Host-guest inclusion systems of podophyllotoxin with β-cyclodextrin derivatives for low cytotoxicity

Abstract

The targeted or responsive systems are appealing therapeutic platforms for the development of next-generation precision medications. In this article the first experimental and theoretical investigation of the complexation between podophyllotoxin (POD) and three β-cyclodextrin (β-CD) derivatives (NH₂-β-CD, SA-β-CD and Bridged β-CD) are presented. The inclusion complexes are characterized by ¹H and 2D nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), and X-ray powder diffraction (XRD). Cytotoxicity tests are furnished by MTT assay. The results show that POD is encapsulated in the cavity of β-CD derivatives to form inclusion complexes with different stability constants (Ks). The cytotoxicity of these inclusion complexes against normal human kidney cell line 293 T is significantly reduced, while the inclusion complexes of SA-β-CD/POD and Bridged-β-CD/POD exhibit the same cytotoxicity against cancerous cell lines HCT116, HepG2 and SY5Y in comparison with native POD. Moreover, the stability constants of the inclusion complexes can affect their cytotoxicity, which provides a strategy for low cytotoxicity systems of POD for clinical application.

Introduction

The solubility and bioactivity of many natural drug candidates are so poor that they cannot be formulated on their own. New methods for increasing their solubility and bioactivity are highly prized. Cyclodextrins (CDs) are cyclic oligosaccharide compounds which consist of glucose units linked by α-1,4 glycoside bonds [1]. The CD molecule has a three-dimensional structure of a slightly conical hollow cylinder [2]. In addition, CDs possess native hydrophobic inner cavity and outer hydrophilic shells, which enable them to be ideal host molecules for suitable objects, such as organic molecules, inorganic ions and gas molecules via van der Waals forces and hydrophobic interaction [3,4]. Therefore, design and development of new drug delivery systems based on CDs has been relentless in recent years. Because of its good inclusion properties [5], it is widely used in nanotechnology [[6], [7], [8]], pharmacy [3,9], medicine [10], and environmental science [11] especially for β-CD among the CD family. β-CD is less soluble in water than other CDs [12], however, derivatization can effectively improve its water solubility. Currently, the most common β-CD derivatives are hydroxypropyl-β-CD and sulfobutylether-β-CD. Despite these obvious advantages, these β-CD derivatives are mixtures of various degrees of substitution and substitution patterns. These uncertainties impede batch-to-batch reproducibility and quality control. The binding ability of natural CD with guest molecules is also very low, which can be improved through chemical modifications. As reported that CD derivatives can improve the binding ability of CD with guest molecules [13]. This has drawn much attention towards the design and synthesis of various new CD derivatives.

Podophyllotoxin (POD, Fig. 1) is an anti-tumor lignan extracted from the roots and stems of genus Podophyllum [14]. It is widely used in treating sexually transmitted diseases as it can effectively inhibit herpes virus and mid-term mitosis [15]. POD has excellent anti-tumor activity due to its ability to inhibit microtubule assembly, such as leukemia [16], non-small cell lung cancer [17,18], non-Hodgkin's malignant lymphoma [19,20], testicular cancer [21,22], etc. It has good biological activity on tumor cells but a strong killing effect on normal cells [23]. The poor bioavailability and low water solubility of POD present significant obstacles for its worldwide acceptance as an effective anti-tumor drug. Inclusion complexes of CDs with POD by host-guest chemistry, can significantly improve the water solubility of POD [[24], [25], [26]]. Recently, we developed acid-sensitive inclusion complexes between acyclic cucurbituril and POD in which drug release is specifically triggered by the acidic tumor environment [27]. In the field of pharmaceuticals inclusion complexation between CDs and drugs is mainly used for solubilization of poorly soluble drugs, reducing their toxicity and increasing bioavailability [[28], [29], [30]]. In order to expand the application range of POD, we investigated the interaction of POD with β-CD derivatives. In this work, we synthesized a series of β-CD derivatives, namely, NH₂-β-CD, SA-β-CD and Bridged β-CD (Fig. 1). Our purpose is to increase the binding ability between POD and CDs. Inclusion complexes of POD with these β-CD derivatives were prepared and characterized，and cytotoxicity tests were conducted by MTT assay. It could provide a strategy for the development of low cytotoxicity systems of POD for clinical applications.