Ribavirin, 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxyamide (
Virazole; Viratek, Inc., Covina, Calif.), has a broad spectrum of
antiviral activity. However, the study of the absorption, metabolism, and excretion of this compound has been limited by the lack of an appropriate assay for
ribavirin and its metabolites. Since
ribavirin has definite potential for
therapeutic use, we developed a radioimmunoassay to measure
ribavirin levels in clinical specimens. To prepare an effective immunogen,
ribavirin was monosuccinylated and coupled to
ovalbumin. The competitive binding radioimmunoassay, in which
tritium-labeled
ribavirin and rabbit antiribavirin serum were used, was quantitative for
ribavirin at concentrations of 1 pmol/100 microliter in urine or plasma samples. The rabbit antibody cross-reacted with the major metabolite of
ribavirin,
1,2,4-triazole-3-carboxamide, at a low level (2 to 5%) which did not interfere with
ribavirin binding until concentrations of
1,2,4-triazole-3-carboxamide 10- to 100-fold higher than
ribavirin were present in mock samples, a condition not present in
biological specimens. We used the
ribavirin radioimmunoassay to determine the
ribavirin concentration in mouse plasma after intraperitoneal administration, in the sera of adults from Sierra Leone after oral or
intravenous administration for treatment of suspected
Lassa fever, and in the sera of children in the United States after small-particle
aerosol administration. Our experience with the radioimmunoassay indicates that it is sensitive, accurate, and reproducible. The assay will permit studies leading to a better understanding of the pharmacology and pharmacokinetics of this potentially useful
antiviral drug.