Pembrolizumab and Paclitaxel in Patients with HR+/HER2- Breast Cancer with HER2-Enriched or Basal-like Subtypes

Hormone receptor-positive (HR+), HER2-negative (HER2-) metastatic breast cancer (mBC) is biologically distinct from early-stage disease, with a higher prevalence of genomically defined nonluminal subtypes, particularly the HER2-enriched (HER2-E) and basal-like subtypes. These tumors are highly proliferative, less dependent on hormone signaling, and associated with poor outcomes and early resistance to endocrine therapy and CDK4/6 inhibition (CDK4/6i). This biological shift highlights the need for biomarker-driven strategies in the CDK4/6i-resistant setting.

The SOLTI-1716 TATEN trial (NCT04251169) is a phase II, single-arm study evaluating the combination of pembrolizumab and paclitaxel in patients with HR+/HER2- mBC classified as HER2-E or basal-like by PAM50 following progression on CDK4/6i. A total of 126 patients were screened using the PAM50 genomic assay; 20 with HER2-E or basal-like subtypes were enrolled and received pembrolizumab (200 mg every 3 weeks) and paclitaxel (80 mg/m2 weekly).

The primary endpoint, overall response rate (ORR), was 61.1% [95% confidence interval (CI), 35.7-82.7], and the clinical benefit rate was 94.4% (95% CI, 72.7-99.9). The median progression-free survival was 8.1 months (95% CI, 5.9-10.4), and the median overall survival was 26 months [95% CI, 18-not reached (NR)]. Three patients achieved durable responses lasting ≥24 months. Gene expression analyses revealed that high expression of proliferation- and immune-related genes predicted improved ORR and survival, whereas luminal-related gene expression was associated with lower clinical benefit.

These results suggest that chemo-immunotherapy may be an effective strategy in genomically defined nonluminal subtypes of HR+/HER2- mBC following CDK4/6i resistance and highlight the value of molecular subtyping to guide post-endocrine treatment decisions.