Anti-tumor activity of non-benzoquinone ansamycin Hsp90 inhibitors in murine xenograft models

5630 Hsp90 is a protein chaperone that acts to stabilize and/or activate a number of proteins required for cellular signaling pathways, including several known determinants of carcinogenesis such as Her2 (ErbB2), B-Raf, c-Kit, Flt-3, and Raf-1. Hsp90 is effectively inhibited by benzoquinone ansamycins, such as geldanamycin, herbimycin, macbecin, and their derivatives, which bind to the ATP binding site in the N-terminal domain. Recent results from clinical trials using the geldanamycin derivatives tanespimycin (17-AAG) and alvespimycin (17-DMAG) suggest that inhibition of Hsp90 could be an important therapeutic mechanism for the treatment of cancer. We have previously reported the production of non-benzoquinone ansamycin Hsp90 inhibitors using genetic engineering of the geldanamycin producing microorganism, Streptomyces hygroscopicus NRRL3602. These molecules retain the exquisite specificity of tanespimycin and alvespimycin for the ATP binding pocket of Hsp90, being the most avid binders reported to date, yet do not rely upon activation by NQO1 for maximal inhibitory activity. They are highly efficacious in the HCT-116 mouse tumor model. We report here that these non-benzoquinone ansamycins are exceptionally potent molecules in additional tumor models, including MV-4-11 and COLO205. The Flt-3 ITD mutation, found in the AML cell line MV-4-11, has been shown to confer sensitivity to tanespimycin in vitro. We demonstrated a similar in vitro response of MV-4-11 to the non-benzoquinone ansamycin KOS-2484 and went on to demonstrate that this in vitro sensitivity correlates with a significant in vivo response. Treatment of mouse MV-4-11 xenografts with KOS-2484 resulted in tumor regression. KOS-2484 was dosed at 7.5, 15, or 30 mg/kg via i.v. bolus on a daily schedule for 5 days with a 2-day holiday for a total of 10 doses, and each of these dose levels was associated with tumor shrinkage. While the response at 7.5 mg/kg was initially not as pronounced as those at the higher doses, after 10 doses, the responses had equalized. This is in accordance with the previously demonstrated long lifetime of KOS-2484 in tumor tissue (half-life ~30 hours), which allows tumor drug levels to build with successive administrations. All doses were well-tolerated with no observable body weight loss. Most remarkably, KOS-2484 showed almost complete regression of very large (1000 mm3) MV-4-11 xenografts after 10 doses. Thus, non-benzoquinone ansamycins produced by genetic engineering are among the most potent Hsp90 inhibitors reported to date and show significant efficacy in diverse mouse xenograft models.