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The Experts for Comprehensive, Convenient Personalized Health!
Summary: With clopidogrel therapy we are interested in the CYP2C19 gene, which encodes the enzyme responsible for the conversion of clopidogrel (prodrug) to its biologically active metabolite. Loss of function variants for the CYP2C19 gene have been shown to impair the formation of active metabolites, resulting in reduced platelet inhibition and increased risks for serious adverse cardiovascular (CV) events among clopidogrel treated patients with acute coronary syndromes (ACSs) undergoing percutaneous coronary intervention (PCI).
CYP2C19
First characterized in 1991, the CYP2C19 gene is one of four CYP2C subfamily genes located on chromosome 10q23.33 (Botton, et al. 2021). CYP2C19 is highly polymorphic with over 35 different star allele haplotypes and 2000 SNPs that have been identified (Ahmed, et al. 2016). Among these, the most common and well-studied allelic variants are CYP2C19*2 and *3 alleles, both of which are categorized as null or no function variants (Lee, et al. 2022). The CYP2C19*2 variant allele is characterized by a splice mutation in exon 5, whereas CYP2C19*3 is marked by a stop codon in exon 4 (Ferguson, 1998; Ibeanu, 1999). In contrast, the variant allele CYP2C19*17 is categorized as an increased function variant and is specified by a double mutation in the promoter region, which leads to increased gene transcription and elevated enzyme activity (Payan, et al. 2015). The CYP2C19 enzyme encoded by CYP2C19is responsible for metabolizing various commonly administered drugs, including proton pump inhibitors (eg, omeprazole, lansoprazole, pantoprazole), antidepressants (eg, sertraline, citalopram, amitriptyline), anticonvulsants (mephenytoin), anxiolytics (diazepam), antiplatelet medications (clopidogrel), and others (Lee, et al. 2013).
CYP2C19 and Clopidogrel
CPIC has published CYP2C19 pharmacogenomic-guided dosing guidelines for several different medications, including clopidogrel, proton pump inhibitors, voriconazole, and certain antidepressant drugs (Lee, 2022; Lima, 2021; Moriyama, 2017; Hicks, 2015; Hicks, 2017). With regard to clopidogrel, the most definitive published studies examining the relationship between CYP2C19 genotype and drug response have primarily been conducted in patients with acute coronary syndrome who underwent percutaneous coronary intervention (PCI). Evidence published in large meta-analyses demonstrate a greater risk for major adverse cardiovascular events and stent thrombosis among clopidogrel-treated patients undergoing PCI with CYP2C19 IM phenotype compared to those with CYP2C19 NM phenotype (Xi, 2019 and Lee, 2022). However, there is growing evidence suggesting a potential genotype-drug-response relationship in scenarios when clopidogrel is used for other indications, such as acute ischemic stroke or transient ischemic attack (Lee, 2022; Pan, 2017). Alternative treatment options, including prasugrel and ticagrelor are recommended in patients identified as PM phenotypes.
Summary: There are three pharmacogenes of interest in this patient to help guide warfarin therapy. The first is CYP2C9 which encodes the enzyme responsible for the metabolism of the more potent S enantiomer to its inactive metabolites. The second is VKORC1 which encodes for the target protein of warfarin responsible for the rate limiting step in vitamin K recycling, which is necessary for the activation of vitamin K dependent clotting factors . The last is CYP4F2 which encodes for the enzyme responsible for the metabolism of vitamin K.
CYP2C9
The CYP2C9 enzyme composes roughly 20% of hepatic CYP450 content and accounts for the highest level of enzyme expression among the CYP2C family (Ingelman-Sundberg, et al. 2007). Approximately 10% of drugs are metabolized by CYP2C9, including a number of clinically important drugs, such as the antidiabetic drugs tolbutamide and glipizide, the anticonvulsants phenytoin and fosphenytoin, warfarin, losartan, torsemide, and a number of non-steroidal anti-inflammatory drugs (NSAIDs), including ibuprofen, celecoxib, diclofenac, meloxicam, and piroxicam (Lee, 2013; Sangkuhl, 2021). The CYP2C9 gene is known to be highly polymorphic, with over 61 identified variant allele and numerous suballeles (Theken, et al. 2020).
CYP2C9 and Warfarin
Polymorphisms associated with CYP2C9 are shown to impact the metabolism of several commonly prescribed drugs that can result in an increased risk for toxicity and may often necessitate dosage adjustments or treatment discontinuation. Notably, warfarin is the most commonly prescribed anticoagulant medication in the western world and is often used in the treatment and prevention of thromboembolic disorders (Johnson, et al. 2017). Warfarin is comprised of roughly equal amounts of two different enantiomers: S-warfarin and R-warfarin. Metabolism of S-warfarin predominantly occurs via CYP2C9, whereas R-warfarin is metabolized by the CYP1A1, CYP2C19, and CYP3A4 enzymes (Kaminsky, et al. 1997). Both in vitro and in vivo studies indicate that CYP2C9*2 and *3 alleles may reduce the metabolism of S-warfarin by approximately 30% to 40% and 80% to 90%, respectively (Lee, et al. 2002). Studies assessing the clinical effects related to these CYP2C9 polymorphisms further demonstrate that individuals harboring one or more CYP2C9*2 or *3 alleles have a greater risk of bleeding when treated with warfarin and may require lower dosages or greater time to achieve a stable INR and the desired anticoagulant effect (Mega, 2015; Aithal, 1999). Other alleles associated with decreased enzyme function, such as CYP2C9*5, *6, *8, and *11, may also contribute to variability in dosing and clinical effect associated with warfarin therapy (Johnson, et al. 2017). CYP2C9polymorphisms account for approximately 10% to 18% of the observed variance in the required therapeutic warfarin dose (Wadelius, 2009; Johnson, 2017).
CYP2C Cluster (rs12777823) and Warfarin
While CYP2C9 and VKORC1variants are shown to explain up to 30% of the total variability in warfarin dose requirements in populations of European or Asian ancestry, polymorphisms associated with these genes explain substantially less variability for individuals of African descent (Perera, et al. 2013). A genome wide association study conducted among individuals of self-reported African ancestry identified the SNP rs12777823 to be associated with a warfarin dose reduction of 7 mg/week in heterozygotes (A/G genotype) and 9 mg/week in homozygotes (A/A genotype). The SNP rs12777823 is located within the CYP2C gene cluster on chromosome 10q23 which also included the CYP2C9, CYP2C8, CYP2C18, and CYP2C19genes. rs12777823 allele variant occurs at a frequency of 25% in African American populations and is estimated to account for roughly 5% of the variability in warfarin dose, exceeding the variability explained by CYP2C9*2and CYP2C9*3 (1-2%).
CYP4F2
CYP4F2is located on chromosome 19p13 and encodes for the CYP4F2 enzyme, a vitamin K epoxidase that catalyzes the metabolism of vitamin K to hydroxy-vitamin K1, effectively removing vitamin K from the vitamin K cycle (Alvarellos, 2016; McDonald, 2009). CYP4F2 is thought to be an important counterpart to VKORC1 to limit excess accumulation of vitamin K. Additionally, CYP4F2 plays a role in oxidation of long chain as well as side chains associated with vitamin E, arachidonic acid, and leukotrienes. Although several different CYP4F2 polymorphisms have been identified, the *3 (rs2108622) allele is the most notable as it relates to warfarin metabolism (Alvarellos, et al. 2016). CYP4A2*3 is a single nucleotide polymorphism in which cytosine is replaced with thymine, resulting in an amino acid change from valine to methionine at position 433. Individuals harboring CYP4F2*3 have a reduced capacity to metabolize vitamin K, resulting in elevated hepatic levels of vitamin K and will therefore likely require higher doses of warfarin to achieve the same anticoagulant response (McDonald, et al, 2009). Study data demonstrate that each CYP4A2*3 allele is associated with a 4-12% increase in the required warfarin dose (Caldwell, et al. 2008).
VKORC1
Located on chromosome 16p11.2, VKORC1 encodes vitamin K epoxide reductase (VKORC1) which catalyzes the conversion of vitamin K epoxide to vitamin K, the rate limiting step in vitamin K recycling (VKORC1_OMIM, 2022; Owen, 2010). Because vitamin K is a required cofactor for several anticoagulation factor proteins, including factor VII, factor IX, and factor X, VKORC1 polymorphisms are of clinical significance (Stafford, et al. 2005). The anticoagulant drug warfarin acts by inhibiting VKORC1, resulting in a reduced amount of vitamin K and thereby limiting production of vitamin K-dependent clotting factors (Rost, 2004; Owen, 2010). Studies have shown VKORC1 polymorphisms to be the biggest predictor or warfarin dose, accounting for approximately 25% of the variance in stabilized warfarin dose (Yin, 2006; Wen, 2008; Cooper, 2008; Wadelius, 2009).
VKORC1, CYP4F2 and Warfarin
Among European populations VKORC1 variants account for the largest variance (11-32%) in warfarin dosage requirements, followed by CYP2C9 (up to 18%) and CYP4F2(up to 11%) (Limdi, 2010; Liang, 2012; Wadelius, 2009; Johnson, 2017). In contrast, VKORC1 polymorphisms are shown to account for only 4% 10% of the variability in warfarin dose among the African American population (Limdi, et al. 2010). Several genetic-based algorithms, including the Gage and International Warfarin Pharmacogenomics Consortium (IWPC) have been commonly used for warfarin dosing in clinical trials (Johnson, et al. 2017). Published CPIC recommendations vary based on ancestry and available pharmacogenetic information. For persons of non-African ancestry, guidelines recommend warfarin dosing to be calculated using genotype information for VKORC1 and CYP2C9*2and *3. For those with African ancestry, genotype-based warfarin dosing is only recommended in cases where CYP2C9*2, *6, *8, and *11 are available; however, for scenarios where this information is unavailable, clinical dosing of warfarin is recommended. Considerations under genotype-based warfarin dosing recommendations for those of African ancestry include: VKORC1and CYP2C9*2 and *3 status; CYP2C9*5, *6, *8 and *11status (15-30% dose reduction for heterozygotes and 20-40% for homozygotes); and rs12777823 status (10-25% dose reduction for A/G or A/A genotypes).
Summary: The SLCO1B1 gene encode the OATP1B1 transporter responsible for the transport of most statins from the portal circulation into hepatocytes for clearance (with the exception of fluvastatin). Variants of the SLCO1B1 gene result in decreased function of the OATP1B1 transporter protein resulting in reduced hepatic clearance of the statin and an increased risk of myopathy. The statin drug most affected by SLCO1B1 variants is simvastatin, whereas pravastatin, rosuvastatin, and fluvastatin are least affected.
SLCO1B1
The SLCO1B1 transporter protein is encoded by SLCO1B1 (located on chromosome 12p12.1) and is primarily expressed in tissues of the liver within the basolateral membrane of hepatocytes (SLCO1B1, 2022; Niemi, 2011). OATP1B1 is thought to play a critical role in the hepatic uptake and clearance of albumin-bound compounds via electroneutral, ATP-independent transport. OATP1B1 facilitates the transport of statin drugs as well as a number of other exogenous and endogenous compounds (eg., unconjugated and conjugated bilirubin, thyroid hormones, various eicosanoids, estradiol-17β-D-glucuronide, see Table 11) (Cooper-DeHoff, 2022; Niemi, 2011. More than 40 nonsynonymous SLCO1B1variants have been identified; however, only a few are known to have a clinically relevant functional impact (Zair, et al. 2008). The c.521T>C variant (rs4149056), contained in exon 5 of the SLCO1B1 transcript, produces a p.V174A substitution that results in a decreased expression of OAT1B1 and decreased transport of drug substrates including rifampin, statins. (Niemi, 2006; Pasanen, 2006; Pasanen, 2007). The common variant, c.388A>G contained in the exon 4 transcript region, is also associated with altered SLCO1B1 transporter activity. Notably, the c.521T>C and c.388A>G variants are both contained in the *5 and *15 haplotypes and are shown to be consistently associated with decreased transporter activity.
SLCO1B1 star allele are categorized according to functionality: normal function (eg., *1, *37); increased function (eg., *14); no function (eg., *5, *9, *15, *23, *31); and uncertain function (*6, *7, *8, *10) (Cooper-DeHoff, et al. 2022; suppl SLCO1B1 allele definitions). Predicted phenotypes are based on the given diplotype and defined accordingly. Individuals expressing heterozygosity for no function alleles (eg., *1/*5) are characterized as a decreased function phenotype, whereas those homozygous for no function alleles are defined as a poor function phenotype.
SLCO1B1 and Statins
Study data demonstrate reduced hepatic uptake of pravastatin among individuals harboring the*15 allele and increased area under the plasma concentration-time curve (AUC) of pravastatin in carriers of *5 or *15 (Nishizato, et al. 2003). Subsequent studies investigating the effects of SLCO1B1 variants on other statin drugs have demonstrated similar results and shown this impact to vary among these agents (Pasenen, 2006; Pasenen, 2007; Niemi, 2006). Notably, the greatest effect of SLCO1B1 genotype is observed with simvastatin (up to a 3.2-fold increase in AUC), followed by atorvastatin; a smaller effect was observed with pravastatin and rosuvastatin, and no significant effect was seen with fluvastatin. Based upon these data and the concentration-dependent muscle toxicity associated with statin drugs, the predicted risk for statin-induced myopathy is highest for simvastatin, followed by atorvastatin, pravastatin, and rosuvastatin.
Symptoms of statin-induced myopathy can range from mild myalgia (5-10% of statin users/year) associated with symptoms, such as fatigue, muscle pain, tenderness, weakness and cramping, to life-threatening rhabdomyolysis (0.001-0.005%) (Niemi, 2011; Staffa, 2022; Graham, 2004). Other factors that can increase the risk of statin-induced myopathy and rhabdomyolysis include a high statin dose, drug-drug interactions, very high age, comorbidities, hypothyroidism, and certain muscle disorders (Thompson, 2003; Neuvonen, 2006; Ghatak, 2010). Because SLCO1B1*5 and *15 variants can lead to a reduced uptake and increased concentrations of simvastatin, it is recommended that high dose simvastatin be avoided in carriers of these variant alleles (Cooper-DeHoff, 2022; Niemi, 2011). Due to the reduced uptake of statins resulting from SLCO1B1*5 and *15 variants, it is also hypothesized that carrier may also have an attenuated cholesterol-lowering effect from statin therapy.