Purpose: Phosphatidylinositol-3-kinase (PI3K) inhibitors have shown activity in relapsed or refractory (R/R) indolent non-Hodgkin lymphoma (iNHL). PI3K inhibitors have been hampered by poor long-term tolerability and toxicity, which interfere with continuous use. Umbralisib, a dual inhibitor of PI3Kδ/casein kinase-1ε, exhibits improved selectivity for PI3Kδ compared with other PI3K inhibitors. This phase IIb trial was designed to evaluate the efficacy and safety of umbralisib in patients with R/R iNHL., Patients and Methods: In this multicohort, open-label, phase IIb study, 208 patients with R/R marginal zone, follicular, or small lymphocytic lymphoma (MZL, FL, or SLL) unresponsive to prior treatments (≥ 1 MZL; ≥ 2 FL/SLL), including ≥ 1 anti-CD20-based therapy, were administered umbralisib 800 mg orally once daily until disease progression, unacceptable toxicity, or study withdrawal. Primary end point is overall response rate; secondary end points include time to response, duration of response, progression-free survival, and safety., Results: The median follow-up is 27.7 months (efficacy) and 21.4 months (safety). The overall response rate was 47.1%, and tumor reduction occurred in 86.4% of patients. The median time to response was 2.7-4.6 months. The median duration of response was not reached for MZL, 11.1 months for FL, and 18.3 months for SLL. Median progression-free survival was not reached for MZL, 10.6 months for FL, and 20.9 months for SLL. At least one grade ≥ 3 treatment-emergent adverse event (TEAE) was reported in 53.4% of patients. TEAEs led to umbralisib discontinuation in 32 patients (15.4%). A total of 31 patients (14.9%) discontinued because of a treatment-related adverse event. Grade ≥ 3 TEAEs reported in ≥ 10% of patients: neutropenia (11.5%) and diarrhea (10.1%). Increased ALT/AST (grade ≥ 3) occurred in 6.7%/7.2% of patients., Conclusion: Umbralisib achieved meaningful clinical activity in heavily pretreated patients with iNHL. The safety profile was manageable, with a relatively low incidence of immune-mediated toxicities and adverse event-related discontinuations., Competing Interests: Nathan H. FowlerEmployment: BostongeneConsulting or Advisory Role: Roche/Genentech, TG Therapeutics, Verastem, Bayer, Celgene, NovartisResearch Funding: Roche, Celgene, Gilead Sciences, TG Therapeutics, Novartis, AbbVie, BeiGene Felipe SamaniegoConsulting or Advisory Role: Astex Pharmaceuticals, ADC Therapeutics, Imbrium Therapeutics Wojciech JurczakResearch Funding: TG Therapeutics Nilanjan GhoshConsulting or Advisory Role: Seattle Genetics, TG Therapeutics, AstraZeneca, Verastem, Pharmacyclics/Janssen, Karyopharm Therapeutics, Genmab, Bristol-Myers Squibb, Gilead Sciences, Adaptive Biotechnologies, Beigene, AbbVie, IncyteSpeakers' Bureau: AbbVie, Janssen Oncology, Kite/Gilead Sciences, Seattle Genetics, AstraZeneca, Bristol-Myers Squibb, Celgene, Pharmacyclics/Janssen, EpizymeResearch Funding: Pharmacyclics, TG Therapeutics, Genentech/Roche, Seattle Genetics, Bristol-Myers Squibb/Celgene, Gilead Sciences Enrico DerenziniConsulting or Advisory Role: AstraZenecaResearch Funding: TG-Therapeutics, ADC Therapeutics James A. ReevesResearch Funding: Sarah Cannon Research Institute, Eli Lilly, Tesaro, TG Therapeutics, Genentech, Celgene, Merck, Bristol-Myers Squibb, Boston Biomedical Inc, AstraZeneca, NovoCure, Calithera Biosciences, Novartis, Guardant Health, Acerta Pharma, Rhizen Pharmaceuticals, Takeda, Onconova Therapeutics, Sanofi, CTI Biopharma, Eisai, Janssen Chan Y. CheahHonoraria: Roche/Genentech, Janssen-Cilag, TG Therapeutics, Loxo/Lilly, AstraZeneca, Bristol-Myers Squibb, Gilead Sciences, Ascentage Pharma, MDSConsulting or Advisory Role: Janssen-Cilag, Roche/Genentech, TG Therapeutics, Loxo/Lilly, Gilead Sciences, AstraZeneca, Bristol-Myers Squibb, Ascentage Pharma, MDSSpeakers' Bureau: Janssen-CilagResearch Funding: Roche, Celgene, AbbVieTravel, Accommodations, Expenses: Roche Tycel PhillipsHonoraria: Seattle Genetics, Incyte, Pharmacyclics, Bayer, Gilead Sciences, GenentechConsulting or Advisory Role: Seattle Genetics, Pharmacyclics, Incyte, Genentech, Bayer, Gilead Sciences, Curis, Kite/Gilead, Celgene, GenmabResearch Funding: AbbVie, Pharmacyclics/Janssen, Bayer Ewa Lech-MarandaConsulting or Advisory Role: Roche, Amgen, AbbVie, Astellas Pharma, Novartis, Janssen-Cilag, Sanofi/Aventis, Takeda, Gilead Sciences Bruce D. ChesonConsulting or Advisory Role: TG Therapeutics, AbbVie, Pharmacyclics/Janssen, Morphosys, Celgene, Karyopharm Therapeutics, Epizyme, Gilead Sciences, SymBio Pharmaceuticals, Parexel, Merck, Glaxo Smith Kline, Beigene, Kite, Reddy Biosimilar, Bayer, CelgeneResearch Funding: TG Therapeutics, Trillium Therapeutics, Seattle Genetics, Bristol-Myers Squibb, Gilead Sciences, Pharmacyclics, AbbVie, AstraZenecaTravel, Accommodations, Expenses: SymBio Pharmaceuticals Paolo F. CaimiConsulting or Advisory Role: Genentech, Seattle Genetics, TG Therapeutics, Kite Pharma, ADC Therapeutics, Bayer, Amgen, Verastem, EpizymeSpeakers' Bureau: CelgeneResearch Funding: Genentech, ADC Therapeutics Lori A. LeslieConsulting or Advisory Role: Kite/Gilead, TG Therapeutics, Celgene/BMS, Seattle Genetics, AbbVie, PCYC/Janssen, ADC Therapeutics, BeiGene, AstraZenecaSpeakers' Bureau: Kite Pharma, Celgene/BMS, Seattle Genetics, AbbVie, PCYC/Janssen, ADC Therapeutics, BeiGene, AstraZeneca, Epizyme, Karyopharm Julio C. ChavezConsulting or Advisory Role: Kite/Gilead, Novartis, Karyopharm Therapeutics, MorphoSys, TeneoBio, AbbVie, JanssenSpeakers' Bureau: AstraZeneca, BeiGene, MorphoSys, EpizymeResearch Funding: Merck Gustavo FonsecaHonoraria: Amgen, CelgeneConsulting or Advisory Role: Amgen, Celgene, Bayer, Abvie/Pharmacyclics, Dava Oncology, KaryopharmSpeakers' Bureau: Amgen, CelgeneResearch Funding: Celgene, Amgen, TG Therapeutics, Verastem OncologyTravel, Accommodations, Expenses: Amgen, Celgene Sunil BabuStock and Other Ownership Interests: Fort Wayne Medical Oncology & Hematology, Lutheran HospitalHonoraria: Bristol-Myers Squibb, Alexion Pharmaceuticals, Lilly, Bayer, AstraZenecaConsulting or Advisory Role: Bristol-Myers Squibb, Alexion Pharmaceuticals, AstraZeneca, argenx, Boehringer Ingelheim, Bayer, Kite Pharma, Janssen OncologySpeakers' Bureau: Alexion PharmaceuticalsResearch Funding: Bristol-Myers Squibb, Novartis, Genentech/Roche, AstraZeneca/MedImmune, Janssen Oncology, Amgen, TG Therapeutics, AbbVie, Lilly, Alexion Pharmaceuticals, Merck, Novartis, Syndax, Nektar, Sanofi, argenxTravel, Accommodations, Expenses: Bristol-Myers Squibb, Alexion Pharmaceuticals, Lilly, Janssen Oncology, Genentech/Roche Daniel J. HodsonResearch Funding: Gilead Sciences John M. BurkeConsulting or Advisory Role: Genentech/Roche, AbbVie, Seattle Genetics, Bayer, AstraZeneca, Adaptive Biotechnologies, Verastem, MorphoSys, Kura Oncology, Epizyme, BeiGene, Kymera, NovartisSpeakers' Bureau: Seattle Genetics, Beigene Jeff P. SharmanLeadership: US OncologyConsulting or Advisory Role: Pharmacyclics, Celgene, TG Therapeutics, Genentech, AbbVie, Acerta Pharma/AstraZeneca, Beigene, PfizerResearch Funding: Pharmacyclics, Genentech, Celgene, Acerta Pharma, Gilead Sciences, Seattle Genetics, TG Therapeutics, Merck, Takeda John M. PagelConsulting or Advisory Role: Gilead Sciences, AstraZeneca, Actinium Pharmaceuticals, BeiGene, Loxo Hari P. MiskinEmployment: TG TherapeuticsLeadership: TG TherapeuticsStock and Other Ownership Interests: TG TherapeuticsTravel, Accommodations, Expenses: TG Therapeutics Peter SportelliEmployment: TG TherapeuticsLeadership: TG TherapeuticsStock and Other Ownership Interests: TG TherapeuticsTravel, Accommodations, Expenses: TG Therapeutics Owen A. O'ConnorEmployment: TG TherapeuticsLeadership: TG TherapeuticsBoard Appointments, Stock, and Other Ownership Interests: TG Therapeutics, Kymera, Myeloid Therapeutics, NomoCan PharmaceuticalsHonoraria: MundipharmaConsulting or Advisory Role: Mundipharma Research, Astex PharmaceuticalsResearch Funding: Merck, Celgene/Bristol-Myers Squibb, Seagen, Mundipharma, Astex Pharmaceuticals, ADC Therapeutics Michael S. WeissEmployment: TG TherapeuticsLeadership: TG TherapeuticsStock and Other Ownership Interests: TG TherapeuticsTravel, Accommodations, Expenses: TG Therapeutics Pier Luigi ZinzaniConsulting or Advisory Role: Sanofi, Verastem, Celltrion, Gilead Sciences, Janssen-Cilag, Bristol-Myers Squibb, SERVIER, Sandoz, MSD, Immune Design, Celgene, Portola Pharmaceuticals, Roche, EUSA Pharma, Kyowa Hakko Kirin, TG Therapeutics, TakedaSpeakers' Bureau: Verastem, Celltrion, Gilead Sciences, Janssen-Cilag, Bristol-Myers Squibb, SERVIER, Sandoz, MSD, Immune Design, Celgene, Portola Pharmaceuticals, Roche, EUSA Pharma, Kyowa Hakko KirinNo other potential conflicts of interest were reported.