Cellular effects and antitumor activity of RET inhibitor RPI-1 on MEN2A-associated medullary thyroid carcinoma

Background: The RET proto-oncogene encodes a receptor tyrosine kinase. RET oncogenes arise through sporadic and inherited gene mutations and are involved in the etiopathogenesis of medullary thyroid carcinoma, a cancer that responds poorly to conventional chemotherapy. Medullary thyroid carcinoma is a component of multiple endocrine neoplasia type 2 or MEN2 syndromes. Methods: We investigated the cellular effects of RPI-1, a novel 2-indolinone Ret tyrosine kinase inhibitor on cells that express RET C634 oncogenic mutants common in the MEN2A syndrome: NIH3T3 fibroblasts transfected with RET^C634R and human medullary thyroid carcinoma TT cells that express endogenous RET^C634W. RPI-1 antiproliferative activity was determined by cell proliferation and anchorage-independent growth assays. Expression and phosphorylation of Ret and of proteins involved in downstream signaling pathways were examined by immunoblotting. Antitumor activity of oral RPI-1 treatment was tested by using two dosing levels in nude mice bearing subcutaneous TT xenograft tumors. All statistical tests were two-sided. Results: The RPI-1 IC₅₀ value for cell proliferation was 3.6 μM (95% confidence interval [CI] = 1.8 to 5.4 μM) in NIH3T3 cells expressing the Ret mutant compared with 16 μM (95% CI = 12.3 to 19.7 μM) in non-transfected NIH3T3 cells, and that for colony formation in soft agar was 2.4 μM (95% CI = 0.8 to 4.0 μM) and 26 μM (95% CI = 17 to 35 μM) in RET mutant-transfected and H-RAS-transfected NIH3T3 cells, respectively. In NIH3T3 cells expressing the Ret mutant, Ret protein and tyrosine phosphorylation were undetectable after 24 hours of RPI-1 treatment. In TT cells, RPI-1 inhibited proliferation, Ret tyrosine phosphorylation, Ret protein expression, and the activation of PLCγ, ERKs and AKT. In mice, oral daily RPI-1 treatment inhibited the tumor growth of TT xenografts by 81% (P