Gene Therapy for Pain: A Perspective

We recently reported results of the first clinical trial of gene transfer as a treatment for pain [1]. Ten subjects with moderate-to-severe intractable pain from cancer received NP2, a replication-defective herpes simplex virus (HSV)-based vector engineered to express human preproenkephalin, injected intradermally into the dermatome(s) corresponding to the radicular distribution of pain. There was no placebo control in this relatively small Phase I trial, but in addition to achieving the primary safety outcome, there was an apparent dose-dependent analgesic effect. Subjects receiving a low dose of NP2 reported no substantive effect, while subjects in the middle and high dose cohorts reported clinically meaningful pain relief as measured by the patient-reported numeric rating scale(NRS) and Short Form McGill Pain Questionnaire (SF-MPQ) for 4 weeks following injection of NP2. Based on these results, we have initiated a Phase II, double-blind, randomized, placebo-controlled, clinical trial [101].

Replication-defective HSV vectors injected into the skin are carried by retrograde axonal transport to dorsal root ganglia (DRG), where the vector establishes a persistent state within the nucleus of those cells as a circular episomal element. Transgene products produced from the otherwise quiescent vector genome are carried by anterograde axonal transport to axon terminals in the spinal cord. The approach of using HSV-mediated gene transfer to treat pain is based on the premise that targeted delivery and expression of genes coding for antinociceptive products to DRG neurons can be used to produce analgesic effects by selectively interfering with nociceptive input at the anatomic level corresponding to the regional distribution of the pain. Among the potential viral-based gene transfer vectors, HSV is particularly well-suited for this purpose since the parental wild-type virus from which the replication-defective gene transfer vectors are constructed naturally targets DRG neurons from skin infection to establish a life-long persistent (latent) state.

The NP2 vector administered to subjects contained the gene for human preproenkephalin. Neurons transduced with this vector release enkephalin peptides, the endogenous agonists for the δ-opioid receptor. The human clinical trial was preceded by preclinical studies that examined the effect of HSV vectors expressing preproenkephalin in rodent models of inflammatory pain, neuropathic pain and pain caused by cancer in bone [2–4]. Similar analgesic effects of preproenkephalin-expressing replication-defective HSV-based vectors have been reported by other investigators in several different models of pain [5–8]. We chose to proceed with a clinical trial in patients with intractable pain resulting from terminal cancer because, despite the evidence of safety from preclinical biodistribution and toxicology studies, this was the first clinical trial using HSV to deliver transgenes to patients.

The strategy of employing targeted gene delivery to the DRG to treat pain is not limited to preproenkephalin. We have constructed a synthetic gene cassette that produces endomorphin peptides, the endogenous agonists for the µ-(morphine) opioid receptor. The endomorphin-expressing replication-defective HSV vector provides significant analgesic effects in models of neuropathic [9] and inflammatory pain [10], and, through actions on the µ-opioid receptor, is likely to have anti-inflammatory effects as well. Transduction of DRG with an HSV vector expressing glutamic acid decarboxylase resulted in constitutive release of γ-aminobutyric acid from afferent terminals in the dorsal horn and reduced pain-related behaviors in animal models of neuropathic pain resulting from nerve injury or from diabetes [11,12]. In addition to inhibitory neurotransmitters, HSV vectors expressing anti-inflammatory proteins IL-4, IL-10 and the TNF soluble receptor have demonstrated reproducible analgesic effects; HSV-delivered RNA interference can be used to knock down expression of genes in the DRG that are involved in maintaining chronic pain.

Herpes simplex virus is not the only vector that can be used to modify pain-related behaviors. In animal studies, gene transfer achieved by intrathecal injection of nonviral plasmid-based gene transfer vectors, as well as adenovirus-, adeno-associated virus (AAV)-, and retroviral-based gene transfer systems have been shown to provide analgesic effects in a wide range of models of pain [13], presumably by producing a continuous release of peptides with analgesic effects from transduced meningeal cells. Transduction of neurons in the DRG has been demonstrated in animal models following direct injection of an AAV-based vector into DRG [14], and by intrathecal inoculation of an AAV type 8 vector [15], but none of these alternatives have yet reached human clinical trials.

NP2 exemplifies a generalizable HSV vector-mediated gene transfer platform that can be employed to deliver genes to DRG neurons by intradermal inoculation, an approach that has been termed the nerve targeting drug delivery system (NTDDS). An NTDDS vector expressing glutamic acid decarboxylase is currently progressing through preclinical evaluation and cGMP manufacturing in prelude to a clinical trial in patients with painful diabetic neuropathy. We have used the same approach to direct the local release of neuroprotective proteins from transduced DRG to protect against the progression of nerve damage [16], and an NTTDS–neurotrophic factor vector is being developed for potentially the first human trial in patients at risk for the development of chemotherapy-induced peripheral neuropathy.

Chronic pain is a difficult clinical problem, and the history of drug discovery in this field over the past several decades has not been encouraging [17,18]. Advances in the basic understanding of acute and chronic pain have greatly expanded the range of potential targets for analgesic therapy. But just as with well-established analgesic drugs, such as opiates, whose use is often limited by the widespread distribution of receptors that result in side effects from systemic administration, even newly discovered targets that may initially appear to be expressed exclusively in nociceptive pathways [19] are often found to be widely distributed. Gene transfer, if it proves successful in placebo-controlled studies, is not an approach that is likely to be useful for all forms of chronic pain. For example, gene transfer to DRG would not be applicable to diffuse pain syndromes, such as that seen in fibromyalgia, and is unlikely to be useful in central pain syndromes, such as those that occur after thalamic stroke. Nonetheless, the results of the Phase I clinical trial mark an important step in the development of this novel platform for the treatment of pain, one that we hope will ultimately add to the therapeutic choices available to physicians who treat patients with chronic pain.

Ethical conduct of research

The authors state that they have obtained appropriate institutional review board approval or have followed the principles outlined in the Declaration of Helsinki for all human or animal experimental investigations. In addition, for investigations involving human subjects, informed consent has been obtained from the participants involved.

Financial & competing interests disclosure

D Fink is co-inventor on patents that are held by the University of Pittsburgh and for which he may receive royalties. He received grant funding from Diamyd for the conduct of the human trial, but has no other financial relationships with Diamyd. D Wolfe is an employee of Diamyd and a co-inventor on patents held by the University of Pittsburgh. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.