Drug Delivery: Absorption and Bioavailability Factors Quiz

Cell membranes are lipophilic. Neutral molecules partition into the lipid bilayer much more effectively than charged species which are surrounded by a hydration shell that hinders passage.

Bioavailability measures the efficiency of delivery. For intravenous administration it is 100 percent. For oral drugs it is reduced by incomplete absorption and metabolic degradation.

After oral ingestion drugs are carried via the portal vein directly to the liver. Hepatic enzymes may metabolize a large portion of the drug before it enters general systemic circulation.

These rules provide a baseline for drug likeness. Violating more than one suggests a molecule may have poor oral bioavailability due to size polarity or extreme lipophilicity.

P glycoprotein acts as a biological vacuum cleaner. It identifies foreign lipophilic molecules and pumps them out of the cell which can significantly limit the absorption of many drugs.

While high lipophilicity aids membrane crossing a drug that is too lipophilic will have extremely poor aqueous solubility. If it cannot dissolve in the GI tract it cannot reach the membrane.

This equation allows chemists to predict where a drug will be absorbed. For example a weak acid will be largely unionized in the acidic stomach and absorbed more effectively there.

This equation describes how quickly a solid drug enters solution. Increasing surface area through micronization can accelerate the dissolution rate and subsequent absorption.

Many drugs are only absorbed in the duodenum or jejunum. If a formulation releases the active ingredient after it has passed this section the bioavailability will drop significantly.

Only the free or unbound drug is small enough to cross biological membranes. Extensive protein binding acts as a reservoir but decreases the immediate concentration available for tissue absorption.

Extended release formulations are designed to bypass or spread out the absorption window to avoid toxic peaks and ensure the drug is absorbed over a longer period in the GI tract.

Drugs can move through the bilayer directly through transport proteins using energy or move between cells through tight junctions depending on their size and polarity.

Prodrugs are inactive compounds that are converted into the active form after absorption or by the liver. They are often used to mask polar groups and increase membrane permeability.

The massive surface area of the small intestine makes it the primary site of absorption for most drugs regardless of whether they are weak acids or weak bases.

The BCS is used by regulatory agencies to predict drug absorption. Class 1 drugs have high solubility and high permeability making them the ideal candidates for oral administration.