No abstract

Quizartinib has clinical activity in patients with relapsed/refractory AML, particularly those with FLT3-ITD, and is associated with an acceptable toxicity profile.

636 Activating mutations in the FLT3 RTK are present in ∼30% of AML patients (pts), who have a significantly worse prognosis than pts with wild type (WT) FLT3. AC220 is a novel 2nd generation RTK inhibitor with potent in vitro and in vivo activity in FLT3- and KIT-dependent tumor cell lines. It is highly selective for both WT and mutant FLT3 with significant activity against KIT. A first-in-human Phase 1 study investigating AC220 in predominantly relapsed or refractory AML pts, unselected for FLT3 mutations, was recently completed, using a standard 3+3 dose escalation with 50% dose increments. AC220 was administered once daily as an oral solution initially with an intermittent dosing (ID) regimen: 14 days on and 14 days off (1 cycle). Starting dose was escalated from 12 to 450 mg/day ID. Additional cohorts were investigated on a continuous dosing (CD) regimen: 200 and 300 mg/day for 28 days (1 cycle). A total of 76 pts (46 male, 30 female) were dosed with AC220. Median age was 60 yrs (23-86), median number of prior therapies was 4 (0-9), 12 pts had prior allogeneic transplant and 3 elderly pts (≥72 yrs) unfit for induction chemotherapy were previously untreated. Eighteen pts (24%) had FLT3 ITD mutations, 47 (62%) were WT, and 11 (14%) were undetermined. Pts were also evaluated for PK, phosphorylated (p) FLT3, pKIT, pSTAT5, and ex vivo plasma inhibitory activity. AC220 plasma exposure was sustained between dose intervals and continued to increase in a dose-proportional manner from 12 to 450 mg with a half-life of ∼1.5 days. An active metabolite, AC886, was detected that has similar potency and activity to AC220. Patient plasma at '12 mg potently inhibited pFLT3 in ex vivo FLT3-ITD cell lines and complete inhibition of pFLT3 in WT cell lines was observed at higher doses. Target inhibition was also observed, with suppression of pFLT3, pSTAT5 and pKIT in peripheral blasts. The most commonly reported possibly drug-related AEs were GI events, peripheral edema, and dysguesia, which were Grade ≤2. DLT was observed at 300 mg CD and 200 mg CD was declared as the MTD. Two pts at 300 mg CD had possibly study drug-related DLTs with grade 3 QTc prolongation, but had confounding factors including concomitant medications known to prolong QTc. Responses (IWG criteria) were observed in 23 (30%) pts. PR and CR were observed as low as the 18 and 40 mg cohorts, respectively. Most responses occurred within cycle 1. Overall, 9 (12%) pts had a complete response (CR) with 2 CR, 5 CRi, and 2 CRp. One of these pts also had complete resolution of leukemia cutis. In addition, 14 (18%) pts achieved PR. Overall median duration of response (MDOR) was 14 (4-62+) weeks and overall median survival (MS) was 14 (1-68+) weeks. In FLT3-ITD pts the MDOR was 12 (4-27+) weeks and the MS was 18 (3-42) weeks. In FLT3-WT pts the MDOR was 32 (8-62+) weeks and the MS was 11 (1-68+) weeks. 10 (56%) of 18 FLT3-ITD pts were responders (1 CR, 3 CRi, 6 PR), 9 (19%) of 47 FLT3-WT pts (1 CRi, 2 CRp, 6 PR), and 4 (36%) of 11 undetermined pts (1 CR, 1 CRi, 2 PR). At 200 mg CD (MTD expansion), 4 of 6 FLT3-ITD pts responded (1 CR, 1 CRp, 1 CRi, 1 PR). Of the 4 responders, 2 failed prior treatment with sorafenib and 2 previously refractory pts went onto transplant. The 2 FLT3-ITD non-responders had 6 and 8 prior lines of therapy, respectively. These encouraging efficacy results and an acceptable safety profile warrant continued evaluation of AC220 as monotherapy and in combination therapy for the treatment of AML. Phase 2 studies in FLT3-ITD positive and WT pts are in progress. Disclosures: Cortes: Ambit Biosciences: Research Funding. Foran:Ambit Biosciences: Research Funding. Ghirdaladze:Ambit Biosciences: Research Funding. DeVetten:Ambit Biosciences: Research Funding. Zodelava:Ambit Biosciences: Research Funding. Holman:Ambit Biosciences: Research Funding. Levis:Ambit Biosciences: Consultancy, Research Funding. James:Ambit Biosciences: Employment. Zarringkar:Ambit Biosciences: Employment. Gunawardane:Ambit Biosciences: Employment. Armstrong:Ambit Biosciences: Employment. Padre:Ambit Biosciences: Employment. Wierenga:Ambit Biosciences: Employment. Corringham:Ambit Biosciences: Employment. Trikha:Ambit Biosciences: Employment.

Introduction: Diffuse large B-cell lymphoma (DLBCL) is the most common histologic subtype of non-Hodgkin Lymphoma (NHL) accounting for approximately 30-40% of NHL cases. Approximately 40% of all newly diagnosed DLBCL patients are either refractory or relapsed following initial response to therapy and represent a population with high unmet need for new therapeutic strategies to achieve or regain disease remission. Because of the near ubiquity and persistence of CD20 expression in B-cell malignancies, there is strong rationale to develop compounds with novel mechanisms of action targeting CD20. However, CD20's non-internalizing nature has, to date, leveraged only cytotoxic mechanisms such as antibody-dependent cellular cytotoxicity to which resistance can arise and has impeded the development of agents that internalize a cytotoxic payload. MT-3724 is a novel engineered toxin body designed to overcome this limitation by combining the specific target selectivity of a single chain variable fragment with the lethality of a genetically fused Shiga-like toxin A subunit that facilitates both internalization and cell killing by inactivating ribosomal protein synthesis. MT-3724 has been shown to specifically bind and kill CD20+ malignant human B-cells in vitro (IC50 <1 nM) and in CB17 SCID and PDX mice (Rajagopalan 2016; Huang 2018). As a direct-kill immunotoxin against CD20, MT-3724 has achieved clinical response in subjects with relapsed NHL regardless of acquired resistance to other treatments. Thus, MT-3724 could be a valuable addition to the armamentarium of treating DLBCL. Study Design and Methods: MT-3724 is being evaluated in this Phase 2 study as monotherapy (NCT02361346) in adult subjects with histologically confirmed, relapsed or refractory DLBCL. The primary objective is to determine the efficacy of MT-3724 as monotherapy based on the overall response rate (ORR) by the revised Lugano Classification for Lymphoma adjusted according to immunomodulatory therapy criteria (LYRIC), hereinafter referred to as "revised Lugano Criteria" (Cheson 2014, 2016). Overall response rate is defined as the proportion of subjects with either a complete response or a partial response as determined by independent, blinded central review. Secondary objectives include safety, progression‐free survival, investigator‐assessed ORR, duration of response, and overall survival as well as pharmacokinetics and pharmacodynamics, immunogenicity, and quality of life. To be eligible, patients must have histologically confirmed, relapsed or refractory DLBCL, have received at least 2 standard of care systemic NHL treatment regimens, and have measurable disease according to the revised Lugano criteria. Since rituximab and MT-3724 compete for the same CD20 domain, subjects must have serum rituximab levels < 500 ng/mL before the start of treatment to allow adequate binding of MT-3724. This single arm phase 2 study is being conducted in three stages, where the first two stages will follow the Simon two-stage optimal design [Simon 1989]. Subjects will be enrolled in successive cohorts with each cohort evaluated for efficacy before opening the subsequent cohort, toward a total sample size of 100 subjects. Subjects will receive MT-3724 as an IV infusion over 1 hour on Days 1, 3, 5, 8, 10 and 12 of a 21-day treatment cycle. All subjects will be treated with a 50 µg/kg/dose of MT-3724, which was the recommended Phase 2 dose, as determined in the Phase 1/1b portion of the trial. Sites are open and recruiting in the US, Canada, and Europe. Disclosures Persky: Sandoz: Consultancy; Debiopharm: Other: Member, Independent Data Monitoring Committee; Bayer: Consultancy; Morphosys: Other: Member, Independent Data Monitoring Committee. Musteata:Arensia EM: Other: Principal Investigator; Institute of Oncology: Employment. Strack:Molecular Templates, Inc.: Employment. Burnett:Molecular Templates, Inc.: Employment. Wilson:Molecular Templates, Inc.: Employment. Baetz:Bristol Meyers Squibb: Other: Advisory board; Merck: Other: Advisory board; Gilead: Other: Advisory board; Roche: Other: Advisory board.

1507 FMS-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) mutations in acute myeloid leukemia (AML) are associated with early relapse after standard chemotherapy and poor survival. Quizartinib is an oral FLT3 receptor tyrosine kinase (RTK) inhibitor that inhibits both ITD-mutant and wild-type FLT3 and displays efficacy in preclinical models of AML. In this first-in-human study, quizartinib was evaluated in patients with relapsed or refractory AML who were enrolled irrespective of FLT3-ITD status. Quizartinib was administered orally on an intermittent schedule (ID, 14 days on, 14 days off) at 12–450 mg/day or continuously (CD) at 200 or 300 mg/day. A total of 76 patients (61% male; 90% white; median age 60 years [23–86]) received quizartinib; median number of prior treatments was 3 (0–12), including 16% with a prior hematopoietic stem cell transplant. FLT3-ITD status was 22% positive (+) (n=17), 49% negative (−) (n=37), and 29% indeterminate/not tested (ind) (n=22). Plasma inhibitory activity assays using plasma samples collected at trough timepoints showed a high degree of FLT3 inhibition at low dose levels (18 mg/day and higher), indicative of potent in vivo FLT3 inhibition. Marked inhibition of FLT3 phosphorylation was also demonstrated in peripheral blood following administration of quizartinib. Responses were observed in 23/76 patients (30%), with 10 patients (13%) having a best response of any type of complete remission (CR=morphologic leukemia-free state with absolute neutrophil count >1×109/L and platelet count ≥100×109/L, and normal marrow differential with <5% blasts with no Auer rods; CRp=CR with incomplete platelet recovery; CRi=CR with incomplete hematologic recovery). Responses included 2 CR, 3 CRp, 5 CRi, and 13 (17%) partial remissions (PR, same hematologic values as CR but with a decrease of '50% in % blasts in the bone marrow aspirate to 5% to 25%; a value of <5% blasts was considered a PR if Auer rods were present). Responses were observed at doses as low as 18 mg/day ID. Nine of 17 FLT3-ITD(+) patients responded (53%; 1 CR, 1 CRp, 2 CRi, 5 PR) compared to 5/37 FLT3-ITD(−) patients (14%; 2 CRp, 3 PR) and 9/22 with FLT3-ITD(ind) status (41%; 1 CR, 3 CRi, 5 PR). Median duration of response was 13.3 weeks (95% confidence interval [CI]: 11.0, 29.0 weeks): 9.7 weeks for FLT3-ITD(+) patients (95% CI: 4.1, 29); 23.7 weeks for FLT3-ITD(−) patients (95% CI: 11, not reached); and 12.4 weeks for FLT3-ITD(ind) patients(95% CI: 8, 53). Median overall survival was 14.0 weeks (95% CI: 10.9, 18.6 weeks): 18.3 weeks for FLT3-ITD(+) patients (95% CI: 10.6, 27.1); 9.7 weeks for FLT3-ITD(−) patients (95% CI: 5.6, 14.0); and 18.7 weeks for FLT3-ITD(ind) patients (95% CI: 14.0, 21.1). Quizartinib was generally well tolerated. The most common drug-related AEs (>10% incidence) were nausea (16%), prolonged ECG QT interval (12%), vomiting (11%), and dysgeusia (11%); most were ≤Grade 2. The maximum tolerated dose was 200 mg/day CD. The dose-limiting toxicity was QTcF interval prolongation, which occurred in 38% of subjects at 300 mg/day CD and in 6% at 200 mg/day CD. QTcF interval prolongation was dose dependent, and there were no associated arrhythmias. Grade 3 AEs related to quizartinib occurring in >1 patient were prolonged ECG QT interval (4 patients [5%]), anemia (3 patients [4%]), and fatigue (2 patients [3%]). Two treatment-related Grade 4 AEs (thrombocytopenia and hypoalbuminemia) were reported in 1 patient each (1%). The encouraging efficacy results in both FLT3-ITD(+) and FLT3 ITD(−) patients and an acceptable safety profile in this high-risk population support continued clinical evaluation of quizartinib with lower doses. A recent 333 patient Phase 2 study in a similar population given quizartinib at 200, 135, or 90 mg/day has been completed, and, considering the clinical activity observed at low doses, quizartinib is also being studied in patients with relapsed or refractory AML at 30 and 60 mg/day. Disclosures: Cortes: Novartis: Consultancy. Kantarjian:Ambit Biosciences: Consultancy. Foran:Ambit Biosciences: Consultancy. Ghirdaladze:Ambit Biosciences: Consultancy. Zodelava:Ambit Biosciences: Consultancy. Borthakur:Ambit Biosciences: Consultancy. Gammon:Ambit Biosciences: Employment. Trone:Ambit Biosciences: Employment. Armstrong:Ambit Biosciences: Employment. James:Ambit Biosciences: Employment. Levis:Ambit Biosciences: Consultancy.