The molecular structure of biologic drugs, which determines its function, is held in place by relatively weak bonds. As such, maintaining stability of biologic drugs is a key aim during the development process (see Chapter 3). The addition of excipients is often used to improve stability and ensure functionality of biologic or biosimilar drugs, as well as for other purposes. Excipients are common in the final formulations of most biologics, biosimilars and small chemical drugs.
An excipient is a substance other than an active drug or pro-drug that is used during manufacturing or contained in the final pharmaceutical formulation. The function performed by excipients can vary depending on the nature of the drug and can include stabilising during transport or bulking to allow accurate measurement. The excipients used in a biosimilar do not have to match the excipients used in the originator product – this is where differences in immunogenicity and stability can arise between biosimilars and their reference biologic product.
This chapter details the classification of excipients, their function, and side effects and interactions with examples throughout.