New 2,N6-Disubstituted adenosines: Potent and selective A1 adenosine receptor agonists

A number of adenosine analogues substituted in the 2- and N⁶-positions were synthesized and evaluated for affinity, functional potency and intrinsic activity at the A₁ and A_2A adenosine receptors (AR). Three classes of N⁶-substituents were tested; norbornen-2-yl (series 1), norborn-2-yl (series 2) and 5,6-epoxynorborn-2-yl (series 3). The halogens; fluoro, bromo, and iodo were evaluated as C-2 substituents. All compounds showed relatively high affinity (nanomolar) for the A₁AR and high potency for inhibiting (−)isoproterenol-stimulated cAMP accumulation in hamster smooth muscle DDT1 MF-2 cells with the 2-fluoro derivatives from each series having the highest affinity. All of the derivatives showed the same intrinsic activity as CPA. At the A2AAR, all of the derivatives showed relatively low affinity and potency (micromolar) for stimulating cAMP accumulation in rat pheochromocytoma PC-12 cells. The intrinsic activity of the derivatives compared to CGS 21680 was dependent upon the halogen substituent in the C-2 position with most showing partial agonist activity. Of particular interest is 2-iodo-N⁶-(2S-endo-norborn-2-yl)adenosine (5e), which is over 100-fold selective for the A₁AR, is a full agonist at this receptor subtype and has no detectable agonist activity at the A_2AAR.

Chemiluminescence from the Sakaguchi reaction

The Sakaguchi reaction between arginine and hypohalites in the presence of α-phenols (Fig. 1) has been extensively employed for the colorimetric determination of this amino acid [1] M.A Parniak, G Lange and T Viswanatha, Quantitative determination of monosubstituted guanidines: a comparative study of different procedures, J. Biochem. Biophys. Methods 7 (1983), pp. 267–276. Abstract | View Record in Scopus | Cited By in Scopus (8)[1] and [2]. There have been a number of modifications to the reaction for the determination of arginine to improve the color stability and sensitivity. The Sakaguchi reaction is much faster with hypobromite than with hypochlorite, but the colored product fades at a higher rate; however, this can be prevented by adding urea to remove the excess hypobromite [3]. Although 1-naphthol was originally used as the chromogen, other phenols including 2,4-dichloro-1-naphthol [4], 8-quinolinol [5], 5-chloro-7-iodo-8-quinolinol [6], and thymol [2] (Fig. 1) have provided superior analytical figures of merit. We have found that the reaction between arginine and hypobromite is chemiluminescent [7], which has been used to develop an analytical procedure that is rapid, simple, and selective for arginine in the presence of other amino acids.

Potential antiarrhythmic and cardioprotective agents based on adenosine

N-Ethylcarboxamidoadenosine (12) was synthesised from adenosine (1) and the 6-chloro-2’,3’-O-isopropylidene-AT-ethylcarboxamidoadenosine (25) was synthesised from inosine (19). Employing molecular modelling techniques and the results from previous structure activity relationships it was possible to design and synthesise a N6-substituted N-ethylcarboxamidoadenosines which possessed an oxygen in the N6-substituent either in the form of an epoxide (which was obtained by cpoxidising an alkene with m-CPBA or dimethyldioxirane) or in the form of a cyclic ether as was the case for N6-((tetrahydro-2H--pyran--2-yl)methyl-N-ethylcarboxamidoadenosine (78). These compounds were tested for their biological activity at the A1 adenosine receptor by their ability to inhibit cAMP accumulation in DDT, MF2 cells. The EC50 values obtained indicated that the N6-(norborn-5-en-2-yl)-N-ethylcarboxamidoadenosines were the most potent. Of theseN6-(S-endo-norbrn-5-en-2-yI)-N-ethylcarboxaniidoadenosine (56) was the most potent (0.2 nM). N6-(exo-norborn-5-en-2-yl)-2-iodo-N-ethylcarboxamidoadenosine (79) was synthesised from guanosine (22) and was also evaluated for its potency at the A, receptor (24.8 ± 1.5 nM). At present 79 is being evaluated for its selectivity for the A1 receptor compared to the other three receptor subtypes (A2a, A2b, A3). A series of N6-(benzyl)-N-ethylcarboxamidoadenosines were synthesised with substitutions at the 4-position of the phenyl ring. Another series of compounds were synthesised which replaced the methylene spacer between the N6H and the N6-aromatic or lipophilic substituent The replacement groups -were carbonyl and trans-2- cyclopropyl moieties. The N6-acyl compounds were obtained by reacting 2’,3’-O- di(tert-butyldimethylsilyl)-AT-ethylcarboxamidoadenosinc (59) with the appropriate acid chloride and then deprotecting with lelrabutylammonium fluoride in tetrahydrofuran. The compound N6-(4-(1,2-dihydroxy)ethyl)benzyl-N- ethylcarboxamidoadenosine (125) was synthesised by the reaction of 4-(1,2-0- isopropylidene-ethyl)benzyl aminc (123) with 6-chloro-2,3-0-isopropylidene-N- ethylcarboxamidoadenosine (25). Compound 123 was synthesised from an epoxidation of vinylbenzyl phthalimide (118) followed by an acidic ring opening to yield the diol which was isopropylidenated to yield 4-(l,2-O-isopropylidene- elhyl)benzyl phlhalimide (122), It was hoped that the presence of the diol functionality in 125 would increase water solubility whilst maintaining potency at the A3 receptor.