Research papers
Novel polymeric microspheres containing norcantharidin for chemoembolization

https://doi.org/10.1016/j.jconrel.2006.08.022Get rights and content

Abstract

Chemoembolization has been found to be a potentially effective method of treating certain types of cancer. It involves arterial embolization of a tumor, in combination with simultaneous or subsequent local delivery of chemotherapeutic agents. In this study, PLGA-alginate microspheres were evaluated for their potential application in chemoembolization. Norcantharidin, which possesses anti-tumor properties, was used to investigate the application of drug-containing microspheres for chemoembolization. The release profiles of alginate, PLGA and PLGA-alginate microspheres were markedly different in phosphate buffered saline, with the composite microspheres showing the most appropriate release rate for chemoembolization. Burst effect decreased while particle size increased with increasing proportion of alginate in the PLGA-alginate microspheres. PLGA-alginate microspheres containing norcantharidin were effective in destroying the cancer cells used in this study. The growth inhibitory effect was concentration and time dependent. These microspheres also exhibited excellent embolization and therapeutic effects on rats with transplanted tumors.

Introduction

Targeted delivery of an anti-cancer drug to a tumor via arterial chemoembolization is attracting increasing attention. It has been found to be more effective than chemotherapy alone in the treatment of cancer. Besides enabling a greater local concentration of drug in the target tissue, it causes hypoxia of the tumor by blocking its blood supply [1], [2]. Systemic side-effects are also reduced due to restricted blood circulation and localized deposition of the drug. Chemoembolization is currently employed for treating malignant hepatic tumors.

The chemoembolization procedure typically involves sequential or concurrent injection of drug and embolization agent [3]. Several materials, both natural and synthetic in origin, had been used for embolization. Examples used included absorbable gelatin powder (Gelform®) and gelatin sponge particles suspended in a liquid medium that might or might not contain the drug [4], [5]. Chemoembolization had also been attempted using microcapsules or microspheres. These were preferred as spherical particles could be transported more distally than particles that were irregular. More importantly, microcapsules or microspheres produced more homogeneous and complete blood vessel occlusion [6]. Degradable starch microspheres (DSM, Spherex®) had been used as a chemoembolization agent. These are spherical particles of about 45 μm in diameter, prepared by cross-linking partly hydrolyzed potato starch with epichlorohydrin. DSM was reported to cause temporary embolism after injection into a vessel, before degrading within 40 min [7]. A slight spread of particle sizes of the embolization agent is desirable for a more extensive embolization. As commercially available DSM has a rather narrow size distribution, its embolization effects are restricted [8]. Hence, it would be ideal if a mixture of drug-loaded particles of greater varying sizes could be formulated to provide more effective embolization, as well as localized drug delivery.

Alginate is a natural polysaccharide and has attracted much attention as a matrix for drugs and immobilized cells since it is non-toxic, biodegradable and can be cross-linked with polyvalent ions [9]. There had been few investigations on the use of alginates for chemoembolization. This is perhaps due to the rapid drug release rates of alginate microspheres [10]. On the other hand, poly (lactic-co-glycolic) acid (PLGA) particles released drug very slowly [11]. They also take a long time to degrade in the body, which may induce the formation of lateral blood vessels to supply the surrounding tissue and tumor [12], [13]. Hence, in this study, PLGA was combined with alginate to form microspheres for use in chemoembolization.

Norcantharidin is synthesized from cantharidin, which is an active constituent obtained from the dried body of the Chinese blister beetle (mylabris) [14]. The use of mylabris as a traditional medicine in China can be traced back to over 2000 years. It was found that norcantharidin possesses anti-tumor properties and has the advantage of inducing the production of leucocytes [15]. Clinical studies showed that norcantharidin was effective against primary liver cancer. However, the significant side-effect of norcantharidin is irritation to the urinary organs, thereby limiting its use [16]. Side-effects may be reduced by decreasing the dose. Hence, the aforementioned limitation of norcantharidin may be overcome by encapsulating the drug in PLGA-alginate microspheres for chemoembolization at the target site as this mode of treatment enables a smaller dose for therapeutic efficacy.

Section snippets

Materials and animals

Sodium alginate (low viscosity, ISP, USA) and PLGA (Resomer RG 502H, 50:50, Mw 10,000, Boehringer Ingelheim, Germany) were used as the matrix polymer and norcantharidin (Pingyuan Pharmaceutical, China) as the drug. Calcium chloride dihydrate (Merck, Germany) was used as the cross-linking agent. Tween 85 (Merck, Germany), Span 85 (Sigma-Aldrich, USA) and polyvinyl alcohol (PVA, 15,000, Fluka Chemie, Switzerland) were employed as emulsifiers. Ethyl acetate (Merck, Germany) was used to dissolve

Morphology and drug content of microspheres

The three types of microspheres prepared were compared. The alginate and PLGA microspheres were prepared by the formation of primary emulsions while PLGA-alginate microspheres involved multiple emulsions. The latter enabled the formation of composite microspheres which were specifically designed to act more effectively as a chemoembolization agent. Using the emulsification method, all the formulations produced free-flowing powders. Upon dispersing the powder in water, relatively discrete and

Conclusion

Composite PLGA-alginate microspheres in the desired size range were successfully prepared by a double emulsification method. The size of the microspheres increased with the concentration of alginate employed. The composite microspheres showed drug release rate that was intermediate between alginate microspheres and PLGA microspheres. The release profile was characterized by initial burst effect, followed by zero order release. The release rate was governed mainly by the entrapped PLGA

Acknowledgement

The authors would like to thank International Specialty Products, especially Dr Yolande Anthony, for the financial support to carry out this study.

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