Sensitive radioimmunoassay for cytarabine and uracil arabinoside in plasma.

A sensitive and specific radioimmunoassay for cytarabine (ara-C) or uracil arabinoside (ara-U) was established by using [3H]ara-C and anti-ara-C antiserum or [3H]ara-U and anti-ara-U antiserum. The antiserum was prepared by immunizing rabbits with ara-C or ara-U hapten conjugated to human albumin. In the present assay, as low as 2.5 ng/ml (0.01 microM) of ara-C or 5 ng/ml (0.02 microM) of ara-U in blood plasma samples could be detected. The cross-reaction of ara-C or ara-U structurally related compounds with each antiserum was so small that the nucleosides could be determined without any purification procedure. The plasma concentration of ara-C and ara-U in mice following oral administration of an antileukemic agent, cytosine arabinoside-5'-stearylphosphate (C18PCA) (100 mg/kg), or ara-C (48 mg/kg) was determined by using this method. In the case of C18PCA administration, ara-C was detectable in blood for at least 24 hours longer than in the case of ara-C administration. This method is suitable for the analysis of ara-C and ara-U, the metabolites of depot drugs of ara-C such as C18PCA.