Medullary thyroid carcinoma (MTC) is a rare endocrine tumor that frequently metastasizes, and treatment with irinotecan (CPT‐11) is limited because of side effects. Mutations in the Rearranged during transfection (RET) proto‐oncogene are considered the causative event of MTC. The objective of this study was to examine whether small interfering RNA (siRNA) and its combined treatment with CPT‐11 could inhibit MTC cell growth in vitro and in vivo. The transfection of RET siRNA suppressed RET expression, reduced proliferation, and increased caspase‐3/7 activity via the down‐regulation of Bcl‐2 expression. Combined treatments with CPT‐11 or SN‐38 significantly increased caspase 3/7 activity compared with RET siRNA, CPT‐11 or SN‐38 treatment alone. Importantly, intratumoral injection of RET siRNA along with intravenous injection of CPT‐11 significantly inhibited the tumor growth of MTC xenografts via an increased apoptotic effect. These findings that RET siRNA enhanced sensitivity for CPT‐11 will provide a novel strategy for the treatment of MTC with RET mutation.
(Cancer Sci 2010; 101: 941–947)