Studies on the selective toxicity of guanazole in mice

Abstract

Guanazole ( 3,5-diamino- 1,2,4-triazole (GZ), an antileukemic agent effective against acute myelocytic leukemia, seems to exert its action through inhibition of ribonucleoside diphosphate reductase. Using recently developed technique for drug infusion in unrestricted mice, 5–30 mg/ml guanazole was infused at a rate of 0.2–0.3 ml/hr in normal mice and mice bearing leukemia L 1210 or a subline resistant to guanazole (L1210/GZ). Incorporation of 14C -uridine (UR) into DNA was considerably reduced in spleen, thymus and bone marrow; incorporation of 3H -thymidine was reduced to a much greater extent in these tissues. The inhibition of precursor incorporation was dependent on the concentration of guanazole used and was somewhat correlated to the intracellular drug concentration in different tissues. This inhibition was in the following order: L 1210 spleen > normal spleen > L1210/GZ spleen. Analysis of free intracellular ribonucleotide pools indicated the presence of a high IMP and a low ATP pool in normal spleen, and a moderate IMP and a high ATP pool in spleen infiltrated by cells from either leukemia; this profile resembled that observed in intraperitoneally growing L 1210 cells. Guanazole infusion resulted in the reversion of pools profile from that characteristic of L 1210 spleen to that of normal spleens. The morphological form and microscopic characteristics of L 1210 spleen from mice treated with GZ also resemble those of spleen from non-leukemic mice. Study of in vitro uptake of guanazole in leukemia L 1210, indicated that the transport occurred by a passive diffusion process. In the subline, L1210/GZ, the initial rate of uptake was 1/3rd to 1/4th of that in the parent line. This suggested that a generalized change in membrane architecture is likely to have accurred in L1210/GZ. The results of this study are consistent with the idea that in different tissues in mice, the mechanism of action of guanazole is dependent upon inhibition of ribonucleotide reductase, whereas its selectivity of action is dependent upon differences in uptake.