Ultraviolet spectra were collected every 30 s during the dissolution experiment to determine the dissolution rate profiles for TPa and TPm in our channel flow cell system. Fig. 7 shows the dissolution profiles for TPa (solid lines) and TPm compacts (dashed lines). From Fig. 7, it can be seen that TPa initially increases to peak values of between 150 and 190 μg/mL, while the TPm reaches concentrations of between 70 and 80 μg/mL.

Subsequently, there is a sharp drop in the first few minutes of the TPa dissolution that is not seen for the TPm dissolution. This change in dissolution behavior is due to a solvent-mediated transformation wherein the dissolving TPa (solubility 12 mg/mL RG-7204 at 25 °C [29]) reaches supersaturation which causes precipitation and growth of the more stable but less soluble TPm (solubility 6 mg/mL at 25 °C [29]) crystals that grow on the surface of the TPa compacts during dissolution. The surface growth of TPm on TPa samples undergoing dissolution has also been observed in other studies, using offline XRPD analysis [17] and inline spontaneous Raman spectroscopy [10] and [30]. The UV data shown in Fig. 7 correlate

well with the CARS images (Fig. 6) that were recorded during the dissolution experiments. The dissolution rate peaked after about 2 min which related to about half of the microscope field www.selleckchem.com/products/scr7.html of view covered in TPm needle-shaped crystals. After about 5 min, the dissolution rate reached a plateau at the same time the crystal growth appeared to completely

cover the field of view. Fig. 7 shows that the TPm dissolution rate quickly reached a steady state after around 1 min and remained there for the duration of the experiment. secondly The steady-state dissolution rates were calculated to be 360 ± 37 μg/min/cm2 and 320 ± 12 μg/min/cm2 for the compacts prepared from TPa and TPm, respectively. The slightly higher dissolution rate (not statistically significant) for the compacts originally composed of TPa after surface conversion to TPm can be attributed to the TPm needle growth resulting in a larger surface area. In situ CARS dissolution imaging identified delayed TPm crystal growth on the surface of TPa compacts undergoing dissolution using a MC solution (0.45% w/v) as the dissolution medium. Fig. 8 shows in situ single-frequency CARS snapshots taken from a dissolution video. The TPm crystal growth was delayed as it was first observed after approximately 300 s (5 min), and the surface coverage with TPm was incomplete after the duration of the experiment (15 min). Additionally, the TPm crystals were of a different morphology than previously seen when using water as the dissolution medium. Instead of the thin needle-like structure seen growing in water, there was a broad almost sheet-like growth along the surface of the compact. The delayed onset of crystal growth and different morphologies suggests that the polymer affects both nucleation and crystal growth.