Paclitaxel-loaded polymeric microparticles: quantitative relationships between in vitro drug release rate and in vivo pharmacodynamics

Abstract

Intraperitoneal therapy (IP) has demonstrated survival advantages in patients with peritoneal cancers, but has not become a widely practiced standard-of-care in part due to local toxicity and sub-optimal drug delivery. Paclitaxel-loaded, polymeric microparticles were developed to overcome these limitations. The present study evaluated the effects of microparticle properties on paclitaxel release (extent and rate) and in vivo pharmacodynamics. In vitro paclitaxel release from microparticles with varying physical characteristics (i.e., particle size, copolymer viscosity and composition) was evaluated. A method was developed to simulate the dosing rate and cumulative dose released in the peritoneal cavity based on the in vitro release data. The relationship between the simulated drug delivery and treatment outcomes of seven microparticle compositions was studied in mice bearing IP human pancreatic tumors, and compared to that of the intravenous Cremophor micellar paclitaxel solution used off-label in previous IP studies. Paclitaxel release from polymeric microparticles in vitro was multi-phasic; release was greater and more rapid from microparticles with lower polymer viscosities and smaller diameters (e.g., viscosity of 0.17 vs. 0.67 dl/g and diameter of 5-6 vs. 50-60 μm). The simulated drug release in the peritoneal cavity linearly correlated with treatment efficacy in mice (r(2)>0.8, p<0.001). The smaller microparticles, which distribute more evenly in the peritoneal cavity compared to the large microparticles, showed greater dose efficiency. For single treatment, the microparticles demonstrated up to 2-times longer survival extension and 4-times higher dose efficiency, relative to the paclitaxel/Cremophor micellar solution. Upon repeated dosing, the paclitaxel/Cremophor micellar solution showed cumulative toxicity whereas the microparticle that yielded 2-times longer survival did not display cumulative toxicity. The efficacy of IP therapy depended on both temporal and spatial factors that were determined by the characteristics of the drug delivery system. A combination of fast- and slow-releasing microparticles with 5-6 μm diameter provided favorable spatial distribution and optimal drug release for IP therapy.

Keywords: AIC; Akaike Information Criterion; Controlled release; GA; HPLC; ILS; IP; In vitro–in vivo correlation; Intraperitoneal therapy; LA; LM; MP; MST; PLGA; PLGA microparticles; Paclitaxel; SF; SS; Tg; VIS; glass transition temperature; glycolic acid; high performance liquid chromatograph; increase in life span; inherent viscosity; intraperitoneal; lactic acid; large microparticles with medium release rate; median survival time; microparticles; poly(d,l-lactide-co-glycolide) acid; small microparticles with fast release rate; small microparticles with slow release rate.

Figure 1

Scanning electron microscopy of paclitaxel-loaded PLGA microparticles.

Figure 2. Relationship between PLG microparticle properties and drug release

Bar: 1 SD (n = 3).

Figure 3. Morphological change of microparticles in the release medium

A. 50:50 PLGA (0.18 dl/g VIS); B. 75:25 PLGA (0.7 dl/g VIS).

Figure 4. Drug release-time profiles

(A) In vitro drug release from three PLGA microparticles, i.e., SF (50:50 LA:GA, 0.17 dl/g, 5–6 μm diameter, fast release), LM (50:50 LA:GA, 0.17 dl/g, about 50 μm, slow release), SS (75:25 LA:GA, 0.67 dl/g, 5–6 μm, slow release), were analyzed. Symbols, experimental data. Lines, best-fitted curves using Eq. 2. (B) Simulated amount of dose released in peritoneal cavity for the nine drug treatment groups. Pac/Crem 40 ×1 refers to a single treatment of paclitaxel/Cremophor at 40 mg/kg. Abbreviations for single type microparticles are: MP(dose)(type of microparticles). For example, MP40SF is small, fast release microparticles at 40 mg/kg dose. Abbreviations for combination microparticles are: MP(dose of first microparticles)(type of first microparticles)(dose of second microparticles)(type of second microparticles). For example, MP40SF80SS is combination of small, fast release microparticles at 40 mg/kg dose and small, slow release microparticles at 80 mg/kg. Abbreviations for repeated treatments are: Pac/Crem 40×3 is 3 weekly treatments of 40 mg/kg on day 0, 7, and 14 days post-treatment (equivalent to 10, 17 and 24 days post tumor-implantation). MP40SF80SSx2 is 2 treatments of MP40SF plus MP80SS given on day 0 and 21 days post-treatment (equivalent to 10 and 31 days post tumor-implantation).

Figure 5. In vivo biological activity

Mice were given IP injections of physiological saline, blank microparticles, or one of the nine treatments as described in Figure 4. Day 0 represents the day of treatment initiation, which corresponded to 10 days post-tumor implantation (about 40% of the MST of controls). Survival over time is shown in Kaplan Meier curves. Toxicity-related death events are indicated by the arrows.

Figure 6. Correlation of simulated drug release in peritoneal cavity and in vivo pharmacodynamics

The drug amount released in peritoneal cavity for individual treatments were obtained from Figure 4. The data points at 0 mg/kg are corresponding to the respective vehicle control for each group (saline and blank microspheres, respectively Open circle: paclitaxel/Cremophor; solid circle: microparticles. Treatment efficacy was expressed in MST. (A) MST vs. dose released in 1 day. Only data from single treatment groups were included. The best-fit linearly regressed lines were: MST=0.68*(Released dose)+19 for microparticles (r²=0.82, p=0.002), and MST=0.33*(Released dose)+14 for paclitaxel/Cremophor (r²=1). (B) MST vs. dose released at MST. All single and repeated treatment groups were included. The best-fit linearly regressed lines were MST=0.39*(Released dose)+19 for microparticles (r²=0.86, p=0.0003) and MST=0.15*(Released dose)+17 for paclitaxel/Cremophor (r²=0.85, p=0.1).