Palbociclib, Trastuzumab, and Endocrine Therapy Significantly Extend Progression-Free Survival in PAM50 Luminal HER2-Positive Advanced Breast Cancer: Results from the SOLTI-1303 PATRICIA Trial

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Superior Progression-Free Survival

The triplet combination of palbociclib, trastuzumab, and endocrine therapy (ET) achieved a significantly better progression-free survival (PFS) compared to the treatment of physician’s choice (TPC), with a stratified hazard ratio of 0.52 (95% CI 0.29-0.95; p=0.03).

Long-term Disease Control

Survival data revealed a striking difference in long-term outcomes, with 24.0% of patients in the triplet arm remaining progression-free at 24 months, compared to only 4.3% in the TPC arm.

Safety and Tolerability

While Grade 3 or higher adverse events occurred in 61.5% of the triplet group (primarily neutropenia at 53.9%), the regimen was well-tolerated with no permanent discontinuations due to toxicity observed.

Background: The Challenge of HER2+ Luminal Breast Cancer

HER2-positive (HER2+) advanced breast cancer (ABC) is a highly heterogeneous disease. While HER2-targeted therapies have revolutionized the treatment landscape, a significant subset of these patients also expresses hormone receptors (HR+), often referred to as ‘triple-positive’ breast cancer. Within this group, intrinsic molecular subtyping via PAM50 has identified that many tumors exhibit a ‘Luminal’ profile (Luminal A or B), which suggests a distinct biological reliance on both the HER2 and the estrogen receptor (ER) signaling pathways.

Clinical evidence has suggested that cross-talk between the ER and HER2 pathways may contribute to treatment resistance. Specifically, the cyclin D1-CDK4/6-RB axis is a critical downstream mediator of both pathways. Previous cohorts (A and B) of the SOLTI-1303 PATRICIA trial suggested that palbociclib, a CDK4/6 inhibitor, combined with trastuzumab could be effective. However, Cohort C was designed to provide a direct comparison against standard-of-care treatments in a strictly defined population of pretreated patients with HER2+, HR+, and PAM50-Luminal A/B advanced disease.

Study Design: A Targeted Phase II Approach

SOLTI-PATRICIA Cohort C was a randomized, multicenter, prospective, open-label, Phase II study. The trial specifically recruited patients with HER2-positive ABC who were also HR-positive and confirmed to have PAM50-Luminal A/B subtypes via central genomic testing.

Participants were randomized 1:1 into two arms:
1. Triplet Arm: Palbociclib (125 mg/day, 3 weeks on/1 week off), trastuzumab (standard dosing), and endocrine therapy (letrozole, fulvestrant, or other per investigator choice).
2. TPC Arm: Trastuzumab-based treatment of physician’s choice, which could include trastuzumab plus chemotherapy, T-DM1 (trastuzumab emtansine), or trastuzumab plus ET.

Notably, the study allowed for re-randomization. Patients in the TPC arm who experienced disease progression could be re-randomized into the study if they still met the inclusion criteria. The primary endpoint was investigator-assessed PFS according to RECIST 1.1 criteria.

Key Findings: Efficacy and Response Rates

Between August 2019 and August 2023, 264 patients were pre-screened, resulting in 73 randomized participants. The baseline characteristics reflected a heavily pretreated population. In the TPC arm, the treatment choices were diverse: 48.5% received trastuzumab plus chemotherapy, 39.4% received T-DM1, and 12.1% received trastuzumab plus ET.

Progression-Free Survival

The study met its primary endpoint. The triplet regimen demonstrated a statistically significant and clinically meaningful improvement in PFS. The stratified hazard ratio (HR) was 0.52, indicating a 48% reduction in the risk of progression or death compared to standard-of-care options. The most notable finding was the tail of the curve; the 24-month PFS rate was 24.0% for the triplet arm versus a meager 4.3% for the TPC arm.

Response and Clinical Benefit

The Overall Response Rate (ORR) was also higher in the triplet group at 18.9% (95% CI 8.6-35.7), compared to 7.1% (95% CI 1.2-25.0) in the TPC group. These data suggest that the biological synergy of targeting the CDK4/6, HER2, and ER pathways simultaneously can overcome resistance mechanisms that render traditional chemotherapy or single-pathway targeted agents less effective in the luminal-intrinsic subtype.

Safety Profile: Predictable and Manageable Toxicity

The safety analysis of the triplet combination was consistent with the known profiles of the individual agents. Grade 3 or higher adverse events (AEs) were reported in 61.5% of patients in the triplet arm. As expected with CDK4/6 inhibitors, neutropenia was the most common high-grade AE, occurring in 53.9% of patients.

Crucially, despite the high incidence of laboratory-defined neutropenia, the clinical impact was manageable. There were no reported cases of permanent treatment discontinuation due to toxicity in the triplet arm, suggesting that dose interruptions or reductions were sufficient to manage side effects without compromising the integrity of the treatment course.

Expert Commentary: Towards Biomarker-Driven De-escalation

The results of the PATRICIA trial Cohort C add to a growing body of evidence supporting the use of CDK4/6 inhibitors in HER2-positive, HR-positive advanced breast cancer. The use of PAM50 intrinsic subtyping is a critical element of this study. By identifying the ‘Luminal’ subtype, clinicians can better select patients who are likely to benefit from a chemotherapy-free, targeted triplet regimen.

In the context of the current treatment landscape, where antibody-drug conjugates (ADCs) like T-DXd (trastuzumab deruxtecan) have become the standard for second-line therapy, the PATRICIA regimen offers a potent alternative for patients who may have progressed on ADCs or for whom chemotherapy-sparing approaches are prioritized. The ability of the triplet to maintain disease control for over two years in nearly a quarter of pretreated patients is a compelling argument for its clinical utility.

However, limitations include the relatively small sample size inherent in Phase II trials and the evolving nature of the ‘Treatment of Physician’s Choice’ arm in an era of rapidly emerging new agents. Future research should focus on the sequencing of these targeted triplets relative to novel ADCs and the potential for even earlier intervention in the metastatic setting.

Conclusion

The SOLTI-1303 PATRICIA trial demonstrates that the combination of palbociclib, trastuzumab, and endocrine therapy is a safe and effective treatment strategy for pretreated patients with HER2-positive, PAM50 luminal advanced breast cancer. By significantly improving PFS and offering a durable response in a subset of patients, this triplet regimen validates the importance of molecular subtyping and dual-pathway inhibition in personalizing breast cancer care.

Funding and Trial Information

The SOLTI-1303 PATRICIA trial was sponsored by the SOLTI Innovative Cancer Research group. ClinicalTrials.gov Identifier: NCT02448420. The study was supported by funding from Pfizer and other institutional grants.

References

Ciruelos E, Pascual T, Villacampa G, et al. Palbociclib, trastuzumab and endocrine therapy in pretreated HER2-positive and PAM50 luminal advanced breast cancer: randomised phase II, SOLTI-1303 PATRICIA trial. Clin Cancer Res. 2025 Dec 3. doi: 10.1158/1078-0432.CCR-25-2882. PMID: 41335375.