18F-PSMA Expression in Amoebic Liver Abscesses Mimics as Metastatic Prostate Cancer.

Using prostate specific membrane antigen (PSMA) expression in clear cell renal cell carcinoma for imaging advanced disease

Background: PSMA is nearly universally expressed by PC, has a correlation with androgen receptor (AR) pathway dysregulation, and some studies have pointed towards PSMA expression as a prognostic marker. Non-invasive measurement of PSMA expression may be biomarker of AR activity in animal models.[Evans et al, PNAS 2011] Anti-PSMA monoclonal antibody J591 has been utilized both for imaging as well as therapeutics and we have published semi-quantitative scoring in the setting of therapeutic clinical trials. Methods: Patients with mCRPC underwent planar gamma camera imaging following radiolabeled (RL) J591 injection (177Lu- or 111In-J591). RL-J591 images were semi-quantitatively scored using a 5-point visual score (VS) system [Tagawa et al, Clin Cancer Res 2013]. Overall survival (OS) was calculated from the date of imaging to death or last follow up. As several life-prolonging treatments (docetaxel, sipuleucel-T, cabazitaxel, abiraterone (Abi), enzalutamide (Enza), radium-223) became available during the study time period, treatment following RL-J591 imaging was recorded and used in multivariate analysis. Results: Between 2000 and 2015, 165 patients with mCRPC were imaged following RL-J591 and semi-quantitatively scored for PSMA expression by VS. Baseline demographics included median age of 71.3 years (44.5-85.9); bone metastases present in 86.7%, 52.7% with LN mets, 17% with lung mets, 7.9% with liver mets, 1.8% other mets. Fifty three (32.2%) had low PSMA expression by imaging (VS 0-1) and 112 (67.8%) had high PSMA expression (VS 2-5). Post RL-J591 imaging, 41.2% (68/165) patients received life prolonging therapy, including taxane chemotherapy (33.9%), Abi/Enza (14.5%), sipuleucel-T (1.2%), or radium-223 (1.2%). At last follow up 12.7% (21/165) patients were alive. Median OS was 22.6 months in patients with low PSMA expression and 16.6 mo with high PSMA expression (P = 0.07). Those with subsequent receipt of life prolonging therapy had OS of 23.6 months vs 14.3 mo (P = 0.002). On multivariable analysis controlling for life prolonging therapy, higher PSMA expression by imaging was a significant predictor for poorer OS (adjusted hazard ratio = 1.60; 95% CI = 1.10, 2.31; P = 0.01). Conclusion: Non-invasive measurement of PSMA expression may be evaluated on a semi-quantitative basis via planar imaging. The level of PSMA expression is inversely associated with survival when controlling for receipt of known life-prolonging therapy. With the introduction of newer quantitative molecular imaging (i.e. PET), quantitative PSMA imaging may prove to be a useful prognostic tool. Citation Format: Kavya Pinto-Chengot, Yuliya Jhanwar, Jaspreet Batra, Beerinder Karir, Shankar Vallabhajosula, Paul Christos, Ana Molina, Himisha Beltran, David M. Nanus, Stanley J. Goldsmith, Neil H. Bander, Scott T. Tagawa. Non-invasive assessment of prostate-specific membrane antigen (PSMA) expression as a prognostic marker in men with metastatic castration-resistant prostate cancer (mCRPC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3971.

Prostate specific membrane antigen (PSMA) directed radioligand therapy (RLT) is a novel therapy for metastatic castration-resistant prostate cancer (mCRPC) patients. However, it is still poorly understood why approximately 40% of the patients does not respond to PSMA-RLT. The aims of this study were to evaluate the pretreatment PSMA expression on immunohistochemistry (IHC) and PSMA uptake on PET/CT imaging in mCRPC patients who underwent PSMA-RLT. We correlated these parameters and a cell proliferation marker (Ki67) to the therapeutic efficacy of PSMA-RLT. In this retrospective study, mCRPC patients who underwent PSMA-RLT were analyzed. Patients biopsies were scored for immunohistochemical Ki67 expression, PSMA staining intensity and percentage of cells with PSMA expression. Moreover, the PSMA tracer uptake of the tumor lesion(s) and healthy organs on PET/CT imaging was assessed. The primary outcome was to evaluate the association between histological PSMA protein expression of tumor in pre-PSMA-RLT biopsies and the PSMA uptake on PSMA PET/CT imaging of the biopsied lesion. Secondary outcomes were to assess the relationship between PSMA expression and Ki67 on IHC and the progression free survival (PFS) and overall survival (OS) following PSMA-RLT. In total, 22 mCRPC patients were included in this study. Nineteen (86%) patients showed a high and homogenous PSMA expression of >80% on IHC. Three (14%) patients had low PSMA expression on IHC. Although there was limited PSMA uptake on PET/CT imaging, these 3 patients had lower PSMA uptake on PET/CT imaging compared to the patients with high PSMA expression on IHC. Yet, no correlation was found between PSMA uptake on PET/CT imaging and PSMA expression on IHC (SUVmax: R2=0.046 and SUVavg: R2=0.036). The 3 patients had a shorter PFS compared to the patients with high PSMA expression on IHC (HR: 4.76, 95% CI: 1.14-19.99; P=.033). Patients with low Ki67 expression had a longer PFS and OS compared to patients with a high Ki67 expression (HR: 0.40, 95% CI: 0.15-1.06; P=.013) CONCLUSION: The PSMA uptake on PSMA-PET/CT generally followed the PSMA expression on IHC. However, heterogeneity may be missed on PSMA-PET/CT. Immunohistochemical PSMA and Ki67 expression in fresh tumor biopsies, may contribute to predict treatment efficacy of PSMA-RLT in mCRPC patients. This needs to be further explored in prospective cohorts.

Prostate specific membrane antigen (PSMA) is a folate gamma glutamyl carboxypeptidase that is oriented on the plasma membrane of normal and prostate cancer cells. A cytosolic version of PSMA, PSM', results from alternative splicing of the PSMA gene. Two additional alternatively spliced variants of PSMA, PSM-C and PSM-D, have been described recently. The ratio of PSMA to PSM' mRNA was higher in a small number of prostate cancer specimens compared to normal prostate cancer and benign prostatic hypertrophy (Su et al. Cancer Res 1995;55:1441). The intent of our study was to measure the gene expression of PSMA and the 3 PSMA splice variants in a large number of patient's tissues. A real-time, quantitative PCR assay was developed to quantify PSMA, PSM', PSM-C and PSM-D. Discrimination among the variants was achieved by designing unique primers and TaqMan probes for each gene. Amplification and detection was specific for the desired splice variant and was sensitive to one gene copy per reaction. The assay was used to quantify the gene expression in specimens of normal, benign, primary and metastatic prostate cancer from 72 patients. The mean PSMA expression (relative to 18S rRNA) was 2- to 3-fold lower in normal prostate (n = 4) compared to primary (n = 55, p = 0.31) and metastatic (n = 20, p = 0.33) prostate cancer. There was no difference in the PSMA expression between benign and cancerous prostate tissue from the same patients (n = 35). The ratio of PSMA to PSM' was lowest in the normal prostate and increased with increasing Gleason score (p < 0.001). The increased ratio in these tissues was a reflection of both increasing PSMA levels and decreasing PSM' mRNA. The expression of PSM-C did not differ in any of the tissue categories studied. The expression of PSM-D was similar in normal and primary prostate cancer but was 2-fold higher in lymph node (p < 0.005) and bone metastases (p < 0.05) compared to the primary tumors. Our results of the first detailed quantitative analysis of PSMA mRNA expression in patient's tissues demonstrate that PSMA and the 3 PSMA splice variants are expressed in normal, benign, cancerous and metastatic prostate cancer. We note increased PSMA expression in some malignant tissues, however, these increases are modest in magnitude. We also report that the expression of a novel splice variant, PSM-D, is elevated in prostate cancer metastases.

Disclosure: R. Sadiq: None. J.O. Russell: Consulting Fee; Self; Baxter. Grant Recipient; Self; Eli Lilly &amp; Company. Research Investigator; Self; Eli Lilly &amp; Company. P.W. Ladenson: Consulting Fee; Self; Veracyte, Inc., Viking Therapeutics. M. Alshalalfa: Employee; Self; Veracyte, Inc.. Stock Owner; Self; Veracyte, Inc. Y. Hao: Employee; Self; Veracyte, Inc.. Stock Owner; Self; Veracyte, Inc. J. Huang: Employee; Self; Veracyte, Inc.. Stock Owner; Self; Veracyte, Inc. J.P. Klopper: Employee; Self; Veracyte, Inc.. Stock Owner; Self; Veracyte, Inc. R.T. Kloos: Employee; Self; Veracyte, Inc.. Stock Owner; Self; Veracyte, Inc. P. Santhanam: Consulting Fee; Self; Carelon Rx. Prostate-specific membrane antigen (PSMA), a transmembrane protein encoded by the gene FOLH1 is a relatively novel target for molecular imaging and therapy in prostate cancer imaging. PSMA-based positron-emission tomography (PET) imaging is approved and recommended in the United States for biochemical recurrence after prostate cancer treatment. This receptor is often expressed on the cell membrane of neovascular endothelial cells of a variety of solid tumors. Immunohistochemical studies reported that several thyroid cancer (TC) subtypes, including papillary and follicular, with intense PSMA staining in the neovasculature had more clinically aggressive behavior than TC with weak PSMA expression. Here we aimed to characterize the expression of PSMA in a large cohort of thyroid nodules. FOLH1 (PSMA) mRNA expression was analyzed across 47,695 thyroid nodules sent for Afirma Genomic Sequencing Classifier (GSC) molecular testing. Differential PSMA expression was explored across Afirma benign and suspicious categories, Bethesda cytology categories, and relative to different molecular alterations identified by the Afirma Xpression Atlas. There were 30,259 cytologically indeterminant nodules classified as GSC-Benign (GSC-B) and 15,815 as GSC-Suspicious (GSC-S); 1,621 were cytologically Bethesda V or VI. Mean expression of PSMA gene was higher in GSC- S and Bethesda V/VI nodules compared to GSC-B (Wilcoxon p-value &amp;lt; 2e-[1]6 for both). Relative to GSC-B, GSC-S and Bethesda V/VI had fold changes (FC) of 1.12 and 1.2, respectively. PSMA expression was higher in Bethesda VI compared to Bethesda V (p=5e-7). Compared to GSC-B, GSC-S samples with PAX8-PPARG, ALK/RET fusions had higher PSMA expression (FC 1.57, 1.28 respectively, p&amp;lt;2e-[1]6 for both). Within Bethesda V/VI nodules, PSMA was higher in samples with PPARG fusions (CREB3L2-PPARG, PAX8-PPARG), ALK, or RET fusions compared to other alterations (p= 0.002). Samples with TSHR variants had the lowest PSMA expression in both GSC-S and Bethesda V/VI samples (FC 1.06, 1.03 respectively with respect to GSC-B). Finally, a negative trend was observed between PSMA and sodium iodide symporter (NIS) expression with samples with highest PSMA having the lowest NIS expression. In conclusion, PSMA expression in indeterminate and malignant thyroid nodules varies across cytology groups and in the presence of different molecular alterations. PSMA expression may provide further thyroid tumor prognostic information in the context of cytology and associated molecular variants. The basis for increased PSMA expression in the setting of PPARG and other fusions is an opportunity for future investigation, as are the roles of preoperative assessment of PSMA expression and PSMA-based imaging and therapy in the setting of advanced thyroid cancer. Presentation: 6/3/2024

35 Background: Imaging-based response to systemic therapies in metastatic prostate cancer is still challenging. The aim of this study was to retrospectively investigate the association between Prostate-Specific Membrane Antigen (PSMA) expression changes on 68Ga PSMA PET/CT and the response to treatment following the start of taxane-based chemotherapies (docetaxel or cabazitaxel) in both metastatic hormonosensitive (mHSPC) and castration-resistant prostate cancer (mCRPC) patients. Methods: We retrospectively reviewed all 68Ga-PSMA-11 PET/CT scans performed at our institution from November 2014 to June 2020. We included mHSPC and mCRPC patients with a baseline 68Ga-PSMA PET/CT performed &lt; 2 months before, and a second 68Ga-PSMA PET/CT performed &lt; 3 months after docetaxel or cabazitaxel (with no modification in therapy between scans). The biological data was used to distinguish PSA-non-responders (increasing PSA or decreasing PSA by &lt; 50% from pretherapy level) and PSA-responders (decreasing PSA &gt; 50% following the start of therapy or the peak thereafter). The PSMA PET/CT response was assessed visually by two independent nuclear medicine physicians blinded to clinical data and patient outcomes. Patients were classified as PSMA-non-responders (≥1 new PSMA-expressing metastases, majority of PSMA-expressing metastases increasing in intensity, or stable PSMA-expression of the disease) or PSMA-responders (complete disappearance of pathologic PSMA uptake, or majority of PSMA-expressing metastases decreasing in intensity). Majority of lesions was defined as &gt; 50% of registered lesions. Descriptive statistics and measures of associations (two-sided Fisher’s exact test and Phi coefficient) were calculated. Results: A total of 37 patients were included (8 mHSPC and 29 mCRPC), 21 patients under docetaxel and 16 cabazitaxel. All patients received at least 3 cycles [median 6 (range 3-10)]. The mean time (±standard deviation) between the first 68Ga-PSMA PET/CT and the start of therapy was 22±16 days. The mean time between the the last cycleand the second 68Ga-PSMA PET/CT was 36±27 days. PSA-response and PSMA-response were concordant in 95% of cases with 18 patients PSA/PSMA-non-responders and 17 PSA/PSMA-responders (Phi = 0.90, p = 0.0001). Two discordant patients presented with PSMA-response but PSA-non-response. No isolated PSMA flare was observed, as PSA-response was always associated with a decrease in PSMA-expression on the second PET/CT. Conclusions: This retrospective study suggests PSMA expression changes on PET/CT at least after 3 cycles of chemotherapy (docetaxel or cabazitaxel) are strongly associated with PSA response. No flare phenomenon was found at this evaluation. Prospective studies are needed to better define the potential role of PSMA-PET/CT in response assessment.

Invited Commentary: Changing Landscape of Imaging in Recurrent Prostate Cancer.

PSA-PSMA profiles and their impact on sera PSA levels and angiogenic activity in hyperplasia and human prostate cancer

Dendritic cells are unique in their ability to stimulate naive T cells. These investigators have developed a prostate cancer vaccine using autologous dendritic cells as a vehicle to present prostate antigens to T cells in vivo.

198 Background: Prostate-specific membrane antigen (PSMA) is a folate hydrolase expressed on the surface of PC cells that has been used as a target to detect disease and selectively deliver cytotoxic agents and radionuclides. The ability to detect PSMA levels on circulating tumor cells (CTCs) may identify patients likely to benefit from such targeted therapy. Technology developed by Epic Sciences utilizes high definition imaging of plated nucleated cells. Methods: Blood samples were obtained from patients (pts) with metastatic castration-resistant prostate cancer (mCRPC). Cells were stained for CK, CD45, PSMA and categorized as classic CTC (CK+, CD45-, intact/morphologically distinct nuclei) or apoptotic CTC (CK+, CD45-, morphology suggesting apoptosis). Clinical data including treatment, metastatic sites, Veridex CellSearch CTC enumeration, prostate-specific antigen, and alkaline phosphatase was collected. Results: Fourteen pts with mCRPC, including eight with serial samples were analyzed (33 samples in total). At the first draw (t1), classic CTC were detected in 13 pts (93%), (median two cells/ml, range 0 to 40 cells/ml) and apoptotic CTC in 14 pts (100%) (median four cells/ml, range 1 to 18 cells/ml), including six pts (42%) with no CTC by Veridex CellSearch. PSMA expression was detected in five pts (36%) with classic CTC of which a median of 32% of cells (range 5 to 100%) expressed the antigen. Similar intra-patient heterogeneity was seen for the 10 pts (71.4%) with PSMA+ apoptotic CTCs (median 33.5%, range 11 to 75% cells). During treatment, often with more complete androgen suppression, PSMA was detected in 3 of the 8 (38%) pts with no PSMA+ classic CTCs at t1. The presence of PSMA expression in apoptotic CTCs did not appear to change while on therapy. Conclusions: A larger percentage of PSMA expression was seen in mCRPC pts in apoptotic CTC (10 out of 14) than classic CTC (5 out of 14) at t1, with intra-patient cell heterogeneity of PSMA expression in both CTC populations. Serial measures suggest dynamic changes in PSMA expression over time. The threshold of detectable cells and proportion and degree of PSMA expression that associates with drug sensitivity is unknown. Larger samples of pts at discrete time points on therapy are underway to further elucidate the potential clinical relevance.

295 Background: While 18F-fluciclovine PET/CT is approved in the US and recommended by the NCCN, prostate-specific membrane antigen (PSMA) PET/CT is more common in Europe/Australia and recommended by the EAU. Less is known about the biology of lesions detected by either modality. 18F-fluciclovine PET relies on radiotracer uptake by amino acid transporters LAT1-4 and ASCT1-2. PSMA PET is dependent on surface expression of PSMA. We compared relative expression of PSMA and fluciclovine transporter genes in radical prostatectomy (RP) samples to determine their distribution across subtypes and correlation with outcomes. Methods: Gene expression data of 19,102 RP samples were analyzed using the Affymetrix Human Exon 1.0 ST microarray. 1,135 patients had long term follow up. Associations between expression of PSMA and fluciclovine transporter genes (LAT1-4 and ASCT1-2) and pathologic variables, molecular subtypes, and clinical outcomes were conducted. Results: All fluciclovine transporter genes (LAT 1-4, ASCT1-2) were expressed at lower levels than PSMA (p &lt;0.0001). PSMA expression was positively correlated with genomic risk score and pathologic Gleason score (p&lt;0.0001), but LAT2-3 and ASCT2 were inversely correlated with genomic risk in primary tumors (p&lt;0.0001) and less expressed in GS 9-10 tumors (p&lt;0.0001). PSMA expression was associated with worse metastasis-free survival (MFS) (HR 1.45, p=0.001) and lymph node involvement (HR 2.14, p&lt;0.0001). Expression of LAT2, LAT3, ASCT2 expression was associated with better MFS (HR 0.85, 0.63, 0.74, p&lt;0.0001-0.04). After multivariable adjustment, PSMA expression remained independently prognostic of poorer MFS (HR 1.3, p=0.028). Luminal B subtype was notable for PSMA overexpression; Luminal A was enriched in ASCT2 and LAT2 (p&lt;0.0001). PSMA expression did not correlate with ERG fusion prostate cancers, but LAT2, ASCT1, and ASCT2 were overexpressed in ERG fusion negative tumors (p&lt;0.0001). Conclusions: PSMA expression is associated with more aggressive disease and poorer clinical outcomes than fluciclovine transporter genes in localized prostate cancer. Molecular subtypes of prostate cancer vary in PSMA and fluciclovine transporter gene expression.

Background: Prostate-specific membrane antigen (PSMA) is a type II transmembrane glycoprotein with folate hydrolase and neurocarboxypeptidase activity. PSMA has been shown to be consistently expressed in prostatic carcinoma as well as benign prostatic tissue. In addition, PSMA is selectively expressed in the neovasculatrization of multiple non-prostatic solid tumours including breast cancers. Inducible PSMA specific expression on angiogenic vasculator suggests that PSMA participates in neovessel growth in developing tumours. Treatment of solid tumours with anti-angiogenic agents has become an established paradigm in cancer therapy. Our study aims at detecting protein expression level of PSMA in breast cancer patients with or without metastases both in the primary tumour and metastases tissues. Materials and methods: 79 patients with histologically proven breast cancer at the department of Gynecology and Obstetrics at the University of Saarland (Germany) were included. 37 patients showed no sign of lymph node or distant metastases (N0M0). 18 patients showed lymph node metastases (N1M0) and 24 patients were diagnosed with distant metastases occurring on different sites (n=5 liver, n=3 lung, n=5 bone, n=7 brain) (M1). Representative formalin-fixed paraffin-embedded tumour blocks from these specimen were obtained from the department of pathology of University of Saarland. Immunohistochemical staining for PSMA and CD31 was performed on parallel representative tumour sections in each case. Results: An immunohistochemical PSMA expression could be detected in tumour cells and in the neovascularization of breast cancer in 36 patients, 16% (n=25) of these patients with primary breast cancer and 45% (n=11) with metastatic disease. The expression is higher in patients with lymph node metastases (40%) and even higher in patients with distant metastases (45%). The distant metastases themselves showed the highest expression of PSMA (67%) compared to the PSMA expression of lymph nodes (30%) or the primary tumor (45%) in patients with metastatic disease. Besides these results we could show that the PSMA expression significantly depends on the tumor stage of the primary tumor. Patients suffering of early stage breast cancer (T1=12%) showed significantly less PSMA expression than patients with T2 primary tumor (29%) (p=0,008). The PSMA expression in T3 and T4 staged primary breast cancer was elevated up to 75%. Conclusion: This data suggests that PSMA might be involved in the pathogenicity of breast cancer and its metastasis. In addition, PSMA may be an ideal target for diagnostic purposes as well as targeted therapies against metastatic breast cancer. Citation Format: von Heesen A, Kasoha M, Solomayer E-F, Unger C, Bohle R, Zaharia C, Ezziddin S, Juhasz I. Prostate-specific membrane antigen (PSMA) expression in breast cancer and its metastases [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-07-25.

It is timely and important to develop new treatment modalities for advanced prostate cancer, because even the newly FDA approved treatments, despite providing significant survival benefits, do not constitute cure of this disease. Antibody drug conjugates (ADCs) represent a promising approach to cancer therapy. Prostate-specific membrane antigen (PSMA) is expressed in advanced prostate cancer and targeting this protein is used for imaging of advanced prostate cancer as well as development of targeting strategies. The objective of our studies was to evaluate the efficacy of PSMA ADC against a series of patient-derived prostate cancer xenografts (LuCaP 58, LuCaP 77, LuCaP 96CR, and LuCaP 105) with different characteristics, including varying levels of PSMA expression and responses to androgen suppression. Mice bearing subcutaneous LuCaP prostate cancer-derived xenografts received PSMA antibody monomethyl auristatin E (MMAE) drug conjugate (PSMA ADC) in which the antibody and MMAE are linked via a protease-cleavable linker. PSMA ADC dose ranged from 1 to 6 mg/kg. Unmodified PSMA mAb + free MMAE at the amount equivalent to those contained in 6 mg/kg PSMA ADC was used as control. All treatments were administered once a week via tail-vein injections and repeated four times once a week and tumor responses were monitored for 10 weeks. IHC analyses were performed to determine PSMA and AR expression and effects on proliferation. Treatment responses varied widely across the tumor models, from complete tumor regressions in LuCaP 96CR to largely unimpeded tumor progression of LuCaP 58, which had the lowest baseline level of PSMA expression. Intermediate antitumor effects were seen for LuCaP 77 and LuCaP 105 tumors, despite their having similar basal expression of PSMA as LuCaP 96CR. Interestingly, we detected substantial differences in responses even within the same model, indicating that PSMA expression is not the only factor involved in treatment outcomes. Our results show high efficacy of PSMA ADC in advanced prostate cancer but also considerable variability in effects despite PSMA expression. Further studies to identify tumor characteristics that are predictive of treatment response are ongoing.

To investigate the association between Prostate-Specific Membrane Antigen (PSMA) expression changes on positron emission tomography-computed tomography (PET/CT) and the response to treatment following the start of enzalutamide or abiraterone in metastatic castration-resistant prostate cancer (mCRPC) patients. All consecutive 68Ga-PSMA-11 PET/CT scans routinely performed at our institution during more than 4years were retrospectively screened for inclusion. We included mCRPC patients with a baseline PSMA PET/CT performed less than 2months before the start of either enzalutamide or abiraterone, and a follow-up PSMA PET/CT performed no more than a year after, while still under those novel antiandrogen drugs (NAD). The associated clinical records were reviewed. Patients were considered treatment responders if they presented decreasing PSA levels > 50% or a radiological response based on RECIST 1.1 criteria. PSMA expression changes on the follow-up PET/CT were assessed using per-patient dominant response criteria to classify patients as PSMA-responders (complete disappearance of pathologic PSMA uptake, or a decreased uptake of the majority of lesions) or PSMA-non-responders (new PSMA-expressing lesions, increased uptake of the majority of lesions, or stable PSMA expression of the disease). Descriptive statistics and measures of associations (two-sided Fisher's exact test and Phi coefficient) were calculated. A total of 11 and 15 patients were included in the enzalutamide and abiraterone groups. Median follow-up was 110 (IQR 76-124) and 87 (IQR 71-242) days, respectively. All treatment responders (3 enzalutamide and 4 abiraterone) were considered PSMA-responders, and all treatment non-responders (8 enzalutamide, 11 abiraterone) were considered PSMA-non-responders. PSMA PET response was thus perfectly associated with conventional response criteria (p = 0.006, Phi = 1 for enzalutamide; p = 0.001, Phi = 1 for abiraterone). In our cohort, no PSMA expression flare phenomenon was detected on follow-up PET/CT scans at a median follow-up of 3months. However, an early and short-lived flare cannot be excluded. This retrospective study suggests that, after a median follow-up of 3months under enzalutamide or abiraterone, PSMA expression changes on PET/CT are strongly associated with response to treatment. Prospective studies are needed to better understand PSMA expression dynamics following the start of enzalutamide and abiraterone, along with the role of PSMA PET/CT in response assessment.

Prostate specific membrane antigen (PSMA) expression correlates with prostate cancer grade and is increased in hormone-refractory prostate cancer. The increased expression of PSMA following androgen deprivation therapy may be a consequence of the down-regulation of PSMA expression by androgen. Moreover, 1alpha,25-dihydroxyvitamin D3 (1,25-VD) has been shown to suppress prostate cancer progression as well as cell motility and invasion. Since PSMA is positively correlated with both of these characteristics, we hypothesized that 1,25-VD would regulate PSMA expression. LNCaP prostate cancer cells were treated with 1,25-VD, followed by analysis of cell surface PSMA expression. The PSMA enhancer, located within the third intron of the PSMA gene, was cloned into a reporter vector and regulation by 1,25-VD was investigated. The role of the androgen receptor (AR) in 1,25-VD mediated suppression of PSMA expression was examined using Casodex and AR specific siRNA. Surface expression of PSMA was significantly decreased in a dose-dependent manner by 10 nM 1,25-VD or greater. Regulation by 1,25-VD occurred at the level of the PSMA enhancer. Over-expression of the vitamin D receptor (VDR) also decreased expression of PSMA. Additionally, suppression of AR translation using siRNA technology blocked the suppressive effect of 1,25-VD on PSMA expression, however inhibition of PSMA expression by 1,25-VD occurred in the absence of androgens. Suppression of PSMA by 1,25-VD occurs at the level of the PSMA enhancer and is elevated by over-expression of the VDR. This regulation involves the AR, but is not dependent on the presence of androgens.