Inclusion criteria

1. Written informed consent obtained prior to any screening procedures
2. Age greater than or equal to 18 years old
3. Patients with biopsy-confirmed stages IB-IVA mycosis fungoides or Sézary syndrome. Patients with SS who have bone marrow involvement are also eligible. Patients with transformed CTCL are eligible.
4. Patients must have been treated with an HDAC inhibitor given for the treatment of CTCL. Patients must have had disease progression on or following treatment with an HDAC inhibitor. Patients are also eligible if they had an inadequate response to an HDAC inhibitor defined as stable disease as the best response after at least 3 months of therapy. Patients previously treated with an HDAC inhibitor are also eligible if they experienced intolerance due to adverse events.

Exclusion criteria

1. Patients with a history of visceral disease including CNS involvement (i.e. stage IVB CTCL). Note, patients who have SS with bone marrow involvement are eligible.
2. Impaired cardiac function
3. Concomitant use of drugs with a risk of causing torsades de pointes
4. Patients who have received chemotherapy or any investigational drug or undergone major surgery ≤ 3 weeks prior to starting study drug or who have not recovered from side effects of such therapy
5. Less than 3 months since prior electron beam therapy
6. Women who are pregnant or breast feeding, or women of childbearing potential (WOCBP) not willing to use a double method of contraception during the study and 3 months after the end of treatment.

Other protocol-defined inclusion/exclusion criteria may apply