Background: Patients (pts) with iNHL treated with front-line immunochemotherapy may benefit from an alternative, chemotherapy-free regimen at relapse. Zandelisib, a potent, selective, and structurally differentiated oral PI3Kδ inhibitor, achieved an 87% response rate, with median duration of response not reached in iNHL when given as a monotherapy or in combination with R. A low rate ( < 10%) of Grade ≥ 3 immune-mediated adverse events of special interest associated with PI3kδ inhibitors is observed in patients administered zandelisib on an intermittent schedule (IS) (JCO 2020 38:15_suppl, 8016). An open-label, phase 2 study (TIDAL, NCT03768505) of zandelisib as monotherapy is ongoing in pts with relapsed/refractory follicular lymphoma (FL) and marginal zone lymphoma (MZL). Methods: The COASTAL study is a randomized, open-label, controlled multicenter phase 3 trial to investigate the safety and efficacy of zandelisib in combination with R versus standard immunochemotherapy in pts with iNHL. Key eligibility criteria: adults with relapsed or refractory FL or MZL who received ≥1 prior lines of therapy which must have included an anti-CD20 antibody in combination with chemotherapy or lenalidomide (L); at least one bi-dimensionally measured lesion > 1.5 cm; adequate bone marrow, renal and hepatic function; ECOG performance status score of 0 to 1. Key exclusion criteria: histologically confirmed diagnosis of FL grade 3b or transformed disease; administration of 2 prior immunochemotherapy regimens; prior PI3K inhibitor therapy; known lymphomatous involvement of the central nervous system. Subjects will be randomized 1:1 to receive R-zandelisib or immunochemotherapy (R-CHOP or R-B) and stratified by type and number of prior treatment regimens, histology, and duration of treatment-free interval after last therapy. Zandelisib will be given in a 28-day cycle comprising of daily dosing for 2 cycles followed by IS dosing on days 1-7 of each 28-day subsequent cycle for a duration of 2 years. Rituximab or immunochemotherapy will be given for a total of 6 cycles. Disease response will be assessed by an Independent Response Review Committee according to the modified Lugano Classification. Radiographic tumor assessment will be performed approximately every 12 weeks for the first 9 months, every 16 weeks for the next 12 months, and every 24 weeks thereafter. The primary efficacy endpoint is progression-free survival. The major secondary endpoints include ORR, complete response rate, overall survival, and safety. The trial will enroll approximately 534 pts in ̃200 sites globally and will begin enrollment in mid-2021. Clinical trial information: NCT04745832