Crystallization Optimization By Enzymatic Control of a Reactive Crystallization

Tunable reactions can be used to control precisely the generation of supersaturation in reactive crystallization systems. Enzymes, with high reaction specificity and predictable rate enhancement, enable fine control over a number of reactions, and have been demonstrated in several reactive crystallization systems. In this study, penicillin G acylase (PGA) is used to synthesize and simultaneously crystallize cephalexin, starting from two cephalexin precursors, in a controlled manner. It was recently shown that cephalexin, which typically forms high-aspect-ratio needle-shaped crystals, can form lower-aspect-ratio crystals when supersaturation is generated in a slow, controlled manner (Li et al., 2019). These lower-aspect-ratio crystals have many desirable properties including faster filtration (by an order of magnitude) and improved powder flowability and handling.

This study aims to show that enzyme control of cephalexin reactive crystallization can produce crystals with a more desirable aspect ratio while also providing the benefits of combined synthesis and separation (greater productivity, conversion). The mechanism of PGA synthesis of beta-lactam antibiotics such as cephalexin is known (McDonald et al. 2017); here an immobilized form of PGA is used to enable recycling of the enzyme. Use of process analytical technology (PAT) combined with a reaction-diffusion-crystallization model (Salami et al. 2020, in preparation) lends insight into the phenomena critical to successful implementation of this novel means of supersaturation control. In situ microscopy and focused beam reflectance measurement (FBRM) show that the amount of crystal surface area is important to both aspect ratio and secondary nucleation. The model suggests that the amount of enzyme, and unexpectedly the immobilized bead size, is critical in dictating the reaction timescale. In general, this study demonstrates how catalysts can lend control over the crystallization process, giving process designers an additional knob to turn in the synthesis of new routes to important pharmaceutical products.

Li, M., et al. (2019). "Optimizing the Aspect Ratio of Cephalexin in Reactive Crystallization by Controlling Supersaturation and Seeding Policy." Transactions of Tianjin University 25(4): 348-356.

McDonald, M. A., et al. (2017). "Enzymatic reactive crystallization for improving ampicillin synthesis." Chemical Engineering Science 165: 81-88.

Salami, H., et al. (2020). “Model development for enzymatic reactive crystallization of β-lactam antibiotics, a reaction-diffusion-crystallization modeling approach.” Computers & Chemical Engineering in preparation