3571 Background: Romidepsin (FK228 or depsipeptide) is a potent histone deacetylase inhibitor (HDI) with cytotoxic activity in preclinical models and significant activity in patients with cutaneous and peripheral T-cell lymphoma. As responses have been infrequently observed in solid tumors it is important to test HDIs in combination therapy. One strategy is to use HDIs to induce expression of genes that can be targeted therapeutically. Metastatic thyroid cancer refractory to RAI offers one such opportunity. In vitro, HDIs restore expression of the sodium iodine symporter in refractory cells and sensitivity to RAI. Methods: We initiated a phase I trial of romidepsin in a new schedule, administered on days 1, 3 and 5 with the goal of enhancing gene expression and evaluating RAI uptake in patients with RAI-refractory thyroid cancer. Results: We have completed the dose escalation stage of the protocol with 26 patients with solid tumors enrolled, including 9 patients with thyroid cancer. Doses tested were 1, 2...