Incorporation of iododeoxyuridine into DNA of granulocytes in patients

Iododeoxyuridine (IdUrd) competes with thymidine for incorporation into DNA. In order to measure the incorporation of the drug in vivo, granulocytes were isolated from peripheral blood of patients at various times during and after 9- to 14-day IdUrd i.v. continuous infusions. DNA was extracted and enzymatically hydrolyzed. Both thymidine and IdUrd were separated and measured by high-performance liquid chromatography. Thymidine substitution by IdUrd was less than 1% prior to the fifth day of infusion and then increased rapidly to achieve a maximal value between 7 and 17% at the end of the infusion. In our protocol, thrombocytopenia was the most frequent dose-limiting systemic toxicity. For our group of patients, there was a clear overall relationship between the extent of substitution by IdUrd and the hematological toxicity. To our knowledge, these data provide the first direct quantitative determination of substitution by a drug into DNA in vivo without the use of radiolabeled compounds. The ability to directly monitor drug incorporation into DNA may provide the basis for monitoring and improving the selectivity of therapy.