LONDON - DNA from a viral vector carrying a "suicide gene" designed to kill brain cancer cells has been found throughout the brains of rats 12 months after administration, raising hopes that it may be possible to develop gene therapy for chronic diseases such as Parkinson's and Alzheimer's.

Less encouraging is the news that chronic inflammation was present in rats' brains three months after the injection of the vector, a finding that will have implications for current clinical trials of this type of gene therapy on humans with brain tumors.

Pedro Lowenstein, Lister Institute professor of molecular medicine and gene therapy at the University of Manchester in the UK, told BioWorld International: "We need to continue with human trials because that is the only way we can further develop these novel therapeutic strategies, but we need to be very careful and look for possible side effects in these patients."

Several clinical trials have begun to evaluate gene therapy for brain tumors called gliomas. People who develop one of these tumors survive on average 12 to 15 months from diagnosis. Traditional treatments such as surgery, chemotherapy and radiotherapy have made little difference to the survival time, which has remained the same for the past 30 years.

During gene therapy, patients are given an injection into the tumor site of genetically modified adenoviruses. These viruses, which can no longer replicate, carry the gene for the herpes simplex virus 1 enzyme, thymidine kinase. When the patient is subsequently given an injection of the prodrug ganciclovir, the thymidine kinase converts it into ganciclovir phosphate, which is then metabolized by cellular kinases to make ganciclovir trisphosphate. This blocks DNA synthesis and effectively causes the cell to commit suicide.

Lowenstein, in collaboration with Maria Castro, professor of molecular medicine at the University of Manchester, and colleagues in Paris, and Vienna, Austria, decided to investigate the long-term consequences of this type of treatment in rats. Their results are reported in a paper in the November issue of Nature Medicine titled, "Chronic brain inflammation and persistent herpes simplex virus 1 thymidine kinase expression in survivors of syngeneic glioma treated by adenovirus-mediated gene therapy: Implications for clinical trials."

Following injection of cells called CNS-1 into the brains of rats, the animals normally die from brain tumors within 30 days. Lowenstein and his colleagues showed that all of the animals were still alive three months later if they were treated by gene therapy using replication-defective recombinant adenovirus expressing herpes simplex virus 1 thymidine kinase, followed by treatment with ganciclovir. At three months, the animals were killed and their brains examined for signs of inflammation.

The researchers found there was a chronic inflammatory process in the brain, which was located in the area where the original injections of tumor cells and virus had been made. In addition, some myelinated fibers surrounding the injection area had died, and this appeared to be the result of tissue injury, not due to an immune response against the brain.

What do these results mean for clinical trials of similar protocols being carried out in humans? Lowenstein said, "It is very difficult to know what would happen in humans - they are a different species and their brain structure is different. What we are saying is that gene therapy has some side effects, so we need to explore what these side effects are, and study these patients very carefully. Gene therapy is pharmacology using nucleic acids as drugs. Just as in pharmacology, side effects are bound to occur. This does not mean one should refrain from further developing gene therapy. On the contrary, reducing side effects will enhance the long-term safety and therapeutic potential of gene therapy."

Conventional treatments such as radiotherapy have severe side effects, too, he pointed out. "Most of these patients die very quickly, so that these long-term effects may never become apparent, but of patients that are treated with radiation, a very small percentage survive. Of this group, many develop dementia in the very long term because they get demyelination due to the radiotherapy."

Commenting on the paper in the same issue of Nature Medicine, Tammy Kielian and William F. Hickey, of the Department of Pathology at Dartmouth Hitchcock Medical Center in Lebanon, N.H., wrote, "To our knowledge this is the first study to document chronic inflammation after [this type of] gene therapy in the brain, and raises important issues to consider in developing strategies for treatment of brain tumors."

Lowenstein is enthusiastic that the transgenic thymidine kinase continued to be expressed throughout the brains of the rats 90 days after injection. He added that subsequent data from his laboratory, which have not yet been published, have shown that expression continues for up to 12 months.

He told BioWorld International: "Interestingly, the inflammation is localized to where we injected the tumor cells and viruses initially, but we have a lot of expression of the transgene throughout the brain, including on the other side of the brain - and there is no inflammation there. This long-term expression is a very encouraging finding and it opens the door for gene therapy of chronic diseases such as Parkinson's and Alzheimer's using adenoviruses. We are already in the process of developing safer next-generation adenoviruses which are less immunogenic and cause less inflammation."

Lowenstein is discussing plans for clinical trials of this type of gene therapy in humans with neurosurgeons in London, Manchester, Paris and Germany. He is also carrying out studies using the new-generation vectors, which he believes should survive even longer in the brain because they are more stable. A paper describing this work has been submitted to the Proceedings of the National Academy of Sciences.