Current Status of Monoclonal Antibodies-Based Therapies in Castration-Resistant Prostate Cancer: A Systematic Review and Meta-Analysis of Clinical Trials

Background Multiple patients with prostate cancer become resistant to castration therapies, which is termed castration-resistant prostate cancer (CRPC). Purpose The purpose of this review is to assess the status of efficacy (≥50% decline in prostate-specific antigen (PSA), progression-free survival (PFS), and overall survival (OS)) and safety (grade 3-4 adverse effects) of monoclonal antibodies in CRPC. Data source We searched databases including PubMed, Embase, Cochrane, Web of Science, and ClinicalTrials.gov. Results Hazard ratios of PFS and OS were 0.77 (95% CI = 0.69-0.87, I2 = 53%) and 0.98 (95% CI = 0.86-1.11, I2 = 40%), respectively, in the favor of monoclonal antibodies as compared to placebo. Risk ratio (RR) of >50% decline in PSA was 1.99 (95% CI = 0.97-4.08, I2 = 53%) in favor of monoclonal antibodies. Pooled incidence of >50% decline in PSA levels was 15% (95% CI = 0.1-0.23, I2 = 83%), 29% (95% CI = 0.14-0.51, I2 = 93%), 63% (95% CI = 0.49-0.76, I2 = 77%), and 88% (95% CI = 0.81-0.93, I2 = 0%) in single, two, three, and four-drug regimens, respectively. Conclusion Monoclonal antibodies are well tolerated and showed better PFS as compared to placebo. However, OS was only improved with ipilimumab. Denosumab delayed skeletal-related adverse events as compared to zoledronic acid. More multicenter double-blind clinical trials may be needed to confirm these results.


Introduction
Prostate cancer is the second most common cause of cancer deaths in men after lung cancer in the United States with both aggressive and slow-growing types identified. More than 20% of the newly diagnosed cases of cancer are prostate cancer [1]. The new cases and estimated deaths for prostate cancer reported in the US in 2019 were 174,650 and 31,620, respectively, with an increase in the trend seen in 2020 with 191,930 new cases and 33,330 estimated deaths [1,2]. Globally, 1,276,106 new cases were estimated in 2018. Developed countries have higher incidence probably due to better use of diagnostic testing [3].
Chemotherapy agents including taxanes, bisphosphonates, immunotherapy agents, and poly (ADP-ribose) polymerase-1 inhibitors have shown anti-tumor activity in patients with castration-resistant prostate cancer (CRPC). Taxane with prednisone is the most common treatment used for CRPC. Despite these treatment options, the prognosis and quality of life of these patients are very poor. There is still room for more combination therapies for the treatment of CRPC, especially for patients who do not tolerate and/or are refractory to first-line therapies [10][11][12][13].
In recent years, monoclonal antibodies have shown promising results in clinical trials. Monoclonal antibodies have been evaluated for their efficacy in CRPC due to their targeted action on various tumor factors that help control cancer progression [4]. The most common antibodies studied include bevacizumab (anti-vascular endothelial growth factor (VEGF)), which decreases angiogenesis and improves vessel penetration of cytotoxic agents like taxanes when used in combination [10,11]. Cixutumumab and ramucirumab act against insulin-like growth factor-1 receptor (IGF-1R)/vascular endothelial growth factor receptor (VEGFR) and can prevent tumor growth. Other monoclonal antibodies, including siltuximab, abituzumab, trastuzumab, and cetuximab, bind to interleukin-6, integrin alpha-V, human epidermal growth factor receptor 2 (HER2), and epidermal growth factor receptor (EGFR), respectively [12][13][14][15]. Checkpoint inhibitors including nivolumab (anti-programmed cell death protein 1 (PD-1)), pembrolizumab (anti-PD-1), and ipilimumab (anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4)) are also tested in clinical trials for anti-tumor activity against CRPC [16,17]. While several of these immunotherapies are under evaluation in clinical trials, denosumab is the major monoclonal antibody approved by the FDA for metastatic bone lesions in CRPC [18].
The aim of this systematic review and meta-analysis is to assess the efficacy and safety of monoclonal antibodies alone or in combination with chemotherapy drugs in CRPC.

Materials And Methods
In conducting this systematic review and meta-analysis, we followed a prespecified protocol registered on the International Prospective Register of Systematic Reviews (PROSPERO) (registration number: CRD42021230102). The protocol was made according to the guidelines established by Cochrane [19] and PRISMA-P (Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols) [20].

Search strategy
A literature search was performed on PubMed, Embase, Web of Science, Cochrane Library, and ClinicalTrials.gov with Medical Subject Heading (MeSH) and Emtree terms "monoclonal antibodies" and "castration-resistant prostate cancer." The search was made from the inception of literature till March 20, 2021, by following the PICO framework (Appendix) [21].

Inclusion and exclusion criteria
We included all clinical trials that provided safety and efficacy data in clinical terms, i.e., objective response (OR), complete response (CR), partial response (PR), ≥50% decline in PSA, progression-free survival (PFS), overall survival (OS), and grade 3-4 adverse effects. We excluded all preclinical studies, case reports, metaanalyses, review articles, observation studies, and clinical studies irrelevant to the study question.

Study selection
Two researchers (WA and TAT) independently reviewed the articles identified through initial search and screened them based on inclusion and exclusion criteria. The differences were addressed by a third researcher (MAA).

Data extraction
Data were extracted by two authors (MS and MYA). The data were extracted for the characteristics of the study, baseline characteristics of participants, treatment drugs, efficacy measures, and toxicity (grade ≥ 3 adverse effects).

Risk of bias assessment
Two researchers (SR and SFB) assessed the risk of bias in randomized clinical trials (RCTs) selected for final inclusion by using the Risk of Bias 2 (RoB 2) tool for risk of bias assessment in RCTs [22]. The third researcher (MAA) addressed the differences.

Statistical analysis
The meta-analysis was performed using the "R" programming language. We used the "meta" package in R for our data analysis [23]. A random-effects model was used, irrespective of the heterogeneity, to keep our results consistent and applicable. All analyses used the DerSimonian-Laird estimator to calculate betweenstudy variance. The risk ratios were pooled using the Mantel-Haenszel method. For studies with zero events in any of the arms, a continuity correction of 0.5 was used. Standard errors and other calculations were done using a 95% confidence interval. For pooling of the results, all the studies were included even if they have zero events in both arms. To estimate the heterogeneity, I 2 was used.

Monoclonal antibodies vs. placebo
In six clinical trials (N = 4,194) [13,[24][25][26][27][28][29][30], monoclonal antibodies were given to 2,225 participants while placebo was given to 1,969 participants. Standard of care (SOC) including luteinizing hormone-releasing hormone agonist/antagonist was given to 180 patients in the study by Hussain et al. [30]. The median ages of participants were ≥65 years in RCTs. Baseline characteristics of participants are given in Table 1 1,904). HR of the first skeletal-related adverse event was 0.82 (95% CI = 0.71-0.95) in favor of denosumab as compared to zoledronic acid. The incidence of total skeletal-related events was 36% in the denosumab group vs. 41% in the zoledronic acid group. Radiation to bone was used in 19% of the people in the denosumab group vs. 21% in the zoledronic acid group. The incidence of adverse events was 97% each in both groups. Greater than or equal to grade 3 adverse events were 72% and 66% in denosumab and zoledronic acid groups, respectively. Osteonecrosis of the jaw was 1% in the zoledronic acid group vs. 2% in the denosumab group. Discontinuation of treatment due to adverse events was reported in 15% of participants in the zoledronic acid group and 17% in the denosumab group.

Discussion
Docetaxel is the most used chemotherapy-based treatment for metastatic CRPC as docetaxel improved OS, PFS, and PSA levels in RCT [59]. Among non-chemotherapy drugs, alpharadin, abiraterone, radium-223 dichloride, etc., showed improvement in survival rates with anti-tumor activity [60]. Among immunotherapies, sipuleucel-T extended OS without improving PFS [61]. However, these therapies are not curative, responses are rarely durable, and are poorly tolerated by some patients. Additional treatment options are needed for better outcomes. In RCTs, majorly monoclonal antibodies were used in combination with docetaxel or in patients refractory to docetaxel therapy. According to the pooled results, monoclonal antibodies improved PFS and PSA response as compared to placebo.
Checkpoint inhibitors, including PD-1, programmed death-ligand 1 (PD-L1), and CTLA-4 inhibitors, have shown efficacy in urothelial and other solid tumors [62][63][64][65]. However, the microenvironment of prostate cancer is more immunosuppressive as compared to other tumors [66,67]. Ipilimumab (CTLA-4 inhibitor) improved PFS and PSA levels in both trials, including docetaxel pre-treated and treatment naïve patients. It was well tolerated in both trials. OS was not prolonged on normal follow-up. However, long-term follow-up of five years showed better OS in the ipilimumab group as compared to placebo [68]. More trials are now conducted on combination therapy of ipilimumab. In the trial conducted by Boudadi et al. (2018) [48], 1 mg/kg of ipilimumab was used with nivolumab and anti-tumor activity was only reported in a small group of patients. However, according to the preliminary results of a trial by Sharma et al. (2020), 3 mg of ipilimumab with nivolumab showed anti-tumor activity in all subsets of patients and a large-scale phase II trial is in progress on ipilimumab + nivolumab in metastatic CRPC patients [16]. Another RCT is in progress to assess the efficacy and safety of ipilimumab in combination with abiraterone acetate, apalutamide, and prednisone. Ongoing clinical trials are also testing nivolumab in combination with ChAdOx1-MVA 5T4 vaccine, ESK981 (Pan-VEGFR/Tie2 tyrosine kinase inhibitor), and DF6002 (binds interleukin 12 (IL-12) receptor).
In a multicohort phase II trial by Antonarakis et al. (2020), pembrolizumab showed anti-tumor activity in docetaxel pretreated patients and the observed survival estimates are promising [17]. Although 5% of the patients showed OR, the response was durable. Pembrolizumab monotherapy was well tolerated, and no unexpected toxicities were reported. A combination of pembrolizumab with olaparib, enzalutamide, and docetaxel is tested in KEYNOTE-365, and the early results have shown anti-tumor activity of these combinations and are well tolerated [51]. According to the results of a phase II trial by Graff et al. (2020), pembrolizumab addition to enzalutamide showed anti-tumor activity in patients refractory to enzalutamide alone, and the response was durable [42]. Another trial was conducted on the addition of pembrolizumab to the anti-tumor DNA vaccine. The addition of pembrolizumab showed better results in terms of PSA decline, OR, and CD-8+ T cell infiltration into tumor lesions as compared to vaccination alone. More trials are in progress to assess the efficacy and safety of pembrolizumab in combination with dipeptidyl peptidase 4 (DPP4) inhibitor BXCL701, HER2 bi-armed activated T cells, talabostat mesylate, lutetium lu 177-PSMA-617, vaccine therapy, ZEN-3694 + enzalutamide, enzalutamide, XmAb22841, and CPI-006 (Table 4). Avelumab, atezolizumab, tremelimumab, cemiplimab, cetrelimab, XmAb20717, PDR001, and durvalumab are other checkpoint inhibitors that are getting tested alone and in combination therapy for the treatment of CRPC.
The anti-angiogenic drug, bevacizumab, also improved PFS and PSA levels without any improvement in OS. Bevacizumab was also tested in combination regimens. Among the combination regimens, the four-drug regimen of bevacizumab with docetaxel + thalidomide + prednisone and lenalidomide + docetaxel + prednisone showed the best efficacy outcomes, and toxicities were manageable ( Figure 5 and Table 2). Early anti-tumor activity was reported with the addition of thalidomide and bevacizumab to docetaxel as compared to docetaxel alone. Bevacizumab in combination with satraplatin has shown promising results in early phase trials in docetaxel refractory patients. The addition of everolimus (mammalian target of rapamycin (mTOR) inhibitor) to docetaxel + bevacizumab did not show better outcomes as compared to docetaxel + bevacizumab in the early-phase trial.
Abituzumab improved the progression of the disease, but the results were not statistically significant. In our analysis, the trial with intetumumab was the outlier and intetumumab caused worsening in PFS or PSA levels. However, intetumumab did not increase adverse events as compared to placebo. Intetumumab might have interacted with docetaxel, resulting in lower efficacy.
Lack of improvement in OS despite changes in PFS and PSA levels might be due to the unique response of CRPC to these drugs. Also, the patients with metastatic CRPC are generally older than patients with other types of cancer, e.g., breast cancer and lung cancer, and comparatively more patients have bone metastasis [69][70][71]. Other possible explanations can be the unique mechanism of action of these drugs or flaws in trial designs. These drugs might show some improvement in OS if followed for longer durations. Further studies should be conducted on how to utilize the anti-tumor activity of these monoclonal antibodies.
Denosumab targets receptor activator of nuclear factor kappa-B ligand (RANKL) and is an anti-bone resorptive agent. It delayed skeletal-related adverse events as compared to zoledronic acid in patients with CRPC with bone metastasis in RCT. Zoledronic acid was proved better than a placebo in an RCT [72]. However, increased incidence of osteonecrosis of the jaw was associated with denosumab as compared to zoledronic acid. A meta-analysis showed similar results for denosumab in the prevention of skeletal-related adverse events as compared to zoledronic acid [73]. Moreover, an RCT by Smith et al. (2012) tested denosumab for the prevention of bone metastasis [25]. Denosumab significantly improved bone metastasisfree survival and time to first bone metastasis as compared to placebo. The major adverse event observed in the denosumab group was the osteonecrosis of the jawbone.
In a non-comparative randomized study, cixutumumab (IGF-1R inhibitor) and ramucirumab (VEGFR inhibitor) were used with mitoxantrone-prednisone. PFS in the cixutumumab group was similar to the projected value, while ramucirumab showed better PFS as compared to the projected value (6.7 months vs. 3.9 months). The incidence of adverse events was similar to expectations. Ramucirumab has shown improvement in OS in RCTs on other solid tumors [74]. Another trial by McHugh et al. (2020) has also shown no activity of cixutumumab with temsirolimus [45].
Among monoclonal antibodies, PD-1 inhibitors, PD-L1 inhibitors, and CTLA-4 inhibitors have the potential to become the drugs of the future for patients with prostate cancer. More multicenter randomized clinical trials should focus on finding the efficacy and appropriate combination of these medications. However, the role of monoclonal antibodies in prostate cancer is still debated.

Conclusions
Monoclonal antibodies were well tolerated and showed better outcomes in terms of PFS and >50% decline in PSA levels compared to placebo. However, OS was only improved with ipilimumab as compared to placebo on long-term follow-up of five years. Denosumab delayed skeletal-related adverse events as compared to zoledronic acid in CRPC with bone metastasis. Denosumab also delayed bone metastasis as compared to placebo in patients with metastatic CRPC. Pembrolizumab, avelumab, atezolizumab, pasotuxizumab, and tremelimumab have shown promising results in the early phase trials. More multicenter, double-blind clinical trials may be needed to confirm these results.

Additional Information Disclosures
Human subjects: All authors have confirmed that this study did not involve human participants or tissue. Animal subjects: All authors have confirmed that this study did not involve animal subjects or tissue.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.