Xanthine Oxidase Inhibition: Mechanism & Use

What is Xanthine Oxidase Inhibition?

Xanthine oxidase inhibition refers to the targeted blocking of the enzyme xanthine oxidase, which plays a central role in purine metabolism. Purines are components of DNA and are released during normal cell breakdown and through dietary intake. The enzyme xanthine oxidase converts the intermediate compounds hypoxanthine and xanthine into uric acid (urate). When this enzyme is inhibited, uric acid production in the body decreases significantly.

Mechanism of Action

Xanthine oxidase inhibitors bind to the active site of the xanthine oxidase enzyme, thereby preventing the oxidation of hypoxanthine to xanthine and xanthine to uric acid. As a result, the more water-soluble precursors hypoxanthine and xanthine accumulate instead and are more readily excreted by the kidneys. The consequence is a significant reduction in blood uric acid levels (serum urate).

There are two types of xanthine oxidase inhibitors:

• Competitive inhibitors: They compete with the natural substrate for the active binding site of the enzyme.
• Non-competitive (allosteric) inhibitors: They bind to a different site on the enzyme and alter its structure, rendering it inactive.

Medical Applications

Gout

Gout is a metabolic disorder in which elevated uric acid levels in the blood (hyperuricemia) lead to the deposition of uric acid crystals in joints and tissues. The resulting painful joint inflammation, most commonly affecting the big toe, is known as a gout attack. Xanthine oxidase inhibitors are the cornerstone of long-term treatment and prevention of further gout attacks.

Hyperuricemia

Even without clinical gout symptoms, persistently elevated uric acid levels can lead to kidney damage, kidney stones, and cardiovascular disease. In such cases, xanthine oxidase inhibition may be indicated to maintain lower uric acid levels over the long term.

Further Indications

In certain situations, such as the treatment of blood cancers or after chemotherapy, rapid cell breakdown can cause a massive release of purines, a condition known as tumor lysis syndrome. Xanthine oxidase inhibitors are also used here to prevent a dangerous uric acid crisis.

Key Active Substances

Allopurinol

Allopurinol is the most commonly used xanthine oxidase inhibitor and is considered the first-line treatment. It is metabolized in the body to its active metabolite oxipurinol, which provides long-lasting inhibition of xanthine oxidase. Allopurinol is taken orally and is generally well tolerated. Potential side effects include skin rashes, gastrointestinal complaints, and in rare cases, severe hypersensitivity reactions.

Febuxostat

Febuxostat is a newer, selective xanthine oxidase inhibitor used primarily in patients who cannot tolerate allopurinol or in whom allopurinol is insufficiently effective. Unlike allopurinol, febuxostat is not a purine analogue, which influences its efficacy and safety profile.

Dosage and Treatment Notes

The dosage of xanthine oxidase inhibitors is determined by individual uric acid levels, kidney function, and clinical response. The therapeutic target is a serum urate level below 6 mg/dL (360 mmol/L), or below 5 mg/dL in severe gout. Treatment is typically long-term. Paradoxically, a gout flare can occur at the start of therapy, which is why colchicine or low-dose NSAIDs are often prescribed concurrently as flare prophylaxis.

Side Effects and Risks

• Skin reactions (rash, rarely severe reactions such as Stevens-Johnson syndrome)
• Gastrointestinal complaints (nausea, diarrhea)
• Elevated liver enzyme levels
• Drug interactions, e.g., with azathioprine (the combination with allopurinol can lead to life-threatening bone marrow suppression)
• With febuxostat: possible cardiovascular risks (under discussion)

References

1. Dalbeth N, Merriman TR, Stamp LK. Gout. The Lancet. 2016;388(10055):2039-2052.
2. Richette P, Doherty M, Pascual E, et al. 2016 updated EULAR evidence-based recommendations for the management of gout. Annals of the Rheumatic Diseases. 2017;76(1):29-42.
3. World Health Organization (WHO): Model Formulary - Antigout medicines. Geneva: WHO Press.