Complete in vivo reversal of the multidrug resistance phenotype by jet-injection of anti-MDR1 short hairpin RNA-encoding plasmid DNA.

Triggering the RNA interference (RNAi) pathway by inducing the expression of short hairpin RNA (shRNA) molecules has become a promising tool for efficient silencing of a given gene in gene therapy applications. In this study, shRNA encoding DNA was utilized to reverse the classical MDR1/P-glycoprotein (MDR1/P-gp)-mediated multidrug resistance (MDR) phenotype in vivo. For the first time, the nonviral jet-injection technology was applied for delivering naked shRNA-vector constructs for direct intratumoral in vivo transfer. The highly efficient anti-MDR1 shRNA expression vectors were applied twice in the human MDR1/P-gp overexpressing MaTu/ADR cancer xenograft-bearing mice, and twice in the corresponding drug-sensitive parental MaTu tumor xenograft bearing mice as well. Two days after anti-MDR1 shRNA vector injection, the expression level of the MDR1 messenger RNA (mRNA) was decreased by more than 90% and the corresponding MDR1/P-gp protein was no longer detectable in the tumors. Two jet-injections of anti-MDR1 shRNA vectors into the tumors, combined with two intravenous (IV) administrations of doxorubicin, were sufficient to achieve complete reversal of the drug-resistant phenotype. The data show that jet-injection delivery of shRNA-expressing vectors is effective in reversing MDR1/P-gp-mediated MDR in vivo, and is therefore a promising strategy for making tumors with an MDR1/Pgp-dependent MDR phenotype revert to a drug-sensitive state.