By David N. Leff
One trouble with cancer gene therapy is that its gene delivery vehicle tends to be a retroviral vector (RVV).
And one trouble with RVVs is that they tend to have dicey safety margins.
So says developmental immunologist Ralph Snodgrass, vice president of research and chief scientific officer of Progenitor Inc., in Menlo Park, Calif. He explained: "One of the interesting issues around retroviral vectors, or other viral vectors, is that they need to get into the target cell's nucleus, and they need to integrate within its genome. There it might integrate within a critical gene, which is not a good process."
Progenitor has developed and tested an alternative tumor ablation strategy, summed up in the title of its paper in the March issue of Human Gene Therapy: "Cancer gene therapy by direct tumor injections of a nonviral T7 vector encoding a thymidine kinase gene."
Snodgrass, a co-author of the journal article, described that nonviral T7 delivery system. "T7," he told BioWorld Today, "is strictly a cytoplasmic vector, which doesn't integrate in the nucleus. It's based on a bacterial viral promoter that's used in phages.
That plasmid vector delivers two genes to cancer cells. One encodes its own RNA polymerase enzyme, which then becomes self-amplifying, for higher and higher levels of expression in the cytoplasm.
"So you could visualize this as a self-perpetuating amplification process," Snodgrass pointed out. "The therapeutic gene you want to express is driven off this T7 promoter, and the vector itself produces more of the enzyme that acts as a feedback loop."
The therapeutic anti-tumor gene is herpes thymidine kinase (HTK), an enzyme produced by the herpes simplex virus -- which itself is not involved in the Progenitor process. But cells that express this HTK turn an antiviral, antiproliferative drug, gancyclovir, into a compound highly toxic to tumor cells.
That's the cast of characters in Progenitor's cancer gene therapy drama. Here's how it played out in the company's animal trial:
Ten nude mice received subcutaneous flank injections of 5 million human osteosarcoma cells, which started to grow voraciously. Plasmid DNA injections came about a week later, when the tumors had reached 40 to 100 cubic millimeters in size. The injections were designed for delivery to as many intratumoral regions as possible. Soon after, the gancyclovir was injected intraperitonealy.
The result, as reported in the journal and summed up by Snodgrass: "Six out of the ten tumors in the vector-injected group demonstrated different degrees of tumor regression. Three of those six tumors completely disappeared, either during or shortly after the end of the gancyclovir injections. The effect was permanent, with no evidence of relapse in a 10- to 12-month observation period."
None of the 20 mice in the untreated or placebo control group attained tumor regression.
Snodgrass recalled that in 1995, following early successful experiments with the T7 gene delivery system, Chiron Corp., of Emeryville, Calif., paid Progenitor $2.5 million to share in the nonviral system's commercialization. The agreement focuses on Chiron's use of the T7 vectors to develop gene therapies for 11 potential products in cancer, cardiovascular disorders and infectious diseases. The deal has a potential value of $50 million. (See BioWorld Today, April 5, 1995, p. 1)
Enter a serendipitous player -- the bystander effect.
"One interesting aspect of those cytotoxic molecules produced within the cells," Snodgrass observed, "is that they actually showed the capacity to reach out of the cells and kill nearby bystander cells."
He went on: "That's very important in our approach, because it does not require you to get your vector into all the target cells. If you get it in a percentage, those cells will produce these toxic byproducts. They not only die themselves, but then their byproducts kill off the surrounding tumor cells, which may not have been transfected."
Projenitor's paper in the journal concluded: "Results of these antitumor efficacy studies suggest that the T7 system may be used as an alternative to viral vectors for cancer gene therapy and other transient gene therapies or biological applications that require temporary but rapid and efficient therapeutic gene expression." *