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Lack of interaction between zidovudine and 4-methyl-amino-antipyrine, the active metabolite of metamizole, in human liver in vitro.

Zidovudine (AZT) is eliminated by extensive metabolism to an ether glucuronide (GAZT). The nonnarcotic analgesic metamizole (dipyrone) is a typical polydrug, the active metabolite being 4-methyl-amino-antipyrine (4-MAA). About 20% of 4-MAA is excreted in the form of glucuronide in the urine. The aim of this study was to investigate whether 4-MAA inhibits the glucuronidation of AZT, by comparing the GAZT formed in the presence and absence of 4-MAA in the microsomal fractions. Microsomal fractions were obtained from 6 human livers. AZT and 4-MAA were added in concentrations of 1 mmole, corresponding to the therapeutically relevant plasma concentrations of both drugs. Incubation time was 20 min. Concentrations of GAZT were measured using reverse-phase HPLC (high performance liquid chromatography). The mean value of GAZT formed in the microsomal samples without the addition of 4-MAA was 1.87 +/- 0.74 pmole/mg protein. In the presence of 4-MAA, the concentrations averaged 1.77 +/- 0.77 pmole/mg protein, and did not differ significantly from those measured without 4-MAA. In conclusion, the glucuronidation of AZT is not inhibited by 4-MAA, the main active metabolite of metamizole. From the in vitro findings it is predicted that concomitant metamizole administration may fail to enhance by metabolic interference the AZT concentrations under therapy.