Abstract 4328: Epigenetic modulation of SSTR2 expression provides the potential to broaden PEN-221 treatment population

The expression of somatostatin receptor subtype 2 (SSTR2) is increased in up to 80% of neuroendocrine tumors (NETs), 20-40% of small cell lung cancers (SCLC) and a number of other tumor types. As a cell surface receptor, SSTR2 is a logical target for a drug conjugate. However, marginal, or loss of, expression of SSTR2 in tumor tissues has potential to limit the benefit from such conjugates. Recent published preclinical studies have demonstrated that epigenetic modulators, such as HDAC inhibitors, can result in increased SSTR2 expression with treatment. Given that the expression of SSTR2 can be variable from patient to patient and in some cases low to absent, we hypothesized that the combination of epigenetic modulators with an SSTR2 target therapy may provide a benefit for patients who are not eligible for these targeted approaches on their own. PEN-221 is a miniature drug conjugate of an SSTR2 targeting peptide attached to the potent cytotoxic DM1 through a cleavable disulfide linker. PEN-221, currently in Phase 1/2a (NCT02936323), is designed as a potent and selective anticancer agent to treat patients whose tumors express SSTR2. The targeting peptide of PEN-221 selectively binds to SSTR2, triggers receptor internalization leading to the accumulation of the DM1 to provide antitumor activity by disrupting microtubule networks causing apoptosis and mitotic catastrophe. In preclinical studies, single agent PEN-221 treatment leads to complete and sustained tumor regression in xenograft models that over-express SSTR2. We hypothesize that increased expression of SSTR2 as a result of epigenetic modulation from HDAC or other epigenetic modulators, may allow for the broadening of PEN-221 treatable tumors. To evaluate these hypotheses, we have combined PEN-221 and epigenetic modulators in preclinical models of cancer. Efficacy studies were carried out in models expressing various degrees of SSTR2, and combinations resulted in greater efficacy than that of the single agent. In addition, a pharmacodynamic assessment of epigenetic modulator treatment on SSTR2 expression levels was performed. These data demonstrate that combination of PEN-221 and epigenetic modulators provide greater efficacy than single agent activity alone and support the evaluation of such combinations in the clinical setting. Citation Format: Samantha Perino, James Quinn, Jessica Freda, Brian White, Scott Bailey, Viren Bhatia, Beata Sweryda-Krawiec, Mark Bilodeau, Jeffrey D. Bloss, Kerry Whalen, Richard Wooster. Epigenetic modulation of SSTR2 expression provides the potential to broaden PEN-221 treatment population [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4328.