3-phenyl-6-(2-pyridyl)methyloxy-1,2,4-triazolo[3,4-a]phthalazines and analogues: high-affinity gamma-aminobutyric acid-A benzodiazepine receptor ligands with alpha 2, alpha 3, and alpha 5-subtype binding selectivity over alpha 1.
Studies with our screening lead 5 and the literature compound 6 led to the identification of 6-benzyloxy-3-(4-methoxy)phenyl-1,2,4-triazolo[3,4-a]phthalazine 8 as a ligand with binding selectivity for the gamma-aminobutyric acid-A (GABA-A) alpha 3- and alpha 5-containing receptor subtypes over the GABA-A alpha 1 subtype (K(i): alpha 2 = 850 nM, alpha 3 = 170 nM, alpha 5 = 72 nM, alpha 1 = 1400 nM). Early optimization studies identified the close analogue 10 (K(i): alpha 2 = 16 nM, alpha 3 = 41 nM, alpha 5 = 38 nM, alpha 1 = 280 nM) as a suitable lead for further study. High-affinity ligands were identified by replacing the 6-benzyloxy group of compound 10 with 2-pyridylmethoxy (compound 29), but binding selectivity was not enhanced (K(i): alpha 2 = 1.7 nM, alpha 3 = 0.71 nM, alpha 5 = 0.33 nM, alpha 1 = 2.7 nM). Furthermore, on evaluation in xenopus oocytes,(22) 29 was discovered to be a weak to moderate inverse agonist at all four receptor subtypes (alpha 1, -7%; alpha 2, -5%; alpha 3, -16%; alpha 5, -5%). Replacement of the 3-phenyl group of 29 with alternatives led to reduced affinity, and smaller 3-substituents led to reduced efficacy. Methyl substitution of the benzo-fused ring of 29 at the 7-, 8-, and 10-positions resulted in increased efficacy although selectivity was abolished. Increased efficacy and retention of selectivity for alpha 3 over alpha 1 was achieved with the 7,8,9,10-tetrahydro-(7,10-ethano)-phthalazine 62. Compound 62 is currently one of the most binding selective GABA-A alpha 3-benzodiazepine-site partial agonists known, and although its selectivity is limited, its good pharmacokinetic profile in the rat (33% oral bioavailability after a 3 mg/kg dose, reaching a peak plasma concentration of 179 ng/mL; half-life of 1 h) made it a useful pharmacological tool to explore the effect of a GABA-A alpha 2/alpha 3 agonist in vivo.
Carling
Robert W
RW
Department of Medicinal Chemistry, The Neuroscience Research Centre, Merck Sharp and Dohme Research Laboratories, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, UK. robert.carling@merck.com
Moore
Kevin W
KW
Street
Leslie J
LJ
Wild
Deborah
D
Isted
Catherine
C
Leeson
Paul D
PD
Thomas
Steven
S
O'Connor
Desmond
D
McKernan
Ruth M
RM
Quirk
Katherine
K
Cook
Susan M
SM
Atack
John R
JR
Wafford
Keith A
KA
Thompson
Sally A
SA
Dawson
Gerard R
GR
Ferris
Pushpinder
P
Castro
José L
JL
D000818:Animals; D014151:Anti-Anxiety Agents / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology; D001667:Binding, Competitive; D001682:Biological Availability; D002460:Cell Line; D058785:GABA-A Receptor Agonists; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D018782:Maze Learning / Q000187:drug effects; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010793:Phthalazines / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D011963:Receptors, GABA-A / Q000378:metabolism / Q000502:physiology; D011994:Recombinant Proteins / Q000378:metabolism; D013329:Structure-Activity Relationship; D014230:Triazoles / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D014981:Xenopus
C484868:3-phenyl-6-(2-pyridyl)methoxy-7,8,9,10-tetrahydro(7,10-ethano)-1,2,4-triazolo(3,4-a)phthalazine; D014151:Anti-Anxiety Agents; D058785:GABA-A Receptor Agonists; D010793:Phthalazines; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D014230:Triazoles