Prostatic Acid Phosphatase (PAP) Antibodies to Treat Castration-Resistant Prostate Cancer.

Prostate cancer (PCa) is the most common cancer and the second leading cause of cancer death in American men. Most patients with metastatic disease respond initially to androgen deprivation therapy (ADT) but almost inevitably progress to castration-resistant prostate cancer (CRPC). Identification of markers and drivers of Metastatic CRPC (mCRPC) that (a) represent a progenitor-type cancer cell population, (b) persist in castration-resistant disease, (c) are actionable targets expressed on the cell surface, and (d) are induced by hypoxia is required to facilitate the development of novel targeted therapies. We identified prostatic acid phosphatase (PAP), particularly the transmembrane form (TMPAP), as one such potential target. PAP is both a phosphatase and a 5'ectonucleotidase that generates adenosine. PAP is a human tumor marker first described in 1936 and is still used as an important prognostic marker for advanced metastatic prostate cancer. Our group recently reported that the transmembrane form of the protein (TMPAP) is expressed in CRPC and can serve as a potential therapeutic target. We identified a lead human anti-TMPAP antibody clone 3D8 (3D8-Ab). 3D8-ADCs (Antibody Drug Conjugates) and 3D8-Ab were tested for their ability to reduce tumor size/volume in a xenograft model. The human PAP-expressing PCa cell line VCaP, originally derived from a vertebral metastasis from a patient with CRPC, was inoculated subcutaneously into SCID mice. Treatment with either 3D8-Ab or 3D8-ADC significantly reduced tumor size and increased animal survival. These data indicate that targeting PAP with monoclonal antibodies either alone or conjugated to toxins has the potential to treat CRPC.