Purdue Pharma L.P. and two affiliated companies are seeking to disqualify a former District of Columbia attorney general from representing the city of Chicago in its lawsuit against several drugmakers for allegedly seeking to misinform the public about the risks and benefits of opium-like pain medications.

According to Purdue, attorney Linda Singer and her law firm law firm Cohen Milstein Sellers & Toll should be disqualified because Singer served as attorney general between Jan. 2, 2007, and Jan. 5, 2008. The District and 26 other jurisdictions reached a $19.5 million settlement with Purdue over the marketing of OxyContin on May 8, 2007, the drugmakers said.