Potent 4-arylalkyl-substituted 3-isothiazolol GABA(A) competitive/noncompetitive antagonists: synthesis and pharmacology.

The GABA(A) agonists muscimol (1), 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP, gaboxadol, 3), and the partial GABA(A) agonist 5-(4-piperidyl)-3-isoxazolol (4-PIOL, 6a) and their respective 3-isothiazolol analogues thiomuscimol (2), thio-THIP (4), and thio-4-PIOL (7a) are ligands at the GABA(A) orthosteric (recognition) site. The structure-activity relationships (SARs) between these structures are key elements of a 3D-pharmacophore model for GABA(A) agonists and competitive antagonists [Frølund, B.; Jørgensen, A. T.; Tagmose, L.; Stensbøl, T. B.; Vestergaard, H. T.; Engblom, C.; Kristiansen, U.; Sanchez, C.; Krogsgaard-Larsen, P.; Liljefors, T. J. Med. Chem. 2002, 45, 2454-2468]. Prompted by this model, we now report the synthesis and SAR of a series of analogues of 7a, in which the 4-position of the 3-isothiazolol was substituted by alkyl or bulky aromatic groups such as naphthylmethyl and diphenylalkyl groups (7b-h). The compounds have been pharmacologically characterized using receptor binding assays and two-electrode voltage-clamped Xenopus oocytes expressing alpha1beta3gamma2S- and alpha4beta3delta-containing receptors. The compounds show SARs comparable with those of 6b-h but are generally 5-15 times more potent. The 2-naphthylmethyl, the 1-bromo-2-naphthylmethyl, and the 3,3-diphenylpropyl analogues, compounds 7e, 7f, and 7h, respectively, show affinity in the low-nanomolar range (K(i) 2-10 nM). Interestingly, 7e and 7h exhibited a mixed antagonist profile consisting of a noncompetitive component in the picomolar range and a competitive component at concentrations above 1 nM. This unique profile was shown not to be due to either use dependence or kinetic effects. This antagonist profile of 7e and 7h was particularly pronounced at alpha4beta3delta-containing GABA(A) receptors, which showed three- and 10-fold selectivity for 7h and 6h, respectively.

Phosphinic, phosphonic and seleninic acid bioisosteres of isonipecotic acid as novel and selective GABA(C) receptor antagonists.

A number of amino acids bioisosterically derived from the specific GABA(A) agonist, isonipecotic acid, were electrophysiologically characterized as antagonists at GABA(C) rho(1) receptors expressed in Xenopus oocytes. The phosphinic acid analogue of isonipecotic acid, piperidin-4-ylphosphinic acid (2), was comparable with the standard GABA(C) antagonist, (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), in terms of potency and GABA(C) versus GABA(A) receptor selectivity. Whereas the phosphonic acid analogue, piperidin-4-ylphosphonic acid (4), was at least an order of magnitude weaker than piperidin-4-ylphosphinic acid as a GABA(C) antagonist, the seleninic acid analogue, piperidin-4-ylseleninic acid (SEPI, 6), was the most potent and selective GABA(C) antagonist within the group of isonipecotic acid derived amino acids studied.

Aza-THIP and related analogues of THIP as GABA C antagonists.

The potency of a series of eight compounds structurally related with 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a potent GABA(A) partial agonist exhibiting GABA(C) rho(1) antagonist effect (K(i)=25 microM), was determined electrophysiologically using homomeric human GABA(C) rho(1) receptors expressed in Xenopus oocytes. Protolytic properties (pK(a) values for the acidic bioisosteric groups) and the presence of steric bulk in the molecules appear to be structural parameters of importance for blockade of the GABA(C) rho(1) receptor. Within this series of moderately potent GABA(C) antagonists, only 4,5,6,7-tetrahydropyrazolo[5,4-c]pyridin-3-ol (Aza-THIP) does not interact detectably with GABA(A) receptors, and Aza-THIP has the potential of being a useful tool for molecular and behavioural pharmacological studies.