Several options are available for the design of pharmacogenetic studies. Firstly, randomised controlled clinical trials (RCTs), both pre- and post-authorisation, provide the opportunity to address several pharmacogenetic questions. Pharmacogenetics in randomized controlled trials: considerations for trial design (Pharmacogenomics 2011;12(10):1485-92) describes three different trial designs differing in the timing of randomization and genotyping, and Promises and challenges of pharmacogenetics: an overview of study design, methodological and statistical issues (JRSM Cardiovasc Dis 2012 5;1(1)) discusses outstanding methodological and statistical issues that may lead to heterogeneity among reported pharmacogenetic studies and how they may be addressed. Pharmacogenetic trials can be designed (or post hoc analysed) with the intention to study whether a subgroup of patients, defined by certain genetic characteristics, respond differently to the treatment under study. Alternatively, a trial can verify whether genotype-guided treatment is beneficial over standard care. Obvious limitations with regard to the assessment of rare adverse drug events are the large sample size required and its related high costs. In order to make a trial as efficient as possible in terms of time, money and/or sample size, it is possible to opt for an adaptive trial design, which allows prospectively planned modifications in design after patients have been enrolled in the study. Such a design uses accumulating data to decide how to modify aspects of the study during its progress, without undermining the validity and integrity of the trial. An additional benefit is that the expected number of patients exposed to an inferior/harmful treatment can be reduced (see Potential of adaptive clinical trial designs in pharmacogenetic research. Pharmacogenomics 2012;13(5):571-8).
Observational studies are the alternative and can be family-based (using twins or siblings) or population-based (unrelated individuals). The main advantage of family-based studies is the avoidance of bias due to population stratification. A clear practical disadvantage for pharmacogenetic studies is the requirement to study families where patients have been treated with the same drugs (see Methodological quality of pharmacogenetic studies: issues of concern. Stat Med 2008;27(30):6547-69).
Population-based studies may be designed to assess drug-gene interactions as cohort (including exposure-only), case-cohort and case-control studies (including case-only, as described in Nontraditional epidemiologic approaches in the analysis of gene-environment interaction: case-control studies with no controls! Am J Epidemiol 1996;144(3):207-13). Sound pharmacoepidemiological principles as described in the current Guide also apply to observational pharmacogenetic studies. A specific type of confounding due to population stratification needs to be considered in pharmacogenetic studies, and, if present, needs to be dealt with. Its presence may be obvious where the study population includes more than one immediately recognisable ethnic group; however in other studies stratification may be more subtle. Population stratification can be detected by Pritchard and Rosenberg’s method, which involves genotyping additional SNPs in other areas of the genome and testing for association between them and outcome. In genome-wide association studies, the data contained within the many SNPs typed can be used to assess population stratification without the need to undertake any further genotyping. Several methods have been suggested to control for population stratification such as genomic control, structure association and EIGENSTAT. These methods are discussed in Methodological quality of pharmacogenetic studies: issues of concern (Stat Med 2008;27(30):6547-69) and Softwares and methods for estimating genetic ancestry in human populations (Hum Genomics 2013;7:1).
The main advantage of exposure-only and case-only designs is the smaller sample size that is required, at the cost of not being able to study the main effects of drug exposure (case-only) or genetic variant (exposure-only) on the outcome. Furthermore, interaction can be assessed only on a multiplicative scale, whereas from a public health perspective additive interactions are very relevant. An important condition that has to be fulfilled for case-only studies is that the exposure is independent of the genetic variant, e.g. prescribers are not aware of the genotype of a patient and do not take this into account, directly or indirectly (by observing clinical characteristics associated with the genetic variant). In the exposure-only design, the genetic variant should not be associated with the outcome, for example variants of genes coding for cytochrome p-450 enzymes. When these conditions are fulfilled and the main interest is in the drug-gene interaction, these designs may be an efficient option. In practice, case-control and case-only studies usually result in the same interaction effect as empirically assessed in Bias in the case-only design applied to studies of gene-environment and gene-gene interaction: a systematic review and meta-analysis (Int J Epidemiol 2011;40(5):1329-41). The assumption of independence of genetic and exposure factors can be verified among controls before proceeding to the case-only analysis. Further development of the case-only design for assessing gene-environment interaction: evaluation of and adjustment for bias (Int J Epidemiol 2004;33(5):1014-24) conducted sensitivity analyses to describe the circumstances in which controls can be used as proxy for the source population when evaluating gene-environment independence. The gene-environment association in controls will be a reasonably accurate reflection of that in the source population if baseline risk of disease is small (<1%) and the interaction and independent effects are moderate (i.e. risk ratio<2), or if the disease risk is low (e.g. <5%) in all strata of genotype and exposure. Furthermore, non-independence of gene-environment can be adjusted in multivariable models if non-independence can be measured in controls.
|Annex 1.||Guidance on conducting systematic revies and meta-analyses of completed comparative pharmacoepidemiological studies of safety outcomes|