Rabusertib

Checkpoint kinase‐1 (CHK1) controls DNA replication and repair. Tumor cells depend on CHK1, whose high levels are associated with worse patient prognosis. We define a bona fide proteolysis‐targeting‐chimera (PROTAC) for CHK1. PROTAC MA203 contains the type I kinase inhibitor rabusertib, which preferentially inhibits activated CHK1, and the cereblon (CRBN) ligand pomalidomide. MA203 accelerates CRBN‐dependent proteasomal degradation of CHK1 in solid tumor‐derived cells and acute leukemia cells. Chemotherapy‐induced DNA replication stress and a consequent activation of CHK1 accelerate this event‐driven process which promotes DNA damage and tumor cell apoptosis. Biochemical and cellular target engagement studies confirm the potency and selectivity of MA203. MA203 does not damage healthy differentiated and primitive hematopoietic cells, stromal cells, and retinal epithelial cells. MA203 is superior to its corresponding kinase inhibitor concerning DNA damage, dysregulation of BCL2 proteins, and apoptosis induction. These processes occur independently of the tumor‐suppressive transcription factor p53. Elimination of CHK1 protein as structural element, but not its inhibition per se, triggers a proteasomal degradation of key DNA replication and repair proteins. Genetic CHK1 elimination confirms that such newly recognized functions of CHK1 rely on functions beyond its well‐known catalytic activity. Thus, kinase‐independent functions of CHK1 can be exploited with innovative pharmacological agents.

Increased kinase activity of leucine-rich repeat kinase 2 (LRRK2) is associated with Parkinson’s disease (PD). Numerous LRRK2-selective type I kinase inhibitors have been developed, and some have entered clinical trials. Here, to our knowledge, we present the first type II kinase inhibitors that target LRRK2. Targeting the inactive conformation of LRRK2 is functionally distinct from targeting the active-like conformation using type I inhibitors. We designed these inhibitors with a combinatorial chemistry approach fusing selective LRRK2 type I and promiscuous type II inhibitors using iterative cycles of synthesis supported by structural biology and activity testing. Our lead compounds are selective and potent toward both LRRK2 and LRRK1, a close relative of LRRK2. Through cellular assays, cryo–electron microscopy structural analysis, and in vitro motility assays, we show that our inhibitors stabilize the open, inactive LRRK2 kinase conformation. These new conformation-specific compounds will be invaluable as tools to study LRRK2’s function and regulation and expand the potential therapeutic options for PD.