Background: Duvelisib (IPI-145), an oral dual inhibitor of PI3K-δ and -γ, is being evaluated as an oral therapeutic for hematologic malignancies as a monotherapy or in combination with anti-CD20 antibodies. Methods: CONTEMPO (NCT02391545) is an ongoing 2-part, 2-arm, Phase 1b/2 study evaluating the safety, pharmacokinetics, pharmacodynamics and efficacy of duvelisib combined with the anti-CD20 antibodies rituximab (DR) or obinutuzumab (DO) in patients with previously untreated CD20+ follicular lymphoma. Duvelisib is administered at 25 mg BID continuously in 28-day treatment cycles combined either with rituximab (375 mg/m²for 4 weekly doses, then 1 dose every 2 cycles) or obinutuzumab (1000 mg for 4 weekly doses, then 1 dose every 2 cycles). We report results from Part 1 which evaluated the safety of the combinations by monitoring adverse events (AEs) and dose limiting toxicities (DLTs). Results: Twelve patients were treated in Part 1 of the study (6 DR and 6 DO). At the time of data cut-off, patients had received duvelisib for a median of 2.5 (DR) and 1.9 (DO) months. No DLT occurred with DR. One DLT of treatment-related Grade 3 elevated lipase occurred with DO (Cycle 1). No serious AEs occurred on either arm, and no patient discontinued treatment. Four patients (1 DR; 3 DO) had ≥ Grade 3 AEs (DR: amylase increased, lipase increased; DO: neutropenia, ALT increased, lipase increased). Duvelisib was rapidly absorbed (t_max ~1 hour; C_max ~910 ng/mL; single dose); after multiple doses, plasma steady state levels were ~500-540 ng/mL and approximately half of the duvelisib was transformed into its metabolite IPI-656. Postdose serum cytokine levels were decreased by Cycle 1 Day 8 compared to baseline. Preliminary activity will be reported at the time of the meeting. Conclusions: Combination of duvelisib 25 mg BID with rituximab or obinutuzumab was well tolerated in these patients, with one DLT observed. Both DR and DO arms were continued to Part 2 of the study, which is ongoing. Clinical trial information: NCT02391545