Key Inclusion Criteria:  Histologically confirmed CD20-positive MZL. Previously received ≥ 1 line of systemic therapy including anti-CD20 agent. Must have a documented failure to achieve at least partial response during the most recent systemic therapy or documented progressive disease after the most recent systemic therapy. Need for systemic therapy for MZL. Measurable disease by computed tomography or magnetic resonance imaging. Adequate bone marrow, liver and renal function  Key Exclusion Criteria:  Transformation to aggressive lymphoma. Requiring ongoing need for corticosteroid treatment. Clinically significant cardiovascular disease. Prior malignancy within the past 2 years. Active fungal, bacterial, and/or viral infection that requires systemic therapy.  Note: Other protocol defined Inclusion/Exclusion criteria may apply.