News Release

A peptide-drug conjugate that targets the acidic environment of cancer cells may improve the efficacy of immunotherapy

Reports and Proceedings

American Association for Cancer Research

PHILADELPHIA – A peptide-drug conjugate that targets the acidic environment of cancer cells enhanced the efficacy of immune checkpoint inhibitors in preclinical cancer models, according to results presented at the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics, held October 7-10, 2021.

Immune checkpoint inhibitors are commonly combined with chemotherapy, but the myelosuppression associated with chemotherapy can reduce the efficacy of immunotherapy, explained presenter Sophia Gayle, PhD, an associate director of biology at Cybrexa Therapeutics. Chemotherapy is also associated with various toxicities, due to its effects on non-cancer cells.

CBX-12 is a peptide-drug conjugate that delivers exatecan, a potent cytotoxic agent, specifically to cells with low pH environments, a trait that is characteristic of all cancer types. CBX-12 utilizes a unique variant of a pH-low insertion peptide (pHLIP) that undergoes a conformational change in the presence of low pH, enabling the peptide to deliver exatecan into the cell.

“The novelty of CBX-12 is that it targets acidity, which is a universal feature of all tumors,” said Gayle. “We are, therefore, able to deliver a potent anticancer therapeutic selectively to cancer cells in a much broader patient population, as opposed to antibody-drug conjugates that are restricted primarily to patients whose tumors express high levels of a target antigen.”

The selective targeting of cancer cells also avoids the toxicities associated with other therapies that non-selectively affect normal cells, she added. Gayle and colleagues recently demonstrated that treatment with CBX-12 did not induce myelosuppression in preclinical cancer models. “Since CBX-12 does not lead to myelosuppression, we hypothesized that combining CBX-12 with immunotherapies could be a promising therapeutic strategy,” she noted.

In this study, Gayle and colleagues used mouse models of colorectal cancer to evaluate the impact of CBX-12 treatment on the efficacy of PD-1- and CTLA4-targeted immune checkpoint inhibitors. Mice receiving the combination treatment exhibited significantly delayed tumor growth, improved survival, and complete tumor regressions compared to mice treated with immune checkpoint inhibition alone. Tumor growth was delayed four times longer when CBX-12 was combined with a PD-1 inhibitor and 10 times longer when combined with a CTLA4 inhibitor.

In addition, mice that experienced complete tumor regressions demonstrated long-term immunological memory after tumor rechallenge. Injection of CBX-12-treated tumor cells into mice led to antitumor immunity upon tumor challenge, indicating that CBX-12 induced immunogenic cell death.

“Our results suggest that combining CBX-12 with immune checkpoint inhibition could extend the benefit of immunotherapies to tumors that do not normally respond to them,” said Gayle, noting that this could benefit a wide range of patients, 80 percent of whom have tumors that do not respond to immune checkpoint inhibitors alone.

A limitation of the study is that all experiments were performed in preclinical models; thus, additional research is required to understand the impact of the treatment in patients. A phase I clinical trial to determine the safety and recommended phase II dose of CBX-12 is currently in progress.

The study was supported by Cybrexa Therapeutics, which developed CBX-12. Gayle is an employee of Cybrexa Therapeutics.


Presentation #: P258       

Title: CBX-12 (alphalex™-exatecan) sensitizes tumors to immune checkpoint blockade in an antigen agnostic manner by immune activation

Immune checkpoint blockade (ICB) in combination with chemotherapy is standard of care for several solid tumors. However, potent chemotherapies such as topoisomerase inhibitors can result in severe dose-limiting toxicities requiring dose reduction, limiting their efficacy. Moreover, leukocytopenia from chemotherapy induces immunosuppression that further limits the full potential of combinations with ICB. The use of targeted chemotherapies such as antibody-drug conjugates (ADCs) demonstrate potential synergy with ICB, however are limited to a defined subset of patients with tumors expressing the target antigen. Cybrexa has reported the development of CBX-12, a peptide-drug conjugate consisting of alphalexTM-exatecan (1). Rather than targeting a specific antigen, alphalexTM consists of a unique variant of pH-Low Insertion Peptide (pHLIP®; 2-4) which targets the low pH environment of the tumor, a universal feature characteristic of all tumors due to the Warburg effect. The alphalexTM component of CBX-12 forms an alpha helix only in low pH conditions, allowing for directional insertion of the peptide within the cancer cell membrane, delivery of C-terminally linked exatecan across the membrane, and subsequent intracellular release of active exatecan via glutathione reduction of the linker, thereby allowing for tumor-specific intracellular delivery in an antigen-independent manner. Here we evaluated the potential for CBX-12 to effectively synergize with PD1 and CTLA4 blockade in multiple syngeneic mouse models without requirement for tumor specific antigens. We found that CBX-12/ICB combination treatment significantly delayed tumor growth, improved survival and led to complete tumor regressions. Mice cured with combined CBX-12/ICB therapy demonstrated formation of long-term immunological memory after in vivo and ex vivo tumor rechallenge. The ability of CBX-12 to induce immunogenic cell death was confirmed by vaccinating syngeneic mice with CBX-12 treated tumor cells and subsequent tumor rechallenge, which demonstrated near total anti-tumor immunity induced by CBX-12. Together, these preclinical data demonstrate the potential for CBX-12 to enhance tumor immunogenicity and potentiate the efficacy of ICB in patients with solid tumors affording a superior, universal tumor targeting mechanism that bypasses the limitations of ADCs. 1. Gayle S et al. 2021.Tumor-selective, antigen-independent delivery of a pH sensitive peptide-topoisomerase inhibitor conjugate suppresses tumor growth without systemic toxicity. NAR Cancer. 2. Rather than targeting a specific antigen, alphalexTM includes a pHLIP® peptide. pHLIP® peptides are a family of pH-Low Insertion Peptides that target acidic cell surfaces. pHLIP® was developed at Yale University and the University of Rhode Island, and is exclusively licensed to pHLIP, Inc. 3. Wyatt LC et al. 2017. Applications of pHLIP Technology for Cancer Imaging and Therapy. Trends Biotech. 4. Wyatt LC et al. 2018. Peptides of pHLIP family for targeted intracellular and extracellular delivery of cargo molecules to tumors. PNAS.

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