Citations

e16265 Background: The pivotal TOPAZ-1 trial established durvalumab plus gemcitabine and cisplatin (GC) as a new first-line standard of care for advanced biliary tract cancer (ABTC). However, weekly administration of GC can lead to significant toxicity limiting its tolerability in clinical practice. Based on previous studies, we implemented biweekly regimen of GC on days 1 and 15 together with durvalumab on day 1 of each 28-day cycle to optimize the treatment delivery and improve tolerability. Here, we report the Mayo Clinic experience with biweekly GC with monthly durvalumab in patients with ABTC. Methods: At our three-site institution, we retrospectively identified patients with ABTC between January 2021 and August 2024, who received biweekly GC with monthly durvalumab. Patient demographics, tumor characteristics, treatment details - treatment related adverse events (trAEs) graded according to the NCTCAE V5.0 were collected. Outcomes included median progression-free survival (mPFS), median overall survival (mOS) and clinically assessed overall response rate (ORR) and disease control rate (DCR). Results: 57 patients were included in our analysis, median age was 68 years (range 35-90), and 57.9% were females. 91.2% of patients had ECOG performance status of 0-1. 71.9% had intrahepatic cholangiocarcinoma, 19.3% extrahepatic cholangiocarcinoma and 8.7% gallbladder carcinoma. 70.2% had metastatic disease, 21 % locally advanced unresectable disease and 8.7% recurrent metastatic disease. Patients were followed for a median of 9.1 months (2.3-28.5). 91.2% of the patients received biweekly GC plus monthly durvalumab while 8.8% switched from weekly to biweekly GC plus monthly durvalumab due to treatment toxicities. Median number of treatment cycles was 5. Maintenance regimens included durvalumab (19.3%), gemcitabine/durvalumab (8.8%) and GC/durvalumab (8.8%). Median duration of follow up was 9.1 months (range 2.3 – 28.5). ORR was 36.9% and DCR was 72% (complete response 5.3%, partial response 31.6% and stable disease 35.1%). mPFS was 7.2 months, while mOS was 16 months. Grade(G) 3 or 4 trAEs included abnormal liver function tests (15.7%), infections (11.5%), anemia (8.7%), thrombocytopenia (5.2%), peripheral neuropathy (1.7%), acute kidney injury (1.7%). Immune-related AEs were noted in 6 patients (10.5%) including colitis, gastritis, hepatitis, myocarditis and hypothyroidism. There was no patient with severe neutropenia or febrile neutropenia. Only 5 patients (8.8%) stopped treatment due to trAEs. Conclusions: Biweekly GC on days 1 and 15 plus durvalumab on day 1 of each 28-day cycle is associated with a more favorable toxicity profile while maintaining efficacy similar to that observed in the TOPAZ-1 trial indicating that a de-escalation strategy is feasible with improvement in patient centered outcomes.

4085 Background: Cholangiocarcinoma (CC) is a rare disease with an increasing incidence among younger adults, which is poorly understood. Genomic profiling of tumors is both prognostic and predictive of benefit for targeted therapies. We investigated if there are clinical and molecular differences between younger vs older patients with CC. Methods: We collected TEMPUS genetic data via retrospective chart review from tumors in young vs old patients seen at our institution, defined as ≤50 vs > 50 years of age at time of diagnosis. We included patients diagnosed with CC between January 2008 and July 2024 with available clinical follow up and TEMPUS genetic sequencing data. We collected mutation data on the following actionable genes: FGFR2, IDH1/2, BRCA1, BRCA2, BRAF, ATM, ERBB2/3, and KRAS. Patient characteristics and gene expression variables were compared using Chi-square, Fisher’s exact and Wilcoxon rank-sum tests. Kaplan-Meier, log rank tests and a multivariable Cox model were used for survival analysis. This study was IRB exempt. Results: We included 410 patients, 84 in the young group with median age at diagnosis of 40.8 years, and 326 in the old group with median age 68.5 years. 91.5% of patients were white. There was no difference in BMI between groups, however the older group had higher rates of hypertension (15.5% vs 57.7%), hyperlipidemia (6.0% vs 49.7%), cardiovascular disease (1.2% vs 20.6%), and type 2 diabetes (6.0% vs 21.5%), (all p < 0.01). Primary sclerosing cholangitis was more common in the young group (26.2% vs 4.3%, p < 0.01). ECOG status of 0 at first treatment was seen in 65.3% of young vs 52.5% of old patients (p = 0.02). FGFR2 alterations were more common in the young group (17.9% vs 8.0%, p < 0.01), while ATM mutations were more common in old vs young (5.5% vs 0%, p = 0.03). There was no age difference seen for the other genetic alterations. Mean tumor mutational burden was higher in the old group (4.1 vs 3.8 mut/mb, p = 0.01). MSI-high was found in 2% of cases with no difference between groups. There was no significant difference in overall survival between age groups. There was a numeric difference in overall survival in stage IV patients, though not statistically significant (17.8 months vs. 16.3 months, p = 0.08). In a multivariable Cox analysis, female sex, earlier stage at diagnosis and clinical trial enrollment were associated with favorable prognostics. Conclusions: Our data highlight relatively low rates of comorbidities associated with metabolic dysfunction in younger adults with CC, suggesting alternative factors are likely to explain the increasing incidence of early-onset disease. FGRFR2 is a more common pathogenic alteration among the young and could inform targeted therapies. Younger patients with CC may not have improved survival outcomes compared to their older counterparts. This underscores the aggressive nature of CC and the need for more effective therapies to improve outcomes.

4166 Background: GN is a front-line therapy for aPDAC. A Phase I/II study demonstrated that GCN has a higher overall response rate and median overall survival (mOS). Our previous study found no difference in outcomes among patients with DNA damage repair gene mutations; however, stromal gene expression correlated with mOS in patients receiving GCN. Here, we further evaluate differences in the outcomes with GCN and GN by site of biopsy. Methods: PDAC samples (n = 4,463) were analyzed by NGS (NextSeq/NovaSeq) or RNA (NovaSeq) (Caris Life Sciences, Phx, AZ). Expression of stromal and related genes ( ACTA2, ADIRF, HAS2, IL-6, MMP-2, MMP-9, SPARC, STAT3, TBGB1, TGFB2, TGFBR3, IDO1, HLA-DRB4, VEGFB) from different biopsy sites [high expression (H) >50% of RNA transcripts per million] was correlated with outcomes to GCN or GN. mOS was obtained from insurance claims and calculated from first treatment to last contact. The hazard ratio (HR) was calculated by the Cox proportional hazards model, and p-values were calculated using the log-rank test. Results: 4325 patients [primary biopsy (PT), n = 1,878; non-liver metastatic biopsy (N-LM), n = 818; Liver biopsy (LM), n = 1,629] received GN while 138 patients (PT, n = 45; N-LM, n = 28; LM, n = 65) received GCN. GCN was associated with longer mOS than GN [∆: 5.2 months (m), HR: 0.76, 95% CI 0.63-0.92, p = 0.01]. GCN was associated with longer mOS compared to GN in LM (∆: 5.6 m, HR: 0.66, 95% CI 0.50-0.87, p = 0.003), but it was not significant in PT (∆: 4.6 m, p = 0.13) and N-LM (∆: 4.4 m, p = 0.35). Median MMP2 (38.5 vs. 163.1 vs. 162.2), VEGFB (14.4 vs. 16.9 vs. 16.2) and TGFBR3 (11.4 vs. 15 vs. 16.4) expression were lower in LM compared to PT and N-LM while TGFB1 (41.6 vs. 34.5 vs. 39.8) and IL6 (1.61 vs. 1.19 vs. 1.41) expression were highest in LM (p < 0.05). In LM, IL6 -H (∆: -11 m, p = 0.037) and TGFB1 -H (∆: -5.9 m, p = 0.10) were associated with worse post-GCN survival compared to GN (p = 0.82 and p = 0.57). Whereas, in N-LM, TGFBR3 -H trended towards longer post-GCN survival (∆: 12 m, p = 0.18), while ADIRF -H trended towards shorter post-GCN survival (∆: -12 m, p = 0.056) compared to GN (p = 0.42 and p = 0.10). While in PT, MMP2 -H (∆: 11.3m, HR: 0.46, p = 0.16), TGFB1 -H (∆: 11.3 m, HR: 0.33, p = 0.09), HLA-DRB4 -H (∆: 12.9 m, HR: 0.44, p = 0.11) and VEGFB -H (∆: 10.7, HR: 0.34, p = 0.09) trended towards longer post-GCN survival compared to GN ( MMP2 -H, ∆: 3.4 m, p = 0.03, TGFB1 -H, p = 0.661, HLA-DRB4 -H, p = 0.67, VEGFB-H , ∆: 1.3 m, p = 0.03). Conclusions: GCN is associated with improved mOS compared to GN, especially in LM.Stromal gene expression in the liver differs from that in N-LM and the pancreas. While high stromal gene expression trends towards worse post-GCN survival in LM, it is associated with improved survival in N-LM and PT. Further validation is needed to understand the impact of stromal gene expression in different tumor sites on survival outcomes and signature development.

10587 Background: The American Society of Clinical Oncology (ASCO) and NCCN recommend germline testing for patients (pts) with biliary tract cancers (BTC), particularly those diagnosed at a young age. Prior studies estimate that approximately 15% of pts with BTC harbor pathogenic germline alterations. Germline testing can identify hereditary cancer syndromes, guide targeted therapy, and inform family counseling. This study aimed to further characterize and describe the prevalence of pathogenic germline variants (PGVs) in patients with young-onset BTC (YO-BTC). Methods: We conducted a retrospective review of medical records for pts diagnosed with YO-BTC (ages 18–55) across all Mayo Clinic tri-sites from January 2014 to October 2024. Data collected included results from germline DNA sequencing, tumor genetic testing, demographics, history of other malignancies, and family history of cancer. The primary outcome was the identification of pathogenic germline mutations associated with cancer predisposition. Results: Among 239 pts with YO-BTC, the median age at diagnosis was 43 years (range 19–55), with 62% female and 65.2% identifying as white. Clinical germline testing (e.g., Invitae, Ambry, Myriad) was performed in 123 pts (51.5%), revealing PGVs in 15 pts (12.1%), variants of uncertain significance (VUS) in 30 pts (24.3%), and likely benign variants in 4 pts (3.2%). Among those with PGVs, DNA repair mutations were the most common findings (10/15), including ATM (4), CHEK2 (2), BRCA2 (2), BAP1 (1), and PALB2 (1). One pt had a Lynch syndrome-associated mutation (MSH6), and four had mutations in other cancer-related genes (MUTYH, HOXB13, RMRP, and PIK3CA). Tumor genomic profiling (via CARIS, TEMPUS, or FoundationOne) was performed in 9 pts with PGVs, detecting the variant in 4 (44%), while liquid biopsy (Guardant) identified the PGV in 4 of 6 cases (66.6%).The majority of PGV carriers had advanced BTC (14/15), with intrahepatic cholangiocarcinoma being the most prevalent (11/15), while 4 had extrahepatic cholangiocarcinoma. Additionally, most PGV carriers had a first- or second-degree relative with a prior history of malignancy (14/15), and 3 pts had a second malignancy. Of the 15 pts with PGVs, 4 had BTC associated with primary sclerosing cholangitis. Conclusions: In this large institutional series of YO-BTC, PGVs were identified in 12.1% of pts, demonstrating a significant yield from germline testing in this population. Notably, tissue and liquid biopsies demonstrated different detection rates for PGVs, underscoring the variability in PGVs reporting across platforms. Germline testing through CLIA-approved methods should remain the standard of care. DNA repair pathway mutations, particularly ATM, CHEK2, and BRCA2, were the most frequently identified PGVs, highlighting their role in the pathogenesis of YO-BTC and their potential to guide treatment decisions.

Abstract Background: Pancreatic ductal adenocarcinoma (PDA) is a disease with guarded prognosis with a 5-year overall survival (OS) of 3.1% in patients with metastatic disease. The need for innovative treatments for this lethal disease is paramount. TTFields is a novel treatment modality that employs low intensity, alternating, and intermediate frequency electrical fields that abrogates cancer cell proliferation through different mechanisms. These include inhibition of mitosis, disruption of cancer cell membranes, metabolomic dysregulation of cancer cells, and immunogenic activation of the tumor microenvironment (TME). TTFields are approved in glioblastoma multiforme, non-small cell lung cancer and malignant mesothelioma. The PANOVA trial demonstrated promising efficacy for TTFields plus chemotherapy in patients with PDA. A trial of GCN in patients with PDA demonstrated exceptional efficacy in patients with metastatic disease. Here in, we present our ongoing clinical trial investigating the safety and efficacy of TTFields plus GCN in patients with PDA (NCT04605913). Methods: This a phase I/II trial with an initial safety run-in phase followed by the phase II portion. The primary objective of the trial is to determine the safety (CTCAE v 5.0) of GCN plus TTFields. Secondary objectives include determining the 6-month progression free survival (PFS) rate, overall response rate (ORR, RECIST v1.1), 12-month survival rate, OS, and QOL (EORTC QLQ-C30, version 3.0.2.3). Inclusion criteria include PDA with liver metastasis (patients can have additional sites of metastasis), ECOG 0-1, measurable disease, and ability to operate the Novo TTF-100L (P)/Novo TTF-200T system. Treatment consisted of TTFields (150 kHz 18 hours/day) and GCN (G: 1000 mg/m2, C 30 mg/m2, and N: 150 mg/m2 every 2 weeks, 28 days cycle), followed by a maintenance phase after 6 months (G: 1000 mg/m2 every 2 weeks) plus TTFields until progression. Six patients were to be enrolled in the phase I portion and if ≤1/6 patients experienced a dose limiting toxicity (DLT, treatment emergent adverse event, or skin toxicity of Gr4), the plan would be to move to a phase II trial enrolling 34 more patients. If 2/6 patients experienced a DLT, 6 patients would be enrolled in dose level -1 (GCN plus TTFields for 9 hours/day followed by the maintenance treatment) and if tolerated the phase II portion would start with dose level -1 regimen. The phase I portion was completed without any DLTs and the trial is currently enrolling to the phase II portion. The total number of patients (40) was powered to the 6-months PFS rate and ORR (estimated using the lower bound of exact one-sided 90% confidence intervals). The goal is to "reject" a 6-month PFS rate of 66% and an ORR of 71% (based on previous GCN trial). Time-to-event endpoints will be assessed using the Kaplan-Meier method. Citation Format: Hani M. Babiker, Jason Starr, Jeremy Jones, Conor O’Donnell, Robert Jones, Jacqueline Boni, Lashundra Stewart, Josh Cameron, Oudom Kour, Kabir Mody, Robert R. McWilliams, Daniel H. Ahn, Tanios Bekaii-Saab, Mitesh J. Borad, Umair Majeed. A phase I/II pilot trial, single arm, open label, of gemcitabine, cisplatin, protein-bound paclitaxel (GCN) combined with Tumor Treatment Fields (TTFields) in patient with metastatic pancreatic adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr CT190.

Abstract BACKGROUND: Intrahepatic cholangiocarcinoma (ICC) is a rare but aggressive cancer with limited curative options outside of surgical resection. Trans-arterial radioembolization (TARE) with yttrium-90 is a promising treatment with retrospective data showing an overall survival (OS) of up to 30 months. The single arm prospective MISHPEC trial combined TARE with gemcitabine and cisplatin showed an overall response rate (ORR) of 39% at 3 months along with a median OS of 22 months. PDL1 inhibitor (Durvalumab) has shown modest benefit in addition to chemotherapy in unresectable cholangiocarcinoma with three-year OS of 15% based on the TOPAZ-1 trial. ICC is a biologically cold tumor; preclinical data indicates that Yttrium-90 TARE can alter tumor microenvironment making it conducive to effector T cell recruitment and function leading to enhanced efficacy of immunotherapy. This provides the biological rationale of combining yittrium-90 TARE with systemic immunotherapy involving dual PDL1 and CTLA-4 inhibitors. Here in, we present our ongoing clinical trial investigating the safety and efficacy of yttrium-90 TARE combined with Durvalumab and Tremelimumab in patients with ICC (NCT06058663). METHODS: In this phase 1 study, yittrium-90 TARE will be combined with Durvalumab and Tremelimumab in 16 patients with unresectable or oligometastatic ICC. Eligible patients with histologically confirmed measurable disease must not have received any prior local therapies and any immunotherapy with ECOG PS of 0 or 1, adequate liver (Child Class A, ALBI 1 or 2) and renal function. Patients should also have a favorable mapping angiography based on an angiosomal analysis. Ablative radioembolization would be performed at least a dose of greater than 205 Gy MIRD to &gt;75% of tumor. Study treatment with Tremelimumab (x 1 dose of 300mg total) + Durvalumab 1500mg monthly will continue until unacceptable toxicities, or disease progression for up to 24 months. Durvalumab and Tremelimumab will be infused in two alternate schedules after yittrium-90 TARE. In cohort 1 (3 patients) immunotherapy will be delivered within 24 hours of TARE. In cohort 2 (3 patients) immunotherapy will be delivered 2 weeks after TARE. Subsequent patients will be enrolled in either cohort 1 or 2 based on the safety signal from the first 6 patients. Primary goal of the study is to determine safety of this combination. Secondary goals include ORR per RECIST 1.1, progression-free survival, overall survival, infield and out of field response rate and downstaging to resection. Correlative studies include ctDNA at baseline along with evolution through treatment, pre and post treatment biopsies to look at tumor microenvironment and post treatment dose volume histograms. Three patients have been accrued in the study thus far. Citation Format: Umair Majeed, Jason Starr, Conor O'Donnell, Robert Jones, Jacqueline Boni, Josh Cameron, Oudom Kour, Mitesh Borad, Hani Babiker, Kabir Mody, Beau Toskich. Yttrium-90 radioembolization in combination with tremelimumab and durvalumab for treating patients with locally advanced, unresectable or oligo-metastatic intrahepatic cholangiocarcinoma who are not candidates for curative therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr CT189.

Intrahepatic cholangiocarcinoma is an aggressive malignancy with rising incidence and poor outcomes. This review examines recent advancements in locoregional therapies for unresectable intrahepatic cholangiocarcinoma, focusing on external beam radiotherapy, transarterial radioembolization (TARE), hepatic artery infusion pump (HAIP) chemotherapy, and liver transplantation. Stereotactic body radiation therapy and proton beam therapy have shown promise in achieving local control and improving survival. TARE, with personalized dosimetry, has demonstrated encouraging results in select patient populations. HAIP chemotherapy, primarily studied using floxuridine, has yielded impressive survival outcomes in phase II trials. Liver transplantation, once contraindicated, is now being reconsidered for carefully selected patients with localized disease. While these locoregional approaches show potential, randomized controlled trials comparing them to standard systemic therapy are lacking. Patient selection remains crucial, with factors such as liver function, tumor burden, and molecular profile influencing treatment decisions. Ongoing research aims to optimize treatment sequencing, explore combination strategies with systemic therapies, and refine phenotype identification and patient selection criteria. As the landscape of intrahepatic cholangiocarcinoma management evolves, a multidisciplinary approach is essential to tailor treatment strategies and improve outcomes for patients with this challenging disease.

5 Background: Metastatic anal squamous cell carcinoma is rare and associated with a poor prognosis. There is limited evidence on the impact of local interventions on survival outcomes in patients with metastatic disease. We aimed to evaluate the survival of patients with liver-only metastatic anal cancer following definitive local management of liver lesions. Methods: This is a single-institution retrospective cohort study of patients with liver-limited metastatic anal cancer who underwent curative-intent local therapy. Clinical characteristics were collected and analyzed. The Kaplan-Meier method was used to calculate disease-free survival (DFS) and overall survival (OS). Prognostic factors associated with DFS and OS were assessed using Cox-proportional hazards models. Results: Twenty-five patients were included, median age was 59 years, and 92% were female. None of the patients had known HIV infection, 92% had HPV-positive disease and 76% had metachronous liver metastases following locoregional disease. All patients received chemoradiation for the primary lesion Unilobar hepatic involvement was present in 76% of the patients, with 56% having a single metastatic lesion. Pre-intervention systemic therapy was provided to 52% of the patients, including 8 who received carboplatin and paclitaxel, with a median duration of 3 months and ORR of 69%. Definitive treatment of metastatic disease included surgical resection (60%), non-surgical interventions (20%), and multimodal therapy (20%). All patients had viable tumor in the pathological specimen. With a median follow-up of 22 months, median DFS was 7.3 months, and median OS was 51.3 months. An ECOG performance status of 2, poorly differentiated histology, and extrahepatic recurrence were identified as significant prognostic factors in the univariate analysis, but not in the multivariate analysis. Conclusions: Our results demonstrate potential for long-term survival in liver-limited metastatic anal squamous cell carcinoma amenable to local therapeutic intervention combined with systemic therapy. These findings warrant further prospective study of local therapy in oligometastatic anal cancer. Characteristics N=25 (%) Median number of liver metastases (range), lesions- 1- 2-3- >3 1 (1-20)14 (56)8 (32)3 (12) Median maximal diameter of liver metastases (range), cm 3 (0.9-11.9) Best response of liver metastases to chemotherapy (N=13)- Complete response- Partial response- Stable disease- Progressive disease 2 (15)7 (54)3 (23)1 (8) Surgical procedure (N=20)- Right/left hemi-hepatectomy- Segmentectomy- Wedge resection 4 (20)14 (70)2 (10) Non-surgical procedure (N=10)- Local ablation- Radiotherapy- Other 5 (50)3 (30)2 (20) Hepatic surgical margins (N=20)- Positive margins- Close margins (<1mm)- Negative margins 02 (10)18 (90) Recurrent disease- Yeso Intrahepatico Extrahepatic- No 20 (80)14 (70)6 (30)5 (20)

537 Background: A+B is the preferred first-line (1L) standard of care for uHCC, with emerging RW evidence supporting its use in a broad patient (pt) cohort. We evaluated pt characteristics, clinical and treatment outcomes in pts treated at five U.S. institutions: Mayo Clinic, Houston Methodist, Moffitt Cancer Center, Mays Cancer Center, and University of Arizona. Methods: This is a retrospective study of uHCC pts who initiated A+B after January 1, 2019. De-identified pt data were extracted by treating oncologists. Overall survival (OS) and real-world progression-free survival (rwPFS) were assessed using Kaplan-Meier methods for all pts and for a “trial-like” subgroup with characteristics similar to those in the IMbrave150 trial (ECOG performance status [PS] 0-1, Child-Pugh [CP] class A, albumin-bilirubin [ALBI] grade 1-2). HCC-related hospitalizations were evaluated within 1 year of A+B initiation. Results: A total of 300 eligible pts were treated with 1L A+B (median age 68 years, 79% male, 79% White, 82% BCLC C, 44% viral etiology,12% ECOG PS >1). Liver function was CP A in 73% (36% A5, 31% A6, 6% not reported), CP B in 26% pts (15% B7, 7% B8, 5% B9), and CP C in 2 (<1%) pts. Compared with CP A, more CP B pts had characteristics of compromised liver function (CP A vs B: cirrhosis: 61% vs 91%; ascites: 7% vs 46%; encephalopathy: 4% vs 17%; esophageal varices: 14% vs 32%), bile duct invasion (2% vs 11%), and ECOG PS>1 (8% vs 22%), all Ps<0.001. Over a median follow-up of 8.7 mos, 244 (81%) pts discontinued A+B, primarily due to disease progression. Toxicity-related treatment discontinuation and hospitalizations due to treatment-related adverse events (TRAE) were similar between the two groups (Table). Median OS (mOS) was 14.4 mos (95% CI: 12.3, 18.2) and median rwPFS was 6.8 mos (95% CI: 5.8, 8.4). In the trial-like subgroup (n=194), mOS was 19.5 mos (95%CI: 14.6, 24.7) and median rwPFS was 8.8 mos (95% CI: 7.6, 12.1). In the CP B subgroup, mOS was 5.6 mos (95% CI: 4.6, 11.3) and median rwPFS was 3.1 mos (95% CI: 2.4, 5.8). Conclusions: This study demonstrates the RW outcomes of 1L A+B in a diverse pt cohort. Results from the “trial-like” pts further confirm the reproducible efficacy of A+B in clinical practice. Although pts with CP-B had higher hospitalization rates due to disease progression or symptoms, consistent with their underlying liver complications, they had similar rates of toxicity-related treatment discontinuation and TRAE-related hospitalizations compared to CP-A. Further characterization of safety for A+B in CP B pts within a prospective, controlled trial is warranted. N (%) CP A (n=219) CP B (n=79) Treatment discontinuation* 167 (76) 75 (95) Atezolizumab-related toxicity 20 (12) 13 (17) Bevacizumab-related toxicity 26 (16) 16 (21) Hospitalization* 87 (40) 58 (73) Symptom-related 42 (48) 35 (60) Progression 8 (9) 11 (19) TRAE 7 (8) 6 (10) *P-value comparing CP A vs CP B <0.001.

10 Background: Cell-free circulating tumor tissue modified viral DNA (ctDNA) has been used as a biomarker to detect recurrence for anal squamous cell carcinoma (ASCC). However, its value in informing treatment modification during definitive chemoradiation (CRT) has yet to be explored. We evaluated ctDNA response during and at completion of definitive CRT to establish patterns of disease response and inform future studies to personalize treatment. Methods: From 11/2022 to 6/2024, 13 consecutive patients with biopsy proven, non-metastatic ASCC received definitive CRT and had ctDNA testing using a commercially available droplet digital quantitative PCR assay. ctDNA testing was done at baseline (pre-CRT), week 4 of CRT, and/or end of CRT, 1-3 months post-CRT, and every 6 months thereafter. CRT included concurrent 5FU/MMC (or capecitabine/MMC) and radiotherapy according to RTOG 0529. Treatment response assessment included: physical exam, anoscopy, and imaging with PET/CT, CT C/A/P +/- MRI abdomen and pelvis. Results: 13 patients had ctDNA testing and 11 patients had HPV-positive ASCC and tested positive for ctDNA. The 11 patients with detectable baseline ctDNA were included in this analysis. The median age at diagnosis was 63 years old. Staging included: 1- Stage IIA, 7- Stage IIB, 1- Stage IIIA, 2- Stage IIIC. 1 patient was cN0, while 11 were cN1. With a median follow-up from completion of CRT of 13 months, 4 patients have persistent or recurrent disease: 2 with distant metastases, 1 with persistent local disease (1 month post-CRT), and 1 with local and distant recurrence. Baseline value of ctDNA did not correlate to stage at diagnosis, rate of clearance during CRT, or likelihood of recurrence. 5 of 11 patients cleared ctDNA by week 4 of CRT. Of 5 patients who cleared ctDNA by week 4 of CRT, 4 remain NED and 1 developed distant metastases. Of 6 patients with persistent ctDNA at week 4 of CRT, 3 have persistent or recurrent disease- 1 local persistence 1 month post-CRT, 1 local and distant recurrence 6 months post-CRT, and 1 with distant recurrence 12 months post- CRT. Among 6 patients who cleared ctDNA by the final week of CRT, 1 patient developed recurrent disease. Among the 4 patients in the series with residual/recurrent disease, 3 of them had not cleared ctDNA by the end of CRT. Of 10 patients with ctDNA testing at 1 month post-CRT, 8 had no detectable ctDNA. 2 patients with persistent (1) or recurrent (1) ctDNA at 1 month post-CRT developed disease recurrence- 1 with distant recurrence and 1 with local and distant recurrence. Conclusions: ctDNA assessment during CRT may provide insight into disease response that can predict patterns of failure and facilitate personalized therapy- treatment intensification, deintensification, or additional testing to detect early metastatic disease. Larger studies and clinical validation are needed.