Abstract 5094: Immunotherapy in advanced cutaneous non-melanomatous spindle cell neoplasms

Abstract Background: Non-melanomatous cutaneous spindle cell neoplasms (NMCSpC) are a rare group of malignancies including pleomorphic dermal sarcoma (PDS) and spindle-cell cutaneous squamous cell carcinoma (SpSCC). They present a diagnostic challenge with overlapping morphological features and variable patterns of immunohistochemical (IHC) staining. There is a lack of consensus on how to best manage advanced NMCSpC and limited reports of immunotherapy (IO) responses. This study reviews the efficacy of IO in NMCSpC and performs comprehensive tumor profiling to identify biological rationale for IO use in this rare group of tumors. Methods: We performed a single-center retrospective review of all locally-advanced or metastatic NMCSpCs treated with IO. Blinded histopathology reviews occurred to confirm each diagnosis. Overall response rates (ORR) were collected as per Response Evaluation Criteria in Solid Tumors 1.1 or Positron Emission Tomography Response Criteria 1.0. Comprehensive tumor profiling included whole exome sequencing for tumor mutational burden (TMB) and ultraviolet(UV)-signatures, and IHC for immune-cell infiltration (CD103/CD4/CD3/CD8/CD20) and immune-checkpoint expression (PD-L1/TIGIT/LAG3). Results: 7 patients were identified. Median follow up was 18.1 months (range: 5.3-34.7). ORR 86% (6/7) with five complete responses (CR). One patient who achieved stable disease progressed. Responses were durable with two patients in CR >30 months after IO commencement. Blinded pathological review identified 3 SpSCC, 2 PDS and 2 spindle cell tumors of unclear sub-classification. All patients had high TMB and UV-signatures. One patient had PD-L1 100% (combined positive score) with abundant immune-cell infiltration and LAG3 expression (Table 1). Conclusion: In advanced NMCSpC, excellent responses to IO with durable disease control was observed. IO is worthy of further exploration in these patients. UV-signatures and high TMB could be used to help select patients for treatment. Immunohistochemistry (IHC)^, tumor mutational burden and ultraviolet signatures Patient T-cell markers B-cell marker Immune-checkpoints Tumor mutational burden (mutations/megabase) Ultraviolet-mutational signature Best response assessment# CD103 CD4 CD3 CD8 CD20 PD-L1 TIGIT LAG3 1 1 20 1 1 0 7 0 0 17.94 Yes Stable metabolic response 2 1 20 30 10 0 100 0 10 35.34 Yes Complete metabolic response 3 1 10 10 5 0 5 0 0 104.87 Yes Complete metabolic response 4 1 0 1 0 0 0 0 0 74.04 Yes Partial metabolic response 5* NA NA NA NA NA NA NA NA 29 Yes Complete metabolic response 6 1 0 0 5 0 0 0 4 12.02 Yes Complete metabolic response 7 1 0 5 1 0 0 0 0 41.17 Yes Complete response ^IHC scores are % of positively stained cells, except for PD-L1 which is combined positive score (CPS). *Insufficient tissue for IHC on Patient 5. #As per PERCIST 1.0 on FDG-PET/CT or RECIST1.1 on CT. Citation Format: Luke S. McLean, Annette M. Lim, Christopher Angel, Richard J. Young, Angela Pizzolla, Stuart Archer, Alesha A. Thai, Benjamin Solomon, Jeremy Lewin, Danny Rischin. Immunotherapy in advanced cutaneous non-melanomatous spindle cell neoplasms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5094.