Differential pharmacodynamic effects of paclitaxel formulations in an intracranial rat brain tumor model

Abstract

Nano- and microparticulate carriers can exert a beneficial impact on the pharmacodynamics of anticancer agents. To investigate the relationships between carrier and antitumor pharmacodynamics, paclitaxel incorporated in liposomes (L-pac) was compared with the clinical standard formulated in Cremophor-EL/ethanol (Cre-pac) in a rat model of advanced primary brain cancer. Three maximum-tolerated-dose regimens given by intravenous administration were investigated: 50 mg/kg on day 8 (d8) after implantation of 9L gliosarcoma tumors; 40 mg/kg on d8 and d15; 20 mg/kg on d8, d11, and d15. Body weight change and neutropenia were assessed as pharmacodynamic markers of toxicity. The pharmacodynamic markers of antitumor efficacy were increase in lifespan (ILS) and tumor volume progression, measured noninvasively by magnetic resonance imaging. At equivalent doses, neutropenia was similar for both formulations, but weight loss was more severe for Cre-pac. No regimen of Cre-pac extended survival, whereas L-pac at 40 mg/kg x2 doses was well tolerated and mediated 26% ILS (p < 0.0002) compared with controls. L-pac at a lower cumulative dose (20 mg/kg x3) was even more effective (40% ILS; p < 0.0001). In striking contrast, the identical regimen of Cre-pac was lethal. Development of a novel semimechanistic pharmacodynamic model permitted quantitative hypothesis testing with the tumor volume progression data, and suggested the existence of a transient treatment effect that was consistent with sensitization or "priming" of tumors by more frequent L-pac dosing schedules. Therefore, improved antitumor responses of carrier-based paclitaxel formulations can arise both from dose escalation, because of reduced toxicity, and from novel carrier-mediated alterations of antitumor pharmacodynamic effects.

Fig. 1.

System pharmacological models of tumor volume progression. Schematic illustrations of pharmacodynamic models that were applied or developed to analyze formulation- and regimen-dependent differences in the therapeutic effects of paclitaxel on intracranial 9L brain tumors. All are based on the concept of signal or cell transduction initiated by exposure to drug. Detailed equations for the final model (D) are provided in Materials and Methods. Model A (Lobo and Balthasar, 2002): drug activates a signal transduction cascade that ultimately exerts lethality in the tumor compartment. The magnitude of drug effect is defined by a Hill-type relationship; E_max is the maximal drug effect, EC₅₀ is the concentration inducing half-maximal response, and c(t) is the plasma concentration of drug at time t. The drug signal propagates through a series of compartments (x₁–x₄) with a mean time of signal propagation (τ). Tumor volume is w(t); the curved arrow represents a tumor growth function, and x₄ denotes that the elimination rate of cells from the tumor is determined by the drug signal from compartment x₄. Model B (Simeoni et al., 2004): drug induces a dose-proportional fraction of cells to progress through stages of commitment to cell death. Compartment x₁ represents the tumor mass, which increases according to a growth function. The concentration-time profile of drug in plasma c(t) induces a fraction of tumor cells to commit to cell death according to second-order killing constant k₂, which was defined in the original publication. Terminally committed cells progress through compartments x₂ to x₄ with a rate constant of k₁. The observed tumor volume is the sum of cells in compartments x₁ to x₄. Model C is identical to model B, with the exception that the tumor growth function is modified to assume that drug induced a fraction of tumor [k_ng(t)] that is transiently nondividing. Model D: hybrid based on models A and B. This model is described in detail, with equations, in Materials and Methods. As in model A, the drug initiates a signal [E′(t)] according to a Hill-type relationship. Drug signal s governs the rate of elimination of cells from the tumor mass x₁. The first-order rate constant (k₁) defines the rate of signal turnover. The model also incorporates the concept that drug treatment causes a transient period of heightened sensitivity to subsequent treatments; f(s) represents the effect of the drug signal on the elimination rate of tumor cells from the tumor mass. It includes the second-order kill constant k₂ of model B, but is modulated by drug signal s (see Materials and Methods). The transient sensitization signal is modeled as an increase in the quantity of tumor-associated drug, but could also represent treatment-induced changes in tumor interstitial density or vascular permeability (Jang et al., 2001; Lu et al., 2007).

Fig. 2.

Formulation-dependent toxicocodynamic effects of paclitaxel. Tumor-bearing rats were treated with Cre-pac or L-pac beginning on day 8 after implantation. Points represent the mean ± S.D. of each group; n indicates the number of animals per group. Symbols on abscissa indicate times of treatment for each group. A, C, change in body weight for animals treated with Cre-pac (A) and L-pac (C). B, D, change in neutrophil counts for animals treated with Cre-pac (B) and L-pac (D).

Fig. 3.

Effects of formulation and treatment regimen on lifespan. Survival of animals bearing intracranial 9L tumors treated with Cre-pac (A) or L-pac (B). Table 2 shows the group sizes and statistical comparisons.

Fig. 4.

Representative serial MR images for treatment groups. Rows shows consecutive 1-mm-thick (T₂)-weighted MR image slices from individual rats treated with saline (top row), or 20 mg/kg ×3 doses of Cre-pac (middle row) or L-pac (bottom row). Tumor appears as a hyperintense mass in the left frontal quadrant.

Fig. 5.

Tumor volume progression in treatment groups by repetitive MR images. Tumor volumes were measured during treatment of animals with Cre-pac (A) and L-pac (B). Symbols represent the measured tumor volume for each individual animal (n = 3/group), and lines represent the best fit of pharmacodynamic model D (Fig. 1D) to the data for each group. ○, dotted line, control group; ♦, solid line, 50 mg/kg ×1 dose; ●, dash-dot line, 40 mg/kg ×2 dose; ◊, dashed line, 20 mg/kg ×3 dose. Asterisks indicate time points at which treated versus control differ significantly at p < 0.002 or lower. C, D, magnitude of cytotoxicity “driving function” for Cre-pac (C) and L-pac (D) treatments, based on model D. Symbols on abscissa are the same as for A and B, and show the treatment times for each group. a.u., arbitrary units.