Cytotoxic purine nucleoside analogues bind to A1, A2A, and A3 adenosine receptors

Fludarabine, clofarabine, and cladribine are anticancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Radioligand binding studies utilizing Chinese hamster ovary cells, stably transfected with adenosine A₁, A_2A, or A₃ receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A_2B receptors. Clofarabine and cladribine both bound to the A_2A receptor with a K _i of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A₃ receptor with a K _i of 10 μM, and none of these compounds bound to the A_2B receptor. Results show that clofarabine, cladribine, and fludarabine bind to the A₁ receptor. In addition, clofarabine, cladribine, and fludarabine were A₁ agonists (IC₅₀ 3.1, 30, and 30 μM, respectively). Neither pyrimidine nucleoside analogues gemcitabine nor cytarabine associated with any of the adenosine receptor subtypes (K _i > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.