(T1330-04-24) Preparation and Characterization of a New Sweet Salt of Berberine

Purpose: Berberine, the active ingredient extracted from Rhizoma Coptidis, is a non-antibiotic anti-diarrheal drug. It has also been used to treat non-insulin-dependent diabetes mellitus. ¹  However, its intensely bitter taste hinders oral delivery. ²  The purpose of this study is to prepare a new sweet salt of berberine to enable the development of a palatable oral tablet.

Methods: Berberine cyclamate was obtained through the anion exchange reaction between berberine chloride and a sweetener, sodium cyclamate. 4.08 g of berberine chloride was dissolved in 30 mL hot water and then 2.01 g of sodium cyclamate was added to the solution, which caused precipitation. The suspension was stirred with a magnetic stirring bar overnight. The single crystal of berberine cyclamate was prepared by vapor diffusion where ethyl ether was allowed to diffuse into a methanol solution in a sealed container (Figure 1a). Single-crystal X-ray diffraction data was collected on a Bruker Photon-II CMOS diffractometer at 150 K, with Mo radiation. The crystal structure was solved using SHELXTL. The phase purity of a bulk powder was evaluated by powder X-ray diffraction. Thermal stability was characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and hot stage microscopy (HSM). An excess amount of berberine cyclamate (100 mg) was suspended in 10 mL water at 25 ^oC and stirred for 72h. The suspension was filtered through a 0.45 μm membrane and the amount of dissolved berberine cyclamate was determined by UV-Vis spectroscopy. Mechanical properties of bulk powders were investigated using a compaction simulator, where tabletability, compressibility, compactibility, and in-die Heckel parameter were obtained.

Results: Berberine cyclamate single crystals are yellow needles (Figure 1b). The crystal structure is triclinic with a space group P (-1). Unit cell parameters are: a = 7.0187(7) Å, b = 12.0460(10) Å, c = 14.3465(14) Å, α = 81.585(3)°, β = 83.428(3)°, γ = 78.096(3)°, Z = 2, and calculated density is 1.461 g/cm³. The unit cell contains two berberine cations and two cyclamate anions (Figure 1c), confirming it is a mono salt. The bulk berberine cyclamate powder is phase pure based on its experimental PXRD pattern, which is in agreement with the calculated pattern (Figure 1d). On heating, berberine cyclamate DSC thermogram exhibits an endotherm with an onset temperature of 239 ˚C, followed immediately by an exotherm (Figure 2a). These thermal events are attributed to thermal degradation, instead of the classical scenario of melting followed by crystallization, because they are accompanied by significant weight loss (Figure 2a). This is supported by HSM data, which show the release of bubbles not melting between 230 ˚C and 400 ˚C (Figure 2b). Thus, the melting point of berberine cyclamate is higher than 230 ˚C, which is higher than that of berberine chloride (192 ˚C). ³  The higher thermal stability of berberine cyclamate is advantageous for pharmaceutical processing when drying is required. The aqueous solubility of berberine cyclamate (3.78 mg/mL) is lower than berberine chloride (4.90 mg/mL) at 25 ˚C.  The reduced solubility limits the dissolution of berberine.  This, along with the simultaneous release of a sweetener, is expected to improve the taste profile of berberine. Compared with berberine chloride, berberine cyclamate exhibits lower tabletability (Figure 3a). However, the tabletability of berberine cyclamate is still acceptable since tablets with > 2 MPa tensile strength can be prepared at a relatively low compaction pressure of 100 MPa (Figure 3a).  The lower tabletability may be a result of a smaller bonding area (BA), a lower bonding strength (BS), or both, according to the BABS interplay model. ⁴  The compressibility profile of berberine cyclamate indicates a smaller BA because it shows a higher porosity than berberine chloride under the same compaction pressure (Figure 3b).   The lower compressibility of berberine cyclamate is attributed to its lower plasticity, as indicated by its higher P_y value than berberine chloride (Figure 3c). ⁵  An analysis of the compactibility profiles reveals a lower BS of berberine cyclamate, because of the lower tensile strength extrapolated to zero porosity (Figure 3d).

Conclusion: Phase pure berberine cyclamate could be readily prepared using an efficient anion exchange reaction in water.  This salt crystallizes into needle crystals that are thermally stable up to 230 ˚C.  Its lower aqueous solubility and the presence of a sweetener suggest an improved taste profile, which is critical for developing a palatable oral tablet of berberine.  Compared to berberine chloride, berberine cyclamate exhibits a lower tabletability as a result of both lower plasticity and weaker bonding between particles.  However, its tabletability remains sufficient for developing a tablet formulation, since tablets with > 2 MPa tensile strength can be prepared at relatively low pressures.  Overall, the thermal and mechanical properties of the cyclamate salt support its potential use for developing a palatable tablet formulation. The cyclamate salt also expands the solid form landscape of berberine, which facilitates a systematic investigation of structure-property relationships of berberine crystals.

References: (1) Wang, C.; Perumalla, S. R.; Lu, R.; Fang, J.; Sun, C. C. Sweet Berberine. Cryst. Growth Des. 2016, 16 (2), 933–939.
(2) Kataoka, M.; Tokuyama, E.; Miyanaga, Y.; Uchida, T. The Taste Sensory Evaluation of Medicinal Plants and Chinese Medicines. Int. J. Pharm. 2008, 351 (1), 36–44.
(3) Wang, L.; Liu, S.; Chen, J.; Wang, Y.; Sun, C. C. Novel Salt-Cocrystals of Berberine Hydrochloride with Aliphatic Dicarboxylic Acids: Odd–Even Alternation in Physicochemical Properties. Mol. Pharm. 2021, 18 (4), 1758–1767.
(4) Sun, C. C. Materials Science Tetrahedron—A Useful Tool for Pharmaceutical Research and Development. J. Pharm. Sci. 2009, 98 (5), 1671–1687.
(5) Vreeman, G.; Sun, C. C. Mean Yield Pressure from the In-Die Heckel Analysis Is a Reliable Plasticity Parameter. Int. J. Pharm. X 2021, 3, 100094.

Acknowledgments: Figure 1a is credited to Margaret Clapham, Ph. D candidate from the University of Minnesota. We thank her for providing this figure to us.

Figure 1. a) Vapor diffusion method for single crystal growth of berberine cyclamate; b) The yellow and needlelike single crystal under an optical microscope; c) Asymmetric unit of berberine cyclamate; d) The experimental and calculated PXRD patterns of berberine cyclamate.



Figure 2. Thermal properties of berberine cyclamate a) DSC and TGA profiles; b) HSM images at different temperatures.



Figure 3. Compaction properties of berberine chloride and berberine cyclamate, a) tabletability, b) compressibility, c) in-die Heckel profiles, and d) compactibility.