Introduction: Addressing the Challenge of PTEN-Deficient Prostate Cancer
The landscape of metastatic hormone-sensitive prostate cancer (mHSPC) has undergone a rapid transformation over the last decade. The standard of care has shifted from androgen deprivation therapy (ADT) alone to intensification strategies involving androgen receptor pathway inhibitors (ARPIs), docetaxel, or triplet therapies. However, despite these advances, a significant subset of patients experiences early progression and poor long-term outcomes. One of the most critical drivers of this resistance is the loss of the phosphatase and tensin homolog (PTEN) tumor suppressor.
PTEN deficiency occurs in approximately 25% to 30% of patients with mHSPC. Loss of PTEN leads to the constitutive activation of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway. This pathway provides an independent survival and proliferative signal to cancer cells, effectively bypassing the blockade of the androgen receptor (AR) pathway. Historically, patients with PTEN-deficient mHSPC have had a worse prognosis and a shorter duration of response to conventional ARPIs. The CAPItello-281 study (NCT04493853) was designed to address this unmet need by evaluating whether the addition of capivasertib, a potent and selective pan-AKT inhibitor, to abiraterone could overcome this resistance.
Highlights of the CAPItello-281 Trial
– The addition of capivasertib to abiraterone and ADT resulted in a statistically significant and clinically meaningful 7.5-month improvement in median radiographic progression-free survival (rPFS).
– PTEN deficiency serves as a robust predictive biomarker for response to AKT inhibition in the mHSPC setting.
– The safety profile of the combination therapy was consistent with the known toxicities of individual agents, with metabolic and gastrointestinal adverse events being the most prevalent.
Study Design and Methodology
CAPItello-281 was a randomized, double-blind, placebo-controlled Phase III trial conducted globally. The study specifically targeted patients with histologically or cytologically confirmed mHSPC whose tumors demonstrated PTEN deficiency. PTEN status was determined via a centralized immunohistochemistry (IHC) assay, with the deficiency cut-off defined as 90% or more of viable malignant cells showing no specific cytoplasmic PTEN staining.
Participants were randomized in a 1:1 ratio to receive either:
1. Capivasertib (400 mg twice daily, 4 days on/3 days off) plus abiraterone (1000 mg once daily) and prednisone/prednisolone.
2. Placebo plus abiraterone and prednisone/prednisolone.
All patients continued on a background of ADT. The primary endpoint of the study was investigator-assessed rPFS in the PTEN-deficient population, defined as the time from randomization to radiographic progression (per PCWG3 criteria) or death from any cause. Key secondary endpoints included overall survival (OS), time to start of first cytotoxic chemotherapy, and time to pain progression.
Key Findings and Clinical Efficacy
The study screened 6003 patients, of whom 1519 (25.3%) were identified as having PTEN-deficient tumors. A total of 1012 patients were randomized (507 to the capivasertib arm and 505 to the placebo arm).
Radiographic Progression-Free Survival (rPFS)
At the time of the primary analysis, the capivasertib combination demonstrated a statistically significant improvement in rPFS. The median rPFS was 33.2 months for the capivasertib plus abiraterone group compared to 25.7 months for the placebo plus abiraterone group. This translated to a hazard ratio (HR) of 0.81 (95% CI 0.66-0.98; P = 0.034). This 7.5-month extension in median rPFS represents a substantial clinical benefit in a population traditionally associated with rapid disease progression.
Analysis by PTEN Loss Thresholds
An intriguing post hoc exploratory analysis evaluated the impact of different PTEN loss cut-offs (≥95%, ≥99%, and 100%). The data revealed that while the capivasertib arm maintained consistent efficacy across these thresholds, the placebo arm performed progressively worse as the degree of PTEN loss increased. This suggests that near-complete or complete loss of PTEN identifies a population with particularly aggressive disease that is most reliant on AKT signaling and, consequently, most likely to benefit from capivasertib.
Overall Survival and Secondary Endpoints
Overall survival data were immature at the time of reporting (26.4% maturity). The HR for OS was 0.90 (95% CI 0.71-1.15; P = 0.401). Long-term follow-up will be required to determine if the rPFS benefit translates into a significant survival advantage. Other secondary endpoints, such as time to chemotherapy, generally favored the capivasertib arm, supporting the overall clinical utility of the combination.
Safety and Tolerability Profile
The addition of an AKT inhibitor introduces a specific set of adverse events (AEs) related to the inhibition of insulin signaling and epithelial turnover. The most common AEs reported in the capivasertib arm were:
– Diarrhea: 51.9% (vs. 8.0% in the placebo arm)
– Hyperglycemia: 38.0% (vs. 12.9% in the placebo arm)
– Rash: 35.4% (vs. 7.0% in the placebo arm)
Grade 3 or higher AEs occurred more frequently in the capivasertib group. Management of hyperglycemia and rash is critical when utilizing AKT inhibitors, often requiring proactive monitoring and, in some cases, dose interruptions or reductions. Deaths associated with an AE were reported in 7.2% of the capivasertib group compared to 5.2% in the placebo group, emphasizing the need for careful patient selection and monitoring.
Expert Commentary: Mechanistic Insights and Clinical Implications
The CAPItello-281 trial provides a definitive validation of the PI3K/AKT pathway as a therapeutic target in mHSPC. The results underscore the importance of precision oncology in prostate cancer; rather than a “one size fits all” approach, clinicians can now look toward molecularly defined subsets to optimize treatment.
Mechanistically, the reciprocal feedback between the AR and PI3K/AKT pathways is well-documented. When the AR is inhibited, the AKT pathway is often upregulated as a compensatory survival mechanism. Conversely, inhibiting AKT can increase AR signaling. By blocking both pathways simultaneously—AR with abiraterone and AKT with capivasertib—the trial demonstrated that it is possible to delay the emergence of resistance more effectively than with AR inhibition alone.
One of the primary challenges for the implementation of this regimen will be the requirement for centralized PTEN IHC testing. Clinicians must ensure that high-quality tissue samples are available and that pathology departments are equipped to perform and interpret the PTEN assay accurately. Furthermore, the management of hyperglycemia is a specific concern, particularly in an older population with a higher prevalence of type 2 diabetes. Close collaboration between oncologists and primary care or endocrinology specialists may be necessary.
Conclusion and Future Directions
Capivasertib plus abiraterone represents a significant step forward for the treatment of PTEN-deficient mHSPC. With a 7.5-month improvement in median rPFS, the CAPItello-281 study successfully met its primary endpoint, proving that dual blockade of the AKT and AR pathways is a viable and effective strategy.
While the OS data are still pending, the rPFS benefit is clear. Future research should focus on refining the biomarker threshold and exploring capivasertib in combination with other ARPIs or in different disease states, such as non-metastatic castration-resistant prostate cancer. For now, CAPItello-281 establishes a new benchmark for targeted therapy in hormone-sensitive disease, reinforcing the necessity of biomarker testing at the time of diagnosis.
Funding and Clinical Trial Information
The CAPItello-281 study was funded by AstraZeneca. The trial is registered at ClinicalTrials.gov under the identifier NCT04493853.
References
1. Fizazi K, Clarke NW, De Santis M, et al. Capivasertib plus abiraterone in PTEN-deficient metastatic hormone-sensitive prostate cancer: CAPItello-281 phase III study. Ann Oncol. 2026;37(1):53-68. doi:10.1016/j.annonc.2025.10.004.
2. de Bono JS, De Giorgi U, Rodrigues DN, et al. Randomized Phase II Study of Ipatasertib in Combination with Abiraterone in Castration-resistant Prostate Cancer. Clin Cancer Res. 2019;25(3):928-937.
3. Sweeney CJ, Chen YH, Carducci M, et al. Chemohormonal Therapy in Metastatic Hormone-Sensitive Prostate Cancer. N Engl J Med. 2015;373(8):737-746.
