A radioimmunoassay for cytosine arabinoside

A radioimmunoassay (RIA) for cytosine arabinoside (AraC) has been developed using antiserum raised in a sheep to an AraC monophosphate-ovalbumin conjugate. The antibody shows only 0.008% cross-reactivity with uracil arabinoside (AraU) and low (0.023%) cross-reactivity with other commonly co-administered drugs such as cytotoxic and antibacterial agents, and also a number of naturally occurring nucleosides and nucleotides. It does however cross-react by 125% with AraC monophosphate and by 109% with AraC triphosphate. As little as 1 ng/ml of AraC can be detected in plasma, serum, urine and cerebrospinal fluid (CSF) with no need for prior extraction. This RIA has been used to follow the disappearance of AraC from the plasma of patients receiving the drug.