In vivo reversal of the multidrug resistance (MDR) phenotype in a multidrug resistant cancer model by jet-injection of anti-MDR1 short hairpin RNA-encoding plasmid DNA

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA 2279 Triggering the RNA interference (RNAi) pathway through the expression of small hairpin RNA (shRNA) molecules has become a promising tool for efficient silencing of a given gene in gene therapeutic applications. In this study, shRNA was used to reverse the classical MDR1/P-glycoprotein (MDR1/Pgp)-mediated multidrug resistance (MDR) phenotype in vivo. For delivery of naked shRNA-vector constructs, for the first time the nonviral jet-injection technology was applied for direct intratumoral in vivo transfer. For this, highly efficient anti-MDR1 shRNA expression vectors were applied twice in a human MDR1/Pgp overexpressing cancer xenograft and the corresponding drug-sensitive parental tumor xenograft bearing mice. The expression level of the MDR1 mRNA was decreased to about 90% and the corresponding MDR1/Pgp protein was no longer detectable in the tumors two days after anti-MDR1 shRNA vector injection. In order to analyze on whether downregulation of MDR1/Pgp leads to chemosensitization towards MDR-related drugs, two jet-injections of anti-MDR1 shRNA vectors into the tumors were combined with two intravenous administrations of doxorubicin. The data show, that jet-injection delivery of shRNA expressing vectors is effective and sufficient for reversing MDR1/Pgp-mediated MDR in vivo. Therefore, this therapeutic approach is a promising strategy to reverse tumors with a MDR1/Pgp-dependent MDR phenotype back to a drug-sensitive one.