Sirolimus, an active metabolite of temsirolimus, is the principal metabolite in humans following intravenous treatment. The remainder of the metabolites account for less than 10% of radioactivity in the plasma.
Temsirolimus is metabolized by hydrolysis to sirolimus, the principal active metabolite. Both temsirolimus and sirolimus also are metabolized by cytochrome P-450 (CYP) isoenzyme 3A4. Although temsirolimus is metabolized to sirolimus, temsirolimus itself exhibits antitumor activity and is not considered a prodrug.
The in vitro metabolism of temsirolimus, (rapamycin-42-[2,2-bis-(hydroxymethyl)]-propionate), an antineoplastic agent, was studied using human liver microsomes as well as recombinant human cytochrome P450s, namely CYP3A4, 1A2, 2A6, 2C8, 2C9, 2C19, and 2E1. Fifteen metabolites were detected by liquid chromatography (LC)-tandem mass spectrometry (MS/MS or MS/MS/MS). CYP3A4 was identified as the main enzyme responsible for the metabolism of the compound. Incubation of temsirolimus with recombinant CYP3A4 produced most of the metabolites detected from incubation with human liver microsomes, which was used for large-scale preparation of the metabolites. By silica gel chromatography followed by semipreparative reverse-phase high-performance liquid chromatography, individual metabolites were separated and purified for structural elucidation and bioactivity studies. The minor metabolites (peaks 1-7) were identified as hydroxylated or desmethylated macrolide ring-opened temsirolimus derivatives by both positive and negative mass spectrometry (MS) and MS/MS spectroscopic methods. Because these compounds were unstable and only present in trace amounts, no further investigations were conducted. Six major metabolites were identified as 36-hydroxyl temsirolimus (M8), 35-hydroxyl temsirolimus (M9), 11-hydroxyl temsirolimus with an opened hemiketal ring (M10 and M11), N- oxide temsirolimus (M12), and 32-O-desmethyl temsirolimus (M13) using combined LC-MS, MS/MS, MS/MS/MS, and NMR techniques. Compared with the parent compound, these metabolites showed dramatically decreased activity against LNCaP cellular proliferation.
CYP3A4 inhibitors: Potential pharmacokinetic interaction (increased plasma concentrations of the principal active metabolite sirolimus). Concomitant use with a potent CYP3A4 inhibitors should be avoided; if no alternative is available, consideration should be given to temsirolimus dosage adjustment.
CYP3A4 inducers: Potential pharmacokinetic interaction (decreased plasma concentrations of the principal active metabolite sirolimus). Concomitant use with potent CYP3A4 inducers should be avoided; if no alternative is available, consideration should be given to temsirolimus dosage adjustment.
