Effect of repetitive administration of Doxorubicin-containing liposomes on plasma pharmacokinetics and drug biodistribution in a rat brain tumor model

Abstract

Purpose: The incorporation of doxorubicin in long-circulating sterically stabilized liposomes (SSL-DXR) alters the pharmacokinetics and biodistribution of doxorubicin and therefore has the potential to alter the pharmacologic properties of doxorubicin. Previously, we showed that repetitive administration of SSL-DXR alters tumor vascular permeability.

Experimental design: Here, we investigated the effect of weekly i.v. injections of SSL-DXR on plasma pharmacokinetics and drug biodistribution in the orthotopic 9L rat brain tumor model.

Results and conclusions: The pharmacokinetics of free doxorubicin (5.67 mg/kg) did not change with repeat dosing. In contrast, drug concentrations in plasma and brain tumor increased and deposition in liver and spleen decreased after administration of the second of two weekly doses of SSL-DXR. Noncompartmental analysis and descriptive pharmacokinetic models were created to test hypotheses relating to the mechanisms responsible for alterations in SSL-DXR deposition. The analysis suggested that weekly administration of SSL-DXR significantly (P < 0.05) decreased the plasma elimination rate of SSL-DXR (34%) and decreased drug deposition in liver (2-fold) and spleen (3.5-fold). The pharmacokinetic model that best captured the observed 2.5-fold increase in tumor uptake of SSL-DXR mediated by repeat dosing was one that hypothesized that the rates of drug influx/efflux into tumor were increased by the first dose of SSL-DXR. Models that accounted only for residual drug deposited in the tissue or blood by the first weekly injection provided inferior fits to the data. Thus, the effects of repetitive dosing on SSL-DXR deposition in tumor are consistent with a treatment-mediated alteration of tumor vascular permeability.

Fig. 1

Biodistribution of doxorubicin after single and repetitive treatments. A, tissue and tumor biodistribution of doxorubicin after single (day 14) and repetitive (days 7 and 14) i.v. doses of 5.67 mg/kg free doxorubicin. B, tissue and tumor biodistribution of doxorubicin after day 14 (d14) and days 7 and 14 (d7&d14) administration of 5.67 mg/kg SSL-DXR. Columns, mean (n = 4-8 animals); bars, SE. For animals treated on days 7 and 14 with SSL-DXR, a significant decrease in liver deposition was observed at 24 and 48 hours and in spleen deposition at 8, 24, and 48 hours. Naive pooled data analysis showed that repetitive administration of SSL-DXR significantly increased the cumulative brain tumor deposition compared with a single treatment. Temporal and treatment-mediated differences in drug deposition were tested in SAS using a stepwise two-way (time and treatment) ANOVA followed by a post hoc Bonferroni t test. *, P < 0.05.

Fig. 2

Observed and model-predicted plasma pharmacokinetics of doxorubicin. A, concentration-time profiles of doxorubicin in plasma after administration of 5.67 mg/kg free doxorubicin (△ and solid lines) or SSL-DXR (• and dashed lines). Filled bars, time of administration. Points, mean of three to five individual animals at each time point; bars, SD. Lines are drawn point-to-point. B, doxorubicin plasma concentrations were analyzed using pharmacokinetic models developed to fit simultaneously the data for both single and two weekly doses of SSL-DXR. •, SSL-DXR-treated animals as shown in (A). Solid line, fit of the model described by Eq. A to the data, in which the initial rate of elimination (k) is hypothesized to decrease incrementally with time (according to the variable α) as a response to the first dose. Dashed line, fit of the model under conditions of stationary kinetics (α = 0), in which the elimination rate is held constant for both doses of SSL-DXR.

Fig. 3

Observed tumor deposition of SSL-DXR and deposition predicted by a pharmacokinetic model. A, a pharmacokinetic model designed to fit simultaneously the brain tumor concentrations observed resulting from single and repetitive treatment with SSL-DXR (SSL-DXR). The concentration of drug in tumor (Ct) was described using a one-compartment model (Eq. B) in series with fixed plasma concentrations (Eq. A), where k_ct and k_tc are the first-order rate constants for uptake and elimination between the plasma and the tumor, respectively. A scaling factor that accommodates stepwise changes in variables from different cohorts permitted simultaneous fitting of all treatment groups and was used to estimate the magnitude of change in the uptake rate (X_ct) or elimination rate (X_tc) mediated by repetitive administration of SSL-DXR. B, observed and model-predicted brain tumor concentrations following single or repeat weekly administration of SSL-DXR. Open symbols, observed tumor concentrations following a single treatment given on day 7 (△) or day 14 (□); filled symbols, tumor concentrations observed following the second of two SSL-DXR administration [i.e., days 7 and 14 (▼)]. Solid line, model prediction after single (day 7) and repetitive (days 7 and 14) treatment with SSL-DXR; dashed line, model fit of data from animals that received a single day 14 treatment; dotted line, model prediction for the residual SSL-DXR in brain tumors 168 to 336 hours after the administration of SSL-DXR on day 7.