Citabol Mechanism of Action

Pharmacology: Pharmacodynamics: Mechanism of Action: Gemcitabine kills cells undergoing DNA synthesis and blocks the progression of cells through the G1/S-phase boundary. Gemcitabine (dFdC), which is a pyrimidine antimetabolite, is metabolized intracellularly by nucleoside kinases to the active diphosphate and triphosphate 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. Cytotoxic activity in cell cultures Gemcitabine shows significant cytotoxic effects against a variety of cultured murine and human tumor cells. Its action is phase-specific such that Gemcitabine primarily kills cells that are undergoing DNA synthesis (S-phase) and under certain circumstances, blocks the progression of cells at the junction of the G1/S phase boundary. In vitro, the cytotoxic effect of Gemcitabine is dependent on both concentration and time.
Pharmacokinetics: Absorption: Peak plasma concentrations (obtained within 5 minutes of the end of the infusion) were 3.2 to 45.5 μg/ml. Plasma concentrations of the parent compound following a dose of 1,000 mg/m²/30-minutes are greater than 5 μg/ml for approximately 30-minutes after the end of the infusion and greater than 0.4μg/ml for an additional hour.
Distribution: The volume of distribution of the central compartment was 12.4 l/m² for women and 17.5 l/m² for men (inter-individual variability was 91.9%). The volume of distribution of the peripheral compartment was 47.4 l/m². The volume of the peripheral compartment was not sensitive to gender. The plasma protein binding was considered to be negligible.
Half-life: This ranged from 42 to 94 minutes depending on age and gender. For the recommended dosing schedule, gemcitabine elimination should be virtually complete within 5 to 11 hours of the start of the infusion. Gemcitabine does not accumulate when administered once weekly.
Biotransformation: Gemcitabine is rapidly metabolised by cytidine deaminase in the liver, kidney, blood and other tissues. Intracellular metabolism of gemcitabine produces the gemcitabine mono, di and triphosphates (dFdCMP, dFdCDP and dFdCTP) of which dFdCDP and dFdCTP are considered active. These intracellular metabolites have not been detected in plasma or urine. The primary metabolite, 2'-deoxy-2', 2'-difluorouridine (dFdU), is not active and is found in plasma and urine.
Elimination: Systemic clearance ranged from 29.2 l/hr/m² to 92.2 l/hr/m² depending on gender and age (interindividual variability was 52.2%). Clearance for women is approximately 25% lower than the values for men. Although rapid, clearance for both men and women appears to decrease with age. For the recommended gemcitabine dose of 1000 mg/m² given as a 30-minute infusion, lower clearance values for women and men should not necessitate a decrease in the gemcitabine dose. Urinary excretion: Less than 10% is excreted as unchanged drug.
Renal clearance was 2 to 7 l/hr/m².
During the week following administration, 92 to 98% of the dose of gemcitabine administered is recovered, 99% in the urine, mainly in the form of dFdU and 1% of the dose is excreted in faeces.
dFdCTP kinetics: This metabolite can be found in peripheral blood mononuclear cells and the information as follows refers to these cells. Intracellular concentrations increase in proportion to gemcitabine doses of 35-350 mg/m²/30-minutes which give steady state concentrations of 0.4-5μg/ml. At gemcitabine plasma concentrations above 5μg/ml dFdCTP levels do not increase suggesting that the formation is saturable in these cells.
Half-life of terminal elimination: 0.7-12 hours.
dFdU kinetics: Peak plasma concentrations (3-15 minutes after end of 30-minute infusion, 1000 mg/m²): 28-52μg/ml. Trough concentration following once weekly dosing: 0.07-1.12μg/ml with no apparent accumulation. Triphasic plasma concentration versus time curve, mean half-life of terminal phase 65 hours (range 33-84 hr).
Formation of dFdU from parent compound: 91%-98%.
Mean volume of distribution of central compartment: 18 l/m² (range 11-22 1/m²).
Mean steady state volume of distribution (Vss): 150 1/m² (range 96-228 1/m²).
Tissue distribution: Extensive.
Mean apparent clearance: 2.5 1/hr/m² (range 1-4 1/hr/m²).
Urinary excretion: All.
Gemcitabine and Paclitaxel combination therapy: Combination therapy did not alter the pharmacokinetics of either gemcitabine or paclitaxel.
Gemcitabine and Carboplatin combination therapy: When given in combination with carboplatin the pharmacokinetics of gemcitabine were not altered.
Renal impairment: Mild to moderate renal insufficiency (GFR from 30 ml/min to 80 ml/min) has no consistent, significant effect on gemcitabine pharmacokinetics.