Water-Soluble Phosphate Prodrugs of 1-Propargyl-8-styrylxanthine Derivatives, A2A-Selective Adenosine Receptor Antagonists

Water-soluble prodrugs of potent, A2A-selective adenosine receptor (AR) antagonists were prepared. 8-(m-Bromostyryl)-3,7-dimethyl-1-propargylxanthine (BS-DMPX, 11) and the analogous 8-(m-methoxystyryl)xanthine derivative (MS-DMPX, 5b) were used as starting points. It was found that polar functional groups suitable for the attachment of a prodrug moiety were tolerated on the styryl ring and even better on the 3-substituent. 8-(m-Hydroxystyryl)-DMPX (7) and 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-1-propargylxanthine (5e, MSX-2) were the most potent and A2A-selective compounds and were selected for prodrug formation. For the preparation of 5e a new ring-closure method was applied. Treatment of 6-amino-1-(3-hydroxypropyl)-5-(m-methoxycinnamoylamino)-3-propargyluracil with hexamethyldisilazane at high temperature resulted in higher yields of the target xanthine than the standard ring-closure procedure using sodium hydroxide. Phosphate prodrugs were prepared by classical phosphorylation using phosphorus oxychloride and alternatively by using a phosphoramidite method. Phosphates of the aliphatic alcohol 5e could be obtained by both methods in similar yields. The phenolic compound 7, however, could be phosphorylated only by using the phosphoramidite method. The disodium salts of the phosphate prodrugs exhibited high water solubility (8-(m-methoxystyryl)-7-methyl-3-[3-O-phosphatylpropyl]-1-propargylxanthine disodium salt, 9b: 17 mM, 9 mg/mL). Prodrug 9b was found to be stable in aqueous solution (pH 7) but readily cleaved by phosphatases to liberate 5e (MSX-2). Compound 5e showed high affinity for rat A2A AR (K i = 8 nM), human recombinant A2A AR (K i = 5 nM), and human native A2A AR (K i = 15 nM) and was highly selective versus rat A1 AR (110-fold), human recombinant A2A AR (500-fold), human A2B AR (>2000-fold), and human A3 AR (>2000-fold).