title: Flexible modulation of agonist efficacy at the human A3 adenosine receptor by the imidazoquinoline allosteric enhancer LUF6000.
abstract: BACKGROUND
  A series of 1H-imidazo- [4,5-c]quinolin-4-amine derivatives, represented by LUF6000 (N-(3,4-dichloro-phenyl)-2-cyclohexyl-1H-imidazo [4,5-c]quinolin-4-amine), are allosteric modulators of the human A3 adenosine receptor (AR). Here we studied the modulation by LUF6000 of the maximum effect (Emax) of structurally diverse agonists at the A3 AR stably expressed in CHO cells.
  
  
  RESULTS
  In an assay of [35S]GTPgammaS binding, the Emax of the A3 AR agonist Cl-IB-MECA at the A3 AR was lower than that of the non-selective AR agonist NECA. LUF6000 exerted an Emax-enhancing effect at a concentration of 0.1 microM or higher, and was shown to increase the Emax of Cl-IB-MECA and other low-efficacy agonists to a larger extent than that of the high-efficacy agonist NECA. Interestingly, LUF6000 converted a nucleoside A3 AR antagonist MRS542, but not a non-nucleoside antagonist MRS1220, into an agonist. LUF6000 alone did not show any effect. Mathematical modeling was performed to explain the differential effects of LUF6000 on agonists with various Emax. A simple explanation for the observation that LUF6000 has a much stronger effect on Cl-IB-MECA than on NECA derived from the mathematical modeling is that NECA has relatively strong intrinsic efficacy, such that the response is already close to the maximum response. Therefore, LUF6000 cannot enhance Emax much further.
  
  
  CONCLUSIONS
  LUF6000 was found to be an allosteric enhancer of Emax of structurally diverse agonists at the A3 AR, being more effective for low-Emax agonists than for high-Emax agonists. LUF6000 was demonstrated to convert an antagonist into an agonist, which represents the first example in G protein-coupled receptors. The observations from the present study are consistent with that predicted by mathematical modeling.
authors.affiliation: Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0810, USA. zg21o@nih.gov
authors.forename: Zhan-Guo
authors.initials: ZG
authors.lastname: Gao
authors.forename: Kai
authors.initials: K
authors.lastname: Ye
authors.forename: Anikó
authors.initials: A
authors.lastname: Göblyös
authors.forename: Adriaan P
authors.initials: AP
authors.lastname: Ijzerman
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Jacobson
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives* / Q000378:metabolism; D058909:Adenosine A3 Receptor Agonists; D058918:Adenosine A3 Receptor Antagonists; D019830:Adenosine-5'-(N-ethylcarboxamide) / Q000378:metabolism*; D000494:Allosteric Regulation; D000634:Aminoquinolines / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006801:Humans; D007093:Imidazoles / Q000494:pharmacology*; D008962:Models, Theoretical; D011869:Radioligand Assay; D043684:Receptor, Adenosine A3 / Q000378:metabolism
chemical_list: D058909:Adenosine A3 Receptor Agonists; D058918:Adenosine A3 Receptor Antagonists; D000634:Aminoquinolines; D007093:Imidazoles; C511637:N-(3,4-dichlorophenyl)-2-cyclohexyl-1H-imidazo(4,5-c)quinolin-4-amine; D043684:Receptor, Adenosine A3; D019830:Adenosine-5'-(N-ethylcarboxamide); D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000241:Adenosine; C090034:2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide