Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘How to collect PK samples correctlyThe success of a bioequivalence trial depends on many factors, such as:• the standardization of study procedures• demographic and dietary factors• analytical workAmong these factors there are two basic study-design issues related to bloodsampling that deserve special attention, since they determine whether the samplescan be used to fully describe the absorption, distribution, and elimination phasesof the drug.Sampling timesThe sampling times at which the blood samples are collected have a decisiveimpact on the calculation of the PK parameters for the study drug. Ideally, thesamples should be collected as frequently as possible during the study period so asto give an accurate PK profile. However, in practice, a relatively small number ofblood samples are usually collected at selected time points due to ethical andfinancial considerations.The US Food and Drug Administration (FDA) requires that sample collectionshould be spaced in such a way that the maximum concentration of the drug in theblood (C max) and the terminal elimination rate constant (λ) can be accuratelyestimated [1–3]. It is important that there are enough sampling times clusteredaround C max. For example, in Figure 1, the blood samples were collected every0.25 hours from 0 to 1.5 hours around the T maxvalue (0.75 hours), meaning that theanagrelide PK profile is correct. However, occasionally, not enough blood samplesare collected around C maxand, consequently, false C max, T maxand AUC values areobtained. Figure 3 shows an incorrect sampling scheme missing the time point at0.75 hours. As a consequence of missing just this one time point, the PK parametersderived from this scheme, such as C max, T max, and AUC 0–t, are severely biased, andtherefore erroneous conclusions about bioequivalence could be drawn from them.The sampling periodThe FDA requires that, to obtain an accurate estimate of λ from linear regression,sampling should continue for at least three terminal half-lives of the drug, and thatat least three to four samples should be obtained during the terminal log-linearphase [1,2]. In the case of anagrelide, for example, empirical studies had shownthat the half-life for this drug ranges from 1 to 2 hours, so a sampling period of12 hours was planned for the anagrelide trial. The profiles for anagrelide from thistrial show that this sampling period was long enough to obtain an accurateestimate of λ because, after 8 hours, the anagrelide plasma concentrations were no125