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, CBPII), 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 PET can be used for imaging also other than prostate cancers (Salas Fragomeri et al., 2018). In detecting hepatocellular carcinoma, [⁶⁸Ga]PSMA-11 PET/CT was superior to [¹⁸F]FDG (Kesler et al., 2019), and had higher accuracy than multiphase contrast-enhanced CT (Hirmas et al., 2021). [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), which activates metabotropic glutamate receptor mGluR3, is catabolized by the glutamate carboxypeptidase function of PSMA; PSMA expression may be involved with glutamate excitotoxicity (Bařinka et al., 2012). In preclinical models of neurodegenerative diseases GCPII inhibitors increase NAAG, activating mGluR₃ and decreasing glutamate. In rat glioma model, high uptake of PSMA radiopharmaceuticals is seen in peritumoral area, presumably caused by activated astrocytes (Oliveira et al., 2020). PET radioligands for PSMA do not cross intact blood-brain barrier, but may be useful for autoradiographic investigations of GCPII expression in the brain (Thomsen et al., 2023).

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.

Pitfalls of PSMA PET

Pitfalls of PSMA PET are reviewed by Sheikhbahaei et al (2019).

PSMA expression in neovascular endothelium may be related to cancer 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). Diffusion-weighted MR imaging may help in identification of sympathetic ganglia (Bialek and Malkowski, 2020).

Salivary glands have high PSMA expression on secretory cells (Wolf et al., 2009). This allows visualization of major and minor and seromucous glands with PSMA PET (Klein Nulent et al., 2018), but may pose challenges to oncological imaging. The high PSMA radioligand uptake in salivary glands may be 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). Yet, salivary gland tissue that is damaged by radiotherapy loses its ability to take up PSMA radioligands in a dose-dependent manner (Valstar et al., 2019; Mohan et al., 2022).

PET tracers for PSMA

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.

In preclinical and in vitro studies it should be noted that molar activity can markedly influence the uptake parameters (Wurzer et al., 2018; Sanchez-Crespo et al., 2018; Soeda 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).

The renal uptake of PSMA radioligands can be reduced with mannitol, infused prior to and at the time of radioligand administration (Matteucci et al., 2017; Baiocco et al., 2020). "Cold" PSMA ligand can substantially reduce the salivary gland and kidney uptake in radiotherapy (Kalidindi et al., 2021).

Urea-based radioligands

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). Glutamate-ureido-lysine (GUL) probes have been found to identify some PSMA negative tumours, too, because it can bind to other proteins, such as NAALADaseL and mGluR8 (Bakht et al., 2022). 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).

[⁶⁸Ga]THP-PSMA utilizes trishydroxypyridinone (THP) as the chelator, offering one-step kit-based radiolabelling (Young et al., 2017; Hofman et al., 2018). [⁶⁸Ga]THP-PSMA has promising results in clinical management of patients with high-risk prostate cancer and patients with biochemical recurrence (Kulkarni et al., 2020).

[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). High tumour burden was seen to cause only minimal decrease in normal organ uptake (SUV "sink effect") (Werner et al., 2020), but at least in some cases marked "sink effect" can be observed in SUVs, but avoided in tumour-to-blood ratio (Cysouw et al., 2020). However, tumour-to-blood ratio depends more on uptake interval than SUV (Bodar et al., 2021). Relatively low inter-patient SUV variability was seen in the liver; the inter-patient variability was high in the spleen, which on the other hand had low intra-patient variability (Sahakyan et al., 2020).

[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. [¹⁸F]PSMA-1007 has been developed on the scaffold of a theranostic ligand PSMA-617 (Giesel et al., 2016). Clinical results from [¹⁷⁷Lu]Lu-PSMA-617 radiotherapy have been promising (Rasul et al., 2020; Violet et al., 2020).

With [¹⁸F]rhPSMA-7, at least equal performance to [⁶⁸Ga]PSMA-HBED-CC has been obtained 3 h after administration (Dietlein et al., 2018; Hohberg et al., 2018).

[¹⁸F]JK-PSMA-7 has the advantage of a very simple manufacturing protocol (Hohberg et al., 2019), but the uptake may also be higher than with some other PSMA radioligands (Zlatopolskiy et al., 2019). Static whole body scan 2 h after administration is used (Dietlein et al., 2020a). [¹⁸F]JK-PSMA-7 has shown high detection rate in patients under ADT with PSA level ≥0.3 ng/mL, similar to [¹⁸F]DCFPyL and [⁶⁸Ga]PSMA-11 (Dietlein et al., 2020b).

Phosphonate-based radioligands

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).

Monoclonal antibodies

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).

Literature

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Tags: PSMA, Folates, Glutamate

Updated at: 2023-09-13
Created at: 2015-03-25
Written by: Vesa Oikonen, Anne Roivainen