Chapter 2: Pharmacokinetics
The term “first-pass effect” refers to the
a. effect a new drug has on the body after its first administration.
b. time it takes for a drug to be detected in the urine or feces after oral administration.
c. ability of the intestines and liver to reduce the bioavailability of a drug.
d. time it takes for a drug to reach therapeutic concentrations in the target tissue.
e. initial effect a drug has on target tissues.
Answer is c. A drug given orally must be absorbed first from the GI tract, and the absorbed drug then passes through the liver, where metabolism and biliary excretion may occur before the drug enters the systemic circulation. Accordingly, a fraction of the administered and absorbed dose of drug will be inactivated or diverted in the intestine and liver before it can reach the general circulation and be distributed to its sites of action. If the metabolic or excretory capacity of the liver and the intestine for the drug is large, bioavailability will be reduced substantially (first-pass effect). This decrease in availability is a function of the anatomical site from which absorption takes place; other anatomical, physiological, and pathological factors can influence bioavailability, and the choice of the route of drug administration must be based on an understanding of these conditions. Moreover, knowledge of drugs that undergo significant metabolism or require active transport across the intestinal and hepatic membranes instructs our understanding of adverse events in therapeutics, because some drugs are substrates for the same drug metabolizing enzymes or drug transporters and thus compete for metabolism and transport.
Diseases such as hepatic cirrhosis may affect “first-pass metabolism” by shunting of blood away from hepatic metabolizing enzymes.
The term “blood-brain barrier” (BBB) refers to a
a. noncellular barrier that prevents drugs from entering the CNS unless transported by specific carriers.
b. cellular barrier that includes brain capillary endothelial cells that limits drug entry into the brain.
c. virtual or conceptual barrier that can explain the behavior of some drug effects on the CNS.
d. physical barrier that prevents blood from entering the brain.
e. device that is used to prevent blood-borne drugs from entering the brain.
Answer is b. The distribution of drugs into the CNS from the blood is unique. One reason for this is that the brain capillary endothelial cells have continuous tight junctions; therefore, drug penetration into the ...