Peripheral Administration of Selective Glycine Transporter-2 Inhibitor, Oleoyl-D-Lysine, Reverses Chronic Neuropathic Pain but Not Acute or Inflammatory Pain in Male Mice.

Aberrations in spinal glycinergic signaling are a feature of pain chronification. Normalizing these changes by inhibiting glycine transporter (GlyT)-2 is a promising treatment strategy. However, existing GlyT2 inhibitors (e.g., ORG25543) are limited by narrow therapeutic windows and severe dose-limiting side effects, such as convulsions, and are therefore poor candidates for clinical development. Here, intraperitoneally administered oleoyl-D-lysine, a lipid-based GlyT2 inhibitor, was characterized in mouse models of acute (hot plate), inflammatory (complete Freund's adjuvant), and chronic neuropathic (chronic constriction injury) pain. Side effects were also assessed on a numerical rating score, convulsions score, for motor incoordination (rotarod), and for respiratory depression (whole body plethysmography). Oleoyl-D-lysine produced near complete antiallodynia for chronic neuropathic pain, but no antiallodynia/analgesia in inflammatory or acute pain. No side effects were seen at the peak analgesic dose, 30 mg/kg. Mild side effects were observed at the highest dose, 100 mg/kg, on the numerical rating score, but no convulsions. These results contrasted markedly with ORG25543, which reached less than 50% reduction in allodynia score only at the lethal/near-lethal dose of 50 mg/kg. At this dose, ORG25543 caused maximal side effects on the numerical rating score and severe convulsions. Oleoyl-D-lysine (30 mg/kg) did not cause any respiratory depression, a problematic side effect of opiates. These results show the safe and effective reversal of neuropathic pain in mice by oleoyl-D-lysine and provide evidence for a distinct role of glycine in chronic pain over acute or short-term pain conditions. SIGNIFICANCE STATEMENT: Partially inhibiting glycine transporter (GlyT)-2 can alleviate chronic pain by restoring lost glycinergic function. Novel lipid-based GlyT2 inhibitor ol-D-lys is safe and effective in alleviating neuropathic pain, but not inflammatory or acute pain. Clinical application of GlyT2 inhibitors may be better suited to chronic neuropathic pain over other pain aetiologies.

Low intrinsic efficacy for G protein activation can explain the improved side effect profiles of new opioid agonists.

Biased agonism at G protein-coupled receptors describes the phenomenon whereby some drugs can activate some downstream signaling activities to the relative exclusion of others. Descriptions of biased agonism focusing on the differential engagement of G proteins versus ß-arrestins are commonly limited by the small response windows obtained in pathways that are not amplified or are less effectively coupled to receptor engagement, such as ß-arrestin recruitment. At the µ-opioid receptor (MOR), G protein-biased ligands have been proposed to induce less constipation and respiratory depressant side effects than opioids commonly used to treat pain. However, it is unclear whether these improved safety profiles are due to a reduction in ß-arrestin-mediated signaling or, alternatively, to their low intrinsic efficacy in all signaling pathways. Here, we systematically evaluated the most recent and promising MOR-biased ligands and assessed their pharmacological profile against existing opioid analgesics in assays not confounded by limited signal windows. We found that oliceridine, PZM21, and SR-17018 had low intrinsic efficacy. We also demonstrated a strong correlation between measures of efficacy for receptor activation, G protein coupling, and ß-arrestin recruitment for all tested ligands. By measuring the antinociceptive and respiratory depressant effects of these ligands, we showed that the low intrinsic efficacy of opioid ligands can explain an improved side effect profile. Our results suggest a possible alternative mechanism underlying the improved therapeutic windows described for new opioid ligands, which should be taken into account for future descriptions of ligand action at this important therapeutic target.