SCINTILLATION PROXIMITY RADIOIMMUNOASSAY FOR THE MEASUREMENT OF ACYCLOVIR

A homogeneous, single-tube scintillation proximity radioimmunoassay (S PRIA) to quantitate acyclovir (Zovirax(R)), ACV, (9-[(2[hydroxyethoxy) ]methylguanine)} in human plasma is described. The reagents for the SP RIA are an anti-ACV monoclonal antibody (WACO4 MAb), tritiated ACV, an d scintillation proximity reagent (goat anti-mouse immunoglobulin G (I gG) coupled to fluoromicrospheres). The ACV standard curve range in th e SPRIA is from 0.7 ng ml(-1) (3.0 nmol l(-1)) to 90.0 ng ml(-1) (0.4 mu mol 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 h uman plasma controls ranges between 90-110%. Intra- and inter-assay re lative standard deviations were < 8%. This high throughput homogeneous assay is a rapid, convenient and simple alternative to the current ra dioimmunoassay that uses ammonium sulfate precipitation as the separat ion method. This technique is particularly attractive because it requi res neither separation of bound from free drug nor use of scintillatio n fluid. The procedure was applied to quantitate ACV in samples from p re-clinical and clinical studies after the administration of valaciclo vir, a prodrug of ACV (256U87, Valtrex(R), L-valyl ester of ACV). Auto mation of this assay will further improve efficiency in processing a l arger number of samples.