A2.1 Pharmacokinetics and pharmacodynamics

Topic

Once a drug enters the body, it follows two paths—studied separately but together forming the cross-cutting physico-mathematical foundation of all oncological drug therapy: what the body does to the drug, on the one hand, and what the drug does to the body, on the other. This dual relationship between drug and body underpins the three dependent modalities—chemotherapy, targeted therapy, and immunotherapy—each of which requires an understanding of the drug's behavior within the body before its effect on the tumor can be described.

What the body does to the drug is the subject of pharmacokinetics: once the drug enters the system, the body absorbs it into the circulation, distributes it to various tissues, metabolizes it by transforming it into other substances, and finally eliminates it. These four processes, acting sequentially on the same drug, determine the amount of drug that remains available within the body and for how long.

Conversely, what the drug does to the body is the subject of pharmacodynamics: the higher the concentration of the drug remaining available in the body—resulting from the aforementioned processes—the greater the biological effect it exerts; thus, drug concentration ultimately governs the intensity of the biological effect.

Together, the combination of pharmacokinetics and pharmacodynamics bridges the gap between the administered dose prescribed by the oncologist and the drug's ultimate effect on tumor cells; without this combination, models describing the effect on tumor cells would rely on free parameters, lacking any real connection to the dose actually administered in a clinical setting. By combining pharmacokinetics and pharmacodynamics, however, each dosing regimen selected by the oncologist generates a specific drug concentration-versus-time curve unique to that regimen.

Ultimately, this concentration-versus-time curve provides the input for the parameters of the chemotherapy and targeted therapy modules, establishing a tangible link to the administered dose rather than leaving them as free parameters. In this way, the combination of pharmacokinetics and pharmacodynamics fulfills its purpose: it transforms the dose prescribed by the oncologist into a physical and mathematical description of the drug's behavior within the body—a description that ultimately makes it possible to explain the drug's effect on tumor cells across chemotherapy, targeted therapy, and immunotherapy.

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