It is impossible not to be aware of the crisis that the opioid epidemic has created in the treatment of chronic pain. While there is no question as to their efficacy, the clinical use of opioids is severely limited due to the rapid development of tolerance, dependence and the production of opioid induced respiratory depression, the major cause of opioid-induced lethality. Research programs are underway nationwide to discover and develop new non-opioid drugs that are effective analgesics without the tolerance and abuse liability ascribed to opioids. Chronic pain is especially difficult to treat due to its complex nature with a variety of different etiologies. For example, chronic pain may be produced by injury, surgery, neuropathy, the inflammation produced by arthritis or by certain drugs such as cancer chemotherapeutics. For these reasons, better management and control of chronic pain continues to be a serious need in medical practice.
Data from both preclinical and clinical studies are consistent with the idea that GABAergic neurotransmission is an important regulatory mechanism for the control of pain. Gabapentin (Neurontin®) and pregabalin (Lyrica®), two commonly used drugs for the treatment of chronic pain, are believed to produce their analgesic effects by enhancing GABAergic neurotransmission. However, although they have received FDA approval, the clinical results have not been overwhelming. In a published review of 37 clinical trials with a total of 5,914 patients experiencing neuropathic pain, there was no difference in the percentage of patients experiencing pain reduction of greater than 50% when comparing gabapentin to placebo. The most common side effects produced by gabapentin were sedation, dizziness and problems walking. It is uncertain whether greater efficacy was not observed because of poor intrinsic pharmacological efficacy or insufficient dosages due to dose limiting side effects.
An alternate approach to enhancing GABAergic neurotransmission is the use of GABAA PAMs. This approach has been under- utilized because of the general lack of efficacy of the BDZ PAMs due to dose limiting side effects. However, a strong case for the potential value of subtype selective GABAA PAMs for the treatment of pain can be made. First, GABAA receptor regulated pathways are integral to pain processing with α2/3 containing GABAA receptor subtypes present on nerve pathways modulating pain sensation and perception. Second, we believe that the analgesic properties of BDZs may be masked by concurrent activation of other GABAA receptor subtypes that mediate the side effects. Diazepam has been reported to produce maximal analgesia in rodents if the side effects are attenuated by GABAA subtype genetic manipulation. Third, KRM-II-81 and predecessor GABAkines made by Dr. Cook, which selectively amplify GABAA receptor subtype signaling, are effective in pain models in rodents at doses much lower than those producing motor side effects.
In a number of laboratory procedures and animal studies, KRM-II-81 has been shown to selectively bind to and enhance GABAergic neurotransmission at α2/3 containing GABAA receptor subtypes. Sub-chronic dosing for 22 days with KRM-II-81 and the structural analogue, MP-III- 80, demonstrated enduring analgesic efficacy without tolerance development. In contrast, tolerance developed to the analgesic effects of gabapentin. At a dose that produces maximal analgesic effect in an inflammatory chronic pain model, KRM-II-81 does not substitute for the BDZ midazolam in a drug discrimination assay, suggesting a reduced abuse liability. Furthermore, KRM-II-81 did not produce the respiratory depression observed with alprazolam, a major problem with BDZs leading to emergency room visits and overdose.
We believe that the ability to attenuate both acute and chronic pain combined with a greatly reduced side effect profile, a lack of tolerance and a reduced abuse potential makes KRM-II-81 a promising clinical lead and a potential advance in pain therapeutics. Results from preliminary chemistry, metabolism and pharmacokinetic studies support its further development.