SENSITIVE AND SPECIFIC RADIOIMMUNOASSAY FOR FIALURIDINE - INITIAL ASSESSMENT OF PHARMACOKINETICS AFTER SINGLE ORAL DOSES TO HEALTHY-VOLUNTEERS

Fialuridine (FIAU) is a halogen-substituted analog of thymidine that w as undergoing clinical investigation as a drug for the treatment of ch ronic hepatitis B viral infection. However, clinical trials of FIAU we re terminated after adverse events occurred following chronic oral adm inistration. Prior to the termination of clinical trials, a sensitive assay was needed for the measurement of FIAU because of the anticipate d low dose administered to patients. We therefore undertook the develo pment of a radioimmunoassay (RIA). A specific antiserum was raised in rabbits following immunization with a 5'-O-hemisuccinate analog of FIA U coupled to keyhole limpet hemocyanin. Radiolabeled FIAU was synthesi zed by a destannylation procedure by using sodium [I-125]iodide. W, de veloped a competitive-binding procedure and used precipitation with po lyethylene glycol as the method for separating the bound and free form s of FIAU. The RIA is sensitive (0.2 ng/ml), specific (negligible inte rference from known metabolites and endogenous nucleosides), and repro ducible (interassay coefficients of variation range from 5 to 19.7% fo r serum controls). We used the RIA to assess the pharmacokinetics of F IAU in healthy adult volunteers following administration of a single 5 -mg oral dose. The sensitivity of the RIA permitted the detection of a prolonged elimination phase for FIAU in healthy volunteers and dogs, with mean elimination half-lives of 29.3 and 35.3 h, respectively. We conclude the RTA is a valid method for the quantification of FIAU in b iological fluids.