Prostate cancer (PCa) is the second most common malignant tumor and the third leading cause of cancer-related death in men worldwide. Although a substantial portion of PCa can be cured by surgery or radiotherapy if detected early, recurrence or metastatic PCa remains a therapeutic challenge. Biochemical recurrence (BCR) occurs in up to 40% of PCa patients after primary treatment (1). Timely detection of disease recurrence is important for curatively intended treatment. Despite recent advances in conventional imaging, such as contrast-enhanced CT or MRI imaging, these imaging modalities exhibit limited sensitivity for recurrence assessment, especially in cases with low serum prostate-specific antigen (PSA) levels (1, 2).

In recent years, prostate-specific membrane antigen (PSMA) has received significant interest as a target for prostate cancer imaging. PSMA is a well-characterized membrane protein expressed 100- to 1000-fold higher on the surface of PCa cells than on benign prostate cells or normal tissue. Several PET agents, such as 68Ga-PSMA-11 and 18F-PSMA-1007, have been increasingly used to diagnose and guide salvage radiation therapy for biochemically recurrent PCa (3–5). A growing body of evidence suggests that PSMA PET has very high sensitivity and specificity for tumor sites in patients with BCR, even at PSA values less than 1 ng/mL (4, 6, 7). Based on its excellent diagnostic performance, several international PCa guidelines, including the European Association of Urology guideline, recommend PSMA PET/CT for patients with recurrent PSA after primary therapy (2, 8, 9).

In addition to small molecule inhibitors suitable for PET imaging, 99mTc-labeled small molecule inhibitor of PSMA has gained increasing interest recently. Given the broad availability of SPECT/CT devices, lower instrumentation and radionuclide costs, SPECT/CT systems have huge prospects for application to improve PSMA imaging capacity as more patients with prostate cancer undergo PSMA imaging. Several 99mTc-labeled PSMA inhibitors have hitherto been developed to detect PCa, including 99mTc-MIP-1404, 99mTc-MIP-1405, 99mTc-PSMA-I&S, and 99mTc-EDDA/HYNIC-iPSMA. Schmidkonz et al. (10–12) and García-Pérez et al. (13) have demonstrated the diagnostic potential of 99mTc-MIP-1404 and 99m Tc-EDDA/HYNIC-iPSMA in detecting BCR of PCa, even at low and very low serum PSA levels.

In recent years, a novel 99mTc-labeled PSMA inhibitor (HYNIC-Glu-Urea-A, 99mTc-HYNIC-PSMA) with specific accumulation in PSMA-positive tumors and low dosimetry has been synthesized (14, 15). Additionally, the labeling method of 99mTc-HYNIC-PSMA is simpler, quicker, and don’t require further purification, thus making it suitable for routine clinical applications. Nonetheless, little evidence is available on its clinical value in PCa patients with BCR. To bridge this knowledge gap, we sought to report our experience on the use of 99mTc-HYNIC-PSMA in 147 PCa patients with BCR.

An increasing body of evidence suggests that progression to metastasis or recurrence of PCa remains an important cause of death (1, 18). The early detection of prostate cancer recurrence following primary therapy is crucial to improve patient outcomes. It has been established that conventional imaging modalities (bone scan, CT, and MRI) exhibit limited ability to detect recurrence accurately, especially at low PSA levels (2). Therefore, an imaging technique that can detect recurrences effectively is urgently needed. Given that PSMA is overexpressed predominantly in 90-100% of PCa lesions, it has gained increasing attention as an attractive target for diagnosis. There is a rich literature available substantiating that 68Ga and 18F labeled PSMA PET-CT imaging is a promising modality for detecting PCa recurrence (3, 4, 6, 7, 19). To evaluate the diagnostic potential of 99mTc-HYNIC-PSMA in PCa recurrence, we retrospectively analyzed the clinical and imaging data from 147 patients with biochemical recurrence of PCa. We provided compelling evidence that 99mTc-HYNIC-PSMA SPECT/CT could effectively detect and localize PSMA-positive lesions with an overall detection rate of 80.3% (118/147). Detection rates were significantly increased at PSA levels above 2 ng/mL (90.2% vs. 48.6%; P < 0.0001). In addition, univariate and multivariable linear regression analysis showed that SUVmax positively correlated with PSA levels, which could potentially reflect disease activity.

Over the years, 99mTc-labeled PSMA inhibitors have been reported to detect BCR (See the summary of studies in Table 4 ), although limited studies have been conducted. Two studies by Schmidkonz and colleagues reported that the overall detection rates of 99mTc-MIP-1404 for BCR of PCa were 77% (174/225) (10) and 70% (42/60) (12), respectively. Similarly, Su et al. (20) and Liu et al. (21) reported detection rates of 78% (39/50) and 73% (151/208) for 99mTc-HYNIC-PSMA in the localization of BCR. A detection rate of 57% (87/152) was reported for 99mTc-PSMA-I&S, which is lower than our finding (22). A prospective study reported a lower detection rate (58%, 21/36) using the new radiopharmaceutical 99mTc-PSMA-T4 (23).

Interestingly, Schmidkonz et al. and Reinfelder et al. observed 54% and 40% detection rates for PSA levels less than 2 ng/mL (10, 12). A similar detection rate of 54% was reported by Liu et al. (21) by analysis of 78 patients with PSA levels less than 2 ng/mL, slightly higher than our findings. Moreover, we found that the detection rate was 40.0% (4/10) and 30.8% (4/13) at PSA levels of 0.5-1 ng/ml and 0.2-0.5 ng/ml. Schmidkonz et al. reported a favorable detection rate of 56% and 44% at low (0.5-1 ng/ml) and very low (0.2-0.5 ng/ml) PSA levels, respectively (11). Nevertheless, 2 studies reported less promising detection rates at PSA levels less than 1 ng/ml (30% and 20% with 99mTc-HYNIC-PSMA and 99mTc-PSMA-I&S, respectively) (20, 22).

An increasing body of evidence suggests that 68Ga-PSMA ligand, a dedicated PET agent, can significantly increase the detection rate of recurrent PCa (24–29). The overall detection rate ranged from 47% to 97%, and at PSA levels above 2 ng/ml, a detection rate ranging from 75% to 97% was observed, while a detection rate of 80% was found in our study. For PSA levels less than 2 ng/mL, a 61% to 69% detection rate was reported, 15% higher than reported in the present study. Furthermore, several studies reported promising detection rates at low PSA levels, ranging from 58% to 75% at 0.5-1 ng/ml and 38% to 62% at <0.5 ng/ml (27, 29–32), which are higher than observed in our study. However, Meredith et al. (33) reported a significantly lower detection rate in patients with low PSA levels. The detection rate was 25% at PSA levels 0.5-1 ng/mL and 18% at PSA levels less than 0.5 ng/mL. The primary advantage of 68Ga-PSMA PET is its relatively high detection rate at low serum PSA levels. It is widely acknowledged that compared with SPECT, PET has the advantages of higher sensitivity and higher spatial resolution when evaluating small lesions associated with low PSA levels. A direct comparison involving the same patient population between 68Ga-PSMA and 99mTc-PSMA was reported by Fallahi et al. (34). They found that lesion size strongly influenced SPECT detection of lesions.

Herein, we showed that the detection rate and SUVmax from patients treated with ADT were significantly high. Consistent with our findings, Einspieler et al. (35) and Afshar-Oromieh et al. (27, 30) demonstrated a strong positive correlation between ADT and 68Ga-PSMA Ligand PET/CT positivity. However, Eiber et al. (29) found no significant difference in detection rates regarding ADT. Indeed, ADT is highly controversial in regards to PSMA ligand uptake. Short-term ADT is known to increase PSMA expression in PCa cells, thereby increasing PSMA ligand uptake (36). In contrast, it has been reported that PSMA uptake decreases with long-term ADT, decreasing the sensitivity of PSMA PET for detecting PCa lesions (37, 38). Furthermore, long-term ADT also reduces tumor cell numbers. Thus, the higher detection rate in patients who received at least 6 months of ADT may be attributed, to a certain extent, to have more advanced-stage disease. The present study found that patients with ADT had a higher percent of advanced disease than those without ADT (81.9% vs. 68.0%), but the difference was not statistically significant (p=0.0526). Accordingly, PSA levels were higher in patients with ADT than in those without such therapy.

The present study found no significant difference between the detection rates at different Gleason scores, consistent with the literature (30, 39). In addition, a significant positive correlation between Gleason scores and SUVmax was observed during univariate linear regression analysis but not during multivariate regression analysis. Schmidkonz et al. found a significant correlation between the PSMA PET positivity and the Gleason score, which was attributed to PSMA upregulation at higher Gleason scores (10, 40).

99mTc-HYNIC-PSMA can detect recurrent lesions in different locations, such as local recurrence, lymph nodes, bone metastases, and visceral metastases. Some patients presented multiple lesions at different sites. The detection rates of 9.5%, 49.7%, 32.7% and 2.0% in the prostate, lymph nodes, bone and visceral regions, respectively. The lymph nodes are one of the most common sites of PCa recurrence after primary therapy, especially in the pelvic region. Pelvic lymph node metastases were detected in 17.4% of patients with early BCR (PSA levels of less than 1 ng/mL). Notably, 99mTc-HYNIC-PSMA detected 35.6% PSMA positive lymph nodes less than 10 mm in diameter, highlighting good detection yield for small lymph node metastases, suggesting that SPECT can be used to diagnose lymph node metastases although it has an inferior resolution than PET.

In addition, 165 PSMA-positive bone lesions could be detected on 99mTc-HYNIC-PSMA SPECT. Of these, 34.5% of PSMA-positive bone lesions were recognized only by SPECT but not by CT. A potential explanation could be that the 99mTc-HYNIC-PSMA SPECT detects metabolic changes in bone tissue during the development of bone metastases at a very early stage, whereas a CT scan shows no obvious soft tissue or bone destruction (41). Notably, early detection of lymph node and bone metastases is beneficial to PCa patients since early treatment of BCR results in better outcomes (42).

This retrospective analysis has certain limitations. The pathological confirmation of positive lesions was available in only 19 patients. Accordingly, it is highly conceivable that false-positive lesions were detected. Nevertheless, the PSA levels decreased in 68 patients with positive imaging results who were referred for follow-up treatment, indicating the PSMA-positive lesions were most likely true positives. Sergieva et al. (23) demonstrated a sensitivity of 84.37% (27/32), a specificity of 84.37% (27/32) and an accuracy of 86.11% (31/36) for detecting BCR using 99mTc-PSMA-T4 in 36 PCa patients. Therefore, it is necessary to perform further analyses to evaluate the sensitivity and specificity of 99mTc-HYNIC-PSMA imaging in detecting BCR. In addition, the present study was retrospective, limiting our ability to obtain more comprehensive data regarding patient treatment and follow-up. Accordingly, prospective studies are warranted to validate our findings.

The high prevalence of prostate cancer emphasizes the need to develop a novel, cost-effective, and easily synthesized 99mTc-labelled PSMA ligand to leverage SPECT’s broad availability. 99mTc-HYNIC-PSMA may fill this clinical need, yielding a high detection rate for 147 PCa patients with biochemical relapse. Further analysis indicated that the detection rate and SUVmax were positively associated with increasing PSA levels and ADT.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by the ethics committee of Henan Provincial People’s Hospital & Zhengzhou University People’s Hospital according to the principles of the Declaration of Helsinki. All patients gave written permission for the use of their data and informed consent for the anonymous publication. The patients/participants provided their written informed consent to participate in this study.

YG, XW, DD and BL designed the retrospective study. BL, XW, LD, JS, WW and YG collected and analyzed the data. YH, XC, AK and YC analyzed the data. BL, YG and XW drafted the manuscript. YG and BL can authenticate all raw data. All authors contributed to the article and approved the submitted version.