Long-term consequences of silencing pain pathways with novel Botulinum constructs.

Pain is a protective mechanism necessary for survival that informs us of tissue damage that will require time to heal. While normally of biological benefit, pain can sometimes persist after healing or because of on-going disease and become chronic, a condition that is particularly difficult to manage with current therapeutic approaches. In Europe, chronic pain affects at least 20-30% of the population, reduces the quality of life and the ability to work, therefore presenting a huge social and economic burden. Less than 50% of chronic pain patients achieve pain relief with current pharmacological approaches and this relief is often only partial and accompanied by side effects. Although research into developing new analgesic drug therapies has been intense, translating knowledge from preclinical observations in animal models to new therapies in the clinic has been challenging.
Research into the control of chronic pain states has, however, identified neural pathways connecting the spinal cord and brain that are key to the regulation of on-going pain. It was shown previously that destroying these pathways alleviated on-going persistent pain in rats and companion dogs suffering with bone cancer pain. This approach has however not translated to human patients primarily because of clinical reluctance to kill nerve cells and the difficulties in producing the neurotoxin at the clinical grade required for this experimental approach. We therefore adopted a new approach and designed conjugates derived from Botulinum toxin A (widely known as Botox) and the neuropeptides substance P and dermorphin that silence but do not kill pain-signaling neurons. Remarkably, these constructs were safe to synthesize, nontoxic, and acted relatively quickly to silence pain-processing neurons in the spinal cord. In a series of experiments we were able to show that in animal models of inflammatory and neuropathic pain a single injection of Botulinum toxin-derived construct can silence pain-processing neurons in the spinal cord. A single injection of either of the Botulinum constructs was effective in managing pain for several weeks and importantly the constructs were as effective as morphine in reducing pain but obviously without the potential side effects of opioids.
Botulinum toxin A injections are widely used in the cosmetics industry for reducing wrinkles but the treatment is reversible lasting perhaps 3-4 months and the injection is then generally repeated. Silencing of pain-signaling neurons with Botulinum-based constructs is also assumed to be reversible but this has not been demonstrated. The aim of the current proposal is to establish the point at which our Botulinum-based silencing of pain pathways wears off and whether reinjection of the Botulinum construct will re-silence these key pain-signaling neurons without damaging spinal neurons. It will also be essential to determine that the constructs are not toxic to neurons. We will also examine the influence of sex in our models as recent research has suggested that chronic pain in female mice may have different roots and therefore that the efficacy of Botulinum based constructs silencing MOR expressing neurons or NK1R projection neurons may vary depending on sex. In summary, the translational potential for treating chronic pain in humans with these new Botulinum constructs is compelling and while we have demonstrated that the silencing of pain-signaling neurons lasts at least 40d in mice, we need to know at what point the treatment reverses and whether it can be repeated.

There is an urgent need for new pain-relieving therapies. We have previously shown in mouse models of persistent pain that a single injection of novel Botulinum toxin constructs silenced pain-processing neurons in the spinal cord and decreased mechanical hypersensitivity. We targeted either NK1R-expressing neurons that relay pain-related information from the spinal cord to the brain or the mu opiate receptor (MOR) expressing dorsal horn neurons that modulate activity in the dorsal horn. To silence NK1R- expressing neurons, we used an SP-Botulinum (SP-BOT) construct, whereas MOR expressing neurons were silenced using a dermorphin-Botulinum (Derm-BOT) construct. We found that the new constructs were selectively internalized after binding to their target receptors and observed a long-term effect on mechanical pain sensitivity on both inflammatory and neuropathic pain states after a single injection of the constructs. We also found that Botulinum construct treatment was as effective as giving an intrathecal opiate. The local paralysis caused by injections of Botulinum neurotoxin A into muscle is reversible over a period of months and may be repeated if desired. The aim of the current proposal is to establish the point at which Botulinum-construct based silencing of pain pathways is lost and whether reinjection of the Botulinum constructs will re-silence these key pain-signaling neurons without damaging neurons. We will also examine the influence of sex in our models as recent research has suggested that the efficacy of morphine-mediated analgesia may be sex dependent and Botulinum based constructs may therefore be more effective in one sex than the other. The evidence for the translational potential of these Botulinum constructs is strong. But while we have demonstrated that the silencing of pain-signaling neurons lasts at least 40d in mice we need to know at what point the treatment reverses and whether it can be repeated without causing damage to spinal neurons.

Pain is a protective mechanism necessary for successful healing of an injury. While normally of biological benefit, pain can sometimes persist after healing and become chronic. In Europe, chronic pain affects at least 20-30% of the population, reduces the quality of life and the ability to work, therefore presenting a huge social and economic burden. Less than 50% of chronic pain patients achieve pain relief and this relief is often only partial and accompanied by side effects, especially in the case of the neuropathic pain that results from nerve injury. Opioid analgesics like morphine are among the most effective medications for pain management (including non-cancer pain), but they are also associated with serious and increasing public health problems, such as abuse, addiction, and deaths from opioid over-dose. The current research proposal comes out of our recent MRC funded research into new ways of treating chronic persistent pain with safe derivatives of Botulinum toxin. These Botulinum based molecules were redesigned to specifically target and silence nerve cells in the spinal cord that send pain-related messages to the brain. Pain was reduced for weeks once these nerve cells were silenced by a single injection of the Botulinum construct.
Botox, the commercial form of Botulinum neurotoxin A is, of course, widely used in cosmetic surgery paralysing facial muscles to reduce wrinkles. This effect is reversible and is often repeated every 3-4 months. Thus while we have demonstrated that the silencing of pain-signaling neurons lasts for weeks in mice we need to know at what point the treatment reverses and whether it can be repeated without causing damage to spinal neurons. We are confident that our approach has a substantial translational potential and benefit for people suffering with chronic pain. Establishing the length of time the procedure is effective, potential toxicity and repeatability are essential pieces of data for this translational journey. Finally, we will also examine the influence of sex in our models as recent research has suggested that pain-relieving treatments may be sex dependent.