What is the difference between drug substance and drug product?
The drug substance, depending on its purity, is mostly composed of the API (active pharmaceutical ingredient), or the ‘naked’ drug without excipients. The API is what will have a therapeutic effect inside the body, as opposed to the excipients, which serve to package and deliver the API.
The drug product is the formulated mixture of the drug substance and excipients, either as a prototype, or as the final marketed dosage form.
What in vitro testing is carried out in drug substance vs. drug product?
The drug substance is obtained after drug candidate selection, very early in development, before drug product development. To develop a drug product (the final formulated drug) from a drug substance, it’s important to understand the drug substance’s physicochemical properties. Biorelevant solubility testing of the drug substance is performed to understand the compound’s solubility and therefore choose the right formulation strategy.
Once the drug substance is developed and formulated into a drug product, i.e. the prototype or final oral dosage form, the drug product can be tested in suitable media (biorelevant/physiologically relevant media) for dissolution testing prior to clinical studies. Biorelevant dissolution testing gives an insight into how a drug product would dissolve in vivo.
Understanding the difference between drug substance and drug product is essential for choosing what in vitro testing to carry out, which also depends on the stage of preclinical development you’re at.