N6-cyclopentyl-3'-substituted-xylofuranosyladenosines: a new class of non-xanthine adenosine A1 receptor antagonists

Synthesis and evaluation of 5-substituted 2'-deoxyuridine monophosphate analogues as inhibitors of flavin-dependent thymidylate synthase in Mycobacterium tuberculosis

A series of 5-substituted 2'-deoxyuridine monophosphate analogues has been synthesized and evaluated as potential inhibitors of mycobacterial ThyX, a novel flavin-dependent thymidylate synthase in Mycobacterium tuberculosis. A systematic SAR study led to the identification of compound 5a, displaying an IC(50) value against mycobacterial ThyX of 0.91 μM. This derivative lacks activity against the classical mycobacterial thymidylate synthase ThyA (IC(50) > 50 μM) and represents the first example of a selective mycobacterial FDTS inhibitor.

Synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine

The synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine is reported. Initial incorporation of N,N-dibenzylformamidino protection of the C2-amino of guanosine masked the reactivity of that group and simplified purification of subsequent analogues. The first key intermediate, 9-(2,5-bis-O-tert-butyldimethylsilyl-beta-D-ribofuranosyl)-2-N-(N,N-dibenzylformamidino)guanine (3a), was prepared in 60% yield after recycling of the undesired 3',5'-bis-O-protected byproduct (4a) by simple equilibration in methanol to a mixture of the two bis-O-protected compounds. Thus, protected, the 3'-position was manipulated to form the 3'-deoxyribo- or 3'-deoxyxylo-3'-azido derivatives (9 or 16, respectively). Further selective manipulations provided the cis-5'-monophosphate (3-chlorophenyl)-1,3-propanyl diester prodrugs (HepDirect prodrugs), 15 and 21. These HepDirect prodrugs were demonstrated to activate to their respective NTPs in rat hepatocytes.

2,6-disubstituted and 2,6,8-trisubstituted purines as adenosine receptor antagonists

Purines have long been exploited as adenosine receptor antagonists. The substitution pattern about the purine ring has been well investigated, and certain criteria have become almost a prerequisite for good affinity at the adenosine A(1) receptor. The adaptation of the pharmacophore and the initial series of pyrimidines developed in an earlier publication resulted in a series of purines with an entirely new substitution pattern. One compound in particular, 8-cyclopentyl-2,6-diphenylpurine (31, LUF 5962) has been shown to be very promising with an affinity of 0.29 nM at the human adenosine A(1) receptor.

Allosteric modulation, thermodynamics and binding to wild-type and mutant (T277A) adenosine A1 receptors of LUF5831, a novel nonadenosine-like agonist

The interaction of a new nonribose ligand (LUF5831) with the human adenosine A1 receptor was investigated in the present study. Radioligand binding experiments were performed in the absence and presence of diverse allosteric modulators on both wild-type (wt) and mutant (T277A) adenosine A1 receptors. Thermodynamic data were obtained by performing these assays at different temperatures. In addition, cyclic adenosine monophosphate (cAMP) assays were performed. The presence of allosteric modulators had diverse effects on the affinity of LUF5831, N6-cyclopentyladenosine (CPA), a full agonist, and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an inverse agonist/antagonist, for the adenosine A1 receptor. PD81,723, for example, increased the affinity of CPA, while the affinity of LUF5831 was decreased. However, the affinity of DPCPX was decreased even more. In addition, LUF5831 was shown to have an affinity for the mutant (T277A) adenosine A1 receptor (Ki=122+/-22 nM), whereas CPA's affinity was negligible. The results of temperature-dependent binding assays showed that the binding of LUF5831 was entropy driven, in between the behaviour of CPA binding to the high- and low-affinity states of the receptor, respectively. The inhibition of the forskolin-induced production of cAMP through activation of the wt adenosine A1 receptor showed that LUF5831 had a submaximal effect (37+/-1%) in comparison to CPA (66+/-5%). On the mutant receptor, however, neither CPA nor LUF5831 inhibited cAMP production. This study indicates that the nonribose ligand, LUF5831, is a partial agonist for the adenosine A1 receptor.

Structure-activity relationships of inverse agonists for G-protein-coupled receptors

It has been recently established that G-protein-coupled receptors (GPCRs) can be constitutively active, i.e., they can be active in the absence of an agonist. This activity can be inhibited by so-called inverse agonists. For a number of GPCRs, such inverse agonists have been developed and studied, now enabling for the first time a study into their structure-activity relationships.

A series of ligands displaying a remarkable agonistic-antagonistic profile at the adenosine A1 receptor

Adenosine receptor agonists are usually variations of the natural ligand, adenosine. The ribose moiety in the ligand has previously been shown to be of great importance for the agonistic effects of the compound. In this paper, we present a series of nonadenosine ligands selective for the adenosine A(1) receptor with an extraordinary pharmacological profile. 2-Amino-4-benzo[1,3]dioxol-5-yl-6-(2-hydroxyethylsulfanyl)pyridine-3,5-dicarbonitrile (70, LUF 5853) shows full agonistic behavior comparable with the reference compound CPA, while also displaying comparable receptor binding affinity (K(i) = 11 nM). In contrast, compound 58 (2-amino-4-(3-trifluoromethylphenyl)-6-(2-hydroxyethylsulfanyl)pyridine-3,5-dicarbonitrile, LUF 5948) has a binding affinity of 14 nM and acts as an inverse agonist. Also present within this same series are compounds that show neutral antagonism of the adenosine A(1) receptor, for example compound 65 (2-amino-4-(4-difluoromethoxyphenyl)-6-(2-hydroxyethylsulfanyl)pyridine-3,5-dicarbonitrile, LUF 5826).

Neutral, acidic, and basic derivatives of anthranilamide that confer different formal charge to reducing oligosaccharides

To facilitate the use of oligosaccharides as analytical tools in biological studies, we have designed, synthesized, and conjugated to maltosaccharides a novel series of homologous small fluorescent moieties that differ in formal charge. These moieties are amide derivatives of anthranilic acid: uncharged N-(2-aminobenzoyl)glycinamide (ABGlyAmide; 2), acidic N,N-dimethyl-N(')-(2-aminobenzoyl)ethylenediamine (ABGlyDIMED; 3), and basic N-(2-aminobenzoyl)glycine (ABGly; 1). Routes for synthesis and optimal reaction conditions for glycoconjugation by conventional reductive amination are presented, as is the compatibility of these adducts with common analytical and preparative chromatographic methods, including RP-HPLC and HPAEC-PAD. These novel anthranilic acid derivatives confer both fluorescence and defined charge to oligosaccharides, and so enhance the repertoire of chromatographic and analytical methods for which anthranilic acid can be used. Furthermore, because glucosaccharides have rigid solution structure, these small fluorescent adducts with different formal charge are ideal tools for molecular sizing studies of membrane pores.

9-Ethyladenine derivatives as adenosine receptor antagonists: 2- and 8-substitution results in distinct selectivities

9-Ethyladenine was used as the basis for a series of non-xanthine adenosine receptor antagonists at human adenosine receptors. The adenine-based compounds were substituted in 2- or 8-position with a variety of side chains including some aryl or arylalkynyl groups previously tested as 2-substituents in adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) for their effect on agonist affinity. The affinity of the novel compounds was tested in radioligand binding assays (A1, A2A and A3) and inhibition of NECA-stimulated adenylyl cyclase activity (A2B) in membranes prepared from CHO cells stably transfected with the respective human receptor subtype. High affinity antagonists were identified for A1 (9-ethyl-8-phenyl-9H-adenine, compound 2; 6-(1-butylamino)-9-ethyl-8-phenyl-9H-purine, compound 3), A2A (8-ethoxy-9-ethyladenine; compound 8) and A3 (9-ethyl-8-phenylethynyl-9H-adenine, compound 5) with selectivities versus other receptor subtypes in the range of 10 to 600. These results demonstrate that adenine is a useful template for further development of high-affinity antagonists with distinct receptor selectivity profiles.

New 2-Alkynyl Derivatives of the Acyclic Nucleoside 9-(2,3-Dihydroxypropyl)adenine and Their 6-Guanidinopurine Counterparts as Potential Effectors of Adenosine Receptors

A series of the new 2-alkynyl derivatives of the acyclic nucleoside 9-(2,3-dihydroxypropyl)- adenine and their 6-guanidinopurine analogues were prepared by the Sonogashira coupling. The effect of the prepared compounds on A1 and A2A receptors was examined.

2- and 8-alkynyladenosines: conformational studies and docking to human adenosine A3 receptor can explain their different biological behavior

Adenosine (Ado) derivatives substituted at the C2 position with an alkynyl chain are endowed with high affinity for A(1), A(2A) and A(3) human adenosine receptors, while being less active at the low affinity A(2B) subtype. On the other hand, the introduction of an alkynyl chain at the C8 position of adenosine is detrimental for the affinity and potency at A(1), A(2A), and A(2B) receptors, while is more tolerated by the A(3) receptor. The evaluation of the stimulation of [35S]GTPgammaS binding revealed that 2-alkynyladenosines behave as adenosine receptors agonists while, on the contrary, 8-alkynyladenosines behave as antagonists. With this work we demonstrated, by means of an NMR-based and a computational conformational analysis, that 8-alkynyladenosines, differently from 2-alkynyladenosines, cannot adopt the sugar-base anti conformation required for adenosine receptor activation.Furthermore, using the recently reported X-ray crystal structure of bovine rhodopsin as template, we built a 3D model of the seven transmembrane domains of the human adenosine A(3) receptor with the homology modeling. After identification of the binding site we carried out docking experiments, demonstrating that the two class of molecules have different binding modes that explain their different degree of affinity and the shift of their activity from agonism to antagonism.

5'-Deoxy congeners of 9-(3-amido-3-deoxy-beta-D-xylofuranosyl)-N(6)-cyclopentyladenine: new adenosine A(1) receptor antagonists and inverse agonists

The synthesis and structure-activity relationship of N(6)-cyclopentyl-3'-substituted-xylofuranosyladenosine analogues with respect to various adenosine receptors were explored in order to identify selective and potent antagonists and inverse agonists for the adenosine A(1) receptor. In particular, the effects of removal of the 5'-OH group and introduction of selected substituents at the 3'-NH(2) position of 9-(3-amino-3-deoxy-beta-D-xylofuranosyl)-N(6)-cyclopentyladenine were probed. A solid phase-assisted synthetic approach was used to optimize the 3'-amide functionality. In view of the general concern of the presence of a 5'-OH moiety with regard to cellular toxicity, the present study describes 5'-deoxy compounds with reasonable affinity for the human adenosine A(1) receptor. Interestingly, this study shows that optimization of the 3'-"up" amide substituent can substantially compensate for the drop in affinity for the adenosine A(1) receptor, which is generally observed upon removal of the 5'-OH group. The fact that for several 3'-amido-substituted (5'-deoxy)-N(6)-cyclopentyladenosine derivatives, guanosine 5'-triphosphate-induced shifts in K(i) values were significantly lower than 1 implies that these analogues behave as inverse agonists. This is further supported by their 1,3-dipropyl-8-cyclopentylxanthine-like capacity to increase forskolin-induced adenosine cyclic 3',5'-phosphate production.

Neoceptor concept based on molecular complementarity in GPCRs: a mutant adenosine A(3) receptor with selectively enhanced affinity for amine-modified nucleosides

Adenosine A(3) receptors are of interest in the treatment of cardiac ischemia, inflammation, and neurodegenerative diseases. In an effort to create a unique receptor mutant that would be activated by tailor-made synthetic ligands, we mutated the human A(3) receptor at the site of a critical His residue in TM7, previously proposed to be involved in ligand recognition through interaction with the ribose moiety. The H272E mutant receptor displayed reduced affinity for most of the uncharged A(3) receptor agonists and antagonists examined. For example, the nonselective agonist 1a was 19-fold less potent at the mutant receptor than at the wild-type receptor. The introduction of an amino group on the ribose moiety of adenosine resulted in either equipotency or enhanced binding affinity at the H272E mutant relative to wild-type A(3) receptors, depending on the position of the amino group. 3'-Amino-3'-deoxyadenosine proved to be 7-fold more potent at the H272E mutant receptor than at the wild-type receptor, while the corresponding 2'- and 5'-amino analogues did not display significantly enhanced affinities. An 3'-amino-N(6)-iodobenzyl analogue showed only a small enhancement at the mutant (K(i) = 320 nM) vs wild-type receptors. The 3'-amino group was intended for a direct electrostatic interaction with the negatively charged ribose-binding region of the mutant receptor, yet molecular modeling did not support this notion. This design approach is an example of engineering the structure of mutant receptors to recognize synthetic ligands for which they are selectively matched on the basis of molecular complementarity between the mutant receptor and the ligand. We have termed such engineered receptors "neoceptors", since the ligand recognition profile of such mutant receptors need not correspond to the profile of the parent, native receptor.

Hydrocarbon activation. Synthesis of beta-cycloalkyl (di)nitriles through photosensitized conjugate radical addition

Photoinduced hydrogen abstraction from aliphatic cyclic hydrocarbons (C(5) to C(7), C(12), as well as adamantane) by triplet aromatic ketones in the presence of alpha,beta-unsaturated (di)nitriles offers a straightforward entry to the corresponding alkylated (di)nitriles via the alkyl radicals. Yields are moderate to good depending on the olefins structure (substitution in beta slows down the addition to mononitriles, but with alpha,alpha-dinitriles electronic activation allows efficient alkylation also of beta,beta-disubstituted substrates). A tandem alkylation-cyclization process has been obtained with (1-methylpent-4-enylidene)malononitrile.

Selective A1-adenosine receptor antagonists identified using yeast Saccharomyces cerevisiae functional assays

Evaluation of a biased "library" of pyrrolo[2,3-d]pyrimidines using yeast-based functional assays expressing human A1- and A2a-adenosine receptors, led to the A1 selective antagonist 4b. A direct correlation between yeast functional activity and binding data was established. Practical compounds with polar residues at C-4 of the pyrrolopyrimidine system required H-bond donor functionality for high potency.