he Food and Drug Administration set the standards for studying drug-drug interactions (DDIs) in the US by drafting sets of guidelines for the industry. The 2006 guidelines 1 focused on study design, data analysis, and implications for dosing and labeling and provided a general orientation to key issues such as study design, study population, choice of substrate and interacting drugs, route administration, dose selection, and end points, sample size, and statistical considerations. However, it is up to pharmaceutical companies to implement specific studies for their drugs. Dr Greenblatt, 2 the Editor of this journal, has recently provided a summary of some methodological requisites for the design and presentation of pharmacokinetic DDI studies to improve the scientific quality of this journal's DDI articles. Certainly, Dr Greenblatt's summary could be used for any pharmacological journal. In the current editorial, the writers point out that the existing scientific standards for pharmacokinetic DDI studies are not sufficient to meet the needs of unusual patients or their prescribing physicians. There are 3 concepts: (1) pharmacological heterogeneity, (2) use of repeated dosing, and (3) translation into a message understandable to practicing clinicians that can be used to illustrate how these studies could be improved.
PHARMACOLOGICAL HETEROGENEITYTo explain what we call pharmacological heterogeneity, we shall begin with what statisticians call heterogeneity; we apologize for having to go back and forth between statistical and pharmacological concepts. As Feinstein 3 (both a physician and a scientist with mathematical training) pointed out, the problem of statistical heterogeneity is largely ignored by statistics textbooks. A statistician analyzing a DDI study would probably assume that the issue of heterogeneity had already been resolved by clinicians or pharmacologists before research data were presented to them. If one calculates the mean and SD of a variable using a sample, one assumes that the sample is homogeneous, including entities of the same type, rather than a heterogeneous mixture of things. The concept of pharmacological heterogeneity in drug metabolism somewhat overlaps with the concept of statistical heterogeneity. For us, pharmacological heterogeneity refers to the concept that some individuals metabolize medications in a way that is different from the average individual. We consider 2 types of pharmacological heterogeneity that have clinical relevance; the first is found in individuals whom statisticians would call outliers and the second in those treated with clinically relevant inhibitors and/or inducers.According to statistics texts, outliers are subjects at the extremes of a distribution, that is, the subjects observed in the tail areas, far beyond the central indexes and the near-central zones of the distribution. 4 This statistical concept has major relevance for pharmacokinetics. To simplify, we will focus on the so-called polymorphic variations. A genetic variation is usually considered a po...