Table 1.

List of GPCII/NAALADase inhibitor (2‐PMPA) studies focussing on neurological disorders

Neurological disorder	Dose	Route	Animal model	End‐point comments	References
Neuropathic pain	50 and 100 mg·kg−1	IV	Sprague–Dawley rats	Significant increase in paw‐withdraw threshold in rats with partial ligation of the left from less than 5 g at 45 min to ~14 g and 19 g, respectively.	(Chen et al., 2002)
				Significant decrease in the spontaneous ectopic discharges recorded from injured sciatic afferent nerves from ~15 imp.·s‐1 for the control group to ~10 imp.·s‐1 and ~5 imp.·s‐1, respectively, for treatment groups
				Significant improved allodynia induced by sciatic nerve ligation
Chronic pain	50 mg·kg−1	IP	Wistar rats/Sprague–dawley rats	After 2‐PMPA dose administration, the maximum concentration achieved in brain ECF was 30 μM.	(Nagel et al., 2006)
				Following 2‐PMPA administration, a steady increase of NAAG in brain ECF was observed, reaching a peak concentration of 3 μM.
				The study concludes that the low concentration of NAAG in the brain ECF is unlikely to have an effect on any known target, and so they say that the effect of inhibitor is unlikely to be mediated by either NAAG or mGluR3.
Diabetic neuropathy	n/a	n/a	n/a	2‐PMPA rescued the dorsal root ganglion neurons from glucose induced programmed cell death	(Berent‐Spillson et al., 2004)
				mGluR3 antagonist EGLU reversed the 2‐PMPA protection profile in three assays testing for apoptosis; caspase‐3 and −9 cleavage, and TUNEL
				This reversal was not seen with either the group I mGluR antagonist AIDA, nor the group 3 mGluR antagonist MSOP.
				At high glucose concentrations, 2‐PMPA was able to maintain neurite growth and viability.
Inflammatory pain	100 μg	IT	Sprague–Dawley rats	In the rat formalin test (model for inflammatory pain), 2‐PMPA attenuated the phase 1 and 2 biphasic flinching when rats were pre‐treated with inhibitor prior to the formalin test.	(Yamamoto et al., 2001b)
				In the post‐treatment test, 2‐PMPA had no effect on biphasic flinching.
				2‐PMPA did not have an effect on the reaction time of the rats in the hot plate test
Inflammatory pain	10 μg	ICV	Sprague–Dawley rats	Significant reduction in the phase 1 and 2 biphasic flinching in the rat formalin test with 2‐PMPA.	(Yamamoto et al., 2008)
				These effects were attenuated following the administration of group II mGluR antagonist LY341495
Pain	100–400 μg	IT	Sprague–Dawley rats	2‐PMPA improved the level of allodynia induced by paw carrageenan injection.	(Yamamoto et al., 2001a)
				However, this improvement was not observed for other models of pain (i.e. skin incision or mild thermal injury.)
Peripheral pain	1–100 μg	SC	Sprague–Dawley rats	Group II mGluR agonists (SLX‐3095‐1 and APDC) and 2‐PMPA reduced the pain response in the rat formalin test.	(Yamamoto et al., 2007)
				Group II mGluR agonists (SLX‐3095‐1, NAAG and APDC) and 2‐PMPA reduced the pain response in the carrageenan model of induced allodynia
				These effects were attenuated following the administration of group II mGluR antagonist LY341495
Brain injury from soman	50 mg·kg−1	IP	Sprague–Dawley rats	The nerve agent soman was shown to reduce NAAG levels in regions of the brain following exposure	(Guo et al., 2015)
				The administration of 2‐PMPA reduced neuronal cell death in some (but not all) regions of the brain following exposure to soman; these regions include the entorhinal cortex, piriform cortex and the amygdala
Spinal cord injury	1–4 μM	subarachnoid space	Sprague–Dawley rats	The co‐administration of somatostatin with 2‐PMPA and dynorphin A (causes ischemic spinal cord injury) along with 2‐PMPA as a spinal subarachnoid injection resulted in improved hindlimb motor scores by 24 h post injection.	(Long et al., 2005)
				Significant reduction of the extracellular concentrations of glutamate in cerebrospinal fluid was observed when 2‐PMPA was co‐administered with dynorphin A, in comparison to mice treated with dynorphin A alone
Ketamine‐induced neurotoxicity	n/a	n/a	n/a	2‐PMPA attenuated the decrease in cell viability seen with 2 mM of ketamine in neuron–glia mixed culture by decreasing the loss of nodes/cell from <0.5 with 50 μM of 2‐PMPA to >1.00 with 100 μM of 2‐PMPA but the same effect was not observed in cultures of neurons alone.	(Zuo et al., 2014)
				The authors conclude that glia cells must be involved in the neuroprotective effect that 2‐PMPA offers.
Neurotoxicity	n/a	n/a	n/a	The degree of neuroprotection offered by 2‐PMPA in an in vitro model of primary neurons (neurotoxicity) from rat embryos was 100%, 46%, 16% and 0% for hypoxia, glutamate, NMDA injury and veratridine‐induced injury	(Tortella et al., 2000)
Alcohol	50, 100 and 200 mg·kg−1	IP	Alcohol preferring P rat	Significant reduction in the consumption of ethanol by alcohol preferring rats by ~25% during their daily 1‐hour access to 10% (v/v) ethanol following treatment with 2‐PMPA	(McKinzie et al., 2000)
Cocaine	0.01–100 μM	Local exposure	Planarians (Dugesia dorotocephala)	2‐PMPA attenuated the C‐like hyperkinesias (i.e., stereotypical counts) induced in planarians by four compounds; glutamate, NMDA, pilocarpine and cocaine	(Tallarida et al., 2012)
Cocaine	100 mg·kg−1 and 30 mg·kg−1	IP / PO	Sprague–Dawley rats	The acquisition of the conditioned place preference (CPP) response was blocked by either 100 mg·kg−1 2‐PMPA i.p. or 30 mg·kg−1 p.o. of GPI 5693; an orally bioavailable NAALADase inhibitor. Administration of either drug in the absence of cocaine did not significantly alter the time spent in each chamber.	(Slusher et al., 2001)
				2‐PMPA did not significantly reduce the CPP response that has been shown to be induced by food.
Cocaine	10, 30 and 100 mg·kg−1	IP	Long‐Evans rats	Inhibition of intravenous self‐administration of cocaine.	(Xi et al., 2010b)
				Dose‐dependent reduction in extracellular dopamine and glutamate after 2‐PMPA administration.
				Successful prevention of cocaine‐induced reinstatement of drug‐seeking behaviour
Cocaine	1, 10, 30 and 100 mg·kg−1	IP	Long‐Evans rats	Significant inhibition of cocaine self‐administration under progressive‐ratio (PR) reinforcement conditions	(Xi et al., 2010a)
				Significant inhibition of cocaine‐enhanced brain‐stimulation reward (BSR) in rats.
				Dose‐dependent reduction in cocaine‐induced extracellular dopamine in the nucleus accumbens (NAc).
Cocaine	100 mg·kg−1	IP	Sprague–Dawley rats	Increased locomotor activity of the rats following 2‐PMPA administration	(Shippenberg et al., 2000)
				No significant difference in the distance travelled between the control (saline) and the mice pre‐treated with 2‐PMPA, following cocaine administration after 15 min
				2‐PMPA prevents the sensitization that develops to the locomotor activity, following cocaine use.
Cocaine‐kindled seizures	10, 30 and 100 mg·kg−1	IP	Swiss‐Webster mice	2‐PMPA was shown to prevent the expression and development of cocaine kindling.	(Witkin et al., 2002)
				Mice pretreated with 2‐PMPA showed a lower % of mice convulsing when challenged with 60 mg·kg−1 of cocaine following 10 days of treatment with 40 mg·kg−1 of cocaine.
				The inhibitor dose did not alter the behaviour of the mice in the inverted screen test and locomotor activity.
Ischemic brain injury	500 mg·kg−1	IV	Sprague–Dawley rats	Reduction in the extent of injury (magnitude of protection was 54%) in an established focal ischemia model.	(Jackson et al., 1996)
				Using microdialysis, it was shown that the levels of glutamate in the mice treated with inhibitor showed no increase following middle cerebral artery occlusion (MCAO).
				In non‐ischemic mice, the inhibitor did not alter basal levels of glutamate
Ischemic strokes	n/a	n/a	n/a	Ischemic strokes result in an increase in the extracellular concentration of glutamate, which tends to overactivate the N‐methyl‐D‐aspartate receptors (NMDARs)	(Khacho et al., 2015)
				The inhibitor was shown to increase the amplitude of the synaptic NMDAR excitatory postsynaptic currents (EPSCs) while decreasing the extrasynaptic NMDAR ESPCs.
Schizophrenia	50–150 mg·kg−1	IP	C57BL/6 and DBA/2 mice	Group II agonist LY354740 was able to moderate the effects of PCP on prepulse inhibition (PPI) of acoustic startle in DBA/2 but not C57BL/6 mice	(Profaci et al., 2011)
				2‐PMPA was unable to affect the PCP‐mediated PPI in either strain.
Schizophrenia	100 mg·kg−1	IP	C57BL/6 mice, DBA/2 mice and Sprague–Dawley rats	2‐PMPA reduced the motor activation in PCP or d‐amphetamine induced models of schizophrenia	(Olszewski et al., 2012)
				These effects were not observed in mice that are homozygous for a deletion in GCPII
				2‐PMPA attenuated the effect of dizocilpine (MK‐801) in an object recognition test; a model for the cognitive impairment seen in schizophrenia
Memory Process	150 mg·kg−1	IP	Swiss‐Webster mice	No negative impact of the dose on long term memory in the passive avoidance task.	(Łukawski et al., 2008)
				Increased latency to enter the dark box on training day from ~24 s in control mice to ~103 s in treated mice.
				The dose impaired spontaneous alternation in the Y‐maze task.
				NAALADase inhibition may impair alternation behaviour
Memory Process	0.2–100 mg·kg−1	IP	C57BL/6 mice	In comparison to the control mice, the mice treated with 2‐PMPA spent significantly more time exploring the novel object compared with the familiar object on day two.	(Janczura et al., 2013)
				Mice that were homozygous for a deletion in GCPII displayed the same results as those treated with the inhibitors.
Neurodegenerative disorders	30 mg·kg−1 for each dose	IN/IP	Wistar rats, Cynomolgus monkey	i.n. administration showed higher levels of 2‐PMPA accumulation in olfactory bulb and the cerebellum compared with i.p. administration, but had similar plasma profiles.	(Rais et al., 2015)
				In a non‐human primate study, 2‐PMPA achieved micromolar concentrations following i.n. delivery.