Low-Dose Pembrolizumab Matches Standard-of-Care Efficacy in Neoadjuvant TNBC: Insights from the PLANeT Trial

Introduction: The Challenge of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) remains one of the most clinically challenging subtypes of breast cancer due to its aggressive nature, high risk of early recurrence, and lack of targeted hormone receptors or HER2 expression. For years, the backbone of treatment was restricted to cytotoxic chemotherapy. However, the advent of immune checkpoint inhibitors (ICIs) has revolutionized the management of localized TNBC. The landmark KEYNOTE-522 trial established the combination of pembrolizumab and neoadjuvant chemotherapy (NACT) as the global standard of care, significantly improving pathological complete response (pCR) and event-free survival.

Despite these advances, the high cost of standard-dose pembrolizumab (200 mg every three weeks) remains a prohibitive barrier for many patients, particularly in low- and middle-income countries (LMICs). The PLANeT trial (Pembrolizumab Low Dose in Addition to NACT in TNBC) was designed to address this disparity by evaluating whether a significantly lower dose of pembrolizumab could yield comparable clinical benefits.

The Scientific Rationale for Dose Reduction

The rationale for using low-dose pembrolizumab is rooted in pharmacodynamic studies of programmed death-1 (PD-1) receptor occupancy. Research has shown that PD-1 receptors on T-cells are nearly saturated at doses as low as 0.3 mg/kg. While the standard 200 mg fixed dose ensures maximum occupancy across a wide range of body weights and metabolic profiles, it likely represents an over-treatment for many patients. If lower doses can achieve similar pCR rates, the financial toxicity of cancer care could be drastically reduced without compromising patient outcomes.

Study Design and Methodology

The PLANeT trial was a Phase II, randomized, open-label study conducted at a premier tertiary cancer center in New Delhi, India. The study enrolled 157 treatment-naive patients with Stage II-III TNBC. These patients were randomized in a 1:1 ratio into two distinct arms:

Control Arm: Dose-Dense NACT

Patients received a standard dose-dense chemotherapy regimen consisting of four cycles of doxorubicin and cyclophosphamide followed by four cycles of paclitaxel.

Experimental Arm: Low-Dose Pembrolizumab + NACT

In addition to the dose-dense NACT regimen, patients received a low dose of pembrolizumab—specifically 50 mg administered every six weeks for a total of three cycles during the neoadjuvant phase.

The primary endpoint was the rate of pathological complete response (pCR), defined as the absence of residual invasive cancer in the breast and axillary lymph nodes (ypT0/is ypN0) at the time of surgery.

Key Findings: Efficacy and Statistical Significance

The results of the PLANeT trial, presented for the period between February 2024 and February 2025, indicate a clear therapeutic advantage for the addition of low-dose pembrolizumab. In the intention-to-treat (ITT) population, the pCR rate was 53.8% in the pembrolizumab arm compared to 40.5% in the chemotherapy-only arm. This represents an absolute improvement of 13.3% (90% CI 0.3-26.3; one-sided p = 0.047).

When analyzing only the patients who successfully completed surgery (n=152), the disparity was even more pronounced. The pCR rates were 56.7% for the low-dose pembrolizumab group versus 41.0% for the control group, yielding an absolute difference of 15.7% (one-sided p = 0.031). These figures are numerically comparable to the 13.6% improvement reported in the KEYNOTE-522 trial, which utilized the much higher standard dose of pembrolizumab.

Safety and Tolerability Profile

Safety is a paramount concern when adding ICIs to intensive chemotherapy. Interestingly, the PLANeT trial reported that Grade 3 or higher toxicities occurred in 50% of the low-dose pembrolizumab arm, which was actually lower than the 59.5% observed in the control arm. This suggests that the low-dose immunotherapy did not exacerbate the toxicity profile of the underlying chemotherapy backbone. Common adverse events were consistent with those expected from dose-dense AC-T regimens, and the lower frequency of pembrolizumab administration (every six weeks) likely contributed to the manageable safety profile.

Expert Commentary: Bridging the Global Oncology Divide

The implications of the PLANeT trial are profound for global health policy. By using 50 mg every six weeks—roughly one-eighth of the total pembrolizumab dose used in the standard neoadjuvant protocol—clinicians may be able to achieve similar pathological outcomes. For healthcare systems in LMICs, where immunotherapy is often an out-of-pocket expense or not covered by public insurance, this low-dose strategy could expand access to life-saving treatment to thousands of patients who would otherwise be excluded.

However, some experts caution that while pCR is a strong surrogate marker for long-term survival in TNBC, longer follow-up is required to confirm that low-dose pembrolizumab provides the same event-free survival (EFS) and overall survival (OS) benefits seen with standard dosing. Furthermore, the open-label nature of the study and its Phase II design suggest that while these results are practice-informing, larger Phase III confirmatory trials could further solidify this approach.

Conclusion: A New Horizon for TNBC Treatment

The PLANeT trial successfully demonstrates that the addition of low-dose pembrolizumab to neoadjuvant chemotherapy significantly enhances pCR rates in patients with localized TNBC. The observed benefit is statistically significant and numerically aligns with results from standard-dose trials. As oncology moves toward personalized and precision medicine, the PLANeT study serves as a critical reminder that ‘more’ is not always ‘better,’ and that dose optimization is a vital frontier in making cancer care both effective and equitable.

References

1. Arora A, et al. A Phase II, Randomized, open-label study to evaluate low-dose Pembrolizumab plus Chemotherapy vs Chemotherapy as Neoadjuvant Therapy for localized Triple Negative Breast Cancer (TNBC). [PLANeT Trial]. Ann Oncol. 2025. 2. Schmid P, et al. Pembrolizumab for Early Triple-Negative Breast Cancer. N Engl J Med. 2020;382(9):810-821. doi:10.1056/NEJMoa1910549. 3. Low-dose pembrolizumab pharmacokinetics and receptor occupancy: Summary of early phase clinical data in solid tumors.