3- and 6-Substituted 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines as A1 adenosine receptor allosteric modulators and antagonists
Graphical abstract
A number of 3- and 6-substituted 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines were prepared and evaluated as potential allosteric modulators at the A1 adenosine receptor. These modifications afforded compounds with the ability to recognize an allosteric site on the agonist-occupied A1AR at relatively high concentrations, but ultimately favoured orthosteric antagonism over allosteric enhancement.
Introduction
The initial studies of Bruns et al.1, 2 on the allosteric modulation of the A1 adenosine receptor (A1AR), described four compounds of interest, including the prototypical modulator PD 81,7231 (Fig. 1). Improving the properties of such leads has received much subsequent attention and, in particular, analogues of the related PD 71,605 led to the discovery of two allosteric modulators, LUF 5484 and T-62 with improved potency over PD 81,723.3, 4, 5 Despite selectively potentiating agonist action at the A1AR via interaction with an allosteric site, however, a characteristic feature of all these modulators is a propensity to also cause antagonism at higher concentrations,3 which is most commonly interpreted as a lower affinity interaction of the modulator with the A1AR’s orthosteric site.1, 2, 3 Thus, there is an ongoing need to gain a better understanding of the structure–activity relationships that govern the allosteric effects of these compounds, on the one hand, and the possible orthosteric effects, on the other. There have also been additional studies with respect to the leads PD 117,975 and PD 78,416, in which the main focus has been variation of the benzoyl substituent in the 3-position.4, 6, 7, 8 Testing of analogs of the two lead compounds PD 117,975 and PD 78,416 conducted by Bruns et al. revealed that the same changes in the 3-position provided comparative enhancement of agonist action, yet, a marked difference was noted if the 6-position was N-methylated, making these compounds virtually inactive.2 Bruns et al.’s reasoning for this was that the N-methyl derivative is more basic than the N-benzyl and therefore protonation causes deleterious effects; this is why the N-ethoxycarbonyl series are potent enhancers, an observation confirmed by Baraldi et al.6
In a recent study conducted in our group, it was shown that amides in the 3-position of 2-aminothiophenes analogous to PD 71,605 supported allosteric enhancement at the A1AR.9 This was the first report in which a detailed SAR study of various carboxamides in the 3-position was performed for the allosteric enhancers at the A1AR. In the current study, we report on synthesis and biological evaluation of analogues of PD 117,975 and PD 78,416, in which we focus on varying the substituent in the 3- and 6-positions of 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines.
Section snippets
Results and discussion
The series of compounds listed in Table 1 were synthesised by the methods depicted in Scheme 1, Scheme 2, Scheme 3. Target 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines with different 6-substitutents were prepared by the Gewald synthesis.10 Ethyl cyanoacetate was reacted with the commercially available N-substituted piperidones 1a–c and elemental sulfur providing 2-aminothiophenes 2a–c (Scheme 1). In the case of 2d–f, the appropriate N-substituted piperidone was first prepared from
Conclusions
Despite finding that 3- and 6-substituted 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines possess the ability to recognize an allosteric site on the agonist-occupied A1AR at relatively high concentrations, we conclude that the structural modifications we have performed on the 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridine scaffold actually favor the expression of orthosteric antagonist properties over allosteric properties. This finding is similar to that made in a recent study by our group
Experimental
Melting points were determined with an Electrothermal melting point apparatus and are uncorrected. All 1H NMR spectra were recorded on a Bruker Avance DPX 300 spectrometer at 300.13 MHz. All 13C NMR spectra were recorded on a Varian Unity Inova 600 spectrometer at 150.8 MHz, unless stated otherwise or on a Bruker Avance DPX 300 spectrometer at 75.4 MHz. Unless stated otherwise, samples were dissolved in CDCl3. High resolution mass spectra were obtained on a Waters LCT Premier XE (TOF) mass
Acknowledgments
This research was supported by Discovery Grant DP0558184 of the Australian Research Council, Project Grant 400134 of the National Health and Medical Research Council (NHMRC) of Australia, and National Institutes of Health Grant R01 HL56111. A.C. is a Senior, and P.M.S. a Principal, Research Fellow of the NHMRC. We are grateful to Dr Michael Crouch, TGR Biosciences, Adelaide, for generously providing the ERK SureFire Alphascreen kit reagents.
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