Obesity Alters the Pharmacokinetics of Vincristine in Mice.

Obesity has been associated with increased risk of development and poor outcome from a variety of hematological malignancies. There is a lack of consensus as to how to dose chemotherapies in obese patients, and therefore inadequate dosing represents a potential factor contributing to poor outcome. We have previously shown that diet-induced obese mice transplanted with p190^BCR/ABL ALL cells and then treated with vincristine (VCR) proportional to body weight (15 mg/kg/wk × 4 wks) have a poorer outcome than non-obese mice (event free survival: Obese = 0.42, Control = 0.75, p=0.07, log rank). The present study was designed to quantify the pharmacokinetic (PK) parameters of vincristine in control and obese mice and determine whether differences in these parameters could explain the poorer outcome in the obese mice. Twenty obese and 20 control mice received tail-vein injections of tritiated vincristine (specific activity 75 mCi/mg; dose 0.5 mg/kg). Blood samples were taken at 5 minutes, and the mice sacrificed at various time-points between 15 minutes and 24 hours post-injection. Whole blood and tissue specimens were solubilized, decolorized, and read on a scintillation counter. Blood vincristine concentrations were fit to a 3-compartment model using the two-stage PK in 3 subsets of VCR concentrations vs. time method. Non-compartmental modeling was used to confirm the results. Due to body weight differences, obese mice received ~ 28% more vincristine per injection than controls. Blood vincristine concentrations in both groups of animals followed a triexponential decay, as has been previously described. Blood levels tended to be higher (by 20±15%) in the obese mice at all time-points tested. While the t_1/2α was longer in the obese mice (10.6 vs. 6.0 minutes), the t_1/2b and t_1/2γ were both shorter (22.7 vs. 27.1 minutes and 114.4 vs. 345.8 hours). Therefore the AUC_{0 → 24} was higher in the obese mice (3,279 vs. 2,776 ng/ml*hr), while the AUC_{0 → ∞} was lower in the obese mice (21,705 vs. 53,581 ng/ml*hr). Vincristine concentrations in spleen and bone marrow were similar at all time-points in the obese and control groups. In summary, obesity caused alterations in vincristine pharmacokinetics, characterized by longer initial half-life but shorter terminal half-lives. Thus, it is possible that the decreased AUC_{0 → ∞} contributed to the poorer outcome in this model. Despite this, blood and tissue concentrations of vincristine were equal or higher in the obese mice during the first 24 hours after injection of the drug when it was dosed proportional to body weight. These data suggest that dosing vincristine in obese patients based on body surface may lead to lower levels in blood and tissue, particularly if the dose is capped. As the prevalence of obesity continues to increase, pharmacokinetics studies should be performed in obese and lean subjects to further optimize chemotherapy dosing regimens in obese patients.