Synthesis of hybrid molecules of caffeine and eudistomin D and its effects on adenosine receptors

Four hybrid molecules (1 and 12-14) of caffeine and eudistomin D, a beta-carboline alkaloid from a marine tunicate, were synthesized, and their affinity and selectivity for adenosine receptors A(1), A(2A), and A(3) were examined. It was found that all the compounds showed better potency as adenosine receptor ligands as compared with caffeine. Among them, a compound (13) possessing a nitrogen at the delta-position of the pyridine ring (delta-N type) showed the most potent affinity for adenosine receptor A(3) subtype, while N-methylation (14) of a pyrrole ring in 13 significantly lowered the potency as adenosine receptor ligands. Compounds (1 and 12) having a nitrogen at the beta-position of the pyridine ring (beta-N type) showed lower affinity than the corresponding delta-N type compounds (13 and 14), while compounds (10, 11, and 17) lacking a pyrrole ring between the pyridine and pyrimidine rings exhibited almost no affinity to the adenosine receptor subtypes examined.     

Synthesis of eudistomin D analogues and its effects on adenosine receptors

Six analogues (1-6) of eudistomin D, a beta-carboline alkaloid from a marine tunicate Eudistoma olivaceum, were synthesized, and their affinity and selectivity for adenosine receptors A(1), A(2A), and A(3) were examined. All the synthetic compounds 1-6 did not show affinity to the adenosine A(1) receptor. Delta-carboline 3 exhibited the most potent affinity to the adenosine receptor A(3) among compounds 1-6. Delta-carbolines 3 and 4 showed better affinity than the corresponding beta-carbolines 1 and 2, respectively, while N-methylation (2, 4, and 6, respectively) of the pyrrole ring in 1, 3, and 5 resulted in the reduced affinity to the adenosine A(3) receptor. On the other hand, an eudistomin D derivative, BED, exhibited modest affinity to all the receptors A(1), A(2A), and A(3) but no selectivity.     

New base-altered adenosine analogues: Synthesis and affinity at adenosine A1 and A2A receptors

N⁶-Substituted adenosine analogues containing cyclic hydrazines or chiral hydroxy (ar)alkyl groups, designed to interact with the S2 and S3 receptor subregions, have been synthesized and their binding to the adenosine A₁ and A_2A receptors have been investigated. Examples of both types of compounds were found to exhibit highly selective binding (K_i in low nM range) to the rat A₁ receptor.     

Synthesis of 3'-ureidoadenosine analogues and their binding affinity to the A3 adenosine receptor

Novel 3'-ureidoadenosine analogues were synthesized from 1,2:5, 6-di-O-isopropylidene-D-glucose in order to lead to stronger hydrogen bonding at the A3 adenosine receptor than the corresponding 3'-aminoadenosine derivatives. However, all synthesized 3'-ureidoadenosine analogues have lost their binding affinities to the all subtypes of adenosine receptors, indicating that bulky 3'-urea moiety led to conformational distortion.     

Cholecystokinin analogues with high affinity and selectivity for brain membrane receptors

A series of CCK analogues in which positions 28 and 31 have been replaced by N-methylnorleucine residues have been synthesized. It has been found that most of these N-methylnorleucine containing analogues of CCK are highly potent and some are extraordinarily selective for the central vs. peripheral receptor in two animal models (guinea pig and rat). [N-MeNle28,31]CCK26-33 nonsulfated exhibited both high potency (IC50 = 0.13 nM) and selectivity for central vs. peripheral receptors. The pancrease to brain cortex binding affinity ratio for this analogue is 5100 in the rat model. NMR studies reveal that there is cis/trans isomerism about the N-methylnorleucine residue that may be related to high selectivity.     

2-(N-acyl) and 2-N-acyl-N(6)-substituted analogues of adenosine and their affinity at the human adenosine receptors

A series of 2-(N-acyl) and 2-(N-acyl)-N(6)-alkyladenosine analogues have been synthesized from the intermediate 2-amino-6-chloroadenosine derivatives (2b and 7) and evaluated for their affinity at the human A(1), A(2A), and A(3) receptors. We found that 2-(N-acyl) derivatives of adenosine showed relatively low affinity at A(2A) and A(3) receptors, while the N(6)-cyclopentyl substituent in 4h and 4i induced high potency [A(1) (K(i))=20.7 and 31.8 nM respectively] at the A(1) receptor and resulted therefore in increased selectivity for this subtype. The general synthetic methods and their binding studies are presented herein.     

Analogs of caffeine: antagonists with selectivity for A2 adenosine receptors

Several analogs of caffeine have been investigated as antagonists at A2 adenosine receptors stimulatory to adenylate cyclase in membranes from rat pheochromocytoma PC12 cells and human platelets and at A1 adenosine receptors inhibitory to adenylate cyclase from rat fat cells. Among these analogs, 1-propargyl-3,7-dimethylxanthine was about 4- to 7-fold and 7-propyl-1,3-dimethylxanthine about 3- to 4-fold more potent than caffeine at A2 receptors of PC12 cells and platelets. At A1 receptors of fat cells, both compounds were about 2-fold less potent than caffeine. These caffeine analogs have an A1/A2 selectivity ratio of about 10-20 and are the first selective A2 receptor antagonists yet reported. The results may provide the basis for the further development of highly potent and highly selective A2 adenosine receptor antagonists.     

Structure-affinity relationships of the affinity of 2-pyrazolyl adenosine analogues for the adenosine A2A receptor

The structure-affinity relationships of two novel 2-substituted adenosine series containing a substituted pyrazole attached at the N-1 or C-4 position for the adenosine (ADO) A2A receptor are described. Compounds in the 2-(N-1-pyrazolyl) adenosine series IV provided the highest affinity for the ADO A2A receptor as compared to the 2-(C-4-pyrazolyl) series V. The main structural differences between the two series point to the N-1 nitrogen of series IV imparting more favorable binding interactions with the receptor than those of series V.     

Synthesis of simple adenosine diphosphate ribose analogues

[See figures]. The synthesis of analogues of adenosine diphosphate ribose and acetylated adenosine diphosphate ribose, modified at the northern pentose, is reported. The stereochemistry at the acetylated centers was chosen to minimize acetyl migration and dictated the overall synthetic strategy.     

Chronic caffeine consumption increases the number of brain adenosine receptors

Caffeine, a potent central stimulant, is known to competitively inhibit the specific binding of both adenosine and benzodiazepine receptor ligands to brain membranes in vitro. In mice receiving a diet containing non-toxic doses of caffeine (200 or 400 mg/kg diet) for periods up to 40 days, a dose-related increase in the number of binding sites for [3H]-CHA and [3H] DPX was observed in whole brain membranes without modifications of the receptors' affinity. Furthermore, a transitory increase in the number of [3H]-DZP binding sites was observed. These preliminary data seem to confirm the involvement of the adenosine receptors in the mode of action of caffeine and may be relevant to the development of both tolerance and dependence to some of the central effects of this compound.     

Synthesis and adenosine receptor affinity and potency of 8-alkynyl derivatives of adenosine

Adenosine derivatives bearing different (ar)alkynyl chains at the 8-position were synthesized and tested at human adenosine receptors. Binding studies showed that all compounds possess affinity for the A3 subtype in the high nM range. Moreover, guanosine 5'-O-(3-[35S]thio)triphosphate binding assay indicated that the 8-alkynyl adenosines behaved as antagonists of NECA at A3 receptors.     

Synthesis and NMDA receptor affinity of fluorinated dioxadrol analogues

A series of dioxadrol analogues with fluorine substituents in position 4 of the piperidine ring has been synthesized and pharmacologically evaluated. The key step in the synthesis was the fluorination of diastereomeric piperidones 6a and 6c as well as diastereomeric alcohols 9a and 9c with DAST. The reaction of the alcohols 9a and 9c took place with inversion of configuration. After removal of the Cbz-protective group, the NMDA receptor affinities of the resulting secondary amines 8a, 8c, 12b, and 12d were investigated in receptor binding studies. It was shown that the like-configuration of the ring junction was crucial for high NMDA receptor affinity. An axially oriented fluorine atom in position 4 led to 2-(2,2-diphenyl-1,3-dioxolan-4-yl)-4-fluoropiperidine (12d, WMS-2517) with a K_i-value of 27 nM. The NMDA receptor affinity of 8c (WMS-2513) with an additional fluorine atom in equatorial 4-position was slightly reduced (K_i = 81 nM). Both fluorinated dioxadrol derivatives 8c and 12d showed high selectivity against σ₁ and σ₂ receptors as well as the polyamine binding site of NR2B receptors.     

Effects of chronic caffeine on brain adenosine receptors: Regional and ontogenetic studies

The effect of chronic caffeine treatment on three different binding sites in five brain areas of mice is characterized. The sites studied were the adenosine receptor, using [³H] diethylphenylxanthine, the benzodiazepine receptor, using [³H] diazepam and the adenosine uptake site, using [³H] nitrobenzylthioinosine. Significant increases were only observed in adenosine receptors with the greatest degree of change seen in the cerebellum and brain stem at both 16 and 23 days of caffeine treatment. The lack of significant effects of chronic caffeine on benzodiazepine receptors and adenosine uptake sites indicates that the caffeine effect is specific. The effect of chronic caffeine treatment on the ontogency of adenosine receptors was also studied with the result showing a significantly accelerated development of the receptor in the caffeine treated animals. The adult adenosine receptor levels were 20–30% higher than those observed in control animals. The observed alterations in adenosine receptor number which occur as a consequence of caffeine consumption may underlie some of the behavioral effects of this cortical stimulant as well as provide insights concerning the mechanisms of tolerance to and dependence on caffeine.     

C-nucleoside analogues of furanfurin as ligands to A1 adenosine receptors

Furanfurin (2-beta-D-ribofuranosylfuran-4-carboxamide) derivatives and analogues were synthesized and their affinity for adenosine receptors was determined. The agonistic behavior of furanfurin against A1 receptors is preserved only when the furan ring is substituted with isosteric pentatomic ring systems such as oxazole, thiazole or thiophene, and the carboxamide group is unsubstituted. Replacement of the hydrogen atoms of the carboxamide group with alkyl, cycloalkyl or arylalkyl groups generates compounds endowed with moderate antagonistic activity.     

Stereochemical effects of L-tryptophan and its analogues on trp repressor's affinity for operator-DNA

We have employed a filter binding assay to help study the mechanism by which bound L-tryptophan enables the Escherichia coli trp repressor to bind its operators. We have prepared variants of the trp repressor using structural analogues of the natural corepressor, L-tryptophan, and measured the affinity of these variants for a 20-base pair oligonucleotide duplex containing a symmetrical idealization of the trp operator from the E. coli trpEDCBA operon. By normalizing for each analogue's previously determined affinity for the trp aporepressor, we have estimated the extent to which each of the functional groups of bound L-tryptophan contributes to operator affinity. We discuss the likely role of these functional groups in the context of the crystal structures of the inactive, unliganded trp aporepressor, the liganded, active repressor, an inactive pseudorepressor (Pseudorepressors are formed by analogues of L-tryptophan that bind at the tryptophan-binding site but form near isomorphs of the repressor that have poor affinity for operator-DNA.) and the trp repressor/operator complex. We find that the alpha-amino group and an unsubstituted amino (-NH-) nitrogen of L-tryptophan's indole ring are essential for operator affinity. The former properly orients the corepressor and the latter interacts directly with the DNA. The alpha-carboxyl group, on the other hand, greatly enhances but is not essential for operator binding. The alpha-carboxylate's role, which is dependent on the corepressor's orientation in the binding pocket, is apparently to position the guanidinium group of Arg-84 for favorable contacts with the operator's sugar-phosphate backbone.     

Synthesis of cyclic analogues of cholecystokinin highly selective for central receptors

Cyclic CCK analogues in which positions 28 and 31 have been replaced by lysine residues and whose side chains are bridged by a succinic moiety, were synthesized. They were tested for their ability to inhibit the binding of 125I-BH-CCK-8 to isolated rat pancreatic acini and to guinea pig brain membranes. These cyclic CCK-analogues were compared to the potent CCK analogue Boc-[Nle28,31]-CCK-7 and to Boc-Trp-Leu-Asp-Phe-NH2, analogue of CCK-4. These cyclic compounds appeared to be highly selective for central CCK receptors.     

Comparative effects of caffeine, its analogues and calcium deficiency on cytokinesis

The effects of caffeine, aminophylline, caffeic acid, and calcium deficiency on cytokinesis were studied by light and electron microscopy. All these treatments blocked cell plate formation, resulting in the formation of binucleate cells. The aggregation and organization of membranous vesicles at the ‘presumptive cell plate’ during these treatments appears similar to that of normal cells, but fusion of the vesicles is insufficient to form a complete cell plate. It is suggested that some aspect of membrane recognition and fusion is the process actually interfered with by these treatments. Greater numbers of binucleate cells and fewer partial cell plates were observed in cells treated with caffeine and aminophylline as compared with those exposed to caffeic acid or calcium deficiency, indicating that the latter treatments do not block cell plate formation as efficiently as the former.