L11 Pharmacogenomics and Personalized Medicine

Variable Drug Response

• Inter-individual drug response varies due to demographic factors, environment, co-morbidities, drug-drug interactions, and genomic variation.
• Drug therapy benefits only 30-60% of patients.
• Adverse drug reactions (ADRs) lead to hospitalization for 6.5% of patients.

Pharmacogenomics

• Pharmacogenomics studies the genetic determinants of drug response.
• CYP families 1-3 metabolize 70-80% of clinically used drugs.
• These CYP families are polymorphic.
• P450, now CYP2B4, was purified in 1978 and cloned in 1981 (CYP2B1 from the rat liver).
• Polymorphic forms of P450s lead to adverse drug reactions (ADRs).
• Treating patients with polymorphic forms of P450s is more expensive.
• 30-60% of subjects do not respond to drug therapy.
• 56% of phase 1 drugs are metabolized by phase 1 enzymes.
• Only 20% of drugs that are substrates for non-polymorphic enzymes are in ADR reports.
• 75% of hospital administrations stem from ADRs.

Personalised Medicine

• Personalised medicine involves precise diagnosis, prediction and prevention, personalized targeted intervention, and patient participation.

10,000 Genomes Project

• Genomics England coordinates this project.
• The project sequences the whole genome of families with rare diseases and patients with common diseases.
• This generates detailed clinical and diagnostic data to enable assessment of genomic variation.

Pharmacogenomic Guidelines

• Clinical Pharmacogenomics Implementation Consortium (CPIC) develops guidelines.
• Dutch Pharmacogenetics Working Group (DPWG) also produces guidelines.
• Other guidelines come from the Canadian Pharmacogenomics Network for Drug Safety (CPNDS) and the French National Network of Pharmacogenomics (RNPGx).

Prevalence of Actionable Pharmacogenetic Variants

• 98.5% of Whites and 99.1% of African-Americans in the US have at least one high-risk diplotype (Dunnenberger et al, 2015).

Clinical Scenario 1: Mr. S, Acute Myocardial Infarction

• Mr. S, a 46-year-old man, is admitted with chest pain and diagnosed with acute myocardial infarction.
• He undergoes primary angioplasty and coronary stenting.
• His cholesterol is high (7.2 mmol/L).
• Several first-degree relatives developed coronary artery disease in their thirties.
• Anti-platelet and statin treatment is recommended.
• Pharmacogenetic considerations are important when choosing anti-platelet and statin therapies.

CYP2C19 and Clopidogrel

• Clopidogrel is a prodrug activated by CYP2C19.
• It blocks the P2Y12 receptor on platelet cell surface which binds ADP and induces platelet aggregation.
• Platelet aggregation is reduced in individuals with *1/*1 alleles when treated with Clopidogrel.
• The reduction is less effective with *1/*2 alleles.

Simon et al, 2009 Study

• Poorer outcomes in patients homozygous for the CYP2C19*2 allele treated with clopidogrel experienced after myocardial infarction.
• Patients homozygous for *2 allele showed less protection from clopidogrel.

CYP2C19 Poor Metabolizers

• Poor metabolizers have two non-functional alleles: *2/*2, *3/*3, or *2/*3.
• This results in significantly reduced clopidogrel active metabolite formation.
• There is increased on-treatment of platelet reactivity and an increased risk of adverse cardiac and cerebrovascular events.
• Clopidogrel should be avoided in these individuals, and prasugrel or ticagrelor should be used instead (if no contraindication).

Statin Options

• Possible statin options include atorvastatin, fluvastatin, lovastatin, pitavastatin, pravastatin, rosuvastatin, and simvastatin.

SLOCB1 and Simvastatin

• SLOC1B1 facilitates the hepatic uptake of statins.
• Decreased function of this transporter increases systemic exposure to statins, linking to statin-associated muscle symptoms (SAMS).

ABCG2 and Rosuvastatin

• ABCG2 encodes the ATP-Binding Cassette G2 transporter.
• It is expressed in various tissues, including the liver, blood-brain barrier, and intestine.
• ABCG2 exports compounds into the extracellular space.
• p.Q141K (c.421C>A, rs2231142) reduces protein expression by 30-40% and leads to increased plasma levels of Rosuvastatin.

CYP2C9 and Fluvastatin

• CYP2C9 is highly polymorphic, with at least 71 variant alleles.
• CYP2C92 (p.R114C; rs1799853) and CYP2C93 (p.I359L; rs1057910) reduce CYP2C9 function by approximately 30-40% and 80%, respectively.
• This leads to increased systemic exposure to fluvastatin.

Mr. S's Genotype

• Genotyping reveals that Mr. S carries the *5 and *6 allele for SLOC1B1.

Statin-Induced Myopathy

• Statin-induced myopathy ranges from mild myalgia to rhabdomyolysis.
• GWAS shows SLOC1B1 variant relates to myopathy with simvastatin.
• SLOC1B1 encodes the main inward hepatic statin transporter.
• Decreased activity of SLOC1B1 leads to higher plasma levels.
• This can result in increased statin uptake into muscle tissue.

GWAS on Statin Myopathy (2008)

• SLOC1B1*5 (Val174Ala) was identified as a risk allele.
• Higher plasma levels of statins cause muscle toxicity.
• Only 80 cases were needed to show this effect with simvastatin.

CPIC Guidelines for Simvastatin

• Possible decreased function with one no function allele plus an uncertain/unknown function allele (e.g., *5/*6, *15/*10, *5/*43).
• Poor function with two nonfunctional alleles (e.g., *5/*5, *5/*15, *15/*15).
• Diplotypes are haplotype pairs on homologous chromosomes.
• Haplotypes are physical groupings of genomic variants inherited together.

SLOC1B1 Transporter Function and SAMS Risk

• High, moderate, or low risk of SAMS based on diplotypes.
• Treatment varies depending on the patient's genotype.

SLOC1B1 Decreased Function: Statin Treatment

• High-intensity statins: 20mg Rosuvastatin (low SAMS risk), 40mg Atorvastatin or Rosuvastatin (moderate SAMS risk), 80mg Atorvastatin (high SAMS risk).
• Moderate-intensity statins: 10-20mg Atorvastatin, 5-10mg Rosuvastatin, 40mg Pravastatin, 1mg Pitavastatin (low SAMS risk); 80mg Pravastatin or Fluvastatin, 2mg Pitavastatin (moderate SAMS risk); 20-40mg Simvastatin, 40-80mg Lovastatin, 4mg Pitavastatin (high SAMS risk).
• Low-intensity statins: 20-40mg Fluvastatin, 10-20mg Pravastatin (low SAMS risk); 20mg Lovastatin, 10mg Simvastatin (moderate SAMS risk).

SLOC1B1 Poor Function: Statin Treatment

• High-intensity statins: 20mg Rosuvastatin (low SAMS risk), 40-80mg Atorvastatin, 40mg Rosuvastatin (moderate SAMS risk).
• Moderate-intensity statins: 10-20mg Atorvastatin, 5-10mg Rosuvastatin, 40mg Pravastatin, 1mg Pitavastatin (low SAMS risk); 80mg Fluvastatin or Pravastatin (moderate SAMS risk); 20-40mg Simvastatin, 40-80mg Lovastatin, 2-4mg Pitavastatin (high SAMS risk).
• Low-intensity statins: 10-20mg Pravastatin, 20-40mg Fluvastatin (low SAMS risk); 20mg Lovastatin, 10mg Simvastatin (high SAMS risk).

Mr. S: Homozygous for ABCG2 rs2231142 (T) Variant

• Prescribe 5-10mg rosuvastatin as a starting dose and adjust doses based on disease-specific and population-specific guidelines.
• If a dose >10mg is needed, consider an alternative statin or combination therapy (e.g., rosuvastatin and non-statin guideline).

Prevention and Targeted Information

• Mr. S and his son are screened for familial hypercholesterolemia (FH).
• Mr. S. is heterozygous for LDL receptor mutation.
• His 13-year-old son has a cholesterol level of 13 mmol/L.
• The son is referred to a specialist clinic and found to be homozygous for the LDL receptor mutation.

Familial Hypercholesterolemia

• Functional mutations in LDLR, APOB, or PCSK9 genes predispose to premature coronary artery disease and death.
• Early lipid-lowering treatment and monitoring in a specialist metabolic clinic is recommended.
• A statin and ezetimibe combination is considered for his son.

Mr. S: Atrial Fibrillation and Anticoagulation

• Mr. S recovers from his myocardial infarction and is discharged with ticagrelor and rosuvastatin.
• A year later, he develops atrial fibrillation.
• A decision is made to anticoagulate him to reduce embolic risk.
• Warfarin can be used, but pharmacogenetic considerations should be considered.

CYP2C9 Substrates

• CYP2C9 substrates include warfarin, fluvastatin, tolbutamide, and NSAIDs (ibuprofen, diclofenac) phenytoin.

CYP2C9 Variant Alleles

• The CYP2C9 variant alleles include CYP2C92 (Arg144Cys) and CYP2C93 (Ile359Leu).

Warfarin

• Warfarin is a widely used anticoagulant with a narrow therapeutic index.
• It carries a risk of bleeding and is a common cause of ADRs resulting in hospitalization.
• Clinical therapeutic dose of warfarin ranges by a factor of 40 (0.5 to 20 mg/day).
• This shows that the dose needs to be individualised
• 0.5-1% of the population in NE England are prescribed this drug.
• Warfarin is a mixture of R and S enantiomers.
• R-warfarin is hydroxylated by CYP3A4 & CYP1A2.
• S-warfarin is predominantly metabolized by CYP2C9.
• The S enantiomer is more biologically active and potent than R-warfarin.
• CYP2C9*2 and *3 homozygosity reduces enzyme activity to 5-12%.
• CYP2C9*3 increases the risk of warfarin-induced bleeding.

Aithal et al, 1999 Study

• CYP2C9 genotypes (homozygous wild-type, heterozygous, homozygous mutant) in healthy and random controls.
• 60-62% of healthy and random controls expressed homozygous wild-type CYP2C9.
• In patients requiring low-dose warfarin, only 20% expressed the wild-type allele.
• 35-36% of healthy volunteers are heterozygous for CYP2C9.
• 60% of patients taking low-dose Warfarin were found to express this at least one defect allele of CYP2C9.
• 5-6% of healthy volunteers expressed CYP2C9 homozygous mutant alleles.
• 20% of patients requiring low-dose warfarin expressed these mutant genotypes.

Warfarin Dose and CYP2C9 Genotype

• Warfarin dose requirement is lower in homozygous *2/*2 or *3/*3 genotypes of CYP2C9 (~3mg for *2 and ~1mg for *3).
• Heterozygous individuals (Wt/*2, Wt/*3, Wt/*11) can tolerate higher doses than mutant genotypes (~3mg , ~2mg , ~3mg).
• Wt/Wt can tolerate higher doses of warfarin (mean ~5mg).

Consequences of Warfarin Combination with CYP2C9

• Presence of one or more variant CYP2C9 alleles is associated with low (