Structure elucidation of the tetrahydrocannabinol complex with randomly methylated β-cyclodextrin

Abstract

The low aqueous solubility of the bioactive cannabinoid tetrahydrocannabinol (THC) is a serious obstacle for the development of more efficient administration forms. In this study the aqueous solubility of THC was tested in the presence of α-, β- and γ-CD, and randomly methylated β-CD (RAMEB). It was found that only RAMEB was able to increase the aqueous solubility of THC to a significant level. A THC concentration of about 14 mg/ml was reached by using a 24% (187 mM) RAMEB solution, which means an increase in solubility of four orders of magnitude. The resulting THC/RAMEB complex was investigated through phase-solubility analysis, complemented by ¹H NMR, NOESY- and UV-studies in order to obtain details on the stoichiometry, geometry and thermodynamics of the complexation. The binding ratio of THC to CD was found to be 2:1, with the second THC molecule bound by non-inclusion interactions. Based on the obtained results a model for the complex structure is presented. Stability of the complex under laboratory room conditions was tested up to 8 weeks. Results show that complexation with RAMEB seems to be promising for the development of water-based THC formulations.

Introduction

The Cannabis plant (Cannabis sativa L.) has a long history of medicinal use and the main constituents, the cannabinoids, are under intensive study (Grotenhermen, 2002). At present a number of medicines based on the biological activities of the cannabinoids are available, such as Marinol^® and Nabilone, and several more are expected to be introduced in the near future. Among them are rimonabant, for treatment of obesity (van Gaal et al., 2005), and the potent analgesic ajulemic acid (Burstein et al., 2004). It seems clear that the Cannabis plant still has highly relevant potential for medicine.

The main psychoactive cannabinoid Δ⁹-tetrahydrocannabinol (THC, Fig. 1a) has been shown to be clinically useful for a large diversity of indications, including nausea and weight-loss associated with chemotherapy and HIV/AIDS, spasms in multiple sclerosis, chronic neuropathic pain and glaucoma (Grotenhermen, 2002). However, the reduced bioavailability of orally administered THC, due to low absorption and high first-pass metabolism (Brenneisen et al., 1996), prompts the development of more reliable administration forms, such as aqueous THC solutions for inhalation, sublingual or injection purposes. However, the solubility of THC was reported to be only 1–2 μg/ml in a 0.9% NaCl solution (Garrett and Hunt, 1974). Recently a water-based preparation of cannabis-extract has been developed for sublingual use (Sativex^®). However, it contains ethanol and propyleneglycol as solubilizing agents, resulting in frequent irritation of the administration site (Sativex product monograph, Bayer Healthcare, Canada). Clearly there is still a need for the development of a more optimal preparation of aqueous THC.

Cyclodextrins (CDs) are natural cyclic oligosaccharides constituted by six (α-CD), seven (β-CD) or eight (γ-CD) d-glucose units (Fig. 1b). The three-dimensional structure of the CD-ring is a truncated cone, with each of the α-, β-, and γ-CDs having a different cavity volume. They can form inclusion complexes with lipophilic guest molecules, thereby improving their aqueous solubility, increasing stability and bioavailability, and reducing side effects (Martin Del Valle, 2004). Various modifications of the natural CDs have been developed, such as the randomly methylated β-CD (RAMEB) and hydroxypropyl (HP)-β-CD.

The use of cyclodextrins for the development of aqueous THC preparations seems to be promising. In a study by Jarho et al. (1998), THC could be solubilized up to about 1 mg/ml, using a 40% HP-β-CD solution with addition of the polymer hydroxypropylmethylcellulose. However, no further details were reported on the chemical structure, stability or kinetics of the complex. In another study complexation with β-CD has been shown to improve the chemical stability of THC (Shoyama et al., 1983). Recently, Mannila et al. (2005) demonstrated that complexation with RAMEB increases both the aqueous solubility and dissolution rate of THC as well as the related compound cannabidiol (CBD). These results also showed that the sublingual administration of a THC/RAMEB complex substantially increases the bioavailability of THC in rabbits. Based on phase-solubility data a binding ratio of 1:2 (guest:CD) was suggested for the complex, but no further elucidation of the structure was performed.

However, there is growing evidence that the stoichiometry of drug/cyclodextrin complexes cannot be derived exclusively from simple phase-solubility studies, as it becomes increasingly clear that they are highly oversimplified descriptions, and ignore important aspects of the formation of cyclodextrin complexes. Cyclodextrins are able to form both inclusion and non-inclusion complexes. Self-association of surface-active drugs, lipophilic drug molecules, and drug/cyclodextrin complexes, as well as drug solubilization through non-inclusion interactions with drug/cyclodextrin complex, will influence both the shapes and mathematical interpretation of phase-solubility diagrams (Loftsson et al., 2002, Loftsson et al., 2004). In several cases a different stoichiometry was obtained when using the phase-solubility studies compared to the more reliable construction of a continuous variation (Job's) plot using techniques such as NMR, UV or potentiometry (reviewed by Loftsson et al., 2004). Therefore, other techniques such as construction of Job's plot, preferably in combination with theoretical computer-simulated modelling are important complementary data for determination of stoichiometry.

In this study the aqueous solubility of THC was tested following binding to α-, β- and γ-CD, and RAMEB and the most efficient CD-type was selected for further study. The resulting complex of THC with RAMEB was investigated through phase-solubility analysis, complemented by ¹H NMR, NOESY and UV studies in order to obtain details on the stoichiometry, geometry and thermodynamics of the complexation. Based on the obtained results a model for the complex structure is presented. Stability of the complex under laboratory room conditions was tested up to 8 weeks.