Intrinsic and acquired drug resistance remain major obstacles in the treatment of cancer. Accumulating evidence indicates that in some malignancies P-glycoprotein can confer resistance, and that its reversal can improve therapeutic outcome. Clinical trials investigating P-glycoprotein antagonists have been hampered by the occurrence of unpredictable pharmacokinetic interactions, which have required dose reductions of the chemotherapeutic agents to avert excessive toxicity. XR9576 is a new P-glycoprotein antagonist that is more potent, has prolonged activity, and is potentially devoid of significant pharmacokinetic interactions. This phase I study seeks to identify the safety of XR9576 administration in combination with vinorelbine and determine the extent, if any, of a pharmacokinetic interaction between these two drugs. Clinical responses will also be determined.