Effect of Basic Amino Acids on Folic Acid Solubility

Abstract

To prevent neural tube defects and other cardiovascular diseases in newborns, folic acid (FA) is recommended in pregnant women. A daily dose of 600 µg FA consumption is widely prescribed for women during pregnancy and 400 µg for women with childbearing potential. FA is a class IV compound according to the Biopharmaceutics Classification System (BCS) due to its low permeability (1.7 × 10^-6 cm/s) and low solubility (1.6 mg/L); therefore, it must be administered via a formulation that enhances its solubility. Studies reported in the literature have proved that co-amorphization and salt formation of a poorly soluble drug with amino acids (AA) can significantly increase its solubility. Although arginine has been used with FA as a supplement, there is no information on the effect of basic AA (arginine and lysine) on the physical and chemical properties of FA-AA binary formulations. The present study implemented a conductimetric titration methodology to find the effective molar ratio to maximize FA solubility. The results showed that a 1:2.5 FA:AA molar ratio maximized solubility for arginine and lysine. Binary formulations were prepared using different methods, which led to an amorphous system confirmed by the presence of a glass transition, broad FTIR bands, and the absence of an X-ray diffraction pattern. Results of FA:AA (1:2.5) solubility increased in the range of 5500-6000 times compared with pure FA. In addition to solubility enhancement, the binary systems presented morphological properties that depend on the preparation method and whose consideration could be strategic for scaling purposes.

Keywords: amino acids; amorphous drug; arginine; ball milling; conductimetry; folic acid; lysine; solubility.

Figure 1

Folic acid structure.

Figure 2

Arginine and lysine structures.

Figure 3

(a,b) potentiometric titration of FA with lysine (Lys) and arginine (Arg) and numerical second derivative of pH (red dashed line), respectively (c,d) conductimetric titration of FA with a concentrated solution of ARG and LYS, respectively; the extrapolation dashed lines cross at the molar ratio of maximum conductivity.

Figure 4

X-ray diffraction of pure components and formulations of (a) arginine and (b) lysine pure components along with the physical mixtures (CPM), amorphous by solvent evaporation (ASE), and the ball milled (BM). For comparative purposes, each diffraction pattern is normalized with respect to its maximum intensity.

Figure 5

Infrared spectra of different FA formulations and pure components, (a) the FA:LYS systems and (b) the FA: ARG systems, are shown and obtained with Perkin Elmer spectrometer model Spectrum 100.

Figure 6

Comparison of solubilities in water of different formulations, with error bars (n = 3), the increment of solubility in comparison with FA alone is above bars. A one-way ANOVA analysis demonstrated that the difference between all media of all FA formulations was not significant with a p-value of 0.014; no matter the different changes in their crystal structure, the solubility of FA has proven not to change significantly. This could be attributed to other unseen root causes.

Figure 7

Thermograms for the FA formulations. The solvent evaporation formulations produce amorphous materials with defined onset glass transitions: 51 °C for FA-LYS ASE and 58 °C for the FA-ARG ASE. In the case of the ball-milled formulations, FA-LYS BM and FA-ARG BM, no clear onset can be determined.

Figure 8

SEM images of the crystalline physical mixture (CPM) of (a) FA-LYS and (b) FA-ARG, amorphous salt by solvent evaporation (AES), (c) FA-LYS and (d) FA-ARG, and amorphous by ball milling (BM) (e) FA-LYS and (f) FA-ARG.