RADIOIMMUNOASSAY OF ZIDOVUDINE - EXTENDED USE AND POTENTIAL APPLICATION

When first approved, the dosing regimens for zidovudine were 1,200-1,5 00 mg/day; however, because toxicity developed. the daily dose had to be reduced to 500-600 mg/day. At these lower doses, plasma concentrati ons for a considerable segment of the dosing interval are often below the assay sensitivity for the high-performance liquid chromatography ( HPLC) method. Although commonly used, the zidovudine radioimmunoassay has had minimal documentation for the quantitative analysis of clinica l samples, especially at current doses. The authors' findings indicate that plasma, urine treated with phosphate buffer, and cerebrospinal f luid samples may be assayed using a commercially available radioimmuno assay. A good correlation was found for clinical samples measured by r adioimmunoassay and HPLC (R-2 = 0.85). The greater assay sensitivity, ability to process multiple specimens, and the relatively rapid turnar ound time suggest that the zidovudine radioimmunoassay may have an imp ortant role in clinical trials evaluating zidovudine pharmacokinetics. This report summarizes the authors' experience with the zidovudine ra dioimmunoassay and focuses on its potential use in studying the role o f therapeutic drug monitoring for zidovudine.