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1.  Northern Ring Conformation of Methanocarba-Adenosine 5′-Triphosphate Required for Activation of P2X Receptors 
Drug development research  2004;61(4):227-232.
Strategy, Management and Health PolicyEnabling Technology, Genomics, ProteomicsPreclinical ResearchPreclinical Development Toxicology, Formulation Drug Delivery, PharmacokineticsClinical Development Phases I-III Regulatory, Quality, ManufacturingPostmarketing Phase IV
Replacement of the ribose moiety of adenosine 5′-triphosphate (ATP) with a carbocyclic ring constrained in either the Northern (N) or Southern (S) conformation produces agonists with widely differing activities at P2Y receptors (Kim et al. [2002] J Med Chem 45:208–218). We have used whole cell patch clamp recording to investigate the agonist activity of these two methanocarba analogs of ATP at four different P2X receptors (P2X1, P2X2, P2X3, and P2X2/3). On dorsal root ganglion neurons, (N) methanocarba-ATP ((1′S,2′R,3′S,4′R,5′S)-4-(6-amino-9H-purin-9-yl)-1-[triphosphoryloxymethyl] bicyclo[ 3.1.0]hexane-2,3-diol; MRS 2340) activated rapidly-desensitizing (P2X3) and slowly-desensitizing (P2X2/3) receptors with a similar potency to ATP. In contrast, (S) methanocarba-ATP ((±)-5-(6-amino-9H-purin-9-yl)-1-[triphosphoryloxymethyl] bicycle [3.1.0]hexane-2,3-diol MRS 2312) was devoid of agonist activity. On nodose ganglion neurones, that express mainly P2X2/3 receptors, ATP evoked a slowly desensitizing inward current with an EC50 value of 26 μM. MRS 2340 was an effective agonist, but less potent than ATP, while MRS 2312 at concentrations up to 100 μM produced a barely detectable response. On mammalian cell lines expressing recombinant hP2X1 and hP2X2 receptors, MRS 2340 evoked inward currents similar in amplitude to those produced by the same concentration of ATP or α,β-mATP. In contrast, MRS 2312 failed to give a detectable response. Although the conformation of the ribose affects agonist activity at P2Y receptors, there is a strong requirement for the (N) conformation for the activation of these P2X receptors. Furthermore, the region of the agonist binding site that accommodates the ribose moiety appears to be highly conserved among different P2X receptors. Drug Dev Res 61:227–232, 2004.
PMCID: PMC3402081  PMID: 22833693
ATP; P2X receptors; structure activity relationship
2.  Selective A3 Adenosine Receptor Antagonists Derived from Nucleosides Containing a Bicyclo[3.1.0]hexane Ring System 
Bioorganic & medicinal chemistry  2008;16(18):8546-8556.
We have prepared 5′-modified derivatives of adenosine and a corresponding (N)-methanocarba nucleoside series containing a bicyclo[3.1.0]hexane ring system in place of the ribose moiety. The compounds were examined in binding assays at three subtypes of adenosine receptors (ARs) and in functional assays at the A3 AR. The H-bonding ability of a group of 9-riboside derivatives containing a 5′-uronamide moiety was reduced by modification of the NH, however these derivatives did not display the desired activity as selective A3 AR antagonists, as occurs with 5′-N,N-dimethyluronamides. However, truncated (N)-methanocarba analogues lacking a 4′-hydroxymethyl group were highly potent and selective antagonists of the human A3 AR. The compounds were synthesized from D-ribose using a reductive free radical decarboxylation of a 5′-carboxy intermediate. A less efficient synthetic approach began with L-ribose, which was similar to the published synthesis of (N)-methanocarba A3AR agonists. Compounds 33b – 39b (N6-3-halobenzyl and related arylalkyl derivatives) were potent A3AR antagonists with binding Ki values of 0.7 − 1.4 nM. In a functional assay of [35S]GTPγS binding, 33b (3-iodobenzyl) completely inhibited stimulation by NECA with a KB of 8.9 nM. Thus, a highly potent and selective series of A3AR antagonists has been described.
PMCID: PMC2593936  PMID: 18752961
G protein-coupled receptor; purines; molecular modeling; structure activity relationship; radioligand binding; adenylate cyclase
3.  (N)-Methanocarba 2,N6-Disubstituted Adenine Nucleosides as Highly Potent and Selective A3 Adenosine Receptor Agonists 
Journal of medicinal chemistry  2005;48(6):1745-1758.
A series of ring-constrained (N)-methanocarba-5′-uronamide 2,N6-disubstituted adenine nucleosides have been synthesized via Mitsunobu condensation of the nucleobase precursor with a pseudosugar ring containing a 5′-ester functionality. Following appropriate functionalization of the adenine ring, the ester group was converted to the 5′-N-methylamide. The compounds, mainly 2-chloro substituted derivatives, were tested in both binding and functional assays at human adenosine receptors (ARs), and many were found to be highly potent and selective A3AR agonists. Selected compounds were compared in binding to the rat A3AR to assess their viability for testing in rat disease models. The N6-(3-chlorobenzyl) and N6-(3-bromobenzyl) analogues displayed Ki values at the human A3AR of 0.29 and 0.38 nM, respectively. Other subnanomolar affinities were observed for the following N6 derivatives: 2,5-dichlorobenzyl, 5-iodo-2-methoxybenzyl, trans-2-phenyl-1-cyclopropyl, and 2,2-diphenylethyl. Selectivity for the human A3AR in comparison to the A1AR was (fold): the N6-(2,2-diphenylethyl) analogue 34 (1900), the N6-(2,5-dimethoxybenzyl) analogue 26 (1200), the N6-(2,5-dichlorobenzyl) and N6-(2-phenyl-1-cyclopropyl) analogues 20 and 33 (1000), and the N6-(3-substituted benzyl) analogues 17, 18, 28, and 29 (700–900). Typically, even greater selectivity ratios were obtained in comparison with the A2A and A2BARs. The (N)-methanocarba-5′-uronamide analogues were full agonists at the A3AR, as indicated by the inhibition of forskolin-stimluated adenylate cyclase at a concentration of 10 µM. The N6-(2,2-diphenylethyl) derivative was an A3AR agonist in the (N)-methanocarba-5′-uronamide series, although it was an antagonist in the ribose series. Thus, many of the previously known groups that enhance A3AR affinity in the 9-riboside series, including those that reducing intrinsic efficacy, may be adapted to the (N)-methanocarba nucleoside series of full agonists.
PMCID: PMC3463111  PMID: 15771421
4.  Nucleotide analogues containing 2-oxa-bicyclo[2.2.1]heptane and L-α-threofuranosyl ring systems: interactions with P2Y receptors 
Bioorganic & medicinal chemistry  2004;12(21):5619-5630.
The ribose moiety of adenine nucleotide 3′,5′-bisphosphate antagonists of the P2Y1 receptor has been successfully substituted with a rigid methanocarba ring system, leading to the conclusion that the North (N) ring conformation is preferred in receptor binding. Similarly, at P2Y2 and P2Y4 receptors, nucleotides constrained in the (N) conformation interact equipotently with the corresponding ribosides. We now have synthesized and examined as P2Y receptor ligands nucleotide analogues substituted with two novel ring systems: (1) a (N) locked-carbocyclic (cLNA) derivative containing the oxabicyclo[2.2.1]heptane ring system and (2) L-α-threofuranosyl derivatives. We have also compared potencies and preferred conformations of these nucleotides with the known anhydrohexitol-containing P2Y1 receptor antagonist MRS2283. A cLNA bisphosphate derivative MRS2584 21 displayed a Ki value of 22.5nM in binding to the human P2Y1 receptor, and antagonized the stimulation of PLC by the potent P2Y1 receptor agonist 2-methylthio-ADP (30nM) with an IC50 of 650nM. The parent cLNA nucleoside bound only weakly to an adenosine receptor (A3). Thus, this ring system afforded some P2Y receptor selectivity. A L-α-threofuranosyl bisphosphate derivative 9 displayed an IC50 of 15.3μM for inhibition of 2-methylthio-ADP-stimulated PLC activity. L-α-Threofuranosyl-UTP 13 was a P2Y receptor agonist with a preference for P2Y2 (EC50 = 9.9μM) versus P2Y4 receptors. The P2Y1 receptor binding modes, including rotational angles, were estimated using molecular modeling and receptor docking.
PMCID: PMC3402881  PMID: 15465340
Nucleoside; Purine; Pyrimidine; G protein-coupled receptor; Carbocyclic; Phospholipase C; Radioligand binding; Molecular model
5.  Truncated Nucleosides as A3 Adenosine Receptor Ligands: Combined 2-Arylethynyl and Bicyclohexane Substitutions 
ACS medicinal chemistry letters  2012;3(7):596-601.
C2-Arylethynyladenosine-5′-N-methyluronamides containing a bicyclo[3.1.0]hexane ((N)-methanocarba) ring are selective A3 adenosine receptor (AR) agonists. Similar 4′-truncated C2-arylethynyl-(N)-methanocarba nucleosides containing alkyl or alkylaryl groups at the N6 position were low-efficacy agonists or antagonists of the human A3AR with high selectivity. Higher hA3AR affinity was associated with N6-methyl and ethyl (Ki 3–6 nM), than with N6-arylalkyl groups. However, combined C2-phenylethynyl and N6-2-phenylethyl substitutions in selective antagonist 15 provided a Ki of 20 nM. Differences between 4′-truncated and nontruncated analogues of extended C2-p-biphenylethynyl substitution suggested a ligand reorientation in AR binding, dominated by bulky N6 groups in analogues lacking a stabilizing 5′-uronamide moiety. Thus, 4′-truncation of C2-arylethynyl-(N)-methanocarba adenosine derivatives is compatible with general preservation of A3AR selectivity, especially with small N6 groups, but reduced efficacy in A3AR-induced inhibition of adenylate cyclase.
PMCID: PMC3491360  PMID: 23145215
G protein-coupled receptor; purines; molecular modeling; structure activity relationship; radioligand binding; adenosine receptor
6.  Design of (N)-Methanocarba Adenosine 5′-Uronamides as Species-Independent A3 Receptor-Selective Agonists 
2-Chloro-5′ -N-methylcarboxamidoadenosine analogues containing the (N)-methanocarba (bicyclo[3.1.0]hexane) ring system as a ribose substitute display increased selectivity as agonists of the human A3 adenosine receptor (AR). However, the selectivity in mouse was greatly reduced due to an increased tolerance of this ring system at the mouse A1AR. Therefore, we varied substituents at the N6 and C2 positions in search of compounds that have improved A3AR selectivity and are species independent. An N6-methyl analogue was balanced in affinity at mouse A1/A3ARs, with high selectivity in comparison to the A2AAR. Substitution of the 2-chloro atom with larger and more hydrophobic substituents, such as iodo and alkynyl groups, tended to increase the A3AR selectivity (up to 430-fold) in mouse and preserve it in human. Extended and chemically functionalized alkynyl chains attached at the C2 position of the purine moiety preserved A3AR selectivity more effectively than similar chains attached at the 3 position of the N6–benzyl group.
PMCID: PMC2430186  PMID: 18424135
nucleoside; G protein-coupled receptor; mouse; adenosine receptor; radioligand binding
7.  2-Substitution of Adenine Nucleotide Analogues Containing a Bicyclo[3.1.0]hexane Ring System Locked in a Northern Conformation: Enhanced Potency as P2Y1 Receptor Antagonists 
Journal of medicinal chemistry  2003;46(23):4974-4987.
Preference for the northern (N) ring conformation of the ribose moiety of adenine nucleotide 3′,5′-bisphosphate antagonists of P2Y1 receptors was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute (Nandanan et al. J. Med. Chem. 2000, 43, 829–842). We have now combined the ring-constrained (N)-methanocarba modification with other functionalities at the 2-position of the adenine moiety. A new synthetic route to this series of bisphosphate derivatives was introduced, consisting of phosphorylation of the pseudoribose moiety prior to coupling with the adenine base. The activity of the newly synthesized analogues was determined by measuring antagonism of 2-methylthio-ADP-stimulated phospholipase C (PLC) activity in 1321N1 human astrocytoma cells expressing the recombinant human P2Y1 receptor and by using the radiolabeled antagonist [3H]2-chloro-N6-methyl-(N)-methanocarba-2′-deoxyadenosine 3′,5′-bisphosphate 5 in a newly developed binding assay in Sf9 cell membranes. Within the series of 2-halo analogues, the most potent molecule at the hP2Y1 receptor was an (N)-methanocarba N6-methyl-2-iodo analogue 12, which displayed a Ki value in competition for binding of [3H]5 of 0.79 nM and a KB value of 1.74 nM for inhibition of PLC. Thus, 12 is the most potent antagonist selective for the P2Y1 receptor yet reported. The 2-iodo group was substituted with trimethyltin, thus providing a parallel synthetic route for the introduction of an iodo group in this high-affinity antagonist. The (N)-methanocarba-2-methylthio, 2-methylseleno, 2-hexyl, 2-(1-hexenyl), and 2-(1-hexynyl) analogues bound less well, exhibiting micromolar affinity at P2Y1 receptors. An enzymatic method of synthesis of the 3′,5′-bisphosphate from the corresponding 3′-monophosphate, suitable for the preparation of a radiophosphorylated analogue, was explored.
PMCID: PMC3408611  PMID: 14584948
8.  Semi-Rational Design of (N)-Methanocarba Nucleosides as Dual Acting A1 and A3Adenosine Receptor Agonists: Novel Prototypes for Cardioprotection 
Journal of medicinal chemistry  2005;48(26):8103-8107.
Ring-constrained adenosine analogues have been designed to act as dualagonists at tissue-protective A1 and A3 adenosine receptors (ARs). 9-Ribosides transformed into the ring-constrained (N)-methanocarba-2-chloro-5′-uronamides consistently lost affinity at A1/A2AARs and gained at A3AR. Among 9-riboside derivatives, only N6-cyclopentyl and 7-norbornyl moieties were extrapolated for mixed A1/A3 selectivity and rat/human A3AR equipotency. Consequently, 2 was balanced in affinity and potency at A1/A3ARs as envisioned and dramatically protected in an intact heart model of global ischemia and reperfusion.
PMCID: PMC2597460  PMID: 16366590
A series of flexible carbocyclic pyrimidine nucleosides has been designed and synthesized. In contrast to previously reported “fleximers” from our laboratory, these analogues have the connectivity of the heterocyclic base system “reversed”, where the pyrimidine ring is attached to the sugar moiety, rather than the five membered imidazole ring. As was previously seen with the ribose fleximers, their inherent flexibility should allow them to adjust to enzyme binding site mutations, as well as increase the affinity for atypical enzymes. Preliminary biological screening has revealed surprising inhibition of adenosine deaminase, despite their lack of resemblance to adenosine.
PMCID: PMC3712750  PMID: 23473101
Fleximers; carbocyclic nucleosides; adenosine deaminase; 5-substituted uracils; pyrimidine based inhibitors
10.  Synthesis and pharmacological characterization of [125I]MRS1898, a high-affinity, selective radioligand for the rat A3 adenosine receptor 
Purinergic Signalling  2008;5(1):31-37.
A known selective agonist of the A3 adenosine receptors (AR), MRS1898 [(1′R,2′R,3′S,4′R,5′S)-4-{2-chloro-6-[(3-iodophenylmethyl)amino]purin-9-yl}-1-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2,3-diol], was synthesized in radioactive form and characterized pharmacologically. This agonist ligand series, based on nucleoside analogues containing a rigid, bicyclic ring system in place of the ribose moiety, was selected for radiolabeling due to its high A3AR affinity across species, with nanomolar binding at both rat and human A3ARs. The radioiodination of MRS1898 on its N6–3-iodobenzyl substituent was accomplished in 76% radiochemical yield by iododestannylation of a 3-(trimethylstannyl)benzyl precursor. [125I]MRS1898 bound to the rat A3AR with a Kd value of 0.17 ± 0.04 nM and a Bmax value of 0.66 ± 0.15 pmol/mg protein. The competition binding profiles for other agonists and antagonists obtained with this radioligand are similar to those previously obtained with other radioligands. The advantages of [125I]MRS1898 compared with previously used radioligands are primarily its high selectivity and affinity for the rat A3AR and also its facile synthesis and radiochemical stability; however, a relatively high level of nonspecific binding presents a limitation. Thus, we have introduced the first selective radioligand for the rat A3AR.
PMCID: PMC2721771  PMID: 18528782
Iodination; G protein-coupled receptor; Binding assay; Purine; Nucleoside; Carbocyclic
11.  Search for New Purine- and Ribose-Modified Adenosine Analogues as Selective Agonists and Antagonists at Adenosine Receptors† 
Journal of medicinal chemistry  1995;38(7):1174-1188.
The binding affinities at rat A1, A2a, and A3 adenosine receptors of a wide range of derivatives of adenosine have been determined. Sites of modification include the purine moiety (1-, 3-, and 7-deaza; halo, alkyne, and amino substitutions at the 2- and 8-positions; and N6-CH2-ring, -hydrazino, and -hydroxylamino) and the ribose moiety (2′-, 3′-, and 5′-deoxy; 2′- and 3′-O-methyl; 2′-deoxy 2′-fluoro; 6′-thio; 5′-uronamide; carbocyclic; 4′- or 3′-methyl; and inversion of configuration). (−)- and (+)-5′-Noraristeromycin were 48- and 21-fold selective, respectively, for A2a vs A1 receptors. 2-Chloro-6′-thioadenosine displayed a Ki value of 20 nM at A2a receptors (15-fold selective vs A1). 2-Chloroadenin-9-yl(β-L-2′-deoxy-6′-thiolyxofuranoside) displayed a Ki value of 8 μM at A1 receptors and appeared to be an antagonist, on the basis of the absence of a GTP-induced shift in binding vs a radiolabeled antagonist (8-cyclopentyl-1,3-dipropylxanthine). 2-Chloro-2′-deoxyadenosine and 2-chloroadenin-9-yl(β-D-6′-thioarabinoside) were putative partial agonists at A1 receptors, with Ki values of 7.4 and 5.4 μM, respectively. The A2a selective agonist 2-(1-hexynyl)-5′-(N-ethylcarbamoyl)adenosine displayed a Ki value of 26 nM at A3 receptors. The 4′-methyl substitution of adenosine was poorly tolerated, yet when combined with other favorable modifications, potency was restored. Thus, N6-benzyl-4′-methyladenosine-5′-(N-methyluronamide) displayed a Ki value of 604 nM at A3 receptors and was 103- and 88-fold selective vs A1 and A2a receptors, respectively. This compound was a full agonist in the A3-mediated inhibition of adenylate cyclase in transfected CHO cells. The carbocyclic analogue of N6-(3-iodobenzyl)adenosine-5′-(N-methyluronamide) was 2-fold selective for A3 vs A1 receptors and was nearly inactive at A2a receptors.
PMCID: PMC3457658  PMID: 7707320
12.  Truncated (N)-Methanocarba Nucleosides as A1 Adenosine Receptor Agonists and Partial Agonists: Overcoming Lack of a Recognition Element 
ACS medicinal chemistry letters  2011;2(8):626-631.
A1 adenosine receptor (AR) agonists are neuroprotective, cardioprotective, and anxiolytic. (N)-Methanocarba adenine nucleosides designed to bind to human A1AR were truncated to eliminate 5′-CH2OH. This modification previously converted A3AR agonists into antagonists, but the comparable effect at A1AR is unknown. In comparison to ribosides, affinity at the A1AR was less well preserved than at the A3AR, although a few derivatives were moderately A1AR selective, notably full agonist 21 (N6-dicyclopropylmethyl, Ki 47.9 nM). Thus, at the A1AR recognition elements for nucleoside binding depend more on 5′region interactions, and in their absence A3AR selectivity predominates. Based on the recently reported agonist-bound AR structure, this difference between subtypes likely correlates with an essential His residue in transmembrane domain 6 of A1 but not A3AR. The derivatives ranged from partial to full agonists in A1AR-mediated adenylate cyclase inhibition. Truncated derivatives have more drug-like physical properties than other A1AR agonists; this approach is appealing for preclinical development.
PMCID: PMC3156476  PMID: 21858244
G protein-coupled receptor; purines; molecular modeling; radioligand binding; adenylate cyclase
13.  Structure-Guided Design of A3 Adenosine Receptor-Selective Nucleosides: Combination of 2-Arylethynyl and Bicyclo[3.1.0]hexane Substitutions 
Journal of Medicinal Chemistry  2012;55(10):4847-4860.
(N)-Methanocarba adenosine 5′-methyluronamides containing known A3 AR (adenosine receptor)-enhancing modifications, i.e. 2-(arylethynyl)adenine and N6-methyl or N6-(3-substituted-benzyl), were nanomolar full agonists of human (h) A3AR and highly selective (Ki ~0.6 nM, N6-methyl 2-(halophenylethynyl) analogues 13, 14). Combined 2-arylethynyl-N6-3-chlorobenzyl substitutions preserved A3AR affinity/selectivity in the (N)-methanocarba series (e.g. 3,4-difluoro full agonist MRS5698 31, Ki 3 nM, human and mouse A3) better than for ribosides. Polyaromatic 2-ethynyl N6-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (Ki hA3 3.1 nM; A1, A2A: inactive) and fluorescent 1-pyrene adduct MRS5704 35 (Ki hA3 68.3 nM) were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A2AAR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, X-ray structure of related A2AAR is useful in guiding design of new A3AR agonists.
PMCID: PMC3371665  PMID: 22559880
G protein-coupled receptor; purines; molecular modeling; structure activity relationship; radioligand binding; adenylate cyclase
14.  Differential furanose selection in the active sites of archaeal DNA polymerases probed by fixed-conformation nucleotide analogues 
Biochemistry  2012;51(45):9234-9244.
DNA polymerases select for the incorporation of deoxyribonucleotide triphosphates (dNTPs) using amino acid side-chains that act as a “steric-gate” to bar improper incorporation of rNTPs. An additional factor in the selection of nucleotide substrates resides in the preferred geometry for the furanose moiety of the incoming nucleotide triphosphate. We have probed the role of sugar geometry during nucleotide selection by model DNA polymerases from Sulfolobus solfataricus using fixed conformation nucleotide analogues. North-methanocarba-dATP (N-MC-dATP) locks the central ring into a RNA-type (C2′-exo, North) conformation near a C3′-endo pucker and South-methanocarba-dATP (S-MC-dATP) locks the central ring system into a (C3′-exo, South) conformation near a C2′-endo pucker. Dpo4 preferentially inserts N-MC-dATP and in the crystal structure of Dpo4 in complex with N-MC-dAMP, the nucleotide analogue superimposes almost perfectly with Dpo4 bound to unmodified dATP. Biochemical assays indicate that the S. solfataricus B-family DNA polymerase Dpo1 can insert and extend from both N-MC-dATP and S-MC-dATP. In this respect, Dpo1 is unexpectedly more tolerant of substrate conformation than Dpo4. The crystal structure of Dpo4 bound to S-MC-dADP shows that poor incorporation of the Southern pucker by the Y-family polymerase results from a hydrogen bond between the 3′-OH group of the nucleotide analogue and the OH group of the steric gate residue, Tyr12, shifting the S-MC-dADP molecule away from the dNTP binding pocket and distorting the base pair at the primer-template junction. These results provide insights into substrate specificity of DNA polymerases, as well as molecular mechanisms that act as a barrier against insertion of rNTPs.
PMCID: PMC3563936  PMID: 23050956
DNA polymerase; nucleotide selection; X-ray crystallography
15.  Functionalized Congeners of P2Y1 Receptor Antagonists: 2-Alkynyl (N)-Methanocarba 2′-Deoxyadenosine 3′,5′-Bisphosphate Analogues and Conjugation to a Polyamidoamine (PAMAM) Dendrimer Carrier 
Bioconjugate chemistry  2010;21(7):1190-1205.
The P2Y1 receptor is a prothrombotic G protein-coupled receptor (GPCR) activated by ADP. Preference for the North (N) ring conformation of the ribose moiety of adenine nucleotide 3′,5′-bisphosphate antagonists of the P2Y1 receptor was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute. A series of covalently linkable N6-methyl-(N)-methanocarba-2′-deoxyadenosine-3′,5′-bisphosphates containing extended 2-alkynyl chains was designed and binding affinity at the human (h) P2Y1 receptor determined. The chain of these functionalized congeners contained hydrophilic moieties, a reactive substituent, or biotin, linked via an amide. Variation of the chain length and position of an intermediate amide group revealed high affinity of carboxylic congener 8 (Ki 23 nM) and extended amine congener 15 (Ki 132 nM), both having a 2-(1-pentynoyl) group. A biotin conjugate 18 containing an extended ε-aminocaproyl spacer chain exhibited higher affinity than a shorter biotinylated analogue. Alternatively, click coupling of terminal alkynes of homologous 2-dialkynyl nucleotide derivatives to alkyl azido groups produced triazole derivatives that bound to the P2Y1 receptor following deprotection of the bisphosphate groups. The preservation of receptor affinity of the functionalized congeners was consistent with new P2Y1 receptor modeling and ligand docking. Attempted P2Y1 antagonist conjugation to PAMAM dendrimer carriers by amide formation or palladium-catalyzed reaction between an alkyne on the dendrimer and a 2-iodopurine-derivatized nucleotide was unsuccessful. A dialkynyl intermediate containing the chain length favored in receptor binding was conjugated to an azide-derivatized dendrimer, and the conjugate inhibited ADP-promoted human platelet aggregation. This is the first example of attaching a strategically functionalized P2Y receptor antagonist to a PAMAM dendrimer to produce a multivalent conjugate exhibiting a desired biological effect, i.e. antithrombotic action.
PMCID: PMC2912410  PMID: 20565071
16.  Structural Sweet Spot for A1 Adenosine Receptor Activation by Truncated (N)- Methanocarba Nucleosides: Receptor Docking and Potent Anticonvulsant Activity 
Journal of medicinal chemistry  2012;55(18):8075-8090.
A1 adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N6-cycloalkylmethyl 4′-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N6-dicyclopropylmethyl, Ki 47.9 nM) as a moderately A1AR-selective full agonist. Two stereochemically defined N6-methynyl group substituents displayed narrow SAR; larger than cyclobutyl greatly reduced AR affinity, and larger or smaller than cyclopropyl reduced A1AR selectivity. Nucleoside docking to A1AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger “A” forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39) and carbon chains of glutamates (EL2), and smaller subpocket “B” between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A1AR agonists. Truncated nucleosides, an appealing preclinical approach, have more drug-like physicochemical properties than other A1AR agonists. Thus, we identified highly restricted regions for substitution around N6 suitable for an A1AR agonist with anticonvulsant activity.
PMCID: PMC3463139  PMID: 22921089
G protein-coupled receptor; purines; molecular modeling; seizures; in vivo
17.  Polyamidoamine (PAMAM) Dendrimer Conjugates of “Clickable” Agonists of the A3 Adenosine Receptor and Coactivation of the P2Y14 Receptor by a Tethered Nucleotide 
Bioconjugate chemistry  2010;21(2):372-384.
We previously synthesized a series of potent and selective A3 adenosine receptor (AR) agonists (North-methanocarba nucleoside 5′-uronamides) containing dialkyne groups on extended adenine C2 substituents. We coupled the distal alkyne of a 2-octadiynyl nucleoside by Cu(I)-catalyzed “click” chemistry to azide-derivatized G4 (fourth-generation) PAMAM dendrimers to form triazoles. A3AR activation was preserved in these multivalent conjugates, which bound with apparent Ki 0.1–0.3 nM. They were substituted with nucleoside moieties, solely or in combination with water-solubilizing carboxylic acid groups derived from hexynoic acid. A comparison with various amide-linked dendrimers showed that triazole-linked conjugates displayed selectivity and enhanced A3AR affinity. We prepared a PAMAM dendrimer containing equiproportioned peripheral azido and amino groups for conjugation of multiple ligands. A bifunctional conjugate activated both A3 and P2Y14 receptors (via amide-linked uridine-5′-diphosphoglucuronic acid), with selectivity in comparison to other ARs and P2Y receptors. This is the first example of targeting two different GPCRs with the same dendrimer conjugate, which is intended for activation of heteromeric GPCR aggregates. Synergistic effects of activating multiple GPCRs with a single dendrimer conjugate might be useful in disease treatment.
PMCID: PMC2845915  PMID: 20121074
G protein; coupled receptor; purines; alkyne; azide; radioligand binding; dendrimer
18.  Functionalized Congeners of A3 Adenosine Receptor-Selective Nucleosides Containing a Bicyclo[3.1.0]hexane Ring System† 
Journal of medicinal chemistry  2009;52(23):7580-7592.
(N)-Methanocarba nucleosides containing bicyclo[3.1.0]hexane replacement of the ribose ring previously demonstrated selectivity as A3 adenosine receptor (AR) agonists (5′-uronamides) or antagonists (5′-truncated). Here, these two series were modified in parallel at the adenine C2 position. N6-3-Chlorobenzyl-5′-N-methyluronamides derivatives with functionalized 2-alkynyl chains of varying length terminating in a reactive carboxylate, ester, or amine group were full, potent human A3AR agonists. Flexibility of chain substitution allowed the conjugation with a fluorescent cyanine dye (Cy5) and biotin, resulting in binding Ki values of 17 and 36 nM, respectively. The distal end of the chain was predicted by homology modeling to bind at the A3AR extracellular regions. Corresponding l-nucleosides were nearly inactive in AR binding. In the 5′-truncated nucleoside series, 2-Cl analogues were more potent at A3AR than 2-H and 2-F, functional efficacy in adenylate cyclase inhibition varied, and introduction of a 2-alkynyl chain greatly reduced affinity. SAR parallels between the two series lost stringency at distal positions. The most potent and selective novel compounds were amine congener 15 (Ki = 2.1 nM) and truncated partial agonist 22 (Ki = 4.9 nM).
PMCID: PMC3109436  PMID: 19499950
19.  Cytotoxic purine nucleoside analogues bind to A1, A2A and A3 adenosine receptors 
Naunyn-Schmiedeberg's archives of pharmacology  2012;385(5):10.1007/s00210-011-0719-6.
Fludarabine, clofarabine and cladribine are anti-cancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A1, A2A, A2B, and A3). Radioligand binding studies utilizing chinese hamster ovary cells, stably transfected with adenosine A1, A2A, or A3 receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A2B receptors. Clofarabine and cladribine both bound to the A2A receptor with a Ki of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A3 receptor with a Ki of 10 μM, and none of these compounds bound to the A2B receptor. Results show that clofarabine, cladribine and fludarabine bind to the A1 receptor. In addition, clofarabine, cladribine, and fludarabine were A1 agonists (IC50 3.1, 30 and 30 μM, respectively). Neither pyrimidine nucleoside analogues, gemcitabine nor cytarabine, associated with any of the adenosine receptor subtypes (Ki > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.
PMCID: PMC3831620  PMID: 22249336
Adenosine receptors; purine analogues; A1; A2A; A2B; A3 ligands; fludarabine; clofarabine; cladribine
20.  Synthesis and characterization of oligonucleotides containing conformationally constrained bicyclo[3.1.0]hexane pseudosugar analogs 
Nucleic Acids Research  2004;32(12):3642-3650.
Oligodeoxyribonucleotides containing pseudorotationally locked sites derived from bicyclo[3.1.0]hexane pseudosugars have been synthesized using adenosine, thymidine and abasic versions of North- and South-methanocarba nucleosides. The reaction conditions for coupling and oxidation steps of oligonucleotide synthesis have been investigated and optimized to allow efficient and facile solid-phase synthesis using phosphoramidite chemistry. Our studies demonstrate that the use of iodine for P(III) to P(V) oxidation leads to strand cleavage at the sites where the pseudosugar is North. In contrast, the same cleavage reaction was not observed in the case of South pseudosugars. Iodine oxidation generates a 5′-phosphate oligonucleotide fragment on the resin and releases the North pseudosugar into the solution. This side reaction, which is responsible for the extremely low yields observed for the incorporation of the North pseudosugar analogs, has been studied in detail and can be easily overcome by replacing iodine with t-butylhydroperoxide as oxidant.
PMCID: PMC484163  PMID: 15247346
21.  Synthesis and Characterization of [76Br]-Labeled High Affinity A3 Adenosine Receptor Ligands for Positron Emission Tomography 
Nuclear medicine and biology  2009;36(1):3-10.
Bromine-76 radiolabeled analogues of previously reported high affinity A3 adenosine receptor (A3AR) nucleoside ligands have been prepared as potential radiotracers for Positron Emission Tomography (PET).
The radiosyntheses were accomplished by oxidative radiobromination on the N6-benzyl moiety of trimethyltin precursors. Biodistribution studies of the kinetics of uptake were conducted in awake rats.
We prepared an agonist ligand {[76Br](1′R,2′R,3′S,4′R,5′S)-4-{2-chloro-6-[(3-bromophenylmethyl)amino]purin-9-yl}-1-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2,3-diol (MRS3581)} in 59% radiochemical yield (RCY) with a specific activity of 19.5 GBq/μmol and an antagonist ligand {[76Br](1R,2R,3S,4R,5S)-4-(6-(3-bromobenzylamino)-2-chloro-9H-purin-9-yl)bicyclo[3.1.0]hexane-2,3-diol. (MRS5147)} in 65% RCY with a specific activity of 22 GBq/μmol). The resultant products exhibited the expected high affinity (Ki ~ 0.6 nM) and specific binding at the human A3AR in vitro. Biodistribution studies in the rat showed uptake in the organs of excretion and metabolism. The antagonist MRS5147 exhibited increasing uptake in testes, an organ that contains significant quantities of A3AR, over a 2 h time course, which suggests the presence of a specific A3AR retention mechanism.
We were able to compare uptake of the [76Br]labeled antagonist MRS5147 to [76Br]agonist MRS3581. The antagonist MRS5147 shows increasing uptake in the testes, an A3AR rich tissue, suggesting that this ligand may have promise as a molecular imaging agent.
PMCID: PMC2713098  PMID: 19181263
Adenosine A3 receptor; G protein-coupled receptor; nucleoside; purines; receptor binding; Bromine-76
22.  Synthesis and biological evaluation of 2′,4′- and 3′,4′-bridged nucleoside analogues 
Bioorganic & medicinal chemistry  2011;19(18):5648-5669.
Most nucleosides in solution typically exist in equilibrium between two major sugar pucker forms, N-type and S-type, but bridged nucleosides can be locked into one of these conformations depending on their specific structure. While many groups have researched these bridged nucleosides for the purpose of determining their binding affinity for antisense applications, we opted to look into the potential for biological activity within these conformationally-locked structures. A small library of 2′,4′- and 3′,4′-bridged nucleoside analogues was synthesized, including a novel 3′,4′-carbocyclic bridged system. The synthesized compounds were tested for antibacterial, antitumor, and antiviral activities, leading to the identification of nucleosides possessing such biological activities. To the best of our knowledge, these biologically active compounds represent the first example of 2′,4′-bridged nucleosides to demonstrate such properties. The most potent compound, nucleoside 33, exhibited significant antiviral activity against pseudoviruses SF162 (IC50 = 7.0 μM) and HxB2 (IC50 = 2.4 μM). These findings render bridged nucleosides as credible leads for drug discovery in the anti-HIV area of research.
PMCID: PMC3348725  PMID: 21840722
23.  Click Modification in the N6 Region of A3 Adenosine Receptor-Selective Carbocyclic Nucleosides for Dendrimeric Tethering that Preserves Pharmacophore Recognition 
Bioconjugate Chemistry  2012;23(2):232-247.
Adenosine derivatives were modified with alkynyl groups on N6 substituents for linkage to carriers using Cu(I)-catalyzed click chemistry. Two parallel series, both containing a rigid North-methanocarba (bicyclo[3.1.0]hexane) ring system in place of ribose, behaved as A3 adenosine receptor (AR) agonists: (5′-methyluronamides) or partial agonists (4′-truncated). Terminal alkynyl groups on a chain at the 3 position of a N6-benzyl group or simply through a N6–propargyl group were coupled to azido derivatives, which included both small molecules and G4 (fourth-generation) multivalent poly(amidoamine) (PAMAM) dendrimers, to form 1,2,3-triazolyl linkers. The small molecular triazoles probed the tolerance in A3AR binding of distal, sterically bulky groups such as 1-adamantyl. Terminal 4-fluoro-3-nitrophenyl groups anticipated nucleophilic substitution for chain extension and 18F radiolabeling. N6-(4-Fluoro-3-nitrophenyl)-triazolylmethyl derivative 32 displayed a Ki of 9.1 nM at A3AR with ~1000-fold subtype selectivity. Multivalent conjugates additionally containing click-linked water-solubilizing polyethylene glycol groups potently activated A3AR in the 5′-methyluronamide, but not 4′ truncated series. N6-Benzyl nucleoside conjugate 43 (apparent Ki 24 nM) maintained binding affinity of the monomer better than a N6-triazolylmethyl derivative. Thus, the N6 region of 5′-methyluronamide derivatives, as modeled in receptor docking, is suitable for functionalization and tethering by click chemistry to achieve high A3AR agonist affinity and enhanced selectivity.
PMCID: PMC3291892  PMID: 22175234
G protein-coupled receptor; PAMAM dendrimer; purines; structure activity relationship; molecular modeling; adenylate cyclase
24.  2-Dialkynyl Derivatives of (N)-Methanocarba Nucleosides: “Clickable” A3 Adenosine Receptor-Selective Agonists 
We modified a series of (N)-methanocarba nucleoside 5 -uronamides to contain dialkyne groups on an extended adenine C2 substituent, as synthetic intermediates leading to potent and selective A3 adenosine receptor (AR) agonists. The proximal alkyne was intended to promote receptor recognition, and the distal alkyne reacted with azides to form triazole derivatives (click cycloaddition). Click chemistry was utilized to couple an octadiynyl A3AR agonist to azido-containing fluorescent, chemically reactive, biotinylated, and other moieties with retention of selective binding to the A3AR. A bifunctional thiol-reactive crosslinking reagent was introduced. The most potent and selective novel compound was a 1-adamantyl derivative (Ki 6.5 nM), although some of the click products had Ki values in the range of 200–400 nM. Other potent, selective derivatives (Ki at A3AR in nM) were intended as possible receptor affinity labels: 3-nitro-4-fluorophenyl (10.6), α-bromophenacyl (9.6), thiol-reactive isothiazolone (102), and arylisothiocyanate (37.5) derivatives. The maximal functional effects in inhibition of forskolin-stimulated cAMP were measured, indicating that this class of click adducts varied from partial to full A3AR agonist compared to other widely used agonists. Thus, this strategy provides a general chemical approach to linking potent and selective A3AR agonists to reporter groups of diverse structure and to carrier moieties.
PMCID: PMC2818678  PMID: 20036562
G protein-coupled receptor; purines; azide; structure activity relationship; radioligand binding
25.  Partial Agonists for A3 Adenosine Receptors 
Selective agonists for A3 adenosine receptors (ARs) could potentially be therapeutic agents for a variety of disorders, including brain and heart ischemic conditions, while partial agonists may have advantages over full agonists as a result of an increased selectivity of action. A number of structural determinants for A3AR activation have recently been identified, including the N6-benzyl group, methanocarba substitution of ribose, 2-chloro and 2-fluoro substituents, various 2’- and 3’-substitutions and 4’-thio substitution of oxygen. The 2-chloro substitution of CPA and R-PIA led to A3 antagonism (CCPA) and partial agonism (Cl-R-PIA). 2-Chloroadenosine was a full agonist, while 2-fluoroadenosine was a partial agonist. Both 2’- and 3’- substitutions have a pronounced effect on its efficacy, although the effect of 2’-substitution was more dramatic. The 4-thio substitution of oxygen may also diminish efficacy, depending on other substitutions. Both N6-methyl and N6-benzyl groups may contribute to the A3 affinity and selectivity; however, an N6-benzyl group but not an N6-methyl group diminishes A3AR efficacy. N6-benzyl substituted adenosine derivatives have similar potency for human and rat A3ARS while N6-methyl substitution was preferable for the human A3AR. The combination of 2-chloro and N6-benzyl substitutions appeared to reduce efficacy further than either modification alone. The A2AAR agonist DPMA was shown to be an antagonist for the human A3AR. Thus, the efficacy of adenosine derivatives at the A3AR appears to be more sensitive to small structural changes than at other subtypes. Potent and selective partial agonists for the A3AR could be identified by screening known adenosine derivatives and by modifying adenosine and the adenosine derivatives.
PMCID: PMC3425644  PMID: 15078216

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