The pH of the solutions after the dissolution test was 2.93, 2.66, 3.28, and 3.00 for HTBZ-HFU, HTBZ-HOX, HTBZ-SAC, and HTBZ-HMA, respectively. The higher pH of HTBZ-SAC and HTBZ-HMA suggests that pH is not the only factor affecting solubility, and different factors, such as crystal structure and hydrogen bonding interactions, play important roles in dissolution rates . After the dissolution test, PXRD spectra were collected and confirmed transition to TBZ free base during the test (Figure ).