Design, Synthesis, and Biological Evaluation of C⁹- and C²-Substituted Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as New A_2Aand A₃Adenosine Receptors Antagonists

Abstract

In the past few years, our group has been involved in the development of A(2A) and A(3) 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 A(2A) receptor subtype, and N(8)-substituted-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines-N(5)-urea or amide (MRE series, b), very selective at the human A(3) adenosine receptor subtype. We now describe a large series of C(9)- and C(2)-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, N(8)-substituted-pirazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines-N(5)-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 C(9)-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.

Substructure‐Based Virtual Screening for Adenosine A2A Receptor Ligands

“…The rationale behind this filtering step was that many A 2A AR antagonists contain a furan-2-yl group, and that substitutions of the 2-furanyl ring generally lead to decreased A 2A binding. [23,24] The high occurrence of this substructure is reflected in the list of substructures used for screening: the furan ring is present in three of the 50 top-ranked substructures (table SI1 in the Supporting Information), two of which are substructures d and e (Table 2). These substructures not only match the unsubstituted furan moiety, but all substitution patterns of the furan ring, hence the extra filtering step.…”

“…The (often unsubstituted) furan moiety is a common structural feature found in many A 2A antagonists. [23,24] It is typically linked to a triazole ring that is part of a ring-fused system of two or three heterocycles, with a single amine group attached. Describing molecules in terms of molecular parts, or fragments, such as rings and ring systems, substituents and functional groups, provides an abstraction for chemists to reason about molecules.…”

Induced fit and pharmacophore generation approach applied to A2A adenosine receptor antagonists

“…[9][10][11][12][13][14][15][16][17][18][19][20] We selected the training set using chosen molecules with human binding data, defined stereochemistry and biological data (e.g. structure with biological data such as Ki>500 were excluded from the training set).…”

“…Some SARs studies 10 suggest that the furanyl ring at the 2-position of the tricyclic structure is a necessary element to guarantee the activity of the antagonists, probably because of the oxygen atom that may produce a favorable electronic interaction with the adenosine receptor. On the other hand, the 3D structure of the antagonist-receptor complex generated via induced fit does not highlight any kind of hydrophilic interaction in the binding site region occupied by this moiety.…”

Synthesis of some new azoles of potential antiviral activity

“…16 Recently, 1,3,4-oxadiazoles, 1,3,4-thiadiazoles, 1,2,4-triazoles and triazines have attracted particular attention due their anti-inflammatory, [17][18][19] analgesic, ulcerogenic and lipid peroxidation activities. 17 Because of our interest in the synthesis of new 1,3,4-oxadiazoles, 1,2,4-triazoles, 1,3,4-thiadiazines and pyrazole derivatives of biological interest, [20][21][22][23][24][25][26] we decided to synthesize the title compounds for the future evaluation of their antiviral activity.…”