Dynamics of the tumor marker (TM) cancer antigen 15.3 (CA 15.3) and its association with overall survival (OS) in patients (pts) with hormone-receptor (HR)-positive/HER2-negative advanced breast cancer (ABC) in a public oncologic institution of Peru.

e13080 Background: CA 15.3 is a conventional TM commonly elevated in HR (+) /HER2 (-) ABC at diagnosis and is associated with a high burden of disease and shorter OS during follow-up for response to treatment. The aim of our study is to evaluate the association of elevated vs. normal CA15.3 at diagnosis with clinicopathological characteristics and OS. Methods: Retrospective and descriptive analysis were made between 2018-2024. The association of initial CA 15.3 levels (normal, elevated: ≥ 32.4 U/ml) with the qualitative variables was evaluated with Chi-square test. Effect of the variables that had a significant association with initial CA15.3 levels were estimated with logistic regression, through odds ratio (OR). Efficacy was measured as OS. A p < 0.05 value (SPSS) will be considered for a significant difference. Survival curves were constructed with the Kaplan-Meier method. Results: 127 female pts were included. Median age was 55 years old (45-63); 50% were postmenopausal and 42% had de novo disease at diagnosis. Most pts were in ECOG 0-1 (93%) and 79% were luminal B subtype. Regarding the first-line therapy, 80% used endocrine therapy (15% used also ribociclib) and the rest used chemotherapy. According site of metastases, 43% and 27% had visceral and bone metastases, respectively. Regarding number of metastases, 61%, 28% and 11% had 1, 2 and ≥ 3 metastases, respectively. 68% of pts had elevated CA 15.3 (median: 65.08) at diagnosis, and 68% had a persistently elevated final CA15.3 during follow-up (p < 0.001). Postmenopausal had more elevated CA 15.3 levels than premenopausal (p = 0.014). Also, pts with ≥ 2 metastases had more elevated CA 15.3 than 1 site (p = 0.013). After a median follow-up of 30 months (2-120), the median OS rates at 12, 36 and 48 months were estimated in 97%, 69% and 45%, respectively; with a median OS of 55 months. Pts with pretreatment normal CA 15.3 (65 vs. 41 months, p = 0.024), luminal B (p = 0.032) and normal last CA 15.3 had longer OS compared to those with persistently elevated CA 15.3 (p = 0.0036). Conclusions: Elevation of TM CA 15.3 is very common in Peruvian pts with ABC HR (+) /HER2 (-) at diagnosis and during follow-up. Postmenopausal status and ≥ 2 metastases are associated with raised CA15.3 at diagnosis. Pts with normal pretreatment baseline CA 15.3, luminal B and who have normal last CA15.3 have a longer OS compared to those with persistently elevated CA 15.3. Estimation of the effect of the characteristics under study on the likelihood of a high initial CA 15.3 level. Univariate Multivariate OR 95% CI p-value OR 95% CI p-value Age group (years) < 50 Ref. Ref. ≥ 50 3.02 1.26-8.14 0.019 3.27 1.33-9.03 0.014 Number of metastases 1 Ref. Ref. ≥ 2 2.53 1.18-5.49 0.017 2.72 1.24-6.10 0.013 OR: odds ratio; CI: confidence interval; Ref.: reference.