Differential Effects of PI3K and Dual PI3K/mTOR Inhibition in Rat Prolactin-Secreting Pituitary Tumors

Aggressive pituitary tumors are rare but difficult to manage, as there is no effective chemotherapy to restrict their growth and cause their shrinkage. Within these tumors, growth-promoting cascades, like the PI3K/mTOR pathway, appear to be activated. We tested the efficacy of two inhibitors of this pathway, NVP-BKM120 (Buparlisib; pan-PI3K) and NVP-BEZ235 (dual PI3K/mTOR), both in vitro on immortalized pituitary tumor cells (GH3) and on primary cell cultures of human pituitary tumors and in vivo on a rat model of prolactin (PRL) tumors (SMtTW3). In vitro, NVP-BEZ235 had a potent apoptotic and cytostatic effect that was characterized by decreased cyclin D/E and Cdk4/2 protein levels and subsequent accumulation of cells in G1 In vivo, the effect was transient, with a decrease in mitotic index and increase in apoptosis; long-term treatment had no significant inhibitory effect on tumor growth. In contrast, while NVP-BKM120 had little effect in vitro, it dramatically limited tumor growth in vivo Increased Akt phosphorylation observed only in the NVP-BEZ235-treated tumors may explain the differential response to the two inhibitors. Primary cell cultures of human PRL pituitary tumors responded to NVP-BEZ235 with reduced cell viability and decreased hormone secretion, whereas NVP-BKM120 had little effect. Altogether, these results show a potential for PI3K inhibitors in the management of aggressive pituitary tumors. Mol Cancer Ther; 15(6); 1261-70. ©2016 AACR.