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Compounds presenting an additional fused ring on the xanthine nucleus have been reported to exhibit antagonistic activity with various levels of affinity and selectivity toward the four adenosine receptors subtypes A1, A2A, A2B, and A3. This paper reports synthesis and biological evaluation of new 1-benzyl-3-propyl-1H,6H-pyrrolo[2,1-f]purine-2,4-diones and 1-benzyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-diones, among which we identified potent and selective A3 adenosine receptors antagonists. In particular, 1-benzyl-7-methyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-dione (11e) shows a Ki (hA3) value from binding assay of 0.8 nM.

In this paper we report the synthesis and biological evaluation of a new class of 2-phenyl-2,5-dihydro-pyrazolo[4,3-c]quinolin-4-ones as A3 adenosine receptor antagonists. We designed a new route based on the Kira-Vilsmeier reaction for the synthesis of this class of compounds. Some of the synthesized compounds showed A3 adenosine receptor affinity in the nanomolar range and good selectivity as evaluated in radioligand binding assays at human (h) A1, A2A, A2B, and A3 adenosine receptor subtypes. We introduced several substituents on the 2-phenyl ring. In particular substitution at the 4-position by methyl, methoxy, and chlorine gave optimal activity and selectivity 6c (KihA1, A2A>1000 nM, EC50hA2B>1000 nM, KihA3 = 9 nM), 6d (KihA1, A2A>1000 nM, EC50hA2B>1000 nM, KihA3 = 16 nM), 6b (KihA1, A2A >1000 nM, EC50hA2B>1000 nM, KihA3 = 19 nM). In conclusion, the 2-phenyl-2,5-dihydro-pyrazolo[4,3-c]quinolin-4-one derivatives described herein represent a new family of in vitro selective antagonists for the adenosine A3 receptor.

A new series of 1-deoxy-1-[(6-(4-(substituted-aminosulfonyl)phenyl)amino)carbonylamino-9H-purin-9-yl]-N-ethyl-β-d-ribofuranuronamides (83−102) have been synthesized and tested at the human A3 adenosine receptor subtype. All the derivatives described in this work displayed affinity versus this receptor in the nanomolar range and good selectivity versus A1 adenosine receptor subtype, confirming that the p-sulfonamido moiety positively affected the activity of the molecules. The best substituents at the sulfonamido nucleus were found to be small alkyl groups, like methyl, isopropyl, ethyl, or allyl moieties (compounds 96−100), whereas monosubstitution at the amino group led to a decrease in A3 affinity values. The selectivity versus A1 adenosine receptor subtype is increased when the amino group in the sulfonamido core is represented by a hydrogenated heterocyclic ring like piperidine, morpholine, or pyrroline. Bulky groups, like adamantane and alkyl chains with more than four carbon atoms, are detrimental for the affinity and the selectivity of the A3 adenosine receptor agonists described here.

Here we report the synthesis of 8-heterocycle-substituted xanthines as potent and selective A2B adenosine receptor antagonists. The structure−activity relationships (SAR) of the xanthines synthesized in binding to recombinant human A2B adenosine receptors (ARs) in HEK-293 cells (HEK-A2B) and at other AR subtypes were explored. The synthesized compounds showed A2B adenosine receptor affinity in the nanomolar range and good levels of selectivity evaluated in radioligand binding assays at human (h) A1, A2A, A2B, and A3 ARs. We introduced several heterocycles, such as pyrazole, isoxazole, pyridine, and pyridazine, at the 8-position of the xanthine nucleus and we have also investigated different spacers (substituted acetamide, oxyacetamide, and urea moieties) on the heterocycle introduced. Various groups at the 3- and 4-positions of phenylacetamide moiety were studied. This study allowed us to identify the derivatives 2-(3,4-dimethoxyphenyl)-N-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yl]acetamide (29b, MRE2028F20) [Ki(hA2B) = 38 nM, Ki(hA1,hA2A,hA3) >1000 nM], N-benzo[1,3]dioxol-5-yl-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]acetamide (62b, MRE2029F20) [Ki(hA2B) = 5.5 nM, Ki(hA1,hA2A,hA3) > 1000 nM], and N-(3,4-dimethoxyphenyl)-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]acetamide (72b, MRE2030F20) [Ki(hA2B = 12 nM, Ki(hA1,hA2A, hA3) > 1000 nM], which showed high affinity at the A2B receptor subtype and very good selectivity vs the other ARs. Substitution of the acetamide with an urea moiety afforded bioisosteric xanthines with good affinity and selectivity comparable to the acetamide derivatives. Substitution at the para-position of a 4-benzyloxy group of the phenylacetamido chain enhanced affinity at the A2B receptor [compound 30b (Ki(hA2B) = 13 nM) vs compound 21b (Ki(hA2B = 56 nM)] but did not favor selectivity. The derivatives with higher affinity at human A2B AR proved to be antagonists, in the cyclic AMP assay, capable of inhibiting the stimulatory effect of NECA (100 nM) with IC50 values in the nanomolar range, a trend similar to that observed in the binding assay (62b, IC50 = 38 nM; 72b, IC50 = 46 nM). In conclusion, the 8-pyrazolo-1,3-dipropyl-1H-purine-2,6-dione derivatives described herein represent a new family of selective antagonists for the adenosine A2B receptor.

In the past few years, our group has been involved in the development of A2A and A3 adenosine receptor antagonists which led to the synthesis of SCH58261 (5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine, 61), potent and very selective at the A2A receptor subtype, and N⁸-substituted-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines-N⁵-urea or amide (MRE series, b), very selective at the human A3 adenosine receptor subtype. We now describe a large series of C⁹- and C²-substituted pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines to represent an extension of structure−activity relationship work on this class of tricyclic compounds. The introduction of a substituent at 9 position of the tricyclic antagonistic structure led to retention of receptor affinity but a loss of selectivity in respect to the lead compounds b, N⁸-substituted-pirazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines-N⁵-urea or -amide. The substitution of the furanyl moiety of compound 61, necessary for receptor binding, with a phenyl or a substituted aromatic ring (compounds 5a−d, 6−8), caused a complete loss of the affinity at all the adenosine receptor subtypes, demonstrating that the furanyl ring is a necessary structural element to guarantee interaction with the adenosine receptor surface. The introduction of an ethoxy group at the ortho position of the aromatic ring to mimic the oxygen of the furan (compound 5c, 5-amino-7-(2-phenylethyl)-2-(2-ethoxyphenyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine) did not enhance affinity. The introduction of the cycloaminomethyl function by Mannich reaction at the 5‘ position of the furanyl ring of 61 and the C⁹-substituted compound 41 (5-amino-8-methyl-9-methylsulfanyl-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine) resulted in complete water solubility but a loss of receptor affinity. We can conclude that modifications or substitutions at the furanyl ring are not allowed and the introduction of a substituent at the 9-position of the core pyrazolo-triazolo-pyrimidine structure caused a severe loss of selectivity, probably due to an increased steric hindrance of the radical introduced.

Many natural and synthetic anticancer agents with the ability to interact with DNA have been discovered, but most have little sequence-specificity and often exhibit severe toxicity to normal tissues. Thus, there has been considerable interest in molecular biology and human medicine to find small molecules that can alkylate the DNA in a sequence-specific manner and modify the function of nucleic acids irreversibly. Analogs of naturally occurring antitumor agents, such as distamycin A, which bind in the minor groove of DNA, represent a new class of anticancer compounds currently under investigation. Distamycin A has driven researchers' attention not only for its biological activity, but also for its nonintercalative binding to the minor groove of double-stranded B-DNA, where it forms a strong reversible complex preferentially at the nucleotide sequences consisting of 4-5 adjacent adenine-thymine (AT) base pairs. The pyrrole-amide skeleton of distamycin A has been also used as DNA sequence-selective vehicles for the delivery of alkylating functions to DNA targets, leading to a sharp increase of its cytotoxicity, in comparison to that, very weak, of distamycin itself.