This is an open-label, multicenter, Phase 2 study to evaluate the safety and tolerability of mogamulizumab given Q4W following initial weekly induction in adult participants with relapsed/refractory MF and SS subtypes of CTCL. The study is composed of a 28-day Screening Period during which participants are screened for entry into this study, followed by a treatment period of up to 2 years from Cycle 1 Day 1.

Inclusion Criteria:

• Histologically confirmed diagnosis of MF or SS

  • Stage IB, II-A, II-B, III, or IV;

• Participants who have failed at least one prior course of systemic therapy (e.g., interferon, bexarotene, photopheresis, anti-neoplastic chemotherapy). Psoralen plus ultraviolet light therapy (PUVA) is not considered a systemic therapy.

Exclusion Criteria:

• Current evidence of large cell transformation;
• Prior treatment with mogamulizumab;
• History of allogeneic transplant.