(646b) Crystallization Kinetics and Solubility of Amoxicillin Trihydrate in the Presence of Synthesis Substrates

Amoxicillin is the most consumed antibiotic in the world in terms of doses and consequently vital to global human health. Originally, the manufacturing of amoxicillin was conducted using a multi-step chemical synthetic pathway, consisting of protecting and deprotecting groups, cryogenic refrigeration units, and hazardous solvents. More recently, the use of biocatalytic processing has been investigated; the process consists of a single synthesis step and can be conducted under mild aqueous conditions, resulting in a greener process. The main drawbacks to biocatalytic amoxicillin production are the unwanted hydrolysis catalyzed by the biocatalyst. Improvements to the base biocatalytic process have been achieved by facilitating the reaction and crystallization of similar antibiotics in the same vessel, termed reactive crystallization[1]. Due to the complex nature of reactive crystallization, models describing the system are valuable in predicting optimum operating conditions, feed concentrations, residence times, reactor configurations, and other process considerations. Multi-module reactive crystallization models have been developed for similar systems, such as with cephalexin monohydrate[2]. On the other hand, neither the crystallization kinetics of amoxicillin trihydrate nor the effect of other reaction components on the solubility and crystallization kinetics of amoxicillin trihydrate also have been reported. It has been revealed that the degradation products of amoxicillin inhibit its crystallization. Also, it is known that reaction components in similar systems interact, causing inhibition of crystallization as well as changes in solubility, both phenomena greatly impact the outcomes of the process[3]. Accordingly, a comprehensive model of the reactive crystallization of amoxicillin trihydrate would only be feasible with improved knowledge of the phenomena occurring between the solid and liquid phase.

In this work, we investigated the solubility and crystallization of amoxicillin trihydrate with and without the presence of various reaction components, such as 4-hydroxyphenylglycine (4-HPG), 6-aminopenicillinoic acid (6-APA), and 4-hydroxyphenylglycine methyl ester (4-HPGME). The effects of pH, temperature, ionic strength, and presence of each component on the solubility of amoxicillin were determined. The solubility was found to increase with pH value, temperature, and ionic strength, and the three reaction components increased amoxicillin solubility, with 6-APA having the largest impact. However, ionic strength alone could not explain the increase in solubility in presence of the other reaction components, and there were possibly interactions occurring between the molecules of each reaction component and amoxicillin in the liquid phase. To confirm such interactions, single components and mixtures were analyzed via Raman and ATR-FTIR spectroscopy to explore possible interactions between the functional groups present in each compound and amoxicillin. The knowledge of amoxicillin solubility was important in determining its supersaturation under various conditions, which is the main driving force for crystallization.

We also conducted several seeded and unseeded batch amoxicillin crystallizations with and without the presence of 6-APA or 4-HPGME. We used offline microscopy and focused-beam reflectance measurement (FBRM) to monitor the solid phase, with high performance liquid chromatography (HPLC) to monitor the liquid phase. Solid and liquid phase data were fit to classical nucleation and growth models. Differences in nucleation and growth parameters with pure amoxicillin and in the presence of 6-APA or 4-HPGME were examined. A model considering the effects of 6-APA and 4-HPGME on the crystallization of amoxicillin hydrate was developed and may be incorporated into a reactive crystallization model to inform future manufacturing processes for the more efficient production of amoxicillin.

1. Encarnación-Gómez, L.G., A.S. Bommarius, and R.W. Rousseau, Reactive crystallization of β-lactam antibiotics: strategies to enhance productivity and purity of ampicillin. Reaction Chemistry & Engineering, 2016. 1(3): p. 321-329.
2. McDonald, M.A., et al., Continuous reactive crystallization of β-lactam antibiotics catalyzed by penicillin G acylase. Part I: Model development. Computers & Chemical Engineering, 2019.
3. Fan, Y., Y. Li, and Q. Liu, Enhanced Dissolution of 7-ADCA in the Presence of PGME for Enzymatic Synthesis of Cephalexin. Applied Biochemistry and Biotechnology, 2021.