BIIB021 (CNF2024) is a purine scaffold HSP90 inhibitor that binds to the ATP-binding pocket for HSP90 and blocks its chaperon function. HSP90 inhibition by BIIB021 leads to degradation of multiple oncogenic proteins and resulted in growth arrest and apoptosis.

BIIB021 is the first synthetic HSP90 inhibitor entered clinical trials and was evaluated in advanced solid tumor, gastrointestinal stromal tumors, B-cell chronic lymphocytic leukemia.