ALBERT

Description: Key Points Depletion of host regulatory T cells with IL2DT improves efficacy of haploidentical NK cell therapy for refractory acute myeloid leukemia. Depletion of Treg and persistence of NK cells for ≥7 days after NK cell adoptive transfer predicts beneficial clinical responses.

Description: DT2219 is a recombinant fusion protein composed of catalytic and translocation domains of diphtheria toxin (DT) fused with bispecific single chain variable fragments (sFv) of antibodies targeting human CD19 and CD22 cell surface receptors. Preclinical testing determined that the combination of two different sFvs and a toxin on the same single-chain molecule resulted in efficient toxin delivery and greater anticancer activity compared to monomeric immunotoxins sFv alone or anti-CD19 and anti-CD22 parental monoclonal antibodies. Murine studies demonstrated clinical efficacy and enhanced therapeutic effect with repetitive dosing (Vallera D, Clin Cancer Res 2009). We conducted a phase 1 study to assess safety, to determine the maximum tolerated dose (MTD) and to evaluate preliminary efficacy in patients with chemotherapy refractory B cell leukemia or lymphoma expressing CD19 and/or CD22. DT2219 was administered in a single course intravenously over 4 hours (1-2 hours for subsequent doses) every other day for 4 total doses. The starting dose was 0.5 µg/kg/dose (2.0 ug/kg/course) (1/1000th of the MTD in rabbits). The dose was escalated by accelerated design until dose limited toxicity (DLT) was observed. Twenty-two patients were enrolled, with a median age of 55 years (range, 34-78). Five patients had pre-B acute lymphoblastic leukemia (4 refractory, 1 with extra-medullary disease, 1 had chronic lymphocytic leukemia (CLL) and 7 had non-Hodgkin lymphoma). All were chemo-refractory with a median of 4 prior therapies. All patients had biopsy-confirmed tumor expression of CD19 and/or CD22. The first 9 patients were treated at doses ranging from 0.5 ug/kg/dose to 20 ug/kg/dose. No drug was detectable in those patients and they experienced no DLT or clinical responses. DT2219 was detectable in the serum at doses ≥40 µg/kg/dose (N=13). Dose levels were 40 µg/kg/day N=5; 60 µg/kg/day N=5; 80 µg/kg/day N=3. The area under the curve (AUC ) range was 1,104 -1,346 and the half-life ranged from 54-84 minutes. After a single course of DT2219 we observed partial remission in a 77 year old patient with rituximab/chemotherapy-refractory CLL (dose level 40 µg/kg). The 40% reduction in the abdominal tumor mass is shown in Figure 1. This patient did not experience any DLT. A second partial remission was observed in 53 year old patient with relapsed marginal zone lymphoma (dose level 60 µg/kg) who experienced a DLT of capillary leak syndrome. After regulatory approval, that patient received a second treatment course 8 weeks later at a reduced dose of 40 µg/kg which resulted in a complete remission (Figure 2). Both patients are alive and in remission at 6 and 4 months, respectively, after therapy. Adverse events were observed in all patients who received ≥40 µg/kg. The most common was grade 1-2 capillary leak syndrome, grade 1-2 hematologic toxicity, elevated liver function tests and fatigue. Two patients experienced DLTs: at the 40 µg/kg dose (grade 3 lower extremity weakness) and 60 µg/kg dose ( grade 4 neutropenia and grade 3 capillary leak). All adverse reactions resolved completely within one week. Immunogenicity and formation of neutralizing antibodies is a major barrier in development of DT-based immunotoxins. Remarkably, none of the 8 patients with B-cell lymphoma/CLL who had received rituximab within weeks preceding therapy with DT2219 had detectable neutralizing antibody. These patients had no detectable B cells at the time of study drug administration. This suggests that rituximab pre-treatment may prevent the development of neutralizing antibodies and allow for repetitive dosing. In conclusion, we have demonstrated promising clinical efficacy of the novel immunotoxin DT2219 in refractory B cell lymphoid malignancies. We determined that the biologically active dose of DT2219 is between 40-80 µg/kg and that repetitive dosing is feasible in patients pre-treated with rituximab. Phase 2 trial is in development. Figure 1. CT images of 77 year old patient with refractory CLL after a single course of DT1922 at dose level 40 µg/kg. Figure 1. CT images of 77 year old patient with refractory CLL after a single course of DT1922 at dose level 40 µg/kg. Figure 2. 53 year old female with CD22+CD19+ relapse marginal zone lymphoma received 2 courses of DT1922 ( 60 µg/kg and 40 µg/kg) and attained a complete resolution of subcutaneous tumor mass. Figure 2. 53 year old female with CD22+CD19+ relapse marginal zone lymphoma received 2 courses of DT1922 ( 60 µg/kg and 40 µg/kg) and attained a complete resolution of subcutaneous tumor mass. Disclosures Vallera: The Lion Fund, William Lawrence and Blanche Hughes Foundation : Other, Research Funding.

Description: Abstract 3611 Adoptive transfer of haploidentical natural killer (NK) cells can induce remissions in patients with refractory myeloid leukemia (AML). However, NK cells do not expand and persist in all patients despite lymphodepleting chemotherapy. In trials of adoptive NK cell therapy in solid tumors or lymphoma, host regulatory T cells (Treg) often expand in response to IL-2 given to stimulate donor NK cell expansion. Although murine studies report that Tregs inhibit NK cells, the influence of human Treg on NK cell proliferation and function is not well characterized. We studied the effect of allogeneic Tregs that were derived from human umbilical cord blood (UCB) as described by our group. Resting CFSE labelled NK cells or Teff were purified from healthy donors, and mixed with UCB Treg at various ratios. Unstimulated NK cells did not proliferate and thus IL-2 or IL-15 were added to the media at concentrations of 0.1, 0.25 and 0.5 ng/ml. In the absence of Treg, both cytokines induced equal NK cell proliferation at 5 days as measured by CFSE dilution in a concentration dependent manner. CFSE dilution was inhibited by Treg at a 1:1 ratio, especially at low cytokine concentrations. There were marked differences between the two cytokine conditions. Following IL-15 induced stimulation, the reduction in NK cell proliferation by Treg ranged from 1–35% (at different concentrations tested), whereas the inhibition of IL-2 stimulated NK cell proliferation ranged from 65–85%. Treg inhibition of NK cell proliferation could be measured at ratios as low as 1:8 in the presence of IL-2, but not IL-15. This inhibitory effect was partially explained by competition from CD25+ Tregs for IL-2. We measured Treg utilization of IL-2 by incubating NK cells with or without Treg in 0.5 ng/ml IL-2 for 4 days. The level of IL-2 with NK cells alone was 40 pg/ml vs. 17 pg/ml with Treg (compared to 330 pg/ml in IL-2-supplemented media without cells). Based on this data, we have incorporated host Treg depletion to enhance NK expansion after adoptive transfer to treat patients with refractory AML. As murine data from Blazar's group shows that CTL therapy is enhanced by Treg depletion, we added one dose of denileukin diftitox (ONTAK®, Eisai Inc) at 12 mg/kg to our lymphodepleting preparative regimen of fludarabine 25 mg/m2 × 5 days, cyclophosphamide 60 mg/kg × 2 days for 12 AML patients. Haploidentical NK cells (CD3- and CD19-depleted PBMCs and overnight activated with IL-2 1000 U/ml) were infused on Day 0, followed by 6 doses subcutaneous IL-2 (9 million units) given every other day to promote in vivo NK cell expansion. Eleven of 12 patients were evaluable, having received at least 4 of 6 planned doses of IL-2. Blood and marrow were collected 7 and 14 days after infusion to assess NK cell and Treg expansion, as well as leukemia clearance. Of the 10 patients with interpretable day 7 chimerism data, 9 had detectable donor DNA (median 68% donor DNA). At day 14, 4 of the 12 patients (33%) had successfully expanded NK cells in vivo, with absolute donor derived NK cell counts of 480, 530, 1470 and 12390 cells/μL blood, improving on our previous 10% rate of in vivo NK cell expansion which was observed with the same regimen, without Treg depletion. In the 4 patients who expanded NK cells in vivo, there were no detectable Treg (defined as a CD25+CD4+FoxP3+ lymphocyte population) at either day 7 or day 14. In contrast, the presence of a bona fide Treg population at either day 7 [range 9.5–53%] or day 14 [27–71%] correlated with a lack of in vivo NK cell expansion at day 14. Clinically, 8 of the 11 evaluable subjects cleared leukemia (72%), 7 of whom recovered neutrophils (63% CRp) and 6 of whom went on to best donor transplant (45%). In summary, we demonstrate in vitro and in vivo suppression of NK cell proliferation by IL-2 stimulated Treg. This effect is not seen in vitro with IL-15. We have shown that the absence of host Treg correlates with in vivo NK cells expansion. Although an increased rate of donor NK expansion was observed with a single dose of denileukin diftitox, it did not completely overcome the IL-2 induced host Treg expansion. Future trials testing additional doses of denileukin difitox or other methods of Treg depletion, as well as the use of IL-15 are planned. Disclosures: No relevant conflicts of interest to declare.

Description: Induced pluripotent stem cell (iPSC)-derived immune effector cells offer distinct advantages over existing patient- and donor-derived therapeutic approaches, including the use of a clonal master engineered iPSC line as a renewable source for the mass production of immune cells, which are available off-the-shelf for broad patient access. FT596 is an investigational, off-the-shelf, multi-antigen targeting, chimeric antigen receptor (CAR) natural killer (NK) cell therapy derived from a human clonal master iPSC line engineered with three anti-tumor modalities: (1) a proprietary CD19-targeting CAR; (2) a novel high-affinity, non-cleavable CD16 Fc receptor that enables tumor targeting and enhanced antibody-dependent cell cytotoxicity in combination with a therapeutic monoclonal antibody (mAb); and (3) interleukin (IL)-15/IL-15 receptor fusion promoting cytokine-autonomous persistence. Preclinical in vivo models demonstrate potent multi-antigen targeting activity of FT596 against both CD19+ and CD19- tumor cell lines when combined with the anti-CD20 agent rituximab (Goodridge et al. 2019). FT596 is currently being investigated as a monotherapy and in combination with the anti-CD20 mAbs rituximab and obinutuzumab in a multicenter, Phase I clinical trial for the treatment of relapsed/refractory B-cell lymphoma and chronic lymphocytic leukemia. The trial will test up to four FT596 dose levels ranging from 30 to 900 million cells. We describe the demonstration of early clinical benefit of FT596 in a patient who received a single administration of FT596 as a monotherapy at the first dose level in the Phase I trial. The patient is a 76-year-old female with diffuse large B-cell lymphoma, germinal center B-cell subtype, who was initially diagnosed in January 2014. The patient received eight prior treatment regimens, including autologous stem cell transplant, rituximab in combination with engineered autologous T cells expressing antibody-coupled T-cell receptor, and, most recently, was refractory to an experimental combination therapy comprised of lymphodepleting chemotherapy followed by ex vivo expanded allogeneic NK cells, IL-2, and rituximab. The patient received fludarabine and cyclophosphamide lympho-conditioning followed by a single administration of 30 million cells of FT596 as monotherapy. During the 28-day follow-up period for safety, no dose-limiting toxicities were observed. No cytokine release syndrome, neurologic toxicity, or graft-versus-host disease of any grade was observed. Grade ≥3 treatment-emergent adverse events (AEs) included decreased white blood cell count, decreased neutrophil count, anemia, urinary tract infection with neutropenic fever, and hypertension. Notably, none of these AEs were considered related to FT596 by the treating investigator physician, except for decreased neutrophil count, which was considered possibly related to FT596 and resolved. Evidence of hematologic recovery was observed by the end of the 28-day follow-up safety period. On Study Day 29, the patient's tumor response assessment showed partial response by Lugano 2014 criteria, with a greater than 70% decrease in 18F-fluorodeoxyglucose uptake and greater than 50% reduction in tumor size. The patient remains on study and, per protocol and in collaboration with the U.S. Food and Drug Administration, is being considered for retreatment with a second cycle of FT596 monotherapy. This is the first-ever demonstration of clinical activity following treatment with an off-the-shelf, iPSC-derived CAR immune cell therapy and provides direct evidence that low doses of FT596 can induce short-term responses, opening the opportunity for multi-dosing strategies to deepen response and duration. Additional clinical, pharmacokinetic, and pharmacodynamic data from this patient will be provided at the time of the meeting. The Phase I trial is ongoing and is registered on clinicaltrials.gov: NCT04245722. Disclosures Bachanova: FATE: Research Funding; Kite: Membership on an entity's Board of Directors or advisory committees; Karyopharma: Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Gamida Cell: Membership on an entity's Board of Directors or advisory committees, Research Funding; Incyte: Research Funding. Janakiram:Takeda, Fate, Nektar: Research Funding. Payne:Fate Therapeutics: Current Employment, Current equity holder in publicly-traded company. Wong:Fate Therapeutics: Current Employment, Current equity holder in publicly-traded company. Cooley:Fate Therapeutics: Current Employment, Current equity holder in publicly-traded company. Valamehr:Fate Therapeutics, Inc: Current Employment, Current equity holder in publicly-traded company. Chu:Fate Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company; Roche Holding AG: Current equity holder in publicly-traded company. Miller:GT Biopharma: Consultancy, Patents & Royalties, Research Funding; Vycellix: Consultancy; Onkimmune: Honoraria, Membership on an entity's Board of Directors or advisory committees; Nektar: Honoraria, Membership on an entity's Board of Directors or advisory committees; Fate Therapeutics, Inc: Consultancy, Patents & Royalties, Research Funding. OffLabel Disclosure: Cyclophosphamide and fludarabine will be used as lympho-conditioning therapy prior to FT596 administration.

Description: Introduction: Relapsed/refractory AML and MDS present a clinical challenge. Despite FDA approval of multiple new targeted agents, many patients lack actionable mutations and have exhausted conventional chemotherapeutic options. We have shown that NK cell infusions after lymphodepleting chemotherapy can induce remissions in relapsed/refractory AML. However, this cell-based therapy lacks antigen specificity. To address this, we developed GTB-3550 TriKE, a novel CD16/IL-15/CD33 Tri-Specific Killer Engager (TriKE). GTB-3550 TriKE is comprised of two single chain variable fragments (scFvs); one that binds CD16 on NK cells and one that binds CD33 on myeloid malignancies, plus an IL-15 linker bridging the CD16 and CD33 scFvs for sustained cell activation. Pre-clinical data shows specific NK cell activation and targeted cytolytic activity in xenogeneic AML mouse models. Methods: Patients with CD33+ malignancies (primary induction failure or relapsed AML with failure of one reinduction attempt or high risk MDS progressed on two lines of therapy) age 18 and older were eligible (NCT03214666) if they had adequate renal, hepatic, cardiac and lung function and absolute lymphocyte count (ALC) ≥ 200 cells/µL or absolute circulating CD56+/CD3- NK cell count 〉25 cells/µL within the 14 days prior to start of therapy. The primary endpoint is to identify the maximum tolerated dose (MTD) of GTB-3550 TriKE. Correlative objectives include the number, phenotype, activation status and function of NK cells and T cells. During phase 1, each patient received the GTB-3550 TriKE at 5-200 mcg/kg/day (in 6 cohorts) for 3 weeks (infusion block #1, #2, and #3) of 96 hours continuous infusion separated by a 72-hour rest. Two patients enrolled per dose cohort. Disease response assessed by bone marrow biopsy between Days 21-42. Results: Four patients have enrolled, two at 5 mcg/kg/day and two at 10 mcg/kg/day, and three have completed therapy. Two patients with stable disease and one had substantial disease progression in the setting of a FLT-3 ITD mutation (Figure 1A). The first patient at 5 mcg/kg/day had stable disease after course 1 and qualified for retreatment with a second course of 3 weeks of GTB-3550 TriKE due to improved platelet transfusion requirements. Of the four complete courses of treatment delivered, all were delivered on schedule without treatment interruptions. The previous MTD of continuous infusion rhIL-15 was 2 mcg/kg/day (Conlon et al, Clin. Ca Res.) with associated fevers, tachycardia and constitutional symptoms. Validating our pre-clinical data showing decreased IL-15 potency when sandwiched between the CD16 and CD33 scFvs, patients treated with GTB-3550 TriKE treatment displayed no signs of clinical immune activation or SAE's in the 5 and 10 mcg/kg/day cohorts. Correlative studies have shown reproducible NK cell activity in all patients. NK cell activation (CD69+) increases early during treatment (Figure 1B). The greatest NK cell proliferation (Ki-67+) starts at day 3, is maximal at Day 8, and maintained above baseline at Day 15 and Day 22 (Figure 1C). This correlated with an increase proportion and absolute number of NK cells during treatment (Figure 1D). Targeted delivery of IL-15 to NK cells through the anti-CD16 engager showed preferential proliferation of NK cells and significantly less effect on CD8+ T cells. Additionally, the frequency of NK cells and absolute lymphocyte count decreased early during the continuous infusion of the GTB-3550 TriKE, which rebounded after the 72-hour rest period to higher levels than the pre-treatment baseline. Conclusion: This novel GTB-3550 TriKE administered by continuous infusion led to NK cell proliferation in all patients at initial dose levels and no clinically significant targeted toxicity. Though no objective responses were seen at the initially low dose cohorts tested, these early data suggest proof of principle that the drug has immune activity in humans. Subsequent patients given higher dose levels will determine if NK cell proliferation translates into clinical efficacy. Disclosures Weisdorf: Incyte: Research Funding; FATE Therapeutics: Consultancy. Vallera:GT Biopharma: Consultancy, Patents & Royalties, Research Funding. Schroeder:GT Biopharma: Consultancy, Current equity holder in publicly-traded company. Felices:GT Biopharma: Consultancy. Miller:Fate Therapeutics, Inc: Consultancy, Patents & Royalties, Research Funding; GT Biopharma: Consultancy, Patents & Royalties, Research Funding; Vycellix: Consultancy; Onkimmune: Honoraria, Membership on an entity's Board of Directors or advisory committees; Nektar: Honoraria, Membership on an entity's Board of Directors or advisory committees. OffLabel Disclosure: GTB-3550 TriKE is a cytokine immune engager binding to CD16 on NK cells, CD33 on myeloid blasts and IL-15 between the two engager components.