Prostate cancer is the second leading cause of cancer-related deaths among American men, claiming almost 35,000 lives annually. Androgens, male sex hormones, fuel the growth of most prostate tumors by binding to the androgen receptor (AR) in prostate cancer cells. Thus, drugs that blocking the synthesis of androgens or preventing androgens from binding AR, known as androgen-deprivation therapy, is initially effective in treating most people with prostate cancer. However, resistance to androgen deprivation therapy develops in almost all patients with relapsed prostate cancer. The resulting castration-resistant prostate cancers (CRPCs) are characterized by a range of genetic changes including loss of AR (AR- tumors) and retinoblastoma transcriptional corepressor 1 (RB1) expression and gain of neuroendocrine characteristics (NE+ tumors). Neuroendocrine prostate cancer (NEPC) is the most aggressive disease subtype. Cytotoxic chemotherapy is the only available treatment option for NEPC patients and their prognosis is poor. Therefore, there is a critical clinical need for novel therapeutic options for these patients.
“Targeting cell-surface antigens through the delivery of cytotoxic agents directly to cancer sites or by generating anti-tumor immune responses are promising therapeutic approaches for advanced cancers”, says Dr. Michael Haffner, a physician-scientist who specializes in genitourinary cancers and lead author of a recently published paper in Precision Oncology. All cell express membrane proteins on their cell surface that often play important roles in the function of that cell type. For example, epithelial cells express cadherins that are important for creating cell-cell junctions that mediate their barrier function. By identifying membrane proteins that are restricted to a particular tumor type, investigators hope to use these targets as a molecular “beacon” that can be used to direct the delivery of a cytotoxic payload or stimulate an immune response to kill the cancer cell. Prostate-specific membrane antigen (PMSA) is currently the most extensively validated cell surface target for prostate cancer. Although PSMA has a relatively prostate-restricted expression, up to 40% of CRPCs have no or heterogeneous PSMA expression. NEPC, for instance, has nearly universal loss of PSMA expression. Thus, there is a need to identify novel targets and characterize their expression patterns in prostate cancer to harness the promise of cell-surface protein targeted-therapies.
In their recent paper, Dr. Michael Haffner and his team analyzed the expression of three cell surface proteins in advanced prostate tumors that are the focus of recent preclinical and clinical trials. Using samples collected by the University of Washington Medical Center Prostate Cancer Donor Rapid Autopsy Program lead by Dr. Colm Morrissey, they analyzed 753 tissue samples from 52 CRPC patients for the expression of delta-like ligand 3 (DLL3), carcinoembryonic antigen-related cell adhesion molecular 5 (CEACAM5), and trophoblast cell-surface antigen 2 (TROP2). DLL3 and CEACAM5 were previously reported to have high expression in the majority of NEPC tumors. Meanwhile, TROP2 is expressed in many types of cancer. TROP2-targeting agents have been approved for triple-negative breast cancer and urothelial carcinoma and are in phase 2 trials for CRPC, however, limited data is available on TROP2 expression in advanced prostate cancer. “Through the unique design of this cohort we were able to assess tissue expression heterogeneity across and within different metastatic tumor deposits,” says Dr. Haffner. As expected, DLL3 and CEACAM5 expression was mostly restricted to AR-/NE+ tumors and those with RB1 loss. Conversely, TROP2 was expressed at high levels in most tumors including those lacking AR expression (AR+/NE-, AR+/NE+, AR-/NE-), but not in NEPC (AR-/NE+) or those with RB1 alterations. TROP2 and DLL3 exhibited relatively limited inter-tumoral heterogeneity, despite their prostate cancer sub-type restricted expression. Four of the 52 patients (8%) showed no expression of TROP2, CEACAM5, DLL3 or PSMA. As further validation of cell surface target expression, Dr. Haffner and his team analyzed histone modifications at the transcriptional start sites and gene bodies of TROP2, DLL3 and CEACAM5 in cells from prostate cancer patient-derived xenografts. As expected, tumors with low or no detectable TROP2, DLL3 or CEACAM5 expression displayed a strong enrichment for repressive H3K27me3 marks at the respective gene locus.
Together, Dr Haffner and his team have examined the expression of clinically relevant cell surface targets in CRPC and generated to date, the most comprehensive analysis of these—TROP2, DLL3 and CEACAM5—in CRPC. Their work highlights the molecular sub-type specific expression of these proteins and provides insights for future development of drugs targeting these cell surface markers. “These studies are important for future clinical trials testing agents that target DLL3, CEACAM5 and TROP2. In addition, our data suggest that epigenetic therapies could be used to augment target expression,” says Dr. Haffner.
The spotlighted research was funded by the National Institutes of Health, US Department of Defense, the Doris Duke Charitable Foundation, the V Foundation, the Prostate Cancer Foundation, the Safeway Foundation, the Richard M. Lucas Foundation, the Seattle Cancer Consortium, the Brotman Baty Institute for Precision Medicine and the UW/FHCC Institute for Prostate Cancer Research.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Michael Haffner, Peter Nelson, Michael Schweizer, Colm Morrissey, Eva Corey, Gavin Ha, Jessica Hawley, Robert Montgomery, Evan Yu, Heather Cheng, Lawrence True contributed to this work.
Ajkunic A, Sayar E, Roudier MP, Patel RA, Coleman IM, De Sarkar N, Hanratty B, Adil M, Zhao J, Zaidi S, True LD, Sperger JM, Cheng HH, Yu EY, Montgomery RB, Hawley JE, Ha G, Persse T, Galipeau P, Lee JK, Harmon SA, Corey E, Lang JM, Sawyers CL, Morrissey C, Schweizer MT, Gulati R, Nelson PS, Haffner MC. 2024. Assessment of TROP2, CEACAM5 and DLL3 in metatstatic prostate cancer: expression landscape and molecular correlates. npj precision oncology. 8:104.
Nick Salisbury is a postdoctoral fellow in the Galloway lab at Fred Hutch. Nick's research focuses on understanding how DNA viruses, such as Merkel cell polyomavirus, cause cancer and developing new targeted therapies to treat malignancies caused by viruses. Originally from UK, he completed his BA and PhD at University of Cambridge before moving to US in 2016.