Therapeutic potential of adenosine kinase inhibition-Revisited

Abstract

Adenosine (ADO) is an endogenous protective regulator that restores cellular energy balance in response to tissue trauma. Extracellular ADO has a half-life of the order of seconds thus restricting its actions to tissues and cellular sites where it is released. Adenosine kinase (AK, ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) is a cytosolic enzyme that is the rate-limiting enzyme controlling extracellular ADO concentrations. Inhibition of AK can effectively increase ADO extracellular concentrations at tissue sites where pathophysiological changes occur. Highly potent and selective nucleoside and non-nucleoside AK inhibitors were discovered in the late 1990s that showed in vivo effects consistent with the augmentation of the actions of endogenous ADO in experimental models of pain, inflammation, and seizure activity. These data supported clinical development of several AK inhibitors for the management of epilepsy and chronic pain. However, early toxicological data demonstrated that nucleoside and non-nucleoside chemotypes produced hemorrhagic microfoci in brain in an apparent ADO receptor-dependent fashion. An initial oral report of these important toxicological findings was presented at an international conference but a detailed description of these data has not appeared in the peer-reviewed literature. In the two decades following the demise of these early AK-based clinical candidates, interest in AK inhibition has renewed based on preclinical data in the areas of renal protection, diabetic retinopathy, cardioprotection, and neurology. This review provides a summary of the pharmacology and toxicology data for several AK inhibitor chemotypes and the resulting translational issues associated with the development of AK inhibitors as viable therapeutic interventions.

Keywords: adenosine; adenosine kinase; analgesia; inflammation; motor activity; seizures.

Figure 1

The Purinergic Cascade. Intracellular and extracellular concentrations of ATP, ADP, AMP, and ADO are highly regulated by multiple intracellular and extracellular enzymes. The phosphorylation of intracellular ADO to AMP is mediated primarily by the action of AK. The sequential hydrolysis of ATP to ADO via the CD39/CD73 pathway can lead to physiologically relevant extracellular ADO concentrations in certain pathological situations such as inflammation17, 27

Figure 2

Chemical structures of different classes of AK inhibitors