XMD15-44, a RET kinase inhibitor, demonstrates potent growth-inhibitory effects on RAT1 cells transformed by RET/C634R and RET/M918T, with IC50 values of 11.5 nM and 8.3 nM, respectively. It effectively inhibits RET kinase activity and signaling in human thyroid cancer cell lines harboring oncogenic RET alleles, thereby reducing cell proliferation.