Fate of nanomedicine in the human body

Medicines used in human health have a primary pharmacological activity, but also undesirable side effects. Improving the performance of a medicine by limiting its potential toxic effects amounts to increasing its benefit/risk balance. Nanotechnologies provide ways of increasing the benefit/risk balance by changing the fate of the medicine in the body. The idea is to increase the amount of active molecules in the tissues or cells of interest and decrease the amount in tissues where the molecule could be toxic. This is very important in the field of cancer treatment, where the aim is to target tumor cells very precisely and not healthy cells. In other fields, such as infectious diseases, the goal of nanomedicines is to protect fragile active molecules from rapid degradation (as in the case of COVID-19 vaccines). The idea behind encapsulating molecules within nanoparticles is to combine the active molecule with a vector that has physicochemical properties (size, surface electrostatic charges, hydrophilicity, etc.) that will determine where it is distributed in the body and how it is eliminated. Thus, the fate of the active molecule, the drug, in the body will no longer depend on its own chemical properties but on those of the vector. This concept is called vectorization. Successful vectorization therefore consists of improving the targeting of drug molecules to the tissues of the body where they are desired to be active, while limiting their diffusion to tissues where they could be toxic. This is achieved by extending their residence time in the tissues of interest in order to prolong the pharmacological effect and increase efficacy. The development of an effective and low-toxicity vector relies on mastery of manufacturing and characterization processes, which can be difficult at the nanoscale, as well as knowledge of the physiological, histological, biological, and biochemical structures of the body's tissues. Indeed, the fate of the vector in the body, which we want to control in order to effectively control the action of the drug, will depend on the interaction between the vector and the living environment. Thus, depending on the route of administration of the drug, the vector will come into contact with different tissues and its path through the body may vary. The discipline that studies the behavior of a drug in relation to the biological structures it encounters is called biopharmaceutics. The purpose of this article is to describe the specific concepts of biopharmaceutics as they apply to nanomedicine vectors, also known as nanomedicines. This article will analyze the fate of nanomedicines by route of administration in order to inform formulators about the cellular and tissue structures to be taken into account for the rational and effective design of nanomedicines.