Salivary Metabolic Characteristics and Response to Neoadjuvant Systemic Therapy in Breast Cancer

Metabolic changes in saliva are known to be closely associated with the presence of non-oral cancers, particularly breast cancer. The diagnostic and prognostic potential of salivary biomarkers in breast cancer has been demonstrated, but their applicability for assessing therapy response has not yet been established. The aim of this study was to comprehensively analyze clinical, pathological, molecular, and salivary characteristics when assessing the response to neoadjuvant chemotherapy for breast cancer. The study included 361 breast cancer patients undergoing their first course of chemotherapy and 127 healthy volunteers without breast pathologies. Saliva samples were collected from all volunteers before treatment. Saliva analysis results for amino acids, lipids, and tumor markers were compared with tumor pathomorphism assessment after breast cancer surgery. The proportion of patients with a complete response to therapy was statistically significantly lower after menopause, and in those with HER2-negative breast cancer, moderate tumor differentiation, and high estrogen and progesterone receptor expression. For the first time, a body mass index (BMI) greater than 25 and low HER2 expression (HER2-low) were shown to have an unfavorable prognosis. The criterion for selecting informative salivary metabolites was a multidirectional change in minimal and complete pathological responses to therapy compared to healthy controls. Thus, prognostically favorable signs were a decrease in the concentration of urea below 7.5 mmol/L (OR = 1.921; 95% CI 1.061–4.270; p = 0.0342), a decrease in the area of the absorption band at 2957 cm−1 below 24 (OR = 3.875; 95% CI 1.160–12.70; p = 0.0003), and an increase in the concentration of cancer antigen CA27.29 above 3 U/L (OR = 2.138; 95% CI 1.021–7.273; p = 0.0343) and CA-15-3 above 39 U/L (OR = 3.896; 95% CI 1.062–14.07; p = 0.0072). With a simultaneous increase in both CA27.29 and CA15-3, the probability of a complete response to therapy increased (OR = 4.288; 95% CI 1.056–17.09; p = 0.0013). Multivariate analysis showed that an independent prognostic indicator, along with the expression status of HER2, estrogen receptors, differentiation degree, BMI, and menopause status, was the concentration of CA15-3 in saliva (AUC = 0.789, 95% CI: 0.737–0.842, p = 0.0001). Identifying new markers will help physicians formulate treatment plans tailored to a patient’s individual risk factors, leading to increased survival and improved quality of life.