Assessing The Impact: Capivasertib+Abiraterone For Metastatic Hormone-sensitive Prostate Cancer and PTEN Deficiency
- “CAPItello-281 I believe, will be practice-changing, getting to clinic immediately, because when we now have patients with PTEN loss, and they’re newly diagnosed with metastatic prostate castrate sensitive cancer, we should be able to combine our current standard of care with abiraterone prednisone,” emphasizes Dr. Alexandra Drakaki, an oncologist at UCLA, California.
- In men with PTEN-deficient metastatic hormone-sensitive prostate cancer, adding the AKT inhibitor capivasertib to abiraterone (on an ADT backbone) significantly improved radiographic progression-free survival (rPFS) compared to abiraterone alone.
- Capivasertib led to more frequent on-target toxicities, notably diarrhea (~52% of patients), hyperglycemia (~38%), and rash (~35%), which often manifested early in the treatment course.
- The trial’s biomarker-driven design enrolled only patients with PTEN-loss tumors, defined by IHC as the absence of PTEN in ≥90% of tumor cells. CAPItello-281 demonstrated that these PTEN-deficient patients derive significant benefit from AKT pathway inhibition, validating PTEN loss as a critical predictive biomarker for patient selection and underscoring its prognostic importance in mHSPC.
CAPItello‑281 establishes that biomarker-selected (PTEN-deficient) mHSPC is a distinct, targetable subset where intensifying frontline therapy with an AKT inhibitor provides meaningful rPFS benefit beyond standard abiraterone + ADT. If OS data matures favorably, this could redefine frontline treatment in this genomically high-risk population and make PTEN IHC testing a standard part of the mHSPC workup.
The Study
This is a biomarker‑selected, phase III, double‑blind trial in de novo metastatic hormone‑sensitive prostate cancer (mHSPC) demonstrating that adding the AKT inhibitor capivasertib to abiraterone/prednisone on a backbone of androgen deprivation therapy (ADT) significantly improved investigator‑assessed radiographic progression‑free survival (rPFS) in patients with PTEN‑deficient disease. The median rPFS was 33.2 months with capivasertib versus 25.7 months with placebo (HR 0.81; 95% CI 0.66–0.98; P=0.034), representing a 7.5‑month median rPFS improvement
CAPItello‑281 is anchored in the concept that PTEN deficiency creates a parallel growth and survival driver via PI3K/AKT signaling that is not fully suppressed by androgen receptor pathway inhibition alone. The CAPItello‑281 investigators explicitly frame PTEN deficiency in mHSPC as producing an “independent proliferative drive” and worse outcomes, motivating dual pathway blockade with capivasertib (AKT inhibition) plus abiraterone (androgen synthesis inhibition) on top of ADT.
Mechanistically, reciprocal feedback between androgen receptor signaling and PI3K pathway signaling in PTEN‑deficient prostate cancer has been described preclinically: PTEN deletion is associated with altered AR transcriptional output, and inhibiting PI3K signaling can increase AR signaling by relieving feedback inhibition. This biology supports a combination strategy rather than single‑pathway inhibition in PTEN‑deficient disease.
PTEN Loss
Prognostically, PTEN loss has been associated with poorer outcomes in de novo metastatic castration‑naïve prostate cancer cohorts assessed by PTEN protein expression, supporting the idea that PTEN‑deficient mHSPC is a clinically high‑risk subset.
“PTEN is a tumor suppressor gene. When you lose a tumor suppressor gene, you get activation of other pathways, PI3K being one, and AKT being another one. We have data that you can target AKT with small molecules. There was a completed phase III trial that showed some evidence of benefits, but whether this will be approved by the FDA for use in patients, we’re still waiting to see. It’s at least proof of principle that PTEN loss does identify a pathway that is targetable and also clearly identifies a subset of patients that will do poorly with current standards of treatment,” says Dr. Stephen Freedland, professor of urology at Cedars-Sinai in Los Angeles, California.
“And the more cells that have lost it [PTEN], the worse the prognosis. For example, if a tumor has a hundred percent of PTEN loss, that’s going to be a bad tumor. It clearly creates this unmet need. I really want to do something for these patients. And again, we are developing targeted therapies to the downstream effects of losing PTEN, this AKT overactivation, whether it’s some other therapy that we need to add, but clearly, tumor cells that have lost PTEN are more aggressive, the more loss, the more aggressive the tumor,” Dr. Freedland adds.
Trial Design
CAPItello‑281 (NCT04493853; sponsor study ID D361BC00001) is a phase III, double‑blind, randomized, placebo‑controlled study testing capivasertib plus abiraterone (+ prednisone/prednisolone) plus ADT versus placebo plus abiraterone (+ prednisone/prednisolone) plus ADT in de novo mHSPC characterized by PTEN deficiency.
The trial’s public registry description specifies a clinically “front‑loaded” population: asymptomatic or mildly symptomatic, histologically confirmed de novo hormone‑sensitive prostate adenocarcinoma (without small‑cell tumors) diagnosed within 180 days prior to randomization; ECOG/WHO 0–1; and ongoing ADT initiated from 0 days up to a maximum of 93 days before randomization.
PTEN deficiency was established centrally by immunohistochemistry (IHC). The Annals of Oncology report defines the diagnostic cut‑off as “≥90% viable malignant cells with no specific cytoplasmic PTEN immunohistochemistry staining.”
Efficacy
CAPItello‑281 achieved its primary endpoint. In PTEN‑deficient mHSPC, capivasertib plus abiraterone improved median rPFS to 33.2 months versus 25.7 months with placebo plus abiraterone (HR 0.81; 95% CI 0.66–0.98; P=0.034), a 7.5‑month improvement in median rPFS.
OS is not yet definitive. In the overall population studied, the interim OS hazard ratio was 0.90 (95% CI 0.71–1.15; P=0.401) at 26.4% maturity, indicating that additional follow‑up is needed before survival impact can be determined.
Post hoc exploratory analyses in CAPItello‑281 suggest that the apparent treatment effect increases as the PTEN‑loss threshold becomes more stringent, because rPFS in the capivasertib arm remained comparatively stable while the placebo arm performed progressively worse at higher PTEN‑loss cut‑offs. Conference reporting provides illustrative estimates for these exploratory subgroups (e.g., ≥95%, ≥99%, and 100% PTEN loss) and emphasizes that these are post hoc analyses rather than the primary trial definition
Safety, Tolerability & Patient‑Reported Outcomes
The core safety signal in CAPItello‑281 is the expected tradeoff of improved disease control (rPFS) at the expense of increased toxicity from adding a targeted third agent. In the Annals of Oncology report, the most common adverse events with capivasertib plus abiraterone were diarrhea (51.9% vs 8.0% with placebo), hyperglycemia (38.0% vs 12.9%), and rash (35.4% vs 7.0%), consistent with the AKT‑inhibition class profile.
Higher‑grade toxicity and discontinuation were meaningfully more frequent with capivasertib: grade ≥3 adverse events occurred in 67.0% versus 40.4%, and discontinuations of the study drug occurred in 18.3% versus 4.8%.
Mortality framing must remain disciplined: CAPItello‑281 has not yet demonstrated a statistically significant OS benefit at current maturity, and adverse‑event‑associated deaths were reported in 36 patients (7.2%) in the capivasertib arm versus 26 patients (5.2%) in the placebo arm. These deaths are not synonymous with overall mortality benefit or harm, but they are a key counseling data point when considering any intensification strategy.
CAPItello‑281’s tolerability discussion is clinically intertwined with metabolic risk. The trial excluded patients with clinically significant abnormalities of glucose metabolism, including diabetes mellitus requiring insulin treatment and HbA1c ≥8.0% (AstraZeneca registry). This matters for implementation because real‑world mHSPC populations often include patients with baseline metabolic disease, and prescribing/monitoring pathways would need to reflect that gap between trial eligibility and clinic reality.
Biomarker Testing & Implementation
The biomarker implementation challenge is not hypothetical; it is a central barrier to translation. ESMO conference reporting explicitly highlights uncertainty about the “right cut‑off” when using IHC and notes that prior studies have suggested NGS may be a more accurate technique for assessing PTEN loss—meaning that the field still needs assay harmonization and clarity on what constitutes clinically actionable PTEN deficiency.
“CAPItello used immunohistochemistry protein staining, which we can do, but it’s not routinely done. So now when we order our DNA sequencing our molecular testing for our patients, we’re going to need to also order immunohistochemistry for PTEN and we need a standardized assay with standardized cutoffs,” highlights Dr. Tanya Dorff, a medical oncologist and the Head of the Genitourinary Cancers Program at City of Hope, California.
Current Practice Changes
CAPItello‑281 pushes providers toward more deliberate biomarker‑driven first‑line selection within mHSPC. Even under current standards, multiple intensification options exist (e.g., abiraterone‑based doublets). CAPItello‑281 adds the possibility that, for a definable PTEN‑deficient subgroup, the first‑line regimen could become a targeted triplet—if the totality of evidence and regulatory status support it.
“This study, I believe, will be practice-changing, getting to clinic on Monday, because when we now have patients with PTEN loss, and they’re newly diagnosed with metastatic prostate castrate sensitive cancer, we should be able to combine our current standard of care with abiraterone prednisone,” emphasizes Dr. Alexandra Drakaki, an oncologist at UCLA, California.
