Future Directions in Pain Management: Integrating Anatomically Selective Delivery Techniques With Novel Molecularly Selective Agents

Comparison of the efficiencies of intrathecal and intraganglionic injections in mouse dorsal root ganglion

Background/aim

Dorsal root ganglia (DRG) are structures containing primary sensory neurons. Intraganglionic (IG) and intrathecal (IT) applications are the most common methods used for viral vector transfer to DRG. We aim to compare the efficiencies and pathological effects of IT and IG viral vector delivery methods to DRG, through in vivo imaging.

Materials and methods

Mice were divided into four groups of six each: IT, IG, IT-vehicle, and IG-vehicle. Adeno-associated virus (AAV) injection was performed for EGFP expression in IT/IG groups. DRGs were made visible through vertebral window surgery and visualized with multiphoton microscopy. After imaging, spinal cords and DRGs were removed and cleared, then imaged with light sheet microscopy.

Results

No neuronal death was observed after IT injection, while the death rate was 17% 24 h after IG injection. EGFP expression efficiencies were 90%-95% of neurons in both groups. EGFP expression was only observed in targeted L2 DRG after IG injection, while it was observed in DRGs located between L1-L5 levels after IT injection.

Conclusion

IT injection is a more suitable method for labeling DRG neurons in neurodegenerative injury models. However, when the innervation of DRG needs to be specifically studied, IT injection reduces this specificity due to its spread. In these studies, IG injection is the most suitable method for labeling single DRG neurons.

Targeting intrinsically disordered regions facilitates discovery of calcium channels 3.2 inhibitory peptides for adeno-associated virus-mediated peripheral analgesia

ABSTRACT

Ample data support a prominent role of peripheral T-type calcium channels 3.2 (Ca V 3.2) in generating pain states. Development of primary sensory neuron-specific inhibitors of Ca V 3.2 channels is an opportunity for achieving effective analgesic therapeutics, but success has been elusive. Small peptides, especially those derived from natural proteins as inhibitory peptide aptamers (iPAs), can produce highly effective and selective blockade of specific nociceptive molecular pathways to reduce pain with minimal off-target effects. In this study, we report the engineering of the potent and selective iPAs of Ca V 3.2 from the intrinsically disordered regions (IDRs) of Ca V 3.2 intracellular segments. Using established prediction algorithms, we localized the IDRs in Ca V 3.2 protein and identified several Ca V 3.2iPA candidates that significantly reduced Ca V 3.2 current in HEK293 cells stably expressing human wide-type Ca V 3.2. Two prototype Ca V 3.2iPAs (iPA1 and iPA2) derived from the IDRs of Ca V 3.2 intracellular loops 2 and 3, respectively, were expressed selectively in the primary sensory neurons of dorsal root ganglia in vivo using recombinant adeno-associated virus (AAV), which produced sustained inhibition of calcium current conducted by Ca V 3.2/T-type channels and significantly attenuated both evoked and spontaneous pain behavior in rats with neuropathic pain after tibial nerve injury. Recordings from dissociated sensory neurons showed that AAV-mediated Ca V 3.2iPA expression suppressed neuronal excitability, suggesting that Ca V 3.2iPA treatment attenuated pain by reversal of injury-induced neuronal hypersensitivity. Collectively, our results indicate that Ca V 3.2iPAs are promising analgesic leads that, combined with AAV-mediated delivery in anatomically targeted sensory ganglia, have the potential to be a selective peripheral Ca V 3.2-targeting strategy for clinical treatment of pain.

Platelet-rich plasma and cytokines in neuropathic pain: A narrative review and a clinical perspective

BACKGROUND AND OBJECTIVE

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. A number of preclinical studies have provided evidence for the involvement of cytokines, predominantly secreted by a variety of immune cells and by glial cells from the nervous system, in neuropathic pain conditions. Clinical trials and the use of anti-cytokine drugs in different neuropathic aetiologies support the relevance of cytokines as treatment targets. However, the use of such drugs, in particularly biotherapies, can provoke notable adverse effects. Moreover, it is challenging to select one given cytokine as a target, among the various neuropathic pain conditions. It could thus be of interest to target other proteins, such as growth factors, in order to act more widely on the neuroinflammation network. Thus, platelet-rich plasma (PRP), an autologous blood concentrate, is known to contain a natural concentration of growth factors and immune system messengers and is widely used in the clinical setting for tissue regeneration and repair.

DATABASE AND DATA TREATMENT

In the present review, we critically assess the current knowledge on cytokines in neuropathic pain by taking into consideration both human studies and animal models.

RESULTS

This analysis of the literature highlights the pathophysiological importance of cytokines. We particularly highlight the concept of time- and tissue-dependent cytokine activation during neuropathic pain conditions.

RESULTS

Conclusion: Thus, direct or indirect cytokines modulation with biotherapies or growth factors appears relevant. In addition, we discuss the therapeutic potential of localized injection of PRP as neuropathic pain treatment by pointing out the possible link between cytokines and the action of PRP.

SIGNIFICANCE

Preclinical and clinical studies highlight the idea of a cytokine imbalance in the development and maintenance of neuropathic pain. Clinical trials with anticytokine drugs are encouraging but are limited by a 'cytokine candidate approach' and adverse effect of biotherapies. PRP, containing various growth factors, is a new therapeutic used in regenerative medicine. Growth factors can be also considered as modulators of cytokine balance. Here, we emphasize a potential therapeutic effect of PRP on cytokine imbalance in neuropathic pain. We also underline the clinical interest of the use of PRP, not only for its therapeutic effect but also for its safety of use.

Peripherally acting opioid analgesics and peripherally-induced analgesia

The management of pain, particularly chronic pain, is still an area of medical need. In this context, opioids remain a gold standard for the treatment of pain. However, significant side effects, mainly of central origin, limit their clinical use. Here, we review recent progress to improve the therapeutic and safety profiles of opioids for pain management. Characterization of peripheral opioid-mediated pain mechanisms have been a key component of this process. Several studies identified peripheral µ, δ, and κ opioid receptors (MOR, DOR, and KOR, respectively) and nociceptin/orphanin FQ (NOP) receptors as significant players of opioid-mediated antinociception, able to achieve clinically significant effects independently of any central action. Following this, particularly from a medicinal chemistry point of view, main efforts have been directed towards the peripheralization of opioid receptor agonists with the objective of optimizing receptor activity and minimizing central exposure and the associated undesired effects. These activities have allowed the characterization of a great variety of compounds and investigational drugs that show low central nervous system (CNS) penetration (and therefore a reduced side effect profile) yet maintaining the desired opioid-related peripheral antinociceptive activity. These include highly hydrophilic/amphiphilic and massive molecules unable to easily cross lipid membranes, substrates of glycoprotein P (a extrusion pump that avoids CNS penetration), nanocarriers that release the analgesic agent at the site of inflammation and pain, and pH-sensitive opioid agonists that selectively activate at those sites (and represent a new pharmacodynamic paradigm). Hopefully, patients with pain will benefit soon from the incorporation of these new entities.

MRI guidance technology development in a large animal model for hyperlocal analgesics delivery to the epidural space and dorsal root ganglion

BACKGROUND

Development of new analgesic drugs or gene therapy vectors for spinal delivery will be facilitated by "hyperlocal" targeting of small therapeutic injectate volumes if spine imaging technology can be used that is ready for future clinical translation.

NEW METHOD

This study provides methods for MRI-guided drug delivery to the periganglionic epidural space and the dorsal root ganglion (DRG) in the Yucatan swine.

RESULTS

Phantom studies showed artifact-corrected needle localization with frequency encoding parallel to the needle shaft, while maximizing bandwidth (125 KHz) minimized needle artifact. A custom constructed 8-12 element surface coil (phased array) wrapped over the spine in conjunction with lateral recumbent positioning achieved diagnostic quality signal to noise ratio at the depth of the DRG and afforded transforaminal access via anterolateral or posterolateral vectors, as well as interlaminar access. Swine epidural anatomy was homologous with human anatomy. Injectate containing 2% gadolinium allowed imaging of injectate volumes in increments as small as 10 microliters and discrimination of epidural flow from intraparenchymal injectate delivery into a DRG. All technical and technological elements of the procedure appear clinically translatable.

COMPARISON WITH EXISTING METHODS

Computed tomographic or fluoroscopic guidance cannot directly visualize drug delivery into the DRG due to contrast medium toxicity, nor reliably identify epidural injection volumes of < 50 microliters.

CONCLUSIONS

MRI-guided hyperlocal delivery in swine provides a translatable and faithful model of future human spinal novel drug- or gene therapy vector delivery.

Laminotomy for Lumbar Dorsal Root Ganglion Access and Injection in Swine

Dorsal root ganglia (DRG) are anatomically well defined structures that contain all primary sensory neurons below the head. This fact makes DRG attractive targets for injection of novel therapeutics aimed at treating chronic pain. In small animal models, laminectomy has been used to facilitate DRG injection because it involves surgical removal of the vertebral bone surrounding each DRG. We demonstrate a technique for intraganglionic injection of lumbar DRG in a large animal species, namely, swine. Laminotomy is performed to allow direct access to DRG using standard neurosurgical techniques, instruments, and materials. Compared with more extensive bone removal via laminectomy, we implement laminotomy to conserve spinal anatomy while achieving sufficient DRG access. Intraoperative progress of DRG injection is monitored using a non-toxic dye. Following euthanasia on post-operative day 21, the success of injection is determined by histology for intraganglionic distribution of 4',6-diamidino-2-phenylindole (DAPI). We inject a biologically inactive solution to demonstrate the protocol. This method could be applied in future preclinical studies to target therapeutic solutions to DRG. Our methodology should facilitate testing the translatability of intraganglionic small animal paradigms in a large animal species. Additionally, this protocol may serve as a key resource for those planning preclinical studies of DRG injection in swine.