UM) Phenotype

increased formation of morphine following codeine administration leads to higher risk of toxicity

Codeine Dosing for 1.0-2.0 (EM) Phenotype

standard dosing dose

Codeine Dosing for 0.5 (IM) Phenotype

standard starting dose; monitor closely for lack of analgesic response due to reduced morphine formation

Codeine Dosing for 0.0 (PM) Phenotype

greatly reduced morphine formation following codeine administration, leading to insufficient pain relief

Patient Case 2: for patients with PM CYP2D6 phenotypes, what drugs should be used for effective relief of severe pain?

Morphine; Codeine is prodrug but it is not being metabolized, leading to ineffective pain relief; by using morphine directly, pain relief can be better achieved

Patient Case 3: a patient started taking a standard dose of codeine. He was treated concomitantly with a macrolide antibiotic and an azole antifungal drug, which are both inhibitors of CYP3A4; on hospital day 4, the patient developed CNS opioid intoxication; his level of consciousness rapidly deteriorated, and he had respiratory depression. Which phenotype of CYP2D6 metabolizer does the patient most likely have?

UM

Patient Case 3: a patient started taking a standard dose of codeine. He was treated concomitantly with a macrolide antibiotic and an azole antifungal drug, which are both inhibitors of CYP3A4; on hospital day 4, the patient developed CNS opioid intoxication; his level of consciousness rapidly deteriorated, and he had respiratory depression. Looking at the metabolic pathway of codeine shown, describe how CYP3A4 inhibitors affect metabolism of codeine

codeine is converted by CYP2D6 to morphine to by CYP3A4 to norcodeine, which is inactive; when CYP3A4 is inhibited, metabolism of codeine is shunted towards the CYP2D6 pathway. As he has a UM CYP2D6 phenotype, codeine is rapidly converted to morphine, resulting in a build-up of morphine in the brain, leading to respiratory depression

tamofixen

a selective estrogen receptor modulator that is used in the treatment of estrogen receptor-positive breast cancer; overall action of tamoxifen is due, in part, to conversion to active metabolites; women with PM or IM phenotypes of CYP2D6 had lower concentrations of the active metabolite endoxifen; in women with advanced breast cancer, approximately 35% of those with estrogen receptor-positive tumors do not respond to tamoxifen therapy

phase II metabolizing enzymes

usually conjugate endogenous molecules into substrates to enhance their elimination; genetic variations of phase II enzymes may diminish drug elimination and increase risks for toxicities

phase II enzyme example

UGT1A1 (uridine diphosphate glucuronosyltransferase) conjugates glucuronide to bilirubin, a toxic metabolite from degradation of red blood cells

glucuronidation transferase enzyme

UDP glucuronosyl-transferase

acetylation transferase enzyme

N-acetyltransferase

glutathione conjugation enzyme

GSH-S-Transferase

glycine conjugation enzyme

acyl-CoA glycinetransferase

sulfation conjugation enzyme

sulfotransferase

Methylation Conjugation Enzyme

transmethylases

water conjugation enzyme

epoxide hydrolase

Biotransformation of anticancer drug Irinotecan

Irinotecan is indicated for treatment of metastatic carcinoma of the colon or rectum; this prodrug is converted by carboxylesterase 2 (CES2) to metabolite SN-38 (active form)

SN-38

active drug that inhibits topoisomerase I and terminated DNA replication, thus producing therapeutic effects and toxicity

UGT1A1

catalyzes conjugation of glucuronide to SN-38

SN-38 conversion

SN-38 is converted to SN-38G through addition of glucuronide, which makes compound inactive

UGT1A1*1

normal allele in the proximal promoter region, there are 6 TA repeats

UGT1A1*28

gilbert syndrome allele; an extra TA repeat leads to decreased gene expression and enzyme activity; 10% european populations are homozygous carriers of UGT1A1 28/28 and have deficit in glucuronidation

Biotransformation and Enterohepatic Recycling Affect Toxicity of Irinotecan

Irinotecan (CPT-11) is converted to SN-38, the active metabolite; SN-38 is glucuronidated by UGT1A1 and UGT1A7 in the GI tract; SN-38G is excreted via the bile

Bacterial b-glucuronidase

in the gut may de-conjugate SN-38G to SN-38

Excessive accumulation of SN-38 cause...?

toxicity in the gut and blood; in patients with deficient UGT1A1 alleles, such toxic effects are more likely

Patients with reduced UGT1A1 Activity

individuals who are homozygous for the UGT1A128 allele are at increased risk for neutropenia following initiation of irinotecan hydrochloride injection treatment; when administered in combination with other agents or as a single agent, a reduction in the starting dose by at least one level of irinotecan hydrochloride injection should be considered for patients known to be homozygous for the UGT1A128 allele

UGT1A1 Testing

a laboratory test is available to determine the UGT1A1 status of patients and can detect the UGT1A1 6/6, 6/7, and 7/7 genotypes

A patient has Gilbert syndrome. Her genotype UGT1A1*28/28, encodes for a protein with 30% of the enzymatic activity of the normal UGT1A1 protein. What is a likely symptom of this patient?

Normal function of UGT1A1 to place glucuronide on bilirubin to increase its excretion; bilirubin will therefore accumulate, leading to mild hyperbilirubinemia