Cohorts of 3 patients are enrolled into each treatment arm at each dose level of gemcitabine. Toxicity for each arm is assessed independently. Once dose limiting toxicity (DLT) is observed at any dose level, 3 more patients are enrolled at the same dose level. The maximum tolerated dose (MTD) is defined as the dose at which no more than 1 of 6 patients experiences grade 4 toxicity, or no more than 4 of 6 patients experience grade 3 or worse toxicity (DLT). After the MTD is determined for each treatment arm, additional patients are treated at that dose level for the phase II portion of the study.