Background: Parsaclisib is a potent and highly selective PI3Kδ inhibitor that has shown clinical benefit in patients with relapsed/refractory (R/R) B-cell malignancies. In this phase 2 study (CITADEL-205; NCT03235544, EudraCT 2017-003148-19), the efficacy and safety of parsaclisib was evaluated in patients with R/R mantle cell lymphoma (MCL)., Methods: Patients ≥18 years old with pathologically confirmed R/R MCL and prior treatment with 1-3 systemic therapies, with (cohort 1) or without (cohort 2) previous Bruton kinase inhibitor (BTKi) treatment, received oral parsaclisib 20 mg once-daily (QD) for 8 weeks, then either parsaclisib 20 mg once-weekly (weekly dosing group [WG]) or parsaclisib 2.5 mg QD (daily dosing group [DG]). The primary endpoint was objective response rate (ORR)., Findings: At the primary analysis data cutoff on January 15, 2021, 53 patients in cohort 1 (BTKi-experienced) (WG, n = 12; DG: n = 41) and 108 patients in cohort 2 (BTKi-naive) (WG, n = 31; DG: n = 77) had received parsaclisib monotherapy. The BTKi-experienced cohort was closed after an interim analysis demonstrated limited clinical benefit. In the BTKi-naive cohort, the ORR (95% CI) for DG (dosing selected for further study) was 70.1% (58.6%-80.0%), with a complete response rate (95% CI) of 15.6% (8.3%-25.6%) and a median duration of response (95% CI) of 12.1 (9.0-not evaluable) months. Treatment-emergent adverse events (TEAEs) occurred among 90.7% (98/108) of all treated patients in the BTKi-naive cohort. Grade ≥3 TEAEs occurred among 62.0% (67/108) of patients, including diarrhoea (13.9%, 15/108) and neutropenia (8.3%, 9/108). Parsaclisib interruption, reduction, or discontinuation due to TEAEs occurred among 47.2% (51/108), 8.3% (9/108), and 25.0% (27/108) of patients, respectively. Fatal TEAEs were experienced by six patients and determined to be treatment-related in one patient., Interpretation: Parsaclisib, a potent, highly selective, PI3Kδ inhibitor demonstrated meaningful clinical benefits and a manageable safety profile (25.0% discontinuation rate, low incidences of individually reported grade ≥3 or serious adverse events) in R/R MCL patients with no prior BTKi therapy. Limited clinical benefit was observed with parsaclisib monotherapy in patients who had previously received BTKi treatment. Future development of PI3K inhibitors for NHL will require further investigation of dose optimisation to improve safety and long-term survival., Funding: Incyte Corporation., Competing Interests: PLZ reports consulting or advisory role, and involvement of speakers bureau for Beigene, Bristol Myers Squibb, Celltrion, EUSA Pharma, Gilead, Incyte Corporation, Janssen-Cilag, Kyowa Kirin, Merck Sharp & Dohme, Novartis, Roche, Servier, Takeda, and TG Therapeutics; advisory role for ADC Therapeutics, Sandoz, and Secura Bio. MTrněný reports honoraria, consulting or advisory role, travel and accommodations expenses for AbbVie, Bristol Myers Squibb, Gilead Sciences, Janssen, Roche, and Takeda; honoraria, consulting or advisory role for Amgen, Incyte Corporation, and MorphoSys. VR reports research funding for ArgenX and Astex; membership on a board or advisory committee for Bristol Myers Squibb, Epizyme, Gilead, GlaxoSmithKline, Incyte Corporation, Infinity, Merck Sharp & Dohme, Nanostring, and Roche; membership on a board or advisory committee, honoraria for PharmaMar; membership on a board or advisory committee, consultancy for Servier. VRZ reports consulting or advisory role, speakers bureau for Gentili, Gilead, Italfarmaco, Janssen, Merck Sharp & Dohme, Roche, Servier, and Takeda. JW reports research funding for GlaxoSmithKline, Novartis, and Roche; advisory role and lecture honoraria for AbbVie, Amgen, Celgene, Gilead, Janssen, Novartis, Roche, Servier, and Takeda. GR reports consulting or advisory role, and involvement of speakers bureau for Bristol Myers Squibb, Celgene, Janssen, Roche, and Takeda; consulting or advisory role for EUSA Pharma. MC reports research funding for Amgen, Astra Zeneca, BeiGene, Bristol Meyers, Celgene, Gilead, Incyte, Innocare, Johnson and Johnson, Merck, Pharmacyclics; consulting for BeiGene, Epizyme, Gilead. CD reports advisory board for AbbVie, AstraZeneca, Beigene, and Janssen; travel and accommodation expenses for AbbVie, AstraZeneca, and Janssen. CP reports advisory board for Abbvie, Incyte Corporation, and Janssen. FP reports consulting or advisory role, speakers bureau, travel and accommodations expenses for AbbVie (Investigator in sponsored clinical trials), Amgen, Jazz Pharmaceuticals, and Novartis Pharma SAS; consulting or advisory role, travel and accommodations expenses for Daiichi Sankyo; travel and accommodations expenses for Janssen; research funding for Novartis Pharma SAS (Institutional). WJurczak reports research funding for Bayer, Gilead, Incyte Corporation, Mei Pharma, and TG Therapeutics. MTaszner reports consulting or advisory role, travel and accommodations expenses for Roche and Takeda. SP reports honoraria for AbbVie, Bristol Myers Squibb, Celgene, Gilead, and Janssen. FZ reports employment and stock ownership for Incyte Corporation. DJD reports former employment and stock ownership for Incyte Corporation. WJiang reports former employment and stock ownership for Incyte Corporation. AG reports employment and stock ownership for Incyte Corporation. AM reports consultancy, membership on an entity's board of directors or advisory committees, research funding, speakers bureau for Seattle Genetics and TG Therapeutics; research funding for Affimed, Celgene/Bristol Myers Squibb, fortyseven Inc/Gilead, Innate Pharmaceuticals, Juno Pharmaceuticals/Bristol Myers Squibb, Kite/Gilead, Merck, OncoTartis, Roche-Genentech, and Takeda; consultancy, membership on an entity's board of directors or advisory committees, research funding, speakers bureau for Incyte Corporation. JHC, VD, and PV report no relevant disclosures to declare., (© 2023 The Authors.)