Chiral Pyrrolo[2,3-d]pyrimidine and Pyrimido[4,5-b]indole Derivatives:  Structure−Activity Relationships of Potent, Highly Stereoselective A1-Adenosine Receptor Antagonists

A series of 33 novel, mostly chiral pyrrolo[2,3-d]pyrimidine and pyrimido[4,5-b]indole derivatives has been synthesized and investigated in radioligand binding assays at the high-affinity adenosine receptor (AR) subtypes A1 and A2a. The compounds can be envisaged as adenine and hypoxanthine analogs lacking the nitrogen in the 7-position (7-deazaadenines and 7-deazahypoxanthines). 7-Deazaadenines were much more potent than 7-deazahypoxanthines at AR with A1AR affinities in the low-nanomolar range, extraordinarily high selectivity for the rat brain A1AR versus the A2aAR (several thousandfold), and high stereoselectivity (up to 96-fold). Pyrimido[4,5-b]indoles were more potent A1AR antagonists compared to pyrrolo[2,3-d]pyrimidines. Compound 34a (APEPI) is one of the most potent and most selective non-xanthine A1AR antagonists known to date (K i = 2.8 nM, >2000-fold A1-selective). A new class of very potent A1AR antagonists has been identified, namely, 2-phenyl-7-deazaadenines bearing a substituent at the exocyclic amino group (N4-substituted pyrrolo[2,3-d]pyrimidines). (R)-N-(1-Phenylethyl)-4-amino-5,6-dimethyl-2-phenyl-7H-pyrrolo[2,3-d]pyrimidine (DPEAP, 17a) showed a K i value of 6.7 nM at A1AR and >4000-fold A1 selectivity. Different binding modes are postulated for the N4-substituted 4-aminopyrrolo[2,3-d]pyrimidines (e.g., 17a) and the 7-substituted derivatives (e.g., 1a), based on a comparison of steric, electronic, and hydrophobic properties of the two classes of compounds. Water solubility and lipophilicity have been determined for selected compounds. 4-Amino-5,6-dimethyl-2-(3-chlorophenyl)-7H-pyrrolo[2,3-d]pyrimidine (4a) showed the highest water solubility/A1AR affinity ratio of 368 in the present series, over 2000-fold A1 selectivity, and 64-fold stereoselectivity (R > S). Therefore, 4a should be an interesting compound for in vivo evaluation.