Combination Therapy, Drug Interactions, and Survival Outcomes in Metastatic Breast Cancer: A Pooled Analysis of Phase...

The relationship between regimen complexity, treatment sequencing, and survival outcomes in metastatic breast cancer (MBC) remains poorly characterized.

To quantify how treatment complexity, line of therapy, and baseline performance status (PS) differentially affect progression-free survival (PFS) and overall survival (OS), and to identify synergistic and antagonistic drug combinations across MBC phase II trials.

A systematic PubMed search (1992-2022) identified 892 eligible studies comprising 1,088 treatment arms and 53,672 patients. Objective response rate (ORR), median PFS, and median OS were extracted per arm along with molecular subtype, treatment size, therapy type, line of therapy, and weighted ECOG PS. Drug-drug interactions were inferred using a null model of non-interacting agents.

Adding agents improved both PFS and OS, but the PFS benefit was approximately twice the OS benefit (effect size, ε² = 0.24 vs. 0.11; both p < 0.0001). Combination therapy progressively reduced the survival disadvantage associated with prior treatment, with PFS effect sizes declining from r_rb = 0.55 (95% confidence interval [CI], 0.44-0.64) for single agents to 0.27 (0.09-0.44) for triplets. OS followed a similar pattern, decreasing from r_rb = 0.58 (0.48-0.66) to 0.32 (0.14-0.48). ECOG PS showed an escalating correlation, from negligible for ORR (r_s = -0.09, 95% CI -0.15 to -0.03) to medium for PFS (-0.23, -0.29 to -0.16) and very large for OS (-0.41, -0.48 to -0.35). Nine combinations showed response-based synergy, eight of which were cytotoxic doublets involving anthracycline-taxane or alkylator-anthracycline pairings; five pairs showed response-based antagonism with no consistent drug-class pattern.

Combination therapy confers a substantially larger benefit for PFS than OS and partially compensates for the treatment-line penalty in patients with pretreated MBC. Patient fitness appears to become increasingly critical for long-term survival and may help guide regimen planning.