Abstract CT155: Clinical biomarker studies with an enhanced potency oncolytic HSV expressing an anti-CTLA-4 antibody, as a single agent and combined with nivolumab in patients with advanced solid tumors indicates potent immune activation
Authors: Harrington Kevin J. , Bommareddy Praveen K. , Middleton Mark R. , Sacco Joseph J. , Olsson-Brown Anna , Chan Tze Y. , Nenclares Pablo , Leslie Isla , Aroldi Francesca , Saleem Imran , Ahlers Christoph M. , Castro Henry , Coffin Robert S. 
Published: June 2022

Introduction: RP2 is a novel, enhanced potency oncolytic HSV1 which expresses GM-CSF, an anti-CTLA-4 antibody-like molecule and the fusogenic gibbon ape leukemia virus membrane R- glycoprotein (GALV-GP R-). RP2 is in clinical development in a range of solid tumors alone and with nivolumab (nivo). RP2 + nivo has resulted in deep and durable responses in patients who failed prior anti-PD1 therapy (SITC 2021). Here we present biomarker data in patients treated with RP2 alone or combined with nivo from an ongoing clinical trial (NCT04336241).

Methods: Tumor biopsies and peripheral blood mononuclear cells (PBMCs) were collected pre-treatment and at D43. The tumor immune microenvironment (TIME) was analyzed by multi-plex (7 color 6-plex - CD8, PD-L1, PD-1, foxp3, CD68 and S100B) immunohistochemistry (IHC) of tumor biopsies using the Opal Human Panel (OHP) 6043 and by gene expression analysis using the NanoString IO360 panel. The tumor inflammation signature score (TIS) was also calculated using an 18 gene signature (Ayers JCI 2017) . Systemic anti-tumor immunity was assessed using PBMCs by sequencing the CDR3 regions of TCRβ chains using immunoSEQ Assay. Correlation analysis of baseline tumor PD-L1 and CD8 status versus clinical responses was also performed.

Results: IHC indicated robust increases in CD8 T cell influx and PD-L1 expression post-RP2 alone and with RP2 + nivo. An increase in the CD8/foxp3+ cell ratio was observed by multi-plex IHC. A consistent increase in CD8 and PD-L1 was observed in most of the tested biopsies (~70%), which generally appeared to be co-located (n=20). These increases in CD8 and PD-L1 expression levels were observed in both superficial and visceral tumors. A particularly striking change was observed in a biopsy obtained from a liver lesion from a tebantafusp and ipi/pembro-failed uveal melanoma patient. Clinical responses were independent of baseline CD8 T cell infiltration, PD-L1 expression levels, and prior anti-PD-1 therapy status. Gene expression analysis of tumor biopsies (n=12) indicated increases in the expression of key genes associated with immune activation, particularly those associated with dendritic cell function, major histocompatibility complex-II and interferon-gamma signature. Increases in expression of genes associated with ARG1, cytotoxicity, IDO1, NK cell and Th1 cell abundance were observed, particularly in responding patients. TCR sequencing of PBMCs revealed expansion of pre-existing T cell clones and the appearance of new clones post-RP2 monotherapy and RP2 + nivo, with ~50% of these changes being newly detected clones. Expansion of pre-existing clones and generation of new T cell clones specific for MART-1 was also observed with RP2 monotherapy and in combination.

Conclusion: The biomarker data presented indicates broad immune activation by RP2 and demonstrates that clinical response does not correlate with baseline PD-L1 and CD8 expression status. Clinical responses were often associated with increases in gene signatures associated with cytotoxic T, NK and Th1 cells. These data indicate the potential for broad utility of RP2 in a range of tumor types, including in patients with primary or acquired resistance to immune checkpoint blockade.

Citation Format: Kevin J. Harrington, Praveen K. Bommareddy, Mark R. Middleton, Joseph J. Sacco, Anna Olsson-Brown, Tze Y. Chan, Pablo Nenclares, Isla Leslie, Francesca Aroldi, Imran Saleem, Christoph M. Ahlers, Henry Castro, Robert S. Coffin. Clinical biomarker studies with an enhanced potency oncolytic HSV expressing an anti-CTLA-4 antibody, as a single agent and combined with nivolumab in patients with advanced solid tumors indicates potent immune activation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT155.