Physiological Role and Therapeutic Target
The mechanism of action of this drug group is based on disrupting a natural physiological process: enterohepatic circulation. Under normal conditions, the liver continuously synthesizes bile acids using endogenous cholesterol as the primary building block. These synthesized acids are secreted into the intestinal lumen to perform their function, after which the vast majority are reabsorbed and returned to the liver.
Bile acids located within the intestinal lumen serve as the primary pharmacological target for sequestrants. Chemically, these drugs are anion-exchange resins. Upon entering the gastrointestinal tract, they act exclusively locally. The resins bind free bile acids, forming large, insoluble, and non-absorbable complexes. As a result, normal recirculation is interrupted, and the bound acids leave the body via fecal excretion.
Compensatory Hepatocyte Response
Massive loss of bile acids through the gastrointestinal tract triggers a powerful compensatory response by the liver. This process can be divided into several sequential steps:
- The liver is forced to urgently increase the synthesis of bile acids de novo. To fuel this biochemical process, hepatocytes actively consume endogenous cholesterol.
- Intracellular cholesterol stores within liver cells are rapidly depleted.
- In response to this deficit, liver cells alter their membrane structure: the expression (total number) of specific LDL receptors on their surface significantly increases.
- Receptor-mediated endocytosis is enhanced—a process where the liver actively captures atherogenic low-density lipoproteins (LDLs) from the systemic circulation.
The end result of this complex cascade is a pronounced decrease in the plasma concentration of atherogenic lipoproteins. Notably, high-density lipoprotein (HDL) levels during such therapy either remain completely unchanged or show a minor increase.
Drug Representatives and Clinical Application
In clinical practice, the main representatives of bile acid sequestrants are cholestyramine and colestipol. Both drugs possess approximately equal therapeutic efficacy.
- Indications: The primary indication for prescribing anion-exchange resins is type IIa hyperlipoproteinemia (HLP).
- Dosing Regimen: The medications are taken orally, 2–3 times a day. A standard treatment course lasts 14 days.
- Temporal Parameters: An initial decrease in lipid levels is registered 24–48 hours after starting therapy. The effect of a single dose lasts from 12 to 24 hours. Maximum clinical efficacy is achieved within one month. Even after complete discontinuation of the drug, the lipid-lowering aftereffect persists for 2–4 weeks.
Safety Profile
Because bile acid sequestrants form non-absorbable complexes, their safety profile is quite specific. The vast majority of adverse reactions are related to the local effect of the resins on the gastrointestinal tract.
Patients most frequently complain of persistent constipation, nausea, heartburn, and marked flatulence. In some cases, therapy may be accompanied by systemic reactions such as headaches.