Abstract 268: Cytotopically modified antibodies to checkpoint proteins can actively reconstitute immune checkpoint blockade and inhibit tumor growth in a prostate cancer mouse model

Abstract

Abstract Introduction & Objectives: Prostate cancer progression can arise by tolerance to, and evasion of tumour cells from immune effector cells such as CD8 T cells and NK cells due to the immunosuppressive tumour microenvironment. Immunotherapeutic drugs can eliminate tumours by breaking this tolerance, and such drugs that can be localized to the prostate will have greater efficacy and less systemic toxicity than drugs administered systemically. It has also been shown that a localized stimulation of the immune system can boost immunity all over the body to clear distant metastases. However, in the highly immunosuppressive prostate cancer microenvironment, most activated effector immune cells are rendered anergic or even regulatory. We have shown that IL-15 in a localizable form retains its ability to activate NK and CD8 T cells in the presence and absence of prostate cancer cells. We have now used a similar approach to modify two antibodies (patent pending) which have been shown previously to be potent in inhibiting checkpoint blockade of the immune response in clinical trials. Material & Methods: The antibodies were modified to enable their conjugation to a cytotopic “tail” molecule. The “tail” consists of three regions - a thiol reactive region which interacts with free cysteines on the protein of interest, a cationic region, which binds to the negatively charged cell membrane surface, and a hydrophobic region that enters the cell membrane. The cytotopic antibodies can then adhere to cell membranes and can thus localize to any lesion where it is potentially injected. The tailed agents were then compared to non-tailed ones in in-vitro assays of T cell inhibition to measure their neutralizing ability and an in-vivo prostate tumour challenge model in C57/BL6 mice. Results: We have shown that the modified antibodies can attach to cell membranes through their cytotopic tails. The two cytotopically modified antibodies were both active in reconstituting T cell activation as shown by IL-2 secretion assays in Jurkats and EL4 by upto 50% (p<0.05 n = 5) which was equivalent to that seen with the non-modified antibodies Furthermore we have also demonstrated in vivo in a TRAMP-C2 tumour challenge model using C57/BL6 wild type mice model that a cocktail of tailed antibodies and cytotopically modified IL-15 can completely clear animals of tumour by day 73 whereas tumour size is only reduced by 50% with the non-tailed cocktail. Conclusion: We have identified that a cocktail of a modified cytokine, IL-15, together with two antibodies directed to immune checkpoint proteins can break tolerance to immunity in the prostate cancer microenvironment and cause disease regression. A tailed form of these molecules, has equivalent or greater activity in in vivo model and makes it therefore a promising new agent for immunotherapy of prostate cancer. Citation Format: Dorota Smolarek, Christina A. Sakellariou, Prokar Dasgupta, Richard A. G. Smith, Christine Galustian. Cytotopically modified antibodies to checkpoint proteins can actively reconstitute immune checkpoint blockade and inhibit tumor growth in a prostate cancer mouse model. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 268. doi:10.1158/1538-7445.AM2015-268