TROP2 as an actionable biomarker for anal cancer.

3519 Background: Chemoimmunotherapy is standard frontline therapy for metastatic anal cancer, with no effective treatment options afterward. Mutation profiling has not yielded matched targeted therapies against anal cancer. Trophoblast cell-surface antigen 2 (TROP2) is overexpressed in other solid tumors, and TROP2 antibody-drug conjugates (ADCs) have demonstrated efficacy in breast and lung cancers. Given the therapeutic availability against TROP2, we evaluated expression of TROP2 in anal cancer as an actionable therapeutic target. Methods: Metastatic anal cancer tumors from 33 patients at MD Anderson were sequenced by bulk RNA sequencing (“Tumor Portrait” assay, Boston Gene) to quantify TACSTD2 (TROP2) gene expression. TACSTD2 expression was compared across all solid tumors and classified according to ranked expression percentile: high (>83 rd percentile), medium (17-83 rd percentile), or low (<17 th percentile). Gene expression (Nanostring) for >18,000 genes was measured using whole-genome digital spatial profiling (DSP) on a separate cohort of 40 chemoradiotherapy-refractory localized anal cancers collected at salvage surgery. TACSTD2 gene expression on tumor cells vs TME cells was compared with a t-test (SPSS). TROP2 protein expression (Invitrogen) was quantified by H-score using immunohistochemistry (IHC) and correlated with TACSTD2 expression via Spearman’s correlation. Three patient-derived xenograft (PDX) models of anal cancer (C1411, C1436, O0026) were treated with normal saline (untreated control, UTC) or the TROP2 ADC sacituzumab govitecan (sac-gov; 10 mg/kg IP twice weekly). Tumor growth inhibition (TGI) was defined as 1 - (mean tumor volumes of sac-gov/UTC) at 21 days. Results: Median TACSTD2 (TROP2) gene expression for patients with metastatic anal cancer was ranked at the 75 th percentile (IQR 63-85) relative to all solid tumors: 13/33 (39%) with high, 20/33 (61%) with medium, and none with low TACSTD2 expression. For localized anal cancers, TACSTD2 gene expression using DSP was significantly higher on tumor segments vs TME segments (log fold change 3.49, adjusted p < 0.0001). 30/39 (77%) anal cancers had high TROP2 expression by IHC, defined by H-score >200. Correlation between TACSTD2 gene expression and TROP2 protein expression by IHC was observed (r= 0.43, p < 0.001). In PDX models, TGI with sac-gov relative to UTC was observed in both models with high TROP2 expression (C1411: TGI 54%, p< 0.001; C1436: TGI 52%, p< 0.001) but not in a model with no TROP2 expression (O0026: TGI 12%, p=n.s.). Conclusions: TROP2 expression is high for localized and metastatic anal cancer. TACSTD2 gene expression correlates with matched TROP2 protein expression for anal cancer. Anti-tumor efficacy of TROP2 ADCs in vivo supports a forthcoming trial of sacituzumab tirumotecan for patients with treatment-refractory metastatic anal cancer, with the ultimate goal of establishing TROP2 as a predictive biomarker for treatment benefit in this rare cancer.