Temsirolimus

Temsirolimus is a selective inhibitor of mTOR (mammalian target of rapamycin). Temsirolimus binds to an intracellular protein (FKBP-12), and the protein/temsirolimus complex binds and inhibits the activity of mTOR that controls cell division.

In vitro, at high concentrations (10-20 M), temsirolimus can bind and inhibit mTOR in the absence of FKBP-12. Biphasic dose response of cell growth inhibition was observed. High concentrations resulted in complete cell growth inhibition in vitro, whereas inhibition mediated by FKBP-12/temsirolimus complex alone resulted in approximately 50% decrease in cell proliferation. Inhibition of mTOR activity results in a G1 growth delay at nanomolar concentrations and growth arrest at micromolar concentrations in treated tumour cells resulting from selective disruption of translation of cell cycle regulatory proteins, such as D-type cyclins, c-myc, and ornithine decarboxylase. When mTOR activity is inhibited, its ability to phosphorylate, and thereby control the activity of protein translation factors (4E-BP1 and S6K, both downstream of mTOR in the P13 kinase/AKT pathway) that control cell division, is blocked.

In addition to regulating cell cycle proteins, mTOR can regulate translation of the hypoxia-inducible factors, HIF-1 and HIF-2 alpha. These transcription factors regulate the ability of tumours to adapt to hypoxic microenvironments and to produce the angiogenic factor vascular endothelial growth factor (VEGF). The anti-tumour effect of temsirolimus, therefore, may also in part stem from its ability to depress levels of HIF and VEGF in the tumour or tumour microenvironment, thereby impairing vessel development.

Absorption

Following administration of a single 25 mg intravenous dose of temsirolimus in patients with cancer, mean C_max in whole blood was 585 ng/ml (coefficient of variation [CV] = 14%), and mean AUC in blood was 1627 ng•h/ml (CV = 26%). For patients receiving 175 mg weekly for 3 weeks followed by 75 mg weekly, estimated C_max in whole blood at end of infusion was 2457 ng/ml during Week 1, and 2574 ng/ml during Week 3.

Distribution

Temsirolimus exhibits a polyexponential decline in whole blood concentrations, and distribution is attributable to preferential binding to FKBP-12 in blood cells. The mean ±standard deviation (SD) dissociation constant (K_d) of binding was 5.1 ± 3.0 ng/ml, denoting the concentration at which 50% of binding sites in blood cells were occupied. Temsirolimus distribution is dose-dependent with mean (10th, 90th percentiles) maximal specific binding in blood cells of 1.4 mg (0.47 to 2.5 mg). Following a single 25 mg temsirolimus intravenous dose, mean steady-state volume of distribution in whole blood of patients with cancer was 172 liters.

Biotransformation

Sirolimus, an equally potent metabolite to temsirolimus, was observed as the principal metabolite in humans following intravenous treatment. During in vitro temsirolimus metabolism studies, sirolimus, seco-temsirolimus and seco-sirolimus were observed; additional metabolic pathways were hydroxylation, reduction and demethylation. 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.

Elimination

Following a single 25 mg intravenous dose of temsirolimus, temsirolimus mean ± SD systemic clearance from whole blood was 11.4 ± 2.4 l/h. Mean half-lives of temsirolimus and sirolimus were 17.7 hours and 73.3 hours, respectively. Following administration of [¹⁴C] temsirolimus, excretion was predominantly via the faeces (78%), with renal elimination of active substance and metabolites accounting for 4.6% of the administered dose. Sulfate or glucuronide conjugates were not detected in the human faecal samples, suggesting that sulfation and glucuronidation do not appear to be major pathways involved in the excretion of temsirolimus. Therefore, inhibitors of these metabolic pathways are not expected to affect the elimination of temsirolimus.

Model-predicted values for clearance from plasma, after applying a 175 mg dose for 3 weeks, and subsequently 75 mg for 3 weeks, indicate temsirolimus and sirolimus metabolite trough concentrations of approximately 1.2 ng/ml and 10.7 ng/ml, respectively.

Temsirolimus and sirolimus were demonstrated to be substrates for P-gp in vitro.

Pharmacokinetic/pharmacodynamic relationship(s)

Inhibition of CYP isoforms

In in vitro studies in human liver microsomes, temsirolimus inhibited CYP3A4/5, CYP2D6, CYP2C9 and CYP2C8 catalytic activity with Ki values of 3.1, 1.5, 14 and 27 μM, respectively.

IC₅₀ values for inhibition of CYP2B6 and CYP2E1 by temsirolimus were 48 and 100 μM, respectively. Based on a whole blood mean C_max concentration of 2.6 μM for temsirolimus in MCL patients receiving the 175 mg dose there is a potential for interactions with concomitantly administered medicinal products that are substrates of CYP3A4/5 in patients treated with the 175 mg dose of temsirolimus. Physiologically-based pharmacokinetic modelling has shown that after four weeks treatment with temsirolimus, the AUC of midazolam can be increased 3-to-4 fold and C_max around 1.5-fold when midazolam is taken within a few hours after the start of the temsirolimus infusion. However, it is unlikely that whole blood concentrations of temsirolimus after intravenous administration of temsirolimus will inhibit the metabolic clearance of concomitant medicinal products that are substrates of CYP2C9, CYP2C8, CYP2B6 or CYP2E1.

Special populations

Hepatic impairment

Temsirolimus should be used with caution when treating patients with hepatic impairment.

Temsirolimus is cleared predominantly by the liver.

Temsirolimus and sirolimus pharmacokinetics have been investigated in an open-label, dose-escalation study in 110 patients with advanced malignancies and either normal or impaired hepatic function. For 7 patients with severe hepatic impairment (ODWG, group D) receiving the 10 mg dose of temsirolimus, the mean AUC of temsirolimus was ~1.7-fold higher compared to 7 patients with mild hepatic impairment (ODWG, group B). For patients with severe hepatic impairment, a reduction of the temsirolimus dose to 10 mg is recommended to provide temsirolimus plus sirolimus exposures in blood (mean AUC sum approximately 6510 ng·h/ml; n=7), which approximate to those following the 25 mg dose (mean AUC sum approximately 6580 ng·h/ml; n=6) in patients with normal liver function.

The AUC_sum of temsirolimus and sirolimus on day 8 in patients with mild and moderate hepatic impairment receiving 25 mg temsirolimus was similar to that observed in patients without hepatic impairment receiving 75 mg (mean AUC_sum mild: approximately 9770 ng*h/ml, n=13; moderate: approximately 12380 ng*h/ml, n=6; normal approximately 10580 ng*h/ml, n=4).

Gender, weight, race, age

Temsirolimus and sirolimus pharmacokinetics are not significantly affected by gender. No relevant differences in exposure were apparent when data from the Caucasian population was compared with either the Japanese or Black population.

In population pharmacokinetic-based data analysis, increased body weight (between 38.6 and 158.9 kg) was associated with a two-fold range of trough concentration of sirolimus in whole blood.

Pharmacokinetic data on temsirolimus and sirolimus are available in patients up to age 79 years. Age does not appear to affect temsirolimus and sirolimus pharmacokinetics significantly.

Paediatric population

In the paediatric population, clearance of temsirolimus was lower and exposure (AUC) was higher than in adults. In contrast, exposure to sirolimus was commensurately reduced in paediatric patients, such that the net exposure as measured by the sum of temsirolimus and sirolimus AUCs (AUC_sum) was comparable to that for adults.

Adverse reactions not observed in clinical studies, but seen in animals at exposure levels similar to or even lower than clinical exposure levels and with possible relevance to clinical use, were as follows: pancreatic islet cell vacuolation (rat), testicular tubular degeneration (mouse, rat and monkey), lymphoid atrophy (mouse, rat and monkey), mixed cell inflammation of the colon/caecum (monkey), and pulmonary phospholipidosis (rat).

Diarrhoea with mixed cell inflammation of the caecum or colon was observed in monkeys and was associated with an inflammatory response, and may have been due to a disruption of the normal intestinal flora.

General inflammatory responses, as indicated by increased fibrinogen and neutrophils, and/or changes in serum protein, were observed in mice, rats, and monkeys, although in some cases these clinical pathology changes were attributed to skin or intestinal inflammation as noted above. For some animals, there were no specific clinical observations or histological changes that suggested inflammation.

Temsirolimus was not genotoxic in a battery of in vitro (bacterial reverse mutation in Salmonella typhimurium and Escherichia coli, forward mutation in mouse lymphoma cells, and chromosome aberrations in Chinese hamster ovary cells) and in vivo (mouse micronucleus) assays.

Carcinogenicity studies have not been conducted with temsirolimus; however, sirolimus, the major metabolite of temsirolimus in humans, was carcinogenic in mice and rats. The following effects were reported in mice and/or rats in the carcinogenicity studies conducted: granulocytic leukaemia, lymphoma, hepatocellular adenoma and carcinoma, and testicular adenoma.

Reductions in testicular weights and/or histological lesions (e.g., tubular atrophy and tubular giant cells) were observed in mice, rats, and monkeys. In rats, these changes were accompanied by a decreased weight of accessory sex organs (epididymides, prostate, seminal vesicles). In reproduction toxicity studies in animals, decreased fertility and partly reversible reductions in sperm counts were reported in male rats. Exposures in animals were lower than those seen in humans receiving clinically relevant doses of temsirolimus.