In the first trial of genes encoding preproenkephalin (PENK), the researchers from the University of Michigan have found that the treatment appears to provide substantial relief from cancer pain.

It has been told by the researchers that injection of PENK genes by neurons of the targeted dorsal root ganglia (DRG) leads to production of preproenkephalin, an opioid precursor protein, which is then cleaved to produce endogenous opioids that inhibit pain signaling in the spinal cord.

The primary aim of this study was to determine the safety of introducing PENK into the DRG. The study conveys that sensory inputs to the spinal cord, using a herpes simplex viral vector unable to replicate.

Around 10 patients participated in the study whose cancer pain has not been relieved by 200mg of morphine or more per day. The human PENK gene region was inserted into the DNA of the replication-defective HSV, and patients were given 10 intradermal injections of the vector within "dermatomes" corresponding to the somatic location of their pain.

During the study, eight patients remained in the study at 28 days after treatment. Patients receiving the lowest dose did not report any considerable changes in pain on the numeric rating scale.