Abstract 1964: Preclinical efficacy of the HSP90 inhibitor SNX-5422 in targeting C481S mutant BTK and ibrutinib resistant CLL

While chronic lymphocytic leukemia (CLL) is effectively treated by the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib, patients that do relapse on ibrutinib (approximately 19%) have an extremely poor clinical outcome without further intervention. Before clinical diagnosis of relapse, signs of molecular relapse can be identified through deep sequencing for mutations in BTK (C481S) and the immediate downstream BTK target, PLCγ2. The BTK C481S mutation (which reduces ibrutinib binding) can be identified at frequencies as low as 5%, and expansion of this mutant close inevitably leads to clinical relapse. Therefore, identifying methods to target this mutant clone before clinical relapse represents a novel and potentially effective therapeutic strategy. HSP90 is a chaperone protein responsible for maintaining the stability of a large number of client proteins involved in cancer development. Cancer cells, particularly those reliant on a mutated oncogene, rely heavily on HSP90 for survival, making this an attractive clinical target. Here we have explored the preclinical efficacy of SNX-5422, a chemically unique, orally bioavailable HSP90 inhibitor, in ibrutinib resistant CLL. We determined that SNX-5422 can degrade C481S mutated BTK by utilizing BTK-null B-cell lines over-expressing wildtype or C481S BTK. Whereas ibrutinib reduces BTK activity in only the wildtype BTK transfected cell line, SNX-5422 degrades both wildtype BTK and C481S mutant BTK and impairs downstream signaling. In fact, C481S mutant BTK shows increased degradation compared to wildtype BTK suggesting that the C481S mutant BTK may rely on HSP90 more for stabilization. We next examined the in vivo efficacy of SNX-5422 in a mouse model of ibrutinib-resistant CLL. Leukemic splenocytes isolated at removal criteria from TCL1 mice continuously treated with ibrutinib were adoptively transferred into syngeneic WT B6 mice. Upon reaching 10% leukemia burden in the peripheral blood, the mice were randomized into 1 of 4 treatment groups: vehicle, SNX-5422 (50 mg/kg MWF), ibrutinib (continuous in drinking water), or SNX-5422 plus ibrutinib (concurrent). Mice treated with the combination of SNX-5422 and ibrutinib exhibit prolonged survival and decreased disease burden in the peripheral blood compared to the vehicle or single agent groups. In summary, we show that SNX-5422 degrades C481S BTK in vitro and prolongs survival when combined with ibrutinib in vivo. We therefore propose that upon molecular relapse,