Final answer:
The capacity of a drug molecule to bind to a key site of action is referred to as its affinity. This binding affinity dictates the drug's ability to achieve its pharmacological effect while avoiding unwanted interactions with other receptors or enzymes, and overcoming physiological barriers like the BBB and PPB.
Step-by-step explanation:
The capacity of a drug molecule to bind to a key site of action refers to its affinity for a specific target or receptor. This encompasses the idea of selectivity and potency, where a drug has a high affinity for its intended target while minimizing interactions with other receptors, which could potentially lead to side effects. The concept of drug-target interaction is integral to pharmacology and can include interactions at an enzyme's active sites, receptor-mediated targeting, and avoidance of barriers like the blood-brain barrier (BBB) and plasma protein binding (PPB). Selective binding is essential for achieving the desired pharmacological effect without harming the host, a concept that is particularly important in the development of antimicrobial drugs
One example of this is enzymatic hydrolysis that prevents drugs from penetrating into tissues. Drugs need to maintain a balance between having sufficient lipophilicity for tissue absorption and having features to avoid being effluxed by transport proteins like P-glycoprotein. This balance is crucial because overly tight binding to plasma proteins can also impede the drug's therapeutic effect by preventing it from reaching its site of action within the tissues or crossing physiological barriers like the BBB. The drug's properties, such as its affinity and the ability to overcome these physiological hurdles, determine its efficacy.