It is estimated that medications are involved in up to 80% of all treatments and impact every aspect of a patient’s life. As a result, physical therapists must recognize that drugs may alter a patient’s clinical presentation, which at times may require that physical therapy interventions be modified. Knowledge of drug classes and their mechanisms of actions is key to understanding patients’ responses to medications. The beneficial clinical effects of drugs occur within specific concentration ranges. These ranges are unique to the different pharmacologic classes of drugs and, for some drugs, unique to the specific individual. Concentrations below the effective range provide no therapeutic benefit, while concentrations above the range almost always result in adverse drug reactions (ADRs). As discussed in the Chapter 3, the goal of dosing regimens is to utilize knowledge of the therapeutic range for each drug to determine the frequency and dose for a specific person.
Both the therapeutic and toxic effects of the majority of drugs result from interactions with their specific molecular targets—receptors. A drug molecule is an exogenous ligand that interacts with a receptor and initiates a chain of biochemical and physiologic events leading to the drug’s observed effects. Pharmacodynamics is the branch of pharmacology concerned with the interaction between drug and receptor and the subsequent results.
A drug’s mechanism of action is based on whether it mimics or inhibits an endogenous ligand or has some other unrecognized effect(s). A drug may directly compete with an endogenous ligand for a specific receptor or modulate the affinity (binding strength) of the receptor for the endogenous ligand. Some drugs may permanently inactivate the receptor to which they bind or stimulate additional cellular homeostatic mechanisms, which can result in a clinical effect lasting after the drug itself is no longer present in the body.
Key principles underlying the receptor concept form the basis of understanding the actions and clinical uses of drugs. These principles also have important practical consequences for drug development. First, receptors largely determine the quantitative relationship between dose or concentration of a drug and its pharmacologic effects. The receptor’s affinity for binding a drug determines the concentration of drug required to form a significant number of drug-receptor complexes. In addition, the total number of receptors may limit the maximal effect a drug may produce. Second, receptors are responsible for the selectivity of drug action. The molecular size, shape, and electrical charge of a drug determine whether it will bind to a particular receptor among the vast array of chemically different binding sites available within the body. Accordingly, changes in a drug’s chemical structure can dramatically alter its affinity for different classes of receptors, with resulting alterations in therapeutic and toxic effects. Finally, receptor activation (by agonists) or receptor blockade (by antagonists) are the primary factors responsible for many clinical effects of drugs. Knowledge of whether a drug is an agonist, antagonist, or partial agonist ...