At a Glance
Dr. Alexander Birbrair, assistant professor in the Department of Dermatology, led a research project aimed at advancing treatments for melanoma. Melanoma was the fifth most common cancer in the United States and had disproportionately higher mortality rates among racial and ethnic minority populations. Recognizing the close relationship between cancer and the nervous system, the team sought to understand how signaling through a receptor called PAC1 affected melanoma progression. The project was intended to inform clinical testing of medications targeting the PAC1 signaling pathway, with the ultimate goal of improving survival rates for all patients with melanoma.
The Challenge
Melanoma is the fifth most common cancer among both men and women in the United States, with approximately 325,000 new cases and 57,000 deaths worldwide each year. Long-standing racial disparities existed among patients with melanoma, in part because of later diagnoses among racial and ethnic minority populations. Despite advances in understanding and treating melanoma, overall survival rates remained poor, with many individuals succumbing to the disease despite appropriate therapy. This highlighted a pressing need for new and more effective treatment approaches.
Project Goals
Cancer and the nervous system shared a close relationship, and the research team’s goal was to understand how signals from nerves within melanoma tumors affected cancer progression. The team was particularly interested in PAC1, a receptor involved in this signaling. The project pursued three specific aims:
- Understand PAC1’s role in tumor cells: The team studied how PAC1 signaling affected certain cells within melanoma tumors.
- Test PAC1 inhibition as a treatment: The team tested a PAC1 inhibitor in combination with immunotherapy in mice to determine whether it effectively treated melanoma.
- Examine PAC1 in human melanoma: The team analyzed human melanoma tissue samples to determine how PAC1 was expressed and assess its importance as a potential treatment target.
Results
The project successfully identified and characterized a previously unrecognized neuro-stromal axis that drives melanoma development and progression. Researchers discovered that melanoma tumors recruit PACAP-positive neurons, which signal directly to PAC1-positive pericytes within the tumor microenvironment. Using PACAP-Cre+ reporter mice, the team found that PACAP-positive neuronal axons aggressively infiltrated both primary and metastatic melanoma tumors as the disease progressed. Reversibly silencing these neurons significantly reduced intratumoral PACAP levels, tumor growth and metastatic dissemination, while injections of PACAP into tumors increased tumor growth.
Analysis of murine and human single-cell RNA-sequencing datasets showed that PAC1 expression was highly enriched in tumor-infiltrating pericytes, identifying these cells as the primary responders to neuronal PACAP. Both a global genetic reduction of PAC1 and deletion of PAC1 specifically in pericytes markedly reduced melanoma growth and metastatic burden in the lungs.
The team also demonstrated the therapeutic potential of targeting this pathway using PA-810-04, a novel and highly selective small-molecule PAC1 antagonist. In laboratory studies, PACAP stimulated pericyte proliferation through the cAMP/PKA pathway, an effect that was completely reversed by PAC1 inhibition. In preclinical models, systemic administration of PA-810-04 profoundly slowed melanoma growth and significantly reduced metastatic nodule formation without overt toxicity. PAC1 inhibition also impaired pericyte-mediated blood vessel formation, reduced intratumoral collagen deposition and significantly increased CD8-positive T-cell infiltration within metastatic lesions, effectively turning immunologically “cold” tumors “hot.”
Findings from human melanoma datasets and tissue specimens further supported the clinical relevance of the research. High expression of ADCYAP1, which encodes PACAP, and ADCYAP1R1, which encodes PAC1, was strongly associated with decreased overall survival, a reduced response to immune checkpoint inhibitor therapy and lower levels of antitumor CD8-positive T-cell infiltration. Spatial transcriptomics and immunohistochemistry confirmed that PAC1 protein was primarily localized to nonmalignant PDGFRβ-positive pericytes in advanced, metastatic melanoma.
Looking to the Future
The team plans to move the PACAP-PAC1 findings from biological discovery toward clinical translation. Immediate next steps include testing PA-810-04 in combination with FDA-approved immune checkpoint inhibitors across a diverse range of advanced melanoma preclinical models, with the goal of advancing this therapeutic strategy toward Phase I clinical trials.
Researchers will expand engagement with clinicians in the UW–Madison Division of Hematology, Medical Oncology and Palliative Care, particularly the Melanoma and Skin Cancer Clinical Research group. They will also continue working with practicing dermatologists at UW–Madison and New York-Presbyterian Hospital to assess the clinical translatability of the findings and design future studies for patients with metastatic melanoma. In parallel, the team plans to consult with WARF Therapeutics regarding drug development, intellectual property optimization and potential commercialization of PAC1-targeted therapies.
To sustain and expand the research, the team used findings as preliminary data for a recently awarded NIH National Cancer Institute (NCI) Method to Extend Research in Time (MERITH) R37 grant. This funding will support the continued development and eventual clinical application of PAC1 inhibition as a strategy for treating advanced melanoma and overcoming resistance to immunotherapy.
Lasting Impact
The project has redefined understanding of how the peripheral nervous system helps orchestrate the tumor microenvironment by identifying the PACAP-PAC1 axis as a driver of melanoma growth, angiogenesis, extracellular matrix remodeling and immune evasion. Because up to half of patients with melanoma do not respond to standard immune checkpoint inhibitors, the findings establish PAC1 antagonism as a promising therapeutic strategy that may both restrict tumor growth and help overcome immunotherapy resistance by restoring CD8-positive T-cell infiltration.
The award also strengthened institutional capacity for multidisciplinary melanoma research. The project provided research training and mentorship to Dr. Debpali Sur, PhD students Brajesh Savita and Taeho Lee and 15 undergraduate researchers. Many of these trainees are using their experience to pursue medical school, physician assistant programs, PhD programs or careers in biotechnology and public health. By engaging a diverse group of scientists, including trainees from racial and ethnic minority groups, the project also helped address barriers to equitable representation in the scientific and medical workforce.
The grant strengthened Dr. Birbrair’s leadership and established a broader network of collaborations across UW–Madison. New and expanded partnerships included the research groups of Drs. Nader Sheibani, Christine M. Sorenson and Justin Wolter, as well as collaborations with Drs. Paul Sondel, Kevin Eliceiri, Vince Ma and Nihal Ahmad. These partnerships have increased the team’s capacity to advance patient-centered and translational melanoma research beyond the grant period.
