An investor in AbbVie Inc. has sued the pharmaceutical company in Delaware Chancery Court to investigate an alleged kickback scheme that boosted sales of its popular immunosuppressant Humira and potentially exposed the company to more than $6 billion in legal liability.

The 28-page filing, filed May 3, claimed that senior officers and directors had allowed the firm to illegally promote Humira, which is used to treat rheumatoid arthritis, Crohn's disease and other inflammatory conditions, by offering doctors benefits in exchange for prescribing the drug.

According to the complaint, Humira executives instituted a system of providing incentives to doctors in exchange for prescribing the medication, and created “ambassador ” positions to interact directly with patients and keep them on the drug, which has been linked to increased risks of fatal infections and certain cancers.