Adenosine kinase inhibitors. 2. Synthesis, enzyme inhibition, and antiseizure activity of diaryltubercidin analogues.

In the preceding article (Ugarkar et al. J. Med. Chem. 2000, 43) we reported that analogues of tubercidin are potent adenosine kinase (AK) inhibitors with antiseizure activity in the rat maximum electroshock (MES) model. Despite the discovery of several highly potent AK inhibitors (AKIs), e.g., 5'-amino-5'-deoxy- 5-iodotubercidin (1c) (IC50 = 0.0006 microM), no compounds were identified that exhibited a safety, efficacy, and side effect profile suitable for further development. In this article, we demonstrate that substitution of the tubercidin molecule with aromatic rings at the N4- and the C5-positions not only retains AKI potency but also improves in vivo activity. Synthesis of such compounds entailed transformation of 4-arylamino-5-iodotubercidin analogues to their corresponding 5-aryl derivatives via the Suzuki reaction. Alternatively, 4-N-arylamino-5-arylpyrrolo[2,3-d]pyrimidine bases were constructed and then glycosylated with appropriately protected alpha-ribofuranosyl chlorides using a phase-transfer catalyst. Several compounds exhibited potent activity in the rat MES seizure assay with ED50s < or = 2.0 mg/kg, ip, and showed relatively mild side effects.

Adenosine kinase inhibitors as a novel approach to anticonvulsant therapy.

Adenosine levels increase at seizure foci as part of a postulated endogenous negative feedback mechanism that controls seizure activity through activation of A1 adenosine receptors. Agents that amplify this site- and event-specific surge of adenosine could provide antiseizure activity similar to that of adenosine receptor agonists but with fewer dose-limiting side effects. Inhibitors of adenosine kinase (AK) were examined because AK is normally the primary route of adenosine metabolism. The AK inhibitors 5'-amino-5'-deoxyadenosine, 5-iodotubercidin, and 5'-deoxy-5-iodotubercidin inhibited maximal electroshock (MES) seizures in rats. Several structural classes of novel AK inhibitors were identified and shown to exhibit similar activity, including a prototype inhibitor, 4-(N-phenylamino)-5-phenyl-7-(5'-deoxyribofuranosyl)pyrrolo[2, 3-d]pyrimidine (GP683; MES ED50 = 1.1 mg/kg). AK inhibitors also reduced epileptiform discharges induced by removal of Mg2+ in a rat neocortical preparation. Overall, inhibitors of adenosine deaminase or of adenosine transport were less effective. The antiseizure activities of GP683 in the in vivo and in vitro preparations were reversed by the adenosine receptor antagonists theophylline and 8-(p-sulfophenyl)theophylline. GP683 showed little or no hypotension or bradycardia and minimal hypothermic effect at anticonvulsant doses. This improved side effect profile contrasts markedly with the profound hypotension, bradycardia, and hypothermia and greater inhibition of motor function observed with the adenosine receptor agonist N6-cyclopentyladenosine and opens the way to clinical evaluation of AK inhibitors as a novel, adenosine-based approach to anticonvulsant therapy.