Discovery and Synthesis of Heterobifunctional Degraders of Rearranged during Transfection (RET) Kinase

We describe the design, synthesis, and structure–activity relationship (SAR) of heterobifunctional RET ligand-directed degraders (LDDs) derived from three different second-generation RET inhibitors. These LDDs are composed of a target binding motif (TBM) that binds to the RET protein, a linker, and a cereblon binding motif (CBM) as the E3 ligase recognition unit. This led to the identification of a series of pyrazolopyridine-based heterobifunctional LDDs, as exemplified by compound 39. LDD 39 demonstrated high in vitro inhibitory and degradation potency against both RET wild-type and the two representative mutants, V804M and G810R. Importantly, in PK/PD studies, 39 exhibited a differentiated and favorable in vivo profile compared to the corresponding tyrosine kinase inhibitor (TKI), compound 3. Robust and sustained degradation of total-RET (tRET) protein and inhibition of phospho-RET (pRET) signaling were observed in TPC-1 xenograft tumors driven by RET and the RET/G810R mutant following a single dose of LDD 39 at 15 and 75 mg/kg, respectively.