Bifunctional, bitopic and fluorescent agonists for the a1 adenosine receptor

Abstract

Recent research has proposed new models of GPCR signal transduction that highlight oligomerization and allosterism as crucial elements in function of these receptors. Ligands designed to exploit these new models have been demonstrated for some GPCRs and include bifunctional and bitopic ligands. Research presented in this thesis explores these new ligand designs, focusing on ligands acting at the A1 adenosine receptor and β2-adrenergic receptor. Bifunctional ligands were prepared by linking agonists for the A1 adenosine receptor and the β2-adrenergict receptor using an alkyl or PEG linker. This preparation proved difficult and a number of synthetic strategies were employed before a successful approach was found. The resulting bifunctional ligands, with variations in linker length, linker nature and catechol amine head group, showed low to medium nM affinity and potency at both the A1 adenosine receptor and β2-adrenergic receptor. These ligands were full agonists at the A1 adenosine receptor and partial agonists at the β2-adrenergic receptor. The effect of pharmacophore and linker structure on potency and affinity measures is discussed. Agonism of the A1 adenosine receptor inhibits intracellular cAMP production while agonism at the β2-adrenergict receptor stimulates this response. Presentation of both agonists at the cell surface has potential to elicit a compromised response and allow physiological partial agonism. When these ligands were able to access both receptor types, the A¬1 adenosine receptor binding pharmacophore dominated intracellular cAMP response. The contribution of the β2-adrenergic receptor binding pharmacophore could be unmasked by treatment with pertussis toxin or by pretreatment with CPA or an irreversible agonist. Bitopic ligands targeting the A1 adenosine receptor were prepared, each containing an orthosteric and allosteric pharmacophore with different lengths of alkyl linker joining the two. Pharmacological analysis of these ligands suggested that they were orthosteric ligands and that the allosteric site may not have been accessed due to interference by the linker moiety. A series of proposed enhancer moieties with various linker attachments were prepared. Evaluation of these compounds highlighted some attachment designs that allow enhancing activity. Structural features employed in the design of both bifunctional and bitopic ligands were also employed in the design of fluorescent agonists targeting the adenosine receptor. Fluorescent agonists for the A1 adenosine receptor have been reported by previous authors and have been employed in technologies such as in confocal microscopy and fluorescence correlation spectroscopy. Novel fluorescent agonists for the A1 adenosine receptor were prepared by us with either long fluorescent lifetime rhenium complexes or solvatochromic naphthalimide fluorophores. The length of an alkyl linker was varied for both design types. Pharmacological evaluation of the two designs showed medium nM binding for the ligands incorporating the rhenium complex fluorophore and recommended these ligands for future microscopic evaluation. Designs incorporating the naphthalimide fluorophore had poor affinity at the A1 adenosine receptor.