PXD101 is a small molecule HDAC inhibitor being investigated for its role in the treatment of a wide range of solid and hematologic malignancies either as a single-agent, or in combination with other active anti-cancer agents, and is currently being evaluated in a Phase II clinical trial for the treatment of multiple myeloma. HDAC inhibitors represent a new mechanistic class of anti-cancer therapeutics that target HDAC enzymes, and have been shown to: arrest growth of cancer cells (including drug resistant subtypes); induce apoptosis, or programmed cell death; promote differentiation; inhibit angiogenesis; and sensitize cancer cells to overcome drug resistance when used in combination with other anti-cancer agents. PXD101 may block the growth of tumors and the growth of blood vessels from surrounding tissue to the tumor. It may also make tumor cells more sensitive to other anticancer drugs. PXD101 is a type of histone deacetylase (HDAC) inhibitor and angiogenesis inhibitor. Preclinical data showing PXD101 down-regulation of thymidylate synthase (TS), an in vivo target of 5-fluorouracil (5-FU) provide a rationale for combination of PXD101 with 5-FU.