Antisense hairpin loop oligonucleotides as inhibitors of expression of multidrug resistance-associated protein 1: Their stability in fetal calf serum and human plasma

Multidrug resistance-associated protein (MRP1) is a transmembrane pump protein responsible for the efflux of chemotherapeutic drugs, an important cause of anticancer treatment failure. Trying to circumvent MRP-mediated resistance we de­signed and synthesized hairpin loops forming antisense oligodeoxyribonucleotides (ODNs), both phosphodiesters (PO-ODNs) and their phosphorothioate analogues (PS-ODNs), to reduce the protein expression by targeting its mRNA in a sequence spe­cific manner. Melting temperature measurements as well as polyacrylamide gel elec- trophoresis supported the preferential formation of a secondary structure, which was expected to protect ODNs against 3-exonuclease degradation. ODNs and PS-ODNs designed in this work were successfully tested as antisense inhibitors of the expres­sion of MRP1 in the leukaemia HL60/ADR cell line. Foreseeing the necessity to per­form clinical studies with such ODNs we investigated their stability against the 3 -exonuclease activity of fetal calf serum and human plasma. Under the conditions, corresponding to physiological ones, we observed high stability of hairpin loop form­ing ODNs, especially those containing longer (e.g. 7 base pair) stems. Comparative studies on the stability of chemically unmodified hairpin loop forming ODNs and their PS-counterparts indicated that endonuclease activity did not play any important role in the process of their nucleolytic degradation. Our studies provide strong evidence for high stability of chemically unmodified hairpin loop ODNs, making them an attractive alternative to phosphorothioate analogues commonly used in antisense strategy.