JNTU B.Pharmacy III BIOPHARMACEUTICS AND PHARMACOKINETICS Previous Question Paper (November/December - 2020 - R17)

The relationship between blood flow and drug-protein binding is primarily governed by the extraction ratio and the restrictive vs. non-restrictive binding classification. For drugs with a high extraction ratio (high hepatic clearance), the rate of drug delivery to the liver is the rate-limiting step. In this case, protein binding does not significantly restrict the hepatic clearance because the liver efficiently removes both free and protein-bound drug from the blood as it passes through the organ. Conversely, for drugs with a low extraction ratio, the liver is less efficient, and only the free (unbound) fraction of the drug is available for metabolism or excretion. Here, protein binding acts as a reservoir, and changes in the fraction unbound significantly influence the clearance rate. Thus, for low-clearance drugs, blood flow has a minimal effect on total clearance, whereas for high-clearance drugs, blood flow is the primary determinant of the rate of drug elimination.

The extent of drug-protein binding is heavily influenced by the physicochemical properties of the drug molecule, which determine its affinity for plasma proteins like albumin and alpha-1-acid glycoprotein. Key factors include:

• Lipophilicity: There is a strong positive correlation between the lipophilicity of a drug (often measured by the log P value) and its binding to plasma proteins. Highly lipophilic drugs tend to bind more extensively to albumin, as they partition more readily into the hydrophobic binding pockets of the protein.
• Ionization (pKa): The degree of ionization at physiological pH (7.4) affects binding. Acidic drugs, such as warfarin and NSAIDs, typically bind strongly to human serum albumin (HSA), while basic drugs, such as propranolol or imipramine, often show higher affinity for alpha-1-acid glycoprotein.
• Molecular Size and Structure: The molecular weight and spatial configuration of the drug influence its fit within the protein binding sites. Larger, more complex molecules may have more extensive van der Waals interactions with protein residues, enhancing binding affinity.
• Stereochemistry: Many proteins exhibit stereoselectivity; therefore, different enantiomers of a racemic drug may show significantly different binding affinities due to their specific spatial orientation within the protein's active site.