Scintillation proximity radioimmunoassay for the measurement of acyclovir

A homogeneous, single-tube scintillation proximity radioimmunoassay (SPRIA) to quantitate acyclovir (Zovirax ®), ACV, (9-[(2[hydroxyethoxy)]methylguanine)} in human plasma is described. The reagents for the SPRIA are an anti-ACV monoclonal antibody (WACO4 MAb), tritiated ACV, and scintillation proximity reagent (goat anti-mouse immunoglobulin G (IgG) coupled to fluoromicrospheres). The ACV standard curve range in the SPRIA is from 0.7 ng ml −1 (3.0 nmol l −1) to 90.0 ng ml −1 (0.4 μmol l −1) with a 50% inhibitory concentration of 5.0 ng ml −1 (22.2 nmol l −1). However, the lower limit of quantification is 7 ng ml −1 at 1:10 dilution of plasma. Analytical recovery of ACV in spiked human plasma controls ranges between 90–110%. Intra- and inter-assay relative standard deviations were < 8%. This high throughput homogeneous assay is a rapid, convenient and simple alternative to the current radioimmunoassay that uses ammonium sulfate precipitation as the separation method. This technique is particularly attractive because it requires neither separation of bound from free drug nor use of scintillation fluid. The procedure was applied to quantitate ACV in samples from pre-clinical and clinical studies after the administration of valaciclovir, a prodrug of ACV (256U87, Valtrex ®, l-valyl ester of ACV). Automation of this assay will further improve efficiency in processing a larger number of samples.