1. Molecular Absorption: Absorption refers to the process by which a drug passes from the site of administration into the systemic circulation. This involves crossing biological barriers, which can include cell membranes and tissue layers. The drug's molecular size, lipophilicity, and ionization state can influence this process.
2. Oral Bioavailability: For orally administered drugs, absorption typically occurs in the gastrointestinal tract. Factors such as the drug's stability in gastric acid, solubility in intestinal fluid, and permeability through the intestinal wall (which can be affected by the presence of transport proteins) play a role in oral bioavailability.
3. Molecular Distribution: Once absorbed, the drug gets distributed throughout the body. The molecular properties of the drug, its affinity for different tissues, and the blood flow to these tissues determine the extent and rate of distribution. Binding to plasma proteins can influence the drug's availability at its site of action.
4. Volume of Distribution: The volume of distribution (Vd) is a theoretical value that indicates the extent to which a drug is distributed in the body tissues. A high Vd suggests extensive tissue binding or sequestration, while a low Vd suggests the drug is primarily confined to the plasma.
5. Molecular Metabolism: Drugs are often metabolized to facilitate their removal from the body. This involves enzymatic transformations, typically in the liver. These transformations can result in drug inactivation, drug activation (for prodrugs), or the formation of toxic metabolites. The Cytochrome P450 enzyme family is particularly important in drug metabolism.
6. Drug-Drug Interactions: Interactions between different drugs can influence their ADME properties. For instance, one drug might inhibit or induce the enzymes that metabolize another drug, altering its pharmacokinetics.
7. Genetic Variability in Metabolism: Individual genetic variation can significantly affect drug metabolism. Polymorphisms in metabolic enzymes can lead to individuals being classified as poor, intermediate, extensive, or ultra-rapid metabolizers, which influences drug response and risk of adverse effects.
8. Molecular Excretion: Excretion is the final stage in drug elimination from the body. Kidneys are the primary organ for drug excretion, but other routes like biliary and lung excretion also exist. The drug's molecular properties, such as its size, charge, and lipophilicity, influence its excretion.
9. Renal Excretion Mechanisms: In the kidneys, drugs can be excreted via glomerular filtration, tubular secretion, and tubular reabsorption. These processes are influenced by the drug's molecular properties and by physiological factors like renal blood flow and urine pH.
10. Enterohepatic Recirculation: Some drugs and their metabolites undergo enterohepatic recirculation, where they are excreted into the bile, reabsorbed from the intestine, and then returned to the liver. This can prolong the drug's presence in the body and affect its pharmacokinetics.