Targeting N-methyl-D-aspartate 2B receptor in painful diabetic neuropathy – mechanisms, challenges, and emerging therapeutics

Table 1 Clinical and preclinical findings of N-methyl-D-aspartate receptor-2B involvement in the pathogenesis of diabetic neuropathy

Ref.	Research	Clinical/preclinical model type	Clinical/preclinical model duration	Sample	Key outcome
Ismail et al[4], 2019	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	Increased p-NMDAR-2B and t-NMDAR-2B protein expression in the spinal cord of DPN rats with aberrant thermal hyperalgesia and formalin-induced chemical hyperalgesia responses, which were significantly suppressed with a seven-day intrathecal injection of ifenprodil (0.5 µg/µL)
Ismail et al[12], 2022	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	Painless DN rats exhibited notably less formalin-induced flinching and licking, which was linked to reduced spinal phosphorylated and total NMDAR-2B protein levels compared to DPN and control groups
Uniyal et al[21], 2022	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	A considerable increment in formalin-induced flinching during Phase 1 (represented by peripheral nociceptive changes) and early and late Phase 2 (represented by central nociceptive changes), which were associated with increased spinal p-NMDAR-2B and t-NMDAR-2B expression; minocycline (a selective microglia OX-42 inhibitor) substantially suppressed the responses and phosphorylated and total NMDAR-2B expression; nevertheless, the substance did not exert any effects on thermal hyperalgesia
Suo et al[23], 2016	Preclinical	High-fat diet-induced prediabetic mice	24 weeks	Mouse spinal cord tissue	Tactile allodynia and thermal hypoalgesia were developed in pre-diabetic wild-type mice, which correlated with elevated NMDAR-2B expression and activation and Fyn-NMDAR-2B interaction in the spinal cord; the ro 25-6981 (selective NMDAR-2B, i.t.) also demonstrated alleviated tactile allodynia but not thermal hypoalgesia in a dose-dependent manner
Lu et al[24], 2020	Preclinical	Type 2 high-fat diet (HFD)-induced DPN rat	8 weeks HFD	Rat spinal cord tissue	Levels of kalirin-7, p-NMDAR-2B, PSD-95, and PSD-95-NMDAR-2B coupling were significantly improved in kalirin-knockout mice with type-2 DPN with diminished mechanical allodynia and thermal hyperalgesia, suggesting that spinally expressed kalirin-7 could contribute to type-2 DPN by regulating PSD-95/NMDAR-2B interaction-dependent NMDAR-2B phosphorylation in the spinal cord
Li et al[25], 2019	Preclinical	STZ-induced type 2 DPN rat (in vivo); BV2 immortalised murine microglia cell line (in-vitro)	14 days; 24 hours incubation	Rat spinal cord tissue; BV2 immortalised murine microglia cell line	The p-JAK2, p-STAT3, total-CAV-1, and p-NMDAR-2B were upregulated in the spinal cord dorsal horn of the DPN rat with enhanced mechanical and thermal hyperalgesia development; intrathecal injection of the JAK2 inhibitor significantly suppressed the expression of the markers and thereby reduced pain responses; in vitro, a high glucose environment markedly induced activation of p-STAT3 in microglia and upregulated p-CAV-1 and p-NMDAR-2B in neurons
Shiers et al[26], 2024; Nakazawa et al[28], 2001	Clinical	-	-	Human DRG tissue	Histological and spatial RNA analyses of human DRGs from DPN donors showed damaged peripherin-positive axons and Nageotte nodules (support and immune cell clusters); ligand-receptor interactions in DRG neurons linked to DN mechanisms were also identified
Shiers et al[26], 2024	Preclinical	STZ-induced diabetic neuropathy rats	3 weeks	Rat spinal cord tissue	The DN rats exhibited lower paw withdrawal threshold, displaying notably higher expressions of NMDAR-2B and p-CREB in the spinal cord dorsal horn, which were abolished by intrathecal injections of baclofen (specific GABAB receptor antagonist), suggesting that the activation of spinal GABAB receptors activation normalises NMDAR-2B expression in DN
Fajrin et al[29], 2020	Preclinical	STZ-induced DPN mice model	28 days	Mouse sciatic nerve and spinal cord tissue	Spinal NMDAR-2B and TRPV1 mRNA expression was significantly downregulated in the mice treated with 6-shogaol, with improved thermal hyperalgesia, allodynia, and pain-induced mechanical pressure
Liu et al[30], 2014	Preclinical	STZ-induced diabetic neuropathic pain rats	3 weeks	Rat spinal cord tissue	Baclofen substantially improved paw withdrawal threshold and thermal withdrawal latency, with significant downregulation of mRNA and reduced protein expression of spinal NMDAR-2B and p-CREB in DPN rats; the data postulated that GABAB activation by baclofen might attenuate diabetic neuropathic pain partly via the downregulation of p-CREB and NMDAR-2B expression

CAV-1: Caveolin-1; DN: Diabetic neuropathy; DRG: Dorsal root ganglia; GABAB: Gamma-amino butyric acid B receptor; NMDAR-2B: N-methyl-D-aspartate-2B; OX-42: CD11b protein; p-CAV-1: Phosphorylated caveolin-1; p-CREB: Phosphorylated cyclic AMP-responsive element binding protein; p-JAK2: Phosphorylated Janus kinase 2; p-NMDAR-2B: Phosphorylated N-methyl-D-aspartate-2B receptor; PDN: Painful diabetic neuropathy; PSD-95: Postsynaptic density-95; STAT3: Signal transducer and activator of transcription 3; t-NMDAR-2B: Total N-methyl-D-aspartate-2B receptor; TRPV1: Transient receptor potential vanilloid-1.

Table 2 Success or failures of clinical investigations on N-methyl-D-aspartate receptor-2B antagonists from previous clinical trials

Ref.	NMDAR-2B antagonist	Clinical indication	Study phase/type of clinical trial	Outcome summary
Preskorn et al[44], 2008; Machado-Vieira et al[45], 2017	Traxoprodil (CP-101, 106)	Treatment-refractory major depressive disorder	Randomised, double-blind study	Traxoprodil produced an exceptional antidepressant response, well-tolerated without producing a dissociative reaction in patients. Nevertheless, further development of traxoprodil was not pursued due to concern for potential cardiovascular toxicity risk via QTc prolongation
Yurkewicz et al[46], 2005	Traxoprodil (CP-101, 106)	Severe traumatic brain injury	Randomised, double-blind study	Traxoprodil failed to demonstrate any mortality rate improvement or a favourable outcome that achieved statistical significance. No definitive claim of efficacy was made for traxoprodil for the treatment of severe TBI
Merchant et al[47], 1999	Traxoprodil (CP-101, 106)	Mild or acute moderate traumatic brain injury or hemorrhagic stroke	Double blind, placebo-controlled study	Minimal adverse effects of traxoprodil were reported in healthy subjects at 3 mg/kg/hour for 72 hours. The 72-hour-infused taxoprodil exerted no psychotropic effects in mild or moderate TBI or hemorrhagic stroke, but is well-tolerated by patients
Kotajima-Murakami et al[48], 2022	Ifenprodil	Methamphetamine use disorder	Exploratory, randomised, double-blind, placebo-controlled study	Ifenprodil at 120 mg/day showed safety and efficacy in reducing emotionality problems but exerted no effect on primary or secondary outcomes (such as drug use status, relapse risk, drug craving, and methamphetamine in urine)
Sugaya et al[49], 2018	Ifenprodil	Alcohol dependence	Prospective, randomised, controlled, rater-blinded study	Ifenprodil (60 mg/day for 3 months) significantly lowers the rate of heavy drinking in patients with alcohol dependence
Sasaki et al[50], 2022	Ifenprodil	Adolescent post-traumatic stress disorder (PTSD)	Randomised, double-blind, placebo-controlled trial	Ifenprodil can be a short-term, effective, safe alternative treatment for adolescent patients with PTSD
Danton et al[51], 2004	Ifenprodil	Stroke	Phase 2 clinical trial	Although eliprodil demonstrated a safer side-effect profile with promising findings in Phase 2 clinical trials; however, no considerable effect was observed in patients with stroke
Jain et al[52], 2000	Ifenprodil	Acute ischaemic stroke	Phase 3 clinical trial	Treatment of eliprodil in 8 hours (time window) for 14 14-day duration of treatment indicated no functional outcome improvement at 3 months. This clinical trial was discontinued, thus unpublished, due to lack of efficacy in sequential analysis
Branchereau and Rouffy[53], 1995	Ifenprodil	Chronic arterial occlusive disease of the legs (Fontaine stage II)	Double-blind randomised controlled trial	Ifenprodil tartrate (60 mg a.d.) improved maximum walking distance in patients. Nonetheless, no considerable evolution of ankle/brachial systolic post detected compared to placebo, ifenprodil exhibited excellent clinical and biological tolerance in the patients

PTSD: Post-traumatic stress disorder; TBI: Traumatic brain injury.