PDTI Mod 13

Basics of Drug Metabolizing Enzymes (DMEs)

• Drug metabolizing enzymes and their function will be reviewed, as well as sources of variation and how interactions impact medication pharmacokinetics.

Drug Biotransformation/Drug Metabolism

• Most orally administered drugs need some level of lipophilicity to be absorbed from the GI tract.
• Lipophilic compounds need to be converted to more water-soluble products for removal, called biotransformation or metabolism.
• Oral drugs undergo the ADME process: absorption, distribution, metabolism, and excretion.
• Drug biotransformation modifies medications by either activating or deactivating them.
• Modification results in the termination of biological activity or bioactivation (increases pharmacologic activity, such as with prodrugs).

Characteristics of Enzymes Involved in Drug Biotransformation

• Broad Substrate Specificity: A signal enzyme can metabolize many compounds.
• Enzyme Multiplicity: Several enzymes may be involved in biotransformation of one drug.
• Product Multiplicity: A drug is metabolized into several different metabolites.
• Polyfunctionality: A drug may undergo several different reaction types, involving multiple enzymes in sequence.
• An oral drug (substrate) can be metabolized by multiple enzymes, each forminga different metabolite.

Enzyme Substrate Complex

• Drug metabolizing enzymes have a large active site.
• The substrate (medication) binds to the active site, forming an enzyme-substrate complex.
• The substrate is then biotransformed into products or metabolites to be eliminated from the body.
• These metabolites can be either inactive or active, depending on the enzyme and medication.
• The kinetic properties of interactions between substrates and enzymes are determined by in vitro experiments using microsomes prepared from human tissue or transformed cell lines.

Biotransformation Reactions: Phase I, Phase II, and Phase III

• Phase I: Introduces or reveals a functional group within the substrate through oxidation, reduction, or hydrolysis using Cytochrome P450s (CYPs).
• These reactions make the compound more hydrophilic and prepare it for Phase II reactions.
• Phase II: Involves conjugation by endogenous substrates such as sulfate, acetate, gluconic acid, glutathione, and glycine using transferase enzymes.
• These reactions further increase compound solubility and involve transferase enzymes such as UGTs, STs, NAT2, and COMT.
• Phase III: Further processing prior to recognition by efflux transporters for elimination.

Top Medications and Biotransformation

• Approximately 75% of drugs are metabolized by the liver through Phase I and Phase II metabolism.
• About 25% of medications are renally eliminated unchanged, and a smaller fraction is eliminated directly in the bile.
• Of the drugs metabolized by the liver, the majority involve CYP enzymes.
• A smaller fraction are conjugated by UGT enzymes, and some other enzymes are involved to a lesser extent.

CYP Enzymes

• CYP3A metabolizes about 50% of medications and includes CYP3A4 and CYP3A5.
• These enzymes are abundantly expressed in the liver and intestine and have the greatest contribution to human drug metabolism.
• CYP2D6 metabolizes about 25% of medications.
• CYP2C19 is also a key enzyme.

Cytochrome P450s (CYPs)

• Main players in Phase I metabolism.
• Some CYPs are involved in the production of endogenous compounds, while others metabolize xenobiotics.

Nomenclature of Cytochrome P450s

• The nomenclature includes CYP as the root, a number for the family, and another number for the subfamily (e.g., CYP2D6).
• P450s from the same family share about 40% homology, while those from the same subfamily share about 55%.
• Alleles for each gene encoding CYP P450 differ based on individual genetics.
• The numbering of enzymes is across species.

Expression of CYPs

• The liver and intestines have the largest role in drug metabolism.
• CYP3A is the most abundantly expressed in the liver and intestine and has the greatest contribution to human drug metabolism.
• CYP3A is widely expressed, with about 40% in the liver and 82% in the intestine.
• CYP2D6 is about 2% of CYP expression in the liver but metabolizes about 25% of medications and is susceptible to many drug interactions.
• CYP2s are expressed in the intestine.
• CYP2D6, CYP2C19, and CYP2C9 are clinically relevant CYP enzymes.

Contribution to Drug Metabolism

• CYP3A metabolizes about half the drugs that undergo CYP metabolism and has the broadest substrate specificity.
• CYP2D6 has low abundance in the liver but a substantial contribution to human drug metabolism, about 25% of all drugs.
• CYP2C includes CYP2C9 and CYP2C19.
• CYP2A6's major contributor is nicotine.

Sources of Inter-Individual Variability

• Intrinsic and environmental factors are key and relevant to variability in metabolism.
• Genetics: Genetic variation can impact enzyme activity level.
• Age and Sex: Can impact variability among different CYP enzymes.
• Disease States: Can impact clearance and metabolism because of less functional CYP enzymes present.
• Environmental Factors: Drugs and diet can change drug clearance by causing induction or inhibition.

Genetics

• Ultra-Rapid Metabolizers: Individuals with a genetic variation that results in higher baseline enzyme activity
• They have more enzyme available, leading to lower drug concentrations and overall lower AUC.
• Poor Metabolizers: Individuals with a genetic variation that results in little to no enzyme activity.
• They have less enzyme available, leading to higher drug exposure and higher AUC.
• Intermediate metabolism results in higher drug exposure depending on the drug pathway, assuming the substrate is active and the products are inactive.

Age and Sex Variables

• Each CYP enzyme has different medications that it pairs to and can differ by age, sex, and genetic polymorphism.
• Infants have increased CYP2C19 activity, almost like ultra-rapid metabolizers.
• Around puberty (11-16 years), enzyme activity levels out to adult levels.
• Some studies show sex-related differences for CYP1A1, CYP1A2, and a few others.

Disease States

• As liver disease progresses, there is less functional mass of the liver, impacting the expression of CYP enzymes and therefore clearance and metabolism.
• Older adults tend to lose phase one metabolism.

Environmental Factors: Drugs and Diet

• Inhibition: Disables the enzyme, reducing the clearance of the substrate.
• It takes time for the body to recreate the enzyme after the inhibitor is discontinued (about 7-10 days to get back to steady state).
• Induction: The drug essentially tells the body to create more enzyme.
• The effect of induction takes longer to see but stops more quickly after the drug is stopped.

Key Points

• Drug metabolizing enzymes play a key role in the effectiveness and risk of side effects for medications.
• Several sources of inner individual variability exist and can impact the pharmacokinetics of drugs.

Pharmacokinetic and Pharmacodynamic Variability:

• Genetic variation impacts drug exposure, affecting Cmax (maximum concentration) and the area under the curve (AUC) of substrate medications.
• Genetic variations can impact how a patient responds to medication, influencing its effectiveness and toxicity.
• The content primarily focuses on oral administration of medications.

CYP Pharmacogenetics:

• Genetic variation is a major cause of inter-individual variability in drug response.
• Genetic variation contributes to variation in major CYP enzymes responsible for drug metabolism.
• The discussion is centered on CYP enzymes important for human drug metabolism, particularly those with clinical consequences due to genetic variation.

Genetics Background and Nomenclature:

• Wild Type Allele: Designated as *1, indicating no variation detected and serves as the reference allele.
• Star Alleles: Sequentially numbered and each has a defined function, but it varies across enzymes.
• Allele Functions: Standard definitions exist for allele functions.
• Functionality Buckets for Star Alleles: Increased, normal, decreased, no function, unknown, and uncertain.

Phenotype Definitions:

• Phenotype is determined by combining allele one and allele two.
• Five standard categories of enzyme phenotypes exist: ultra-rapid, rapid, normal, intermediate, and poor.

Phenotype Groupings:

• Ultra-rapid and rapid: increased activity.
• Normal: fully functional standard enzyme activity.
• Intermediate: decreased activity.
• Poor: little to no enzyme activity.
• Genetic Definition: Ultra-rapid metabolizers have two increased functions or more than two normal functions, sometimes due to duplication variations.
• It is important to understand and translate the relationship between the number of function alleles and the resulting metabolizer phenotype.

Enzyme Activity Level and Phenotypes:

• Poor metabolizers have little to no enzyme activity compared to normal metabolizers and normal should be the reference point.
• Enzyme activity translates to pharmacokinetic effects.

Therapeutic Range:

• The goal is to maintain drug concentrations within the therapeutic range, avoiding the toxic range (side effects) and sub-therapeutic range (ineffectiveness).
• For drugs given in active form and metabolized into inactive metabolites:
  • Poor metabolizers are more likely to experience side effects due to higher concentrations of the active drug.
  • Ultra-rapid metabolizers tend to be in the sub-therapeutic range due to more inactive metabolite and less active drug.
• For prodrugs: ultra-rapid metabolizers will have increased enzyme activity, leading to higher concentrations of the active metabolite. Poor metabolizers have less active metabolite due to less metabolism occurring.

Pharmacokinetic Variables:

• Time of maximum concentration (Tmax) depends on the elimination rate constant (KE), assuming absorption rate (KA) is fixed.
• Increased KE (increased elimination) results in increased drug elimination, with Tmax occurring sooner.
• As elimination rate increases (from poor to normal metabolizer), Cmax and AUC decrease, and Tmax occurs earlier.
• Bioavailability: Low bioavailability: less than 30% and High bioavailability: greater than 70%.
• High Bioavailability Drugs: Half-life increases in poor metabolizers, but Cmax does not change significantly.
• Low Bioavailability Drugs: Poor metabolizers have increased Cmax, but half-life is not as affected.
• In both cases, poor metabolizers have an overall increase in AUC.

CYP Enzyme Categorization:

• Broadly categorized into well-conserved versus highly polymorphic.
• Pharmacogenetics focuses on highly polymorphic CYPs, like CYP2D6.

Genetics vs. Phenotyping:

• Genetics predicts enzyme activity, but enzyme activity can also be measured through phenotyping using probe drugs.
• Probe drugs have high affinity and are metabolized mostly by one CYP enzyme.
• By administering a probe drug and measuring drug concentrations after metabolism, enzyme activity can be measured.

Study Example:

• A study involving 100 individuals given debrisoquine and caffeine to measure CYP2D6 and CYP1A2 activity, respectively.

Independence of CYP Enzymes:

• CYP2D6 enzyme activity does not predict CYP1A2 activity.
• CYP enzymes are unique to the individual based on genetics and are regulated by different mechanisms.
• There are very specific gene-drug relationships.

CYP1A2 Considerations:

• CYP1A2 is not highly polymorphic, so it's not a focus.
• Diet and environmental exposure can induce this enzyme.
• Smoking induces CYP1A2, increasing its activity by about 1.6-fold.

VIPs (Very Important Pharmacogenes):

• VIPs play a role in the metabolism of many drugs or contain variants that potentially contribute to severe drug responses.
• The lecture focuses on phase one drug metabolizing enzymes.

CYP2 Family Overview:

• Includes CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, and CYP2D6.
• Collectively, CYP2 subfamilies contribute to metabolizing about half of all drugs.

CYP2A6:

• Known for nicotine metabolism.
• Metabolizes about 3% of drugs.
• Environmental factors can impact its expression.
• Poor metabolizers: Approximately 1% of Caucasians and 20% of Asians.
• Target for new drugs aimed at inhibiting nicotine metabolism to change smoking rates.
• The most common variant allele in Asians is CYP2A6*4, a gene deletion resulting in a no-function allele.
• Duplications exist, causing rapid or ultra-rapid phenotypes.
• Smokers can titrate nicotine concentrations through smoking behavior.
• Individuals with low CYP2A6 activity are less likely to become smokers or will smoke less.

CYP2B6:

• Important for the metabolism of CNS-acting drugs. Classical substrates: methadone, sertraline, efavirenz.
• Highly polymorphic.
• CYP2B6*6 is a clinically relevant polymorphism.
• Several normal, decreased, and no function alleles exist, as well as two increased function alleles.
• Inverse relationship with clearance in AUC.
• Efavirenz is a classic substrate and the *6 allele results in decreased CYP2B6 expression.
• CYPC guidelines are available for efavirenz, sertraline, and methadone.

CYP2C Gene Family:

• Includes CYP2C8, CYP2C9, and CYP2C19.
• Collectively make up about 20% of the protein content in the human liver.

CYP2C8:

• In strong linkage disequilibrium (LD) with CYP2C93.
• Guidelines primarily exist for NSAIDs for CYP2C9 and not a clinically common gene to test for directly.
• Two functional polymorphisms: CYP2C8*2 and *3.
• No activity level or phenotype classification has been assigned; there are not current guidelines.
• *3 is a variant that has increased expression and is associated with increased metabolism of substrates, such as glitazones.
• Rifampin and St John's wort are inducers.
• Variants exist, more emerging evidence for substrates, and it is in strong LD with CYP2C9.

CYP2C9:

• Primarily expressed in the liver and responsible for clearing about 15 to 20% of all drugs undergoing phase one metabolism.
• Genetic information added to warfarin's label due to polymorphism in CYP2C9, as it has a very narrow therapeutic index.
• S-warfarin is more impacted by CYP2C9 variation Key substrates to know: NSAIDs, phenytoin, and warfarin.
• Rifampin is a classic inducer and amiodarone/fluconazole are inhibitors.
• Variant alleles include decreased and no function.
• Poor metabolizer phenotype was updated to now be defined as little to no enzyme activity with an activity score of 0 to 0.5.
• Activity value of *2 is 0.5, and *3 is 0.
• Enzyme with polymorphisms that have substrate-specific effects; CYP2C9 structure suggests it can accommodate multiple substrates to bind.
• CYP2C9 polymorphisms learned about from warfarin.
• In white populations, about one out of every three patients carries some type of variant allele.
• In African Americans, at least about 13% carry at least one *2 allele. Important CYP2C9 star alleles in African American populations: star five, six, and eight.
• The majority of Asians have normal CYP2C9 enzyme activity.
• *3 variant is functionally more detrimental than that *2.
• Clinical guidelines relevant: warfarin, NSAIDs, and phenytoin.

CYP2C19:

• Contributes to the metabolism of a large number of clinically relevant drugs and different classes, such as antidepressants, benzodiazepines, proton pump inhibitors, and clopidogrel.
• Requires activation by CYP2C19 to work and was the first gene to be implemented here at UF Health.
• The *2, three and 17 are really the most common genetic variations.
• *2 and three are no function and *17 is increased function.
• Frequency of poor metabolizers is quite high in our Asian population here.
• Poor metabolizers is highest in East Asian versus the rapid is most common in more of our African Americans as well as Europeans.
• Example of CYP2C19 and clopidogrel: poor metabolizer having the least amount of activation.
• Poor metabolizers, it really does not pharmacodynamically that translate to platelet reactivity reduction.
• Omeprazole: Poor metabolizers have about 15 to 30-fold higher drug exposure, which some of the literature shows is beneficial.
• Clinical recommendations to increase the dose in our normal metabolizers in proton pump inhibitors.
• Voriconazole: Normal metabolizers are the lowest, potentially at risk for therapeutic failure.

CYP2D6:

• Highly polymorphic, one of the most extensively studied genes.
• Over 100 variants known to date and phenotypes were discovered before the genotype.
• Metabolizes about 25% of drugs, substrates being heart and head medications.
• Opioids and Tamoxifen require CYP2D6 to become active (prodrug).
• Beta blockers and antidepressants depend on CYP2D6 to get inactivated and out of the body.
• No inducers are known for CYP2D6.
• *4 is the single most common variant.
• Duplications are present, and gene can be duplicated with a copy number variation, which will confer that ultra-rapid metabolizer phenotype.
• Population frequencies vary, such as Ethiopians having a very high proportion of ultra-rapid metabolizers.
• CYP2D6 and CYP2A6 are the two genes with duplication.
• Codeine to morphine by CYP2D6 is considered a prodrug and the active metabolite, respective.
• Anyone with increased CYP2D6 activity is going to have increased concentrations of morphine and is dangerous and leading to potential overdoses.
• The individual with 13 copies here is gonna have very high concentrations of the metabolite and low of the parent compound.
• Codeine has been associated with deaths in different population, one in children after tonsillectomy.
• Tramadol as well, because very similar activation that has to occur there.

CYP3A Family

• Four genes: CYP3A4, CYP3A5, 3A7, and 3A43 and share about 80% homology.
• CYP3A4 and CYP3A5 are the most important for clinical practice and account for 40-82% in the liver and intestine, respectively.
• Responsible for the metabolism of about 50 to 60% of clinical drugs.
• Key substrates for pharmacogenetics are tacrolimus and quetiapine.
• Key variant with CYP3A4 is really *22 and explains about 12% of the variability in CYP3A4 enzyme among individuals.
• For this gene or for this variant, the most evidence is with quetiapine and have higher quetiapine drug exposure in the *22 allele.
• In the majority of the population, CYP3A5 is not expressed.
• It is more common to actually express CYP3A5 in African Americans.
• Examples with tacrolimus being the immunosuppressant.
• CYP3A5*1 is more common in our African American populations.
• For Caucasians it's more common to have CYP3A5*3, which is a reduced variant expression or variants.

DPYD:

• A gene that encodes for dihydropyrimidine dehydrogenase, or DPD enzyme and is another Phase I VIP even though it's not a CYP.
• Recommend testing for specific genetic variations.
• If you were taking fluoropyrimidines, so 5-FU and capsamine, those are fluoropyrimidines, that are used in oncology, mostly GI cancers.
• People can have a variation that will result in decreased DPD activity and then that increases exposure to the drug and it has life-threatening fluorouracil toxicity.

Pharmacogenomics: Phase II Drug Metabolism and Genetic Variation

• The role of genetics in interindividual variability in medication response and the impact of genetic variation on the pharmacokinetics of medications will be explained.

Phase II Reactions

• Phase II reactions involve glucuronidation, sulfation, acetylation, methylation, and conjugation.
• Some drugs undergo only glucuronidation, while others go through both Phase I and Phase II metabolism.
• These reactions are catalyzed by transferase enzymes that transfer an endogenous moiety to the drug molecule.
• The focus will be on glucuronidation (UGTs), thiopurine methyltransferase (TPMT), and N-acetyltransferase 2 (NAT2) due to their pharmacogenetic relevance.

Enzymes Responsible for Phase II Metabolism

• UGTs metabolize the most drugs; some drugs are directly conjugated without Phase I metabolism.
• TPMT has a small overall contribution to metabolism, but genetic variation in TPMT has significant clinical consequences for thiopurines.
• NAT2 also contributes to Phase II metabolism.

Classification and Nomenclature

• Similar to cytochrome P450s, the classification includes superfamily, family, and subfamily.
• For UGTs, there are two major families (1 and 2) and three subfamilies. UGT3 and UGT8 are not well characterized.

UGT1A1

• UGT1A1 has the largest contribution to metabolism and is responsible for the glucuronidation of many compounds, including hormones, flavonoids, and environmental mutagens.
• Most UGT family members are expressed in the liver, intestines, stomach, and breast tissue. Some members like UGT1A7, UGT1A10, and UGT2A1 are expressed extrahepatically.
• Acetaminophen is a classic substrate that is both glucuronidated and sulfated.

UGT1A1 Substrates and Genetic Variations

• UGT1A1 is relevant for drugs and bilirubin.
• *1 is the normal function allele, and *6 and *28 are key decreased function alleles.
• An individual with one normal and one decreased function allele is an intermediate metabolizer, while an individual with two decreased function alleles is a poor metabolizer.
• The 28 allele is widely studied and associated with reduced gene expression and glucuronidation in human liver microsomes.
• *28 is most frequent in African Americans, followed by Caucasians and Asians, meaning a large portion of African Americans are intermediate or poor metabolizers.

Clinical Implications of UGT1A1 Variations

• Decreased enzyme activity can lead to Gilbert's syndrome or neonatal hyperbilirubinemia and altered drug response.
• Sengers and Gilbert's syndromes are associated with impaired bilirubin metabolism leading to hyperbilirubinemia.
• Gilbert's syndrome is mild and common, with mildly elevated bilirubin.
• Crigler-Najjar syndrome is more significant, with Type 1 being the most severe and often fatal in childhood without treatment. Type 2 is less severe with low but detectable UGT1A1 enzyme activity.
• Gilbert's syndrome is most common with poor metabolizers.

UGT Substrate Overlap and Drug Metabolism

• UGTs are similar to CYP3A in that there's high overlap.
• Genetic polymorphisms may not be as important due to multiple pathway concept.
• However, some drugs are predominantly metabolized by a single UGT enzyme, leading to a greater impact when there is a polymorphism, such as Irinotecan.

Irinotecan

• Used for cancer treatment, toxicity includes neutropenia, diarrhea, and death.
• UGT1A1 intermediate or poor metabolizers will have more active metabolite (SN-38) at at risk for severe toxixity.

Atazanavir

• Atazanavir, an antiviral drug for HIV, is a CYP3A4 substrate and inhibits UGT1A1.
• It prevents the elimination of bilirubin, leading to indirect hyperbilirubinemia with jaundice, which can cause premature discontinuation of the drug.
• CPIC guidelines recommend an alternative in poor metabolizers due to the risk of bilirubin-related atazanavir discontinuation.

N-Acetyltransferase (NAT)

• Homozygous variants are slow acetylators, and heterozygous or homozygous variants are fast acetylators.
• Homozygous wild type or heterozygous are fast acetylators, homozygous variant is poor metabolizers.
• 50% of the population are slow acetylators, varying between populations, and responsible for Phase II hepatic metabolism of many drugs.
• Isoniazid and procainamide are most often associated with NAT2 status. Hydralazine is a major substrate.

Hydralazine

• Parent drug and active, it undergoes first pass hepatic metabolism by NAT2 into an inactive metabolite.
• No dosing adjustments in the package label.
• Slow acetylators have higher plasma levels and more drug exposure, requiring lower doses to maintain blood pressure.

Isoniazid

• Fast acetylators have lower drug concentrations, while slow acetylators have higher concentrations.
• Higher isoniazid levels result in greater neuropathy and hepatitis in poor metabolizers.
• Faster appearance of antinuclear antibodies with procainamide and hydralazine-induced lupus erythematosus is also common in poor metabolizers.

Amifampridine

• Approved for Lambert-Eaton myasthenic syndrome and is metabolized to an inactive metabolite by NAT2.
• Drug exposure is increased in NAT2 poor metabolizers.
• Dose adjustments are available based on genetics in drug resources.

Thiopurine Methyltransferase (TPMT)

• TPMT catalyzes the s-methylation of thiopurine drugs (6-mp, azathioprine, thioguanine).
• Azathioprine, mercaptopurine (6-mp), and thioguanine are most clinically relevant.
• TPMT is polymorphic, with about 28 variants identified.
• CPIC guidelines are available that take into account both TPMT and NUDT15 to dose adjust.
• Despite higher drug exposures in intermediate metabolizers, only about 30 to 60% tolerate full doses of mercaptopurine or azathioprine.

NUDT15

• NUDT15 variants strongly influence thiopurine toxicity. Intermediate and poor metabolizers are more common in Asian populations.
• Less TPMT and less NUDT enzyme activity, more toxicity.
• NUDT15 converts a toxic metabolite to a less toxic one.

COMT (Catechol O-Methyltransferase)

• COMT encodes for catecholamine methyltransferase, which breaks down synaptic dopamine and norepinephrine.
• Variants have been linked to psychiatric disorders, schizophrenia, opioid receptor-mediated pain perception, and breast cancer.
• Most highly studied variant is a G-to-A SNP at codon 158 (Val-to-Met).
• Individuals with the A allele (Met) have about 25% of the protein activity compared to wild type.
• Mixed evidence with COMT in ADHD, antipsychotics, antidepressants, pain.

Summary of Phase II Enzymes

• Genetic variation in conjugating Phase II enzymes can be associated with variability in drug response, adverse drug reactions, and disease susceptibility.
• There is more interest in Phase II enzymes, particularly with cancer and environmental exposure.

Introduction to Drug Interactions

• Drug interactions can affect drug efficacy and safety, making understanding them crucial for optimizing treatment.

Why Worry About Drug Interactions?

• Multiple medications: Patients, especially the elderly or hospitalized, often take multiple medications.
• Morbidity and mortality: Unanticipated drug interactions are a significant cause.

Definition of a Drug Interaction

• A drug interaction is a modification of the expected drug response due to exposure to another drug, food, supplements, or a medical condition.
• A disease-drug interaction is when an existing medical condition or disease causes a drug interaction.

Effects of Drug Interactions

• Efficacy: Interactions with food or other drugs may enhance or reduce therapeutic effects.
• Side effects: Interactions can lead to unexpected side effects or adverse reactions.

Types of Drug Interactions

• Synergistic: A drug's effect is increased.
• Antagonistic: A drug's effect is decreased.
• Drug interactions can result from alterations in pharmacokinetics (PK) or pharmacodynamics (PD).

Clinically Relevant Interactions

• Change the intended response to the medication.
• Cause toxicity.

Drug-Drug Interactions at the Pharmacokinetic (PK) Level

• Interactions occur if a drug changes the GI tract pH, affecting the dissolution of another drug.
• Example: Proton pump inhibitors.
• Both metabolism and transporter-mediated processes are prone to drug-drug interactions.
• Drug-drug interactions can impact first-pass metabolism or drug clearance.
• Drug interactions at the distribution level occur when one drug displaces another from its plasma protein binding site, increasing the free drug fraction.
• Drug-drug interactions can be mediated by active transporters in the kidneys.

Enzyme-Mediated Drug-Drug Interactions

Can involve inhibition or induction.

Inhibition

• Competitive Inhibition: The substrate and inhibitor compete for the same active site on the enzyme.
• Non-competitive Inhibition: The inhibitor binds to the enzyme-substrate complex at a different site, inactivating the enzyme.
• Irreversible Inhibition: The inhibitor binds irreversibly to the enzyme through a covalent bond, causing permanent inhibition.

Induction:

• Inducers increase the expression level of enzymes or transporters.
• Mechanism: increasing transcription, decreasing mRNA degradation, increasing apoprotein production.
• It may take a few days of treatment with the inducer to observe a clinically relevant interaction.

Examples of CYP-Mediated Drug-Drug Interactions

• CYP3A4: Ritonavir (strong inhibitor) impairs triazolam clearance.
• CYP2D6: Fluoxetine (potent inhibitor) increases desipramine AUC.
• CYP2C9: Miconazole (moderate inhibitor) increases S-warfarin AUC more than R-warfarin AUC.
• CYP1A2: Fluvoxamine (potent inhibitor) significantly increases tizanidine.

Classification of Inhibitors

• Classification is based on the effect on the AUC of the substrate.
• Inhibitor Strength: Weak (1.25 to 2-fold increase), Moderate (2 to 5-fold increase), Strong (Greater than a 5-fold increase)

Examples of Drug-Metabolizing Inducers

• Smoking (CYP1A2), Rifampicin (CYP2C9), Ethanol (CYP2E1), Carbamazepine (CYP3A4), Phenobarbital (CYP3A4), Rifampin (CYP3A4), Phenytoin (CYP3A4), St. John's Wort (CYP3A4), Troglitazone (CYP3A4)

Classification of Inducers

• Classification based on changes in the AUC of the substrate.
• Inducer Strength: Weak (20-50%), Moderate (50-80%), Potent (More than 80%)

Example of CYP3A4 Induction

Triazolam becomes ineffective during rifampicin treatment due to increased metabolism in the gut wall and liver.

Onset and Offset of Interactions

• Enzyme inhibition: Fast onset and offset (direct chemical effect).
• Enzyme induction: Slow onset and offset (requires protein synthesis).

Food-Drug Interactions

• Bisphosphonates: Absorption is negligible when taken with food; take with plain water 30 minutes before the first food, drink, or medication of the day.
• Cephradine: Food intake reduces C-max and increases T-max, but the overall AUC does not change; can result in subtherapeutic levels when the drug is given with food.
• Pantoprazole: Food increases T-max and decreases AUC and C-max, so better to administer under fasting conditions.
• Ciprofloxacin: Co-administration with dairy products reduces AUC and C-max. If a patient takes dietary supplements containing calcium, they should stop unnecessary supplements during ciprofloxacin treatment.
• Theophylline: Dose-dumping effect, resulting in serum levels that are twofold higher; the increased C-max can lead to toxicity.
• Amoxicillin: Fiber affects the bioavailability, resulting in a lower AUC, increasing gastric emptying and intestinal motility.

Food/Nutrient Effect on Drug Excretion: Impact of Urine pH

• Variations in urine pH can significantly impact drug reabsorption in the renal tubules.
• Acidic and alkaline foods may affect urine pH through ionization of weak acids and weak bases.
• Salicylic Acid Example: more ionized in alkaline urine, less reabsorption; non-ionized in acidic urine, more reabsorption.

Food/Nutrient Effects on Drug Metabolism: Grapefruit Juice

• Grapefruit juice inactivates intestinal enzymes and transporters, resulting in clinically relevant drug interactions.
• Consumption reduces the intestinal concentration of CYP3A4 protein in enterocytes but does not alter liver CYP3A4 activity.
• The mechanism for the effect of grapefruit juice on oral felodipine kinetics is the selective downregulation of CYP3A4 in the small intestine and not in the liver.
• Simvastatin: Avoid concomitant use of grapefruit juice and simvastatin, at least in large amounts.
• Drugs that show clinically relevant interactions with grapefruit juice are generally CYP3A substrates with high first-pass metabolism.

Herb-Drug PK Interactions

• Herb-drug interactions are a growing medical concern, but the FDA does not regulate herbal supplements as rigorously as conventional medications.
• Saint John's Wort: Use reduced the AUC of indinavir.

Saint John's Wort Interactions

• Most HIV proteinase inhibitors, including indinavir, are substrates for CYP3A4, and Saint John's Wort induces CYP3A4, therefore increasing clearance and reducing the AUC of indinavir.
• Saint John’s Wort is an inducer of CYP3A4 expression and P-glycoprotein expression and both are significantly involved with the metabolism and absorption of cyclosporine and other immunosuppressants.
• Saint John's Wort requires long-term use to observe an induction effect on enzyme activity.
• drugs are metabolized by CYP3A4 as a major route of metabolism or as a secondary pathway.
• Also induces other enzymes, such as CYP2C9 and CYP2C19 in vivo.
• Herbal supplements contain active compounds that may cause relevant drug interactions mediated by both enzymes and transporters, including both inhibition and induction.

Drug-Herbal Interaction at the Pharmacodynamics Level

• The anticoagulant effects of warfarin may increase when combined with coumarin-containing herbal medicines or antiplatelet herbs.
• The anticoagulant effects of warfarin may decrease when combined with vitamin K-containing herbs.
• Warfarin decreases INR ratio, which can be a subtherapeutic level.

Summary of Key Points

• It is important to determine the magnitude of changes in PK parameters or in PD response to determine the clinical significance of the observed drug interaction and to inform how to appropriately manage and prevent these clinically significant drug interactions.
• Enzyme-mediated interactions, non-enzyme-mediated interactions, and transporter-mediated interactions are all important in food-drug and herb-drug interactions.

Introduction

• Drug-drug-gene interactions involve the interplay between drug-drug interactions and drug-gene interactions, incorporating both drugs and an individual's genotype to predict pharmacokinetic changes.

Basic Principles

• A drug-drug interaction plus a drug-gene interaction equals a drug-drug-gene interaction.
• Regardless of whether it is a drug-gene interaction or a drug-drug interaction, the result is reduced metabolism of the substrate.

Enzyme Inhibition

• Strong inhibitors are typically non-competitive, while moderate and weak inhibitors are often competitive.
• Strong inhibitors decrease enzyme activity to nearly null or a poor metabolizer.

AUC Ratio and Fold Increase

• The AUC (Area Under the Curve) ratio is used to quantify the magnitude of drug interactions.
• The magnitude of the AUC fold increase depends on genetic variation.

Clinical Data Examples

• The fraction of clearance for substrates increases with enzyme activity.
• The magnitude of AUC fold changes increases with enzyme activity; ultra-rapid metabolizers have the biggest magnitude of change in AUC, as poor metabolizers have little to no change.

Storelli and Colleagues Study

• Study examined the effects of inhibitors on dextromethorphan, with participants stratified by genetics: normal and intermediate metabolizers.
• With the strong inhibitor paroxetine, 100% of intermediate metabolizers phenoconverted to poor metabolizers, while only about half of normal metabolizers did.
• When dextromethorphan was the substrate, more inhibition was observed in individuals with more enzyme activity.

Key Findings from Storelli Group

• Individuals with normal metabolizer status will have the highest AUC curve.
• More enzyme activity leads to a higher fold increase in AUC or a higher AUC ratio.

Drug-Drug Interactions vs. Poor Metabolizers

• A drug-drug interaction and a poor metabolizer have similar impacts from a pharmacokinetic standpoint, impacting (in increasing order): ultra-rapid, normal, and intermediate metabolizers.

Mirabegron Example

• Moderate or weak CYP2D6 inhibitors may have blunted inhibitory pharmacokinetic properties in patients with reduced CYP2D6 activity.

Endoxifen Formation Example

• As enzyme activity increases, the impact of strong inhibitors becomes more pronounced, leading to lower concentrations of endoxifen.

Voriconazole and Ritonavir Interaction

• In CYP2C19 poor metabolizers, voriconazole metabolism shifts to CYP3A.
• When ritonavir is present, both CYP2C19 (due to genetics) and CYP3A (due to drug interaction) pathways are blocked.

Debrisoquine and Quinidine Interaction

• A study showed that all individuals with functional copies of CYP2D6 were converted to presenting as poor metabolizers when given quinidine, named phenoconversion.

Codeine and