st 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. Gemcitabine plasma protein binding is negligible.
The pharmacokinetics of gemcitabine were examined in 353 patients, about 2/3 men, 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 Gemzar dose varied from 500 to 3600 mg/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 gender. Clearance was affected by age and gender. 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 1 shows plasma clearance and half–life of gemcitabine following short infusions for typical patients by age and gender.
Table 1: Gemcitabine Clearance and Half–Life for the “Typical” Patient *
Half–life for patients receiving a short infusion (<70 min).
Age
Clearance
Men
(L/hr/m2)
Clearance
Women
(L/hr/m2)
Half–Life*
Men
(min)
Half–Life*
Women
(min)
29
92.2
69.4
42
49
45
75.7
57.0
48
57
65
55.1
41.5
61
73
79
40.7
30.7
79
94
Gemcitabine half–life for short infusions ranged from 42 to 94 minutes, and the value for long infusions varied from 245 to 638 minutes, depending on age and gender, reflecting a greatly increased volume of distribution with longer infusions. The lower clearance in women and the elderly results in higher concentrations of gemcitabine for any given dose.
The volume of distribution was increased with infusion length. Volume of distribution of gemcitabine was 50 L/m2 following infusions lasting<70 minutes, indicating that gemcitabine, after short infusions, is not extensively distributed into tissues. For long infusions, the volume of distribution rose to 370 L/m2, reflecting slow equilibration of gemcitabine within the tissue compartment.
The maximum plasma concentrations of dFdU (inactive metabolite) were achieved up to 30 minutes after discontinuation of the infusions and the metabolite is excreted in urine without undergoing further biotransformation. The metabolite did not accumulate with weekly dosing, but its elimination is dependent on renal excretion, and could accumulate with decreased renal function.
The effects of significant renal or hepatic insufficiency on the disposition of gemcitabine have not been assessed.
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.
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