The dramatic results of the PATINA trial reignited conversation around the crucial role and optimal utilization of endocrine therapy in HER2-positive, ER-positive metastatic breast cancer, a topic previously overshadowed by anti-HER2 treatments.
Experts challenge the default use of induction chemotherapy. For patients with asymptomatic or low-burden disease, moving directly to a more tolerable, non-chemo maintenance regimen is an emerging and valid consideration, especially given the efficacy of modern maintenance options.
DESTINY-Breast09 data shows that complete responses with TDXD+pertuzumab can take 8-10 months to develop. This suggests the regimen is not just a short induction strategy and may offer the greatest benefit for durable remission in lower-burden patients, countering the instinct to de-escalate.
For a patient on TDXD plus pertuzumab who develops severe, treatment-limiting diarrhea, a reasonable strategy is to discontinue pertuzumab and continue with TDXD monotherapy. This preserves the most active agent while mitigating toxicity, as the incremental benefit of pertuzumab is not yet reported.
While the PATINA regimen is effective after THP induction, no data supports switching to this maintenance after first-line TDXD-based therapy. Clinicians making this switch are operating in a "data-free zone" and extrapolating from different trial populations.
Data from the PERTAIN trial's non-induction subgroup shows a significant PFS benefit (26.6 months) with dual HER2 blockade plus endocrine therapy. This supports a chemotherapy-free initial strategy for carefully selected patients with indolent, clinically stable disease.
The degree of hormone receptor positivity can guide maintenance strategy. In patients with strongly ER/PR-positive tumors, an endocrine-based approach like the PATINA regimen may be favored. Conversely, in low ER/PR expressing tumors, a more HER2-focused regimen could be more effective.
The DESTINY-Breast09 trial compared continuous TDXD+pertuzumab against THP, which was largely treated as a short induction followed by maintenance. This design difference—continuous therapy versus induction/maintenance—may contribute to the superior outcomes seen with the TDXD arm, making it a slightly uneven comparison.
Despite the individual efficacy of tucatinib-based and CDK4/6 inhibitor-based maintenance regimens, experts do not recommend combining them. The potential for overlapping toxicities and lack of supporting data mean clinicians should "choose one horse to ride" rather than stacking these strategies.