Design, Synthesis, and Structure–Activity Relationship Studies of (4-Alkoxyphenyl)glycinamides and Bioisosteric 1,3,4-Oxadiazoles as GPR88 Agonists

Abstract

Increasing evidence implicates the orphan G protein-coupled receptor 88 (GPR88) in a number of striatal-associated disorders. In this study, we report the design and synthesis of a series of novel (4-alkoxyphenyl)­glycinamides (e.g., 31) and the corresponding 1,3,4-oxadiazole bioisosteres derived from the 2-AMPP scaffold (1) as GPR88 agonists. The 5-amino-1,3,4-oxadiazole derivatives (84, 88–90) had significantly improved potency and lower lipophilicity compared to 2-AMPP. Compound 84 had an EC50 of 59 nM in the GPR88 overexpressing cell-based cAMP assay. In addition, 84 had an EC50 of 942 nM in the [35S]­GTPγS binding assay using mouse striatal membranes but was inactive in membranes from GPR88 knockout mice, even at a concentration of 100 μM. In vivo pharmacokinetic testing of 90 in rats revealed that the 5-amino-1,3,4-oxadiazole analogues may have limited brain permeability. Taken together, these results provide the basis for further optimization to develop a suitable agonist to probe GPR88 functions in the brain.

Evaluation of Amide Bioisosteres Leading to 1,2,3-Triazole Containing Compounds as GPR88 Agonists: Design, Synthesis, and Structure–Activity Relationship Studies

“…Synthesis of 1-substituted 1 H -1,2,4-triazole 21 , 1-substituted 1 H -1,2,3-triazoles 25 and 26 , and 5-substituted 2 H -1,2,3,4-tetrazole 28 is depicted in Scheme . O -alkylation of 18 with 2-methylpentanol, cyclobutylmethanol, or ( S )-2-methylbutanol under Mitsunobu conditions afforded ethers 19a , 19b , and 19c , respectively. The ester in 19a – c was reduced with freshly prepared LiBH 4 to give the corresponding alcohol, which was converted into bromides 20a – c by treatment with CBr 4 .…”

“…Our Lance time-resolved fluorescence resonance energy transfer (TR-FRET) cAMP assay with stable CHO-GPR88 cells was used to assess the agonist potency (EC 50 ) and efficacy ( E max ) at the GPR88 receptor as described previously. , In these assays, RTI-13951-33 ( 2 , Figure ) was used as the reference compound and had a collectively average EC 50 of 45 nM ( n > 100). E max values (%) of test compounds were evaluated with RTI-13951-33 as 100%.…”

“…The data are summarized in Table . The previously reported 1,3,4-oxadiazole 6 is included to highlight SAR comparison. In this SAR, both 2-methylpentyl and cyclobutylmethyl groups were used in the alkoxy side chain as the cyclobutylmethyl analogue has a lower lipophilicity (in general an order of magnitude of clog P ) than that of the 2-methylpentyl counterpart.…”

“…In this vein, our medicinal chemistry campaign to understand the signaling mechanism and biological functions of GPR88 showed that 2-PCCA [(1 R ,2 R )-2-(pyridin-2-yl)­cyclopropane carboxylic acid ((2 S ,3 S )-2-amino-3-methylpentyl)-(4′-propylbiphenyl-4-yl)­amide, 1 , Figure ] can activate GPR88 via a Gα i -coupled signaling pathway in the functional cAMP assay in GPR88 overexpressing CHO cells. , Afterward, our systematic modifications of the 2-PCCA scaffold have resulted in the discovery of a brain-penetrant agonist, RTI-13951-33 ( 2 ), that enabled in vivo studies of GPR88 in operant alcohol self-administration in rats, demonstrating that GPR88 agonists are potential drug candidates for the treatment of alcohol addiction . Other than the 2-PCCA scaffold, the 2-AMPP [(2 S )- N -((1 R )-2-amino-1-(4-(2-methylpentyloxy)­phenyl)­ethyl)-2-phenylpropanamide ( 3 , Figure )] scaffold has also shown promise for GPR88 agonist development. ,, It has been illustrated that the 2-AMPP scaffold can tolerate modifications at three distinct sites and the amino group can be replaced by amide ( 4 ), among other functional groups, with improved potency. , Recently, we have reported our rational modifications of 4 by investigating the structure–activity relationship (SAR) at the alkoxy side chain and the N -methyl amide functionality . Although replacement of the N -methyl amide group with a bioisosteric 1,3,4-oxadiazole moiety provided 5 with a moderate potency, notable improvement in potency was achieved upon the addition of a methylene linker between the oxadiazole moiety and the benzylic carbon (e.g., 6 and 7 , Figure ).…”

“…5-Amino-1,3,4-oxadiazole 7 had an EC 50 of 59 nM in the cAMP assay using CHO-GPR88 cells, which is 5-fold more potent than 4 . Additionally, 7 had GPR88-specific agonist signaling activity when tested in the [ 35 S]­GTPγS binding assay using striatal membranes prepared from WT mice and GPR88 KO mice . However, in vitro ADME and pharmacokinetic (PK) assessments revealed that 1,3,4-oxadiazole analogues have poor aqueous solubility and limited blood–brain barrier (BBB) permeability, thus requiring further optimization to improve druglike properties.…”