University of Pisa, Department of Chemistry and Industrial Chemistry, via Risorgimento 35, 56126 Pisa, Italy.
J Pharm Biomed Anal. 2010 Aug 1;52(4):479-83. doi: 10.1016/j.jpba.2010.01.037. Epub 2010 Feb 1.
Kavalactones represent the active constituents of kava-kava (Piper methysticum G. Forster), endowed with sedative and anaesthetic properties. Kavalactones are polar constituents, but poorly soluble in water with a low bioavailability. In this study, the formation of inclusion complexes of one of the most representative kavalactone isolated from kava-kava extract, (S)-7,8-dihydrokavain (DHK), with beta-cyclodextrin (beta-CyD) was investigated mainly by spectroscopic methods. NMR experiments were extensively used for the complete characterization of the complex and included (1)H NMR complexation shifts analysis, (1)H NMR diffusion measurements (DOSY), and ROESY experiments. In particular DOSY experiments demonstrated that in the presence of beta-CyD the translational diffusion of kavalactone is sizably slowed down (2.5x10(-10)m(2)/s) with respect to the free drug (4.4x10(-10)m(2)/s) according to the inclusion of DHK in the cavity of (beta-CyD). ROESY experiments confirmed the inclusion of DHK in the hydrophobic pocket of beta-CyD through the primary hydroxyl rim, being the most relevant interactions between the H3' of beta-CyD and the ortho protons on the phenyl ring of the DHK, and between H5' of beta-CyD and the meta/para protons of DHK phenyl ring. The inclusion of the phenyl ring of DHK, leaving the lactone moiety outside of CyD was also confirmed by the induced CD effects. The binary solution DHK/beta-CyD shows a 50% intensity increase of the negative band of the pi-pi* transitions of the phenyl ring with respect to the absorption observed with DHK alone. Molecular dynamics simulations results corroborated and further clarify observed spectroscopic data. It was found that the phenylethyl substituent at C6 has a preferential equatorial position in the free state, and an axial one in the complex, justifying the large downfield shift experienced by H6 of DHK upon binding. Finally the influence of beta-CyD on water solubility of DHK was investigated by phase-solubility studies. In the range 2-4mM of host, solubility of DHK was increased only two-fold, but being beta-CyD also a penetration enhancer, in vivo studies will be performed to clarify a possible role of the complex on the bioavailability of DHK.
卡瓦内酯是卡瓦胡椒（Piper methysticum G. Forster）中的活性成分，具有镇静和麻醉作用。卡瓦内酯是极性成分，但在水中溶解度低，生物利用度低。在这项研究中，主要通过光谱方法研究了从卡瓦胡椒提取物中分离出的一种最具代表性的卡瓦内酯（S）-7,8-二氢卡瓦酮（DHK）与β-环糊精（β-CyD）形成包合物的情况。NMR 实验广泛用于复合物的完全表征，包括（1）H NMR 络合位移分析、（1）H NMR 扩散测量（DOSY）和 ROESY 实验。特别是 DOSY 实验表明，在β-CyD 的存在下，卡瓦内酯的平动扩散明显减慢（相对于游离药物 4.4x10(-10)m(2)/s，为 2.5x10(-10)m(2)/s），这表明 DHK 被包合在β-CyD 的腔中。ROESY 实验证实了 DHK 通过初级羟基边缘被包合在β-CyD 的疏水性口袋中，β-CyD 的 H3'与 DHK 苯环上的邻位质子以及β-CyD 的 H5'与 DHK 苯环的间/对位质子之间存在最相关的相互作用。DHK 苯环的包合，使内酯部分留在 CyD 之外，也通过诱导 CD 效应得到证实。DHK/β-CyD 二元溶液与单独使用 DHK 相比，DHK 苯环的 pi-pi*跃迁的负带强度增加了 50%。分子动力学模拟结果证实并进一步澄清了观察到的光谱数据。结果发现，C6 上的苯乙基取代基在游离状态下具有优先的赤道位置，在复合物中具有轴向位置，这解释了 DHK 的 H6 在结合时经历的大的场位移。最后，通过相溶解度研究考察了β-CyD 对 DHK 水溶性的影响。在 2-4mM 宿主范围内，DHK 的溶解度仅增加了两倍，但由于β-CyD 也是一种渗透增强剂，因此将进行体内研究以阐明复合物对 DHK 生物利用度的可能作用。
