Poly(ethylene oxide)-modified poly(β-amino ester) nanoparticles as a pH-sensitive biodegradable system for paclitaxel delivery

Abstract

The main objective of this study was to develop and characterize a pH-sensitive biodegradable polymeric nanoparticulate system for tumor-selective paclitaxel delivery. A representative hydrophobic poly(β-amino ester) (poly-1) was synthesized by conjugate addition of 4,4′-trimethyldipiperidine with 1,4-butanediol diacrylate. Poly-1 (M_n 10,000 daltons) nanoparticles were prepared by the controlled solvent displacement method in an ethanol–water system in the presence of Pluronic^® F-108, a poly(ethylene oxide) (PEO)-containing non-ionic surfactant. Control and PEO-modified nanoparticles were characterized by Coulter counter, scanning electron microscopy (SEM), zeta potential measurements, and electron spectroscopy for chemical analysis (ESCA). Polymer degradation studies were performed in phosphate-buffered saline (PBS, pH 7.4) at 37 °C. Paclitaxel loading capacities and efficiencies were determined and release studies were performed in Tween^®-80 (0.1%, w/v)-containing PBS at 37 °C. Control and PEO-modified nanoparticles, labeled with rhodamine-123, were incubated with BT-20 cells to examine the uptake and cellular distribution as a function of time. PEO-modified nanoparticles with an average size of 100–150 nm and a positive surface charge of 37.0 mV were prepared. SEM analysis showed distinct smooth, spherical particles. The ether (–C–O–) peak of the C_1s envelope in ESCA confirmed the surface presence of PEO chains. Polymer biodegradation studies showed that almost 85% of the starting material degraded after 6 days. The maximum paclitaxel loading efficiency attained was 97% at 1.0% (w/w) of the drug. Paclitaxel release studies showed that approximately 10% was released in the first 24 h, 80% after 3 days, and the entire content was released in approximately 5 days. After 1 h of incubation, a large fraction of the administered control and PEO-modified poly-1 nanoparticles was internalized in BT-20 cells. Results of this study demonstrate that PEO-modified poly-1 nanoparticles could provide increased therapeutic benefit by delivering the encapsulated drug to solid tumors.

Introduction

Although significant advances have been made in our understanding of tumor origin, growth, and metastasis and many different types of pharmacological agents have been developed over the years to treat tumors, the problem of optimum delivery remains a formidable challenge [1], [2], [3], [4], [5]. For anti-tumor drug therapy to be effective, the agent must be able to reach the tumor mass in sufficient concentration, traverse through the tumor microcirculation, diffuse into the interstitium, and remain at the site for the duration to induce a tumoricidal effect. The tumor vasculature consists of blood vessels formed by secreted pro-angiogenic factors, such as vascular endothelial growth factor, from the tumor cells and those recruited from the pre-existing network of the host vasculature [6]. The resulting blood capillaries around the tumor become dense, highly disorganized, tortuous, unpredictable in both structure and function, and leaky [7]. Since the hydrostatic pressure inside the tumor mass is significantly higher than in the vasculature, many drugs cannot diffuse evenly within the interstitial matrix. In addition, the tumor metabolic profile is different due to poor oxygen perfusion, resulting in elevated levels of lactic acid and a reduction in pH from 7.4 to about 6.0 [8]. Finally, in most normal tissues, the extravasated drug is taken up by the lymphatic system and returned to the circulation. Due to the lack of a functional lymphatic system in tumors, the drug oozing out of the tumor mass will be diluted in the tissue space that surrounds the tumor, thereby reducing the efficacy of the agent against the tumor [9].

Due to the porosity of the tumor vasculature and the lack of lymphatic drainage, blood-borne macromolecules and colloidal particles are preferentially distributed in the tumor due to the enhanced permeability and retention (EPR) effect. Concentrations of polymer–drug conjugates in tumor tissues can reach levels 10 to 100 times higher than would be seen after administration of the free drug [10], [11]. Using long-circulating liposomes, it has been found that the effective pore size of most peripheral human tumors range from 200 to 600 nm in diameter, with a mean of about 400 nm [12]. Additionally, colloidal particles with a positive surface charge are preferentially taken up by the tumor and retained for a longer duration than negatively charged or neutral particles [13].

Polymeric nanoparticles offer a number of advantages for drug delivery to tumors [14]. First, by selecting the type of polymer, one could encapsulate drugs with different physical-chemical properties. Second, the loading efficiency of drugs in nanoparticles tends to be significantly higher than in liposomes or micelles. Third, the drug is dispersed throughout the matrix, which will prevent “burst release” in the plasma. Fourth, based on the choice of polymer type, such as one with pH-responsive solubility, the encapsulated drug can be released at the desired site (solid tumor) for greater efficacy. Lastly, nanoparticle surfaces can be easily modified with targeting ligands for site specificity.

In the present study, we examined the encapsulation of paclitaxel in a representative hydrophobic PEO-modified poly(β-amino ester) (poly-1) nanoparticles. Previous studies have shown that poly-1 is a biodegradable poly(β-amino ester) with a pH-sensitive aqueous solubility profile. The polymer becomes rapidly soluble at a pH below 6.5 [15], [16]. Paclitaxel (Taxol^®, Bristol-Meyers Squibb) is a complex diterpenoid natural product with anti-tumor activity [17]. Paclitaxel is a unique antimicrotubule agent that promotes the assembly of microtubules from tubulin dimers and stabilizes microtubules by preventing depolymerization during the late G2 mitotic phase of the cell cycle. In addition to its cytotoxic effects, paclitaxel can also sensitize tumor cells to ionizing radiation and is found to inhibit angiogenesis, probably by blocking endothelial cell division [18], [19]. Paclitaxel is indicated as a first-line and subsequent therapy for the treatment of advanced carcinoma of the breast and ovaries. It is also indicated for the second-line treatment of AIDS-related Kaposi’s sarcoma. In a recent review article, Dhanikula and Panchagnula [20] suggested that paclitaxel would be an excellent candidate for tumor selective delivery based on its physicochemical and pharmacokinetic properties.