PX-12 (1-methylpropyl 2-imidazolyl disulfide) is a novel small molecule inhibitor of thioredoxin-1, a small protein over-expressed in many human cancers that is associated with aggressive tumor proliferation, angiogenesis, and drug resistance. This study is being conducted to determine the maximally tolerated dose of PX-12 delivered as a 72-hour infusion over days 1, 2, and 3 of a 21-day cycle in patients with advanced or metastatic cancer.