Fluorosulfonyl- and Bis-(β-chloroethyl)amino-phenylamino Functionalized Pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine Derivatives: Irreversible Antagonists at the Human A3 Adenosine Receptor and Molecular Modeling Studies

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Abstract
A series of pyrazolotriazolopyrimidines was previously reported to be highly potent and selective human A3 adenosine receptor antagonists (Baraldi et al. J. Med. Chem. 2000, 43, 4768−4780). A derivative having a methyl group at the N8 pyrazole combined with a 4-methoxyphenylcarbamoyl moiety at N5 position, displayed a Ki value at the hA3 receptor of 0.2 nM. We now describe chemically reactive derivatives which act as irreversible inhibitors of this receptor. Electrophilic groups, specifically sulfonyl fluoride and nitrogen mustard (bis-(β-chloroethyl)amino) moieties, have been incorporated at the 4-position of the aryl urea group. Membranes containing the recombinant hA3 receptor were preincubated with the compounds and washed exhaustively. The loss of ability to bind radioligand following this treatment indicated irreversible binding. The most potent compound in irreversibly binding to the receptor was 14, which contained a sulfonyl fluoride moiety and a propyl group at the N8 pyrazole nitrogen. The bis-(β-chloroethyl)amino derivatives displayed a much smaller degree of irreversible binding than the sulfonyl fluoride derivatives. A computer-generated model of the human A3 receptor was built and analyzed to help interpret these results. The model of the A3 transmembrane region was derived using primary sequence comparison, secondary structure predictions, and three-dimensional homology building, using the recently published crystal structure of rhodopsin as a template. According to our model, sulfonyl fluoride derivatives could dock within the hypothetical TM binding domain, adopting two different energetically favorable conformations. We have identified two amino acids, Ser247 and Cys251, both in TM6, as potential nucleophilic partners of the irreversible binding to the receptor.