Drug Response Pharmacogenetics for 200,000 UK Biobank Participants
Introduction Pharmacogenetics promises to revolutionize patient care by offering personalized drug selection and dosage based on an individual’s genetics . Variations in the genes that encode proteins involved in drug pharmacokinetics and pharmacodynamics are known to lead to interindividual heterogeneity in drug response and can greatly affect clinical outcome. Dosage guidelines have been developed by organizations such as the Clinical Implementation of Pharmacogenetic Consortium (CPIC; cpicpgx.org ) to aid physicians in incorporating pharmacogenetics into their practice, however the adoption of pharmacogenetics by practicing physicians has not lived up to the optimism in the field , . Doctor’s may not directly be using pharmacogenetics to inform practice, but genetics influences how patients respond to drugs nonetheless. Some drugs, such as warfarin, have a narrow therapeutic index and blood concentration of the drug must be frequently measured to ensure patient safety . The ultimate dose at which the patient achieves the appropriate, stable blood concentration of the drug is the maintenance dose. For warfarin, this dose is strongly influenced by genetic factors such including variations in the metabolizing enzymes CYP2C9 and CYP4F2, as well as the drug target VK0RC1. In other instances genetic variation may lead patients to be at higher risk for side effects. The frequently prescribed drug simvastatin has well known pharmacogenetic interactions with SLCO1B1 that can lead to simvastatin-induced myopathy . While this is a rare side effect, individuals with poor functioning SLCO1B1 are at higher risk for simvastatin-induced myopathy. CPIC guidelines for simvastatin recommend that individuals with poor functioning SLCO1B1 take a reduced simvastatin dose or a different drug altogether. Numerous pharmacogenetic drug-gene relationships have been discovered, but most pharmacogenetic studies are small and narrowly focused. The use of electronic health record and biobank scale data as a means for pharmacogenetic discovery and validation of known relationships has been proposed, but until recently databases linking clinical data with genetic data for a large number of patients were unavailable , . Biobanks offer an opportunity to retrospectively assess known drug-gene relationships in a clinical setting as well as offer the opportunity to discover new drug-gene associations. Biobanks and electronic health records have been used to perform targeted association studies between genomics and response to individual drugs as well as characterize frequency of pharmacogenetic alleles in populations , , but studies of drug response across a large number of drugs have not yet been performed. The UK Biobank has been widely used to perform genome-wide association studies on a wide variety of traits, but it also includes primary care data from the United Kingdom’s National Health System . This dataset offers longitudinal, structured clinical data for more than 220,000 participants that includes diagnoses, laboratory tests, and prescription data. This dataset offers a unique opportunity to identify associations between drug response phenotypes and genetics. Here we present a retrospective pharmacogenetic analysis linking drug exposure for 200 drugs to clinical outcome using the UK Biobank primary care data. We focus on two types of clinical outcomes of interest: maintenance dose and differential drug response. 

Methods 2.1. Pharmacogenetic Allele Calling We investigated drug-gene relationships for nine important pharmacogenes in the UK Biobank for 222,114 participants using primary care data from the National Health System, provided by the UK Biobank . The pharmacogenetic alleles used in this study were derived from a previously reported procedure, described here in brief . We used imputed genotypes from the Axiom Biobank Array released by the UK Biobank . We included nine genes in our analysis: CYP2B6, CYP2C19, CYP2C9, CYP2D6, CYP3A5, CYP4F2, SLCO1B1, TPMT , and UGT1A1 . The proteins encoded by these genes play critical roles in drug pharmacokinetics and each is included in a CPIC dosing guideline for a drug. We assigned pharmacogenetic phenotypes for each gene using PGxPOP, a tool designed for high throughput mapping of pharmacogenetic alleles and phenotypes ( https://github.com/PharmGKB/PGxPOP ). The analysis was limited to individuals of European descent. This included participants who self reported as European and were confirmed as European using principal component analysis. 2.2. Drug Dosage Association with Pharmacogenetics Drugs used in this study were derived from the PharmGKB curated drug list ( https://www.pharmgkb.org/downloads , drugs.zip) . For each drug, we extracted prescription information from the UK Biobank primary care prescription data by matching the drug name and brand names in the prescription data. Dosage information and drug quantity was extracted using regular expressions that searched within the drug description. We excluded combination therapies from the analysis. We calculated maintenance dose by determining the average milligrams of drug per day for the last five prescriptions of each drug. This was done by calculating the total milligrams of drug administered for a single prescription divided by the number of days until the next prescription. We then averaged the milligrams of drug per day over the five most recent prescriptions. Prescriptions with a quantity outside two standard deviations from the mean quantity across all participants for that drug were excluded. Subjects were required to receive a minimum of five prescriptions to be included in the analysis. We required drugs to have a minimum of 50 subjects with a maintenance dose to be included in the analysis. We divided the analysis of maintenance dose associations into three groups of drug-gene pairs. First, we investigated the relationship between drug-gene pairs that have an existing CPIC guideline. This indicates a strong level of evidence of a relationship between a drug and a gene. Second, we investigated drug-gene pairs which have some level of evidence in PharmGKB, but no existing CPIC guideline. These pairs still have some prior evidence indicating an association, but not enough to develop a dosage guideline. Third, we investigated all other drug-gene pairs where an interaction is indicated in DrugBank . These pairs have no prior evidence of a pharmacogenetic association. Data was grouped within each gene by predicted phenotype. For example, for CYP2C9 participants were put into bins by metabolizer class (normal metabolizers (NM), intermediate metabolizers (IM), and poor metabolizers (PM). Phenotype groups with less than ten participants for a drug are excluded from analysis. Association between maintenance dose and pharmacogenetic phenotypes was tested for 200 drugs using two types of non-parametric statistical association tests. We used both a Kruskal-Wallis one-way analysis of variance and Jonckheere-Terpstra trend tests to test for associations between each drug and gene pair. Both types of tests are necessary to detect various relationships between dosage and genetics. First, the Kruskal-Wallis test was used to identify any pharmacogenetic phenotype (e.g. CYP2C9 PMs) that have a significant difference in the dosage from other metabolizer classes. Second, Jonckheere-Terpstra tests for an ordered relationship in ranked groups. This is a natural fit for pharmacogenetic phenotypes since there is an inherent order in function which may lead to a linear relationship with dosage (e.g. NM > IM > PM). Resulting p-values are adjusted using a Bonferroni correction. We used a covariate-adjusted dose as the response variable for each test. To do this we fit a linear regression model to the dosage using several covariates: age (at time of last prescription), sex, BMI, genotyping array, and the first for principal components of a principal component analysis (PCA) using genotype data (UK Biobank Data-Field 22009). We tested the impact of the intronic CYP2C19 variant rs3814637 on warfarin dose. We used a two-sided Jonckheere-Terpstra test on the allele dosage against the warfarin maintenance dose. Allele dosage was determined as the sum of the alternate alleles for rs3814637. 2.3. Differential Drug Response Phenotype Association In a separate analysis, we tested the relationship between pharmacogenes and drug response for all drugs using diagnosis codes in primary care data. We sought to identify pharmacogenomic phenotypes that would lead to a differential drug response phenotype, for example, instances where poor metabolizers have an increased risk of developing some side effect compared to normal metabolizers. For each drug included in the dosage analysis we identified all diagnoses in the primary care data in the year following the first exposure to the drug. Diagnosis codes in the primary care data are provided as Read Codes (version 2 and version 3), we mapped the Read Codes to ICD-10 codes including only the first three digits (the chapter and first two numerals). ICD-10 codes from chapters V, W, X, Y, and Z were excluded from analysis. Codes were required to have at least 100 events per drug to be included in the analysis. Diagnosis codes may represent the primary disease indication for the drug, side effects, comorbidities, or other unrelated events. We used logistic regression to test the association between gene phenotypes and ICD-10 code incidence for each drug. This was set up using a binary indicator as the response variable and a one-hot encoding of gene phenotype. We included age (at time of first prescription), sex, genotyping array, and the first four principal components from a genotype PCA as covariates. We evaluated three tiers of drug-gene relationship, as in the maintenance dose analysis. Drug-gene pairs with CPIC guidelines, drug-gene pairs with any level of evidence in PharmGKB but no CPIC guideline, and an exploratory analysis. For the exploratory analysis of side effect relationships we limited our search to drugs known to interact with CYP2C9 , CYP2C19 , and CYP2D6 , as indicated by DrugBank. These genes were selected because they are promiscuous metabolizing enzymes with well defined pharmacogenetics. 

Pharmacogenetic Allele Calling We investigated drug-gene relationships for nine important pharmacogenes in the UK Biobank for 222,114 participants using primary care data from the National Health System, provided by the UK Biobank . The pharmacogenetic alleles used in this study were derived from a previously reported procedure, described here in brief . We used imputed genotypes from the Axiom Biobank Array released by the UK Biobank . We included nine genes in our analysis: CYP2B6, CYP2C19, CYP2C9, CYP2D6, CYP3A5, CYP4F2, SLCO1B1, TPMT , and UGT1A1 . The proteins encoded by these genes play critical roles in drug pharmacokinetics and each is included in a CPIC dosing guideline for a drug. We assigned pharmacogenetic phenotypes for each gene using PGxPOP, a tool designed for high throughput mapping of pharmacogenetic alleles and phenotypes ( https://github.com/PharmGKB/PGxPOP ). The analysis was limited to individuals of European descent. This included participants who self reported as European and were confirmed as European using principal component analysis. 

Drug Dosage Association with Pharmacogenetics Drugs used in this study were derived from the PharmGKB curated drug list ( https://www.pharmgkb.org/downloads , drugs.zip) . For each drug, we extracted prescription information from the UK Biobank primary care prescription data by matching the drug name and brand names in the prescription data. Dosage information and drug quantity was extracted using regular expressions that searched within the drug description. We excluded combination therapies from the analysis. We calculated maintenance dose by determining the average milligrams of drug per day for the last five prescriptions of each drug. This was done by calculating the total milligrams of drug administered for a single prescription divided by the number of days until the next prescription. We then averaged the milligrams of drug per day over the five most recent prescriptions. Prescriptions with a quantity outside two standard deviations from the mean quantity across all participants for that drug were excluded. Subjects were required to receive a minimum of five prescriptions to be included in the analysis. We required drugs to have a minimum of 50 subjects with a maintenance dose to be included in the analysis. We divided the analysis of maintenance dose associations into three groups of drug-gene pairs. First, we investigated the relationship between drug-gene pairs that have an existing CPIC guideline. This indicates a strong level of evidence of a relationship between a drug and a gene. Second, we investigated drug-gene pairs which have some level of evidence in PharmGKB, but no existing CPIC guideline. These pairs still have some prior evidence indicating an association, but not enough to develop a dosage guideline. Third, we investigated all other drug-gene pairs where an interaction is indicated in DrugBank . These pairs have no prior evidence of a pharmacogenetic association. Data was grouped within each gene by predicted phenotype. For example, for CYP2C9 participants were put into bins by metabolizer class (normal metabolizers (NM), intermediate metabolizers (IM), and poor metabolizers (PM). Phenotype groups with less than ten participants for a drug are excluded from analysis. Association between maintenance dose and pharmacogenetic phenotypes was tested for 200 drugs using two types of non-parametric statistical association tests. We used both a Kruskal-Wallis one-way analysis of variance and Jonckheere-Terpstra trend tests to test for associations between each drug and gene pair. Both types of tests are necessary to detect various relationships between dosage and genetics. First, the Kruskal-Wallis test was used to identify any pharmacogenetic phenotype (e.g. CYP2C9 PMs) that have a significant difference in the dosage from other metabolizer classes. Second, Jonckheere-Terpstra tests for an ordered relationship in ranked groups. This is a natural fit for pharmacogenetic phenotypes since there is an inherent order in function which may lead to a linear relationship with dosage (e.g. NM > IM > PM). Resulting p-values are adjusted using a Bonferroni correction. We used a covariate-adjusted dose as the response variable for each test. To do this we fit a linear regression model to the dosage using several covariates: age (at time of last prescription), sex, BMI, genotyping array, and the first for principal components of a principal component analysis (PCA) using genotype data (UK Biobank Data-Field 22009). We tested the impact of the intronic CYP2C19 variant rs3814637 on warfarin dose. We used a two-sided Jonckheere-Terpstra test on the allele dosage against the warfarin maintenance dose. Allele dosage was determined as the sum of the alternate alleles for rs3814637. 

Differential Drug Response Phenotype Association In a separate analysis, we tested the relationship between pharmacogenes and drug response for all drugs using diagnosis codes in primary care data. We sought to identify pharmacogenomic phenotypes that would lead to a differential drug response phenotype, for example, instances where poor metabolizers have an increased risk of developing some side effect compared to normal metabolizers. For each drug included in the dosage analysis we identified all diagnoses in the primary care data in the year following the first exposure to the drug. Diagnosis codes in the primary care data are provided as Read Codes (version 2 and version 3), we mapped the Read Codes to ICD-10 codes including only the first three digits (the chapter and first two numerals). ICD-10 codes from chapters V, W, X, Y, and Z were excluded from analysis. Codes were required to have at least 100 events per drug to be included in the analysis. Diagnosis codes may represent the primary disease indication for the drug, side effects, comorbidities, or other unrelated events. We used logistic regression to test the association between gene phenotypes and ICD-10 code incidence for each drug. This was set up using a binary indicator as the response variable and a one-hot encoding of gene phenotype. We included age (at time of first prescription), sex, genotyping array, and the first four principal components from a genotype PCA as covariates. We evaluated three tiers of drug-gene relationship, as in the maintenance dose analysis. Drug-gene pairs with CPIC guidelines, drug-gene pairs with any level of evidence in PharmGKB but no CPIC guideline, and an exploratory analysis. For the exploratory analysis of side effect relationships we limited our search to drugs known to interact with CYP2C9 , CYP2C19 , and CYP2D6 , as indicated by DrugBank. These genes were selected because they are promiscuous metabolizing enzymes with well defined pharmacogenetics. 

Results The pharmacogenetic analyses presented here included a total of 201,498 participants, after removing 20,615 participants not of European descent. More than 57 million prescriptions are contained within the primary care data, an average of 262 prescriptions per participant. Our initial drug list included 3,358 drugs. Of this, 200 were found in the UK Biobank prescription data with sufficient counts to be included in subsequent analysis. 3.1. Drug Dosage Association with Pharmacogenetics We sought to evaluate methods for testing the relationship between maintenance dose and pharmacogenes at a biobank scale. We performed this analysis using three groups of drug-gene pairs. Of the drugs with CPIC guidance for any of the nine genes queried, there were 24 that had the minimum of 50 participants for whom a maintenance dose could be calculated. We find that nine of the drug-gene pairs have a significant difference in the dosage across gene phenotypes (Kruskal-Wallis or Jonckeere-Terpstra p < 0.05, ). We do not adjust for multiple tests because these are known relationships not discoveries. Warfarin and CYP2C9 phenotypes had the most significant relationship ( p ≅ 0, Jonckeere-Terpstra). The remaining twenty drug-gene pairs did not have a significant relationship between maintenance dose and gene phenotype. We then investigated association between maintenance dose and gene phenotype for drug-gene pairs with any level of evidence in PharmGKB but no CPIC guideline. We found two drug-gene pairs with a p-value less than 0.05 for either the Kruskal-Wallis test or Joncheere-Terpstra trend test . The most significant was the Kruskal-Wallis test for warfarin and CYP2C19 phenotype. Investigating the dose relationship with phenotype reveals that CYP2C19 normal metabolizers have a decreased maintenance dose compared to the other CYP2C19 metabolizer classes ( , second row, first column). We followed up on this finding by interrogating the association between rs3814637 and warfarin maintenance dose. The intronic variant rs3814637 within CYP2C19 has been previously reported to be associated with warfarin response – . This variant is contained within several CYP2C19 star alleles: CYP2C19*1.004, CYP2C19*1.005 , and CYP2C19*15.001 , all of which are normal functioning alleles. We observed that normal metabolizers had an average daily dose of 4.8 mg (compared to 5.3 mg for the other metabolizer classes). We then tested the association between rs3814637 and warfarin maintenance dose. We find a significant relationship between rs3814637 dosage and warfarin maintenance dose ( p <= 1.0 x 10 −46 , two-sided Jonckheere-Terpstra, ). We then analyzed the relationship between maintenance dose and gene phenotype for drug-gene pairs that had no previous indication of a pharmacogenetic relationship but are known to interact. We tested 581 drug-gene pairs and found two significant relationships between dose and gene phenotype: cyclosporine and CYP2C19 , and nicotine and CYP2B6 ( p < 8.6 x 10 −6 , Jonckheere-Terpstra, bonferroni adjusted, table 3: Novel associations). 3.2. Differential Drug Response Phenotype Association We investigated the degree to which adverse drug reactions related to pharmacogenetics could be discovered by performing a statistical analysis of pharmacogene phenotypes and coded medical events within a one year window following the first administration of a drug. We again evaluated three drug-gene groups starting with drug-gene pairs with CPIC guidelines ( , CPIC Guidance Group). The most significant side effect is a decreased incidence of herpes zoster diagnoses among CYP2C19 intermediate metabolizers ( p <= 8.76 x 10 −5 ). Next we looked to see if there are any differential drug response phenotypes enriched among drug-gene pairs with any level of evidence but no CPIC guideline. The top five results are shown in under “No Guidance”. We find several phenotypes enriched among CYP2D6 intermediate metabolizers taking citalopram, including respiratory issues and heart failure. We also find an increased risk of sinus infections among CYP2C9 poor metabolizers on simvastatin, and an increased risk of puerperal infections among CYP2D6 intermediate metabolizers on propranolol. We interrogated all other drugs known to be metabolized by CYP2C9, CYP2C19, or CYP2D6 for differential drug response phenotypes. This resulted in 4,806 independent association tests across 81 drugs. After multiple hypothesis corrections one side effect was significantly associated with a drug-gene pair: increased incidence of osteoarthritis in CYP2C9 poor metabolizers after taking diazepam. We show the top five results from the exploratory analysis in . 

Drug Dosage Association with Pharmacogenetics We sought to evaluate methods for testing the relationship between maintenance dose and pharmacogenes at a biobank scale. We performed this analysis using three groups of drug-gene pairs. Of the drugs with CPIC guidance for any of the nine genes queried, there were 24 that had the minimum of 50 participants for whom a maintenance dose could be calculated. We find that nine of the drug-gene pairs have a significant difference in the dosage across gene phenotypes (Kruskal-Wallis or Jonckeere-Terpstra p < 0.05, ). We do not adjust for multiple tests because these are known relationships not discoveries. Warfarin and CYP2C9 phenotypes had the most significant relationship ( p ≅ 0, Jonckeere-Terpstra). The remaining twenty drug-gene pairs did not have a significant relationship between maintenance dose and gene phenotype. We then investigated association between maintenance dose and gene phenotype for drug-gene pairs with any level of evidence in PharmGKB but no CPIC guideline. We found two drug-gene pairs with a p-value less than 0.05 for either the Kruskal-Wallis test or Joncheere-Terpstra trend test . The most significant was the Kruskal-Wallis test for warfarin and CYP2C19 phenotype. Investigating the dose relationship with phenotype reveals that CYP2C19 normal metabolizers have a decreased maintenance dose compared to the other CYP2C19 metabolizer classes ( , second row, first column). We followed up on this finding by interrogating the association between rs3814637 and warfarin maintenance dose. The intronic variant rs3814637 within CYP2C19 has been previously reported to be associated with warfarin response – . This variant is contained within several CYP2C19 star alleles: CYP2C19*1.004, CYP2C19*1.005 , and CYP2C19*15.001 , all of which are normal functioning alleles. We observed that normal metabolizers had an average daily dose of 4.8 mg (compared to 5.3 mg for the other metabolizer classes). We then tested the association between rs3814637 and warfarin maintenance dose. We find a significant relationship between rs3814637 dosage and warfarin maintenance dose ( p <= 1.0 x 10 −46 , two-sided Jonckheere-Terpstra, ). We then analyzed the relationship between maintenance dose and gene phenotype for drug-gene pairs that had no previous indication of a pharmacogenetic relationship but are known to interact. We tested 581 drug-gene pairs and found two significant relationships between dose and gene phenotype: cyclosporine and CYP2C19 , and nicotine and CYP2B6 ( p < 8.6 x 10 −6 , Jonckheere-Terpstra, bonferroni adjusted, table 3: Novel associations). 

Differential Drug Response Phenotype Association We investigated the degree to which adverse drug reactions related to pharmacogenetics could be discovered by performing a statistical analysis of pharmacogene phenotypes and coded medical events within a one year window following the first administration of a drug. We again evaluated three drug-gene groups starting with drug-gene pairs with CPIC guidelines ( , CPIC Guidance Group). The most significant side effect is a decreased incidence of herpes zoster diagnoses among CYP2C19 intermediate metabolizers ( p <= 8.76 x 10 −5 ). Next we looked to see if there are any differential drug response phenotypes enriched among drug-gene pairs with any level of evidence but no CPIC guideline. The top five results are shown in under “No Guidance”. We find several phenotypes enriched among CYP2D6 intermediate metabolizers taking citalopram, including respiratory issues and heart failure. We also find an increased risk of sinus infections among CYP2C9 poor metabolizers on simvastatin, and an increased risk of puerperal infections among CYP2D6 intermediate metabolizers on propranolol. We interrogated all other drugs known to be metabolized by CYP2C9, CYP2C19, or CYP2D6 for differential drug response phenotypes. This resulted in 4,806 independent association tests across 81 drugs. After multiple hypothesis corrections one side effect was significantly associated with a drug-gene pair: increased incidence of osteoarthritis in CYP2C9 poor metabolizers after taking diazepam. We show the top five results from the exploratory analysis in . 

Discussion Biobanks offer a powerful solution for enabling the study of relationships between drugs and genes. Large datasets linking genetic and longitudinal clinical data are becoming more broadly available and allow interrogation of the relationship between drug response and pharmacogenetic phenotypes. Here we derived drug phenotypes in the form of maintenance dose and differential drug response phenotypes for more than 200,000 participants across 200 drugs in the UK Biobank and tested their association with well established pharmacogenetic phenotypes for nine genes. Pharmacogenetic testing is not yet common practice, but for some drugs the standard clinical procedures used to determine maintenance dose are influenced by genetics. We find evidence to support existing pharmacogenetic associations with maintenance dose. Among 24 drugs with CPIC guidance in our study we find evidence for a genetic influence on maintenance dose for nine drugs. For the remaining pairs with guidance, it is possible we are not likely to observe an association with maintenance dose because efficacy is difficult to measure or side effects are rare. Among drugs with any prior evidence of a pharmacogenetic relationship but no CPIC dosage guideline we find that maintenance dose supports the association for two drug-gene pairs. Most notably, carriers of the CYP2C19 intronic variant rs3814637 have a significantly decreased warfarin maintenance dose. The causal mechanism through which this effect occurs is unclear, and this variant itself may not be causal, rather in linkage disequilibrium with a causal variant. In GTEx, rs3814637 is associated with increased expression of CYP2C9 (the gene typically associated with warfarin response) in several tissues, although importantly not in liver. There is a gap in the amount of warfarin dosing variability that can be explained by genetics among individuals of African descent . rs3814637 has nearly twice the allele frequency in the African population as it does in the European population (11.6% vs 6.7%) . Although this study focuses on Europeans, this variant may explain some of the missing heritability of warfarin response among Africans, but further study is needed to confirm this relationship. We discovered potential novel pharmacogenetic associations with maintenance dose for two drugs: cyclosporine with CYP2C19 , and nicotine with CYP2B6 . Both drugs are known to be metabolized by their respective associated enzymes, however there is no prior literature evidence suggesting a pharmacogenetic relationship. For both drugs, we find a decreasing association between dose and metabolizer class of their associated enzymes, where individuals with higher rates of metabolism tend to be on lower doses. Our analysis of differential drug response phenotypes reveals associations with side effects among drug-gene pairs. This analysis is limited due to the large number of tests requiring a strict multiple hypothesis testing threshold, but produces interesting hypotheses. At first glance many of the differential phenotype associations seem unlikely, but literature evidence exists for many of the findings. For example, the most significant association among drugs with CPIC guidelines was a decreased incidence of herpes zoster among CYP2C19 intermediate metabolizers compared to CYP2C19 normal metabolizers treated with citalopram. However, two previous studies have demonstrated that SSRIs can lead to increased resistance to herpes , . CYP2C19 intermediate metabolizers have an increased blood concentration of citalopram and may have an increased resistance to a herpes infection. We also find CYP2C19 rapid metabolizers on clopidogrel have a decreased risk of viral skin lesions compared to CYP2C19 normal metabolizers. There is evidence that clopidogrel may inhibit viral clearance . It may be possible that CYP2C19 rapid metabolizers have a lower concentration of clopidogrel and therefore the degree to which they are able to fight off viral infections is higher than that of CYP2C19 normal metabolizers. The most significant association is between CYP2C9 poor metabolizers on diazepam having an increased incidence of osteoarthritis. There is no literature that suggests osteoarthritis may be a side effect of diazepam, although there are studies that suggest diazepam could be used to treat pain as a result of rheumatoid arthritis. Without further evidence we cannot say whether this relationship results from pharmacogenetics and not a correlation with the drug indication or a statistical artifact. This work has several limitations. First, we use pharmacogenetic alleles called from data imputed from genotyping arrays. We previously reported limitations in accuracy of the ability to accurately call alleles in several pharmacogenes from imputed data, notably in CYP2D6 . The lack of structural variants in the dataset in addition to the inability to call rare variants may lead to inaccurate prediction of CYP2D6 phenotypes. Second, we broadly apply our maintenance dose algorithm to drugs in the UK Biobank. While this is effective for some drugs, better clinical end points may provide an improved representation of patient response. For example, a dose response curve may provide more fine grained insight into individual response and yield better insight into the genetics of drug response. It is challenging to broadly define response across drugs from numerous classes with varying indications and therapeutic indices. Even a single drug can be used for different indications and may require different doses to treat each indication. Additionally, this approach will miss patients who take a drug once and experience side effects that lead them to immediately switch drugs. No catch-all definition will suffice, but maintenance dose does reveal insight into patient response. Third, the data we used to define drug usage is in the form of prescription orders. We do not know whether the prescriptions were filled or if the patient took the drug as prescribed. Finally, we do not provide any clinical validation of the predictions presented here; further followup is needed. Biobanks are an immense resource that allow for pharmacogenetic association testing at an unprecedented scale. Longitudinal clinical data is critical to be able to define drug response phenotypes in order to accurately assess patient response to treatment and ultimately test genetic associations. As access to biobanks continue to expand and more data is available, the ability to perform pharmacogenetic studies at large scale will increase. We believe that these resources offer a promising avenue for discovery and will further advance the field of pharmacogenetics. 

