Retrospective analysis of the efficacy and safety of disitamab vedotin (RC48) in Chinese patients with HER2-positive or HER2-low metastatic breast cancer.

e13002 Background: Antibody-drug conjugates (ADCs) have emerged as a novel and effective treatment option for breast cancer patients with varying levels of HER2 expression. Disitamab vedotin (RC48) is a HER2-targeting ADC composed of hertuzumab conjugated to monomethyl auristatin E (MMAE) via a cleavable linker. This study retrospectively evaluates the efficacy and safety of RC48 in Chinese patients with HER2-positive or HER2-low metastatic breast cancer (mBC). Methods: This single-center, observational study was conducted at the Cancer Hospital of Shantou University Medical College, China. Patients with HER2-positive or HER2-low mBC who received RC48 treatment were included. Data on demographics, treatment patterns, clinical outcomes, and adverse events were collected and analyzed. Results: From August 2021 to November 2024, 28 female patients meeting the study criteria were enrolled, with a median age of 56 years (range: 40-73). Of these, 15 patients were HER2-positive, and 13 had HER2-low expression. The median number of prior treatment lines was five (range: 2nd-line to 17th-line). The overall median progression-free survival (PFS) was 7.0 months, with 11 patients (39.3%) achieving partial response (PR) and 15 patients (53.6%) having stable disease (SD). Eleven patients (39.3%) received RC48 in combination with other therapies, resulting in a median PFS of 10.0 months, with 8 patients achieving PR. Seventeen patients (60.7%) were treated with RC48 monotherapy, achieving a median PFS of 6.0 months. The median PFS for HER2-positive and HER2-low patients was 7.0 months each. Eight patients (28.6%) received RC48 as either 2nd or 3rd-line treatment, with a median PFS of 8.5 months. Adverse events occurred in 13 patients (46.4%), primarily including manageable pain (17%), peripheral sensory neuropathy (13%), and fatigue (13%). No grade 3 or higher adverse events or treatment discontinuations due to toxicity were reported. Conclusions: RC48 demonstrates promising antitumor activity and a manageable safety profile in patients with HER2-positive or HER2-low mBC. The combination of RC48 with other therapies, such as endocrine or anti-angiogenic agents, may offer synergistic benefits and warrants further investigation in larger clinical trials. Efficacy summary. Median PFS, mo (95% CI) Overall (n=28) 7.00 (6.00-8.50) HER2-positive (n=15) 7.00 (6.70-10.35) HER2-low (n=13) 7.00 (5.62-10.38) RC48 treatment patternMono (n=17)Com (n=11) 6.00 (4.90-7.80)10.00 (7.86-13.77) RC48 treatment lines ≤ 3 (n=8) ＞3 (n=20) 8.50 (5.37-14.38)6.50 (5.82-8.98) Prior Pyrotinib (n=10) 8.00 (6.18-11.49) Prior CDK4/6 inhibitors (n=4) 14.50 (3.27-21.73)