Introduction: Previously reported results from an ongoing study of polatuzumab vedotin (PoV), an antibody drug conjugate (ADC) containing the anti-mitotic MMAE targeting CD79b, in combination with rituximab, cyclophosphamide, doxorubicin, and prednisone (R-CHP) demonstrated an acceptable safety profile in patients (pts) with previously untreated DLBCL. The dose escalation (esc) phase established a recommended phase 2 dose of PoV at 1.8 mg/kg. We report updated safety and efficacy results for this multicenter, open-label Phase Ib/II study of PoV combined with R-CHP in previously untreated DLBCL pts (ClinicalTrials.gov NCT01992653).Methods: In the esc phase of this study, pts aged 60-80 years, with newly diagnosed or relapsed/refractory B-cell NHL were given PoV intravenously 1.0, 1.4, or 1.8 mg/kg with R-CHP every 21 days for a total of 6 or 8 cycles. In the dose expansion (exp) phase, pts were restricted to those with previously untreated DLBCL and aged 18-80 years. Investigator assessments for anti-tumor activity by Cheson 2007 were performed following 4 cycles of study treatment (tx) and at the end of study tx (EOT).Patients: DLBCL patient characteristics as of February 5, 2016 are included in Table 1. The majority of the subjects were IPI 4-5.Results: Of the 36 safety evaluable pts, the most common adverse events (AEs) in > 20% of pts were fatigue (36%) and diarrhea (30%), nausea (31%), and neutropenia (25%). 44% had at least one Gr 3/4 AE including neutropenia (22%), febrile neutropenia (14%), hypertension (8%), thrombocytopenia (4%), leukopenia (4%), oral fungal infection (4%), dysphagia (4%), vomiting (4%), asthenia (4%), fatigue (4), decreased appetite (4%), hyponatremia (4%), malnutrition (4%), chylothorax (4%), back pain (4%), motor dysfunction (4%), and confusional state (4%).Serious AEs were reported in 33% of pts, with 17 different events. These included 4 episodes of febrile neutropenia, 2 of neutropenia, 2 of pneumonia, and 1 each of E. coli UTI, oral fungal infection, hyponatremia, malnutrition, chylothorax, pulmonary embolism, pyrexia, pathologic femur fracture, and worsening of rheumatoid arthritis. 33% of pts experienced peripheral neuropathy (PN). The reported terms included PN, paresthesia, muscular weakness, neuralgia, and motor dysfunction. All neurologic AEs attributed to PoV were Gr 1 except one PN that was Gr 2 and improved to Gr 1 after PoV dose reduction. There were 2 pts that discontinued study treatment due to AEs (one with a Gr 2 tremor who is still in CR, and the other with Gr 2 UTI who ultimately progressed) while 5 (14%) had PoV dose reductions (2 PN, 2 neutropenia, 1 weight decrease and asthenia). No Gr 5 AEs were reported among the exp cohort.At the time of this report, response assessments were available in 26 pts. All pts in the esc phase have completed study tx and 8 in exp cohort have completed study tx. Interim and EOT PET response is summarized in Table 2. Among the 2 pts that developed progressive disease in the exp phase, one pt had a new axillary lesion and subsequently died despite salvage tx; the other patient had a CR at interim evaluation, but stopped all tx after 5 cycles due to E. coli UTI and subsequently developed recurrent disease.Conclusions: Early results show that PoV plus R-CHP has an acceptable safety profile in pts with previously untreated DLBCL. Although the protocol-specified MTD was not formally reached, the recommended phase 2 dose of PoV was established at 1.8 mg/kg based on the overall safety and tolerability profile at that dose. The pts treated at the phase 2 dosing were strongly weighted in the high risk category by IPI and in that context, efficacy is promising and will be updated at the time of presentation. [Display omitted] [Display omitted] DisclosuresMorschhauser:Janssen: Honoraria; Roche: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Gilead Sciences: Consultancy, Honoraria; Servier: Consultancy, Honoraria. Bartlett:Gilead: Consultancy. Sharman:Gilead: Research Funding; Seattle Genetics: Research Funding; Celgene: Research Funding; Pharmacyclics: Research Funding; Acerta: Research Funding; TG Therapeutics: Research Funding. Haioun:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sandoz: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees. Kolibaba:Celgene: Research Funding; Acerta: Research Funding; Janssen: Research Funding; Amgen: Research Funding; Cell Therapeutics: Research Funding; GSK: Research Funding; Novartis: Research Funding; Gilead: Consultancy, Research Funding; Genentech: Research Funding; Pharmcyclics: Research Funding; Seattle Genetics: Research Funding; TG Therapeutics: Honoraria, Research Funding. Chen:Seattle Genetics: Consultancy. Jones:Genentech, Inc.: Employment. Penuel:Genentech/Roche: Employment. Lee:Genentech, Inc.: Employment. Salles:Gilead: Honoraria, Research Funding; Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Mundipharma: Honoraria; Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Roche/Genentech: Consultancy, Honoraria, Research Funding.