Each treatment cycle will consist of 6 doses of MT-3724 to be administered over 12 days followed by at least 9 days of observation but longer intervals between dosing cycles will be allowed at the investigator's discretion (e.g., 6 doses of MT-2734 over 12 days followed by 16 days observation). Subjects in this extension study may continue to receive up to 6 additional cycles in the absence of clear disease progression or toxicity. If a subject achieves complete remission, two additional cycles will be attempted, after which dosing may be suspended prior to completion of the maximum 6 cycles.