Aspirin enhances the anti-tumour activity of lapatinib in HER2-positive breast cancer.

Resistance to human epidermal growth factor receptor 2 (HER2)-directed therapies remains a challenge in HER2-positive breast cancer. We evaluated whether aspirin could enhance the activity of lapatinib, a HER2/EGFR tyrosine kinase inhibitor, in HER2-positive breast cancer models. In BT-474 and SK-BR-3 cells, aspirin plus lapatinib inhibited cell viability, clonogenic growth and DNA synthesis more effectively than either agent alone, with combination index values supporting a synergistic interaction under the tested fixed-aspirin combination conditions. Cyclooxygenase-2 inhibition or PTGS2 knockdown did not fully recapitulate this potentiation, suggesting that cyclooxygenase-2 suppression alone is insufficient to explain the interaction. The combination was associated with increased AMP-activated protein kinase and acetyl-CoA carboxylase phosphorylation and reduced HER2 expression and phosphorylation of phosphoinositide 3-kinase/protein kinase B/mechanistic target of rapamycin pathway components. Compound C partially attenuated growth inhibition and pathway suppression, consistent with possible, non-exclusive involvement of AMPK, whereas rapamycin partially reproduced the growth-inhibitory pattern, suggesting a possible contribution of mTOR pathway attenuation without establishing it as the primary mediator. In BT-474 xenografts, co-administration significantly suppressed tumour growth and tumour weight, reduced Ki-67 positivity and increased cleaved caspase-3 expression. No marked systemic toxicity signals were detected based on body weight, major-organ histology and serum liver and renal function markers, although mild gastrointestinal histological changes occurred in aspirin-treated mice. Because the principal in vitro assays used millimolar aspirin concentrations and the in vivo dose exceeded conventional low-dose antiplatelet exposure, these findings should be interpreted as preclinical proof-of-concept evidence requiring further PK/PD-guided validation under clinically feasible conditions.