The initial evidence supporting an association between the 2155T>C SNP and either CHD or statin response consists of genetic association studies conducted retrospectively as post hoc analyses in large clinical trials [35,36,37,38,39]. These studies have recently been viewed with some skepticism, in part since they were published by authors closely affiliated with the sole commercial distributor of a KIF6 test in the US, but more so due to the underlying methodology of a “candidate gene” approach instead of a more impartial “genome-wide association” study [40,41]. Furthermore, subsequent independent retrospective association studies failed to replicate the results found in the initial analyses [42,43,44]. Finally, two meta-analyses of 19 case-control studies of nonfatal CHD that did not assess the effect of statin treatment on disease progression failed to show the previously reported association between KIF6 and disease progression [30,45].

The studies evaluating the utility of KIF6 testing can roughly be broken into two categories: those assessing its utility as a predictive marker of a patient’s response to statin therapy, and those assessing its utility as a prognostic marker of disease progression.

With traditional adherence-promoting interventions, much of the benefit comes from improving a patient’s self-efficacy or belief that their actions will have meaningful impact on their outcomes. Logically, if KIF6 testing were able to identify those patients with the greatest benefit from statin therapy, it might also have utility as an adherence-promoting intervention via its ability to improve a patient’s sense of self-efficacy [32]. The potential utility of routine KIF6 testing to promote adherence was evaluated in the Additional KIF6 Risk Offers Better Adherence to Statins (AKROBATS) trial. AKROBATS was a non-randomized comparative effectiveness study where enrolled patients were offered KIF6 testing, and their subsequent adherence to treatment was compared with concurrent untested controls via a prescription database [27]. This hypothesis-generating study indicated that patients who were aware of their KIF6 status were approximately twice as likely to be adherent to therapy at 6 months based upon a proportion of days covered (PDC) value greater than 0.80 [46].

AKROBATS was limited by its non-randomized and relatively uncontrolled design. The portion of the improvement in adherence as due to the patient’s knowledge of their KIF6 carrier status was difficult to determine. In fact, it is likely that much of the improvement that was seen was due to the fact that in participating in the trial, patients were in conversation with a pharmacist about their coronary risk and the need for good adherence. In itself, such a conversation is an adherence-promoting intervention independent of any genomic risk information that was disclosed.

Although the evidence from the AKROBATS trial may not fully support the utility of KIF6 testing, the concept is compelling. The trial presents an important proof-of-principle that interventions aimed at genomic personalization of statin therapy have the potential to improve adherence, and thereby patient outcomes of patients on statin therapy. Other markers, such as SLCO1B1, are now emerging as alternative pharmacogenomic markers that have clear evidence and biological plausibility and may operate in robust ways.

Although statins have well established efficacy in lowering atherosclerotic cardiovascular event and death rates and are generally regarded as safe drugs, muscle pain and weakness (myalgia/myopathy) are common side effects in this class, occurring in up to 10% of patients [47]. The concern over statin-related myopathy (SRM) is exemplified by the drug cerivastatin, which the FDA removed from the market due to its high risk of rhabdomyolysis and subsequent deaths [4]. Recently, the concern over SRM motivated the FDA to place new warnings on simvastatin formulations due to the significant risk of both myopathy and rhabdomyolysis with high doses [5,48].

SRM is an extremely heterogeneous condition. Patients with SRM can present with muscle complaints ranging from weakness, aches and/or pain without elevated creatine kinase (CK) levels, (i.e., myalgia), to more significant discomfort with mild CK elevations and myositis, to life threatening rhabdomyolysis [47]. Myalgia is the least severe but most common presentation of muscle toxicity, and rhabdomyolysis with potential renal failure is the most severe but least common presentation occurring in only a small percentage of the patients who develop myopathy. Although the exact rate of myopathy is unclear, outcomes data suggest that it may be more common than originally thought, based on strictly-controlled pre-approval clinical trials. Studies of intensive statin therapy report that roughly 3% and 2% of patients will experience myalgia and myopathy, respectively [49]. In studies that incorporate patient reported outcomes, however, the prevalence of muscle related side effects are as high as 10%–25% [50,51].

Although SRM actually relatively infrequently leads to hospitalization or disability, its milder clinical effects are an important cause of statin intolerance and discontinuation [52,53]. Previous reports have repeatedly suggested that side effects or the perception of side effects may be among the most significant obstacles to optimal adherence on statin treatment, and a major source of random switching as well [54,55,56]. Thus, while myopathy may be dismissed by some prescribers as minor aches and pains, it may have a more profound consequence in terms of treatment discontinuation. The results from the USAGE study (Understanding Statin Use in America and Gaps in Patient Education) which was an internet-based survey of more than 10,000 statin users highlight this issue. Twenty-nine percent of all participants had experienced muscle-related side effects, and of those who had discontinued their medication due to a side effect, approximately one third did so without speaking with their physician [50]. In other words, some patients who would likely benefit from statins discontinued therapy outside of the healthcare system. The gravity of this problem may be invisible to many health care providers.

For statins to perform their function they must first reach the liver. The uptake of statins from portal blood into hepatocytes across the phospholipid bilayer occurs primarily through the organic anion-transporting polypeptide 1B1 (OATP1B1) influx transporter which is expressed on the basolateral membrane of human hepatocytes (Figure 2). OATP1B1 transport appears to be rate limiting for hepatocyte uptake and hence distribution and metabolism of many statins. Consequently, modifications in this transporter have been significantly associated with the risk of SRM [26,58,59]. OATP1B1 is encoded by the gene SLCO1B1 whose *5 allele (Val174Ala, 521T>C) has been shown to interfere with localization of the transporter to the plasma membrane, leading to decreased liver uptake and greater systemic statin concentrations and hence greater muscle statin exposure [60].

The genotypic frequencies for the variants of SLCO1B1 vary by ethnicity and some reduced function alleles are relatively common, such as the presence of *5 in 8%–20% of Caucasians. The best-documented haplotypes thought to play an important role in modulating the risk of SRM are shown in Table 3. Of note, the *15 haplotype, with an allele frequency of 10% in Japanese, carries the same 521T>C substitution as *5 in combination with the 388A>G SNP and represents another risk allele for myopathy in patients receiving statin therapy.

Reduced function mutations in SLCO1B1 limit the transport of these molecules by OATP1B1 into liver cells, which leaves an increased concentration of the statin in the bloodstream. Elevated plasma concentrations of statins increase the risk of adverse drug reactions, of which SRM is one of the most common. This effect has been best documented for simvastatin where the Area Under the Concentration-response Curve [68] (AUC_0-∞ (ng∙hr/mL)) is greater than three times higher in those patients homozygous for the *5 variant than wild types after a single 40-mg dose [57]. By contrast, fluvastatin has been shown to have pharmacokinetic properties that are independent of genetic variation at the SLCO1B1 locus, and notably has correspondingly lower rates of reported SRM [69].

Although most statins are substrates of the transporter OATP1B1 [69], the effects of SLCO1B1 polymorphism still vary based on the pharmacologic profile of the specific statin. Statins each possess unique absorption, distribution, metabolism, and excretion properties that affect kinetics and treatment response (Table 4).

If toxicity is correlated with muscle exposure to drug levels, then logically, increased drug concentrations in the blood should be reflected in an altered side effect profile in patients with reduced transport. Several safety studies have evaluated how the rates of adverse drug reactions, most commonly myopathy, vary by genotype. To date, five studies have evaluated the risk of SRM as a function of genetic variation in SLCO1B1 (Table 5).

The myopathy risk associated with SLCO1B1 was first reported in a study reported by the Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine (SEARCH) Collaborative Group [59]. The authors studied two cohorts of clinically severe cases and controls from large trials involving approximately 12,000 and 20,000 participants who were treated with 80 mg and 40 mg of simvastatin per day, respectively. The investigators observed a significant association between SRM and a single marker in the SLCO1B1 gene (rs4363657, p = 3 × 10⁻²⁸). This association was then confirmed in a second cohort which included patients who were randomly assigned to 40 mg of simvastatin per day (see Table 6).

The results from the retrospective genetic association study in SEARCH and HPS were subsequently validated in the prospective randomized STRENGTH (Statin Response Examined by Genetic Haplotype Markers) study [58]. In STRENGTH, subjects (n = 509) were randomized to ascending doses of atorvastatin 10→80 mg, simvastatin 20→80 mg, or pravastatin 10→40 mg. A composite adverse event (CAE) was defined as discontinuation for any side effect, myalgia, or CK>3 times upper limit of normal (ULN) during follow-up. Of the five candidate genes evaluated, including CYP2D6, CYP2C8, CYP2C9, CYP3A4, and SLCO1B1, only SLCO1B1*5 was associated with CAE (37% vs. 25% in carriers and wild type patients respectively, p = 0.03) and more significantly for those with CAE exclusive of significant CK elevation (p ≤ 0.03). Furthermore, a gene-dosage effect was observed (percent with CAE in those with 0, 1, or 2 of the variant (*5) alleles: 19%, 27%, and 50%, p = 0.01 for the trend). Importantly, only allele carriers receiving ascending doses of simvastatin showed significantly heightened risk of CAE compared to patients who carried no alleles (16% vs. 34%, p = 0.01). This is in contrast to patients receiving atorvastatin and pravastatin who showed non-significant changes in CAE risk based on allele carriage (19% vs. 27%, p = 0.3 and 22% vs. 22%, p = 0.97 for atorvastatin and pravastatin respectively).Since carriers of 521T>C mutations experienced higher rates of myalgia, a significant obstacle to optimal adherence, these same patients should have prescribing patterns reflective of intolerance such as switching to a different statin at a lower or equivalent dose, reducing the dose of the same statin, or discontinuation of statin therapy. This hypothesis was the aim of the GO-DARTS (Genetics of Diabetes Audit and Research) study which examined whether SLCO1B1 variants were associated with general statin intolerance in a large population of patients with type 2 diabetes receiving statins as part of routine clinical care. This observational incident cohort analysis used information from 4,196 genotyped patients in the GO-DARTS database, which is part of an ongoing research initiative in the Tayside, Scotland (population 400,000) community to track the treatment and health outcomes of individuals with diabetes [26]. Information captured in this database included detailed clinical information for individuals with diabetes from 1990 to the present including all pharmacy records, lab test results, and other clinical data related to diabetes care. This study particularly focused on mild manifestations of myopathy, and patients with CK > 3×ULN^iv [71] were excluded from analysis. For purposes of this study, intolerance was defined as a composite measure of abnormal lab values, alanine transaminase (ALT) and CK, and relevant adjustments to the prescription of each patient.

This study confirmed the association of the *5 allele with statin intolerance (OR = 2.05, 95% CI: 1.02–4.09, p = 0.04), and further showed that *5 allele carriers have a doubled risk for intolerance to their originally prescribed statin. These results were observed in a population where moderate and severe cases of myopathy were excluded, therefore representing better the sub-pathological end of the spectrum of statin related muscle effects likely to be the more significant driver of correlated non-adherence in terms of numbers. This study suggests that the muscle toxicity associated with SLCO1B1 is represented in prescribing patterns suggestive of intolerance, and may ultimately prove to be useful as a prospective intervention.

Although the majority of evidence for SLCO1B1-related SRM has been around simvastatin, cerivastatin, a drug that was recalled due to its risk of rhabdomyolysis [4], has also recently been shown to be effected by this locus. In an analysis by Marciante et al., a candidate gene study (examining CYP2C8, UGT1A1, UGT1A3, and SLCO1B1) and a GWAS study were performed on 185 cerivastatin-induced rhabdomyolysis cases matched to statin-using controls from Cardiovascular Health Study (n = 374) and Vascular Health Study (n = 358) [70], A subsequent in vitro functional analysis for 521T>C was also performed in stable HEK293 cells. Permutation test results showed an association between cerivastatin-induced rhabdomyolysis and the *5 allele (OR = 1.89, p = 0.002). In functional studies, this variant reduced transport by 40% compared with the reference transporter (p < 0.001). This study extends the results of simvastatin-centered trials to cerivastatin and functional studies provide a potential causal association.

Clinical evidence shows a strong association between carriage of alleles of SLCO1B1 and both mild myalgia and clinically severe myopathy [58,59]. Furthermore, SLCO1B1 induced muscle toxicity has also been associated with lower levels of drug tolerance [26]. Since there is a gradient of effect for variations in this transporter across the statin class [72], it may be possible to personalize statin treatment for the patient’s effectiveness goals as well as their predisposition to myopathy according to SLCO1B1 genotype. In fact various groups, including the Clinical Pharmacogenomics Implementation Consortium, have drafted specific treatment recommendations that can provide clinicians with a practical starting point for how to implement a patient’s 521T>C status into their treatment [72,73].

Although the evidence from the AKROBATS trial may not fully support the utility of genomic testing in itself in improving patient adherence [27], the concept is compelling and the use of SLCO1B1 may go one step further by not only affecting a patient’s sense of self-efficacy [32] but also reducing the probability of myopathy, an independent barrier to optimal adherence. Most importantly, decisions based on this intervention could lead to less atherosclerosis and cardiovascular events. Currently there is no direct clinical evidence that the personalized prescribing of statins using a patient’s SLCO1B1*5 status will improve their medication adherence; however, previous analyses suggest that this is a logical conclusion and will be an important hypothesis to evaluate in future analyses.