Species dependence of A3 adenosine receptor pharmacology and function

Abstract

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A 3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A 3 AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A 3 AR are inactive or weakly active at the rat and mouse A 3 ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A 3 AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A 3 AR undergoes desensitization faster than the human A 3 AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A 3 AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A 3 ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A 3 AR pharmacology.

[1,2,4]Triazolo[1,5‐c]pyrimidines as Tools to Investigate A3 Adenosine Receptors in Cancer Cell Lines

“…Hydrazides (35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48), when not commercially available, were synthetized from the corresponding carboxylic acid or ethyl ester via classical Fisher esterification followed by reaction with hydrazine, or directly reacting with hydrazine, respectively (data not shown). The obtained N'-(pyrimidin-4yl)hydrizides (49-62) were then cyclized under drying and silylating conditions (i. e. phosphorus pentoxide and hexamethyldisiloxane, HDMSO, in dry xylene) leading to the corresponding [1,2,4]triazolo [4,3-c]pyrimidines, that immediately underwent a Dimroth rearrangement to the more stable 1,2,4triazolo [1,5-c]pyrimidines (63-76). The [1,2,4]triazolo [4,3-c]pyrimidines had never been isolated in all reactions performed in this work.…”

“…The obtained N'-(pyrimidin-4yl)hydrizides (49-62) were then cyclized under drying and silylating conditions (i. e. phosphorus pentoxide and hexamethyldisiloxane, HDMSO, in dry xylene) leading to the corresponding [1,2,4]triazolo [4,3-c]pyrimidines, that immediately underwent a Dimroth rearrangement to the more stable 1,2,4triazolo [1,5-c]pyrimidines (63-76). The [1,2,4]triazolo [4,3-c]pyrimidines had never been isolated in all reactions performed in this work. 5-Thiomethyl derivatives 63, 66, 67, 70-73, 75 and their corresponding intermediates were already described in a previous work of our group.…”

“…[1] Our target of interest in this work is the A 3 AR subtype, which regulates intracellular pathways coupling essentially to Gα i3 proteins even if also the coupling to Gα i1-2 , G q/11 , and Gα 0A-B proteins has been demonstrated. [2] The A 3 AR is involved in inflammation, and with the A 2A AR subtype mediates the immunosuppressive and anti-inflammatory effects of methotrexate, [3] but, in contrast to the A 2A AR, it does not display the side effects of A 2A AR agonism like hypotension, in fact, the A 3 AR is not widely expressed in our body like the A 2A AR.…”

“…[10] The pharmacological variability observed upon engagement of synthetic ligands with the receptor is also related to the significant interspecies differences in the expression of the A 3 AR in healthy cells as well as in cancer cells. [2] It is now well-known that adenosine is present at high levels in the tumor microenvironment (TME) under hypoxic conditions, a typical feature of solid tumors. The A 3 AR has been found overexpressed in different tumors but whether its effect is either pro-or anti tumoral remains an open question and needs further investigations.…”

“…In this work we have developed a series of A 3 AR antagonists with a [1,2,4]triazolo [1,5-c]pyrimidine (TP) scaffold, [12][13][14] starting from previous data on series of non-selective compounds (best compound represented by compound 3 in Figure 2). [15,16] Our interest in this class of compounds came from the observation that most of the high potent and selective hA 3 AR antagonists were tricyclic complex molecules, [17] with poor physicochemical properties that can hamper their biological investigation.…”