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A comparative in vitro metabolic study of methaphenilene and pyribenzamine
1Department of Pharmacology, University of California, Los Angeles 90024-1735.
Abstract:
1. In vitro metabolism of methaphenilene (MFN) and pyribenzamine (PBZ) was compared to that of methapyriline (MPH) in rat, because chronic treatment with MPH causes cancer in rats, whereas MFN and PBZ cause no cancer. 2. G.l.c. and mass spectrometry were used to identify 7 metabolites of MFN and 6 of PBZ in extracts of rat liver microsome incubations. 3. Quantification of the metabolic pathways revealed that N-oxide formation is considerably more important for both MFN and PBZ than for MPH, and only MPH forms an amide as a metabolic product. 4. Quantitative balance studies show that a lower recovery is apparent for metabolic experiments with MPH than for either MFN or PBZ under all conditions examined, indicating that significant metabolic pathways for MPH exist which are not being measured under these conditions.
Insights
Comparing the in vitro metabolism of methaphenilene (MFN) and pyribenzamine (PBZ) to methapyriline (MPH) in rats revealed distinct metabolic pathways. N-oxide formation is key for MFN and PBZ, unlike MPH, suggesting differing toxicological profiles.
Area of Science:
- Pharmacology and Toxicology
- Drug Metabolism
- Carcinogenesis Research
Background:
- Methapyriline (MPH) causes cancer in rats after chronic treatment.
- Methaphenilene (MFN) and pyribenzamine (PBZ) do not exhibit carcinogenic effects in rats.
- Understanding the metabolic differences may elucidate the varying toxicological outcomes.
Purpose of the Study:
- To compare the in vitro metabolism of MFN and PBZ with MPH in rat liver microsomes.
- To identify and quantify the metabolites formed from MFN and PBZ.
- To correlate metabolic pathways with the known carcinogenic potential of these compounds.
Main Methods:
- In vitro incubation of MFN and PBZ with rat liver microsomes.
- Gas-liquid chromatography (G.l.c.) and mass spectrometry for metabolite identification.
- Quantitative analysis of metabolic pathways and recovery studies.
Main Results:
- Seven MFN metabolites and six PBZ metabolites were identified.
- N-oxide formation was a significantly more important pathway for MFN and PBZ compared to MPH.
- Only MPH produced an amide as a metabolic product.
- Lower quantitative recovery was observed for MPH metabolism, indicating unmeasured pathways.
Conclusions:
- MFN and PBZ metabolism differs significantly from MPH, particularly in the prominence of N-oxide formation.
- The metabolic profile of MPH suggests the existence of uncharacterized pathways contributing to its carcinogenicity.
- These findings provide insights into the differential toxicity of these related compounds.