Expression of TK1 and CDK9 in plasma-derived exosomes is associated with clinical response to CDK4/6 inhibitors in breast cancer

Abstract

Background: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) improve PFS in patients with hormone receptor-positive (HR+) advanced breast cancer (Finn et al., 2016). In order to better characterize the response to these agents and increase our knowledge on the pharmacogenetic profile of CDK4/6i, the aim of this study was to analyse the expression of targets relevant to the activity of CDK4/6i in plasma-derived exosomes. Methods: Blood samples were collected from patients affected by HR+, HER2- advanced breast cancer receiving palbociclib/ribociclib in association with hormonal therapy. Three ml of plasma were taken at the beginning of treatment (baseline) and at the first clinical evaluation (after 3months). Objective responses were defined following the RECIST criteria v.1.1. RNA from plasma-derived exosomes was extracted by the ExoRNeasy kit (Qiagen) and analysed for the expression of thymidine kinase 1 (TK1), CDK 4, 6 and 9 by digital droplet PCR (BioRad). Mann-Whitney test was applied. Results: Thirty-four metastatic breast cancer patients were prospectively enrolled in this study. The comparison of mRNA levels of TK1, CDK4, 6 and 9 between baseline and the first clinical evaluation was available in 22 patients treated with letrozole/anastrozole+ palbociclib and 22 patients given fulvestrant + palbociclib. 18 patients had newly diagnosed advanced breast cancer while 16 patients received ≥1 line of treatment. Objective responses were: 1 (2,9%) CR, 4 (11,8%) PR, 16 (47,1%) SD and 13 (38,2%) PD. The comparison of changes in the expression between TK1, CDK 4, 6 and 9 at baseline compared to first evaluation was statistically significant for TK1 (PR+SD vs. PD p=0.009), CDK4 (PR+SD vs. PD p=0.020), CDK6 (PR+SD vs. PD p=0.047) and CDK9 (PR+SD vs. PD p=0.008). The univariate analysis didn't find any significant correlation between patients clinical variable and PFS (i.e. type of hormonal treatment, the line of treatment, performance and menopausal status, visceral metastasis, bone only metastasis, number of metastasis, previous hormonal or lines of chemotherapy received). Conclusions: Exosomal expression of CDK4, CDK6 and in particular of TK1 and CDK9 may be useful to early identify patients who are likely to respond to CDK4/6i.