Targeting B7-H3 in the small cell lung carcinoma (SCLC) tumor-immune microenvironment (TIME)

Introduction: Despite incorporation of immune checkpoint blockade into first line treatment of SCLC, patients have a poor prognosis with limited treatment options. Recent profiling of the SCLC TIME has revealed distinct immunologic subtypes: non-neuroendocrine (non-NE) SCLC, which exhibits robust upregulation of MHC I and immune infiltration, and neuroendocrine (NE) SCLC subpopulations which downregulate MHC I and show an “immune desert” TIME. Therapeutic strategies targeting B7-H3 using the antibody-drug conjugate (ADC), ifinatamab deruxtecan (I-DXd), has shown recent promise in SCLC. In the IDeate-Lung01 trial, patients treated with I-Dxd after ≥1 prior line of chemotherapy showed a disease control rate of 87.6% (NCT05280470). Yet, the mechanism of action on the constituent cells of the SCLC TIME remains relatively unexplored. In this study, we aimed to determine the expression of B7-H3 in the SCLC TIME and consequent susceptibility to I-DXd, of SCLC subpopulations and stromal cells using a SCLC TIME 3D microphysiological system (MPS). Methods: We initially profiled B7-H3 expression using a human SCLC cell line panel representative of the NE MHC I low (n=3) or non-NE MHC I high (n=4) phenotype, as well as stromal components (endothelial cells, fibroblasts) by flow cytometry. We validated these findings in patient SCLC samples (n=31) using a mIF panel. We then evaluated the direct cytotoxic effect of the I-DXd on SCLC cell lines and stromal components using 2D and 3D in vitro cytotoxicity assays. Further, we evaluated I-DXd in a model of vascular networks using an ex vivo MPS. Finally, using the MPS, we assessed the functional impact on vascular structure via permeability assays. Results: NE MHC I low SCLC cell lines showed low/moderate levels of B7-H3 expression, while non-NE MHC I high SCLC upregulated B7-H3. B7-H3 was also highly expressed by stromal TIME components such as vascular endothelial cells (HUVEC) and human lung fibroblasts (hLFB). While SCLC expression of B7-H3 was more variable in patient samples, and did not show a clear correlation with tumor MHC I expression, stromal cells, particularly endothelial cells showed strong B7-H3 expression, and, in the majority of cases, were the major contributors of B7-H3 expression in the TIME. Non-NE SCLC cells showed sensitivity to I-DXd in a B7-H3 dependent manner in a 2D viability assay, while NE SCLC cells exhibited relative resistance. Experiments using vascular networks in the MPS demonstrated that I-DXd caused increased vascular permeability. Conclusions: Our study suggests that B7-H3 expression is heterogenous in SCLC and may differ by immunologic subtype. Moreover, our findings suggest that B7-H3 may serve as a target on stromal components of the SCLC TIME. Our results suggest that in addition to the direct targeting of tumor cells, a potential mechanism of action underlying the efficacy of I-DXd may be an increase in vascular permeability.