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Selective deactivation of ICR mutagens as related to their distinctive pulmonary carcinogenicity
Abstract:
The mutagenicity patterns and the metabolic behaviour of 4 structurally related ICR compounds were investigated using the Salmonella/microsome test. ICR 191 and ICR 170 reverted 4 his- strains (TA1537 greater than TA100 greater than TA98 and TA1538), while ICR 191-OH and ICR 170-OH reverted TA1537 only, with a narrow range of mutagenic activity. Although all the products tested were shown to contain a common mutagenic impurity, analysis of chemicals fractionated by h.p.l.c. provided evidence that pure ICR 191-OH and ICR 170-OH also induce frameshift errors. The mutagenic activity of all the 4 ICR compounds, as well as of their common impurity, was decreased in the presence of S-9 mix containing rat liver S-9 fractions, a trend confirmed by h.p.l.c. of ICR/S-9 mixtures. Despite the greater mutagenic potency in the absence of metabolic systems, ICR 191 was deactivated far more efficiently and rapidly than ICR 170 by a variety of mouse (liver greater than lung) and rat (liver greater than testis greater than kidney greater than lung greater than striated muscle greater than spleen) S-9 fractions. Mouse preparations were more effective than the corresponding rat preparations. Aroclor 1254 strongly stimulated tha activity of th NADPH-requiring enzymes responsible for ICR deactivation. H.p.l.c. analyses ruled out interconversion among the 4 ICR compounds in the presence of liver S-9 fractions, while revealing the appearance of new distinct peaks for ICR 191, ICR 170 and ICR 170-OH. The selective metabolic deactivation of ICR compounds may be related to the findings of previous carcinogenicity studies (1-3) which showed the induction of lung adenomas following i.v. but not i.p. administration of ICR 170. Conversely, ICR 191 and ICR 191-OH showed no activity even when administered i.v. Since intercalation of ICR compounds into DNA is the molecular basis for both mutagenic and antitumor activities (4), their distinctive metabolic reactivity may be also responsible for the differential efficacy in pharmacological assays.
Insights
The mutagenicity of four ICR compounds varied, with ICR 191 and ICR 170 being more potent mutagens. Metabolic deactivation by liver enzymes differed significantly between ICR 191 and ICR 170, influencing their pharmacological effects.
Area of Science:
- Toxicology
- Molecular Biology
- Pharmacology
Background:
- The mutagenicity and metabolic fate of intercalating agents are crucial for understanding their biological effects.
- Structurally related ICR compounds are known for their mutagenic and potential antitumor activities.
- Investigating metabolic deactivation pathways can elucidate differential efficacy and toxicity.
Purpose of the Study:
- To compare the mutagenicity patterns of four related ICR compounds.
- To investigate the metabolic behavior and deactivation of these ICR compounds using liver enzyme fractions.
- To correlate metabolic reactivity with previously observed in vivo carcinogenicity and pharmacological data.
Main Methods:
- Salmonella/microsome assay (Ames test) to assess mutagenicity.
- High-performance liquid chromatography (HPLC) to analyze compound integrity and metabolic products.
- Incubation with various S-9 fractions (rat and mouse liver, lung, kidney, testis, spleen) to study metabolic deactivation.
- Assessment of NADPH-dependent enzyme activity stimulation by Aroclor 1254.
Main Results:
- ICR 191 and ICR 170 showed broader mutagenic activity than ICR 191-OH and ICR 170-OH, which primarily reverted TA1537.
- All compounds, including a common impurity, exhibited frameshift mutagenicity.
- Metabolic deactivation by S-9 mix reduced mutagenicity for all compounds.
- ICR 191 was deactivated more rapidly and efficiently than ICR 170 by mouse and rat S-9 fractions, with mouse preparations being more effective.
- HPLC analysis confirmed metabolic deactivation and formation of new metabolites, ruling out interconversion.
Conclusions:
- Differential metabolic deactivation of ICR compounds by liver enzymes significantly impacts their mutagenic potential.
- The selective metabolic reactivity of ICR compounds likely explains their varied carcinogenicity and pharmacological efficacy observed in previous studies.
- Understanding these metabolic pathways is key to predicting the in vivo behavior and therapeutic utility of ICR derivatives.