In recent years, cucurbit [7]uril (CB [7]) has attracted great attention in drug delivery. Though the effect of CB [7] in enhancing the solubility of water insoluble drugs has been validated, the underlying mechanism remains poorly understood, particularly at a molecular level. This study is designed to evaluate a CB [7]-based pharmaceutical formulation to improve solubility and bioavailability of triamterene (a mild potassium-sparing diuretic). Two polymorphs of triamterene@CB [7] were obtained, and their crystal structures were determined by single crystal X-ray diffraction. The CB [7] molecule forms a stable host−guest complex with triamterene (K a = 1.69 ± 0.34 × 10 4 M −1 ) in aqueous solution (pH = 1.0). The results of dissolution study demonstrate that the apparent solubility value of triamterene@CB [7] complex in 0.1 M HCl is 1.6 times as large as that of triamterene, while free triamterene was released from triamterene@CB [7] complex in phosphate buffer of pH 6.8. Pharmacokinetic studies in rats reveal that the AUC 0−∞ value of triamterene@CB [7] complex increases 2.8-fold compared with that of free triamterene, and t 1/2 is prolonged from 1.42 to 2.61 h (P < 0.05) after oral administration. The increased solubility and oral bioavailability are attributed to the formation of a hydrophilic capsule composed of two CB [7] molecules, in which two insoluble triamterene molecules are encapsulated. These results demonstrate that triamterene@CB [7] complex is a stable and effective pharmaceutical formulation. KEYWORDS: cucurbit [7]uril, triamterene, crystal structure, solubility, pharmacokinetics
■ INTRODUCTIONPoor aqueous solubility is a major bottleneck in drug discovery and drug development: more than 40% of drug candidates are poorly water-soluble, and poor aqueous solubility is the main factor to restrict the bioavailability of drugs.1,2 Various approaches have been investigated to improve solubility of drugs, including cocrystal, 3,4 salts, 5 prodrugs, 6 solid dispersion, 7 macrocyclic hosts (e.g., cyclodextrins, cucurbiturils), 8,9 etc. Triamterene, 2,4,7-triamino-6-phenylpteridine (Scheme 1) is a mild potassium-sparing diuretic used either alone or in combination with potassium-wasting diuretics such as hydrochlorothiazide.10,11 It displays low oral bioavailability, and there exists wide intersubject variation because of its poor aqueous solubility (45 mg/L). 12,13 Though triamterene contains three primary amine groups, various methods to obtain soluble salts failed, probably due to its weak basicity. 12 To date, only a few formulations have been proposed to improve the solubility of triamterene, including cyclodextrins 14,15 and solid dispersions. 16,17 However, parenteral administration of β-cyclodextrin results in renal toxicity, 18 and solid dispersions are often physically unstable upon storage at elevated temperature and humidity, 19 and neither of the two triamterene formulations has been applied for clinical therapy. Therefore, the development of new pharmaceutical formulation of triam...