In vitro pharmacological profiling of R406 identifies molecular targets underlying the clinical effects of fostamatinib.

Off-target pharmacologymay contribute to both adverse and beneficial effects of a new drug. In vitro pharmacologicalprofiling is often applied early in drug discovery; there are fewer reports addressing the relevance of broad profiles to clinical adverse effects. Here, we have characterized the pharmacologicalprofile of the active metaboliteof fostamatinib, R406, linking an understanding of drug selectivity to the increase in blood pressure observed in clinical studies. R406 was profiled in a broad range of in vitro assays to generate a comprehensive pharmacologicalprofile and key targets were further investigated using functional and cellular assay systems. A combination of traditional literature searches and text-miningapproaches established potential mechanistic links between the profile of R406 and clinical side effects. R406 was selective outside the kinase domain, with only antagonist activity at the adenosine A3 receptorin the range relevant to clinical effects. R406 was less selective in the kinase domain, having activity at many protein kinases at therapeutically relevant concentrations when tested in multiple in vitro systems. Systematic literature analyses identified KDR as the probable target underlying the blood pressure increase observed in patients. While the in vitro pharmacologicalprofile of R406 suggests a lack of selectivity among kinases, a combination of classical searching and text-miningapproaches rationalized the complex profile establishing linkage between off-target pharmacologyand clinically observed effects. These results demonstrate the utility of in vitro pharmacologicalprofiling for a compound in late-stage clinical development.