Tucatinib (TUC) combined with trastuzumab and pertuzumab (HP) as first-line (1L) maintenance therapy for HER2+ metastatic breast cancer (MBC): An in-depth safety analysis of HER2CLIMB-05.

1042 Background: The primary analysis of the phase 3 HER2CLIMB-05 study (NCT05132582) showed the addition of TUC to 1L maintenance HP yielded a statistically significant improvement in progression-free survival (per investigator) versus control treatment (hazard ratio: 0.641, P < 0.0001) in patients with HER2+ MBC. Here we report our findings from an in-depth safety analysis of HER2CLIMB-05. Methods: Patients with centrally confirmed HER2+ MBC without evidence of progression following chemotherapy-based induction treatment (taxane + HP) were randomly assigned 1:1 to TUC (300 mg) or placebo (PBO) BID, both in combination with HP. The safety analysis set included all randomly assigned patients who received ≥1 dose of any study treatment. Treatment-emergent adverse events (TEAEs), laboratory values, and TUC/PBO dose modifications and discontinuations were examined. Results: Among patients in the safety analysis set (TUC arm = 326; PBO arm = 324), median treatment durations of TUC and PBO were 17.1 months (range: 0.4–36.5) and 15.5 months (range: 0.5–41.3), respectively. Grade (G) ≥3 TEAEs occurred in 42.3% of patients in the TUC arm and 24.4% in the PBO arm; the most frequent G ≥3 TEAEs in the TUC arm were elevated alanine aminotransferase (ALT; 13.5%) and aspartate aminotransferase (AST; 7.1%). TUC was discontinued in 13.5% of patients due to a TEAE. Cardiac TEAEs were similar between TUC and PBO arms (4.3% vs 6.2%). TEAEs of hepatic events (43.6% vs 15.7%) and diarrhea (72.7% vs 51.2%) occurred at higher incidence in the TUC arm versus the PBO arm (Table). In the TUC arm, increased ALT (28.2%) and AST (25.8%) accounted for the majority of hepatic TEAEs. Most hepatic and diarrhea TEAEs were managed with TUC dose modifications and/or discontinuation; 7.7% of patients discontinued TUC due to hepatic TEAEs and 1.5% discontinued due to diarrhea (Table). Among the patients with a TEAE of diarrhea, 57.7% and 29.9%, respectively, in the TUC and PBO arms used antidiarrheals; the most used medication was loperamide. Further details of the in-depth safety analysis will be presented. Conclusions: TUC addition to 1L HP maintenance therapy may be an effective option for HER2+ MBC with a clinically manageable safety profile. Clinical trial information: NCT05132582 . TEAEs* Any Any hepatic Increased ALT/AST Diarrhea Arm TUC PBO TUC PBO TUC PBO TUC PBO Any G, % 99.1 96.6 43.6 15.7 28.2/25.8 7.1/9.0 72.7 51.2 G ≥3 42.3 24.4 18.1 1.2 13.5/7.1 0.6/0.6 6.1 4.0 Any G: Median time to onset (days) 34.5 85.0 11.0 32.0 Median time to resolution (days) 25.0 22.0 2.0 5.0 Any G: TUC/PBO dose hold, % 49.4 25.3 19.3 2.5 12.6/4.0 0.6/0.6 8.6 3.4 TUC/PBO dose reduction, % 29.1 11.1 16.3 0.9 <jats:td colspan="1" row