A Study of Topical Duvelisib in Participants With Early-stage Mycosis Fungoides

This study will evaluate the safety and preliminary efficacy of duvelisib topical gel in participants with early-stage mycosis fungoides.

Key Inclusion Criteria: * Have a clinical diagnosis of Stage IA, Stage IB, or Stage IIA mycosis fungoides (cutaneous T-cell lymphoma), without clinically significant lymphadenopathy (≥1.5 centimeters). * Have 3 to 5 active, treatment-accessible mycosis fungoides lesions; ulcerated lesions and lesions to the face, neck, inguinal folds and intergluteal cleft are not considered suitable for administration of study drug. * Have baseline organ function (without transfusion support within 30 days of Cycle 1 Day 1) as defined by the protocol. Key Exclusion Criteria: * Have Stage IIB to IVB mycosis fungoides or Sezary syndrome. * Have histopathologically confirmed lymph node involvement. * Received systemic treatment with a phosphoinositide 3-kinases inhibitor in the 60 days prior to starting study drug. * Received topical steroids or other topical treatment (for example, nitrogen mustard) on mycosis fungoides lesions in the 2 weeks prior to starting study drug. Participants who receive such agents may become eligible for the study after a 14-day washout period. * Received systemic steroids, immunosuppressants (for example, cyclosporine, prednisone), psoralen ultraviolet A radiation therapy, narrow-band ultraviolet B light therapy, carmustine or other systemic therapies for mycosis fungoides in the 3 weeks prior to starting study drug. * Received electron beam irradiation within the 3 months prior to starting study drug. Note: other protocol-defined criteria apply.