Synthesis of paraxanthine analogs (1,7-disubstituted xanthines) and other xanthines unsubstituted at the 3-position: structure-activity relationships at adenosine receptors

X-ray structures of two xanthine inhibitors bound to PEPCK and N-3 modifications of substituted 1,8-dibenzylxanthines

The analysis of the X-ray structures of two xanthine inhibitors bound to PEPCK and a comparison to the X-ray structure of GTP bound to PEPCK are reported. The SAR at N-1, N-7 and developing SAR at C-8 are consistent with information gained from the X-ray structures of compounds 1 and 2 bound to PEPCK. Representative N-3 modifications of compound 2 that led to the discovery of 3-cyclopropylmethyl and its carboxy analogue as optimal N-3 groups are presented.

Modified 3-alkyl-1,8-dibenzylxanthines as GTP-competitive inhibitors of phosphoenolpyruvate carboxykinase

The first non-substrate like inhibitors of human cytosolic phosphoenolpyruvate carboxykinase (PEPCK) competitive with GTP are reported. An effort to discover orally active compounds that improve glucose homeostasis in Type 2 diabetics by reversibly inhibiting PEPCK led to the discovery of 1-allyl-3-butyl-8-methylxanthine (5). We now report modifications at N-1 and C-8 that improved the in vitro activity of the initial xanthine HTS hit by 100-fold and a developing SAR for this class of inhibitor.

Synthesis of series of 1- and 3-differently substituted xanthines from imidazoles

A new and general method is described for the synthesis, in three steps and in good overall yields, of tetrasubstituted xanthines from an easily prepared imidazole precursor. The method is especially useful for the preparation in standardized conditions of series of xanthines combining a broad variety of primary or secondary alkyl, benzyl or aryl groups at N1 and of alkyl or arylmethyl groups at N3, that are not readily available by other methods.

Structure-activity relationships at human and rat A2B adenosine receptors of xanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions

In the search for improved selective antagonist ligands of the A2B adenosine receptor, which have the potential as antiasthmatic or antidiabetic drugs, we have synthesized and screened a variety of alkylxanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions. Competition for 125I-ABOPX (125I-3-(4-amino-3-iodobenzyl)-8-(phenyl-4-oxyacetate)-1-propylxanthine) binding in membranes of stably transfected HEK-293 cells revealed uniformly higher affinity (<10-fold) of these xanthines for human than for rat A2B adenosine receptors. Binding to rat brain membranes expressing A1 and A2A adenosine receptors revealed greater A2B selectivity over A2A than A1 receptors. Substitution at the 1-position with 2-phenylethyl (or alkyl/olefinic groups) and at N-3 with hydrogen or methyl favored A2B selectivity. Relative to enprofylline 2b, pentoxifylline 35 was equipotent and 1-propylxanthine 3 was >13-fold more potent at human A2B receptors. Most N-7 substituents did not enhance affinity over hydrogen, except for 7-(2-chloroethyl), which enhanced the affinity of theophylline by 6.5-fold to 800 nM. The A2B receptor affinity-enhancing effects of 7-(2-chloroethyl) vs 7-methyl were comparable to the known enhancement produced by an 8-aryl substitution. Among 8-phenyl analogues, a larger alkyl group at the 1-position than at the 3-position favored affinity at the human A2B receptor, as indicated by 1-allyl-3-methyl-8-phenylxanthine, with a K(i) value of 37 nM. Substitution on the 8-phenyl ring indicated that an electron-rich ring was preferred for A2B receptor binding. In conclusion, new leads for the design of xanthines substituted in the 1-, 3-, 7-, and 8-positions as A2B receptor-selective antagonists have been identified.

Synthesis and use of FSCPX, an irreversible adenosine A1 antagonist, as a 'receptor knock-down' tool

A new preparative synthetic route for the irreversible adenosine A1 antagonist 8-cyclopentyl-3-N-[3-((3-(4-fluorosulphonyl)benzoyl)-oxy)-propyl]-1-N-propyl-xanthine (FSCPX, 1) is described. The availability of ample amounts of the irreversible antagonist FSCPX allowed us to use FSCPX as a research tool for adenosine A1 receptors in in vivo experiments. After verification of the irreversible antagonistic function of FSCPX in in vitro experiments, FSCPX was used successfully as a 'receptor knock-down' tool in in vivo experiments on conscious rats.

Structure-activity relationships for G2 checkpoint inhibition by caffeine analogs

Caffeine inhibits the G2 checkpoint activated by DNA damage and enhances the toxicity of DNA-damaging agents towards p53-defective cancer cells. The relationship between structure and G2 checkpoint inhibition was determined for 56 caffeine analogs. Replacement of the methyl group at position 3 or 7 resulted in loss of activity, while replacement at position 1 by ethyl or propyl increased activity slightly. 8-Substituted caffeines retained activity, but were relatively insoluble. The structure-activity profile did not resemble those for other known pharmacological activities of caffeine. The active analogs also potentiated the killing of p53-defective cells by ionizing radiation, but none was as effective as caffeine.

8-(Sulfostyryl)xanthines: water-soluble A2A-selective adenosine receptor antagonists

8-(Sulfostyryl)xanthine derivatives were synthesized as water-soluble A2A-selective adenosine receptor (AR) antagonists. meta- and para-sulfostyryl-DMPX (3,7-dimethyl-1-propargylxanthine) derivatives 11a and 11b exhibited high affinity to rat A2A-AR in submicromolar concentrations, and were 20- to 30-fold selective versus rat A1-AR. Styryl-DMPX derivatives were inactive at human A2B- and A3-AR. 1,3-Dipropyl-8-p-sulfostyrylxanthine (13) or only a 7-methyl derivative (14) showed similar (13) or higher (14) A2A affinity than 11a and 11b but showed no (13) or only a low degree (14) of selectivity versus A1-, A2B-, and A3-AR. The A2A-selective sulfostyryl-DMPX derivatives exhibit high water-solubility and may be useful research tools for in vivo studies.

Aza-analogs of 8-styrylxanthines as A2A-adenosine receptor antagonists

In the present study we synthesized aza-analogs of 8-styrylxanthines, in which the ethenyl bridge is replaced by an imine, amide, or azo function, in order to investigate structure-activity relationships of the 8-substituent of A2A-selective xanthine derivatives. Thus, various 8-substituents were combined with theophylline or caffeine, respectively, and affinities of the novel compounds for adenosine A1- and A2a-receptors were determined and compared with those of analogous 8-styrylxanthine derivatives. 8-(Benzylideneamino)caffeine derivatives exhibited high affinity and selectivity for A2A-adenosine receptors, but were unstable in aqueous buffer solution at physiological pH values. 8-(Phenylazo)caffeine derivatives were less potent than corresponding 8-styrylcaffeine derivatives at adenosine receptors. The most potent azo compound of the present series was 8-(m-chlorophenylazo)caffeine (14b) exhibiting a Ki value of 400 nM at A2A-adenosine receptors and 20-fold selectivity versus A1-receptors. Due to the facile synthetic access to 8-(phenylazo)xanthine derivatives, which are obtained by coupling of 8-unsubstituted xanthines with phenyldiazonium salts, 14b may be an interesting new lead compound for the development of more potent and selective A2A-antagonists with azo structure.

Adenosine receptors and their modulators

The identification and characterization of adenosine receptors and the development of potent, receptor subtype-selective agonists and antagonists has been an active area of research for the past 20 years. Major recent advances in the field have been the cloning of several adenosine receptor subtypes of different species, including the discovery of a new subtype, designated A3, the discovery and development of new agonists and antagonists, particularly those with selectivity for the A2a adenosine receptor, the characterization of signal transduction pathways, and the development of agents which act indirectly on the adenosine receptor system. The present article focusses on aspects of pharmaceutical/medicinal chemistry related to adenosine receptors.