Yield enhancement and phase behaviors of cyclic peptide drug crystallization in the presence of polyethylene glycol additive

Crystallization of peptide drugs typically requires a lengthy period to attain reasonable yields due to slow crystallization kinetics of macromolecules. Herein we investigated the use of polyethylene glycol (8 kDa) (PEG 8k) as an additive to enhance the yield of salting-out crystallization of cyclic peptide drug vancomycin (Van). Our preliminary study showed the addition of PEG 8k increased the 24-h yield from 46 wt% to 70 wt% attributed to the polymer's volume-exclusion effects. PEG 8k addition, however, resulted in modified Van crystal habits, thereby phase behavior study was performed at different pH, salt, polymer, and Van concentrations. The phase behavior study showed PEG 8k and salt concentrations must be meticulously determined to produce stable Van crystals because their excessive concentrations resulted in heavy precipitates. Different crystal habits (e.g., acicular, blade-like) were produced with PEG 8k additive depending on the pH, distinct from octahedral crystals produced without additive. Bulk crystallization with PEG 8k additive resulted in platy Van crystals with higher crystallinity, lower thermal stability and purity, but with similar dissolution rates compared to octahedral crystals prepared without PEG 8k. PEG 8k addition had catalytic effects on the salting-out yield whereby lower salt concentration was needed. In short, PEG 8k addition enhanced the yield without compromising Van crystals’ qualities.