Strickler, John H.

PURPOSE This study aimed to investigate the clinicomolecular profiles and the efficacy of human epidermal growth factor receptor 2 (HER2)-targeted therapy in HER2 -amplified biliary tract cancer (BTC). METHODS This study was an international collaboration that used combined data from the prospective SCRUM-Japan GOZILA and MONSTAR-SCREEN in Japan and retrospective reviews in the United States; patients with advanced BTC who had received systemic therapy were included. The clinicomolecular profiles were evaluated in an exploratory cohort, whereas the efficacy of HER2-targeted therapy was assessed in a biomarker-selected cohort. RESULTS Of the 439 patients in the exploratory cohort, 43 (10%) had HER2 amplification. The frequencies of coalterations were higher in patients with HER2 amplification versus patients without HER2 amplification including HER2 mutations (26% v 5%, P < .001), TP53 mutations (84% v 61%, P = .003), and BRAF amplification (9% v 2%, P = .030). There were no KRAS mutations identified in patients with HER2- amplified BTC. No significant difference in overall survival (OS) was observed between patients with and without HER2 amplification (median, 17.7 v 16.9 months; hazard ratio [HR], 0.95 [95% CI, 0.65 to 1.40]). Of the 60 patients with HER2 -amplified BTC in the biomarker-selected cohort (43 from Japan and 17 from the United States), the OS was significantly longer in 29 patients who received HER2-targeted therapy than in those who did not receive HER2-targeted therapy (median, 24.3 v 12.1 months; HR, 0.39 [95% CI, 0.23 to 0.82]). Multivariate analysis identified HER2-targeted therapy as an independent prognostic factor for OS (HR, 0.29 [95% CI, 0.14 to 0.58]; P < .001). CONCLUSION HER2 amplification was found in 10% of advanced BTC and was not identified as an independent prognostic factor for OS. Patients with HER2 -amplified BTC derive significant benefit from HER2-targeted therapy.

243 Background: HER2-amplified aCRC is associated with poor prognosis despite new anti-HER2 regimens. Insight on biomarkers that predict response to these therapies is limited. This study aims to evaluate whether baseline alterations in oncogenic drivers along the HER2 pathway predict response to anti-HER2 therapy in HER2-amplified aCRC. Methods: In this international multi-center study involving Mayo Clinic, Duke Cancer Institute and National Cancer Center Hospital East (Japan), we collected baseline genomic alterations from tissue or blood samples using next-generation sequencing (NGS) prior to anti-HER2 therapy initiation. We defined PIK3CA, RAS, RAF, MAP2K, and MET as oncogenic drivers of resistance within the HER2 pathway. Overall survival (OS) was evaluated with the Kaplan-Meier method and a log-rank test was used to compare median OS (mOS) between pts with and without alterations in the above genes. A chi-square test (significance level 0.05) was performed to compare overall response rate (ORR) and disease control rate (DCR). Results: We identified 39 pts with HER2-amplified aCRC and baseline NGS. Median age was 57 (range 30-77), 54% were male. Baseline alterations in HER2 pathway oncogenic drivers were identified in 17 pts (43.5%). 18 (46%) received monoclonal antibodies (mAbs) plus tyrosine kinase inhibitors, 12 (30%) received dual mAbs and 7 (18%) received trastuzumab deruxtecan. The mOS was 41.9 months (mos) (95%CI 23.7-60.1) for other alterations vs 24.2 mos (95%CI 14.1-34.3) in pts with HER2 pathway alterations (p = 0.1). ORR was 47% vs 41% (p = 0.7) and DCR 82% vs 73% (p = 0.4), for pts with HER2 pathway alterations and without HER2 pathway alterations, respectively. 4 pts receiving trastuzumab/tucatinib had complete response, none of which had HER2 pathway alterations. Conclusions: Pts with baseline alterations in oncogenic drivers of the HER2 pathway had clinically worse outcomes compared to pts without such alterations despite receiving HER2-targeted therapy, though results lacked statistical significance. Larger studies are needed to confirm the lack of benefit from anti-HER2 therapy in this subgroup.

225 Background: Novel targeted therapies entered the treatment landscape in patients with HER2-positive metastatic colorectal cancer (mCRC). To ensure that these therapies are used effectively, clinicians must assess tumor HER2 status and recognize how HER2-directed treatment modalities fit into the current paradigm. To further enable clinicians to integrate HER2-targeted therapies, an educational program was designed in 2023. Methods: A one-hour online, video-based program was designed for clinicians and their teams and hosted on MedLive from February 2023 for one year. Knowledge and competence questions were administered pre-, and immediate post-activity. Additional questions assessed attitudes, barriers, and intended practice changes related to HER-positive mCRC. Results: Over 4,000 participants engaged in the educational activity, 67% of whom were physicians, and 63% noting their specialty as oncology. Approximately 68% identified as treaters seeing 14 patients with CRC per week with 78% of those patients have HER2-positive CRC. All pre-post questions showed significant improvements in knowledge and competence related to HER2 amplification testing methodologies, trial eligibility criteria, identification of patients who would benefit from anti-HER2 targeted regimens, and adverse events associated with anti-HER2 agents. Following the activity, 51% were more likely to incorporate testing for HER2 expression level or amplification prior to the selection of targeted therapy for their patients. The greatest challenges faced when managing patients with HER2-positive mCRC were affordability of therapy for patients (50%), adherence to treatment schedules (25%) and patient anxiety about treatment efficacy (24%). The most common barriers to patient enrollment in clinical trials identified by clinicians were lack of trials at their institutions (35%), lack of trials in their geographic region (21%), and patient lack of interest (20%). Qualitative insights on intended practice changes were shared by learners post-activity. Conclusions: The outcomes of this educational activity demonstrate the effectiveness of education in improving knowledge and confidence from testing to adverse event management to better integrate new regimens into practice. Encouraging practice changes were also seen from the write-in responses from learners. Additional needs were also identified to further solidify HER2 testing as a prerequisite for anti-HER2 targeted therapies, selecting eligible patients, and timely management of adverse events.

277 Background: While HER2-targeted therapies benefit patients (pts) with advanced HER2-positive mCRC, the development of resistance remains inevitable. This study aims to explore the landscape and frequency of genetic alterations putatively associated with acquired resistance to anti-HER2 therapy in HER2-positive mCRC. Methods: In this retrospective-multicenter international study involving Mayo Clinic, Duke Cancer Center and National Cancer Center Hospital East (Japan), we descriptively compared comprehensive genomic analyses from tissue or blood samples of pts with HER2-positive CRC pre-treatment and post-treatment with HER2-targeting agents. Results: A total of 36 pts were included in this study. 29 (80%) had HER2-positive mCRC as confirmed by immunohistochemistry (IHC) and fluorescent in situ hybridization (ISH), the remaining pts had HER2 amplification detected on blood or tissue . 34/36 (94%) pts were administered their first anti-HER2 agent after progressing on ≥2 prior lines of treatment. The most common anti-HER2 regimen administered was trastuzumab/pertuzumab (50%, 18/36), followed by trastuzumab-tyrosine kinase inhibitors (TKIs) (tucatinib or neratinib) (30%, 11/36) and trastuzumab deruxtecan (14%, 5/36). Putative genomic mechanisms of acquired resistance were detected in 72% (26/36), while the remaining pts lacked new genomic alterations that might explain resistance to anti-HER2 therapy, suggesting alternative escape mechanisms. Acquired alterations in downstream or alternate bypass pathways were identified in 72% (26/36) of pts; most commonly in the RAS/RAF pathway and in EGFR (34.6%, 9/26 each), followed by 27% (7/26) of pts with MET point mutations (mts) and amplification; activating muts in PI3K/AKT and in 27% (7/26), most commonly CDK12 (60%, 4/7) . HER2 receptor alterations were detected in 3 pts (8.3%), all of whom received trastuzumab-tucatinib and included HER2 L755S, HER2 V777L, HER2 D769Y, HER2 G727A and HER2 S310F, which are known to cause resistance to , however, information about tucatinib resistance is not yet available. Other altered pathways included AR, RB, NOTCH and HRD. Two pts lost HER2 expression on blood and tissue NGS post anti-HER2 therapy. Conclusions: The emergence of resistance might be in part related to acquired alterations that constitutively activate signaling pathways parallel or downstream of HER2, bypassing HER2 inhibition. Additional histologic studies, deep mutational scanning, larger populations and correlative analysis are needed to validate these findings.

3509 Background: HER2 overexpression/amplification (HER2+) occurs in ~3%-5% of pts with metastatic colorectal cancer (mCRC) and up to ~10% of pts with RAS/BRAF wild-type (WT) mCRC. The primary results of the MOUNTAINEER study (NCT03043313) demonstrated that the combination of tucatinib (TUC), a highly selective HER2-directed TKI, and trastuzumab (Tras) was well tolerated with clinically meaningful activity in pts with chemo-refractory HER2+ mCRC. Confirmed ORR was 38.1%, median DOR was 12.4 months, and median OS was 24.1 months. Here, we present results from the final analysis of MOUNTAINEER. Methods: MOUNTAINEER is a multi-center, open-label, randomized, phase 2 trial. Eligible pts had HER2+ and RAS WT mCRC with progression on or intolerance to last systemic therapy. Previous HER2-directed therapies were not permitted. The trial consisted of Cohort A, comprising pts treated with TUC (300 mg PO BID) + Tras (8 mg/kg IV then 6 mg/kg IV Q3W), as well as pts randomized 4:3 to receive TUC+Tras (Cohort B) or TUC monotherapy (Cohort C). Pts randomized to Cohort C were allowed to crossover and receive TUC+Tras on radiographic progression or if they did not achieve a complete or partial response by Week 12. Study endpoints evaluated at final analysis include DOR per BICR, PFS per BICR, OS, and safety. Results: The median duration of follow-up was 32.4 months (IQR, 25.1-46.7), with an additional 16.1 months of follow-up since the primary analysis (data cutoff, 02-Nov-2023). A total of 116 pts received at least 1 dose of study treatment (Cohorts A+B, N = 86; Cohort C, N = 30). In Cohorts A+B, median DOR per BICR was 15.2 months (95% CI, 8.9-20.5), median PFS per BICR was 8.1 months (95% CI, 4.2-10.2), and median OS was 23.9 months (95% CI, 18.7-28.3). In Cohort C post-crossover (N = 28), median DOR was not reached (95% CI, 2.8 months-NE). Median OS in Cohort C was 21.1 months (95% CI, 17.0-NE). The most common TEAEs in Cohorts A+B were diarrhea (66.3%), fatigue (44.2%), and nausea (34.9%). The majority of TEAEs were low grade, and rates were stable with longer follow-up. The most common grade ≥3 TEAE was hypertension (7.0%). TEAEs leading to TUC discontinuation in Cohorts A+B occurred in 5.8% of pts, which was unchanged from the primary analysis. In Cohort C, the most common TEAEs post-crossover were diarrhea (39.3%), back pain (21.4%), and pyrexia (21.4%); the most common grade ≥3 TEAE was AST increase (10.7%). No deaths resulted from TEAEs in any cohort and no new safety signals were identified. Conclusions: TUC+Tras continued to be well tolerated with sustained efficacy in pts with chemo-refractory RAS WT HER2+ mCRC, demonstrating clinically meaningful activity of the combination on longer follow-up. TUC+Tras is an important chemotherapy-free treatment option in HER2+ mCRC. TUC+Tras is being further evaluated in the first-line setting in combination with chemotherapy in the ongoing phase 3 MOUNTAINEER-03 trial (NCT05253651) for pts with HER2+ mCRC. Clinical trial information: NCT03043313 .

538 Background: In biliary tract cancer (BTC), TP53 is the most commonly altered gene and may co-occur with KRAS alteration. The presence of a co-alterations in of TP53 and KRAS is associated with poor prognosis in many cancers. In patients (pts) with advanced BTC, the prognostic implications of such co-alterations have not been fully investigated. Methods: This is a pooled analysis of SCRUM-Japan GOZILA and MONSTAR-SCREEN-1 and -2 studies. Genomic profiling was conducted with Guardant360 for plasma in GOZILA, FoundationOne CDx for tissue and FoundationOne Liquid CDx for plasma in MONSTAR-SCREEN-1, and CARIS MI Profile for tissue in MONSTAR-SCREEN-2. Pts with advanced BTC receiving systemic therapy were included in this study. Results: Among 636 pts with advanced BTC, 85 had TP53/KRAS co-alteration, 293 had TP53, 44 had KRAS, and 214 had neither KRAS nor TP53 alteration (WT/WT). Tumors with TP53 alteration had significantly higher level of tumor mutation burden (TMB) compared to the other groups (3.8 mut/Mb for TP53/KRAS, 5.0 mut/Mb for TP53, 2.8 mut/Mb for KRAS, and 2.5 mut/Mb for WT/WT, respectively, P<0.001). In addition, the frequency of microsatellite instability-high (MSI-High) in the TP53/KRAS group (11.0%) was significantly higher compared to the TP53 group (3.1%, P<0.05), and the WT/WT group (0.6%, P<0.001), respectively. The objective response rate of first-line therapy in the TP53/KRAS group (8.2%) was significantly lower than in the other groups (20.8% for TP53, 23.3% for KRAS, and 19.7% for WT/WT groups, P=0.035). There was no pts treated with durvalumab plus gemcitabine and cisplatin. Two pts with TP53/KRAS alteration and MSI-High received pembrolizumab monotherapy, and partial response was achieved with a progression-free survival of 9.4 and 22.0 months. The median overall survival (OS) for pts treated with first-line therapy of the TP53/KRAS, KRAS, TP53, and WT/WT groups were 13.6 (95% confidence interval [CI], 11.5-15.5), 14.3 (95%CI, 9.7-25.9), 18.8 (95%CI, 16.9-20.6), and 22.7 months (95%CI, 20.4-25.9), respectively. Multivariate analysis identified TP53/KRAS alteration (hazard ratio [HR] 2.14, 95%CI 1.43-3.22, P<0.001), number of detected variants (≥2) (HR 1.50, 95%CI 1.04-2.16, P=0.029), locally advanced (HR 1.72, 95%CI 1.05-2.83, P=0.033), and metastatic status (HR 1.78, 95%CI 1.39-2.28, P<0.001) as independent prognostic factors for shorter OS. Conclusions: Patients with advanced BTC with co-occurring TP53/KRAS alterations have worse prognosis. Interestingly, the presence of these co-alterations was associated with a higher likelihood of MSI-High phenotype.

100 Background: HER2 is overexpressed in various GI tumors. Tucatinib (TUC), which is approved in combination with trastuzumab (Tras) in the US for previously treated RAS WT HER2+ metastatic colorectal cancer (mCRC), is a tyrosine kinase inhibitor highly selective for HER2. Preliminary safety and antitumor activity results of TUC + trastuzumab + FOLFOX in patients (pts) with HER2+ GI cancers are presented. Methods: The phase 1b/2 SGNTUC-024 (NCT04430738) study evaluated TUC + Tras + FOLFOX in pts with HER2+ metastatic GI cancers in Cohorts 1A, 1B, 1D (pts in Japan), and 2B. For Cohorts 1A, 1B, and 1D (enrolled sequentially), eligible pts had unresectable or metastatic HER2+ GI malignancies, including mCRC. Cohort 2B is still enrolling, and pts with HER2+ mCRC are eligible. Pts received a combination of TUC 150 mg orally twice a day (PO BID; Cohort 1A) or TUC 300 mg PO BID (Cohorts 1B, 1D, and 2B) + Tras + FOLFOX. Pts in Cohort 2B were enrolled to further assess the study regimen. Antidiarrheal prophylaxis was required for Cohort 1D. Results: As of July 10, 2023, 25 pts were treated (5, 11, 7, and 2 in Cohorts 1A, 1B, 1D, and 2B, respectively). In Cohort 1A (TUC 150 mg PO BID), the most common TEAEs were diarrhea, fatigue, nausea, and proteinuria (each in 60.0% [3/5]). For pts receiving TUC 300 mg PO BID, the most common TEAEs were diarrhea (90.0% [18/20]) and fatigue (65.0% [13/20]). Table 1 presents the overall summary of adverse events. In Cohort 1A, Treatment-emergent adverse events (TEAEs) leading to any treatment discontinuation were reported in 40.0% (2/5) pts, with 20.0% (1/5) discontinuing tucatinib. For pts treated with TUC 300 mg, TEAEs leading to treatment discontinuation were reported in 35.0% (7/20), with 15.0% (3/20) discontinuing tucatinib. One TEAE leading to death (aspiration) in Cohort 1A was unrelated to study treatment. Grade ≥3 diarrhea was observed in 45.5% (5/11) pts in Cohort 1B; most were observed in elderly pts or pts with gastric, gastroesophageal, or esophageal cancer or pts who were generally noncompliant with antidiarrheal treatment. No grade ≥3 diarrhea was observed in Cohort 1D. Among pts receiving TUC 300 mg, confirmed objective response rate was 83.3% (5/6) for mCRC, 40.0% (4/10) for gastroesophageal cancers, and 0% (0/4) for biliary tract cancer. Conclusions: TUC + Tras + FOLFOX showed manageable safety in the enrolled pt population and preliminary antitumor activity in pts with HER2+ mCRC and gastroesophageal cancer. This regimen will be compared with the standard of care (FOLFOX with or without bevacizumab/cetuximab) in the ongoing randomized, phase 3 study (MOUNTAINEER-03; NCT05253651) for pts with HER2+ RAS WT mCRC. Clinical trial information: NCT04430738 . [Table: see text]

544 Background: Biliary tract cancer (BTC) is an aggressive malignancy with poor prognosis and limited treatment options. The prevalence of HER2 amplification in BTC has been reported to be 5-20%, and clinical trials suggested the clinical benefit of HER2-targeted treatment in this disease subpopulation. In a global collaboration, we investigated the comprehensive clinico-molecular characterization as well as the efficacy of anti-HER2 therapy in HER2-amplified BTC. Methods: Patients with advanced BTC who received systemic therapy were included from the GOZILA and MONSTAR studies in Japan and from the COLOMATE trial in the US. Genomic alterations were detected by tissue next-generation sequencing (NGS), including FoundationOne CDx and TempuslxT, and plasma circulating tumor DNA NGS including Guardant360 and FoundationOne Liquid CDx. The clinico-molecular characteristics were evaluated in an exploratory cohort comprised of patients from Japan, whereas the efficacy of anti-HER2 therapy was assessed in a biomarker selected cohort with patients from both Japan and the US. Results: Of 439 patients included in the exploratory cohort, 43 (10%) had HER2 amplification. BTC with primary tumor location in the gallbladder accounted for 58% of primary sites of patients with HER2 amplification. TP53 mutation (84% vs 61%, p=0.003) and BRAF amplification (9% vs 2%, p=0.030) were significantly more frequent in patients with HER2 amplification compared to those without, and no KRAS mutation co-occurred with HER2 amplification. There was no significant difference in overall survival (OS) between patients with and without HER2 amplification (17.7 months [M] vs 16.9 M, hazard ratio [HR] 0.95, 95% confidence interval [CI] 0.65-1.40, p=0.799). Of 58 patients with HER2-amplified BTC included in the biomarker selected cohort (43 Japan, 15 US), 27 (47%) received anti-HER2 therapy. The median OS of patients treated with anti-HER2 therapy was significantly longer than that of patients who did not receive anti-HER2 therapy (24.3 M vs. 12.1 M, HR 0.43, 95% CI 0.23-0.82, p=0.011). Multivariate analysis also identified anti-HER2 therapy as an independent prognostic factor for OS (HR 0.42, 95% CI 0.20-0.85, p=0.015). Conclusions: HER2 amplifications were found in 10% of advanced BTC and did not represent an independent predictive factor for OS. Of clinical significance, patients with HER2-amplified BTC derive a significant benefit from anti-HER2 therapy.