ACPP (PSAP) is a dephosphorylating enzyme produced in prostatic glandular epithelium. Increased serum levels of ACPP were shown to be associated with prostate cancer in the 1930’s, particularly in patients with bone metastases. This enzyme was one of the first tumor serum markers, and it was the main prostate cancer biomarker until the discovery of PSA (Gutman, 1938; Muniyan, 2013; Rao, 2008). Higher ACPP expression in tumors has been correlated with increasing tumor stage (Gunia, 2009), although ACPP is thought to be a tumor suppressant by means of vitamin D-associated slowing of prostate growth (Lin, 1992). Lower expression in normal tissue is therefore thought to be a risk factor for the development of cancer (Stewart, 2005; Kong, 2013). Recently, this target has been used successfully to sensitize patient dendritic cells in order to stimulate an immune response to cancer (Fong, 2001). Although it is highly expressed in prostatic glandular epithelium, ACPP is not as specific as PSA, and has been occasionally associated with other tumors such as lung carcinoid tumors (Azumi, 1991).

AMACR/P504S is a mitochondrial and peroxisomal enzyme that is overexpressed in prostate intraepithelial neoplasia and prostate cancer (Luo, 2002; Evans, 2003; Lloyd, 2013). AMACR functions to oxidize fatty acids, drugs such as ibuprofen, and bile acid intermediates. AMACR stimulates prostate cancer growth, but unlike other prostate cancer markers, the effects of this target have been demonstrated to be independent of androgen signaling (Zha, 2003). Because of the enzymatic activity of this target, it is a potential mechanistic contributor to the relationship between high consumption of dietary branched chain fatty acids (present in meats and dairy products) and increased risk of prostate cancer (Lloyd, 2008; Zhu, 2005; Stacewicz-Sapuntzakis, 2008; Capurso, 2017).

CDKN1B, commonly known as p27 (or KIP1), is an important nuclear regulator of cell cycle progression. p27 protein is specifically involved in g1 arrest, where it functions to stop or slow down the cell division cycle. Loss of p27 nuclear expression and/or translocation from the nucleus to cytoplasm has been associated with poor prognosis and potential disease progression in a variety of cancers, such as squamous carcinomas of head and neck, melanomas, and lung carcinomas (Hnit, 2015; Vallonthaiel, 2016; Dobashi, 2017; Chu, 2008; Denicourt, 2007; Tsihlias, 1998). This target may also be associated with hereditary cancers: mutations in CDKN1B have been linked to an increased risk of prostate cancer (Chang, 2004). Decreased expression of p27 has been associated with growth in pituitary adenomas, and circulating p27 autoantibodies (hence decreased p27 expression) have been correlated with poor prognosis in osteosarcoma (Li, 2016; Martins, 2016; Bamberger, 1999; Teixeira, 2000). Conversely, the indolent course of thyroid papillary carcinomas has been attributed to the presence of p27 (Garcia-Rendueles, 2017). Tumor cells that are quiescent (non-dividing) are protected from the cytotoxicity of many chemotherapeutic agents. Therefore, disrupting the normal nuclear expression of p27, which prevents cells from entering the mitotic pathway, may enhance response rates to chemotherapy, making this target important in the search for new cancer treatments (Becker, 2017; Barzegar, 2017).

ELAC2 (HPC2 / Prostate Cancer, Hereditary, 2) is a nuclear and mitochondrial endonuclease enzyme that functions in tRNA processing (Rossmanith, 2011). Missense variants in ELAC2 are associated with a small percentage of familial prostate cancers (Rebbeck, 2000; Tavtigian, 2000; Xu, 2000; Xu, 2001).

Folate Hydrolase 1 (FOLH1, also known as Prostate-Specific Membrane Antigen or PSMA) is a membrane-associated protein that is highly expressed in prostatic epithelium. It increases in expression progressively with increasing grade in prostatic intraepithelial neoplasia and prostatic carcinoma. Decreased expression is associated with poor survival in prostate cancer (Bostwick, 1998; Murphy, 1998). The function of FOLH1 is not well understood (Kaittanis, 2018). Recently, this target has been shown to be expressed in neovascular endothelium in a number of non-prostatic carcinomas, including lung, pancreatic, and renal cell carcinomas, and glioblastomas. Positive expression in endothelium may predict a positive response to chemotherapy (Baccala, 2007; Nguyen, 2016; Stock, 2017; Wang, 2015; Wernicke, 2011).

Fork-head box protein A1 is a nuclear transcription factor that is activated in embryogenesis of the liver. It is associated with the function of nuclear hormone receptors including estrogen, and is integral to tamoxifen effectiveness in breast cancer. FOXA1 is also associated with the function and distribution of androgen receptors in prostate tissue, and mutations in FOXA1 have been demonstrated in prostate cancer (Sahu, 2011; Hurtado, 2011; Barbieri, 2012). This target has also been associated with aggressive prostate cancers, and may be an independent predictor of recurrence (Tsourlakis, 2017).

Beta-microseminoprotein (MSMB), also known as prostate secretory protein 94 (PSP94), is a secreted immunoglobulin binding protein present in seminal fluid that is synthesized in prostate epithelium (Hara, 1989). The single nucleotide polymorphism rs10993994 in MSMB has been associated with an increased risk for prostate carcinoma (Whitaker, 2010), leading to the theory that MSMB plays a protective role in prevention of prostate carcinoma (Anklesaria, 2018).

Prostein (also known as prostate cancer-associated protein 6 / P501S / SLC45A3) is a protein present in the golgi apparatus of benign and malignant prostatic glandular epithelium, and shows perinuclear cytoplasmic localization in immunohistochemical experiments (Xu, 2001; Sheridan, 2007). Because it is highly specific for prostate glandular cells, this target is useful for differentiating extra-prostatic metastases from other carcinomas such as urothelial carcinomas or colorectal carcinomas (Xu, 2001; Lane, 2008; Chuang, 2007; Sheridan, 2007). Although it may show diminished expression in some aggressive prostate cancers, this target is sometimes expressed in PSA-negative prostate carcinomas, and these two targets used in combination can lead to increased sensitivity in the identification of prostate cancer metastases (Perner, 2013; Sheridan, 2007).

Most commonly known as prostate specific antigen (PSA), Kallikrein-Related Peptidase 3 (KLK3) is a secreted protease produced in prostate glandular epithelium that is thought to function in the liquefaction of seminal fluid. PSA is a highly sensitive and specific clinical marker for recurrent prostate carcinoma after prostatectomy (Oesterling, 1991). It is also useful for confirming suspected prostatic origin in the setting of metastatic carcinoma (Stamey, 1987; Oesterling, 1988).

VEGFA (vascular endothelial growth factor A) is a secreted mitogenic cytokine that stimulates endothelial cells, promoting angiogenesis and increased vascular permeability through interactions with VEGF receptors. VEGFA is the target of antiangiogenic chemotherapueutic agents such as bevacizumab (Grothey, 2008), and measurement of serum VEGFA has been explored as a potential noninvasive means of monitoring efficacy of antiangiogenic therapy in cancers (Caporarello, 2017; Secord, 2014). VEGFA is expressed in osteoblasts and is important in normal bone development. It is thought to play a significant role in the unique propensity for bone involvement of metastatic-prostate cancer, which produces characteristic osteoblastic bone lesions with increased bone density rather than lytic bone lesions associated with other types of cancer (Lee, 2017; Roberts, 2013; Hu, 2016).