227 Association of K-Ras Genotype with Xenograft Tumor Response to the Investigational Proteasome Inhibitor MLN9708

Abstract

The investigational drug MLN9708 is a potent, reversible and specific inhibitor of the proteasome. MLN9708 is currently being evaluated in clinical trials of hematologic malignancies (Phase 1, 2, and 3) as well as solid tumors (Phase 1 and 2). Upon exposure to aqueous solutions or plasma, MLN9708 immediately hydrolyzes to MLN2238, the biologically active form. In vitro, MLN2238 is potent across a broad range of solid tumor cell types evaluated with standard cell viability and colony formation assays. However, not all the cells which respond to MLN2238 in vitro are responsive in vivo when grown as xenografts in immunocompromised mice. In order to understand the contributing factors that determine sensitivity to MLN2238, we evaluated tumor PK, tumor PD (proteasome inhibition), and sequencing of commonly mutated gene in a panel of xenograft tumors. Tumor PK and PD did not show any difference between MLN2238 sensitive and resistant models. A survey of a number of NSCLC and colon xenografts, including both cell line-derived and primary human xenografts, showed a striking correlation between KRas genotype and MLN2238 response. Tumors with WT Kras showed increased sensitivity to MLN2238 when compared to KRas mutant tumors. This relation was not evident with any other common gene mutations included in Oncocarta panel. To confirm our finding that KRas mutation impacts MLN2238 activity, we used SW48 isogenic colon cell lines, in which KRas-G13D mutation was introduced in SW48 cells (KRas WT) to generate stable SW48-KRas-G3D cells. Although in vitro sensitivity to MLN2238 was similar between the paired cell lines, in vivo studies showed a difference in sensitivity. MLN2238 dosed at MTD (13mg/kg IV BIW) showed significant antitumor activity in SW48 xenografts (T/C =0.45), but no antitumor activity in SW48-KRasG13D xenografts (T/C =1.08), thus confirming the association of KRas status and response to MLN2238. In order to understand the mechanism of resistance, we are evaluating the regulation a variety of markers related to KRas signaling. Recent literature has shown that KRas mutant cells exhibit altered glucose metabolism and glycolysis and can proliferate and survive under low glucose conditions by changing glucose transporter expression. We observed significantly higher protein levels of the glucose transporter Glut4 in MLN2238 resistant xenografts compared to the MLN2238 sensitive xenografts. Work is ongoing to understand if Glut4 expression may account for the differential survival of KRas WT vs. mutant xenograft tumors exposed to MLN2238. Ras mutation status will be evaluated in patient samples from future MLN9708 solid tumor trials.