Cholesterol is the major, and probably the sole precursor of bile acids. During normal digestion, bile acids are secreted via the bile from the liver and gall bladder into the intestines. Bile acids emulsify the fat and lipid materials present in food, thus facilitating absorption. A major portion of the bile acids secreted is reabsorbed from the intestines and returned via the portal circulation to the liver, thus completing the enterohepatic cycle. Only very small amounts of bile acids are found in normal serum.

Colestipol hydrochloride binds bile acids in the intestine forming a complex that is excreted in the feces. This nonsystemic action results in a partial removal of the bile acids from the enterohepatic circulation, preventing their reabsorption. Since colestipol hydrochloride is an anion exchange resin, the chloride anions of the resin can be replaced by other anions, usually those with a greater affinity for the resin than the chloride ion.

Colestipol hydrochloride is hydrophilic, but it is virtually water insoluble (99.75%) and it is not hydrolyzed by digestive enzymes. The high molecular weight polymer in colestipol hydrochloride apparently is not absorbed. In humans, less than 0.17% of a single ¹⁴C-labeled colestipol hydrochloride dose is excreted in the urine when given following 60 days of dosing of 20 grams of colestipol hydrochloride per day.

The increased fecal loss of bile acids due to colestipol hydrochloride administration leads to an increased oxidation of cholesterol to bile acids. This results in an increase in the number of low- density lipoprotein (LDL) receptors, increased hepatic uptake of LDL and a decrease in beta lipoprotein or LDL serum levels, and a decrease in serum cholesterol levels. Although colestipol hydrochloride produces an increase in the hepatic synthesis of cholesterol in man, serum cholesterol levels fall.

There is evidence to show that this fall in cholesterol is secondary to an increased rate of clearance of cholesterol-rich lipoproteins (beta or low-density lipoproteins) from the plasma. Serum triglyceride levels may increase or remain unchanged in colestipol hydrochloride treated patients.

The decline in serum cholesterol levels with colestipol hydrochloride treatment is usually evident by one month. When colestipol hydrochloride is discontinued, serum cholesterol levels usually return to baseline levels within one month. Periodic determinations of serum cholesterol levels as outlined in the National Cholesterol Education Program (NCEP) guidelines, should be done to confirm a favorable initial and long- term response.¹

In a large, placebo- controlled, multiclinic study, the LRC- CPPT², hypercholesterolemic subjects treated with cholestyramine, a bile-acid sequestrant with a mechanism of action and an effect on serum cholesterol similar to that of colestipol hydrochloride, had reductions in total and LDL- C. Over the 7- year study period the cholestyramine group experienced a 19% reduction (relative to the incidence in the placebo group) in the combined rate of coronary heart disease (CHD) death plus nonfatal myocardial infarction (cumulative incidences of 7% cholestyramine and 8.6% placebo). The subjects included in the study were middle-aged men (aged 35-59) with serum cholesterol levels above 265 mg/ dL, LDL-C above 175 mg/ dL on a moderate cholesterol-lowering diet, and no history of heart disease. It is not clear to what extent these findings can be extrapolated to other segments of the hypercholesterolemic population not studied.

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