Despite extensive basic research and clinical trials, chronic pain is a condition that often remains refractory to available treatments11-13
. Innovative approaches have largely focused on the search for novel targets for analgesic drugs, based on insights gained from broad based investigations into the neurobiology of chronic pain14
. Unfortunately, it has been difficult to identify amenable targets for systemically delivered small molecules that are located exclusively within nociceptive pathways. Not unlike more conventional analgesic agents15, 16
, even drugs designed to interact with newly identified apparently “nociceptive specific” targets such as the vanilloid receptor TRPV1 have been found to have biological roles unrelated to nociception that pose a significant constraint to their use17, 18
HSV is a ubiquitous, naturally neurotropic virus spread by contact of skin or mucous membranes that establishes a persistent latent state in neurons of sensory ganglia. By the same cellular transduction mechanisms, replication defective HSV-based vectors delivered subcutaneously or intradermally transfer genetic material into DRG neurons in vivo19
. The human PENK gene carried by NP2 encodes preproenkephalin, a precursor protein that is processed to produce 6 met-enkephalin and 1 leu-enkephalin moieties; endogenous opioid peptides that are the naturally occurring high-affinity ligands of the delta opioid receptor10
. In this clinical trial the mechanism of action of the vector was not directly confirmed, but in animal studies the analgesic effects achieved by HSV-mediated expression of PENK are blocked by opioid receptor antagonists naloxone and intrathecal naltrexone 4-6
suggesting a site of action at spinal opioid receptors, and in vitro biological effects of vector-produced enkephalins are blocked by 10 pM naltrindole 20
consistent with in vitro activity at the delta opioid receptor. While met-enkephalin has a higher affinity for the delta compared to the mu opioid receptor 21
it is certainly possible that the analgesic effects observed are mediated through mu as well as delta opioid receptors.
Conceptually, HSV-mediated delivery of PENK represents a logical extension of the technique of delivering opiate drugs by intrathecal infusion22
to maximize effects achieved at the spinal level while minimizing systemic side effects. Following delivery vector particles are taken up by nerve terminals in the skin and transported to the DRG where the circularized vector genome establishes a persistent state as an intranuclear episomal element. Vector derived expression of human PENK in DRG following skin inoculation of HSV vectors in rodents has been confirmed by in situ hybridization, RT-PCR, radioimmunoassay and immunocytochemistry 4, 23
. While we were not able to study vector biodistribution in this clinical trial, preclinical studies indicate that replication defective vector genomes are constrained to the injection site and related DRG. This was confirmed by a preclinical GLP biodistribution study of NP2, in which 16 different tissues were examined between 1 and 91 days after inoculation.
Other gene transfer approaches including intrathecal injection of adenovirus24
, adeno-associated virus25
, or naked plasmids26
have been demonstrated to reduce pain-related behaviors in animal models of pain, but none of these approaches have yet been brought to a human clinical trial. In preclinical animal studies, skin inoculation of HSV vectors expressing PENK reduce acute hyperalgesic responses27
, and reduce pain-related behaviors in models of arthritis28
, formalin injection4
, peripheral nerve damage6
and bone cancer5
. Because this was the first human trial employing HSV vectors to achieve gene transfer, we elected to carry out the phase 1 clinical trial for safety and dose-finding in patients with pain caused by cancer.
The safety profile observed in this study was not unanticipated. Oncolytic recombinant HSV-1 viruses without transgenes that are intended to kill malignant cells by limited replication have been injected directly into tumors in brain, liver, and skin in more than 200 subjects to date with no reported test agent-related SAEs29-33
. Replication-competent HSV recombinants have also been examined in clinical trials as potential vaccines against genital herpes. While these approaches have in some cases generated quite high anti-HSV antibody levels (though not effectively preventing HSV infection), no drug related SAEs have been observed34-36
This Phase I clinical trial primarily addressed the question of whether intradermal delivery of NP2 to skin would prove to be safe and well tolerated by subjects. The small number of patients and the absence of placebo controls warrant circumspect interpretation of the secondary outcome measures. But the observation that subjects in the low dose cohort had little change in the NRS or SF-MPQ while subjects in the higher dose cohorts reported substantial reduction in NRS and improvement in SF-MPQ is encouraging. Based partially upon these results, Diamyd has initiated a randomized, double- blind placebo-control Phase II clinical trial in a similar patient population (A phase II, randomized, double blind, placebo-controlled, multicenter study to investigate the impact of NP2 in subjects with intractable pain due to malignancy; ClinicalTrials.gov #NCT01291901).
While pain associated with cancer is a significant clinical problem with unmet medical need37
, the potential utility of HSV-mediated gene transfer to the DRG from skin inoculation is not limited to treatment of cancer pain. The recombinant replication defective HSV approach represents a platform technology - nerve targeting drug delivery system (NTDDS) - that can be used to deliver and express any one of a number of genes in the nervous system. A related NTDDS vector, NG2 that expresses human glutamic acid decarboxylase (GAD) to effect the release the inhibitory neurotransmitter γ aminobutyric acid reduces pain-related behaviors in preclinical models of neuropathic pain from nerve injury38
and diabetes (European Journal of Pain, in press
). Clinical trials of NTTDS vectors in pain have the advantage that the biological effect of the transgene product can be assessed continuously in real time. NTDDS gene transfer to the DRG to express neurotrophins locally prevents the progression of polyneuropathy in relevant preclinical models39-41
suggesting that the NTDDS platform may be used to treat degenerative polyneuropathies as well.