The decision of how to put a transgene into a cell depends as much on the concerns about doing so as it does on the ease of doing so. Will the vector be able to find and enter its desired target? Will the transgene insert randomly into the genome, or become diluted out as the host cell replicates? Is the vector immunogenic? Can sufficient quantity of sufficient quality vector be produced?
What is perhaps transfection’s ultimate use—modifying the human organism—was front and center for researchers gathered at this year’s American Society for Gene and Cell Therapy annual meeting.
Effective analgesics have been in use for thousands of years, yet our ability to use them to treat localized pain is limited by interaction of the drug with other than the desired receptors. High doses of opiates, for example, will cause confusion, constipation, and urinary retention. Yet if the drugs could be delivered directly to the sites where they are needed, it may be possible to bypass such off-target effects.
David Fink, M.D., chair of the department of neurology at the University of Michigan, and his colleagues engineered NP2, a herpes simplex virus (HSV)-based vector, to express a naturally occurring opioid peptide. When injected into the skin the vector is picked up and transported by the sensory nerves to the dorsal root ganglia (DRG), where they take up residence in the nuclei as circular episomes. Enkephalin peptides—endogenous agonists for the delta opioid receptor—are produced, carried to axonal terminals, and released to the spinal cord.
“We chose herpes because, really, herpes is unique in being a virus that naturally targets DRGs,” explained Dr. Fink. Most people have HSV latent in their ganglia, and people live with recurrent cold sores (indicating the presence of wild type HSV in their sensory ganglia), for life. And the amount of nonreplicating recombinant vector that is being put into the skin—the natural site that the virus uses to infect—is orders of magnitude less than would be found in a cold sore. So “we don’t think it should cause problems, but that’s why we did a Phase I trial in patients with a terminal disease.”
Small molecule Phase I studies are traditionally done in normal peers, but gene therapy trials have to be carried out in a patient population. Because theirs was the first trial to use HSV to deliver a transgene, terminal cancer patients with intractable focal pain were chosen as the first recipients of NP2. There were no serious adverse events.
Patients receiving a high dosage of NP2 “had a very substantial reduction in their pain,” recounted Dr. Fink, with the average level dropping from almost 8 to less than 2 on a 10-point scale. “But there was no placebo control.” The results were encouraging enough for the company, Diamyd Medical, to sponsor a Phase II trial, which is currently under way with more patients and a placebo control.
In his most optimistic assessment, NP2 exemplifies for Dr. Fink a platform technology that could be used to deliver genes into sensory neurons to treat inflammatory pain, or neuropathic pain, or to prevent nerve damage; HSV-based vectors for these applications are currently being investigated.
The boundary of the approach, Dr. Fink says, is that it is only appropriate for problems that are focal in nature. It is “not a solution for somebody who has whole-body pain, like fibromyalgia.”