Long-term in vitro treatment with telomerase inhibitor MST-312 induces resistance by selecting long telomeres cells

Telomerase is a good target for new anticancer drug development because it is present in over 85% of human tumours. However, despite chronic therapy is a condition for anti-telomerase approach, the effects of long-term treatment with telomerase inhibitors remain not well understood. In this work, it was evaluated the effects of long-term treatment of human MDA-MB-231 breast cancer cells with the telomerase inhibitor MST-312. Cells were treated for 72 hours or 140 days, and it was accessed their viability, proliferation rate, morphology, telomeric DNA content, and resistance mechanism. The drug had a clear short-term effect, including chemosensitizing cells for docetaxel and irinotecan, but the chronic exposition led to selection of long telomeres clones, changing characteristics of original cell line. This effect was confirmed in a clonal culture with homogenous karyotype. MRP-1 expression and alternative lengthening of telomeres (ALT) were discarded as additional mechanisms of resistance. This data suggest that, considering the intra-tumour heterogeneity (ITH), what is already a big challenge for treatment of cancer, chronic exposition to telomerase inhibitors can promote tumour adaptations with potential clinical repercussion, drawing attention to ongoing clinical trials and pointing important considerations most times neglected on studies about use of these inhibitors on cancer therapy.Significance of the study:Antitumour action of telomerase inhibitors is wellknown, but it depends on a long‐term exposition because cells will undergo telomereerosion only after many duplication cycles. Recently, the frustrating results of clinicaltrials with these inhibitors aroused the interest of the scientific community to under-stand the mechanisms of resistance to anti‐telomerase therapy. In this study, we con-ducted an 18‐week experiment to show that telomerase inhibition can lead to celladaptations and selection of long‐telomeres clones, leading to acquisition of resis-tance. However, we also showed that this inhibitor can sensitize cells to the chemo-therapeutic drugs docetaxel and irinotecan