4.12 : Factors Affecting Protein-Drug Binding: Protein-Related Factors

Drug binding to proteins is a key aspect of pharmacokinetics and can influence a drug's distribution, absorption, and elimination in the body. Several factors, including the drug's physiochemical properties, protein concentration, disease states, and the number of binding sites on the protein, influence this process.

The physicochemical properties of a drug play a significant role in its ability to bind to proteins. Lipophilic drugs, which dissolve in fats, oils, and lipids, can be bound by lipoproteins and adipose tissue. These substances dissolve the lipophilic drugs in their lipid core, influenced by the physiologic pH.

The concentration of proteins in the plasma can affect how much a drug binds. Due to its high concentration in plasma, albumin predominantly binds drugs. It is a large protein found in high abundance in the bloodstream, making it a significant player in drug-protein interactions.

During certain disease states, the amount of several proteins and tissue components available for binding can change. This can alter the drug-binding process, potentially affecting how the drug is distributed and eliminated in the body.

The number of binding sites on a protein also impacts drug binding. For instance, albumin has many binding sites compared to other proteins, making it a high-capacity binding component. This means it can bind a larger drug quantity at any given time.

Some drugs, such as flucloxacillin, flurbiprofen, and tamoxifen, can bind to more than one site on albumin. For example, indomethacin is known to bind to three different sites on albumin. This multi-site binding can impact the drug's distribution and elimination in the body.

In contrast to albumin, α₁-acid glycoprotein has limited binding capacity due to its low concentration and smaller molecular size. It binds fewer drugs and plays a less significant role in drug-protein interactions.

Understanding these factors is crucial for predicting how drugs behave in the body and can help design and select drugs for specific therapeutic applications.