In vivo antitumor efficacy of purine-scaffold Hsp90 inhibitors in basal-like breast cancer (BLBC)

5637

BLBC, that account for ~15% of breast cancer cases, are highly aggressive tumors that confer a poor prognosis and shorten the disease-free and overall survival. Moreover, BLBC are negative for ER and HER2 limiting the therapeutic options of targeted agents. Our previous findings suggest that BLBC, although somewhat resistant to geldanamycin-derivatives, are sensitive to purine-scaffold Hsp90 inhibitors. We synthesized a purine-scaffold Hsp90 inhibitor called PU-H71, that when administered to BLBC cells, have shown to potently inhibits cell proliferation, produces cell cycle arrest, downregulates the nodal Akt and Raf-1 oncoproteins, and induces significant apoptosis. Here we report that administration of PU-H71 to BLBC xenotransplanted mice results in pharmacologically relevant concentrations in the tumors where can be detected for up to 48 h later. Whereas levels of PU-H71 in plasma and normal tissues are below 0.1 mg/mL at 6 h, in the MBA-MD-468 xenografted tumors remain above 2 mg/mL even at 36 h later. Administration of PU-H71 to BLBC tumors results in efficient down-regulation of Hsp90 client proteins such as Raf-1 and Akt (70-95% of control levels), and induces apoptosis as measured by cleavage of PARP. In the MDA-MB-468 xenografted tumors, Raf-1 degradation, Akt inactivation and cleavage of PARP are detected as early as 12 h after PU-H71 administration, and the effects remain evident at 48 h, irrespective of Hsp70 upregulation. Similar profiles are detected after PU-H71 administration in other BLBC xenografted tumors such as HCC1806 and MDA-MB-231. In vivo activity of PU-H71 compares favorable to geldanamycin-derivatives (such as 17DMAG), as detected by the level of Raf-1 downregulation in MDA-MB-231 xenografted tumors (70-95% versus 20%). Administration of PU-H71 to mice bearing MDA-MB-468 xenografted tumors resulted in tumor response in all the mice (with a mean of 61% tumor volume reduction in the treated versus control mice), with 25% of mice presenting complete remission. Long term follow-up showed that the tumor was completely eradicated in these mice (25%) while in the remaining mice tumors re-grew after the 60 day-treatment intermission. Remarkably, relapsed tumors were responsive to a new cycle of the same dose of PU-H71, suggesting that there is sustained sensitivity to PU-H71. No systemic or organ toxicity was observed during treatment. Tumor remmants showed lower proliferation rate (Ki67), lower mitotic index (phospo-H3), lower phospho-Akt cells and higher apoptotic cells (TUNEL) compared to the the controls. Taken together with the reported drug-like features of purine-scaffold Hsp90 inhibitors, such as insensitivity to MDR, oral bioavailability and metabolic stability, these data support the future clinical translation of these agents to the treatment of BLBC patients.