Idasanutlin is a potent inhibitor of the p53-MDM2 interaction that enables re-activation of the p53 pathway which induces cell cycle arrest and/or apoptosis in tumor cells expressing functional p53. It was investigated for the treatment of solid tumors and several hematological indications such as relapsed/refractory acute myeloid leukemia, polycythemia vera or non-hodgkin lymphoma. For safety reasons it cannot be given in healthy volunteers for drug-drug interaction (DDI) explorations. This triggered the need for in silico explorations on top of the one available CYP3A clinical DDI study with posaconazole in solid tumor patients. Idasanutlin's clearance is dependent on CYP3A4/2C8, forming its major circulating metabolite M4, with contributions from UGT1A3 and biliary excretion. Idasanutlin and M4 have low permeability, very low clearance and extremely low unbound fraction in plasma (<0.001) which makes in vitro data showing inhibition on CYP3A4/2C8 enzymes challenging to translate to clinical relevance. PBPK models of idasanutlin and M4 have been established to simulate perpetrator and victim DDI scenarios and to evaluate whether further DDI studies in oncology patients are necessary. Modelling indicated that idasanutlin and M4 would show no or weak clinical inhibition of selective CYP3A4/2C8 substrates. Co-administered strong CYP3A and CYP2C8 inhibitors might lead to weak or moderate idasanutlin exposure increases and the strong inducer rifampicin might cause moderate exposure reduction. Since the simulated idasanutlin systemic exposure changes would be within the range of observed intrinsic variability, the target population can take co-medications which are either CYP2C8/3A4 inhibitors or weak/moderate CYP2C8/3A4 inducers without dose adjustment. Significance Statement Clinical trials for idasanutlin are restricted to cancer patients, which imposes practical, scientific and ethical challenges on DDI investigations. Furthermore, idasanutlin and its major circulating metabolite have very challenging ADME profiles including high protein binding, low permeability and a combination of different elimination pathways each with extremely low clearance. Nonetheless, PBPK models could be established and applied for DDI risk assessment and were especially useful to provide guidance on concomitant medications in patients.

Small molecules that present complex absorption, distribution, metabolism, and elimination (ADME) properties can be challenging to investigate as potential therapeutics. Acquiring data through standard methods can yield results that are insufficient to describe the in vivo situation, which can affect downstream development decisions. Implementing in vitro - in vivo - in silico strategies throughout the drug development process is effective in identifying and mitigating risks while speeding up their development. Risdiplam (EVRYSDI^®) - an orally bioavailable, small molecule approved by the U.S. Food and Drug Administration and more recently by the European Medicines Agency for the treatment of patients {greater than or equal to}2 months of age with spinal muscular atrophy (SMA), is presented here as a case study. Risdiplam is a low turnover compound whose metabolism is mediated through a non-cytochrome P450 enzymatic pathway. Four main challenges of risdiplam are discussed: predicting in vivo hepatic clearance, determining in vitro metabolites with regard to metabolites in safety testing guidelines, elucidating enzymes responsible for clearance, and estimating potential drug-drug interactions. A combination of in vitro and in vivo results was successfully extrapolated and used to develop a robust physiologically based pharmacokinetic model of risdiplam. These results were verified through early clinical studies, further strengthening the understanding of the ADME properties of risdiplam in humans. These approaches can be applied to other compounds with similar ADME profiles, which may be difficult to investigate using standard methods. Significance Statement Risdiplam is the first approved, small molecule, survival of motor neuron 2 mRNA splicing modifier for the treatment of spinal muscular atrophy. The approach taken to characterize the absorption, distribution, metabolism and excretion (ADME) properties of risdiplam during clinical development incorporated in vitro-in vivo-in silico techniques, which may be applicable to other small molecules with challenging ADME. These strategies may be useful in improving the speed at which future drug molecules can be developed.