Figure 12. Single mTOR inhibition by chronic rapamycin treatment effectively reduces SEGA growth.

(A and B) Chronic treatment with rapamycin and the dual PI3K-mTOR inhibitor PI-103 both resulted in impairment in the growth of pNSC-derived mouse SEGAs with respect to treatment with vehicle up to 63 days after transplantation (n = 3 replicates). Later, only rapamycin yielded sustained antitumor activity, whereas PI-103 was no longer effective. Results are expressed as the mean ± SEM. Two-group comparisons were performed with the independent-samples t test. *P < 0.05 and **P < 0.01. (C) When analyzed 71 days after transplantation, vehicle-treated tumors contained proliferating cells, which were atypical with either large eosinophilic cytoplasm or a spindle-shaped morphology with elongated hyperchromatic nuclei and foci of neoplastic necrosis. IHC showed strong and diffuse positivity for pS6, pAkt_T308, pAkt_S473, and pNDRG1 in most of the neoplastic cells. PI-103–treated tumors were characterized by moderate cellularity with features similar to those of vehicle-treated tumors. They were mainly composed of medium-sized cells with eosinophilic cytoplasm and atypical nuclei with a minor component of spindle-shaped cells, sparse mitotic figures, and only focal necrotic areas. Rapamycin-treated tumors did show strongly reduced cellularity as compared with vehicle- and dual inhibitor–treated tumors. They mostly comprised cells with large eosinophilic cytoplasm and round or oval nuclei with 1 or more prominent nucleoli. Spindle-shaped cells were rare, mitotic figures were only sporadic, and no necrotic foci were found. IHC for pS6 showed heterogeneous signal with only a few cells being faintly positive. pAkt_T308-IR, pAkt_S473-IR, and pNDRG1-IR cells were barely detected and only lightly stained. Original magnification, ×400.