Optimizing Immunotherapy in Triple-Negative Breast Cancer: Reduced-Dose Folate Receptor Alpha Vaccine Shows Robust Efficacy Without Cyclophosphamide Pretreatment

Highlights

The Phase II clinical trial evaluating the Folate Receptor Alpha (FRα) vaccine in patients with triple-negative breast cancer (TNBC) yielded several critical insights for the future of cancer immunotherapy:

• High Immunogenicity: Vaccination elicited a robust immune response in 83% of evaluable patients, with T-cell levels comparable to those seen in response to the tetanus toxoid vaccine.

• Dose Optimization: A lower dose of the vaccine (825 μg) was found to be just as effective as the higher dose (2.5 mg) in generating persistent immunity.

• Simplified Regimen: Pretreatment with cyclophosphamide (CP), traditionally used to deplete regulatory T cells, did not enhance the immune response, suggesting it can be omitted to simplify clinical administration.

• Excellent Safety Profile: The vaccine was well-tolerated across all cohorts, with no treatment-related deaths and a manageable safety profile consistent with prior peptide vaccine studies.

The Clinical Challenge of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) remains one of the most challenging subtypes of breast cancer to treat due to its lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression. This lack of targets renders traditional hormonal therapies and HER2-targeted agents ineffective, leaving systemic chemotherapy as the primary treatment modality. Despite initial sensitivity to chemotherapy, TNBC is characterized by a high risk of early recurrence and a more aggressive clinical course compared to other subtypes.

Recent advances in immuno-oncology have highlighted the importance of the tumor microenvironment and the role of endogenous immunity in controlling disease progression. Specifically, the presence of CD4+ T-cell immunity has been strongly associated with improved survival outcomes in breast cancer patients. Folate receptor alpha (FRα) is a glycosylphosphatidylinositol-anchored glycoprotein that is overexpressed in approximately 70% to 80% of TNBC cases but has limited expression in normal tissues, making it an ideal candidate for targeted immunotherapy. Building on promising Phase I data, this Phase II trial sought to refine the vaccine strategy for broader clinical application.

Study Design and Methodology

The primary objective of this randomized Phase II trial was to evaluate the safety and immunogenicity of two different doses of a multi-epitope FRα peptide vaccine and to determine the necessity of cyclophosphamide (CP) pretreatment. The study enrolled patients with TNBC who had successfully completed all standard-of-care systemic and local therapies (including surgery, chemotherapy, and radiation) and were in clinical remission.

Patients were randomized into four treatment arms to examine the interplay between peptide dosage and the use of CP:

• Arm 1: Low-dose vaccine (825 μg) with CP pretreatment.

• Arm 2: Low-dose vaccine (825 μg) without CP pretreatment.

• Arm 3: High-dose vaccine (2.5 mg) with CP pretreatment.

• Arm 4: High-dose vaccine (2.5 mg) without CP pretreatment.

The vaccine consisted of a pool of five FRα-derived MHC class II-restricted peptides, administered alongside Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) as an adjuvant. The treatment schedule involved six 4-week cycles. For those in the CP arms, a low dose of cyclophosphamide was administered orally prior to vaccination to potentially mitigate the suppressive effects of regulatory T cells (Tregs). The primary endpoints focused on safety and the magnitude of FRα-specific T-cell responses measured via ELISpot assays at baseline and following the completion of the vaccine series.

Key Findings: Safety and Immunological Potency

A total of 80 patients received the vaccine, with 58 patients completing the full protocol and remaining evaluable for immunogenicity. The results provided clear evidence for the clinical viability of the FRα vaccine.

Safety and Tolerability

The vaccine was exceptionally well-tolerated. The most common adverse events were Grade 1 and 2 local injection site reactions (e.g., erythema, induration), which are typical for vaccines containing GM-CSF. No Grade 4 or 5 toxicities related to the vaccine were reported. This safety profile is particularly encouraging for an adjuvant therapy intended for patients who have already undergone intensive conventional treatments.

Immunogenicity and Dose Response

The vaccine successfully induced immunity in 83% of the evaluable population. One of the most significant findings was that the magnitude of the FRα-specific T-cell response did not differ significantly between the 825 μg and 2.5 mg dose groups. Both doses elicited high frequencies of interferon-gamma (IFN-γ) producing T cells. Furthermore, these responses were found to be persistent, maintaining high levels throughout the study period. When compared to the immune response against tetanus toxoid—a gold standard for vaccine-induced memory—the FRα-specific responses were of comparable magnitude, indicating a potent activation of the cellular immune system.

The Role of Cyclophosphamide

Historically, low-dose cyclophosphamide has been used in cancer vaccine trials under the hypothesis that it can selectively deplete Tregs, thereby “releasing the brakes” on the vaccine-induced immune response. However, in this trial, the addition of CP did not result in a statistically significant increase in the frequency or potency of FRα-specific T cells. This suggests that for this specific multi-epitope vaccine, the inherent immunogenicity is sufficient to overcome baseline immunosuppression in the adjuvant setting.

Clinical Outcomes

While the study was primarily powered for immunogenicity and safety, the researchers monitored for disease recurrence. Recurrences occurred in eight evaluable patients. Critically, these events were distributed across the treatment arms, suggesting that neither the dose nor the use of CP was a determining factor for early relapse in this cohort. Remarkably, no patients died due to their disease during the study period, which, while requiring longer follow-up for definitive survival analysis, is a positive signal in a high-risk TNBC population.

Expert Commentary: Implications for Future Trials

The findings from this Phase II trial represent a significant step toward the standardization of cancer vaccines. By demonstrating that a lower dose is equally effective, the researchers have identified a way to reduce manufacturing costs and potential side effects without sacrificing potency. More importantly, the finding that cyclophosphamide is unnecessary simplifies the treatment protocol significantly. For clinicians and patients, a simpler regimen means fewer clinic visits, fewer medications, and a lower burden of treatment.

From a mechanistic standpoint, the high rate of immunogenicity (83%) suggests that FRα is a highly relevant target in TNBC. The use of MHC class II-restricted peptides specifically targets CD4+ helper T cells, which are essential for orchestrating a durable and effective anti-tumor immune response, including the recruitment and activation of CD8+ cytotoxic T cells. The lack of benefit from CP might be explained by the relatively healthy immune status of patients in the adjuvant setting compared to those with advanced, metastatic disease where Treg infiltration is more pronounced.

However, limitations must be acknowledged. As a Phase II trial, the sample size is relatively small, and the study was not designed to provide a definitive assessment of recurrence-free survival or overall survival. The “evaluable” population (58 out of 80) also highlights the challenges of maintaining patients through a multi-cycle vaccine protocol in a real-world clinical setting. Future Phase III trials will be necessary to confirm if this robust immune response translates into a statistically significant reduction in recurrence rates for TNBC patients.

Conclusion

The Phase II trial of the FRα vaccine in TNBC confirms that a reduced-dose strategy (825 μg) is highly immunogenic and safe, and that cyclophosphamide pretreatment is not required to achieve maximal T-cell activation. These results provide a clear roadmap for the design of future efficacy trials. By simplifying the vaccine regimen, this research brings us closer to a viable, target-specific immunotherapy that could potentially transform the adjuvant treatment landscape for triple-negative breast cancer and other FRα-expressing malignancies.

Funding and ClinicalTrials.gov

This study was supported by grants from the National Cancer Institute (NCI) and various philanthropic organizations. Clinical trial registration can be found at ClinicalTrials.gov (NCT02593227 – inferred based on study context).

References

Knutson KL, Abu-Fares H, Keating P, Block MS, Norton N, Erskine CL, Cogen D, Assad H, Graff SL, Hamilton EP, Han HS, McIntyre KJ, Nassar A, Sparano JA, Tiersten A, Tkaczuk KH, Ward PJ, Wilson G, Garrett G, Kenney RT, Sharma P. A Phase II Trial to Evaluate the Safety and Immunogenicity of Two Doses of a Folate Receptor Alpha Vaccine in Patients with Triple-Negative Breast Cancer. Clin Cancer Res. 2026 Jan 6;32(1):106-117. doi: 10.1158/1078-0432.CCR-25-2763.