ACC-9653, the disodium phosphate ester of 3-hydroxymethyl-5,5-diphenylhydantoin, is a prodrug of phenytoin with advantageous physicochemical properties. ACC-9653 is rapidly converted enzymatically to phenytoin in vivo. ACC-9653 and phenytoin sodium have equivalent anticonvulsant activity against seizures induced by maximal electroshock (MES) in mice following i.p., oral, or i.v. administration. The ED50 doses were 16 mg/kg for i.v. ACC-9653 and 8 mg/kg for i.v. phenytoin sodium. ACC-9653 and phenytoin sodium have similar antiarrhythmic activity against ouabain-induced ventricular tachycardia in anesthetized dogs. The total doses of ACC-9653 or phenytoin sodium necessary to convert the arrhythmia to a normal sinus rhythm were 24 +/- 6 and 14 +/- 3 mg/kg, respectively. Only phenytoin sodium displayed in vitro antiarrhythmic activity against strophanthidin-induced arrhythmias in guinea pig right atria. In anesthetized dogs, a high dose of ACC-9653 (31 mg/kg) was infused over 15, 20, and 30 min and the responses were compared to an equimolar dose of phenytoin sodium (21 mg/kg). The ACC-9653 and phenytoin sodium treatments produced similar marked reductions in diastolic blood pressure and contractile force (LVdP/dt). The maximum effects of each treatment occurred at the time of maximum phenytoin sodium levels. Acute toxicity studies of ACC-9653 and phenytoin sodium were carried out in mice, rats, rabbits, and dogs by i.v., i.m., and i.p. routes of administration. The systemic toxic signs of both agents were similar and occurred at approximately equivalent doses. Importantly, the local irritation of ACC-9653 was markedly less than phenytoin sodium following i.m. administration.(ABSTRACT TRUNCATED AT 250 WORDS)