P-glycoprotein (P-gp), a member of the
ATP-binding cassette transporter family, is overexpressed in a number of different
cancers and some studies show that P-gp overexpression can be correlated to poor prognosis or therapeutic resistance. Here we sought to elucidate if
PF-3758309 (PF-309), a novel p-21 activated
kinase inhibitor, efficacy was influenced by
tumor P-gp. Based on in vitro proliferation data, a panel of
colorectal cancer cell lines were ranked as sensitive or resistant and ABCB1 (P-gp) expression was evaluated by microarray for these cell lines. P-gp expression was determined by western blot and activity determined by
rhodamine efflux assay. Knock down of P-gp and pharmacologic inhibition of P-gp to restore PF-309 activity was performed in vitro. PF-309 activity was evaluated in vivo in cell line xenograft models and in primary patient derived
tumor xenografts (PDTX). Mice were treated with 25 mg/kg PF-309 orally, twice daily. On the last day of treatment,
tumor and plasma were collected for PF-309 analysis. Here we show that ABCB1 gene expression correlates with resistance to PF-309 treatment in vitro and the expression and activity of P-gp was verified in a panel of resistant cells. Furthermore, inhibition of P-gp increased the sensitivity of resistant cells, resulting in a 4-100-fold decrease in the IC50s. Eleven cell line xenografts and 12 PDTX models were treated with PF-309. From the cell line xenografts, we found a significant correlation between ABCB1 gene expression profiles and
tumor response. We evaluated
tumor and plasma concentrations for eight
tumor models (three cell line xenografts and five PDTX models) and a significant correlation was found between
tumor concentration and response. Additionally, we show that
tumor concentration is approximately fourfold lower in
tumors that express P-gp, verified by western blot. Our in vitro and in vivo data strongly suggests that PF-309 efficacy is influenced by the expression of
tumor P-gp.