Structure-Based Design, Synthesis, and In Vivo Antinociceptive Effects of Selective A1 Adenosine Receptor Agonists

The Hydrazine Moiety in the Synthesis of Modified Nucleosides and Nucleotides

Synthetic nucleoside mimics are re-emerging as crucial contenders for antiviral and anticancer medications. While, Ribavirin stands out for its unique antiviral properties, predominantly associated with its distinctive triazole heterocycle as a nucleobase, the exploration of alternative nitrogen-based aromatic heterocycles hold great promises for the discovery of novel bioactive nucleoside mimics. Although nucleoside derivatives synthesized from hydrazine-ribose units have been in development for many decades, they have been little evaluated biologically and even less for their antiviral properties. With the aim of taking a closer look at these under-explored derivatives and investigating their synthetic pathways, this review provides an overview of the molecular design, the chemical synthesis, and the biological activity, when available, of these nucleoside analogues. Overall, the entire body of work already done motivates further exploration of these analogues and encourages us of formulating structurally novel nucleoside drug candidates featuring innovative mode of action.

Curcumin Stereoisomer, Cis-Trans Curcumin, as a Novel Ligand to A1 and A3 Adenosine Receptors

Adenosine receptors (ARs) are being explored to generate non-opioid pain therapeutics. Vanilloid compounds, curcumin, capsaicin, and vanillin possess antinociceptive properties through their interactions with the transient receptor potential channel family. However, their binding with adenosine receptors has not been well studied. The hypothesis in this study was that a vanilloid compound, cis-trans curcumin (CTCUR), binds to each of the two Gi-linked AR subtypes (A₁AR and A₃AR). CTCUR was synthesized from curcumin (CUR) using the cavitand-mediated photoisomerization technique. The cell lines transfected with the specific receptor (A₁AR or A₃AR) were treated with CTCUR or CUR and the binding was analyzed using competitive assays, confocal microscopy, and docking. The binding assays and molecular docking indicated that CTCUR had Ki values of 306 nM (A₁AR) and 400 nM (A₃AR). These values suggest that CTCUR is selective for Gi-linked ARs (A₁AR or A₃AR) over Gs-linked ARs (A_2AAR or A_2BAR), based on our previous published research. In addition, the docking showed that CTCUR binds to the toggle switch domain of ARs. Curcumin (CUR) did not exhibit binding at any of these receptors. In summary, CTCUR and other modifications of CUR can be developed as novel therapeutic ligands for the Gi-linked ARs (A₁AR and A₃AR) involved with pain and cancer.

Discovery and Structure-Activity Relationship Studies of Novel Adenosine A1 Receptor-Selective Agonists

A series of benzyloxy and phenoxy derivatives of the adenosine receptor agonists N6-cyclopentyl adenosine (CPA) and N6-cyclopentyl 5'-N-ethylcarboxamidoadenosine (CP-NECA) were synthesized, and their potency and selectivity were assessed. We observed that the most potent were the compounds with a halogen in the meta position on the aromatic ring of the benzyloxy- or phenoxycyclopentyl substituent. In general, the NECA-based compounds displayed greater A1R selectivity than the adenosine-based compounds, with N6-2-(3-bromobenzyloxy)cyclopentyl-NECA and N6-2-(3-methoxyphenoxy)cyclopentyl-NECA showing ∼1500-fold improved A1R selectivity compared to NECA. In addition, we quantified the compounds' affinity and kinetics of binding at both human and rat A1R using a NanoBRET binding assay and found that the halogen substituent in the benzyloxy- or phenoxycyclopentyl moiety seems to confer high affinity for the A1R. Molecular modeling studies suggested a hydrophobic subpocket as contributing to the A1R selectivity displayed. We believe that the identified selective potent A1R agonists are valuable tool compounds for adenosine receptor research.

General Approach to N6,C5'-Difunctionalization of Adenosine

Among the C6-halo purine ribonucleosides, the readily accessible 6-chloro derivative has been known to undergo slow S_NAr reactions with amines, particularly aryl amines. In this work, we show that in 0.1 M AcOH in EtOH, aryl amines react quite efficiently at the C6-position of 2',3',5'-tri-O-(t-BuMe₂Si)-protected 6-chloropurine riboside (6-ClP-riboside), with concomitant cleavage of the 5'-silyl group. These two-step processes proceeded in generally good yields, and notably, reactions in the absence of AcOH were much slower and/or lower yielding. Corresponding reactions of 2',3',5'-tri-O-(t-BuMe₂Si)-protected 6-ClP-riboside with alkyl amines proceeded well but without desilylation at the primary hydroxyl terminus. These differences are likely due to the acidities of the ammonium chlorides formed in these reactions, and the role of AcOH was not desilylation but possibly only purine activation. With 50% aqueous TFA in THF at 0 °C, cleavage of the 5'-silyl group from 2',3',5'-tri-O-(t-BuMe₂Si)-protected N⁶-alkyl adenosine derivatives and from 6-ClP-riboside was readily achieved. Reactions of the 5'-deprotected 6-ClP-riboside with alkyl amines proceeded in high yields and under mild conditions. Because these complementary methodologies yielded N⁶-aryl and -alkyl adenosine derivatives containing a free 5'-hydroxyl group, a variety of product functionalizations were undertaken to yield N⁶,C5'-doubly modified nucleoside analogues.

Acute visceral pain relief mediated by A3AR agonists in rats: involvement of N-type voltage-gated calcium channels

ABSTRACT

Pharmacological tools for chronic visceral pain management are still limited and inadequate. A3 adenosine receptor (A3AR) agonists are effective in different models of persistent pain. Recently, their activity has been related to the block of N-type voltage-gated Ca2+ channels (Cav2.2) in dorsal root ganglia (DRG) neurons. The present work aimed to evaluate the efficacy of A3AR agonists in reducing postinflammatory visceral hypersensitivity in both male and female rats. Colitis was induced by the intracolonic instillation of 2,4-dinitrobenzenesulfonic acid (DNBS; 30 mg in 0.25 mL 50% EtOH). Visceral hypersensitivity was assessed by measuring the visceromotor response and the abdominal withdrawal reflex to colorectal distension. The effects of A3AR agonists (MRS5980 and Cl-IB-MECA) were evaluated over time after DNBS injection and compared to that of the selective Cav2.2 blocker PD173212, and the clinically used drug linaclotide. A3AR agonists significantly reduced DNBS-evoked visceral pain both in the postinflammatory (14 and 21 days after DNBS injection) and persistence (28 and 35 days after DNBS) phases. Efficacy was comparable to effects induced by linaclotide. PD173212 fully reduced abdominal hypersensitivity to control values, highlighting the role of Cav2.2. The effects of MRS5980 and Cl-IB-MECA were completely abolished by the selective A3AR antagonist MRS1523. Furthermore, patch-clamp recordings showed that A3AR agonists inhibited Cav2.2 in dorsal root ganglia neurons isolated from either control or DNBS-treated rats. The effect on Ca2+ current was PD173212-sensitive and prevented by MRS1523. A3AR agonists are effective in relieving visceral hypersensitivity induced by DNBS, suggesting a potential therapeutic role against abdominal pain.

Adenosine receptors as promising targets for the management of ocular diseases

The ocular drug discovery arena has undergone a significant improvement in the last few years culminating in the FDA approvals of 8 new drugs. However, despite a large number of drugs, generics, and combination products available, it remains an urgent need to find breakthrough strategies and therapies for tackling ocular diseases. Targeting the adenosinergic system may represent an innovative strategy for discovering new ocular therapeutics. This review focused on the recent advance in the field and described the numerous nucleoside and non-nucleoside modulators of the four adenosine receptors (ARs) used as potential tools or clinical drug candidates.

Synthesis of Ribavirin, Tecadenoson, and Cladribine by Enzymatic Transglycosylation

Despite the impressive progress in nucleoside chemistry to date, the synthesis of nucleoside analogues is still a challenge. Chemoenzymatic synthesis has been proven to overcome most of the constraints of conventional nucleoside chemistry. A purine nucleoside phosphorylase from Aeromonas hydrophila (AhPNP) has been used herein to catalyze the synthesis of Ribavirin, Tecadenoson, and Cladribine, by a “one-pot, one-enzyme” transglycosylation, which is the transfer of the carbohydrate moiety from a nucleoside donor to a heterocyclic base. As the sugar donor, 7-methylguanosine iodide and its 2′-deoxy counterpart were synthesized and incubated either with the “purine-like” base or the modified purine of the three selected APIs. Good conversions (49–67%) were achieved in all cases under screening conditions. Following this synthetic scheme, 7-methylguanine arabinoside iodide was also prepared with the purpose to synthesize the antiviral Vidarabine by a novel approach. However, in this case, neither the phosphorolysis of the sugar donor, nor the transglycosylation reaction were observed. This study was enlarged to two other ribonucleosides structurally related to Ribavirin and Tecadenoson, namely, Acadesine, or AICAR, and 2-chloro-N6-cyclopentyladenosine, or CCPA. Only the formation of CCPA was observed (52%). This study paves the way for the development of a new synthesis of the target APIs at a preparative scale. Furthermore, the screening herein reported contributes to the collection of new data about the specific substrate requirements of AhPNP.

The role of adenosine and P2Y receptors expressed by multiple cell types in pain transmission

The role of extracellular nucleotides and nucleosides as signaling molecules in cell-to-cell communication has now been clearly established. This is particularly true in the central and peripheral nervous system, where purines and pyrimidines are involved in both physiological and pathological interactions between neurons and surrounding glial cells. It can be thus foreseen that the purinergic system could represent a new potential target for the development of effective analgesics, also through the normalization of neuronal functions and the inhibition of glial cell activation. Research in the last 15 years has progressively confirmed this hypothesis, but no purinergic-based analgesics have reach the market so far; in the present review we have collected the more recent discoveries on the role of G protein-coupled P2Y nucleotide and of adenosine receptors expressed by both neurons and glial cells under painful conditions, and we have highlighted some of the challenges that must be faced to translate basic and preclinical studies to clinics.

Design and in Vivo Characterization of A1 Adenosine Receptor Agonists in the Native Ribose and Conformationally Constrained (N)-Methanocarba Series

(N)-Methanocarba ([3.1.0]bicyclohexyl) adenosines and corresponding ribosides were synthesized to identify novel A1 adenosine receptor (A1AR) agonists for CNS or peripheral applications. Human and mouse AR binding was determined to assess the constrained ring system's A1AR compatibility. N6-Dicyclobutylmethyl ribose agonist (9, MRS7469, >2000-fold selective for A1AR) and known truncated N6-dicyclopropylmethyl methanocarba 7 (MRS5474) were drug-like. The pure diastereoisomer of known riboside 4 displayed high hA1AR selectivity. Methanocarba modification reduced A1AR selectivity of N6-dicyclopropylmethyl and endo-norbornyladenosines but increased ribavirin selectivity. Most analogues tested (ip) were inactive or weak in inducing mouse hypothermia, despite mA1AR full agonism and variable mA3AR efficacy, but strong hypothermia by 9 depended on A1AR, which reflects CNS activity (determined using A1AR or A3AR null mice). Conserved hA1AR interactions were preserved in modeling of 9 and methanocarba equivalent 24 (∼400-fold A1AR-selective). Thus, we identified, and characterized in vivo, ribose and methanocarba nucleosides, including with A1AR-enhancing N6-dicyclobutylmethyl-adenine and 1,2,4-triazole-3-carboxamide (40, MRS7451) nucleobases.