Generic drugs are simple molecules that are easy to characterise and have a small, well-defined structure (approximately 180 daltons on average). However, biosimilars are very complex molecules with many post-translational modifications. They can exceed 150,000 daltons in size in the case of monoclonal antibodies and require a lot of analytical work for their structural and functional characterisation. Furthermore, generic drugs are very stable molecules, making them easy to store. However, just like many biological molecules, biosimilars are very sensitive to storage and handling conditions, so they need to be stored under adequate conditions starting from when they are developed. Another difference between the two products is that generic drugs have virtually no immunogenic potential, which means they cannot generate an immune response, while biosimilars, just like biological reference drugs, could be immunogenic.
Given the simplicity of generic drugs and no need for other complex modifications, it is easy to produce exact copies of them, which makes the manufacturing process easy and predictable. On the other hand, both biosimilars and their reference drugs are not synthesised through a simple chemical reaction like generic drugs. They require a complex biotechnological process in a cellular environment like any protein from the body. As living cells, each manufacturing process is inherently variable, which is why similar and not identical molecules are obtained, despite the process being controlled.