Gene therapy for chronic neuropathic pain: how does it work and where do we stand today?

Recent Advances in Management of Neuropathic, Nociceptive, and Chronic Pain: A Narrative Review with Focus on Nanomedicine, Gene Therapy, Stem Cell Therapy, and Newer Therapeutic Options

PURPOSE OF REVIEW

This manuscript summarizes novel clinical and interventional approaches in the management of chronic, nociceptive, and neuropathic pain.

RECENT FINDINGS

Pain can be defined as a feeling of physical or emotional distress caused by an external stimulus. Pain can be grouped into distinct types according to characteristics including neuropathic pain, which is a pain caused by disease or lesion in the sensory nervous system; nociceptive pain, which is pain that can be sharp, aching, or throbbing and is caused by injury to bodily tissues; and chronic pain, which is long lasting or persisting beyond 6 months. With improved understanding of different signaling systems for pain in recent years, there has been an upscale of methods of analgesia to counteract these pathological processes. Novel treatment methods such as use of cannabinoids, stem cells, gene therapy, nanoparticles, monoclonal antibodies, and platelet-rich plasma have played a significant role in improved strategies for therapeutic interventions. Although many management options appear to be promising, extensive additional clinical research is warranted to determine best practice strategies in the future for clinicians.

Gene therapy for diabetic peripheral neuropathy: A randomized, placebo-controlled phase III study of VM202, a plasmid DNA encoding human hepatocyte growth factor

VM202 is a plasmid DNA encoding two isoforms of hepatocyte growth factor (HGF). A previous phase II study in subjects with painful diabetic peripheral neuropathy (DPN) showed significant reductions in pain. A phase III study was conducted to evaluate the safety and efficacy of VM202 in DPN. The trial was conducted in two parts, one for 9 months (DPN 3-1) with 500 subjects (VM202: 336 subjects; and placebo: 164) and a preplanned subset of 101 subjects (VM202: 65 subjects; and placebo: 36) with a noninterventional extension to 12 months (DPN 3-1b). VM202 or placebo was administered to calf muscles on days 0 and 14, and on days 90 and 104. The primary end point in DPN 3-1 was change from baseline in the mean 24-h Numerical Rating Scale (NRS) pain score. In DPN 3-1b, the primary end point was safety, whereas the secondary efficacy end point was change in the mean pain score. VM202 was well-tolerated in both studies without significant adverse events. VM202 failed to meet its efficacy end points in DPN 3-1. In DPN 3-1b, however, VM202 showed significant and clinically meaningful pain reduction versus placebo. Pain reduction in DPN 3-1b was even greater in subjects not receiving gabapentin or pregabalin, confirming an observation noted in the phase II study. In DPN 3-1b, symptomatic relief was maintained for 8 months after the last injection suggesting that VM202 treatment might change disease progression. Despite the perplexing discrepancy between the two studies, the safety and long-lasting pain-relieving effects of VM202 observed in DPN 3-1b warrant another rigorous phase III study. Study Highlights WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC? Current therapies for painful diabetic peripheral neuropathy (DPN) are palliative and do not target the underlying mechanisms. Moreover, symptomatic relief is often limited with existing neuropathic pain drugs. Thus, there is a great medical need for safer and effective treatments for DPN. WHAT QUESTION DID THIS STUDY ADDRESS? Can nonviral gene delivery of hepatocyte growth factor reduce pain in patients with DPN and potentially modify progression of the disorder? WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE? Nonviral gene therapy can be used safely and practically to treat DPN. HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE? As the first gene medicine to enter advanced clinical trials for the treatment of DPN, this study provides the proof of concept of an entirely new potential approach to the disorder.

Poly(lactic-co-glycolic acid) nanomaterial-based treatment options for pain management: a review

Neuropathic pain is one of the most intense types of chronic pain; it constitutes a pervasive complaint throughout the public health system. With few effective treatments, it remains a significant challenge. Commercially available drugs for neuropathic pain are still limited and have disappointing efficacy. Therefore, chronic neuropathic pain imposes a tremendous burden on patients' quality of life. Recently, the introduction and application of nanotechnology in multiple fields has accelerated the development of new drugs. This review highlights the application of poly(lactic-co-glycolic acid) nanomaterial-based vehicles for drug delivery and how they improve the therapeutic outcomes for neuropathic pain treatment. Finally, future developments for pain research and effective management are presented.

Ex vivo nonviral gene delivery of μ-opioid receptor to attenuate cancer-induced pain

Virus-mediated gene delivery shows promise for the treatment of chronic pain. However, viral vectors have cytotoxicity. To avoid toxicities and limitations of virus-mediated gene delivery, we developed a novel nonviral hybrid vector: HIV-1 Tat peptide sequence modified with histidine and cysteine residues combined with a cationic lipid. The vector has high transfection efficiency with little cytotoxicity in cancer cell lines including HSC-3 (human tongue squamous cell carcinoma) and exhibits differential expression in HSC-3 (∼45-fold) relative to HGF-1 (human gingival fibroblasts) cells. We used the nonviral vector to transfect cancer with OPRM1, the μ-opioid receptor gene, as a novel method for treating cancer-induced pain. After HSC-3 cells were transfected with OPRM1, a cancer mouse model was created by inoculating the transfected HSC-3 cells into the hind paw or tongue of athymic mice to determine the analgesic potential of OPRM1 transfection. Mice with HSC-3 tumors expressing OPRM1 demonstrated significant antinociception compared with control mice. The effect was reversible with local naloxone administration. We quantified β-endorphin secretion from HSC-3 cells and showed that HSC-3 cells transfected with OPRM1 secreted significantly more β-endorphin than control HSC-3 cells. These findings indicate that nonviral delivery of the OPRM1 gene targeted to the cancer microenvironment has an analgesic effect in a preclinical cancer model, and nonviral gene delivery is a potential treatment for cancer pain.

Efficacy of Herpes Simplex Virus Vector Encoding the Human Preproenkephalin Gene for Treatment of Facial Pain in Mice

AIMS

To determine whether herpes simplex virus-based vectors can efficiently transduce mouse trigeminal ganglion (TG) neurons and attenuate preexisting nerve injury-induced whisker pad mechanical hypersensitivity in a trigeminal inflammatory compression (TIC) neuropathic pain model.

METHODS

Tissue transduction efficiencies of replication-conditional and replication-defective vectors to mouse whisker pads after topical administration and subcutaneous injection were assessed using quantitative real-time PCR (qPCR). Tissue tropism and transgene expression were assessed using qPCR and reverse-transcriptase qPCR following topical application of the vectors. Whisker pad mechanical sensitivities of TIC-injured mice were determined using graduated von Frey fibers before and after application of human preproenkephalin expressing replication-conditional vector (KHPE). Data were analyzed using one-way analysis of variance (ANOVA) and post hoc tests.

RESULTS

Transduction of target TGs was 8- to 50-fold greater after topical application than subcutaneous injection and ≥ 100-fold greater for replication-conditional than replication-defective vectors. Mean KHPE loads remained constant in TGs (4.5-9.8 × 10(4) copies/TG) over 3 weeks but were below quantifiable levels (10 copies/tissue) within 2 weeks of application in other nontarget cephalic tissues examined. Transgene expression in TGs was maximal during 2 weeks after topical application (100-200 cDNA copies/mL) and was below quantifiable levels (1 cDNA copy/mL) in all nontarget tissues. Topical KHPE administration reduced TIC-related mechanical hypersensitivity on whisker pads 4-fold (P < .05) for at least 1 week.

CONCLUSION

Topically administered KHPE produced a significant antinociceptive effect in the TIC mouse model of chronic facial neuropathic pain. This is the first report in which a gene therapeutic approach reduced trigeminal pain-related behaviors in an established pain state in mice.

Intrathecal administration of mesenchymal stem cells reduces the reactive oxygen species and pain behavior in neuropathic rats

BACKGROUND

Neuropathic pain induced by spinal or peripheral nerve injury is very resistant to common pain killers, nerve block, and other pain management approaches. Recently, several studies using stem cells suggested a new way to control the neuropatic pain. In this study, we used the spinal nerve L5 ligation (SNL) model to investigate whether intrathecal rat mesenchymal stem cells (rMSCs) were able to decrease pain behavior, as well as the relationship between rMSCs and reactive oxygen species (ROS).

METHODS

Neuropathic pain of the left hind paw was induced by unilateral SNL in Sprague-Dawley rats (n = 10 in each group). Mechanical sensitivity was assessed using Von Frey filaments at 3, 7, 10, 12, 14, 17, and 24 days post-ligation. rMSCs (10 µl, 1 × 10(5)) or phosphate buffer saline (PBS, 10 µl) was injected intrathecally at 7 days post-ligation. Dihydroethidium (DHE), an oxidative fluorescent dye, was used to detect ROS at 24 days post-ligation.

RESULTS

Tight ligation of the L5 spinal nerve induced allodynia in the left hind paw after 3 days post-ligation. ROS expression was increased significantly (P < 0.05) in spinal dorsal horn of L5. Intrathecal rMSCs significantly (P < 0.01) alleviated the allodynia at 10 days after intrathecal injection (17 days post-ligation). Intrathecal rMSCs administration significantly (P < 0.05) reduced ROS expression in the spinal dorsal horn.

CONCLUSIONS

These results suggest that rMSCs may modulate neuropathic pain generation through ROS expression after spinal nerve ligation.

Comparative effectiveness of antinociceptive gene therapies in animal models of diabetic neuropathic pain

Peripheral neuropathic pain is one of the most common and debilitating complications of diabetes. Several genes have been shown to be effective in reducing neuropathic pain in animal models of diabetes after transfer to the dorsal root ganglion using replication-defective herpes simplex virus (HSV)1-based vectors, yet there has never been a comparative analysis of their efficacy. We compared four different HSV1-based vectors engineered to produce one of two opioid receptor agonists (enkephalin or endomorphin), or one of two isoforms of glutamic acid decarboxylase (GAD65 or GAD67), alone and in combination, in the streptozotocin-induced diabetic rat and mouse models. Our results indicate that a single subcutaneous hindpaw inoculation of vectors expressing GAD65 or GAD67 reduced diabetes-induced mechanical allodynia to a degree that was greater than daily injections of gabapentin in rats. Diabetic mice that developed thermal hyperalgesia also responded to GAD65 or endomorphin gene delivery. The results suggest that either GAD65 or GAD67 vectors are the most effective in the treatment of diabetic pain. The vector combinations, GAD67+endomorphin, GAD67+enkephalin or endomorphin+enkephalin also produced a significant antinociceptive effect but the combination did not appear to be superior to single gene treatment. These findings provide further justification for the clinical development of antinociceptive gene therapies for the treatment of diabetic peripheral neuropathies.

The fundamental unit of pain is the cell

The molecular/genetic era has seen the discovery of a staggering number of molecules implicated in pain mechanisms [18,35,61,69,96,133,150,202,224]. This has stimulated pharmaceutical and biotechnology companies to invest billions of dollars to develop drugs that enhance or inhibit the function of many these molecules. Unfortunately this effort has provided a remarkably small return on this investment. Inevitably, transformative progress in this field will require a better understanding of the functional links among the ever-growing ranks of "pain molecules," as well as their links with an even larger number of molecules with which they interact. Importantly, all of these molecules exist side-by-side, within a functional unit, the cell, and its adjacent matrix of extracellular molecules. To paraphrase a recent editorial in Science magazine [223], although we live in the Golden age of Genetics, the fundamental unit of biology is still arguably the cell, and the cell is the critical structural and functional setting in which the function of pain-related molecules must be understood. This review summarizes our current understanding of the nociceptor as a cell-biological unit that responds to a variety of extracellular inputs with a complex and highly organized interaction of signaling molecules. We also discuss the insights that this approach is providing into peripheral mechanisms of chronic pain and sex dependence in pain.