Characterization of [3H]LUF5834: A novel non-ribose high-affinity agonist radioligand for the adenosine A1 receptor

Abstract

The adenosine A(1) receptor is a promising therapeutic target for neurological disorders such as cognition deficits and is involved in cardiovascular preconditioning. Classically adenosine receptor agonists were all derivatives of adenosine, and thought to require a D-ribose moiety. More recently, however, the discovery of non-adenosine agonists for the human adenosine A(1) receptor (hA(1)R) has challenged this dogma (Beukers et al., 2004). In this study we characterize the tritiated form of one of these compounds, [(3)H]LUF5834, as the first non-ribose partial agonist radioligand with nanomolar affinity for the hA(1)R. Due to its partial agonist efficacy, [(3)H]LUF5834 labeled both G protein-coupled and uncoupled receptors with a similar high affinity. Using [(3)H]LUF5834 we performed competition binding experiments to characterize a range of A(1)R ligands varying in efficacy from the full agonist CPA to the inverse agonist DPCPX. Surprisingly, in the control condition both agonists and inverse agonists displayed biphasic isotherms. With the addition of 1mM GTP the high affinity isotherm of agonists or the low affinity isotherm of inverse agonists was lost revealing the mechanism of action of such inverse agonists at the A(1)R. Consequently, [(3)H]LUF5834 represents a novel high affinity radioligand for the A(1)R and may prove a useful tool to provide estimates of inverse agonist efficacy at this receptor.

Ligand-Dependent Activation and Deactivation of the Human Adenosine A2AReceptor

“…In addition, both simulations show little interaction of LUF with the residues in the upper region, suggesting no subtype selectivity that is in agreement with recent experiments. 30,31 …”

Molecular probes for the human adenosine receptors

“…[ 3 H]LUF5834 is a non-nucleoside partial agonist radioligand ( Fig. 1; Table 1) with nanomolar affinity (K D = 2.03 ± 0.52 nM) for the hA 1 R [12]. Its partial agonistic nature allows this radioligand to bind to both G protein-coupled anduncoupled receptors.…”

Non-Nucleoside Agonists of the Adenosine Receptors: An Overview

“…This compound was also developed as a radioligand useful for Positron Emission Tomography (PET) imaging in the brain, given its good pharmacokinetics profile related to the ability to cross the Blood Brain Barrier (BBB) [60,61]. Even the compound 2-amino-4-(4-hydroxy phenyl)-6-(1H-imidazol-2ylmethylsulfanyl)-pyridine-3,5-dicarbonitrile [31], then named LUF5834 (13; Figure 1; Table 1), was developed as a radioligand able to bind to A 1 AR in both G-protein-coupled and uncoupled conditions with a similar high affinity [34]. Since this molecule showed nanomolar affinity also for the A 2A AR, it was employed for a mutagenesis study at this AR subtype, in comparison with the nucleoside derivative CGS21680.…”

“…The early data are related to non-selective agonists ranging from partial to full agonist profiles. Subsequent reports described further non-nucleoside derivatives endowed with low nanomolar potency and improved selectivity for the A 1 , A 2A , or A 2B AR subtypes [30][31][32][33][34][35][36][37][38]. Structural features and biological activity of these molecules, synthetic approaches, and molecular modelling studies simulating the interaction between these compounds and AR targets are reviewed in this work.…”