Reversing the Irreversible: miRNA-Targeting Mesyl Phosphoramidate Oligonucleotides Restore Sensitivity to Cisplatin and Doxorubicin of KB-8-5 Epidermoid Carcinoma Cells

<b>Background:</b> Chemotherapy remains one of the main approaches for treating malignant tumors, but repeated exposure to cytostatics leads to multidrug resistance (MDR), increasing tumor aggressiveness and reducing therapeutic efficacy. Identifying adjuvant agents that restore tumor sensitivity to drugs while minimizing toxicity is a cornerstone challenge today. This study aimed to investigate the potential of mesyl phosphoramidate antisense oligonucleotides (µ-ASOs) targeting miR-17, miR-21, and miR-155 as agents for enhancing the efficacy of cisplatin (Cis) and doxorubicin (Dox) in MDR-positive human epidermoid carcinoma KB-8-5 cells. <b>Methods:</b> Optimal regimens for the simultaneous application of µ-ASOs and Dox or Cis in KB-8-5 cells, including a concentration-dependent analysis and the type of compound interaction in combinations (synergy/additivity/antagonism), were studied using the MTT assay. Antiproliferative effects of the combinations were assessed using the real-time cell monitoring xCELLigence system. The potential molecular mechanism underlying KB-8-5 cell sensitization to cytostatics was investigated using RT-PCR and Western blot hybridization, supported by bioinformatic reconstruction of the gene network. <b>Results:</b> The most effective combinations including µ-ASOs targeting miR-21 and miR-17 together with Cis or Dox demonstrated additive to moderately synergistic effects on KB-8-5 cell viability (HSA synergy score = 4.8-8.7). The co-application of µ-ASOs allowed a 5- to 20-fold reduction in the dose of cytostatics, while maintaining a strong antiproliferative effect of 70-95%. Sensitization of KB-8-5 cells to Cis or Dox following µ-ASO treatment was mediated by a 1.5- to 3-fold decrease in the levels of the well-known MDR marker ABCB1 as well as the newly identified MDR-associated targets ZYX, TUBA4A, and SEH1L. <b>Conclusions:</b> miRNA-targeted mesyl phosphoramidate oligonucleotides are effective tools for overcoming resistance to the clinically approved chemotherapeutics cisplatin and doxorubicin. The relationship between miR-21, miR-17, and miR-155 and the novel MDR markers such as SEH1L, TUBA4A, and ZYX was revealed, thereby expanding the current understanding of the molecular mechanisms underlying tumor cell resistance to chemotherapy.