Introduction: From Population Pharmacology to Individualized Prescribing

• Classical pharmacology describes drug effects in populations resulting in average responses at average doses in average patients.

  • But no patient is average.

    • Substantial inter-individual variability in drug response has always been observed clinically, and for most of pharmacology's history that variability was attributed to differences in age, organ function, body composition, and co-medications.

• Pharmacogenomics, the study of how genetic variation influences drug response, provides deeper insights into sources of this variability.

  • Inherited differences in drug-metabolizing enzymes, drug transporters, and drug targets alter the pharmacokinetics and pharmacodynamics of drugs.

    • Some of these differences lead to an understanding of the clinical consequences in ways that may medical benefits to the patient.

  • The promise of pharmacogenomics is prescribing precision which entails, using personalized genetic information, selecting the right drug at the right dose for that patient.^1,2

Genetic Polymorphisms: The Molecular Basis of Variable Drug Response

• A genetic polymorphism is a variation in DNA sequence that occurs in more than 1% of the population. Polymorphisms relevant to pharmacogenomics occur in three categories of genes:³

  • Drug-metabolizing enzyme genes

    • These variants that alter CYP enzyme function, affecting the rate at which drugs are metabolized and therefore their plasma concentrations and duration of action.

  • Drug transporter genes

    • These variants in genes encoding membrane transporters (P-glycoprotein, organic anion transporters, SLCO/OATP transporters) that alter drug absorption, distribution, and excretion.

  • Drug target genes

    • These variants in receptor, ion channel, or enzyme genes that alter the pharmacodynamic response to a drug at a given concentration.

      • The clinical consequences of these polymorphisms are expressed as metabolizing phenotypes which is the  functional classification of an individual's drug-metabolizing capacity based on their genotype.

CYP Metabolizer Phenotypes

• The CYP enzyme system displays clinically important genetic polymorphisms, particularly in CYP2D6, CYP2C9, and CYP2C19.

  • These enzymes are responsible for the metabolism of a large fraction of commonly prescribed drugs.

    • Four metabolizer phenotypes are recognized:^3,4

      • Poor Metabolizers (PM)

        • Carry two non-functional alleles.

        • Drug metabolism via the affected CYP isoform is absent or severely impaired.

        • Substrates accumulate to high concentrations which may result in toxicity at standard doses or manifest as an inability to convert prodrugs to active metabolites.

      • Intermediate Metabolizers (IM)

        • Carry one functional and one reduced-function allele.

        • Drug metabolism is reduced compared to normal exhibiting intermediate plasma concentrations and effects.

      • Extensive (Normal) Metabolizers (EM)

        • Carry two functional alleles.

        • Standard drug metabolism representing population "norm" against which standard drug doses are calibrated.

      • Ultrarapid Metabolizers (UM)

        • Carry multiple copies of a functional allele (gene duplication), typically for CYP2D6.

        • Drug metabolism is dramatically accelerated resulting plasma drug concentrations that are very low at standard doses.

          • These very low drug levels may result in inadequate therapeutic effect.

            • For prodrugs, conversion to active metabolites may be dangerously rapid.

      • Frequency of these phenotypes varies significantly by ethnic population, a consideration of practical importance in diverse clinical populations.⁴

CYP2D6: The Most Clinically Important Pharmacogenomic Enzyme

• CYP2D6 metabolizes approximately 20–25% of commonly used drugs and displays the most extensive pharmacogenomic variation of any CYP enzyme, with over 100 identified alleles, ranging from null (no function) to highly amplified (ultrarapid activity).

  • CYP2D6 is not inducible by drugs or dietary factors with its activity is determined almost entirely by genotype.⁵

    • Structure of CYP2D6

      Attribution BorisTM, Public domain, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:CYP2D6_structure.png March 14, 2006

   

  • Codeine and the CYP2D6 PM/UM spectrum

    • Clinical consequences of CYP2D6 polymorphism are apparent when describing codeine metabolism.

      • Codeine is a prodrug requiring O-demethylation by CYP2D6 to morphine for analgesic activity.

    • In poor metabolizers: CYP2D6 activity is absent or negligible.

      • Codeine is not converted to morphine.

        • The patient receives no analgesia.

        • This circumstance represents the pharmacogenomic basis for  inter-individual variability in analgesic efficacy associated with codeine administration.

    • In ultrarapid metabolizers: CYP2D6 converts codeine to morphine far more rapidly and completely than anticipated.

      • At standard doses, plasma morphine concentrations reach levels that in a normal metabolizer would require much higher doses.

        • Deaths have been reported in ultrarapid metabolizer neonates breastfed by ultrarapid metabolizer mothers prescribed codeine postpartum.

          •  These cases prompted regulatory restrictions on codeine use in pediatrics and nursing mothers in multiple countries.⁵

  • Tamoxifen and CYP2D6

    •  Tamoxifen, the cornerstone of adjuvant treatment for estrogen receptor-positive breast cancer, is metabolized by CYP2D6 to endoxifen, its most pharmacologically active metabolite.

      • Endoxifen exhibits about 100-fold greater affinity for the estrogen receptor compared to tamoxifen.

        • In CYP2D6 poor metabolizers, endoxifen plasma concentrations are dramatically reduced, correlating in some studies with worse clinical outcomes.

        • This pharmacogenomic interaction also means that CYP2D6 inhibitors co-prescribed with tamoxifen, such as fluoxetine or paroxetine, both potent CYP2D6 inhibitors often prescribed for hot flushes associated with tamoxifen therapy, may pharmacologically convert an extensive metabolizer into a phenotypic poor metabolizer.⁴

CYP2C9 and Warfarin: The Benchmark Pharmacogenomic Interaction

• The vitamin K antagonist warfarin has become one of the benchmark drugs for pharmacogenomics.

  • Two factors appear responsible for this designation:

    • (1) Its narrow therapeutic indes and

    • (2) Because two genes together explain a large fraction of the variance in stable therapeutic dose requirements across individuals.⁶

• CYP2C9:

  • The principal enzyme responsible for metabolism of the more pharmacologically active S-enantiomer of warfarin.

  • The CYP2C92 and CYP2C93 alleles, which are among the most common pharmacogenomic variants in European populations, encode enzymes with substantially reduced activity.

    • Carriers require lower warfarin doses to avoid supratherapeutic anticoagulation and have a significantly elevated risk of serious bleeding complications, particularly during dose initiation.

• VKORC1

  • The gene encoding Vitamin K Oxide Reductase Complex subunit 1 which is the pharmacodynamic target of warfarin.

    • VKORC1 polymorphisms alter the sensitivity of the target enzyme, with certain variants producing a VKORC1 enzyme that is more easily inhibited by warfarin (lower dose required) or less easily inhibited (higher dose required).

    • VKORC1 polymorphisms and CYP2C9 polymorphisms together explain approximately 40–50% of the variance in stable warfarin dose requirements,  a level of pharmacogenomic predictability that has driven the development of dosing algorithms incorporating genotype alongside clinical variables (age, body surface area, co-medications).⁶

HLA Alleles and Immune-Mediated Adverse Reactions

• Pharmacogenomics extends beyond drug metabolism to the pharmacodynamics of immune-mediated adverse reactions.

  • Certain severe Type B adverse drug reactions are strongly associated with specific HLA alleles, allowing prospective identification of at-risk patients.^1,7

• Abacavir hypersensitivity syndrome (HLA-B*57:01)

  Abacavir, a nucleoside reverse transcriptase inhibitor used in HIV treatment, causes a potentially life-threatening hypersensitivity syndrome in approximately 5–8% of patients.

  • The reaction is almost exclusively confined to carriers of the HLA-B57:01 allele. Prospective HLA-B57:01 screening before abacavir initiation has essentially eliminated abacavir hypersensitivity syndrome in practice.

    • The FDA label requires HLA-B*57:01 testing before prescribing abacavir.

      • This represents one of the most successful translations of pharmacogenomic knowledge into clinical practice.⁷

• Carbamazepine and Stevens-Johnson Syndrome (HLA-B15:02, HLA-A31:01)

  •  Carbamazepine causes Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), life-threatening severe cutaneous adverse reactions, at greatly elevated frequency in carriers of HLA-B15:02 (predominantly Southeast Asian populations).

  • HLA-A31:01 is associated with carbamazepine hypersensitivity reactions across multiple ethnic groups.

    • Many regulatory authorities in affected populations now require HLA screening before carbamazepine initiation.

• Allopurinol and SJS/TEN (HLA-B*58:01)

  • The gout medication allopurinol causes severe cutaneous reactions at elevated frequency in HLA-B58:01 carriers.

    • Mandatory pre-prescription screening has been implemented in several Asian countries with high HLA-B58:01 allele frequency.

TPMT and Thiopurine Toxicity

• Thiopurine methyltransferase (TPMT) is the enzyme responsible for metabolizing thiopurine drugs such as azathioprine, mercaptopurine, and thioguanine.

  • These drugs are used as immunosuppressants and in the treatment of childhood leukaemia.

    • TPMT genetic polymorphisms have direct, well-established clinical consequences.⁸

• Approximately 1 in 300 individuals is a TPMT poor metabolizer (homozygous for non-functional alleles); approximately 10% of the population is heterozygous (intermediate metabolizer).

  • In poor metabolizers, the thiopurine drugs cannot be adequately metabolized and accumulate as cytotoxic thioguanine nucleotides in bone marrow, causing life-threatening myelosuppression at standard doses.

• Pre-treatment TPMT genotyping (or phenotypic enzyme activity testing) allows dose reduction in intermediate metabolizers and avoidance of thiopurine drugs in poor metabolisers in favour of alternative immunosuppressive strategies.

  • This approach is likely appropriate when prescribing azathioprine or mercaptopurine.⁹

Module Summary