CXCR-4 activation mediates resistance to mTOR-directed therapy via regulation of VEGF expression

B35 An explant pancreas adenocarcinoma xenograft model was used to determine potential mediators of resistance to mTOR-directed therapy. A panel of 16 individual patient tumors where treated with temsirolimus for 28 days identifying 7 xenografts as resistant and 9 as sensitive to temsirolimus. Baseline real-time mRNA analysis of these tumors demonstrated that high levels of SDF-1α expression, not CXCR4, correlated with resistance to temsirolimus (correlation coefficient 0.586, p-value 0.017). Treatment of mTOR resistant xenograft Panc159 with the combination of CXCR4 inhibitor (AMD3465) and temsirolimus synergistically inhibited Panc159 growth kinetics compared to treatment with either single agent. This response was associated with an inhibition of p70 phosphorylation as well as cyclin D and cMyc expression. Furthermore, the activation of CXCR4 by exogenous SDF-1α converted the temsirolimus sensitive tumor Panc410 to a resistant xenograft. The inhibition of CXCR4 signal transduction by AMD3465 negated the effect of exogenous SDF-1α on Panc410 growth kinetics in the presence of temsirolimus. Exposure of the xenograft Panc410 to SDF-1α in the presence of temsirolimus specifically induced a 3 fold induction in cyclin D and cMyc expression. Next, modulation in VEGF expression was analyzed as angiogenesis has been demonstrated to be an important determinant of response to mTOR-directed therapy. Treatment of the mTOR resistant xenograft Panc159 with temsirolimus resulted in an induction of VEGF expression. The addition of AMD3465 to temsirolimus abrogated VEGF mRNA and protein expression. Conversely, induction of an mTOR resistant phenotype in Panc410 xenografts was associated with the specific induction of VEGF mRNA and protein expression. Together these data provide initial evidence that CXCR4 activation by SDF-1α is a mediator of resistance to mTOR-directed therapy via regulation of VEGF production.