12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Pemetrexed for Injection is a folate analog metabolic inhibitor that disrupts folate-dependent metabolic processes essential for cell replication. In vitro studies show that pemetrexed inhibits thymidylate synthase (TS), dihydrofolate reductase, and glycinamide ribonucleotide formyltransferase (GARFT), which are folate-dependent enzymes involved in the de novo biosynthesis of thymidine and purine nucleotides. Pemetrexed is taken into cells by membrane carriers such as the reduced folate carrier and membrane folate binding protein transport systems. Once in the cell, pemetrexed is converted to polyglutamate forms by the enzyme folylpolyglutamate synthetase. The polyglutamate forms are retained in cells and are inhibitors of TS and GARFT.
Pemetrexed inhibited the in vitro growth of mesothelioma cell lines (MSTO-211H, NCI-H2052) and showed synergistic effects when combined with cisplatin.
Based on population pharmacodynamic analyses, the depth of the absolute neutrophil counts (ANC) nadir correlates with the systemic exposure to pemetrexed and supplementation with folic acid and vitamin B12. There is no cumulative effect of pemetrexed exposure on ANC nadir over multiple treatment cycles.
The pharmacokinetics of pemetrexed when administered as a single agent in doses ranging from 0.2 to 838 mg/m2 infused over a 10-minute period have been evaluated in 426 cancer patients with a variety of solid tumors. Pemetrexed total systemic exposure (AUC) and maximum plasma concentration (Cmax) increased proportionally with increase of dose. The pharmacokinetics of pemetrexed did not change over multiple treatment cycles.
Pemetrexed has a steady-state volume of distribution of 16.1 liters. In vitro studies indicated that pemetrexed is 81% bound to plasma proteins.
The total systemic clearance of pemetrexed is 91.8 mL/min and the elimination half-life of pemetrexed is 3.5 hours in patients with normal renal function (creatinine clearance of 90 mL/min). As renal function decreases, the clearance of pemetrexed decreases and exposure (AUC) of pemetrexed increases.
Pemetrexed is primarily eliminated in the urine, with 70% to 90% of the dose recovered unchanged within the first 24 hours following administration. In vitro studies indicated that pemetrexed is a substrate of OAT3 (organic anion transporter 3), a transporter that is involved in the active secretion of pemetrexed.
Age (26 to 80 years) and sex had no clinically meaningful effect on the systemic exposure of pemetrexed based on population pharmacokinetic analyses.
The pharmacokinetics of pemetrexed were similar in Whites and Blacks or African Americans. Insufficient data are available for other ethnic groups.
Patients with Hepatic Impairment
Pemetrexed has not been formally studied in patients with hepatic impairment. No effect of elevated AST, ALT, or total bilirubin on the PK of pemetrexed was observed in clinical studies.
Patients with Renal Impairment
Pharmacokinetic analyses of pemetrexed included 127 patients with impaired renal function. Plasma clearance of pemetrexed decreases as renal function decreases, with a resultant increase in systemic exposure. Patients with creatinine clearances of 45, 50, and 80 mL/min had 65%, 54%, and 13% increases, respectively in systemic exposure (AUC) compared to patients with creatinine clearance of 100 mL/min [see Dosage and Administration (2.3) and Warnings and Precautions (5.2)].
Drug Interaction Studies
Drugs Inhibiting OAT3 Transporter
Ibuprofen, an OAT3 inhibitor, administered at 400 mg four times a day decreased the clearance of pemetrexed and increased its exposure (AUC) by approximately 20% in patients with normal renal function (creatinine clearance >80 mL/min).
In Vitro Studies
Pemetrexed is a substrate for OAT3. Ibuprofen, an OAT3 inhibitor inhibited the uptake of pemetrexed in OAT3-expressing cell cultures with an average [Iu]/IC50 ratio of 0.38. In vitro data predict that at clinically relevant concentrations, other NSAIDs (naproxen, diclofenac, celecoxib) would not inhibit the uptake of pemetrexed by OAT3 and would not increase the AUC of pemetrexed to a clinically significant extent [see Drug Interactions (7)].
Pemetrexed is a substrate for OAT4. In vitro, ibuprofen and other NSAIDs (naproxen, diclofenac, celecoxib) are not inhibitors of OAT4 at clinically relevant concentrations.
Aspirin, administered in low to moderate doses (325 mg every 6 hours), does not affect the pharmacokinetics of pemetrexed.
Cisplatin does not affect the pharmacokinetics of pemetrexed and the pharmacokinetics of total platinum are unaltered by pemetrexed.