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MT-3724, a novel engineered toxin body comprised of an anti-CD20 single-chain variable fragment genetically fused to Shiga-like Toxin A subunit, is capable of binding to and internalizing against CD20, inducing cell killing via permanent ribosomal inactivation. This study evaluated MT-3724 in patients with relapsed/refractory B-cell non-Hodgkin lymphoma (r/rNHL). This open-label, multiple-dose phase Ia/b trial included a dose escalation in patients with r/rNHL according to a standard 3+3 design. Primary objectives were to determine the MTD and pharmacokinetics/pharmacodynamics. In a dose expansion study at MTD in serum rituximab-negative patients with diffuse large B-cell lymphoma (DLBCL), primary objectives were safety, tolerability, and pharmacokinetics/pharmacodynamics. Twenty-seven patients enrolled. MTD was 50 μg/kg/dose with 6,000 μg/dose cap. Thirteen patients experienced at least one grade ≥3 treatment-related adverse events; the most common grade ≥3 event was myalgia (11.1%). Two patients (75 μg/kg/dose) experienced grade 2 treatment-related capillary leak syndrome. Overall objective response rate was 21.7%. In serum rituximab-negative patients with DLBCL or composite DLBCL (n = 12), overall response rate was 41.7% (complete response, n = 2; partial response, n = 3). In patients with detectable baseline peripheral B cells, treatment resulted in dose-dependent B-cell depletion. The proportion of patients with anti-drug antibodies (ADA) increased during treatment and the majority appeared to be neutralizing based on an in vitro assay; nevertheless, tumor regression and responses were observed. MT-3724 demonstrated efficacy at the MTD in this population of previously treated patients with r/rDLBCL, with mild-to-moderate immunogenic safety events. Significance: This work describes the safety and efficacy of a new pharmaceutical pathway that could provide a treatment option for a subset of patients with a critical unmet therapeutic need. The study drug, MT-3724, is capable of targeting B-cell lymphomas via a unique, potent cell-killing mechanism that appears to be promising.

Supplementary Table 1. Additional Study Eligibility Criteria. Supplementary Table 2. Pharmacokinetic Serum Sample Collection. Supplementary Table 3. Summary of Adverse Events and Discontinuations. Supplementary Table 4. Summary of Serum MT-3724 PK Parameters by Treatment on Cycle 1 Day 1. Supplementary Table 5. Summary of Serum MT-3724 PK Parameters by Treatment on Cycle 1 Day 12. Supplementary Table 6. Best Overall Response, Objective Response Rate, and Disease Control Rate (FAS). Supplementary Table 7. Summary of ADA Incidence by Actual Dose. Supplementary Figure 1. Study Design for Dose Escalation and Dose Expansion. Supplementary Figure 2. CONSORT diagram. Supplementary Figure 3. Individual CD19+ Nadir Percent Change from Baseline.

MT-3724, a novel engineered toxin body comprised of an anti-CD20 single-chain variable fragment genetically fused to Shiga-like Toxin A subunit, is capable of binding to and internalizing against CD20, inducing cell killing via permanent ribosomal inactivation. This study evaluated MT-3724 in patients with relapsed/refractory B-cell non-Hodgkin lymphoma (r/rNHL). This open-label, multiple-dose phase Ia/b trial included a dose escalation in patients with r/rNHL according to a standard 3+3 design. Primary objectives were to determine the MTD and pharmacokinetics/pharmacodynamics. In a dose expansion study at MTD in serum rituximab-negative patients with diffuse large B-cell lymphoma (DLBCL), primary objectives were safety, tolerability, and pharmacokinetics/pharmacodynamics. Twenty-seven patients enrolled. MTD was 50 μg/kg/dose with 6,000 μg/dose cap. Thirteen patients experienced at least one grade ≥3 treatment-related adverse events; the most common grade ≥3 event was myalgia (11.1%). Two patients (75 μg/kg/dose) experienced grade 2 treatment-related capillary leak syndrome. Overall objective response rate was 21.7%. In serum rituximab-negative patients with DLBCL or composite DLBCL (n = 12), overall response rate was 41.7% (complete response, n = 2; partial response, n = 3). In patients with detectable baseline peripheral B cells, treatment resulted in dose-dependent B-cell depletion. The proportion of patients with anti-drug antibodies (ADA) increased during treatment and the majority appeared to be neutralizing based on an in vitro assay; nevertheless, tumor regression and responses were observed. MT-3724 demonstrated efficacy at the MTD in this population of previously treated patients with r/rDLBCL, with mild-to-moderate immunogenic safety events.Significance:This work describes the safety and efficacy of a new pharmaceutical pathway that could provide a treatment option for a subset of patients with a critical unmet therapeutic need. The study drug, MT-3724, is capable of targeting B-cell lymphomas via a unique, potent cell-killing mechanism that appears to be promising.

Background: Engineered toxin bodies (ETBs), composed of an engineered Shiga-like Toxin A subunit genetically fused to an antibody-binding domain, can force receptor internalization, induce potent cell-kill via enzymatic and permanent inactivation of ribosomes, and may not be subject to resistance mechanisms of other targeted agents. MT-5111, a de-immunized 55 kD ETB targeting HER2 in solid tumors, also binds to an epitope distinct from trastuzumab and pertuzumab, which may permit combination strategies with other HER2 targeting agents. Methods: The primary objective is to determine maximum tolerated dose (MTD) of MT-5111 monotherapy in adult patients (pts) with advanced HER2+ solid tumors. Secondary objectives are PK, efficacy, and immunogenicity. Using a modified 3+3 design, the dose-escalation part of the study includes the following 7 cohorts: 0.5, 1, 2, 3, 4.5, 6.75, and 10 µg/kg. Three dose-expansion cohorts will follow for HER2+ breast cancer, gastro-esophageal cancer, and any other HER2+ tumors. All pts will receive MT-5111 weekly as a 30-min IV infusion in each 21-d treatment (tx) cycle (C) until disease progression (PD), unacceptable toxicity, death, or withdrawn consent (NCT04029922). Results: As of the data cut in December 2020, 16 pts were treated; cancer types included breast (n=6), gastric (n=1), colon (n=1), gallbladder (n=5), and other solid tumors (n=3). Mean age was 64 years (range, 34-78); 37.5% were male. Pts received a median of 4 prior lines of systemic therapies (range, 1-8). No G4 or G5 TEAEs occurred. Six pts had 11 G3 TEAEs; the most common were increased AST and dyspnea (both n=2). Three pts had tx-emergent serious adverse events (abdominal distension [n=1]; dyspnea [n=2]). Tx-related TEAEs occurred in 8 (50%) pts; the most common was fatigue (n=5, 31.3%) and all were ≤ grade 2 in nature, except for one grade 3 event of dyspnea. No cardiac TEAEs, clinically significant changes in cardiac biomarkers (troponin, electrocardiogram, left ventricular ejection fraction), or cases of capillary leak syndrome were observed. Fifteen pts discontinued with PD; 1 pt in cohort 5 (4.5 µg/kg) is on tx with stable disease. To date, no DLTs have been observed and the MTD has not been reached. One pt in cohort 2 (1 µg/kg) had resolution of all hepatic lesions (sub-centimeter lesions pre-tx) at the end of C8; however, the pt came off study due to clinical progression at the end of C10. PK data for the first 5 cohorts matched simulations based on non-human primate studies. Conclusions: MT-5111 was well tolerated with no clinically significant cardiotoxicity. Continued dose escalations are ongoing. Citation Format: Zev A. Wainberg, Monica M. Mita, Minal A. Barve, Erika P. Hamilton, Andrew J. Brenner, Frances Valdes, Daniel Ahn, Joleen Hubbard, Jason Starr, Christine Burnett, Joshua Pelham, Eric T. Williams, Banmeet S. Anand, Thomas Strack, Andrés Machado Sandri, Brian A. Van Tine. Phase 1 study of the novel immunotoxin MT-5111 in patients with HER-2+tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT130.

TPS258 Background: Engineered toxin bodies (ETBs) are comprised of a proprietarily engineered form of Shiga-like Toxin A subunit genetically fused to antibody-like binding domains. ETBs work through novel mechanisms of action & are capable of forcing internalization, self-routing through intracellular compartments to the cytosol & inducing potent cell-kill via the enzymatic & permanent inactivation of ribosomes. MT-5111 is a de-immunized ETB targeting HER2+ solid tumors. Its novel mechanism of action, via enzymatic ribosome inactivation, may not be subject to resistance mechanisms that exist for tyrosine kinase inhibitors, antibody-drug conjugates, or antibody modalities. MT-5111 binds an epitope on HER2, distinct from trastuzumab or pertuzumab, that may provide for combination potential with other HER2-targeting agents. MT-5111 is a 55 kilodalton protein & may have improved tumor penetration capability. The objective of this trial will be to determine the safety, tolerability, & maximum tolerated dose (MTD) of MT-5111 in patients (pts) with advanced HER2+ solid tumors. Methods: This Phase 1, first-in-human, open-label, dose escalation & expansion study will evaluate MT-5111 monotherapy in pts with HER2-positive solid tumors. The primary objective is to determine the MTD; secondary objectives include pharmacokinetics, tumor response & immunogenicity. Part 1 consists of MT-5111 dose escalation (0.5, 1.0, 2.0, 3.0, 4.5, 6.75, 10µg/kg/dose) based on a modified 3+3 design (n≤42 pts); Part 2 (dose expansion) will evaluate MT-5111 at the MTD in ≤98 pts. All pts will be administered MT-5111 over 30 min via IV infusion on Days 1, 8, & 15 of each 21-day cycle until disease progression, unacceptable toxicity, death, withdrawal of consent, or another reason for withdrawal. Part 1 will include pts with any HER2+ solid cancers. Part 2 will enroll 3 expansion cohorts: HER2+ breast (BC), HER2+ gastric or gastroesophageal junction adenocarcinomas (collectively referred as gastroesophageal adenocarcinomas [GEA]) & other HER2+ solid cancers. Immunohistochemistry (IHC) status must be 2+ or 3+, regardless of in situ hybridization (ISH) results; if no IHC is available for pts with BC or GEA, ISH criteria per the American Society of Clinical Oncology College of American Pathologists guidelines will be used. In metastatic cases, HER2 positivity must be demonstrated on metastatic lesions. Pts with HER2+ BC should have had ≥2 lines of HER2-directed therapy; pts with HER2+ GEA should have received or been intolerant to trastuzumab. Pts with evaluable disease may be included in Part 1; in Part 2, all pts must have ≥1 measurable lesion per Response Evaluation Criteria in Solid Tumors v1.1. Further details can be found on clinicaltrials.gov (NCT04029922). Enrollment, which began in September 2019, is ongoing. Clinical trial information: NCT04029922.

Background: Engineered toxin bodies (ETBs) are comprised of a proprietarily engineered form of Shiga-like toxin A subunit (SLT-A) genetically fused to antibody-like binding domains. ETBs work through novel mechanisms of action and are capable of forcing internalization, self-routing through intracellular compartments to the cytosol, and inducing potent cell-kill via the enzymatic and permanent inactivation by SLT-A of ribosomes. MT-3724 represents a novel ETB modality comprised of an anti-CD20 single-chain variable fragment genetically fused to SLT-A, is capable of efficient internalization once bound to CD20 and can induce potent direct cell-kill via enzymatic ribosome inactivation. In a Phase 1/1b dose escalation/expansion study of MT-3724 monotherapy in subjects with heavily pretreated (including CD20 monoclonal antibodies) relapsed or refractory B-cell non-Hodgkin lymphoma (r/rNHL) the most common grade ≥3 treatment-related adverse events were myalgia and neutropenia (n=3 each). Dose-limiting toxicities (DLTs) were indicative of innate immune response. In subjects with negative rituximab serum concentrations there was a 38% objective response rate (Hamlin et al. ASH 2019). MT-3724 is currently being studied in five ongoing (three actively recruiting, two in development) Phase 2 studies for r/rNHL. This study will evaluate the safety, tolerability, recommended phase 2 dose (RP2D), efficacy, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of MT-3724 in two cohorts of subjects with r/r mantle cell lymphoma (r/rMCL). Study Design and Methods: This phase 1, multicenter, open-label, single arm study will include adults ≥18 yrs with histologically confirmed r/rMCL (histology plus expression of Cyclin D1 in association with CD20 and CD5 or evidence of t(11:14)) who have received ≥2 prior systemic therapies (including anti-CD20 antibody alone or in combination with other agents) and have ≥1 measurable lesion (Lugano criteria). Subjects with CNS involvement or recent treatment with rituximab (within 84 days of study initiation; if received within 12-37 weeks of start of treatment, serum rituximab level must be confirmed to be negative [ Disclosures Wang: Juno:Consultancy, Research Funding;Dava Oncology:Honoraria;Kite Pharma:Consultancy, Other: Travel, accommodation, expenses, Research Funding;InnoCare:Consultancy;MoreHealth:Consultancy;Targeted Oncology:Honoraria;OMI:Honoraria, Other: Travel, accommodation, expenses;Oncternal:Consultancy, Research Funding;Pulse Biosciences:Consultancy;Molecular Templates:Research Funding;OncLive:Honoraria;Verastem:Research Funding;Acerta Pharma:Research Funding;Celgene:Consultancy, Other: Travel, accommodation, expenses, Research Funding;AstraZeneca:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;Janssen:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;Pharmacyclics:Consultancy, Honoraria, Other: Travel, accommodation, expenses, Research Funding;BioInvent:Research Funding;Guidepoint Global:Consultancy;VelosBio:Research Funding;Loxo Oncology:Consultancy, Research Funding;Lu Daopei Medical Group:Honoraria;Nobel Insights:Consultancy;Beijing Medical Award Foundation:Honoraria.Burnett:Molecular Templates, Inc.:Current Employment.Strack:Molecular Templates, Inc.:Current Employment.

Background: Engineered toxin bodies (ETBs) are a distinct class of targeted immunotoxins in development by Molecular Templates as anticancer therapeutics. ETBs have a novel mechanism of action that drives a potent and targeted response mediated by antibody-like binding, cellular internalization, and enzymatic ribosomal inhibition via the delivery of a Shiga-like toxin subunit A (SLTA). MT-3724 comprises an anti-CD20 single chain variable fragment genetically fused to SLTA with an approximate molecular weight of 55 kDa and is being developed for the treatment of relapsed or refractory diffuse large B-cell lymphoma (r/rDLBCL). MT-3724 is currently being studied in three ongoing phase 1/2 studies for r/rDLBCL. Methods: MT-3724 is being evaluated in this phase 2a study (NCT03645395) in combination with LEN in adult patients with histologically confirmed, relapsed, or refractory CD20+ B-cell NHL. The primary objectives of this study are to determine the safety and tolerability, including the maximum tolerated dose (MTD), of MT-3724+LEN. Secondary objectives include pharmacokinetics, pharmacodynamics, immunogenicity, and tumor response. The study is being conducted in two parts. Part 1 includes MT-3724 dose escalation over 5 dose cohorts according to the modified 3+3 design and will include up to 24 subjects with CD20+ NHL. Part 2 is designed to assess the safety and tolerability of MT-3724+LEN in the MTD Expansion Cohort, where the dose declared as MTD of MT-3724 in Part 1 will be given in combination with LEN in up to 40 subjects with CD20+ r/rDLBCL. Eligible subjects must have received at least one approved therapy for NHL and must have measurable disease by Lugano criteria. Subjects who have progressed following CAR T-cell therapy, autologous or allogeneic stem cell transplant are also eligible. Serum rituximab level must be negative ( Citation Format: Jason Tache, Deborah A. Katz, Amitabha Mazumder, David Peace, Christine Burnett, Thomas Strack, Elizabeth Bay, Roger Waltzman, Seung Lee. A phase 2a open-label study of MT-3724, a novel CD20-targeting engineered toxin body, in combination with lenalidomide (LEN) in subjects with relapsed or refractory B-cell non-Hodgkin lymphoma (NHL) [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-63.

e15567 Background: Engineered toxin bodies (ETBs), composed of a Shiga-like Toxin A subunit genetically fused to antibody-like binding domains, can force receptor internalization, self-route through intracellular compartments to the cytosol, and induce potent cell-kill via the enzymatic and permanent inactivation of ribosomes. MT-5111, a de-immunized 55 kD ETB targeting HER2 in solid tumors, binds to an epitope distinct from and non-competitive with trastuzumab and pertuzumab. Methods: The primary objective is to determine the maximum tolerated dose (MTD) of MT-5111 monotherapy in adults with advanced HER2+ solid tumors. Secondary objectives are pharmacokinetics, efficacy, and immunogenicity. Part 1 will identify the MTD via MT-5111 dose escalation (0.5, 1.0, 2.0, 3.0, 4.5, 6.75, 10 µg/kg/dose) according to a modified 3+3 design. Part 2 will further evaluate the safety of MT-5111 at the recommended phase 2 dose (RP2D) in 3 separate cohorts of subjs with a HER2 immunohistochemistry score ≥2 in a metastatic lesion of primary breast, gastroesophageal, or other cancers. All subjs receive MT-5111 as a 30-minute IV infusion on Days 1, 8, and 15 of each 21-day treatment (tx) cycle (C) until disease progression, unacceptable toxicity, death, or withdrawn consent. Details can be found on clinicaltrials.gov (NCT04029922). Results: The first cohort (0.5 µg/kg/dose) enrolled 4 subjs (metastatic breast cancer, n = 2; metastatic cholangiocarcinoma, n = 2). Three subjs were female and the mean age was 69 years (median 65, range 64-78). Subjs received a mean of 5 prior lines of therapy (median 4.5, range 3-8). Three subjs completed C1 of tx without dose-limiting toxicities; 1 subj was inevaluable. Two subjs had progressive disease in C2. A total of 23 AEs occurred in 4 subjs; all were grade (G) 1-2 except one G3 event of hypertension in a subj with a history of hypertension. There were 2 tx-related AEs (G1 chills; G2 aspartate aminotransferase increased in the setting of progressive liver metastases). There was 1 serious, non-tx-related AE (G2 dyspnea) that occurred in the inevaluable subj. No cardiac AEs were noted, nor clinically significant changes in cardiac biomarkers, an important safety parameter given non-human primate toxicity. Conclusions: MT-5111 appears to be well tolerated at the lowest dose with no apparent cardiotoxicity to date. Drug concentrations are expected to be below the level required for in vitro tumor cell killing. Safety and efficacy data from subsequent dose escalation cohorts are expected by May 2020. Clinical trial information: NCT04029922.

TPS8074 Background: Engineered toxin bodies (ETBs) are comprised of a proprietarily engineered form of Shiga-like Toxin A subunit (SLT-A) genetically fused to antibody-like binding domains. ETBs work through novel mechanisms of action and are capable of forcing internalization, self-routing through intracellular compartments to the cytosol, and inducing potent cell-kill via the enzymatic and permanent inactivation of ribosomes. MT-3724 represents a novel ETB modality comprised of an anti-CD20 single-chain variable fragment genetically fused to SLT-A. It is capable of efficient internalization once bound to CD20 and can induce potent direct cell-kill via enzymatic ribosome inactivation. MT-3724 is currently being studied in three ongoing Phase 2 studies for relapsed or refractory diffuse large B-cell lymphoma (r/rDLBCL). Methods: The primary objective of this single-arm, Phase 2 study (NCT02361346) is to determine the efficacy of MT-3724 monotherapy in r/rDLBCL based on overall response rate (ORR), defined as the proportion of subjects with a complete/partial response according to the Lugano criteria, as assessed by independent, central review. Key secondary objectives include safety, progression-free survival, investigator‐assessed ORR, duration of response, overall survival, and pharmacodynamics. Adverse events will be assessed and documented according to Common Terminology Criteria for Adverse Events version 5.0. Key eligibility criteria include adult subjects with histologically confirmed, r/rDLBCL, with ≥2 prior standard of care systemic NHL treatment regimens, and ≥1 measurable lesion. As rituximab and other CD20-targeting antibodies compete with MT-3724 for the same CD20 domain, minimum washout periods from these agents must be observed. Subjects remain eligible post stem cell transplant or chimeric antigen receptor T-cell therapy. Subjects will receive 50 µg/kg MT-3724 IV over 1 hour on Days 1, 3, 5, 8, 10 and 12 of a 21-day treatment cycle. The anticipated sample size is N = 100. Interim analyses will be performed to confirm minimum efficacy thresholds based on the encouraging data observed in the completed phase 1 portion of the study [Hamlin et al. Blood 2019;134(Suppl 1):4098]. Multiple global sites are enrolling subjects. Clinical trial information: NCT02361346 .

TPS465 Background: Engineered toxin bodies (ETBs) are proprietarily engineered from a Shiga-like Toxin A subunit fused to antibody-like binding domains. ETBs can force receptor internalization, self-route to the cytosol, and induce cell-kill via inactivation of ribosomes. MT-5111 is a 55 kDa de-immunized ETB targeting HER2, and may not be subject to resistance mechanisms that exist for TKI, ADC, or antibodies. It binds a HER2 epitope distinct from trastuzumab or pertuzumab, could be combined with other HER2 targeting agents, and may have improved tumor penetration. Methods: MT-5111 is evaluated as monotherapy in subj with confirmed HER2+ locally advanced or metastatic cancers. The primary objective is to determine the maximum tolerated dose in subjects (subj) with advanced HER2-positive tumors. Secondary endpoints are PK, tumor response and immunogenicity. Part 1 will escalate doses to identify MTD in up to 42 subj. Part 2 will further evaluate MT-5111 at the MTD in up to 98 subj. All subj will receive MT-5111 on Days 1, 8, and 15 of each 21-day cycle until disease progression, unacceptable toxicity, death, withdrawal of consent or another reason for withdrawal. Part 1 will include subj with any HER2+ solid cancers. Part 2 will enroll 3 expansion cohorts: HER2+ breast (BC), HER2+ gastroesophageal cancer (GEA), and other HER2+ solid cancers. HER2+ must be demonstrated on metastatic lesions in case of metastases. Tumors tested by immunohistochemistry (IHC) must have IHC status of 2+ or 3+, regardless of in-situ hybridization (ISH) results; for BC and GEA, if no IHC is available, ISH per ASCO-CAP guidelines is used. Subj with HER2+ BC should have had at least 2 lines of HER2-directed therapy; subj with HER2+ gastric cancer should have received trastuzumab or have been intolerant to trastuzumab. Subj with evaluable disease may be included in Part 1; in Part 2, all subj must have at least 1 measurable lesion per RECIST 1.1. ECOG should be 0-1, and bone marrow, hepatic, renal, cardiac function should be adequate. Further details can be found on clinicaltrials.gov (NCT04029922). Enrollment has begun in September 2019. Clinical trial information: NCT04029922.

Introduction: Approximately 40% of all newly diagnosed Diffuse large B-cell lymphoma (DLBCL) patients are either refractory or relapsed following initial 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 (MOA) 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. 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 Given MT-3724's unique ability to kill CD20+ cells by ribosomal inhibition, combining MT-3724 with standard cytotoxic chemotherapy may provide enhanced benefit in NHL treatment. To evaluate this possibility, MT-3724 was tested in combination with selected cytotoxic compounds including gemcitabine and cisplatin in various CD20+ cell lines (Raji, SU-DHL-4, Daudi, HBL-1). MT-3724 showed synergistic cytotoxic effects with gemcitabine and mixed additive/synergistic or synergistic effects with cisplatin in all 4 cell lines. Based on these results and the known safety profile of MT-3724, gemcitabine and oxaliplatin (GEMOX) were chosen as combination agents to maximize anti-tumor effects in patients with CD20+ B-cell NHL while minimizing overlapping toxicity risks. Study Design and Methods: MT-3724 is being evaluated in this Phase 2a study (NCT03488251) in combination with GEMOX, in adult subjects with histologically confirmed, relapsed or refractory CD20+ B-cell NHL. The primary objective of this study is to determine the safety and tolerability including the maximum tolerated dose (MTD) of MT-3724+GEMOX. Secondary objectives include the assessment of pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity and tumor response. The study will be conducted in two parts. Part 1 will include MT-3724 dose escalation according to a modified 3+3 design to identify the MTD of MT-3724 in combination with GEMOX and will include up to 24 subjects with histologically confirmed NHL. All subtypes of B-cell NHL are eligible for Part 1. Part 2 is designed to confirm the safety and tolerability of MT-3724 in the MTD Expansion Cohort, MT-3724 would be given in combination with GEMOX in up to 40 subjects with CD20+ DLBCL. In addition, the PK, PD, immunogenicity and tumor response of MT-3724 in combination with GEMOX. Eligible subjects will have histologically confirmed, relapsed or refractory CD20+ B-cell NHL that could benefit from MT-3724+GEMOX therapy. Subjects must have received all approved therapies for NHL that are applicable for the subject in the opinion of the treating physician and must have measurable disease by Lugano criteria. Subjects that are refractory to treatment are eligible, as well as patients who have progressed following CAR T-cell therapy, autologous- or allogeneic stem cell transplant. Residual rituximab causes MT-3724 to be ineffective as per the previous study experience therefore serum rituximab level must be Subjects will receive MT-3724 IV infusions over 1 hour in 3 successive dose cohorts (at 10, 20, and 50-µg/kg/dose) on Days 1, 3, 5, 8, 10 and 12 and will receive GEMOX (GEM 1000 mg/m2, and OX 100mg/m2) on Days 2 and 16 of each 28-day treatment cycle during cycles 1 and 2. Thereafter, MT-3724 will be administered weekly on Days 1, 8, 15, and 22 of each cycle with GEMOX on Days 2 and 16 for cycles 3 and 4. The study is currently ongoing. Eligible subjects will be recruited across multiple sites in North America; additional sites in other global regions may be included if necessary. Disclosures Burnett: Molecular Templates, Inc.: Employment. Strack:Molecular Templates, Inc.: Employment. Lehner:Molecular Templates, Inc.: Employment. Higgins:Molecular Templates, Inc.: Employment, Equity Ownership. Katz:Dova: Consultancy; Stemline: Speakers Bureau.

Background: The near ubiquity and persistence of CD20 expression in B-cell malignancies provides a strong rationale for novel CD20-directed therapies. MT-3724 is a novel engineered toxin body comprised of an anti-CD20 single chain variable fragment genetically fused to Shiga-like toxin A subunit, which forces internalization of CD20-bound MT-3724, irreversibly inactivating ribosomes and triggering apoptosis. As MT-3724 competes with rituximab (RTX) for the same CD20 binding domain, undetectable or low (< 500 ng/mL) RTX levels (RTX-neg) allow potential response to MT-3724. We present final safety and efficacy results from dose escalation to maximum tolerated dose (MTD) in Non-Hodgkin Lymphoma (NHL) subjects (subj) and dose expansion in RTX-neg DLBCL subj. Methods: In Part 1, MT-3724 dose was escalated in 21 subj with B-cell NHL according to the 3+3 design based on ≤1 subj exhibiting DLT during Cycle 1 (28 days) in 6 sequential dose cohorts (5, 10, 20, 50, 75, and 100 μg/kg/dose IV over 2 hours 3 times per week for 2 of 4 consecutive weeks) for up to 5 cycles. In Part 2, the safety and efficacy of MT-3724 was further evaluated in 6 RTX-neg subj with DLBCL. Tumor response was assessed by the International Working Group Response Criteria for Clinical Trials (Cheson 2007). Results: Twenty-seven subj with NHL (DLBCL:16, composite DLBCL/Follicular Lymphoma (FL): 3, FL: 6, Mantle Cell Lymphoma: 2 were enrolled. Seventeen (63%) were female, mean age was 65 yrs (34-78). Performance status was ECOG 0: 44%, 1: 44%, 2: 11%; median follow-up was 50 (11-372) days; median cycles 2 (1-11). The most common AEs (%) of all grades were peripheral edema (63), fatigue (41), diarrhea (41), myalgia (41), muscle weakness (33), insomnia (30); 27 subj had AEs (%) with related causality, the most common were: peripheral edema (41), myalgia (33), fatigue (26). The most common related grade (G) ³3 AEs included neutropenia, myalgia, and infections (all 11%); There were 9 related SAEs among 6 subj. One subj died on study from disease progression. MT-3724 was not tolerated at 100 µg/kg/dose (DLTs: G3 pneumonia, G2 ileus). After 2 obese (BMI >35) subj treated at 75 µg/kg/dose in the expansion cohort had G2 capillary leak syndrome (CLS), the MTD was set at 50 µg/kg/dose with a 6000 mg/dose cap. Innate immune responses such as CLS or manifestations thereof (hypotension, hypoalbuminemia, peripheral edema) were noted with increasing frequency with higher doses of MT-3724. No cases of >G2 CLS have been observed. Of the 13 RTX-neg DLBCL subj, 5 (38%) responded across the range of 5 to 50 μg/kg/dose. All responses were partial responses (PRs) including 1 subj with composite DLBCL/FL post autologous stem cell transplantation (SCT) who had a complete metabolic response of a large mesenteric mass and proceeded to allogeneic SCT. Three subj had stable disease (SD) including two with 49% and 47% tumor reductions, one of whom had FL transformed from DLBCL post autologous SCT; another 5 had progressive disease. Given a limited prescribed period of treatment with MT-3724, duration of response cannot be calculated, however, generally the tumor burden in responders decreased with repeat treatment. Among the responders, mean age was 61 (50-76) yrs, and the median number of prior lines of NHL therapy received was 3 (1-8). Subj with detectable baseline RTX levels did not benefit from MT-3724. Of all 20 subj measured, 6 developed anti-drug antibodies (ADAs) during the first cycle of therapy; 5 developed ADAs at a later timepoint; 6 did not develop ADAs. ADAs were observed in all subj who showed a response, and in 4 of 5 subj with SD. Increasing MT-3724 dose was associated with a dose-dependent peripheral B-cell depletion, evidenced by decreasing CD19+ cells. Conclusions MT-3724 is the first CD20 targeted immunotoxin to enter clinical trials. Safety events were mostly mild-moderate, and DLTs were in line with the mechanism of action of MT-3724. A tolerable dose and schedule have been identified: 50 μg/kg/dose up to a maximum of 6000 μg/dose infused over 1 hour on days 1, 3, 5, 8, 10, and 12 of a 21-day cycle. A 38% objective response rate has been observed with monotherapy in a heavily pretreated, RTX-neg DLBCL population; of these subj, 5 were treated at 50 ug/kg (MTD) and 3 of whom responded. The development of ADAs did not preclude benefit of MT-3724. An ongoing phase 2 monotherapy study to confirm efficacy and safety in RTX-neg subj with DLBCL (NCT02361346) is open for recruitment. Disclosures Musteata: Institute of Oncology: Employment; Arensia EM: Other: Principal Investigator. Park:BMS: Consultancy, Research Funding; Rafael Pharma: Membership on an entity's Board of Directors or advisory committees; G1 Therapeutics: Consultancy; Teva: Consultancy, Research Funding; Gilead: Speakers Bureau; Seattle Genetics: Research Funding, Speakers Bureau. Burnett:Molecular Templates, Inc.: Employment. Dabovic:Molecular Templates, Inc.: Employment. Williams:Molecular Templates, Inc.: Employment. Higgins:Molecular Templates, Inc.: Employment, Equity Ownership. Persky:Sandoz: Consultancy; Morphosys: Other: Member, Independent Data Monitoring Committee; Debiopharm: Other: Member, Independent Data Monitoring Committee; Bayer: Consultancy.

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 Given MT-3724's unique ability to kill CD20+ cells by ribosomal inhibition, combining MT-3724 with standard cytotoxic chemotherapy and/or immune modulatory agents may provide enhanced benefit in NHL treatment algorithms. To evaluate this possibility, MT-3724 was tested in combination with Lenalidomide (LEN) to assess additive, synergistic or antagonistic effects in CD20-positive Daudi cell lines. This combination significantly (Ki = 0.40) potentiated cytotoxicity. Based on these results and the known safety profile of MT-3724, LEN was chosen as a combination agent to maximize anti-tumor effects in patients with B-cell NHL while minimizing overlapping toxicity risks. Study Design and Methods: MT-3724 is being evaluated in this Phase 2a study (NCT03645395) in combination with LEN in adult patients with histologically confirmed, relapsed or refractory NHL. The primary objective of this study is to determine the safety and tolerability, including the maximum tolerated dose (MTD), of MT-3724 + LEN. Secondary objectives include pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity and tumor response in subjects treated with MT-3724 + LEN. The study will be conducted in two parts. Part 1 will include MT-3724 dose escalation according to the modified 3+3 design to identify the MTD of MT-3724 in combination with standard doses of LEN and will include up to 24 subjects with histologically confirmed NHL. Part 2 is designed to confirm the safety and tolerability of MT-3724 + LEN in the MTD Expansion Cohort, where the dose declared as MTD of MT-3724 in Part 1 would be given in combination with LEN in up to 40 subjects with CD20+ DLBCL. In addition, the PK, PD, immunogenicity and tumor response of MT-3724 + LEN will be evaluated in Part 2. Eligible subjects will have histologically confirmed, relapsed or refractory CD20+ B-cell NHL that, in the investigator's opinion, could benefit from MT-3724 + LEN therapy. Patients must have received all approved therapies for NHL that are applicable for the patient in the opinion of the treating physician and must have measurable disease by Lugano Classification for NHL. Subjects that are refractory to treatment are eligible, as well as patients who have progressed following CAR T-cell therapy, autologous- or allogeneic stem cell transplant. Serum rituximab level must be negative ( Subjects will receive MT-3724 IV infusions over 1 hour on Days 1, 3, 5, 8, 10 and 12 and will receive LEN on days 1-21 of each 28-day treatment cycle during cycles 1 and 2. Thereafter, MT-3724 will be administered weekly on Days 1, 8, 15, and 22 of each cycle with continued LEN dosing on Days 1-21 for cycles 3 to 6. The study is currently ongoing. Eligible subjects will be identified and treated through competitive enrollment at multiple study centers in North America. Disclosures Burnett: Molecular Templates, Inc.: Employment. Gordon:Abbott: Equity Ownership; Abbvie: Equity Ownership. Strack:Molecular Templates, Inc.: Employment. Lehner:Molecular Templates, Inc.: Employment. Higgins:Molecular Templates, Inc.: Employment, Equity Ownership.

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 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.

7580Background: MT-3724 is a novel recombinant fusion protein consisting of a CD20 binding variable fragment (scFv) fused to Shiga-like toxin-I A1. The SLT-I A1 forces MT-3724 internalization and i...