A Study of Aplidin (Plitidepsin) 3 h iv in Subjects With Relapsing or Refractory Multiple Myeloma

This is a phase II study to determine the efficacy following treatment with Aplidin® 5 mg/m2, given as a 3 h iv infusion every 2 weeks, in patients with relapsed or refractory multiple myeloma (MM) and to obtain the following : * Additional pharmacokinetic information for Aplidin® given as 3-hour IV infusion every 2 weeks in patients with MM. * To obtain additional genomic and pharmacodynamics information on MM and Aplidin. * To assess the safety and tolerability of Aplidin® given as 3-hour IV infusion every 2 weeks in patients with MM alone or in combination with dexamethasone given orally as a 20 mg daily for 4 days * To determine the response rate in the second cohort of patients following treatment with Aplidin®, given as a 3 hour infusion every 2 weeks, plus dexamethasone given orally as a 20 mg daily for 4 days, starting the same day of Aplidin® administration, as a second treatment stage in patients with suboptimal response to Aplidin® as single agent (progressive disease after three cycles or stable disease after four cycles).