Discovery of Drugs to Treat Chronic Pain

• Glycine is an inhibitory neurotransmitter found in the CNS, predominantly in the spinal cord
• Stimulation of GlyR with glycine causes Cl- channels to open, causing hyperpolarisation
• Glycine transporters expressed on the pre-synaptic neuron clearance of glycine from the synapse to be repackaged and reused
• Increased GlyR stimulation can be used to treat chronic pain
• complete inhibition of the glycine transporter would disable neurotransmission as glycine from the synapse would not be able to be recycled
• N-Arachidonyl Glycine is an endogenous analgesic functioning through inhibition of the glycine transporter
• N-Arachidonyl Glycine is structurally related to endocannabinoid anandamide, but does not have any activity on cannabinoid receptors
• N-Arachidonyl Glycine is an endogenous lipid produced and found in highest concentrations in the spinal cord
• N-Arachidonyl Glycine reduces mechanical allodynia and thermal hyperalgesia in neuropathic pain
• Problems with N-Arachidonyl Glycine

  • Not very potent
  • Readily metabolised
  • Acts on a range of targets
• How do we measure inhibition of glycine transporters?

  The transport of glycine is coupled with 1x Cl- and 2x Na+, so we can measure the movement of charge
• The lipid tail of N-Arachidonyl Glycine, containing four double bonds, was susceptible to oxidation (and therefore inactivation), so was reduced to having just one double bond
• This gave rise to oleoyl-D-lysine, which is significantly more potent
• Changes to improve metabolism:

  • The double bonds in the arachidonoyl tail are readily oxidised
  • L-amino acids are readily hydrolysed from the acyl tail
• Oleoyl-D-lysine is stable in the liver and plasma
• To get a drug registered, you need to test for potential off-target side effects
• Oleoyl-D-lysine was screened for activity at:

  • Other neurotransmitter transporters
  • Na+ and K+ channels
  • GCPRs
  • Enzymes
• Low level, weak activity was observed by oleoyl-D-lysine at mu opioid receptors
• Oleoyl-D-Lysine was shown to reduce pain in a rat model of chronic pain
• Oleoyl-D-Lysine has less side effects than other glycine transporter inhibitors
• Oleoyl-D-Lysine was found to accumulate in the brain
• Oleoyl-D-Lysine is a non-competitive partial inhibitor of glycine transporters
• AI based screening of many compounds to identify drug-like molecules that bind to the lipid binding site of the glycine transporter
• Thermal hyperalgesia: Heightened experience of temperature
•  Mechanical allodynia: Painful sensation caused by little stimulation