Pharmaceutical nanotechnology
Paclitaxel isomerisation in polymeric micelles based on hydrophobized hyaluronic acid

https://doi.org/10.1016/j.ijpharm.2014.03.024Get rights and content

Abstract

Physical and chemical structure of paclitaxel (PTX) was studied after its incorporation into polymeric micelles made of hyaluronic acid (HA) (Mw = 15 kDa) grafted with C6 or C18:1 acyl chains. PTX was physically incorporated into the micellar core by solvent evaporation technique. Maximum loading capacity for HAC6 and HAC18:1 was determined to be 2 and 14 wt.%, respectively. The loading efficiency was higher for HAC18:1 and reached 70%. Independently of the derivative, loaded HA micelles had spherical size of approximately 60–80 nm and demonstrated slow and sustained release of PTX in vitro. PTX largely changed its form from crystalline to amorphous after its incorporation into the micelle’s interior. This transformation increased PTX sensitivity towards stressing conditions, mainly to UV light exposure, during which the structure of amorphous PTX isomerized and formed C3single bondC11 bond within its structure. In vitro cytotoxicity assay revealed that polymeric micelles loaded with PTX isomer had higher cytotoxic effect to normal human dermal fibroblasts (NHDF) and human colon carcinoma cells (HCT-116) than the same micelles loaded with non-isomerized PTX. Further observation indicated that PTX isomer influenced in different ways cell morphology and markers of cell cycle. Taken together, PTX isomer loaded in nanocarrier systems may have improved anticancer activity in vivo than pure PTX.

Introduction

Paclitaxel (PTX) is one of the most successful cancer therapeutic agent, which is currently used for chemotherapy of patients with lung, ovarian, breast, head and neck cancer and advanced forms of Kaposi’s sarcoma (Lee et al., 2012). The major disadvantage, limiting PTX application in cancer chemotherapy, is its poor aqueous solubility (approximately 1 μg/mL) and low therapeutic index (Kim et al., 2006). Currently, in common commercial composition, PTX solubility is enhanced by using a Cremophor EL:ethanol mixture (50:50 v/v), which is diluted with saline or dextrose solution prior administration. However, there are some problems employing this formulation. The major problem is associated with the presence of Cremophor EL, which serves as surfactant and stabilizer, but at the same time causes a number of undesirable side effects including hypersensitivity reactions, nephrotoxicity and neurotoxicity (Shuai et al., 2004). Cremophor EL was also noted to influence endothelial and vesicular muscle function and cause vasodilation, labored breathing, lethargy and hypotension (Singla et al., 2002). In addition, Cremophor formulation was reported to be incompatible with components of infusion sets. Both ethanol and Cremophor were found to leach diethylhexylphtalate (DHEP) from polyvinyl chloride infusion bags and administration sets. The amount of DHEP leakage depends on the concentration of PTX vehicle, length of contact time with container and the type of administration set (Singla et al., 2002).

For the above-mentioned reasons, innovative nanocarrier systems have been often investigated as alternative vehicles of chemotherapeutics. These nanocarrier systems are water soluble and they are able to dissolve hydrophobic drugs including chemotherapeutics within their hydrophobic domains. Taking into account only vehicles with non-covalent PTX binding, PTX has been so far incorporated in polymeric micelles, liposomes, microspheres and nanoparticles (Wei et al., 2009, Zhou et al., 2013). Due to the fact that most of nanocarrier systems are reported to increase PTX aqueous solubility, prolong blood circulation time of drug and reduce nonspecific uptake by reticuloendothelial system (Vlerken et al., 2007), they are very promising candidates for drug delivery systems. However, what is not usually concerned is the stability of PTX after incorporation into the carries.

PTX is known to be relatively unstable molecule in solution (Chen et al., 1994, MacEachern-Keith et al., 1997). In cell culture media, PTX was found to be susceptible to hydrolysis and epimerization. The concentration of original PTX in these media was decreasing with time, and PTX was primarily converted to 7-epipaclitaxel (Ringel and Horwitz, 1987), a thermodynamically more stable isomer. Similar isomerization was observed upon PTX heating in the dry state and in organic solvents (MacEachern-Keith et al., 1997). In another study (Volk et al., 1997) focused on profiling PTX degradants, 7-epipaclitaxel together with baccatin III, PTX side chain methyl ester and 10-deacetylpaclitaxel were formed as degradation products when PTX was exposed to basic conditions. Stressing PTX with acid conditions resulted in the formation of 10-deacetylpaclitaxel and oxene ring opened product. Exposure to high intensity light produced a number of degradation products, mainly pentacyclic PTX isomer with a bond between C3 and C11 (Chen et al., 1994, Volk et al., 1997).

Despite the reported instability of PTX in solutions, there is only limited information on the chemical stability of PTX in drug delivery vehicles. In general, the drug may even change its phase (from crystalline to amorphous) when it is incorporated into drug delivery vehicles (Hu et al., 2007, Nepal et al., 2010) and in this way may become more susceptible towards stressing conditions. It is the aim of this work to compare the phase and structure of PTX before and after physical incorporation in polymeric micelles based on hydrophobized (acylated) hyaluronic acid (HA) and to see whether incorporated PTX is more susceptible to common stress conditions used during polymeric micelle preparation and/or lab sterilization practices. A basic analytical characterization of PTX–HA systems, including loading capacity, morphology and drug release study will be also provided.

Section snippets

Materials

Hyaluronic acid (Mw = 15 kDa) was provided by Contipro Pharma, Dolní Dobrouč, Czech Republic. 4-dimethylaminopyridine (DMAP) was obtained from Merck. Tetrahydrofurane (THF), isopropanol (IPA), triethylamine (TEA), cis-oleic acid (OA) and sodium chloride were obtained from Lach-ner (Czech Republic). Hexanoic acid and 2,4,6-trichlorobenzoyl chloride (TCBC) are commercially available products from Sigma–Aldrich. Paclitaxel was obtained from LC Laboratories (New Boston, MA). D2O and deuterated

Polymeric micelle characterization

Two acylated HA derivatives, caproyl (HAC6) and oleyl (HAC18:1) HA derivative, varying in the length of acyl chains were used for PTX encapsulation. PTX was successfully loaded into polymeric micelles by solvent evaporation method. As shown in Fig. 1, PTX-loaded polymeric micelles exhibited homogeneous spherical shapes. The polymeric micelle core was formed by aggregated acyl chains, while the shell mainly consisted from hydrophilic functional groups of HA. The average micellar size, drug

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