Recommendations on the Use of Multiple Labels in Human Mass Balance Studies

The administration of radiolabeled drug candidates is considered the gold standard in absorption, distribution, metabolism and excretion (ADME) studies for small molecule drugs, since it allows facile and accurate quantification of parent drug, metabolites and total drug related material independent of the compound structure. The choice of the position of the radiolabel, typically ¹⁴C or ³H, is critical to obtain relevant information. Sometimes a biotransformation reaction may lead to cleavage of a part of the molecule. As a result only the radiolabeled portion can be followed while information on the fate of the non-labeled metabolite may be lost. Synthesis and administration of two or more radiolabeled versions of the parent drug as a mixture or in separate studies may resolve this issue, but comes with additional challenges. In this paper we address the questions that may be considered to help make the right choice whether to use single or a multiple radiolabel approach, discuss pros and cons of different multiple labeling strategies that can be taken as well as alternative methods that allow the non-labeled part of the molecule to be followed. Significance Statement Radiolabeled studies are the gold standard in drug metabolism research, but molecules can undergo cleavage with loss of the label. This often results in discussions around potential use of multiple labels which seem to be occurring with increased frequency since an increasing proportion of the "small molecule drugs" are tending towards larger molecular weights. This review provides insight and decision criteria in considering a multiple label approach as well as pros and cons of different strategies that can be followed.