The biological basis for this synergy is unclear as we did not observe changes in apoptosis, DNA repair, troxacitabine incorporation into DNA or troxacitabine metabolism in the presence of gemcitabine. In vivo testing against a human pancreatic (AsPC-1) xenograft mouse tumor model indicated that both drugs were more than additive at well-tolerated doses and schedule. The amount of troxacitabine incorporated into DNA was also not affected by the presence of gemcitabine. The effects of drug exposures on troxacitabine and gemcitabine nucleotide pools were analyzed, and although gemcitabine reduced phosphorylation of troxacitabine when cells were exposed at equal drug concentrations, there was no effect on phosphorylated pools at drug combinations that were synergistic. Synergy, evaluated using the CalcuSyn Software, was observed in all four cell-lines at multiple drug concentrations resulting in combination indices under 0.7 at Fa of 0.5 (50% reduction of cell growth). Statistical analysis was calculated by a one-way ANOVA with Dunnett as a post-test and the two-tailed unpaired t test using GraphPad prism software. For in vivo studies, we evaluated the effect of both drugs, alone and in combination, on the growth of established human pancreatic (AsPC-1) tumors implanted subcutaneously in nude mice. Mechanistic studies addressed incorporation of troxacitabine into DNA and intracellular levels of troxacitabine and gemcitabine metabolites. Synergistic efficacy was determined by the isobologram and combination-index methods of Chou and Talalay. The human pancreatic adenocarcinoma cell lines, AsPC-1, Capan-2, MIA PaCa-2 and Panc-1, were exposed to troxacitabine or gemcitabine alone or in combination, for 72 h, and the effects on cell growth were determined by electronic particle counting. Troxacitabine was recently evaluated as a first-line therapy in 54 patients with advanced adenocarcinoma of the pancreas and gave comparable overall results to those reported with gemcitabine in recently published randomized trials. Troxacitabine (Troxatyl™) is the first unnatural L-nucleoside analog to show potent preclinical antitumor activity and is currently under clinical investigation. Advanced pancreatic cancer thus remains a highly unmet medical need and new therapeutic agents are required for this patient population. Gemcitabine, a deoxycytidine nucleoside analog, is the current standard chemotherapy used as first-line treatment for patients with locally advanced or metastatic cancer of the pancreas, and extends life survival by 5.7 months.

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