Transportability of overall survival estimates from the USA to France in patients with metastatic triple-negative breast cancer

<h3>Background</h3> Metastatic triple-negative breast cancer (mTNBC) represents an aggressive breast cancer subtype with limited treatment options. The external validity of transportability for real-world evidence (RWE) applied across health care systems remains uncertain. This study evaluated the transportability of overall survival (OS) from the USA to France. <h3>Patients and methods</h3> We conducted a retrospective cohort study using de-identified or aggregated electronic health record-derived data from the USA and France. The study included adult female patients diagnosed with mTNBC initiating first-line systemic therapy before the widespread use of poly(ADP-ribose) polymerase (PARP) and programmed cell death (ligand) protein 1 (PD-(L)1) checkpoint inhibitors. To account for population differences, we reweighted the United States cohort to match the French population characteristics. OS was assessed from the start of first-line therapy using Kaplan–Meier curves. <h3>Results</h3> After adjustment, the United States (<i>n</i> = 812) and French (<i>n</i> = 2797) cohorts were well-balanced (standardized mean differences close to 0). Median OS was comparable between the unadjusted United States cohort [13.8 months, 95% confidence interval (CI) 12.6-15.0 months], the adjusted United States cohort (13.2 months, 95% CI 11.6-14.6 months) and the French cohort (13.8 months, 95% CI 13.2-14.6 months). OS rates at 1, 2, and 3 years in the French cohort were 55%, 30%, and 18%, respectively, and did not differ by >3% from those in the USA, regardless of adjustment. <h3>Conclusions</h3> This study shows evidence that OS estimates for mTNBC derived from United States RWE are transportable to the French setting. Findings support the use of international RWE in health technology assessment contexts, potentially facilitating more efficient and evidence-based decision making for novel therapies.