US generic drug maker Ivax is challenging patents held by UK-headquartered drug giant AstraZeneca for its asthma drug, the inhaled corticosteroid Pulmicort Respules (budesonide inhalation suspension).

The agent is used to help prevent asthma attacks in children aged 12 months to eight years old, and generated sales of $561 million dollars in the year to June 30, Ivax said, quoting data from IMS Health.

Ivax, which could soon become part of the biggest generic drugmaker in the world following a proposed merger with Israel’s Teva [], said it initially filed its application to produce a copycat version of the drug with the Food and Drug Administration in late 2004, but added an amendment last month alleging invalidity and non-infringement alleging of certain of AstraZeneca’s patents relating to the product.

In response, the Anglo-Swedish drug major filed a suit against Ivax in the US District Court in New Jersey last week to defend the validity of its patents.