12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Gemcitabine kills cells undergoing DNA synthesis and blocks the progression of cells through the G1/S-phase boundary. Gemcitabine is metabolized by nucleoside kinases to diphosphate (dFdCDP) and triphosphate (dFdCTP) nucleosides. Gemcitabine diphosphate inhibits ribonucleotide reductase, an enzyme responsible for catalyzing the reactions that generate deoxynucleoside triphosphates for DNA synthesis, resulting in reductions in deoxynucleotide concentrations, including dCTP. Gemcitabine triphosphate competes with dCTP for incorporation into DNA. The reduction in the intracellular concentration of dCTP by the action of the diphosphate enhances the incorporation of gemcitabine triphosphate into DNA (self-potentiation). After the gemcitabine nucleotide is incorporated into DNA, only one additional nucleotide is added to the growing DNA strands, which eventually results in the initiation of apoptotic cell death.
The pharmacokinetics of gemcitabine were examined in 353 patients with various solid tumors. Pharmacokinetic parameters were derived using data from patients treated for varying durations of therapy given weekly with periodic rest weeks and using both short infusions (<70 minutes) and long infusions (70 to 285 minutes). The total gemcitabine dose varied from 500 mg/m2 to 3,600 mg/m2.
The volume of distribution was increased with infusion length. Volume of distribution of gemcitabine was 50 L/m2 following infusions lasting <70 minutes. For long infusions, the volume of distribution rose to 370 L/m2.
Gemcitabine pharmacokinetics are linear and are described by a 2-compartment model. Population pharmacokinetic analyses of combined single and multiple dose studies showed that the volume of distribution of gemcitabine was significantly influenced by duration of infusion and sex. Gemcitabine plasma protein binding is negligible.
The active metabolite, gemcitabine triphosphate, can be extracted from peripheral blood mononuclear cells. The half-life of the terminal phase for gemcitabine triphosphate from mononuclear cells ranges from 1.7 to 19.4 hours.
Gemcitabine disposition was studied in 5 patients who received a single 1,000 mg/m2 of radiolabeled drug as a 30-minute infusion. Within one week, 92% to 98% of the dose was recovered, almost entirely in the urine. Gemcitabine (<10%) and the inactive uracil metabolite, 2´-deoxy-2´,2´-difluorouridine (dFdU) accounted for 99% of the excreted dose. The metabolite dFdU is also found in plasma.
Clearance of gemcitabine was affected by age. The lower clearance in geriatric patients results in higher concentrations of gemcitabine for any given dose. Differences in either clearance or volume of distribution based on patient characteristics or the duration of infusion result in changes in half-life and plasma concentrations. Table 15 shows plasma clearance and half-life of gemcitabine following short infusions for typical patients by age and sex.
Gemcitabine half-life for short infusions ranged from 42 to 94 minutes and for long infusions varied from 245 to 638 minutes, depending on age and sex, reflecting a greatly increased volume of distribution with longer infusions.
Male and Female Patients
Females have lower clearance and longer half-lives than male patients as described in Table 15.
Patients with Renal Impairment
No clinical studies have been conducted with gemcitabine in patients with decreased renal function.
Drug Interaction Studies
When gemcitabine (1,250 mg/m2 on Days 1 and 8) and cisplatin (75 mg/m2 on Day 1) were administered in patients with NSCLC, the clearance of gemcitabine on Day 1 was 128 L/hr/m2 and on Day 8 was 107 L/hr/m2. Data from patients with NSCLC demonstrate that gemcitabine and carboplatin given in combination does not alter the pharmacokinetics of gemcitabine or carboplatin compared to administration of either single agent; however, due to wide confidence intervals and small sample size, interpatient variability may be observed.
Data from metastatic breast cancer patients shows that gemcitabine has little or no effect on the pharmacokinetics (clearance and half-life) of paclitaxel and paclitaxel has little or no effect on the pharmacokinetics of gemcitabine.