Abstract Background: HER2 point mutations are found in ~1.4% of solid tumors, but prevalence varies across tumor types. Trastuzumab emtansine is an antibody-drug conjugate approved in HER2-overexpressing metastatic breast cancer. Here we present efficacy and safety data of trastuzumab emtansine in pts with HER2mut advanced/metastatic solid tumors (excluding HER2 amplifications) from Cohort F of the TAPISTRY trial (NCT04589845). Methods: TAPISTRY is a phase II, global, open-label, multi-cohort basket trial evaluating the efficacy and safety of multiple therapies in pts with advanced/metastatic solid tumors. Pts in Cohort F were ≥12 years old, with tumors harboring an activating HER2 mutation identified by NGS, and measurable disease by RECIST v1.1. Trastuzumab emtansine 3.6 mg/kg IV was administered every 21 days; tumor assessments were performed at screening, every 6 weeks from day 1/cycle 1 for one year, and every 9 weeks thereafter. Primary endpoint: objective response rate (ORR) by independent review committee (IRC). Key secondary endpoints: ORR by investigator; duration of response; progression-free survival; overall survival; safety. Results: At data cut-off (16 Jul 2023), 35 pts with 10 different tumor types were efficacy evaluable; the most common tumor type was non-small cell lung cancer (NSCLC; 31%). Baseline characteristics and key outcomes are presented in the Table. After a median follow-up of 7 months, ORR by IRC in efficacy-evaluable pts was 14.3% (n/N=5/35; 95% CI 4.8-30.3]). Responses were observed in 3 tumor types: NSCLC (n/N=1/11), breast (n/N=3/5), endometrial (n/N=1/1). The most frequent adverse events were decreased appetite, fatigue, and nausea (25.7% each). No new safety signals were identified. Conclusions: Trastuzumab emtansine in pts with HER2mut tumors (excluding HER2 amplifications) did not meet its primary endpoint. Trastuzumab emtansine was generally well tolerated and the safety profile was consistent with the known profile of the drug. TABLE 1. NAND Baseline characteristics Trastuzumab emtansine N=35 Median age, years (range) 67 (29-83) Female, n (%) 22 (62.9) Prior lines of therapy, n (%) - No prior lines of therapy/1 prior line of therapy/≥2 prior lines of therapy 1 (2.9)/8 (22.9)/26 (74.3) Efficacy N=35 Objective response rate by IRC, n (%) [95% CI] 5 (14.3)* [4.8-30.3] Complete response 0 Partial response 5 (14.3) Stable disease 14 (40.0) Progressive disease 10 (28.6) Missing† 6 (17.1) Clinical benefit rate by IRC, n (%)‡ 8 (22.9) Objective response rate by investigator, n (%) 2 (5.7) Median duration of response, months (95% CI) - By IRC/By investigator 9.4 (2.8-NE)/11.8 (8.3-NE) Median progression-free survival, months (95% CI) - By IRC/By investigator 3.3 (1.4-4.3)/2.7 (1.5-3.9) Median overall survival, months (95% CI) 10.0 (6.2-13.4) Safety, n (%) N=35 ≥1 AE 33 (94.3) Grade 3-5 AEs 19 (54.3) AE-related deaths 4 (11.4) Serious AEs 13 (37.1) AEs leading to withdrawal from study 4 (11.4) Treatment-related AEs 25 (71.4) Treatment-related AEs leading to withdrawal from study 1 (2.9) *By tumor type: non-small cell lung cancer (n=1), breast (n=3), and endometrial (n=1); †Six patients had missing data due to: death (n=4), clinical progression (n=1), or withdrawal of consent (n=1) prior to first scan; ‡Criteria for clinical benefit: either a response (confirmation not required) and/or stable disease or better for at least 24 weeks. AE, adverse event; CI, confidence interval; NE, not estimable. Citation Format: Jeong Eun Kim, David Thomas, Shirish Gadgeel, Marcelo Corassa, Tira J. Tan, Eugenia Girda, Donald Richards, Sabine Tejpar, David Chen, Junhan Fang, Sid Patel, Timothy R. Wilson, Fabrice Barlesi. Trastuzumab emtansine in patients (pts) with HER2 mutation-positive (HER2mut) tumors: TAPISTRY study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT265.

LBA2509 Background: Studies have suggested that pts with TMB-high tumors could derive clinical benefit from atezolizumab, a PD-L1 inhibitor; however, these studies used inconsistent TMB cutoffs. We report efficacy and safety data of atezolizumab in adult and pediatric pts with TMB-high advanced/metastatic solid tumors from Cohort D of the TAPISTRY trial (NCT04589845), using two TMB cutoffs: ≥13 mutations [mut]/Mb and ≥16 mut/Mb. Methods: TAPISTRY is a phase II, global, open-label, multicohort basket trial evaluating the efficacy and safety of multiple therapies in pretreated pts with advanced/metastatic solid tumors. Pts in Cohort D had advanced unresectable/metastatic, PD-L1 inhibitor-naïve, TMB-high (≥13 mut/Mb) solid tumors. Atezolizumab was given every 21 days, at 1200 mg in pts ≥18 years old, and at ≥15 mg/kg (up to 1200 mg) in pts <18 years old. Tumor responses were assessed per RECIST v1.1. Primary endpoint: objective response rate (ORR) by independent review committee (IRC) in pts with TMB ≥16 mut/Mb. Secondary endpoints included ORR by IRC in pts with TMB ≥13 mut/Mb, duration of response (DoR), progression-free survival (PFS), overall survival (OS) and safety. Results: At data cut-off (Nov 9, 2023), 150 pts with TMB ≥13 mut/Mb were enrolled. In the safety-evaluable population (n = 148), median age was 63 years (range 11–86); 56% of pts were male, and 56% had received ≥2 prior lines of therapy (median 2; range 0–14). The efficacy-evaluable population included 129 pts with TMB ≥13 mut/Mb (TMB ≥16 mut/Mb; n = 111); the most common tumor types were colorectal (n = 40; 31%), breast, and gastroesophageal cancer (n = 11 each; 9%). Key outcomes are presented (Table). After a median follow-up of 9.8 months, ORR by IRC was comparable between pts with TMB ≥16 mut/Mb (22.5%) and pts with TMB ≥13 mut/Mb (20.2%). Responses were seen across a variety of tumor types. DoR 6- and 12-month event-free rates were 79% and 72%, respectively. Median PFS was short, suggesting fast disease progression in non-responders. Fatigue (22%) and anemia (20%) were the most common adverse events. Safety of atezolizumab was consistent with its known profile. Conclusions: Atezolizumab was well tolerated and led to antitumor activity in pts with TMB-high solid tumors. Responses were seen across a variety of tumor types. Clinical trial information: NCT04589845 . [Table: see text]

Background ROS1 tyrosine kinase inhibitors (TKIs) have demonstrated significant clinical benefit for ROS1+ NSCLC patients. However, TKI resistance inevitably develops through ROS1 kinase domain (KD) modification or another kinase driving bypass signaling. While multiple TKIs have been designed to target ROS1 KD mutations, less is known about bypass signaling in TKI-resistant ROS1+ lung cancers. Methods Utilizing a primary, patient-derived TPM3-ROS1 cell line (CUTO28), we derived an entrectinib-resistant line (CUTO28-ER). We evaluated proliferation and signaling responses to TKIs, and utilized RNA sequencing, whole exome sequencing, and fluorescence in situ hybridization to detect transcriptional, mutational, and copy number alterations, respectively. We substantiated in vitro findings using a CD74-ROS1 NSCLC patient's tumor samples. Last, we analyzed circulating tumor DNA (ctDNA) from ROS1+ NSCLC patients in the STARTRK-2 entrectinib trial to determine the prevalence of MET amplification. Results CUTO28-ER cells did not exhibit ROS1 KD mutations. MET TKIs inhibited proliferation and downstream signaling and MET transcription was elevated in CUTO28-ER cells. CUTO28-ER cells displayed extrachromosomal (ecDNA) MET amplification without MET activating mutations, exon 14 skipping, or fusions. The CD74-ROS1 patient samples illustrated MET amplification while receiving ROS1 TKI. Finally, two of 105 (1.9%) entrectinib-resistant ROS1+ NSCLC STARTRK-2 patients with ctDNA analysis at enrollment and disease progression displayed MET amplification. Conclusions Treatment with ROS1-selective inhibitors may lead to MET-mediated resistance. The discovery of ecDNA MET amplification is noteworthy, as ecDNA is associated with more aggressive cancers. Following progression on ROS1-selective inhibitors, MET gene testing and treatments targeting MET should be explored to overcome MET-driven resistance.

BackgroundCoagulation factor VIII (FVIII) inhibitor titer quantification is vital for optimizing care in people with hemophilia A. ObjectivesThis study analyzed the impact of the different kinetic profiles of four FVIII monoclonal antibodies on inhibitor titer quantification using the modified Nijmegen-Bethesda assay. MethodsConcentration-related and time-related profiles of FVIII antibodies (4A4, BO2C11, 2-54, ESH-8) were evaluated in vitro. FVIII residual activity was measured using a one-stage clotting assay and chromogenic substrate assay. Profiles of the FVIII antibodies were compared with the theoretical kinetic model: the ideal log (residual activity)-linear (inhibitor concentration) relationship. Different theoretical kinetic model-dependent and -independent criteria to calculate FVIII inhibitor titer were compared. ResultsFactor VIII monoclonal antibodies had different concentration-related and time-related profiles, ideal for comparative analysis using the modified Nijmegen-Bethesda assay. The kinetic profile of 4A4 was similar to the theoretical kinetic model, while BO2C11 showed a steeper curve, and 2-54 and ESH-8 a flatter curve, than the model. In the modified Nijmegen-Bethesda assay, conversion of measured FVIII residual activities for different inhibitor dilutions into FVIII inhibitor titer is based on the theoretical kinetic model. Therefore, titer calculations for FVIII inhibitors that deviate from the model are prone to underestimation or overestimation. Calculating a theoretical dilution at 50% FVIII residual activity by sigmoidal regression reflecting different kinetic inhibition profiles can provide a more accurate titer result. ConclusionKinetic profiles of FVIII antibodies can deviate from the theoretical kinetic model in the modified Nijmegen-Bethesda assay, leading to differences in FVIII inhibitor titer quantification.

Emicizumab bridges activated factor IX (FIX) and FX to restore the tenase function mediated by activated FVIII (FVIIIa), which is deficient in people with haemophilia A (PwHA). Unlike FVIII, emicizumab does not require activation to function; thus, in coagulation assays, the behavior of emicizumab may differ from that of FVIII. The objective of this study was to assess the effect of emicizumab on coagulation assays, including potential interference behavior that may produce inaccurate or misleading results. A variety of clotting-based, amidolytic/chromogenic, latex particle-enhanced turbidometric, and enzyme-linked immunosorbent methods were investigated. As expected based on its pharmacologic mechanism of action, emicizumab exhibited strong activity on the activated partial thromboplastin time (aPTT), which resulted in interference with several aPTT-based assays, most importantly the one-stage FVIII activity assay; these assays are not recommended for PwHA receiving emicizumab therapy. Pharmacodynamic activity of emicizumab, as measured by FVIII chromogenic assays, was species-dependent due to the binding specificity of the drug antibody. Outside of FVIII assays, emicizumab did not interfere with assays based on immunologic or chromogenic principles, nor with clotting assays based on nonintrinsic pathway activators, thus offering alternative choices where aPTT-based assays might otherwise be used. The observed interferences are in line with the unique mechanism of action of emicizumab. Potential interferences should be taken into account in the selection of coagulation assays and interpretation of coagulation assay test results for PwHA receiving emicizumab therapy.

3040 Background: Baseline (BL) pVEGFA level has been investigated as a predictive biomarker for the clinical efficacy of anti-VEGF therapy with Bv in 13 trials (7 indications).Three studies, AVADO (breast cancer [BC]), AVAGAST (gastric cancer), and AVITA (pancreatic cancer [PC]), reported that a high BL pVEGFA level was associated with prolonged PFS or OS, but contrasting data were obtained in other Bv clinical trials. Therefore, pVEGFA as a potential predictive factor of Bv activity in specific oncological indications and individual pts remains poorly understood. Methods: To investigate the potential value of BL pVEGFA level (from EDTA-plasma) as a predictive biomarker for Bv activity, sensitivity analyses were carried out on pVEGFA assay data from 13 pivotal Bv trials. In all trials, a VEGFA assay with preferred sensitivity toward the short VEGFA isoforms was used (IMPACT platform, currently used for pt stratification in the MERiDiAN BC trial). Results: In the AVADO study, a high BL pVEGFA level was associated with improved PFS in pts treated with Bv 7.5mg/kg but not Bv 15mg/kg. Additionally, while BL pVEGFA levels were predictive of OS and PFS in the AVITA trial, these results were not replicated in a second PC trial (CALGB 80303). Parallel sensitivity analyses of the IMPACT VEGFA assay and assays that recognize all VEGFA isoforms similarly, suggested that preferential detection of the short isoform of VEGFA was not required to assess outcome correlations (AVADO trial). Sensitivity analysis of pVEGFA data from the AVADO trial also indicated an 11% difference in Spearman's rank correlation between different VEGFA assays, which resulted in a loss of the interaction p-value. Finally, longitudinal analysis of VEGFA level in plasma samples from healthy donors showed substantial intra-pt variability in pVEGFA concentration over time. Conclusions: These data suggest that pVEGFA level is not a robust predictive biomarker for Bv activity; stratification of pts based on a single BL measurement of pVEGFA is unlikely to be successfully implemented in clinical practice. Further assessment of this hypothesis awaits the prospective analysis of pVEGFA data from MERiDiAN.