An Integrated Experimental and Modeling Approach for Crystallization of Complex Biotherapeutics

Crystallization of proteins, specifically proteins of medical relevance, is performed for various reasons, such as to understand the protein structure and to design therapies. Obtaining kinetic constants in rate laws for nucleation and growth of advanced biotherapeutics such as capsids, an assembly of macromolecules, is challenging and essential to the design of crystallization processes. In this work, coupled population balance and species balance equations are developed to extract nucleation and growth kinetics for the crystallization of recombinant adeno-associated virus (rAAV) capsids. A comparison of model results with that of experimental data for capsid crystallization in a hanging-drop vapor diffusion system shows that the slow rate of vapor diffusion from the droplet controls the initial nucleation and growth processes, and the capsid nucleation occurs via heterogeneous nucleation in the microdroplet. Results also show that the capsids, which are of very high molecular weight (∼3.6 MDa), have a similar tendency to nucleate as small organic molecules such as glycine (∼75 Da), low-molecular-weight proteins, and small-molecule active pharmaceutical ingredients due to their ball-shaped outer structure/shape. Capsids also show a prolonged nucleation period as for proteins and other macromolecules but have a slow growth rate with a growth rate prefactor seven orders of magnitude smaller than that of lysozyme. The capsid crystal growth rate is weakly sensitive to supersaturation compared to lysozyme and is limited by the transport of capsids due to slow Brownian motion resulting from the very high molecular weight.