Structure-activity relationship studies of pyrazolo[3,4-d]pyrimidine derivatives leading to the discovery of a novel multikinase inhibitor that potently inhibits FLT3 and VEGFR2 and evaluation of its activity against acute myeloid leukemia in vitro and in vivo

We describe the structural optimization of a hit compound, 1-(4-(1H-pyrazolo[3,4-d]pyrimidin-4-ylamino)phenyl)-3-(3-methoxyphenyl)urea (1), which exhibits inhibitory activity but low potency against FLT3 and VEGFR2/KDR/Flk-1. A series of pyrazolo[3,4-d]pyrimidine derivatives were synthesized, and structure-activity relationship analysis using cell- and transgenic-zebrafish-based assays led to the discovery of a number of compounds that exhibited both high potency against FLT3-driven human acute myeloid leukemia (AML) MV4-11 cells and a considerable antiangiogenic effect in transgenic-zebrafish-based assays. The compound 1-(4-(1H-pyrazolo[3,4-d]pyrimidin-4-yloxy)phenyl)-3-(4-chloro-3-(trifluoromethyl)phenyl)urea (33), which exhibited the highest activity in preliminary in vivo anti-AML assays, was chosen for further anti-AML studies. The results demonstrated that compound 33 is a multikinase inhibitor that potently inhibits FLT3 and VEGFR2/KDR/Flk-1. In an MV4-11 xenograft mouse model, a once-daily dose of compound 33 at 10 mg/kg for 18 days led to complete tumor regression without obvious toxicity. Western blot and immunohistochemical analyses were performed to determine the mechanism of action of compound 33.