Prostate-specific membrane antigen (PSMA, GCPII)
Prostate-specific membrane antigen is a type II transmembrane glycoprotein with zinc-depending enzymatic functions. Because of its different roles it is also known as glutamate carboxypeptidase II (GCPII), N-acetyl-L-aspartyl-L-glutamate peptidase I (NAALADase I), NAAG peptidase, and folate hydrolase 1 (FOLH1). It is encoded by the FOLH1 gene, and shares structural homology with transferrin receptor TfR1. The protein (in humans) has a 707-amino-acid extracellular portion, 24-amino-acid transmembrane portion, and 19-amino-acid intracellular portion. PSMA is highly glycosylated with oligosaccharides, which are essential for proper protein folding and enzymatic activity (Mesters et al., 2006). The enzymatic functions of PSMA can be inhibited with specific antibodies or small molecule inhibitors; after binding, these are internalized with PSMA by endocytosis (Liu et al., 1998; Ghosh and Heston, 2004; Eder et al., 2013).
Prostate-specific membrane antigen is expressed in prostate tissue (epithelium), salivary and seromucous glands, proximal tubular cells in the kidneys, intestine (especially proximal small intestine), and in the central and peripheral nervous system. In other mammals (except for the dog) PSMA expression in prostate is minimal (O’Keefe et al., 2018). In prostate tumours, their metastases, and in neovasculature the PSMA is highly overexpressed, providing a target for imaging and therapy (Sweat et al, 2008; Mannweiler et al., 2009; Haberkorn et al., 2016; Baum et al., 2016; Eiber et al., 2017; Ahn et al., 2019). Only 5-10% of primary prostate cancer lesions are PSMA-negative (Schwarzenboeck et al., 2017), but PSMA expression in primary tumours and metastases is heterogeneous and not directly correlated with histological parameters (Mannweiler et al., 2009). PSMA expression in neovascular endothelium (also in other than prostate tumours) may be related to cell invasion and angiogenesis (Conway et al., 2006; Van de Wiele et al., 2019). PSMA is expressed in neovasculature of non-cancerous tissues, too, which may lead to uptake of PSMA radioligands for instance in sites of bone remodeling (Artigas et al., 2016). Benign schwannomas can mimic prostate cancer metastasis in PSMA PET (Wang et al., 2009; Rischpler et al., 2016). PSMA expression in peripheral ganglia can lead to wrong interpretation of lymph node metastasis (Krohn et al., 2015; Kanthan et al., 2017; Werner et al., 2017; Rischpler et al., 2018; Bialek & Malkowski, 2019), and even benign lymph nodes are often visible in PSMA PET (Afshar-Oromieh et al., 2018; Rauscher et al., 2020). Pitfalls of PSMA PET are reviewed by Sheikhbahaei et al (2019). The high PSMA ligand uptake in salivary glands is mostly non-specific: uptake does not correlate with PSMA expression in submandibular glands, and PSMA inhibitors do not markedly reduce the uptake there like in tissues with high PSMA expression, but instead botulinum toxin injection does transiently reduce the uptake (Rupp et al., 2019).
PSMA PET can be used for imaging also other than prostate cancers (Salas Fragomeri et al., 2018). [68Ga]PSMA-11 PET/CT was superior to [18F]FDG in detecting hepatocellular carcinoma (Kesler et al., 2019). Adenoid cystic carcinoma (AdCC) starts from the epithelium of secretory glands in the head and neck region; [68Ga]PSMA-11 PET/CT could detect local recurrent and distant metastatic AdCC (Klein Nulent et al., 2017). Also breast cancer, certain types of renal carcinomas, and high-grade gliomas express PSMA.
PSMA hydrolyses carboxy-terminal glutamate residues (thus the name GCPII); this activity manifests as folate hydrolase in the small intestine (in duodenal brush border cells), freeing folic acid to be used in the body as a vitamin. Mutation of FOLH1 gene may lead to impaired intestinal absorption of folate. PSMA may also play a role in the folate metabolism of the prostate (Pinto et al., 1996; Heston, 1997; Chang, 2004), contributing also to cancer progression. In kidneys PSMA may participate in folate reabsorption.
In the brain, neuropeptide N-acetyl-L-aspartyl-L-glutamate (NAAG) is a substrate of glutamate carboxypeptidase; PSMA expression may be involved with glutamate excitotoxicity (Bařinka et al., 2012). In preclinical models of neurodegenerative diseases GCPII inhibitors increase NAAG, which activates mGluR3 and decreases glutamate.
Matrix metalloproteinase-2 (MMP-2) and PSMA can participate in a proteolytic pathway in extracellular matrix generating small, pro-angiogenic laminin peptides, which enhance endothelial cell adhesion and migration, and activate adhesion via integrins, leading to increased angiogenesis (Conway et al., 2006 and 2013).
Circulating PSA levels are widely used for screening of prostate cancer. Therefore attempts for using PSMA as a serum-based marker have been made. Although researchers have detected circulating PSMA in the serum of patients with prostate cancer, even when PSA levels are low (Xiao et al., 2001), the assessment is not sufficiently accurate for clinical use (Chang et al., 2000 and 2004). Instead, PSMA has been detected in circulating prostate cancer cells, and used for collecting the cells (Diamond et al., 2012; Paller et al., 2019). The amount of PSMA in circulation is probably so low that it does not affect kinetics of PSMA-targeted radioligands.
Several PSMA PET radioligands are already in clinical use (Lütje et al., 2017; Wester & Schottelius, 2019), and many of those are designed as theranostic agents, being suitable for therapy and diagnostic imaging (Weineisen et al., 2014 and 2017; Wurzer et al., 2020). EANM has published guidelines for radionuclide therapy with 177Lu-labelled PSMA ligands (Kratochwil et al., 2019).
Administration of androgen deprivation therapy (ADT) initially increases PSMA expression until tumour sizes are reduces due the treatment. The increase is heterogeneous and most prominent in bone metastases (Ettala et al., 2020). Although the effect of timing on interpretation of PSMA PET is minor, it should be accounted for when assessing the detection sensitivities.
Monoclonal antibodies, conjugated with DOTA and labelled with 64Cu, have shown promise in PSMA-positive tumour model in mice (Elsässer-Beile et al., 2009; Alt et al., 2010; Fung et al., 2016). Banerjee et al (2014) compared different chelators and found that CB-TE2A provided better contrast than NOTA, PCTA, Oxo-DO3A or DOTA, probably due to its higher stability. Intact antibodies usually have poor tumour penetration and slow blood-pool clearance, as demonstrated by the SPECT tracer [111In]capromab pendetide.
Development of specific and strongly binding monoclonal antibodies to PSMA is difficult because of the high glycosylation of native PSMA. Capromab pendetide (ProstaScint, Wong et al., 2005) and some other mAbs bind to the portion of PSMA that is not exposed on the outer cell surface, such as 7E11, and thus cannot be used to target viable cells. J591 binds the extracellular domain of PSMA, and has been labelled with 89Zr and 124I for PET imaging and also used in radioimmunotherapy. 99mTc-labelled diabody of mAb J591 has been used for SPECT imaging (Kampmeier et al, 2014).
Because of the limitations of mAbs, smaller anti-PSMA affibodies may be better suited for tumour imaging (Chatalic et al., 2015). [89Zr]Df-IAB2M minibody has been used for radioimmunotherapy (Joraku et al., 2019). Also single chain antibodies have faster tumour uptake, and conjugation to nanoparticles reduces their urinary excretion and enables carrying chemotherapeutic load (Wong et al., 2017).
Several urea-based PSMA inhibitors, with low molecular weight, have been developed to be used as SPECT and PET radiopharmaceuticals (Eder et al., 2012, Foss et al., 2012). Many of the proposed PSMA ligands contain a chelator, which can bind a variety of radioisotopes. While 18F and 68Ga are most often used, radioisotopes with longer half-lives, such as 64Cu, 89Zr, and 55Co, have been used for delayed imaging.
One of most used PET tracers for imaging PSMA expressing tumours is [68Ga]PSMA-HBED-CC ([68Ga]PSMA-11). This tracer binds to the extracellular domain of PSMA and is then assumed to be internalized. Image contrast, sensitivity, and specificity, are better than with [18F]fluoromethylcholine (Afshar-Oromieh et al., 2014; Perera et al., 2016). It is used for initial staging and detecting recurrent prostate cancer (Bailey & Piert, 2017). [Al-18F]PSMA-11 is less stable than [68Ga]PSMA-11, leading to bone uptake in mice (Lütje et al., 2019).
[18F]DCFBC has potential for detection of primary and metastatic prostate cancer (Mease et al., 2008; Cho et al., 2012; Rowe et al., 2015 and 2016). Blood clearance is relatively slow, leading to high background. Tracer is mainly excreted via kidneys, leading to high radioactivity concentration in the urinary bladder. The fraction of radioactive metabolites in plasma is very low (Cho et al., 2012).
[18F]DCFPyL (Chen et al., 2011) has lower blood activity and better sensitivity than [18F]DCFBC and [68Ga]PSMA-11 (Rowe et al., 2016 and 2017; Dietlein et al., 2017; Kesch et al., 2017; Mena et al., 2018; Wondergem et al., 2019). Distribution of [18F]DCFPyL and [68Ga]PSMA-11 in normal organs is similar (Ferreira et al., 2019).
[18F]PSMA-1007 has shown comparable performance to [68Ga]PSMA-HBED-CC, with mainly hepatobiliary excretion and thus reduced urinary tract and bladder activity but higher uptake in the liver, gallbladder, and gastrointestinal tract.
Phosphonates and phosphoramidates have been used as PSMA inhibitors, and labelled for PSMA imaging. PET radioligands of this group includes for instance (2RS,4S)-2-[18F]fluoro-4-phosphonomethyl-pentanedioic acid ([18F]BAY1075553) (Lesche et al., 2014; Beheshti et al., 2015), [64Cu]ABN-1 (Nedrow et al., 2016), and [18F]CTT1057 (Ganguly et al., 2015; Behr et al., 2019).
For radiotherapy, long circulation times and low excretion into urine are preferred. Adding an albumin-binding moiety to the PSMA ligands can be used optimize the tissue distribution profile, while still enabling internalization into PSMA overexpressing cancer cells (Benešová et al., 2018; Kelly et al., 2018 and 2019).
- [68Ga]PSMA-11 ([68Ga]PSMA-HBED-CC)
- Prostate cancer
- Pretargeted PET imaging
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Updated at: 2020-02-14
Created at: 2015-03-25
Written by: Vesa Oikonen, Anne Roivainen