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Abstract Introduction: Myeloid/lymphoid neoplasms (MLNs) with rearrangement of FGFR1 on chromosome band 8p11 are rare but aggressive neoplasms characterized by heterogeneous presentation with myeloid and/or lymphoid proliferation, extramedullary involvement, and rapid progression to blast phase (Strati P, et al., Leuk Lymphoma. 2018;59:1672-1676). FGFR1 gets constitutively activated through fusion genes involving various partner genes, most frequently ZMYM2-FGFR1 or BCR-FGFR1 as consequence of a t(8;13)(p11;q12) or a t(8;22)(p11;q11), respectively. Chemotherapy is usually ineffective, effective targeted treatment has not been described, and allogeneic hematopoietic stem cell transplant (alloHSCT) is the only potentially curative option. Pemigatinib, a selective, potent, oral inhibitor of FGFR1, 2, and 3, has shown efficacy in patients with FGF/FGFR-activated tumors, including cholangiocarcinoma and urothelial carcinoma. We report interim results from the ongoing fight-203 study (NCT03011372) of pemigatinib in patients with FGFR1-rearranged MLNs. Methods: Fight-203 is a phase 2, open-label study enrolling patients ≥ 18 years of age with FGFR1-rearranged MLN. Patients enrolled in the study must have progressed on ≥ 1 prior treatment and be ineligible for alloHSCT. Patients receive a daily oral dose of pemigatinib 13.5 mg on a 21-day cycle (2 weeks on, 1 week off) until disease progression or unacceptable toxicity. The primary endpoint is overall clinical benefit rate, which includes complete clinical (CR) or partial clinical response (PR), and either complete or partial cytogenetic response (CCyR, PCyR). Secondary endpoints include duration of response/benefit, progression-free survival, overall survival, and safety/tolerability. Efficacy is assessed by evaluation of bone marrow histomorphology changes, standard cytogenetic and FISH evaluation of the FGFR1 rearrangement, and PET/CT scan. Results: At data cutoff (July 23, 2018), 14 patients were enrolled. Ten patients who had ≥ 1 response assessment were included in the analysis (Table). Patients received an average of 6.9 cycles of pemigatinib (range, 2-12 cycles). Median number of prior lines of therapy was 3 (range, 0-5), including 2 patients who received alloHSCT. Eight patients (80%) had a major CyR, including 6 patients with CCyR and 2 with PCyR. Eight patients (80%) had a CR or PR in bone marrow, peripheral blood, and extramedullary disease. One patient died of progression to myeloid blast crisis, 2 patients were bridged to alloHSCT, and 7 patients are ongoing. The most common treatment-emergent adverse events (AEs) were hyperphosphatemia (n=7 [70%]), diarrhea (n=5 [50%]) and anemia (n=5 [50%]); hyperphosphatemia was managed with diet and phosphate binders. Nine events in 4 patients (40%) were grade 3/4; 2 of these events (diarrhea and leukopenia) in 2 patients were related to pemigatinib. There were no drug-related AEs leading to dose interruption, dose reduction, or discontinuation. Conclusions: Pemigatinib showed promising efficacy, with an 80% major CyR rate accompanied by complete or partial remission, and was generally well tolerated by patients with FGFR1-rearranged MLN. The protocol was amended to allow continuous dosing, and the study is currently enrolling. Disclosures Verstovsek: Celgene: Membership on an entity's Board of Directors or advisory committees; Italfarmaco: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Gotlib:Blueprint Medicines: Consultancy, Honoraria, Research Funding; Deciphera: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Gilead: Consultancy, Research Funding; Promedior: Research Funding; Kartos: Consultancy; Incyte: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding. Mead:Celgene: Research Funding; Bristol-Myers Squibb: Consultancy; Evotek: Research Funding; ARIAD: Consultancy; Cell Therapeutics: Consultancy; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Elstar: Research Funding. Hochhaus:Bristol-Myers Squibb: Research Funding; Novartis: Research Funding; Incyte: Research Funding; Takeda: Research Funding; Pfizer: Research Funding. Kiladjian:AOP Orphan: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees. Hernandez Boluda:Incyte: Consultancy; Novartis: Consultancy. Asatiani:Incyte: Employment, Equity Ownership. Lihou:Incyte: Employment, Equity Ownership. Zhen:Incyte: Employment, Equity Ownership. Reiter:Incyte: Consultancy, Honoraria.