L-655,708 (FG-8094) is a
nootropic drug invented in 1996 by a team working for
Merck, Sharp and Dohme, that was the first compound developed which acts as a subtype-selective inverse agonist at the
α5 subtype of the
benzodiazepine binding site on the
GABAAreceptor.[1] It acts as an
inverse agonist at the
α1,
α2,
α3 and
α5 subtypes, but with much higher affinity for α5, and unlike newer α5 inverse agonists such as
α5IA, L-655,708 exerts its subtype selectivity purely via higher
binding affinity for this receptor subtype, with its efficacy as an inverse agonist being around the same at all the subtypes it binds to.[2]
A
radiolabelled form of L-655,708 was used to map the distribution of the GABAA α5 subtype in the brain, and it was found to be expressed predominantly in the
hippocampus,[3] an area of the brain involved with learning and memory. Activation of this subtype is thought to be largely responsible for producing the cognitive side effects displayed by many benzodiazepine and
nonbenzodiazepine drugs, such as
amnesia and difficulties with learning and memory, and so this led researchers to conclude that a drug acting as an inverse agonist at this subtype should have the opposite effect and enhance learning and memory.[4][5]
L-655,708 was indeed found to produce improved cognitive performance in animal studies, without producing the side effect of
convulsions which is produced by non-selective inverse agonists like
DMCM.[6] However it was found to be
anxiogenic at doses which enhanced cognition,[7] most likely because of its inverse agonist effects on other subtypes such as α2 and α3, making it unlikely that this drug would be suitable for use as a
nootropic in humans. Still, L-655,708 may find use in the clinic to combat
postoperative cognitive dysfunction since administration of sub-nootropic doses of L-655,708 prevented persistent memory impairment in mice anesthetized with
isoflurane.[8]
^Quirk K, Blurton P, Fletcher S, Leeson P, Tang F, Mellilo D, et al. (1996). "[3H]L-655,708, a novel ligand selective for the benzodiazepine site of GABAA receptors which contain the alpha 5 subunit". Neuropharmacology. 35 (9–10): 1331–5.
doi:
10.1016/S0028-3908(96)00061-5.
PMID9014149.
S2CID21608179.
^Casula MA, Bromidge FA, Pillai GV, Wingrove PB, Martin K, Maubach K, et al. (April 2001). "Identification of amino acid residues responsible for the alpha5 subunit binding selectivity of L-655,708, a benzodiazepine binding site ligand at the GABA(A) receptor". Journal of Neurochemistry. 77 (2): 445–51.
doi:
10.1046/j.1471-4159.2001.00289.x.
PMID11299307.
S2CID84325916.
^Sur C, Fresu L, Howell O, McKernan RM, Atack JR (March 1999). "Autoradiographic localization of alpha5 subunit-containing GABAA receptors in rat brain". Brain Research. 822 (1–2): 265–70.
doi:
10.1016/S0006-8993(99)01152-X.
PMID10082908.
S2CID54351270.
^Chambers MS, Atack JR, Broughton HB, Collinson N, Cook S, Dawson GR, et al. (May 2003). "Identification of a novel, selective GABA(A) alpha5 receptor inverse agonist which enhances cognition". Journal of Medicinal Chemistry. 46 (11): 2227–40.
doi:
10.1021/jm020582q.
PMID12747794.
^Chambers MS, Atack JR, Carling RW, Collinson N, Cook SM, Dawson GR, et al. (November 2004). "An orally bioavailable, functionally selective inverse agonist at the benzodiazepine site of GABAA alpha5 receptors with cognition enhancing properties". Journal of Medicinal Chemistry. 47 (24): 5829–32.
doi:
10.1021/jm040863t.
PMID15537339.
^Atack JR, Bayley PJ, Seabrook GR, Wafford KA, McKernan RM, Dawson GR (November 2006). "L-655,708 enhances cognition in rats but is not proconvulsant at a dose selective for alpha5-containing GABAA receptors". Neuropharmacology. 51 (6): 1023–9.
doi:
10.1016/j.neuropharm.2006.04.018.
PMID17046030.
S2CID2549642.
^Navarro JF, Burón E, Martín-López M (December 2002). "Anxiogenic-like activity of L-655,708, a selective ligand for the benzodiazepine site of GABA(A) receptors which contain the alpha-5 subunit, in the elevated plus-maze test". Progress in Neuro-Psychopharmacology & Biological Psychiatry. 26 (7–8): 1389–92.
doi:
10.1016/S0278-5846(02)00305-6.
PMID12502028.
S2CID53188364.