Co-Amorphous Andrographolide–Lysine with Unexpectedly Enhanced Solubility

Andrographolide (ADG) is a typical poorly water-soluble drug, and a co-amorphous strategy was used here to improve its aqueous solubility. Co-amorphous systems of ADG and amino acids with a 1:1 molar ratio were screened via the neat ball milling method. L-lysine (Lys) and L-tryptophan (Trp) can be used as co-formers with ADG, forming a co-amorphous phase, which was confirmed by powder X-ray diffraction, IR and Raman spectroscopy. ADG-Trp showed poor solubility at 37 °C, which was close to that of raw ADG (0.08 mg·mL−1). ADG-Lys showed unexpectedly enhanced solubility, at 0.5 mg·mL−1in the media of water and PBS (pH 7.4) and 0.3 mg·mL−1in the medium of HCl buffer (pH 1.2) at 37 °C. ADG-Lys showed good storage stability for 5 months, but its thermal stability was poor and it could recrystallize at 100 °C. Compared with ADG-Trp, ADG-Lys has weaker hydrogen bonding interactions and stronger hydrophobic interactions related to ADG molecules, which might cause the unusual enhancement in solubility. To our knowledge, ADG-Lys prepared in this work shows the maximum ADG content (70 wt.%) and the highest ADG solubility among the reported ADG amorphous solid dispersions and co-amorphous systems.