Moving PARP Inhibition Upfront: Why the Phase 3 TALAPRO-3 Trial Rewrites Treatment for HRR-Deficient mCSPC

From Late to Early Intervention: Advanced Prostate Cancer Treatment.

Study Used Combination of Talazoparib and Enzalutamide to Treat mCSPC: Talazoparib (TALZENNA) is a potent PARP inhibitor used to treat metastatic castration-resistant prostate cancer (mCRPC). In a study presented at the 2026 ASCO Annual Meeting (Abstract #TPS23), the Brief used a combination of TALZENNA and enzalutamide (XTANDI), the standard-of-care androgen receptor pathway inhibitor (ARPI), to treat 599 patients with mCSPC who also had homologous recombination repair (HR) gene-mutations identified by next-generation DNA-sequencing in a clinical setting. The trial, TALAPRO-3, was a global Phase 3 study.

For healthcare providers treating patients with genitourinary cancers, urological cancers and other advanced cancers, the TALAPRO-3 study is practice changing.

Why It Matters

Delaying the Transition to Lethality

Metastatic castration sensitive prostate cancer (mCSPC) is a disease state between the time of localized prostate cancer and its metastatic progression while the patient is on active therapy and still on androgen suppression therapy (or testosterone lowering therapy). This is a critical therapeutic window and the best intervention during this time will delay the inevitable transition to mCRPC (the most symptomatic, aggressive and lethal phase of prostate cancer). HRR gene altered mCSPC patients are particularly in need of novel and effective therapies as their tumors are highly aggressive and associated with poor outcomes with the current frontline treatments and overall survival for all mCRPC patients is relatively poor.

The rationale for the study was based on the concept of synergism between the two classes of drugs. The AR pathway controls the expression of the DNA damage response genes that are involved in the HRR pathway. Inactivation of the AR pathway by an AR inhibitor (enzalutamide) decreases the expression of the HRR genes in prostate cancer cells, inducing a state of BRCAness in the tumors even in the absence of mutations in the HRR genes. In turn, the PARP inhibitor (talazoparib) is a downstream effector of the AR pathway. Therefore, by combining talazoparib with enzalutamide, the two drugs have a dual effect on prostate cancer cells: they inhibit the cells’ ability to repair their DNA and they destroy the cells’ principal survival engine.

The Definitive Data

The results of the TALAPRO-3 study show that in patients with mCSPC and HRR gene alterations early treatment with talazoparib in addition to the standard first-line castration therapy leads to unprecedented cancer control for extended periods of time and preserves the quality of life for years by delaying aggressive disease.

• A 52% Reduction in Progression Risk: At a median follow-up of over 37 months, the combination of talazoparib plus enzalutamide demonstrated a highly statistically significant and clinically meaningful improvement in investigator-assessed radiographic progression-free survival (rPFS), reducing the risk of radiographic progression or death by 52% compared to enzalutamide plus placebo (Hazard Ratio \[HR\], 0.481; 95% Confidence Interval \[CI\], 0.357–0.647; 2-sided P<0.0001).
• Median rPFS Not Reached: While patients on the standard-of-care enzalutamide arm experienced a median rPFS of 45.8 months, the median rPFS for the talazoparib combination arm had not yet been reached at the time of data cutoff.
• Three-Year Survival Landmarks: Landmark analyses estimated that at 3 years, 77% of patients in the talazoparib plus enzalutamide group remained progression-free, compared to just 56% in the control group.
• Secondary Endpoints and OS Trends: The combination also led to a 49% improvement in both time to PSA progression (HR, 0.51; 95% CI, 0.37–0.71; P<0.0001) and time to subsequent anti-cancer therapy (HR, 0.51; 95% CI, 0.38–0.70; P<0.0001). Furthermore, an interim analysis of overall survival (OS)—an alpha-protected key secondary endpoint—already favors the combination group, demonstrating a strong descriptive trend toward prolonged life, though it has not yet reached statistical significance (74 deaths vs. 91 deaths; HR, 0.767; 95% CI, 0.564–1.044).

The use of talazoparib as part of the early treatment of advanced prostate cancer can bring significant and long lasting benefits to patients, changing the expected course of their disease.

2026 ASCO Abstract Overview

Title: TALAPRO-3: Talazoparib (TALA) + Enzalutamide (ENZA) vs Placebo + ENZA in Men with Metastatic Castration-Sensitive Prostate Cancer (mCSPC) and Homologous Recombination Repair (HRR) Gene Alterations: A Randomized Phase 3 Trial

For TALAPRO-3 at 2026 ASCO with a median follow-up of approximately 37.6 months for the 599 patients with mCSPC and HRR gene alterations, the combination of talazoparib plus enzalutamide compared with placebo plus enzalutamide showed a statistically significant improvement in the primary end point of investigator-assessed rPFS. For the TALA + ENZA arm, the median rPFS had not yet been reached. For the placebo + ENZA arm, the median rPFS was 45.8 months. The corresponding hazard ratio for rPFS was 0.481; two-sided P value less than 0.0001.

For patients with BRCA mutations, the hazard ratio for rPFS was 0.368 (P<.0001). For patients with non-BRCA mutations, the hazard ratio for rPFS was 0.567 (P<.0001). Interim results for overall survival were also favorable for TALA+ENZA compared with placebo+ENZA, with a hazard ratio of 0.767 (P = .1104); these results were not statistically significant and should be interpreted with caution.

Regarding several potential issues of talazoparib (TALA) including myelosuppression, the data from TALAPRO-3 supported the use of talazoparib in combination with enzalutamide (ENZA) for patients with metastatic castration-sensitive prostate cancer (mCSPC) with no evidence of new safety signals and with stable incidence of all-grade and high-grade 3 or higher myelosuppression. For TALA + ENZA treated patients, the most common (≥50%) all-grade TEAEs were followed by severity level: Hematologic events: Anemia (71.2%), Decreased neutrophil count (51.7%), Neutropenia (57.8%); and Non-Hematologic events: Fatigue (63.2%), Asthenia (51.7%), and Pyrexia (51.4%). Treatment discontinued due to adverse events (AEs) occurred in 18.7% of patients on TALA + ENZA. The results of TALAPRO-3 provide strong clinical evidence that the addition of talazoparib to first-line androgen receptor inhibitors (ARIs) to treat mCSPC in men with HRR gene alterations should be considered for inclusion in clinical practice guidelines.

Who It Affects

Identifying the HRR-Deficient Cohort

These data will be most relevant to patients with advanced prostate cancer who have a deficiency in Homologous Recombination Repair (HRR) genes. TALAPRO-3 included 599 patients with mCSPC and HRR gene alterations in 12 genes. The 12 genes tested for HRR deficiency in this study were: ATM, ATR, BRCA1, BRCA2, CDK12, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2, and RAD51C. Patients were also stratified by other clinical variables, including whether they were newly diagnosed with mCSPC (de novo) or had relapsed on androgen deprivation therapy prior to enrollment (relapse), by the volume of disease, and by BRCA status.

The 599 patients in the TALAPRO-3 trial were all stratified by 5 factors: 1) de novo mCSPC vs. mCSPC following prior castration-sensitive PC (relapse) 2) high volume of metastases (M1c) vs. low volume of metastases (M1b) 3) BRCA vs. non-BRCA.

Broad Efficacy Across Genomic Subgroups

In addition to patients with BRCA1 and BRCA2 mutations, patients with other HRR-gene alterations also derived benefit from the study’s experimental treatment.

• The BRCA-Mutated Subgroup (n=207): As expected, patients with BRCA alterations experienced the most profound treatment effect, demonstrating a 63% reduction in the risk of radiographic progression or death (HR, 0.368; 95% CI, 0.222–0.609). The 3-year rPFS rate was estimated at 77% for the combination arm versus 49% for the control arm.
• The Non-BRCA HRR Subgroup (n=392): Remarkably, patients harboring alterations in the other 10 non-BRCA HRR genes (such as ATM, CDK12, or CHEK2) also derived a profound, statistically significant benefit. This group achieved a 43% reduction in the risk of progression or death (HR, 0.567; 95% CI, 0.392–0.819), with a 3-year rPFS rate of 76% versus 60% in the control arm.

As the results of the trial show, any patient with a HRR gene alteration in prostate cancer can be treated with the combination of TALA and ENZA. Therefore, PARP inhibitors are no longer to be considered as only “BRCA drugs” for the treatment of prostate cancer.

What Changes

Redefining Practice Protocols

With these new results the treatment of newly diagnosed or relapsing mCSPC patients will have to change.

Mandatory Upfront Genomic Testing

The findings from TALAPRO-3 provide significant clinical utility for practitioners of prostate cancer (PC) early identification of the HRR-deficient CSPC patient and treatment with a regimen of intensified PARP inhibitors in combination with androgen inhibition. Consequently, early, mandatory germline and, where possible, somatic genetic testing for all newly diagnosed patients with mPC, as part of their initial metastatic diagnostic work up, is critical and tissue biopsies and/or circulating tumor DNA (ctDNA) liquid biopsies, where possible, should be integrated into the baseline diagnostic work up for all patients with metastatic prostate cancer.

Tissue as well as liquid biopsies of circulating tumor DNA (ctDNA) must be integrated into the initial diagnostic work-up of patients with metastasized PCa in order to optimize patient outcomes.

Proactive Management of Toxicities

Although it is in the clinic as a single agent, talazoparib in combination with other agents has been shown to be safe and manageable in the outpatient setting monitoring for TEAEs. The safety profile of the combination of talazoparib with enzalutamide did not identify any new or unexpected signals and was consistent with the known individual drug profiles. As with all PARP inhibitors talazoparib can cause hematologic toxicity.

Most Common TEAEs All Grades (talazoparib + enzalutamide vs enzalutamide).

• Anemia: 71.2%
• Fatigue: 28.4%
• Decreased Neutrophil Count: 27.1%
• Neutropenia: 22.1%
• Asthenia: 21.4%
• Decreased White Blood Cell Count: 21.4%

Common adverse effects experienced by patients with cancer can be easily managed. The main hematologic toxicities of PARP inhibitors such as talazoparib include: thrombocytopenia, anemia and leucopenia. In the study TALAPRO-3 the safety profile of talazoparib in combination with enzalutamide was consistent with the known side effect profile of the two individual drugs. No new or unusual side effects were identified. Monitoring of blood counts for patients receiving a PARP inhibitor in combination with an androgen receptor inhibitor will enable the clinician to identify any cytopenias at an early stage, in particular within the first two months of treatment and allow dose modification or treatment interruption as necessary. By doing so, severe complications can be prevented and treatment can be continued with minimal disruption to the patient’s life and treatment schedule.

Hematologic toxicity can also occur with talazoparib, so patients should be monitored on an ongoing basis for signs and symptoms of this side effect. Patients should have a complete blood count (CBC) analyzed on a regular basis; typically, on a weekly basis for the first two months of treatment, then as needed. The dose of talazoparib may need to be decreased in some cases, and should be decreased as indicated and according to local guidelines.

A New Benchmark for First-Line Care

TALAPRO-3, a Phase 3 study for mCSPC, is a pivotal step for precision oncology for genitourinary cancers, as the results define a new benchmark for potentially the most intense first-line regimen for mCSPC in patients with HRR alterations, reducing the risk of PROG or death by 52% when talazoparib is added to enzalutamide.

Results of TALAPRO-3 will likely be used as the basis for potential registration of talazoparib in combination with an androgen receptor inhibitor for treatment of HRR gene altered mCSPC as first-line therapy. Early identification of alterations in HRR genes in newly diagnosed mCSPC patients on androgen deprivation therapy and subsequent treatment with a PARP inhibitor-based regimen is critical for potentially extending the time patients are on therapy for mCSPC prior to clinical onset of resistance.

References

• Agarwal, N., Matsubara, N., Azad, A., Saad, F., Mateo, J., Jiang, S., Ye, D., Voog, E., Shore, N. D., Cil, T., Vulsteke, C., Chung, H.-J., Zschaebitz, S., Laird, D., Zhang, X., Nandoskar, P., Fenech Chetcuti, S., Wang, F., & Fizazi, K. O. (2026). TALAPRO-3: Talazoparib (TALA) + enzalutamide (ENZA) compared with placebo (PBO) + ENZA for the treatment of patients (pts) with metastatic castration-sensitive prostate cancer (mCSPC) harboring homologous recombination repair (HRR) gene alterations. Journal of Clinical Oncology, 44(17\_suppl), LBA5007.
• Pfizer Inc. (2026, May 26). TALZENNA® plus XTANDI® improves radiographic progression-free survival by more than 50% in metastatic prostate cancer. Pfizer Press Release. https://www.pfizer.com/news/press-release/press-release-detail/talzenna-plus-xtandi-improves-radiographic-progression-free