The use of gene therapy in the treatment of brain cancer is hindered by drug delivery problems. Large molecules such as monoclonal antibodies and viral vectors cannot cross the blood-brain barrier. A featured article in the June 1 issue of Clinical Cancer Research [Pardridge et al., vol. 10, pp. 3667-3677 (2004)] now describes the successful delivery of a nonviral RNAi gene therapy to brain tumors in mice, resulting in a significant increase in survival times.

The study, overseen by William Pardridge, MD, of the University of California Los Angeles Medical School, was initiated from a drug delivery angle. "We're a drug delivery lab, not an RNAi lab," Pardridge says. "It seemed like none of the RNAi people could learn drug delivery, so we learned RNAi." Their team designed short hairpin RNAs (shRNAs) against endothelial growth factor receptor (EGFR) and encoded it in a DNA plasmid. The plasmid was then encapsulated in an anionic liposome conjugated with polyethylene glycol (PEG), thus restricting uptake by the reticuloendothelial system. In addition, 1 to 2% of the PEG strands were conjugated with one of two monoclonal antibodies.

These antibodies represented a two level "lock and key" system for getting past the blood-brain barrier (BBB) and the tumor cell membrane (which behaves much like the BBB). The first antibody, which acts as a secret password for getting past the mouse BBB, was targeted to the mouse transferrin receptor (TfR). After the TfR admits the "trojan horse" liposome, it is targeted to the tumor cell by a human insulin receptor.

"The reason this work is significant," says Pardridge, "is that this area of science is not going to be converted into a therapeutic until the problems of drug delivery are solved."

In addition to mice, this technique has also been tested in rats and rhesus monkeys. Pardridge estimates that with funding, this method could be used in clinical trials in two years. A potential hurdle along the way would be eliminating ectopic expression of the gene in noncancer cells. Possible solutions include careful engineering of the DNA promoter and designing shRNAs that target only mutant forms of EGFR, which are frequently expressed by cancer cells.

By Catherine Shaffer