Prostate cancer (PCa) is the world’s second leading cause of cancer-related mortality in men (1). China has the sixth-highest rate of incidence and mortality due to PCa (2). Androgen deprivation therapy (ADT) is one of the most important therapies for patients with hormone-sensitive PCa. Unfortunately, most patients with PCa eventually develop metastatic castration-resistant prostate cancer (mCRPC) within 2–3 years of undergoing ADT (3, 4). Currently, multiple approved therapies can prolong the survival of patients with mCPRC, including new-generation hormone drugs, such as abiraterone and enzalutamide, and chemotherapeutic drugs, such as docetaxel and cabazitaxel, targeted therapy drugs, and immunotherapy drugs (Sipuleucel-T) (5). However, despite the efficacy of these drugs, cancer cells inevitably develop resistance to them (6). Once patients fail the second-line endocrine therapy, there is a lack of a standard treatment model for subsequent treatments. A clinical trial suggested that a common subset of mCRPC, characterized by defects in DNA repair, could be treated using the poly (ADP-ribose) polymerase (PARP) inhibitor olaparib in patients with mCRPC who had developed resistance to standard treatments (7, 8). However, this subset only accounted for approximately 11.8% of all sporadic mCRPC (2). Most patients could not achieve durable responses with available treatments. Thus, it was a fundamental requirement to identify novel strategies to improve the survival of patients with mCRPC after the failure of second-line endocrine therapy.

The introduction of immunotherapy-targeted programmed death protein 1 (PD-1) and programmed cell death-ligand 1 (PD-L1) has altered the treatment paradigm for various types of malignancies. Unfortunately, immunotherapy has only shown modest efficacy against PCa (9). As per the results of two recently published clinical studies, two anti-PD-1 antibodies, pembrolizumab and atezolizumab were well tolerated and safe in patients with mCRPC. However, complete response (CR) was achieved in only a few patients (10, 11). Tislelizumab, an investigational anti-PD-1 antibody, has been shown to be significantly efficacious in (85.7% objective response rate (ORR)) patients with relapsed/refractory classical Hodgkin’s lymphoma (12). A recently published research demonstrated that tislelizumab showed substantial clinical benefits and an acceptable safety profile in patients with urothelial carcinoma (13). Another study found that tislelizumab had disease stabilization capacity for various tumor types and in patients who had undergone different types of long-term treatments (14). Thus, it was speculated that tislelizumab might act as an effective salvage treatment strategy to improve the outcomes in patients with mCRPC. However, due to the “cold tumor” characteristics of PCa, the response to immunotherapy in PCa might not be as strong compared with other tumors (1). Thus, a combination of immunotherapy and other current treatment strategies, such as chemotherapy, targeted therapy, and radiotherapy (RT), could improve the immune response to “cold tumors.” Previous studies have reported that ipilimumab monotherapy or the addition of atezolizumab to enzalutamide for treating patients with mCRPC could not provide a satisfactory primary endpoint for overall survival (OS) (15, 16). However, using ipilimumab plus RT showed improved outcomes compared with placebo plus RT in patients with postdocetaxel mCRPC (17). Consequently, RT could be a promising strategy for the synergistic enhancement of immunotherapeutic efficacy.

Numerous clinical trials have supported the use of RT in the modification of antitumor immune responses, enhanced expression of antigens on the surface of tumor cells, as well as tumor antigen crosspresentation in the draining lymph nodes, directly resulting in the activation and proliferation of tumor-specific cytotoxic T cells (18–21). Consequently, this might result in a modified tumor microenvironment along with the expansion of immunotherapeutic capacity (22, 23). Multisite stereotactic body radiotherapy (SBRT) has emerged as an altering paradigm for treating solid metastatic tumors (24). A phase I study indicated that multisite SBRT combined with pembrolizumab for solid metastatic tumors was well tolerated with acceptable levels of toxicity (25). However, there is a scarcity of sufficient studies examining the therapeutic effects of combined anti-PD-1 and multisite SBRT in mCRPC treatment, necessitating further research. Another phase II trial demonstrated that avelumab combined with SBRT exhibited elevated activity and acceptable toxicity in treatment-refractory mCRPC (26). Although a study reported that SBRT with a few fractionations was the best choice for the abscopal effect (27), an appropriate RT technique should be chosen based on the symptoms and condition of the patient with mCRPC. Previous studies have demonstrated that low-dose radiation (e.g., doses below 3 Gy) could promote immune cell infiltration into the stroma and the tumor bed of distant tumors, resulting in an improved rate of the systemic response to metastatic disease (28).

Furthermore, low-dose radiotherapy against established metastases has also been shown to significantly enhance the abscopal response to hypofractionated RT plus immune checkpoint inhibitors (29). Low-dose radiation also carries the potential to amplify the antitumor immune effects. Another study suggested that low-dose radiation (a maximum dose of 8–10 Gy/fraction) could induce interferon signaling, resulting in RT-induced abscopal outcomes (30). Several studies also indicated that multiple dose-fractionation schedules of RT resulted in an enhanced abscopal effect compared to a single dose (31, 32). However, further research is required to explore whether the reported doses of RT would exhibit the activation impact in combination with immunotherapy. Thus, combination trials of immunotherapy and RT could be designed to optimize the choice of optimal dose and fractionation.

Here, we aimed to analyze the safety and efficacy of multisite radiotherapy combined with tislelizumab for patients with mCRPC who have experienced failure of at least one second-line treatment. Additionally, we planned to explore the predictive biomarkers of the efficacy of this combined regimen to facilitate clinical studies.

This study presents the first investigational analysis of the safety and efficacy of tislelizumab combined multisite RT for patients with mCRPC who had experienced failed ADT and at least one second-line endocrine therapy failure. Until now, the poor responses of immunotherapy against PCa might be attributed to its characteristics of low immune infiltration, low tumor mutation load, and low antigen presentation (38). Additionally, PCa evades and inhibits antitumor immunity via elevated expression of PD-L1 and enrichment of Tregs in both tumor and peripheral blood [19-21]. Interestingly, various studies have confirmed that a combination of immunotherapy and RT could constitute a promising strategy for the synergistic enhancement of treatment efficacy. In the last few years, several studies on various types of tumors have explored the combination of radiotherapy and immunotherapy, such as breast cancer, melanoma, nonsmall-cell lung cancer (NSCLC), and esophageal squamous cell carcinoma (39). All studies showed promising antitumor activity and acceptable tolerability. In recent years, there have been significant advances in the treatment of PCa, and several new treatment strategies for mCRPC with clinically proven survival benefits for mCRPC have been developed (10, 11, 40). However, there is still a lack of appropriate strategies for patients with mCRPC who have experienced ADT failure and second-line endocrine therapy. A recent study revealed that avelumab with SABR showed promising activity and acceptable toxicity in treatment-refractory mCRPC (26), indicating that immunotherapy combined with RT was still the best area of research. However, the data were limited to only one combination of tislelizumab and RT, limiting the treatment potential of mCRPC. Therefore, the combination treatment of tislelizumab plus multisite radiotherapy represents a potential approach and needs further investigation for patients with mCRPC who had experienced failure of ADT and second-line endocrine therapy.

The present study has been designed for three treatment phases. Tislelizumab monotherapy aims to observe the efficacy of tislelizumab monotherapy for patients with mCRPC by measuring changes in patients’ PSA levels and symptoms. Due to the “cold tumor” characteristics of PCa, we predict that the 2-cycle efficacy of tislelizumab monotherapy may be insignificant. But it may show effectiveness in some patients who might benefit from immunotherapy in a short period, and those patients are worth being screened for biomarkers for immunomonotherapy.

Some patients with an immediate immune reaction to immunotherapy may result in irAEs. Previous studies reported that patients who experienced irAEs demonstrated marked improvements in immunotherapy efficacy compared to those with low toxicity (41). However, if irAEs occur prematurely (≤8 weeks), immunotherapy is likely to be discontinued due to toxicity. Therefore, we designed two cycles of tislelizumab monotherapy. Furthermore, recent retrospective studies have indicated that “early” irAEs were associated with poor prognosis, and the immunosuppressive treatment for irAEs may hinder anti–PD-1 monotherapy efficacy (42, 43). Therefore, the monotherapy phase would also help understand whether the early irAEs will occur in tislelizumab monotherapy, thus assessing the safety of tislelizumab monotherapy. Followed by the tislelizumab combined RT, comparing the efficacy and safety of tislelizumab monotherapy, the safety and synergistic effect of multisite RT combined with immunotherapy can be better observed. Furthermore, the safety and efficacy of RT can still be fully observed due to a delayed effect from RT, even if entering the tislelizumab monotherapy maintenance phase. Notably, suppose patients with mCRPC obtain a good survival benefit from this study, then the treatment value of tislelizumab as a maintenance therapy method in these patients could obtain preliminary verification.

Until now, there has been a lack of consensus regarding the ideal dose of RT in combination with immunotherapy. SBRT, as a novel RT method, is essential in the treatment of early primary cancer and oligometastatic disease, such as oligometastatic (≤5 lesions) PCa, early-stage nonsmall-cell lung cancer, and liver cancer (44, 45). It has the potential to deliver a small amount of ultra-high doses of radiation to relatively small target lesions, achieving local control with a low risk of toxicity (46). For advanced cancer patients with multiple metastases, the dose of irradiated lesions might be different to achieve excellent local control with a low risk of toxicity and more potent immune activation effects. Therefore, individualized RT will be performed in this study. We will still preferentially select treatment with SBRT, 40 Gy in five fractions, every other day for primary lesions.

On the one hand, the hypofractionated SBRT regimen of 40 Gy/5 is delivered to accommodate the radiation tolerance of organs at risk. On the other hand, the hypofractionated SBRT regimen facilitates immunogenic cell death (ICD), leading to the release of tumor antigens, thus amplifying the efficacy of immunotherapy (47). However, the majority of patients with PCa usually present with multiple distant metastases. In such cases, the dose regimens, guidelines, and normal tissue constraints determined in carefully conducted, high-quality prospective trials should be adopted (44). According to the ASTRO guidelines and the SABR-COMET study (48), 30 Gy in 10 fractions was preferred to treat bone metastases in the present study, which plays a role in palliative pain relief and modulates the immune response microenvironment (49). For liver metastases, 20 Gy in 5 fractions was standard institutional practice. According to the 2011 consensus guidelines, the radiation dose fractionation for lung metastases mainly included 8 Gy in 1 fraction, 20 Gy in 5 fractions, and 30 Gy in 10 fractions (50), and 20 Gy in 5 fractions was preferred for the treatment of bone metastases in the present study.

Furthermore, the present study combined immunotherapy treatment for patients with mCRPC who had failed multiline therapy and had a relatively long survival time. Thus, irAEs are important safety parameters to consider, especially for fatal irAEs such as pneumonitis, neurologic toxicity, colitis/diarrhea, and hepatitis (51) as the significant life-threatening factors for elderly patients. Using the combination of higher radiation doses with anti-PD-1 immunotherapy may cause irAEs to occur in the long term. Therefore, SBRT with relatively low radiation doses was performed based on security considerations in this study.

Tislelizumab is a novel IgG4 anti-PD-1 mAb monoclonal antibody that minimizes binding to FcγR on the surface of macrophages to eliminate antibody-dependent phagocytosis, resulting in a higher affinity for PD-1 compared with pembrolizumab and nivolumab (12). Both clinical literature and pharmacokinetics (PK) analysis have demonstrated that tislelizumab is well tolerated for multiple advanced tumor types and supports fixed dosing (200 mg) (52). Therefore, in the present study, we used a fixed-dose instead of dose-escalation exploration, which avoided the uncertainty caused by dose exploration and improved the effectiveness of this study.

Additionally, the most important aim was to maximize the therapeutic benefits by developing predictive biomarkers of immunotherapy responsiveness. Several biomarkers have been associated with the treatment effect of anti-PD-1 therapy, such as TMB, mismatch repair deficiency (dMMR), PD-1 expression, and TIL number (53). At the same time, these have been reported to be relatively rare in patients with mCRPC. TMB, a biomarker independent of PD-L1 expression, has been revealed to have a significant association with ORR across multiple cancer types (54). However, the application of TMB in mCRPC needs further validation. A previous study suggested that tumors with dMMR are susceptible to PD-1 and PD-L1 inhibitors.

Meanwhile, dMMR tumors exhibit a dense infiltration of CD⁸⁺ TILs that have been shown to induce a better and more durable response (55). Several clinical studies have indicated the association between dMMR and immunotherapy-related responses and better prognosis in other solid tumors (55–57). However, this correlation needs further exploration in mCRPC. Numerous clinical trials have investigated that PD-L1 expression is the most widely adopted predictor, and high PD-L1 expression is associated with clinical benefit and response rate improvement in anti-PD-1/anti-PD-L1 therapy (53). TIL is a vital component that influences the tumor immune microenvironment and is used for the prediction of immunotherapy combined with the expression of PD-L1 expression. Elevated levels of baseline TIL and PD-L1 expression in breast cancers were found to be associated with an increased probability of pathologic complete response (58). However, in the immunotherapy combination of multisite RT for mCRPC treatment, the predictive value of PD-L1 expression and TIL counts is vague and deserves further investigation.

Furthermore, studies have demonstrated that genomic alterations might elicit a broad impact on the tumor microenvironment, contributing to the promotion and maintenance of responses to immunotherapy (59–61). Thus, a genomic analysis needs to be performed in this study to determine the impact of genomic alterations (such as mutations in the exonuclease domain of the DNA polymerase epsilon (POLE), high tumor mutational burden, and the presence of biallelic loss of CDK12, among others) on immunotherapy for PCa, for the early detection and identification of novel therapeutic targets. Thus, it would be crucial to establish a comprehensive assessment framework involving multiple biomarkers for interrogating the tumor immune landscape and selecting sensitive patients.

However, this study has several limitations. It is a nonrandomized study with a small sample size. Therefore, the results of this study would provide preliminary support for future randomized, controlled trials to assess the combined therapeutic regimen for patients with mCRPC.

Thus, this study is the first attempt to evaluate the efficacy and safety of tislelizumab with multisite radiotherapy for patients with mCRPC who have failed ADT and second-line endocrine therapy, in an attempt to provide an accurate and effective combined treatment for patients with mCRPC and improve the survival status of patients.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

The studies involving human participants were reviewed and approved by Ethics Review Committee of West China Hospital, Sichuan University (Ethical approval number: 2021203). The patients/participants provided their written informed consent to participate in this study.

KC, YQW, YC, YCW, and ZL were involved in the study conception and design. ZL provided administrative support. KC, YQW, and YC provided materials and samples. KC, JZ, and XQ participated in data collection. KC, YQW, YC, YCW, ZL, JZ, and XH contributed to analysis and interpretation of the data and wrote the manuscript. All authors were involved in the final approval of the manuscript. All authors agreed to be accountable for all aspects of the work and approved the final manuscript.

This study was partly supported by the Sichuan Science and Technology Department Key Research and Development Project (22YFS0336) and 1.3.5 project for disciplines of excellence, West China Hospital, Sichuan University (ZYJC18048).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.