The purpose of this study is to determine the overall cutaneous response rate (participants who achieve a complete response or partial response) based on the modified severity weighted assessment tool criteria.

This is a single-arm (group), open-label (all people know the identity of the intervention), and multicenter study. The study consists of 3 phases including, the screening phase (within 28 days prior to the start of study medication), treatment phase (participants will receive quisinostat) and follow-up phase (30 days after the last dose of study medication until death or clinical cutoff). Clinical cut-off is defined as when the last participant will be assessed with progressive disease or died, or 6 months after the last participant enrolled, whichever occurs first. However, if any participants are still receiving study treatment at the time of clinical cut-off, these participants will enter a long-term extension phase and will continue to receive study medication until a reason for discontinuation is met (ie, disease progression, toxicity, availability of other effective medications that the participant may receive, or treating physician advice). The long-term extension phase will continue for a maximum of 2 years beyond the clinical cut-off for primary analysis. Safety will be evaluated by the assessment of adverse events, vital signs, physical examination, 12-lead electrocardiogram, and clinical laboratory tests which will be monitored in this study. Any participants responding to treatment at the end of the 2-year long-term extension phase will be able to continue to receive quisinostat as long as they are assessed by the investigator as not meeting treatment discontinuation criteria; during this time, only information related to serious adverse events will be collected.

Inclusion Criteria:

• Histopathologically confirmed cutaneous T-cell lymphoma (CTCL), either mycosis fungoides or sezary syndrome Stage Ib-IVa
• Relapsed or refractory (unresponsive) disease following at least 1 prior systemic therapy for CTCL, except psoralen and long-wave ultraviolet radiation (it is considered skin-directed therapy and not systemic therapy)
• Stable anti-pruritus regimen (topical corticosteroids or antihistamine) in the preceding 28 days
• Measurable disease with at least 1 skin lesion (patch, plaque, or tumor) 1 cm or greater than 1 cm in the longest diameter laboratory function tests and bone marrow test
• Agrees to protocol defined use of effective contraception
• Adequate laboratory function tests and bone marrow test

Exclusion Criteria:

• Prior histone-deacetylase inhibitor therapy for CTCL
• Concurrent systemic corticosteroid dose greater than 10 mg per day of prednisone or equivalent (stable use of 10 mg or less than 10 mg per day of prednisone for 1 month or more before study entry is allowed)
• Major surgery or radiotherapy within 3 weeks before the start of the study medication
• Unstable angina or heart attack within the preceding 12 months, congestive heart failure New York Heart Association Class II-IV, known presence of dilated, hypertrophic, or restrictive cardiomyopathy
• Inadequate gastrointestinal absorption status
• Use of potent inhibitors of CYP3A4/A5
• Positive human immunodeficiency virus