12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Temsirolimus is an inhibitor of mTOR (mammalian target of rapamycin). Temsirolimus binds to an intracellular protein (FKBP-12), and the protein-drug complex inhibits the activity of mTOR that controls cell division. Inhibition of mTOR activity resulted in a G1 growth arrest in treated tumor cells. When mTOR was inhibited, its ability to phosphorylate p70S6k and S6 ribosomal protein, which are downstream of mTOR in the PI3 kinase/AKT pathway was blocked. In in vitro studies using renal cell carcinoma cell lines, temsirolimus inhibited the activity of mTOR and resulted in reduced levels of the hypoxia-inducible factors HIF-1 and HIF-2 alpha, and the vascular endothelial growth factor.
Effects on Electrocardiogram: There were no clinically relevant QT changes observed at the recommended dose for TORISEL. In a randomized, single-blinded, crossover study, 58 healthy subjects received TORISEL 25 mg, placebo, and a single oral dose of moxifloxacin 400 mg. A supratherapeutic TORISEL dose was not studied in this randomized QT trial. The largest difference between the upper bound 2-sided 90% CI for the mean difference between TORISEL and placebo-corrected QT interval was less than 10 ms. In a different trial in 69 patients with a hematologic malignancy, TORISEL doses up to 175 mg were studied. No patient with a normal QTcF at baseline had an increase in QTcF >60 ms. Additionally, there were no patients with a QTcF interval greater than 500 ms.
Following administration of a single 25 mg dose of TORISEL in patients with cancer, mean temsirolimus Cmax in whole blood was 585 ng/mL (coefficient of variation, CV = 14%), and mean AUC in blood was 1627 ng∙h/mL (CV = 26%). Typically Cmax occurred at the end of infusion. Over the dose range of 1 mg to 25 mg, temsirolimus exposure increased in a less than dose proportional manner while sirolimus exposure increased proportionally with dose. Following a single 25 mg intravenous dose in patients with cancer, sirolimus AUC was 2.7-fold that of temsirolimus AUC, due principally to the longer half-life of sirolimus.
Following a single 25 mg intravenous dose, mean steady-state volume of distribution of temsirolimus in whole blood of patients with cancer was 172 liters. Both temsirolimus and sirolimus are extensively partitioned into formed blood elements.
Cytochrome P450 3A4 is the major isozyme responsible for the formation of five temsirolimus metabolites. 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. In human liver microsomes temsirolimus was an inhibitor of CYP2D6 and 3A4. However, there was no effect observed in vivo when temsirolimus was administered with desipramine (a CYP2D6 substrate), and no effect is anticipated with substrates of CYP3A4 metabolism.
Elimination is primarily via the feces. After a single IV dose of [14C]-temsirolimus approximately 82% of total radioactivity was eliminated within 14 days, with 4.6% and 78% of the administered radioactivity recovered in the urine and feces, respectively. Following a single 25 mg dose of TORISEL in patients with cancer, temsirolimus mean (CV) systemic clearance was 16.2 (22%) L/h. Temsirolimus exhibits a bi-exponential decline in whole blood concentrations and the mean half-lives of temsirolimus and sirolimus were 17.3 hours and 54.6 hours, respectively.
Drug-Transport Systems - P-glycoprotein
Temsirolimus is a substrate of the efflux transporter P-glycoprotein (Pgp) in vitro. If TORISEL is administered with drugs that inhibit Pgp, increased concentrations of temsirolimus are likely and caution should be exercised.
In vitro, temsirolimus inhibited human Pgp (IC50 value of 2 µM). If TORISEL is administered with drugs that are substrates of Pgp, increased concentrations of the substrate drug are likely and caution should be exercised.
Effects of Age and Gender
In population pharmacokinetic-based data analyses, no relationship was apparent between drug exposure and patient age or gender.
Effect of Temsirolimus on CYP2D6 or CYP3A
The concentration of desipramine, a CYP2D6 substrate, was unaffected when 25 mg of temsirolimus was co-administered. No clinically significant effect is anticipated when 25 mg of temsirolimus is co-administered with agents that are metabolized by CYP2D6 or CYP3A.