Effects of antileukemic agents on nucleic acid metabolism of human leukemic leukocytes and hematological effects of the agents

Abstract

Effects of antileukemic agents of nucleic acid metabolism of human leukemic leukocytes were studied by chemical determination of incorporation of radioactive nucleic acid precursors into nucleic acids of the cells and by radioautography using thymidine-³H in vitro in relation with hematological effects of the agents.
Leukocyte suspension was prepared from venous blood of patients with various types of leukemias using the method of Skoog and Beck. Nucleic acid biosynthesis in the leukemic leukocytes was determined by adenine-¹⁴C or hypoxanthine-¹⁴C incorporation into DNA and RNA of the cells during a 3 hour incubation. Inhibition of the nucleic acid biosynthesis by antileukemic agents was observed by measuring the reduction of the incorporation in the presence of the agents. Antileukemic agents such as cytosine arabinoside (CA), 6 mercaptopurine (6 MP) and prednisolone were shown to have a good correlation between the rate of inhibition and hematological effects of the agents. When an agent produced a high rate of inhibition in the majority of cases of a certain type of leukemia, it was found that the agent was effective for that type of leukemia. In the same way, CA was suggested to be effective for a large portion of cases of acute leukemia, but only for a small number of chronic myelocytic leukemia. In the clinical course of patients with acute leukemia treated with CA or 6 MP, the rate of the inhibition was in a good correlation with the hematological effects of the agents. These findings suggest that determination of the rate of inhibition of nucleic acid biosynthesis by the antileukemic agent is valuable for predicting hematological effects of the agent in the chemotherapy of leukemia.
In the clinical course of acute leukemias, the ratio of dividing compartment to non-dividing compartment in leukemic blast cells was shown to be variable according to the chemical determination of DNA biosynthesis or observation of labeling index in the radioautography. The ratio was high in relapse, especially when the leukocyte count was rapidly increasing. DNA inhibitors were effective in this period. On the other hand, the ratio was low in remission. Steroid was effective for small blast cells which had low DNA synthesis or low labeling index in radioautography with thymidine-³H and were suspected to be cells belonging to non-dividing compartment.
Haematological effects of 6 MP were in a good correlation with the ratio of hypoxanthine-¹⁴C/adenine-¹⁴C incorporation. From the results, a possibility that low activity of inosinic and guanylic pyrophosphorylase resulted in deficiency of 6 MP-ribotide formation was suggested as a mechanism of resistance against 6 MP. Rapid conversion of 6 MP to thiouric acid by xanthine oxidase was also suggested to be one of other possible mechanisms of 6 MP resistance in a case of chronic myelocytic leukemia in which a combination of hydroxypyrazolopyrimidine, xanthine oxidase inhibitor, and 6 MP was effective for the resistance.