Discovering interactions in polypharmacy: Impact of metamizole on the metabolism of quetiapine.

<bold>Aims:</bold> Metamizole is quite an old drug with analgesic, antipyretic and spasmolytic properties. Recent findings have shown that it may induce several cytochrome P450 (CYP) enzymes, especially CYP3A4 and CYP2B6. The clinical relevance of these properties is uncertain. We aimed to unravel potential pharmacokinetic interactions between metamizole and the CYP3A4 substrate quetiapine.<bold>Methods:</bold> Plasma concentrations of quetiapine from a large therapeutic drug monitoring database were analysed. Two groups of 33 patients, either receiving quetiapine as a monotherapy (without CYP modulating comedications) or with concomitantly applied metamizole, were compared addressing a potential impact of metamizole on the metabolism of quetiapine being reflected in differences of plasma concentrations of quetiapine and dose‐adjusted plasma concentrations.<bold>Results:</bold> Patients comedicated with metamizole showed >50% lower plasma concentrations of quetiapine (median 45.2 ng/mL, Q1 = 15.5; Q3 = 90.5 <italic>vs</italic>. 92.0 ng/mL, Q1 = 52.3; Q3 = 203.8, <italic>P</italic> = .003). The dose‐adjusted plasma concentrations were 69% lower in the comedication group (<italic>P</italic> = .001). Subgroup analyses did not suggest a dose dependency of the metamizole effect or an influence of quetiapine formulation (immediate <italic>vs</italic>. extended release). Finally, the comedication group exhibited a significantly higher proportion of patients whose quetiapine concentrations were below the therapeutic reference range (78.8% in the metamizole group <italic>vs</italic>. 54.4% in the control group, <italic>P</italic> = .037) indicating therapeutically insufficient drug concentrations.<bold>Conclusion:</bold> The combination of metamizole and quetiapine leads to significantly lower drug concentrations of quetiapine, probably via an induction of CYP3A4. Clinicians must consider the risk of adverse drug reactions, especially treatment failure under quetiapine when adding metamizole. [ABSTRACT FROM AUTHOR]