SUN-340 Prolactinoma Response to Bevacizumab Is Influenced by Tumor Progression Stage at the Beginning of Treatment

Anti-VEGF therapy is being implemented as second line drug for aggressive pituitary adenomas that do not respond to conventional treatments. Although it is approved for treatment of several cancers, many of them acquire resistance. In the present work, we evaluated Bevacizumab (BVZ) effect in an in vivo model of prolactinoma in different stages of tumor progression. Immunodeficient nu/nu mice were injected subcutaneously with MMQ tumoral lactotrophs and once xenografts developed, mice were divided in two groups: ‘Early Onset Treatment (EOT)’ or ‘Late Onset Treatment (LOT)’ depending on tumor size at the start of treatment (smaller than 30 mm(3) or larger than 60 mm(3), respectively). BVZ (25 mg/kg) or saline solution as control were administered twice a week for 2 weeks. When BVZ was administered in EOT group, treated mice showed a slower tumor growth rate than controls since the second week of treatment, although no significant differences in tumor volume were determined at the moment of sacrifice. On the contrary, this beneficial effect was not observed in LOT mice, which increased significantly tumor size in response to BVZ treatment (N=8 per condition in each group, p<0,05). In order to study the mechanisms of resistance involved in late stages of tumor progression, we analyzed VEGF serum levels, which augmented in LOT mice but were not modulated in EOT mice under BVZ treatment (ELISA, N=8, p<0,05). Furthermore, we detected a significant correlation between serum VEGF levels and tumor growth when all animals were considered (r=0,53, N=32, p<0,05). However, no differences were found when analyzing VEGF and bFGF intratumoral content between BVZ treated and controls animals from neither of both groups (ELISA, N=8, ns). A slight increase in SMA+ vessel size but not in their number was determined in BVZ treated animals from EOT group. On the other side, vessels of LOT treated mice tended to be smaller than controls (IHQ, N=8, ns). PCNA proliferation marker expression was increased in tumors which augmented their size in response to BVZ (WB, N=8, p<0,05) and it correlated with tumor growth when mice were evaluated altogether (r=0,4, N=32, p<0,05). Furthermore, we found that proliferation associated proteins as β-CATENIN and C-MYC were increased in BVZ treated-tumors from LOT but not from EOT group (WB, N=8, p<0,05). Tumor necrotic area showed a trend of reduction in BVZ-treated LOT mice but not in EOT mice regarding to controls (H&E, N=8, ns). Our results suggest that tumor progression stage impacts on BVZ response in prolactinomas and that the resistance mechanisms developed in later stages could be associated, not only with the activation of VEGF autocrine loop as observed, but also with the regulation of other cell processes as proliferation and cell death. Further insight into the mediators involved in these mechanisms could help to predict which patients could respond or not to anti-VEGF treatment.