Application of a radioimmunoassay for determination of levels of zalcitabine (ddC) in human plasma, urine, and cerebrospinal fluid.

A specific and sensitive radioimmunoassay for the determination of levels of zalcitabine in human plasma, urine, and cerebrospinal fluid has been developed. Commercially available radiolabel and antiserum (Sigma Chemicals) were used after dilution in assay buffer. Prior to the immunoassay, standard and patient samples were subjected to solid-phase extraction on silica columns in order to obtain purified samples. The lower limit of quantitation was determined to be 1 ng/ml. Intra- and interassay variations were less than 11% for a number of quality control samples of drug in plasma and urine. Results from parallelism studies with plasma and urine demonstrated that samples outside the standard range (1 to 30 ng/ml) could be diluted by blank plasma or assay buffer, respectively. A large number of related compounds and potentially coadministered drugs were tested for cross-reactivity. Stability tests showed that heat treatment for 30 min at 60 degrees C or storage for 1 month at -30 degrees C did not affect zalcitabine concentrations in plasma or urine. The radioimmunoassay with solid-phase extraction for sample cleanup discussed here has been successfully applied in a pharmacokinetic study of a single patient, demonstrating its applicability for clinical pharmacokinetic research with zalcitabine.