Introduction
Prostate cancer remains the second most common malignancy among men worldwide [
1]. In localized prostate cancer, primary treatments typically include radical prostatectomy or radiation therapy. However, recurrence occurs in 15 to 30% of treated patients [
2,
3], who subsequently undergo androgen deprivation therapy (ADT) as a standard treatment [
4]. Unfortunately, some patients progress to non-metastatic castration-resistant prostate cancer (nmCRPC), which is associated with increased risks of metastasis and mortality. Median overall survival (OS) and bone metastasis–free survival in nmCRPC patients treated with ADT have been reported as 43.5 and 31.5 months, respectively [
5].
Recently, three phase Ⅲ trials—SPARTAN, ARAMIS, and PROSPER—demonstrated that apalutamide (APA), darolutamide (DARO), and enzalutamide (ENZ) significantly extend metastatic-free survival (MFS) and OS for patients with nmCRPC [
6,
7,
8]. The NCCN guidelines recommend offering androgen receptor signaling inhibitors (ARSIs) to nmCRPC patients if their PSA doubling time (PSADT) is ≤10 months [
9]. Japanese public health insurance covers ENZ, APA, DARO, and abiraterone acetate (ABI) with prednisone for nmCRPC. Real-world clinical practice shows that PSA progression–free survival (PSA-PFS) and MFS are significantly improved with ENZ and ABI compared to first-generation antiandrogen therapy like flutamide [
10]. Novel ARSIs have been associated with better oncological outcomes in nmCRPC patients compared to those not receiving ARSIs [
11]. However, studies on the efficacy and safety of ARSIs in nmCRPC patients and comparisons among ENZ, APA, DARO, and ABI in real-world clinical settings remain limited. Furthermore, while some reports suggest PSADT as a prognostic factor for nmCRPC, the optimal cutoff value remains unclear [
5,
12]. Therefore, additional effective biomarkers are needed.
In this study, we conducted a retrospective evaluation of the clinical outcomes and prognostic factors among patients with nmCRPC treated with first-line ARSIs in real-world clinical practice in Japan.
Discussion
This study of nmCRPC patients receiving ARSI revealed three key findings. First, ARSI demonstrated favorable efficacy for nmCRPC patients. Second, there were no significant differences in clinical outcomes among different types of ARSI therapy for nmCRPC. Third, a high baseline PSA at the initiation of ARSI was significantly associated with poor prognosis for OS.
Recent three phase Ⅲ clinical trials, such as SPARTAN, ARAMIS, and PROSPER, have demonstrated that ARSIs significantly improve OS and MFS in nmCRPC [
6,
7,
8]. The phase Ⅱ IMAAGEN study demonstrated that ABI resulted in a significant ≥50% PSA reduction in nmCRPC [
15]. Real-world settings have also shown that ENZ and APA are effective and safe, with nmCRPC patients starting ENZ treatment in Japan experiencing a median time to PSA progression of 27.0 months [
16], and an 86% PSA ≥50% reduction reported in nmCRPC patients initiating APA treatment [
17].
Recent multi-institutional real-world studies have indicated that ARSI treatment, including ENZ, APA, DARO, and ABI, significantly improves oncological outcomes compared to conventional hormonal treatments for nmCRPC patients [
11]. Our single-institutional study also supports the favorable oncological outcomes of novel ARSIs in real-world clinical settings.
A systematic review and network meta-analysis focusing on OS and AEs associated with APA, ENZ, and DARO ranked DARO as the most effective OS benefit related to ADT, followed by ENZ and APA, in that order. DARO also showed the most favorable profile regarding Grade ≥3 AEs [
18]. There were no significant differences in PSA-PFS among the four novel ARSIs (APA, ENZ, DARO, ABI) in our cohort. While more patients experienced AEs with APA compared to the other ARSIs, the proportion of severe AEs did not significantly differ among all ARSIs.
A multivariate network meta-analysis suggested that AEs associated with novel ARSI therapy do not significantly differ, except for ENZ being ranked as more toxic regarding hypertension and headache [
19]. However, the comparative oncological outcomes and safety profiles among ARSIs for nmCRPC patients remain unclear. Therefore, additional studies involving larger cohorts of nmCRPC patients treated with ARSIs are needed.
Some risk factors affecting metastasis and survival outcomes in nmCRPC patients have been analyzed. Higher PSA concentrations at diagnosis, shorter PSADT, higher Gleason score, a history of primary intervention, and a shorter interval from ADT initiation to the diagnosis of CRPC have all been associated with shorter time to metastasis [
20]. Another study found that metastasis was associated with higher PSA levels at diagnosis, nadir PSA after ADT, rapid ALP rise, and shorter PSADT [
21]. Shorter bone metastatic–free survival was observed as PSADT decreased below 8 months [
22]. Trials like SPARTAN, PROSPER, and ARAMIS used 6 months as a cutoff point for PSADT in subgroup analyses [
6,
7,
8]. However, while PSADT <3 months was associated with the highest risk of metastasis and poorer survival outcomes, the optimal PSADT cutoff for predicting oncological outcomes and risk stratification in nmCRPC patients remains unclear [
12].
ALP has also been suggested as a significant prognostic marker for MFS in nmCRPC patients, although these patients were treated with not only ARSIs, but also antineoplastic therapies such as docetaxel and cabazitaxel. Elevated ALP is thought to indicate pre-existing activated micro-metastases [
23]. Our study indicated that a baseline PSA ≥3.67 ng/mL at the initiation of first-line ARSI for nmCRPC patients was the strongest predictor of poor OS in multivariate analysis. Previous reports have associated baseline PSA with survival outcomes [
5,
24,
25,
26], as PSA levels reflect tumor volume. High baseline PSA and a short PSADT in nmCRPC may be the most important factors to predict progression to metastatic disease and reduced OS [
20,
22,
25,
26,
27]. Therefore, preventing or delaying metastatic progression using these prognostic biomarkers should be a primary therapeutic goal for treating nmCRPC [
28]. To the best of our knowledge, this is the first report to demonstrate that higher baseline PSA levels are associated with poorer survival outcomes in nmCRPC patients treated with ARSIs. Baseline PSA level as a risk factor has been previously reported in nmCRPC patients undergoing ADT therapy.
We acknowledge several limitations in our study. First, it was retrospective with a relatively small cohort from a single institution, potentially introducing bias. The small sample size may have impacted the robustness of the results, especially in comparing efficacy and safety among different ARSI therapies. Moreover, our cohort included patients diagnosed with nmCRPC based on PSA levels ≤2.0 ng/mL at the start of ARSI therapy, despite the conventional definition of CRPC requiring a PSA rise of ≥2.0 ng/mL [
15]. In our real-world setting, PSADT took precedence over absolute PSA levels in 31 patients (24%). This inclusion of patients with baseline PSA <2.0 ng/mL might have influenced survival outcomes and prognostic factors. Lastly, we defined nmCRPC using conventional imaging like CT and bone scintigraphy. Whole-body diffusion-weighted MRI (WBMRI) has been reported to have better sensitivity and similar specificity compared to bone scintigraphy completed with targeted x-rays (sensitivity: 98–100% vs. 86%, specificity: 98% vs. 98-100%) for detecting bone metastases [
29]. Additionally, another retrospective study indicated that prostate -specific membrane antigen (PSMA)-PET showed positivity in 196 out of 200 patients with nmCRPC, revealing pelvic disease in 44% and metastatic disease in 55% of patients despite negative conventional imaging [
30]. This suggests that some nmCRPC patients may have already progressed to metastatic CRPC detectable by PSMA-PET scan or WBMRI. However, the availability of WBMRI remains limited, and PSMA-PET scans have not yet been introduced in Japan. Therefore, external validation will be necessary after the introduction of these new imaging techniques in the future.