General

Adenosine is ubiquitously present in the extracellular medium in all tissues and acts as a local modulator by activating four well-defined G protein-coupled receptors [16-18,45,47]. These receptors are denoted adenosine receptors, but may also be called P1 receptors to distinguish them from the receptors for nucleotides, which belong either to the family of transmitter-gated ion channels (P2X receptors) or the family of G protein-coupled receptors (P2Y receptors) [18].

The four adenosine receptors, called A₁, A_2A, A_2B and A₃, have been cloned from numerous mammalian species, including man. As seen in Figure 1 there is extensive sequence similarity between species for the A₁, and A_2A receptors, whereas A_2B and A₃ receptors are more variable.

Binding sites

There are reports of binding sites with a pharmacology that is not typical for any of the cloned subtypes [12-13,27-28,39]. These data could indicate that there are as yet undiscovered adenosine receptors. However, no evidence for additional adenosine receptors has evolved from the extensive cloning efforts of several laboratories. The A_2A receptor has been crystallized and its structure has been determined in complex with several agonist and antagonist ligands [11,15,24-25,34,59]. This structure has been used effectively for in silico screening of chemically diverse libraries to identify antagonists that have novel chemotypes [10,29,33,35,53,58] and also for the construction of reliable homology models of other adenosine receptor subtypes [6,32,49].

Ligands

Over the past decades highly selective agonists and antagonists for adenosine receptors have been developed. Some examples of such compounds are listed in Table 1. For structures and additional information on compounds the reader is referred to recent papers and reviews [2-3,14,26,38,46,51]. For reasons of space only a few representative compounds are given in the following data tables. Combined site-directed mutagenesis and homology-based modelling have identified amino acids in the third and seventh transmembrane domains (TM) as being critically important in agonist binding [30,48]. Much of the previous molecular modelling, as informed by studies of mutagenesis and structure activity relationships (SAR), is an agreement with data from the X-ray crystallographic determination of the A_2A adenosine receptor structure.

There is good evidence for the adenosine receptors (as for other 7TM receptors) that agonist potency is strongly dependent on receptor number [1,19,50] and on the assay used. Hence, data for both absolute and relative potencies for agonists must be used cautiously. In particular, the large differences in the number of receptors of different types in vivo could lead to much higher or lower selectivity than suggested by in vitro data.

There is solid evidence that there are partial agonists for all four adenosine receptor subtypes [5,40,55-57]. As for other G protein-coupled receptors, some of the antagonists have been proven to be inverse agonists [4,22,37]

Allosteric site

Besides the traditional agonist recognition site there is evidence, in the case of the adenosine A₁ receptor, for an allosteric site that regulates agonist binding and biological effects [8-9,36,41]. The positive allosteric modulator (PAM) PD81723, a thienopyridine derivative, may act by stabilising the receptor-G protein interaction [7], and the same may apply for analogues that are superior in potency and/or selectivity [54]. It is possible that one reason why such allosteric enhancement of A₁ receptor agonist binding can be found is related to the fact that these receptors are coupled in a somewhat unusual way to G proteins that may involve some other protein factors [43]. PAMs of the adenosine A₃ receptor have also been reported [20,23,31]. Recently bitopic ligands, most likely occupuying orthosteric and allosteric binding sites in the adenosine A₁ receptor simultaneously, have been introduced [44,52].

Adenosine in inflammation

All immune cells express receptors for adenosine [21]. It is suggested that adenosine exhibits pro-inflammatory effects through its A₁ receptor, and anti-inflammatory effects through A_2A receptor. It is noteworthy that A₁ receptor expression peaks during leukocyte recruitment, and this drives increased expression of A_2A receptor which facilitates resolution of inflammation [42].

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