NUCLEOSIDE PRODRUGS OF A₃ ADENOSINE RECEPTOR AGONISTS AND ANTAGONISTS

9-(β-D-Ribosfuranosyluronamide)adenine derivatives that are selective agonists and antagonists of the A₃ adenosine receptor (AR) have been derivatized as prodrugs for in vivo delivery. The free hydroxy groups at the 2' and 3' positions of the agonists 2-chloro-N ⁶-(3-iodobenzyl)-9-(N-methyl-(β-D-ribosfuranosyluronamide)adenine 2b, the corresponding 4'-thio nucleoside 2c, and antagonists 4a and 4b (5'-N,N-dimethylamides related to 2b and 2c, respectively) were derivatized through simple acylation reactions. The prodrug derivatives were tested in radioligand binding assays at ARs and in a functional assay of adenylate cyclase at the A₃AR and found to be considerably less active than the parent drugs. The hydrolysis of nucleoside 2',3'-diesters to regenerate the parent compound in the presence of human blood was demonstrated. 2',3'-Dipropionate esters of 2b and 4a were readily cleaved in a two-step reaction to regenerate the parent drug, on a time scale of two hours. The cleavage of a 2',3'-dihexanoate ester occurred at a slower rate. This indicates that the prodrugs are suitable as masked forms of the biologically active A₃AR agonists and antagonists for future evaluation in vivo.