Valdes, Frances

1098 Background: Leucine-rich repeat-containing protein 15 ( LRRC15) has emerged as a potential biomarker and therapeutic target for various cancers due to its high expression in cancer-associated fibroblasts (CAFs) and role in tumor progression. High LRRC15 expression is associated with poor prognosis in TNBC. This study aims to define the multiomic profile of LRRC15 in TNBC. Methods: 3,038 TNBC samples were analyzed via Next-Generation Sequencing (592, NextSeq; Whole Exome Sequencing, NovaSeq) and Whole Transcriptome Sequencing (NovaSeq; Caris Life Sciences, AZ). Immune cell fractions were estimated using WTS deconvolution (Quantiseq). Stromal cell abundance in the tumor microenvironment (TME) was estimated from RNA expression profiles using MCP Counter. LRRC15 -high (H) and -low (L) tumors were classified by RNA expression above or below the 25th percentile. Real-world overall survival (OS) and treatment-related survival were derived from insurance claims and calculated from tissue collection or treatment initiation to last contact using Kaplan-Meier. Statistical significance was assessed using chi-square and Mann-Whitney U tests with multiple comparison adjustments (q < .05). Results: LRRC15 -H TNBC tumors had higher frequency of PIK3CA (25.9% vs 16.8), PIK3R1 (6.2% vs 1.6%), PTEN (11.3% vs 5.8%), but lower frequency of RB1 (7.8% vs 12.1%) and KMT2D (2% vs 4.4%) compared to LRRC15 -L, all q < 0.05. LRRC15 -H had higher PD-L1 positivity (32.3% vs 24.5%, q < 0.05). Analysis of immune cells showed LRRC15 -H TNBC had higher infiltration of B cells (4.2% vs 3.5%), M1 macrophages (4.3% vs 2%), M2 macrophages (4% vs 2.5%), Tregs (1.9% vs 1.1%), neutrophils (4.9% vs 3.9%), CD8 + T cells (0.4% vs 0.1%), but lower dendritic cells (2.5% vs 3%), all q < 0.05. LRRC15 -H tumors had greater abundance of CAFs (575.6 vs 93.78, 6.14 fold change (FC)) and endothelial cells (7.3 vs 3.7, 1.97 FC), all q < 0.05. LRRC15 -H had higher T-cell inflamed score (71.5 vs -77) and IFNg score (-0.14 vs -1.72), all q < 0.05. LRRC15 -H tumors had higher expression of immune checkpoint genes ( CD274, PDCD1, PDCD1LG2, CTLA4, LAG3, HAVCR2, FOXP3, IDO1, TNFSF14, TIGIT, BTLA, CEACAM1, CD47, CD80, CD86, CD160, CD274 ; FC 1.2-2.5, q < 0.05). LRRC15 -H was associated with better OS (mOS: 24.7 vs 13.6 months; HR 0.61, 95% CI 0.53-0.7, p < 0.001). Post-pembrolizumab survival was longer for LRRC15 -H patients (mOS: 27.2 vs 19.4 months; HR 0.61, 95% CI 0.42-0.89, p = 0.01). Conclusions: LRRC15 -H TNBC exhibited better outcomes with pembrolizumab, likely due to higher immune cell fractions and increased CAFs. These findings highlight TNBC heterogeneity and position LRRC15 as a potential biomarker for tumor stratification, a possible adverse prognostic biomarker and a positive predictive biomarker. Ongoing phase I trials targeting LRRC15 show promise. Combining LRRC15 -targeted therapies with immunotherapy may improve TNBC outcomes, warranting further validation in breast cancer models.

2505 Background: ATRC-101 monoclonal antibody targets tumor-specific ribonucleoprotein complex (RNP) and elicits anti-tumor immunity, innate and adaptive, in preclinical models. Methods: Phase 1b is evaluating ATRC-101 as monotherapy, every 3 (3M) or 2 (2M) weeks (wk), and ATRC-101 in combination with pembrolizumab (3P) in participants (pts) with advanced solid tumors. ATRC-101 target expression is determined by CAP-CLIA IHC assay. For pts described herein, pre-treatment (tx) tumor biopsies were assessed for target expression by IHC retrospectively or prospectively. Enrollment in 3M and 3P is currently open to pts who are target positive (RNP+ve) by IHC with an H-score ≥ 50. Objectives are safety (primary), PK, immunogenicity, recommended dose for expansion, activity by RECIST1.1 and biomarker analyses in pre-tx and on-tx tumors. Results: As of Nov 18th, 2022, 67 pts received ≥1 dose of ATRC-101 (median prior tx 5, range 1-12). Tumor types were colorectal (CRC, 28), ovarian (10), breast (9), melanoma (8), non-small cell lung (NSCLC, 6), head and neck squamous cell (3), urothelial (1), esophageal (1) and small bowel (1). Tx emergent adverse events (TEAEs) Grade ≥3 occurred in 22 (33%) pts and were related to ATRC-101 in 2 (3%). No tx discontinuations or dose reductions related to ATRC-101. RECIST1.1 best overall responses in efficacy evaluable 61 pts included 1 complete response (CR) in RNP+ve melanoma (3P, 10mg/kg), and 1 partial response (PR) in RNP+ve NSCLC (3M, 30mg/kg). Further, disease stabilized (SD) in 24 pts (39%) and 35 pts had disease progression (PD, 57%) with evidence of association between anti-tumor activity and target expression. Of 24 SD, 22 were in monotherapy cohorts and included 9 CRC pts with highly refractory disease. Conclusions: ATRC-101 alone and in combination with pembrolizumab continues to be well tolerated with evidence of anti-tumor activity. Interim data support ongoing enrollment of pts with RNP+ve pre-tx tumors. Updated and new data including durability of disease control will be presented. Clinical trial information: NCT04244552 . [Table: see text]

2583 Background: MT-5111 is a 55kD engineered toxin body (ETB) targeting HER2 in solid tumors that binds to an epitope distinct from trastuzumab and pertuzumab, offering potential combination strategies with other HER2-targeting agents. MT-5111 may demonstrate efficacy in patients (pts) resistant to other HER2-targeting agents, as its mechanism of action induces direct cell kill via enzymatic and permanent ribosome destruction. Methods: This is a phase 1 study in adults with advanced HER2+ solid tumors. The dose-escalation portion (Part A) enrolls pts into sequential dose cohorts, followed by Part B expansion cohorts for HER2+ breast cancer (BC), gastroesophageal adenocarcinoma (GEA), and any other HER2+ cancer (CA). MT-5111 is dosed weekly IV over 30 min in each 21-day treatment (tx) cycle until disease progression, unacceptable toxicity, death or withdrawn consent. Results: As of Jan 2022, 27 pts had enrolled in Part A cohorts (0.5 to 10 µg/kg/dose) with completed DLT assessments: 9 (33%) pts were male and 18 (67%) female, median age 67 and a median of 4 prior systemic and 2 prior HER2-targeting tx. Common tissue types were BC (9/30%), biliary CA (6/22%), GEA (4/15%). The following safety data reflect 33 treated pts to date including ongoing 13 µg/kg/dose Part A and 10 µg/kg/dose BC expansion cohorts. No Grade (G) 4/5 tx-emergent adverse events (AEs) or DLTs occurred. Tx-related AEs occurred in 17 (52%) pts, most commonly G1/2 fatigue (8/24%). 3 pts had G1 troponin elevations without clinical signs or symptoms of cardiac distress: 1 at 6.75 µg/kg/dose, 2 at 10 µg/kg/dose. 2 pts (3 and 4.5 µg/kg/dose) had reversible G2 and G1, respectively, infusion-related reactions (IRR)s. A comparison of cytokines from baseline to on-treatment timepoints reveals no evidence of significant changes, even in pts with IRR. Best response per RECIST thus far was stable disease (SD) in 7 pts or non-CR/non-PD in 2 pts: 1 pt had SD for 12 weeks (wks) (4.5 μg/kg, pancreatic CA); 1 pt (1 μg/kg/dose, BC) had non-CR/non-PD for 30 wks; 1 pt (10 μg/kg/dose, GEA) has ongoing SD for 18 wks. AUClast data match PK simulations in non-human primate studies. Cmax at 10 µg/kg/dose is ≥5 times the IC50 values of high HER2 expressing gastric CA and BC cell lines while approaching the IC50 of a moderately HER2 expressing liver CA cell line. Conclusions: MT-5111 is well tolerated to-date with no clinically significant immuno/cardiotoxicity. Dose escalation is ongoing at a dose of 13µg/kg, expected to be required for efficacious exposure. Clinical trial information: NCT04029922.

Abstract Background: Engineered toxin bodies (ETBs) are composed of a de-immunized Shiga-like Toxin A subunit genetically fused to an antibody-like binding domain. ETBs 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 therapeutics. MT-5111 is a 55 kD ETB targeting HER2 in solid tumors that binds to an epitope distinct from trastuzumab and pertuzumab, offering potential combination strategies with other HER2-targeting agents. MT-5111 may demonstrate efficacy in patients (pts) resistant to other HER2-targeting agents, as its mechanism of action does not rely on inhibition of kinase signaling or cytoskeletal or DNA damage. Methods: The primary objective is to determine the maximum tolerated dose (MTD) of MT-5111 monotherapy in adult pts with advanced HER2+ solid tumors. Secondary objectives are pharmacokinetics (PK), efficacy, and immunogenicity. Using a modified 3+3 design, the dose-escalation part of the study enrolls pts with HER2+ tumors into 7 cohorts: 0.5, 1, 2, 3, 4.5, 6.75, and 10 µg/kg/dose. Three dose-expansion cohorts will follow for HER2+ breast cancer, gastro-esophageal cancer, and other HER2+ tumors. All pts receive MT-5111 weekly as 30-min IV infusions in each 21-day treatment (tx) cycle (C) until disease progression (PD), unacceptable toxicity, death, or withdrawn consent (NCT04029922). Results: Per data cut in June 2021, 21 pts (mean age 64 years, range 34-78; 38% male) in cohorts 1-6 with breast (n=7), biliary (n=6), gastric (n=3), pancreatic (n=2), lung (n=2), and colon (n=1) cancer were treated (Table 1). Pts had a median of 4 prior lines of systemic therapies (range, 1-8) and 2 prior lines of HER2-targeting treatments (range, 0-6). No Grade (G)4 or 5 tx-emergent (TE) adverse events (AEs) occurred. Tx-related AEs occurred in 11 (52%) pts; the most common was fatigue (n=7, 33%). One pt (4.5 µg/kg) had a possibly (per PI) related G3 serious AE (SAE) of dyspnea and hypoxia, but also lymphangitic carcinomatosis and H. influenzae infection. The other related SAE occurred in a pt (6.75 µg/kg) who had a G1 transient troponin increase without concomitant cardiac symptoms or ECG/ECHO changes. All other related AEs were ≤G2. There were no clinically significant changes in cardiac biomarkers (troponin, ECG, left ventricular ejection fraction) nor were there cases of capillary leak syndrome. Two pts (3 µg/kg and 4.5 µg/kg) had reversible G2 infusion-related reactions. Best response to date has been stable disease. AUClast data matched PK simulations based on non-human primate studies, and Cmax data at 6.75 µg/kg indicated that current in-human exposure was between the IC50 values of high and medium HER2-expressing cell lines (approximately 10-89 ng/mL). Thus, a dose of at least 10 µg/kg may be required to achieve effective exposure. To date, no dose-limiting toxicities have been observed and the 10 µg/kg/dose cohort is now accruing. Conclusions: MT-5111 was well tolerated with no clinically significant immuno- or cardiotoxicity. Dose escalation is ongoing and nearing levels expected to be required for efficacious exposure. Table 1.Metastatic HER2 status and MT-5111 treatment by cohortDose0.5 µg/kg1.0 µg/kg2.0 µg/kg3.0 µg/kg4.5 µg/kg6.75 µg/kgCohort123456Number of patients treated with MT-5111433335Metastatic HER2 2+ by IHC: ALL/BC1/11/11/01/10/04/0Metastatic HER2 3+ by IHC: ALL/BC3/12/02/12/13/01/1BC, breast cancer. Citation Format: Brian A. Van Tine, Monica Mita, Minal A. Barve, Erika P. Hamilton, Andrew J. Brenner, Frances Valdes, Daniel Ahn, Joleen Hubbard, Jason Starr, Angela Georgy, Joshua Pelham, Banmeet S. Anand, Thomas Strack, Andrés Machado Sandri, Zev A. Wainberg. Interim results of a phase 1 study of the novel immunotoxin MT-5111 in patients with HER2+tumors [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-45.

297 Background: MT-5111 is a 55 kD engineered toxin body (ETB) targeting HER2 in solid tumors that binds to an epitope distinct from trastuzumab and pertuzumab, offering potential combination strategies with other HER2-targeting agents. MT-5111 may demonstrate efficacy in patients (pts) resistant to other HER2-targeting agents, as its mechanism of action induces direct cell kill via enzymatic and permanent ribosome destruction and does not rely on inhibition of kinase signaling or cytoskeletal or DNA damage. Methods: This is a phase 1 study in adult pts with advanced HER2+ solid tumors. The dose-escalation portion (modified 3+3 design) enrolls pts into sequential cohorts followed by expansion cohorts for HER2+ breast cancer (BC), gastric or gastroesophageal junction adenocarcinoma (GEA), and other HER2+ tumors. MT-5111 is dosed weekly IV over 30 min in each 21-day treatment (tx) cycle until disease progression, unacceptable toxicity, death, or withdrawn consent. Results: As of Sep 2021 (contains preliminary data), 24 pts (mean age 64 yrs) were treated, 13 (54%) of whom had gastrointestinal (GI) tumors (6 biliary, 3 GEA, 2 pancreatic, 2 colo/rectal) (Table). Pts with GI tumors had a median of 3 prior systemic tx and 1 prior HER2-targeting tx. No Grade (G) 4/5 tx-emergent adverse events (AEs) occurred. Tx-related AEs occurred in 13 (54%) pts, most commonly fatigue (n=7, 29%). One pt with biliary cancer and concurrent lymphangitic carcinomatosis and H. influenzae infection (4.5 µg/kg) had a possibly related G3 serious AE (SAE) of dyspnea, which resolved 9 days later. Another related SAE occurred in a pt with GEA (6.75 µg/kg) who had a G1 transient troponin increase that resolved during hospitalization, with no clinical symptoms or ECG/ECHO changes; the pt withdrew from study before further dosing. All other related AEs were ≤G2. No other clinically significant changes in cardiac biomarkers (troponin, ECG, LVEF) or cases of capillary leak syndrome occurred. Two pts (3 and 4.5 µg/kg) had reversible G2 infusion-related reactions. Best response to date has been stable disease. AUClast data matched PK simulations based on non-human primate studies. Cmax data at 10 µg/kg indicate that current in-pt exposure was between IC50 values of high and medium HER2-expressing cell lines (approx 10-89 ng/mL). Thus, at least 10 µg/kg may be required to achieve effective exposure. No dose-limiting toxicities have been observed. The 10 µg/kg cohort is now accruing. Following this cohort, an expansion cohort for pts with BC will open. Conclusions: MT-5111 was well tolerated with no clinically significant immuno/cardiotoxicity. Dose escalation is ongoing and is nearing levels expected to be required for efficacious exposure. Clinical trial information: NCT04029922. [Table: see text]

Abstract 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.