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Ion exchange polymers


General information

Ion exchange polymers are insoluble organic polymers, which trap ions and in doing so release others [ ]. There are four main types:

  • strongly acidic compounds, which typically contain sulfonic acid groups (for example, the polystyrene sulfonates, which are covered in a separate monograph);

  • strongly basic compounds (for example, colestyramine and colesevelam), which contain quaternary amino groups, such as trimethylammonium;

  • weakly acidic compounds, which mostly contain carboxylic acid groups;

  • weakly basic compounds (for example, colestipol), which contain primary, secondary, and/or ternary amino groups.

They are used to soften and purify water, to purify fruit juices, in the separation of metals from each other (for example, separating plutonium and uranium in nuclear reactors), in the manufacture and purification of sugars, and in the manufacture of pharmaceutical products. The ion exchange polymers colestyramine, colestipol, and colesevelam are also known as bile acid sequestrants and are used to lower serum cholesterol concentrations. They are not absorbed from the intestine, where they bind bile acids, reducing their reabsorption after biliary excretion. The pool of bile acids becomes depleted, resulting in upregulation of cholesterol 7-α-hydroxylase, which increases conversion of cholesterol to bile acids.

The main adverse reactions to the bile acid sequestrants affect the gastrointestinal tract. They can also interfere with the absorption of other drugs or fat-soluble vitamins. Colesevelam is more selective for bile acids than colestyramine and colestipol and is less likely to interfere with the absorption of other drugs [ ].

Drug studies

Observational studies

In an open multicenter extension study of colesevelam hydrochloride 3.75 g/day for 52 weeks in 509 subjects with type 2 diabetes, 361 (71%) had an adverse event, mild or moderate in intensity in 88% [ ]. There were 56 drug-related adverse events (11%), the most frequent being constipation and dyspepsia; 35 (6.9%) discontinued treatment because of an adverse event; 54 (11%) had a serious adverse event, but only one was considered drug-related (diverticulitis). There was hypoglycemia in 17 (3.3%), in most cases mild or moderate.

Systematic reviews

According to the manufacturer's product information [ ], colesevelam has been associated constipation in 8.7–11% of patients, dyspepsia in 3.9–8%, hypertriglyceridemia in 4.1%, nasopharyngitis in 3%, weakness in 4%, and myalgia in 2%. They advise that patients with pre-existing constipation are at an increased risk of fecal impaction; that colesevelam is contraindicated in patients with gastrointestinal obstruction; that hemorrhoids may be aggravated if constipation occurs; that because the tablets are large, colesevelam can cause dysphagia or esophageal obstruction; that it should be used with caution in patients with dysphagia, swallowing disorders, severe GI motility disorders (e.g., ileus), or gastroparesis, and after major gastrointestinal tract surgery; and that patients taking oral vitamin supplements should take their vitamins at least 4 hours before colesevelam.

Organs and systems

Metabolism

The bile acid sequestrants tend to increase serum triglyceride concentrations, especially in patients with hypertriglyceridemia [ ].

In a pooled analysis of 696 patients with type 2 diabetes mellitus who were taking metformin monotherapy or metformin combined with other therapies, 355 were randomly assigned to colesevelam and 341 to placebo; colesevelam increased triglyceride concentrations by 13% [ ].

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