US regulators have warned healthcare professionals to use caution when administering GlaxoSmithKline’s Promacta, known as Revolade in Europe, to patients with risk factors for thromboembolism.

The warning comes after a clinical trial of the drug, which is used to address low blood platelet counts, was halted after six patients (4%) in the group receiving Promacta (eltrombopag) experienced a thrombotic event of the portal venous system, compared with 1% of the placebo arm.

Five of the six patients who experienced a thrombotic side effect had platelet counts above 200,000/_L, i.e. within the normal range, prompting a posting on the US Food and Drug Adminstration’s website to stress that treatment with Promacta should aim to boost platelet counts to a level that reduces the risk of bleeding and not as an attempt to normalise platelet count.

Promacta, which was developed with US group Ligand Pharmaceuticals, has received clearance on both sides of the Atlantic for the treatment of thrombocytopenia in adult patients with chronic immune thrombocytopenic purpura (ITP).

But in a letter to healthcare professionals, the UK drugmaker stressed that the drug is not indicated for the treatment of thrombocytopenia in patients with chronic liver disease, and should be used “with caution” in patients with hepatic disease.