Comparative analysis of MIB1 and SP6 antibodies for Ki-67 assessment in breast carcinoma with focus on the influence of molecular subtyping

Abstract

Ki-67 is a nuclear protein used as a proliferation marker in breast carcinoma, aiding in tumor classification and prognostic assessment. However, its clinical utility is limited by variability in immunohistochemical assessment, influenced by numerous factors, including differences in antibody clones. This study aims to compare the performance of two widely used Ki-67 antibody clones, MIB1 and SP6, and to evaluate their impact on molecular subtyping in breast cancer. Sections from formalin-fixed, paraffin-embedded biopsy samples from 35 primary breast carcinoma cases were stained using MIB1 and SP6 antibodies. Histological tumor characteristics and receptor statuses were evaluated for each case. The Ki-67 proliferative index was assessed following the latest recommendations of the International Ki-67 in Breast Cancer Working Group. Statistical analysis included Pearson correlation, paired t-test, Welch's ANOVA, and McNemar's test. A strong correlation was observed between Ki-67 indices obtained using MIB1 and SP6 (r = 0.755, p < 0.001), with no statistically significant difference in mean values (p = 0.288). Ki-67 index correlated significantly with tumor grade for both antibodies. Among Luminal HER2-negative tumors, 17 % showed discordant molecular subtyping between MIB1 and SP6 using a 20 % cut-off. Similar discrepancies were observed at 14 % and 25 % cut-offs, suggesting that antibody choice may affect subtype classification. In conclusion, both MIB1 and SP6 provide reliable assessment of proliferative index in breast carcinoma, however, discrepancies in individual cases may impact molecular subtyping and subsequent clinical decision-making. Therefore, reporting the antibody clone used in Ki-67 evaluation may be advisable, pending further validation.