Nucleoside transport inhibition by draflazine in unstable coronary disease

Kinetic profiling of novel spirobenzo-oxazinepiperidinone derivatives as equilibrative nucleoside transporter 1 inhibitors

Evaluation of kinetic parameters of drug-target binding, kon, koff, and residence time (RT), in addition to the traditional in vitro parameter of affinity is receiving increasing attention in the early stages of drug discovery. Target binding kinetics emerges as a meaningful concept for the evaluation of a ligand's duration of action and more generally drug efficacy and safety. We report the biological evaluation of a novel series of spirobenzo-oxazinepiperidinone derivatives as inhibitors of the human equilibrative nucleoside transporter 1 (hENT1, SLC29A1). The compounds were evaluated in radioligand binding experiments, i.e., displacement, competition association, and washout assays, to evaluate their affinity and binding kinetic parameters. We also linked these pharmacological parameters to the compounds' chemical characteristics, and learned that separate moieties of the molecules governed target affinity and binding kinetics. Among the 29 compounds tested, 28 stood out with high affinity and a long residence time of 87 min. These findings reveal the importance of supplementing affinity data with binding kinetics at transport proteins such as hENT1.

A semi-quantitative translational pharmacology analysis to understand the relationship between in vitro ENT1 inhibition and the clinical incidence of dyspnoea and bronchospasm

Adenosine contributes to the pathophysiology of respiratory disease, and adenosine challenge leads to bronchospasm and dyspnoea in patients. The equilibrative nucleoside transporter 1 (ENT1) terminates the action of adenosine by removal from the extracellular environment. Therefore, it is proposed that inhibition of ENT1 in respiratory disease patients leads to increased adenosine concentrations, triggering bronchospasm and dyspnoea. This study aims to assess the translation of in vitro ENT1 inhibition to the clinical incidence of bronchospasm and dyspnoea in respiratory disease, cardiovascular disease and healthy volunteer populations. Four marketed drugs with ENT1 activity were assessed; dipyridamole, ticagrelor, draflazine, cilostazol. For each patient population, the relationship between in vitro ENT1 [³H]-NBTI binding affinity (K_i) and [³H]-adenosine uptake (IC₅₀) to the incidence of: (1) bronchospasm/severe dyspnoea; (2) tolerated dyspnoea and; (3) no adverse effects, was evaluated. A high degree of ENT1 inhibition (≥13.3x K_i, ≥4x IC₅₀) associated with increased incidence of bronchospasm/severe dyspnoea for patients with respiratory disease only, whereas a lower degree of ENT1 inhibition (≥0.1x K_i, ≥0.05x IC₅₀) associated with a tolerable level of dyspnoea in both respiratory and cardiovascular disease patients. ENT1 inhibition had no effect in healthy volunteers. Furthermore, physicochemical properties correlative with ENT1 binding were assessed using a set of 1625 diverse molecules. Binding to ENT1 was relatively promiscuous (22% compounds K_i<1μM) especially for neutral or basic molecules, and greater incidence tracked with higher lipophilicity (clogP >5). This study rationalises inclusion of an assessment of ENT1 activity during early safety profiling for programs targeting respiratory disorders.

The therapeutic potential of agents acting via purine receptors

A host of physiological processes associated with the cardiovascular (CV) system, central nervous system (CNS), and a variety of other organ systems and tissues are regulated by agents, primarily adenosine (ado) and adenosine triphosphate (ATP), that act via cell-surface purine receptors. These receptors have therefore been the focus of a variety of programmes directed at the discovery and development of new therapeutic agents, most notably for the treatment of disorders of the CV system. Currently, only a handful of agents, including ado, theophylline, dipyridamole, and ticlopidine, are approved for clinical use. A variety of new agents intended for use in CV disease, disorders of the CNS, such as Parkinson's disease, treatment of pain, inflammatory disorders, and diverse other pathophysiological conditions are in clinical development. Historically, ado receptors have been the primary target. Recent research efforts have begun to examine alternative strategies including agents that modulate endogenous levels of extracellular ado and agents that act via P(2) receptors.

Cardioprotective effects of adenosine transport inhibition during reversible ischaemia in patients with coronary artery disease

BACKGROUND: Adenosine plays a major role in protecting ischaemic myocardium and may potentiate ischaemic preconditioning. Nucleosine transport inhibition may enhance these favourable effects. DESIGN: Randomized, double blind, placebo controlled study, to investigate the haemodynamic and cardioprotective effects of nucleoside transport inhibition during ischaemia in patients with coronary artery disease. PATIENTS AND METHODS: Elective left anterior descending (LAD) coronary angioplasty was used to produce reversible ischaemia in 24 patients with stable angina and a single LAD lesion. They were randomized to receive either the nucleoside transport inhibitor draflazine or placebo. The study medication was infused between the 2nd and 3rd balloon inflation. The primary endpoint was ischaemia-induced wall motion abnormalities as measured by left septal echo amplitude, which was plotted against time to produce an area under the curve. RESULTS: No differences were observed in the systemic haemodynamics or the myocardial collateral circulation of the two groups. The ischaemia-induced regional wall motion abnormalities improved significantly after draflazine, while no difference was observed in the placebo group. This improvement was even more pronounced in patients with low caffeine levels compared with those with high caffeine levels. CONCLUSIONS: Draflazine, in the dose and route used, is associated with a significant improvement in regional myocardial function of the ischaemic area, without affecting systemic or collateral circulation, when compared with placebo. This implies that draflazine has a cardio-protective effect in ischaemic myocardium. High caffeine blood levels reduce these effects.

Adenosine and cardioprotection during ischaemia and reperfusion--an overview

Adenosine is a local hormone, with numerous tissue-specific biological functions. In the myocardium, adenosine is released in small amounts at constant basal rate during normoxia. During ischaemia the production of adenosine increases several fold due to breakdown of adenosine triphosphate (ATP). Increased production of adenosine causes coronary vasodilatation. Thus, adenosine couples myocardial metabolism and flow during ischaemia and is called a homeostatic or "retaliatory metabolite". Furthermore, adenosine has electrophysiological effects in supraventricular tissue, causing a decrease in heart rate. In 1985 it was discovered that adenosine also exerts cardioprotective effects directly on cardiomyocytes. The aim of this review is to give an overview of the role of adenosine as a directly cytoprotective agent during myocardial ischaemia and reperfusion. We will focus on its effects on the myocytes, elicited by stimulation of adenosine receptors in sarcolemma, which triggers intracellular signalling systems. We will also address the new aspect that adenosine can influence regulation of gene expression. There is evidence that the myocardium is capable of endogenous adaptation in response to ischaemia, namely "hibernation" and early and late phases of "preconditioning". Endogenous substances produced during ischaemia probably trigger these responses. We will discuss the role of adenosine in these different settings. Adenosine can be given exogenously through intravasal routes; however, this review will also focus on the effects of endogenously produced adenosine. We will discuss pharmacological ways to increase endogenous levels of adenosine, and the effects of such interventions during ischaemia and reperfusion. Finally, we will review results from studies in humans together with relevant experimental studies, and indicate potential therapeutic implications of adenosine.