Chinese hamster ovary (CHO) cells are used for the production of the majority of biopharmaceutical drugs, and thus have remained the standard industry host for the past three decades. The amino acid composition of the medium plays a key role in commercial scale biologics manufacturing, as amino acids constitute the building blocks of both endogenous and heterologous proteins, are involved in metabolic and non-metabolic pathways, and can act as main sources of nitrogen and carbon under certain conditions. As biomanufactured proteins become increasingly complex, the adoption of model-based approaches become ever more popular in complementing the challenging task of medium development. The extensively studied amino acid metabolism is exceptionally suitable for such model-driven analyses, and although still limited in practice, the development of these strategies is gaining attention, particularly in this domain. This paper provides a review of recent efforts. We first provide an overview of the widely adopted practice, and move on to describe the model-driven approaches employed for the improvement and optimization of the external amino acid supply in light of cellular amino acid demand. We conclude by proposing the likely prevalent direction the field is heading towards, providing a critical evaluation of the current state and the future challenges and considerations.

Majority of biopharmaceutical drugs today are produced by Chinese hamster ovary (CHO) cells, which have been the standard industry host for the past decades. To produce and secrete a substantial amount of the target recombinant proteins the CHO cells must be provided with suitable growth conditions and provided with the necessary nutrients. Amino acids play a key role in this as the building blocks of proteins, playing important roles in a large number of metabolic pathways and being important sources of nitrogen as well as carbon under certain conditions. In this study exploratory analysis of the amino acid requirements of CHO cells was carried out using metabolic modelling approaches. Flux balance analysis was employed to evaluate the optimal distribution of fluxes in a genome-scale model of CHO cells to gain information on the cells' metabolic response in silico.The results showed that providing non-essential amino acids (NEAAs) has a positive effect on CHO cell biomass production and that cysteine as well as tyrosine play a fundamental role in this.This implies that extracellular provision of NEAAs limits the extent of energy loss in amino acid biosynthetic pathways and renders additional reducing power available for other biological processes.Detailed analysis of the possible secretion and uptake of D-serine in the CHO model was also performed and its influence on the rest of the metabolism mapped out, which revealed results matching various existing literature. This is interesting since no mention of D-serine in regard to CHO cells was found in current literature, as well as the fact that this opens up the possibility of using the model for better understanding of certain disorders in higher up organisms that have been implicated with D-serine, such as motor neuron and cognitive degeneration. Finally, outcome from the model optimisation of different recombinant proteins demonstrated clearly how the difference in protein structure and size can influence the production outcome. These results show that systematic and model-based approaches have great potential for broad de novo exploration as well as being able to handle the cellular burden associated with the production of different types of recombinant protein.

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