7‐Amino‐2‐aryl/hetero‐aryl‐5‐oxo‐5,8‐dihydro[1,2,4]triazolo[1,5‐a]pyridine‐6‐carbonitriles: Synthesis and adenosine receptor binding studies

A series of novel 7‐amino‐5‐oxo‐2‐substituted‐aryl/hetero‐aryl‐5,8‐dihydro[1,2,4]triazolo[1,5‐a]pyridine‐6‐carbonitriles ( 4a–4t ) was synthesized, characterized and evaluated for their binding affinity and selectivity towards hA₁, hA_2A, hA_2B and hA₃ adenosine receptors (ARs). Compound 4a with a phenyl ring at 2‐position of the triazolo moiety of the scaffold showed high affinity and selectivity for hA₁ AR (K_i hA₁ = 0.076 μM, hA_2A = 25.6 μM and hA₃ > 100 μM). Introduction of various electron donating and withdrawing groups at different positions of the phenyl ring resulted in drastic reduction in affinity and selectivity towards all the ARs, except compound 4b with a 4‐hydroxyphenyl group at 2‐position. Interestingly, the replacement of the phenyl ring with a smaller heterocyclic thiophene ring (π excessive system) resulted in further improvement of affinity for hA₁ AR of compound 4t (K_i hA₁ = 0.051 μM, hA_2A = 9.01 μM and hA₃ > 13.9 μM) while retaining the significant selectivity against all other AR subtypes similar to compound 4a . The encouraging results for compounds 4a and 4t indicate that substitution at 2‐position of the scaffold with π‐excessive systems other than thiophene may lead to even more potent and selective hA₁ AR antagonists.     

Evaluation of 2‐benzylidene‐1‐tetralone derivatives as antagonists of A1 and A2A adenosine receptors

Antagonists of the adenosine receptors (A₁ and A_2A) are thought to be beneficial in neurological disorders, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore 2‐benzylidene‐1‐tetralone derivatives as antagonists of A₁ and/or A_2A adenosine receptors. In general, the test compounds were found to be selective for the A₁ adenosine receptor, with only three test compounds possessing affinity for both the A₁ and A_2A adenosine receptor. The 2‐benzylidene‐1‐tetralones bearing a hydroxyl substituent at either position C5, C6 or C7 of ring A displayed favourable adenosine A₁ receptor binding, while C5 hydroxy substitution led to favourable A_2A adenosine receptor affinity. Interestingly, para‐hydroxy substitution on ring B in combination with ring A bearing a hydroxy at position C6 or C7 provided the 2‐benzylidene‐1‐tetralones with both A₁ and A_2A adenosine receptor affinity. Compounds 4 and 8 displayed the highest A₁ and A_2A adenosine receptor affinity with values below 7 μm . Both these compounds behaved as A₁ adenosine receptor antagonists in the performed GTP shift assays. In conclusion, the 2‐benzylidene‐1‐tetralone derivatives can be considered as lead compounds to design a new class of dual acting adenosine A₁/A_2A receptor antagonists that may have potential in treating both dementia and locomotor deficits in Parkinson's disease.     

Synthesis,adenosine receptor binding and molecular modelling studies of novel thieno[2,3‐d]pyrimidine derivatives

A series of new molecules containing a thieno[2,3‐d]pyrimidine scaffold was synthesized and characterized by adopting an efficient synthetic scheme. The effect of a free or substituted amino group at 2‐position as well as an oxo‐group, imidazole or 1,2,4‐triazole ring at 4‐position of the scaffold on the affinity and selectivity towards adenosine receptors (ARs) was evaluated. Compounds 17–19 with a free amino group at 2‐position along with the presence of an imidazole/1,2,4‐triazole ring at 4‐position of the scaffold showed selective binding affinities for hA_2A AR, whereas carbamoylation of the amino group at 2‐position (in the presence of an oxo‐group at 4‐position of the scaffold) increased the affinity and selectivity of certain compounds ( 7–10 ) for hA₃ AR. Molecular dynamic simulation study of one of the most active compound 8 (K_i hA₁ > 30 μm , hA_2A = 0.65 μm , and hA₃ = 0.124 μm ) revealed the role of important amino acid residues for imparting good affinity towards hA₃ and hA_2A ARs. Molecular docking studies were carried out for other compounds using the crystal structure of hA_2A AR and a homology model of hA₃ AR to rationalize their structure–activity relationships. The molecular docking results were in agreement with the experimental binding affinity data of ARs.     

Tritium‐labelled 8‐cyclopentyl‐3‐(3‐fluoropropyl)‐1‐propylxanthine ([3H]CPFPX), a potent and selective antagonist for the A1 adenosine receptor

The reduction of 1‐allyl‐8‐cyclopentyl‐3‐(3‐fluoropropyl)xanthine, 7 , with tritium gas catalyzed by 10% Pd‐C gave 8‐cyclopentyl‐3‐(3‐fluoropropyl)‐1‐[2,3‐³H]propylxanthine ([³H]CPFPX), 8^* , a potent and selective antagonist for the A₁ adenosine receptor (A₁AR). The synthesis of 7 proceeded from 6‐aminouracil, 1 , which underwent silylation and alkylation with allyl bromide to form 6‐amino‐3‐allyluracil, 2 . Nitrosation led to the 5‐nitroso compound, 3 , which underwent reduction to the 4,5‐diaminouracil, 4 , and carbodiimide‐mediated acylation with cyclopentanecarboxylic acid produced 3‐allyl‐6‐amino‐5‐cyclopentylcarboxamidouracil, 6 . Alkylation at N?1 with 3‐fluoro‐1‐bromopropane and cyclization with alkali completed the synthesis of 7 . [³H]CPFPX had a radiochemical purity of > 98% and a specific activity of >2.1 TBq/mmol (57 Ci / mmol). [³H]CPFPX bound to the rat, pig and human A₁AR with a K_D of 0.63, 1.37 and 0.71 nM, respectively. The K_D at the rat and human A_2AAR was 812 and 940 nM, respectively, thus giving selectivities of >1200‐ and >700‐fold. Copyright © 2003 John Wiley & Sons, Ltd.     

Adventitia removal does not modify the α1D‐adrenoceptors response in aorta during hypertension and ageing

1 The aim of the current study was to characterize the α₁‐adrenergic receptors (α₁‐ARs) present in the isolated tunica media of aorta, in normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats during the course of ageing and hypertension (rats of 1, 3, 6 and 12 months of age). In all vessels, endothelium was removed. 2 In isolated aortic rings, phenylephrine increased contraction in a concentration‐ and age‐dependent manner and was impaired in old SHR compared with WKY rats. 3 The α₁‐AR selective antagonist prazosin showed high affinity (pA₂) in vessels from both rat strains. 4 The potency of the α_1A‐AR selective antagonists, RS 100329 (5‐methyl‐3‐[3‐[4‐[2‐(2,2,2,‐trifluoroethoxy) phenyl]‐1‐piperazinyl] propyl]‐2,4‐(1H)‐pyrimidinedione) and 5‐methylurapidil in antagonizing aortic phenylephrine‐responses was low. 5 The α_1D‐AR selective antagonist, BMY 7378 (8‐[2‐[4‐(2‐methoxyphenyl)‐1 piperazynil] ethyl]‐8‐azaspiro [4.5] decane‐7,9‐dione) potently blocked phenylephrine‐induced responses in aorta from both strains and at all ages. 6 Adventitia removal decreased E_max in older rats and modified the relative affinity (pD₂), but did not affect the affinity of the selective antagonists. 7 The results suggest that aorta tunica α_1D‐AR is the main subtype involved in phenylephrine‐induced contraction of rat aorta, while α_1A‐AR plays only a minor role. 8 Ageing and hypertension did not modify α₁‐ARs in the blood vessel and the tunica adventitia does not seem to participate in contraction, even though α₁‐ARs are expressed.     

N‐[9‐(ortho‐Fluorobenzyl)‐2‐Phenyl‐8‐Azapurin‐6‐yl]‐Amides as Potent and Selective Ligands for A1 Adenosine Receptors

A series of N‐[9‐(ortho‐fluorobenzyl)‐2‐phenyl‐8‐azapurin‐6‐yl]‐amides were synthesized and tested for their affinity toward A₁, A_2A, and A₃ adenosine receptor subtypes. Biological results demonstrated that the introduction of a fluorine atom at the ortho position of the 9‐benzyl group generally enhanced affinity toward A₁ subtype and did not significantly affect A_2A and A₃ affinity. Very interesting is the compound bearing a meta‐fluorophenyl substituent on the carbonyl carbon of the amide group, which shows significantly high A₁/A_2A‐A₃ selectivity. Compounds of this new series, together with the previously published analogs without the fluorine atom on the 9‐benzyl group, constituted the starting dataset for the development of QSAR models. The models obtained were able to rationally describe the affinity trends resulting from biological testing and to enable investigation of the role of different substituents on the 8‐azapurine scaffold, as well as the influence of the newly introduced fluorine atom on the benzyl moiety. The said QSAR models can also assist in the design of new compounds selectively active on A₁ adenosine receptors. Furthermore, a molecular docking study was carried out to assess hypothetical binding mode of N‐[9‐(ortho‐fluorobenzyl)‐2‐phenyl‐8‐azapurin‐6‐yl]‐amides to A₁ adenosine receptors.     

Adenosine A1 modulators: a patent update (2008 to present)

Introduction: In 2008, we published our review titled ‘Therapeutic potential of A₁ adenosine receptor ligands - a survey of recent patent literature' that reported the compounds active on A₁ adenosine receptors (ARs) and the applications of A₁ AR ligands patented in the period 2005 – 2008.

Areas covered: This article is a discussion of the patents about the same subjects, issued in the period 2008 to present. It is organized similarly to the first one, with a section about new compounds, subdivided on the basis of their functional activity (agonists, antagonists and allosteric modulators) and a section regarding new therapeutic applications.

Expert opinion: The main novelty is represented by the patenting of A₁ AR ligands with dual selectivity which may show, in some conditions, better efficacy and fewer side effects. Moreover, while the way to arrive into the market appears full of obstacles for selective A₁ ligands that need systemic administration for long-term therapy, better chances are foreseen in applications requiring topical administration.     

Further pharmacological characterization of the adenosine receptor subtype mediating inhibition of oxidative burst in human isolated neutrophils

The aims of this study were to characterize the adenosine receptor subtype mediating inhibition of superoxide anion generation induced by N-formyl-methionyl-leucylphenylalanine (fMLP) in human neutrophils and to test the hypothesis that adenosine 3′:5′-cyclic monophosphate (cAMP) is the second messenger mediating such inhibition. Superoxide anion generation induced by a submaximal concentration of fMLP (1 μM) was inhibited in a concentration-dependent manner by adenosine receptor agonists with a rank order of potency ofN-ethylcarboxamidoadenosine (NECA) > 2(4-[(2- carboxyethyl)phenyl]ethylamino)-5′-N-ethylcarboxamido adenosine (CGS 21680) > (R)-N⁶-phenyl-2-propyladenosine ((R)-PIA) > 2-Chloro-N⁶-(3-iodobenzyl)9-[5-methylcarbamoyl)-β-D-ribofuranosyl] adenine (2-Cl-IB-MECA) > N⁶-cyclopentyladenosine (CPA) > (S)-N⁶-phenyl-2-propyladenosine ((S)-PIA) ≥ N⁶-(4-amino-3-iodobenzyl) adenosine-5′-N-methyl-uronamide (AB-MECA); this order of potency is consistent with the activation of A_2A adenosine receptors. The nonselective A₁, A_2A, and A_2B receptor antagonist 8-p-(sulphophenyl) theophylline (8-SPT; 10 μM) produced blockade of each of the agonists (pK_B values 4.79–5.68). The selective A₁ adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM) and the selective A₃ adenosine receptor antagonist 3-(3-iodo-4-aminobenzyl)-8-(4-oxyacetate)phenyl-1-propylxanthine (I-ABOPX; 200 nM) also produced blockade of all the agonists (PK_B values 7.37–7.61 and 6.75–7.52, respectively); however, the concentrations required were higher than those which are considered selective for the A₁ or A₃ receptors. The selective A_2A receptor antagonist, 4-(2-[7-amino-2-(2-furyl) [1,2,4] triazolo [2,3-a] [1,3,5] triazin-5-yl amino]ethyl) phenol (ZM 241385; 100 nM), powerfully suppressed the inhibition of the oxidative burst induced by each of the agonists. A Schild analysis of the effects of ZM 241385, 1–100 nM, against NECA and CGS 21680 was carried out. ZM 241385 produced concentration-dependent, parallel shifts of the concentration–effect curves to both NECA and CGS 21680, with pA₂ values of 9.62 and 9.59, respectively. Together, these data establish that inhibition of the oxidative burst in human isolated neutrophils, induced by adenosine receptor agonists, is mediated by the A_2A receptor. NECA (0.01–10 μM) induced a concentration-dependent increase in the intracellular cAMP content of neutrophils. This effect was inhibited in a dose-dependent manner by ZM 241385 (0.001–10 μM), consistent with activation of A_2A adenosine receptors. The results clearly demonstrate that in human neutrophils inhibition of the fMLP—induced oxidative burst by adenosine receptor agonists is mediated via activation of A_2A adenosine receptors linked positively to cAMP. No evidence of A₁, A_2B, or A₃ adenosine receptor-mediated modulation of oxidative burst was found. Drug Dev. Res. 43:214–224, 1998. © 1998 Wiley-Liss, Inc.     

Targeting adenosine receptors with coumarins: synthesis and binding activities of amide and carbamate derivatives

Objectives With the aim of finding the structural features governing binding activity and selectivity against adenosine receptors (ARs), several 3‐subtituted coumarins with amide (compounds 3 – 6 ) and carbamate ( 7 – 9 ) functions were synthesized. To study its possible influence on the binding activity and selectivity, a hydroxyl substituent was also introduced at position 4 of the coumarin moiety. Methods A new series of coumarins ( 3 – 9 ) were synthesized and evaluated by radioligand binding studies towards ARs. Key findings None of the 4‐hydroxy derivatives ( 4 , 8 and 9 ) showed binding affinity for any of the ARs. None of the compounds interacted with the hA_2B AR (K_i > 100 000 nm ). Compounds 3 , 5 , 6 and 7 had different activity profiles with dissimilar binding affinity and selectivity towards human A₁, A_2A and A₃ ARs. Conclusions The most remarkable derivative is compound 7 , which presents the best affinity and selectivity for the A₃ adenosine receptor (K_i = 5500 nm ).     

C2-linked alkynyl poly-ethylene glycol(PEG) adenosine conjugates as water-soluble adenosine receptor agonists

A series of 12 novel polyethylene-glycol(PEG)-alkynyl C2-adenosine(ADN) conjugates were synthesized using a robust Sonogashira coupling protocol and characterized by NMR spectroscopy and mass spectrometry analysis. The ADN-PEG conjugates showed null to moderate toxicity in murine macrophages and 12c was active against Mycobacterium aurum growth (MIC = 62.5 mg/L). The conjugates were not active against Mycobacterium bovis BCG. Conjugates 10b and 11b exhibited high water solubility with solubility values of 1.22 and 1.18 mg/ml, respectively, in phosphate buffer solutions at pH 6.8. Further, 10b and 11b induced a significant increase in cAMP accumulation in RAW264.7 cells comparable with that induced by adenosine. Analogues 10c , 11c and 12c were docked to the A₁, A_2A, A_2B and A₃ adenosine receptors (ARs) using crystal-structures and homology models. ADN-PEG-conjugates bearing chains with up to five ethyleneoxy units could be well accommodated within the binding sites of A₁, A_2A and A₃ ARs. Docking studies showed that compound 10b and 11b were the best A_2A receptor binders of the series, whereas 12c was the best binder for A₁ AR. In summary, introduction of hydrophilic PEG substituents at the C2 of adenine ring significantly improved water solubility and did not affect AR binding properties of the ADN-PEG conjugates.     

Synthesis of (R)‐ and (S)‐[O‐methyl‐11C]N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐(4‐methoxy‐phenyl)‐urea as candidate high affinity β1‐adrenoceptor PET radioligands

Molecular imaging and quantification of myocardial β₁‐adrenoceptor (AR) rather than total β‐AR density is of great clinical interest since cardiac biopsy studies suggest that myocardial β₁‐AR density is reduced in patients with chronic heart failure whereas cardiac β₂‐AR density may vary. Positron emission tomography (PET), with appropriate radioligands, offers the possibility to assess β‐AR density non‐invasively in humans. However, no PET radioligand for the selective imaging of cardiac β₁‐ARs is clinically available. Here some derivatives of the well characterized β₁‐AR selective antagonist, ICI 89,406, namely the enantiomers of N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐(4‐hydroxy‐phenyl)‐urea ( 5a and 5b ) were synthesized and evaluated in vitro. The (R)‐isomer 5a was more β₁‐selective but has lower affinity than its (S)‐enantiomer 5b (β₁‐AR selectivity: 6100 vs 1240; β₁‐affinity: K₁ = 0.288 nM vs K₁ = 0.067 nM). Etherification of the analogous desmethyl precursors, 5e and 5f , respectively, with [¹¹C]iodomethane gave ¹¹C‐labelled versions of 5a and 5b , namely 5g and 5h , in 44 ± 5% radiochemical yield (decay‐corrected) and 97.4 ± 1.3% radiochemical purity with specific radioactivities of 26.4 ± 9.4 GBq/µmol within 41.2 ± 3.4 min from the end of bombardment (n = 14). 5g and 5h are now being evaluated as candidate radioligands for myocardial β₁‐ARs. Copyright © 2005 John Wiley & Sons, Ltd.     

Novel Mannich Bases, 5‐Arylimidazolidine‐2,4‐dione Derivatives with Dual 5‐HT1A Receptor and Serotonin Transporter Affinity

A computer aided ligand design study of imidazolidine‐2,4‐dione derivatives was conducted in order to obtain compounds with dual 5‐HT_1A receptor and serotonin transporter (SERT) affinity. According to molecular modeling results, series of Mannich bases were chosen and synthesized. Investigated compounds were tested for 5‐HT_1A, 5‐HT_2A, α₁ and SERT affinity. Two selected compounds ( 5 , 9 ) were characterized in functional experiments and possessed a pharmacological profile which may enhance SERT blocking efficacy – 5‐HT_1A partial agonism and 5‐HT_2A antagonism in one molecule. Furthermore these compounds displayed satisfactory selectivity over adrenergic α₁ receptors. The most promising compounds, 5‐arylimidazolidine‐2,4‐dione derivatives with 4‐(3‐chlorophenyl)piperazinylmethyl moiety were tested for antidepressant and anxiolytic activity. In particular, compound 5 (5‐(2‐methoxyphenyl)‐3‐{1‐[4‐(3‐chlorophenyl)piperazin‐1‐yl]methyl}‐imidazolidine‐2,4‐dione), tested in the forced swim test in mice, exhibited a favorable antidepressant‐like profile without affecting spontaneous locomotor activity.     

The synthetic cathinone psychostimulant α‐PPP antagonizes serotonin 5‐HT2A receptors: In vitro and in vivo evidence

Synthetic cathinones (SCs) are β‐keto analogs of amphetamines. Like amphetamines, SCs target monoamine transporters; however, unusual neuropsychiatric symptoms have been associated with abuse of some SCs, suggesting SCs might possess additional pharmacological properties. We performed radioligand competition binding assays to assess the affinities of nine SCs at human 5‐HT_2A receptors (5‐HT_2AR) and muscarinic M₁ receptors (M₁R) transiently expressed in HEK293 cells. None of the SCs exhibited affinity at M₁R (minimal displacement of [~K_d] [³H]scopolamine up to 10 μM). However, two SCs, α‐pyrrolidinopropiophenone (α‐PPP) and 4‐methyl‐α‐PPP, had low μM K_i values at 5‐HT_2AR. In 5‐HT_2AR–phosphoinositide hydrolysis assays, α‐PPP and 4‐methyl‐α‐PPP displayed inverse agonist activity. We further assessed the 5‐HT_2AR functional activity of α‐PPP, and observed it competitively antagonized 5‐HT_2AR signaling stimulated by the 5‐HT₂R agonist (±)‐2,5‐dimethoxy‐4‐iodoamphetamine (DOI; K_b = 851 nM). To assess in vivo 5‐HT_2AR activity, we examined the effects of α‐PPP on the DOI‐elicited head‐twitch response (HTR) in mice. α‐PPP dose‐dependently blocked the HTR with maximal suppression at 10 mg/kg (P < 0.0001), which is a moderate dose used in studies investigating psychostimulant properties of α‐PPP. To corroborate a 5‐HT_2AR mechanism, we also tested 3,4‐methylenedioxy‐α‐PPP (MDPPP) and 3‐bromomethcathinone (3‐BMC), SCs that we observed had 5‐HT_2AR K_is > 10 μM. Neither MDPPP nor 3‐BMC, at 10 mg/kg doses, attenuated the DOI HTR. Our results suggest α‐PPP has antagonist interactions at 5‐HT_2AR in vitro that may translate at physiologically‐relevant doses in vivo. Considering 5‐HT_2AR antagonism has been shown to mitigate effects of psychostimulants, this property may contribute to α‐PPPs unpopularity compared to other monoamine transporter inhibitors.     

Effect of a novel partial adenosine A1 receptor agonist VCP102 in reducing ischemic damage in the mouse heart

Investigations into the use of A₁ adenosine receptor (A₁AR) agonists in reducing post‐ischemic damage to the heart have been of significant interest to cardiovascular research. The present study examined the cardioprotective effects of the partial A₁AR agonist, N⁶‐[4‐[1,1,3,3‐tetramethylisoindolin‐2‐yloxyl‐5amido]ethyl]phenyl]adenosine (VCP102) and the full A₁AR agonist, N⁶‐cyclopentyl adenosine (CPA), in murine isolated hearts perfused in Langendorff mode. Following 30‐min ischemia, hearts were treated with the A₁AR agonists during early reperfusion for 15 min (IR protocol). Significant increases in left ventricular developed pressure (LVDP) and contractility (dP/dt_max) were observed at the final reperfusion values for VCP102 (100 nM)‐treated hearts, as compared with the control tissues (P<0.05). EDP values in VCP102‐treated hearts were also lower than control tissues (P<0.05). The A₁AR antagonist, 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX, 10 nM) and the A₃AR antagonist 3‐ethyl‐5‐benzyl‐2‐methyl‐4‐pheylethynyl‐6‐phenyl‐1,4‐(±)‐diphydropyridine‐3,5‐dicarboxylate (MRS1191, 200 nM) attenuated the protective actions of 15 min VCP102 during IR (P<0.05). An improvement in functional recovery was not observed with CPA (100 nM, P>0.05). Infarct size was also significantly reduced in VCP102‐ or CPA‐treated hearts as compared with control tissues (P<0.05). DPCPX and MRS1191 attenuated VCP102‐induced decreases in infarct size (P<0.05), but did not alter the responses to CPA (P>0.05). Significant reductions in cardiac troponin I release and expression of the pro‐apoptotic zymogen caspase‐3 were observed in both treatment groups (P<0.05). In summary, both A₁ARs and A₃ARs may play a role in mediating cardioprotection and improving functional recovery in the IR heart with VCP102. Furthermore, treatment with the partial agonist appeared to confer improved functional cardiac protection when compared with CPA. Drug Dev Res 68:529–537, 2007. © 2008 Wiley‐Liss, Inc.     

Phenyl‐substituted N6‐phenyladenosines and N6‐phenyl‐5′‐N‐ethylcarboxamidoadenosines with high activity at human adenosine A2B receptors

A series of phenyl‐substituted N⁶‐phenyladenosines and N⁶‐phenyl‐5′‐N‐ethylcarboxamidoadenosines were synthesized and tested at adenosine receptor subtypes. EC₅₀ values were determined for cyclic AMP production in CHO cells expressing human A_2B receptors. Binding affinities were determined for rat A₁ and A_2A receptors and human A₃ receptors. N⁶‐phenyladenosine displayed an EC₅₀ value at A_2B receptors of 6.3 μM. Several N⁶‐phenyladenosine derivatives were more active than N⁶‐phenyladenosine, while two analogs were also more potent than 5′‐N‐ethylcarboxamidoadenosine (NECA, 0.76 μM), i.e., the 4‐iodophenyl ( 10 , 0.37 μM) and the 4‐aminosulfonylphenyl ( 20 , 0.44 μM) derivatives. N⁶‐phenyl‐NECA derivatives were as active as their analogous adenosine derivatives. Drug Dev. Res. 49:85–93, 2000. © 2000 Wiley‐Liss, Inc.