Structure–activity studies of 5-substituted pyridopyrimidines as adenosine kinase inhibitors
Introduction
Endogenously produced adenosine (ADO) serves a number of roles in the body, but it is especially important as an extracellular messenger where it acts at specific receptors on the cell surface to modulate neuronal activity and inflammation.1 In spite of extensive effort, the direct pharmacological modulation of adenosine receptors with agonists has not yielded useful drug candidates for human use, due to the prevalence of mechanism based side effects (prominently hemodynamic effects). A rationale for a therapeutic approach targeting an indirect modulation of ADO receptors has been proposed as providing ‘site and event selectivity’, with an enhanced therapeutic window.2 Damaged tissues produce elevated levels of ADO, and inhibition of the metabolism of endogenously produced ADO may selectively amplify its local action in comparison with undesired systemic actions.3 AK plays a major regulatory role in metabolizing and inactivating ADO by rephosphorylation to AMP. ADO and ADO receptor agonists have shown analgesic actions in both clinical and preclinical models.3 Our interest has been to prepare AK inhibitors for use as a novel class of analgesics, free of the side effects of more traditional analgesics such as opiates.
The substrate ADO 1 is highly polar and rapidly metabolized, but has nonetheless been used as the starting point for the design of AK inhibitors through rational drug design (Scheme 1).4 However, we have more recently described a novel series of pyridopyrimidine AK inhibitors Scheme 1, Scheme 2, fx1 developed by optimization of the high throughput screening hit 3 (IC50 400 nM).5 We anticipated improved membrane penetration and greater metabolic stability for compounds based on Scheme 1, Scheme 2, fx1, since these are more lipophilic and lack the hydroxyl groups seen in nucleoside-like inhibitors. In this report, we describe large gains in potency upon optimization of the R5 position in Scheme 1, Scheme 2, fx1, in which the best compounds exhibited up to a 2000-fold boost in potency, in comparison with the unsubstituted analogues.
Section snippets
Chemistry
The synthesis of 5-aryl pyridopyrimidines6 was carried out as has been described, where aryl aldehydes 4 were condensed with aryl ketones 5 in the presence of NH4OAc in ethanol or benzene to produce the amino cyano pyridine intermediates 6 (R5=Ar, Scheme 2). For compounds 6, these were subsequently cyclized by heating in formamide to give target Scheme 1, Scheme 2, fx1.
However these conditions failed on two counts when attempting to prepare compounds Scheme 1, Scheme 2, fx1 containing an alkyl
Biological Results and Discussion
The importance of the R5 group in modulating cytosolic AK inhibition can be readily seen in Table 1.8 The trend was for AK inhibitory potency to rapidly increase as the size of the substituent at R5 was increased from H (13=733 nM and 27=562 nM ) to methyl (14=47 nM), to cyclopropyl (15=55 nM), to butyl (16=11 nM), to phenyl (18=7 nM), to cyclohexyl (17=8 nM, 39=3 nM) and 2-bromobenzyl (43=0.17 nM). Variation of the substitution of the aryl groups led generally to good potency, with a trend for the
Acknowledgements
We gratefully acknowledge Dr. Tim van Biesen for assistance with the in vitro assay, and the repreparation of some compounds by J. Robert Koenig, and Ernie Paight.
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Present address: Celgene Corporation, Signal Research Division, 5555 Oberlin Drive, San Diego, CA 92121, USA