ICI Safety in Early Breast Cancer: Meta-Analysis of 3977 PatientsImmune checkpoint inhibitors show specific side effects in early breast cancer

This meta-analysis evaluates the safety and tolerability of immune checkpoint inhibitors (ICIs) targeting the PD-1/PD-L1 axis in patients with early-stage breast cancer (eBC). Pooling data from 3977 patients (3138 receiving ICI-containing regimens and 839 controls), the analysis provides a comprehensive profile of adverse events (AEs) and immune-related adverse events (irAEs) in this curative-intent setting.

The most frequent all-grade AEs were alopecia (86.49%, 95% CI: 59.25-96.57) and leukopenia (52.09%, 95% CI: 15.14-86.89). These rates are notably high, likely reflecting the combination of ICIs with chemotherapy, as alopecia and leukopenia are typical chemotherapy toxicities. The wide confidence intervals suggest considerable heterogeneity across trials, possibly due to differences in chemotherapy backbones and patient characteristics.

Grade ≥3 AEs were less common but clinically significant. Decreased neutrophil count occurred in 17.88% (95% CI: 10.65-28.44) and neutropenia in 18.19% (95% CI: 10.81-28.96). These severe hematologic toxicities underscore the need for vigilant blood count monitoring, especially during the first cycles of treatment. The risk of febrile neutropenia and subsequent infections should be managed proactively with growth factors and antibiotics as per guidelines.

Immune-related AEs (irAEs) were a key focus. The most common irAE was hypothyroidism (12.70%, 95% CI: 9.27-17.16), which is consistent with the known endocrine toxicity of ICIs. The most frequent severe (grade ≥3) irAE was adrenal insufficiency (1.47%, 95% CI: 0.68-3.12). While these rates are relatively low, they are clinically important because endocrine irAEs can be life-threatening if unrecognized. Patients should be educated about symptoms such as fatigue, weight changes, and hypotension, and routine thyroid and adrenal function tests are recommended.

Overall, the tolerability of ICIs in eBC appears manageable, with most AEs being low-grade and reversible. However, the significant risk of endocrine irAEs highlights the need for proactive monitoring and early intervention. Clinicians should have a low threshold for evaluating endocrine symptoms and consider referral to endocrinologists when abnormalities are detected.

This meta-analysis has limitations, including potential heterogeneity among included trials and lack of long-term safety data. The analysis does not provide a mechanism for the observed toxicities, but the association between ICI use and specific AEs is clear. Future research should focus on identifying biomarkers to predict which patients are at highest risk for severe irAEs, and on developing optimal management strategies.

In practice, these findings reinforce the importance of a multidisciplinary approach when using ICIs in early breast cancer. Oncology teams should integrate baseline endocrine assessments, regular monitoring during treatment, and patient education to mitigate the impact of irAEs. With appropriate vigilance, the benefits of ICIs in improving outcomes can be achieved while maintaining acceptable safety.