Dose escalation proceeds independently for each risk group. Cohorts of 3 patients are entered into each risk group. If 1 of 3 patients at a dose level experiences dose limiting toxicity (DLT), then 3 additional patients are accrued into this level. If 2 of 6 patients at a dose level experience DLT, then dose escalation stops and the maximum tolerated dose (MTD) is defined at the previous dose level. At least 6 patients must be treated at the MTD.