The maker of the blockbuster medication Humira created a “patent thicket” to illegally maintain a monopoly over the drug so it could be sold at an ever-increasing rate to the U.S. market, a major union has claimed in a newly filed antitrust class action lawsuit.

UFCW Local 1500 Welfare Fund, which is the largest union of grocery store workers in New York state, filed the proposed class action lawsuit against AbbVie and numerous other companies Monday, contending that AbbVie abused the patent process and colluded with other drug companies to improperly stave off competition for Humira in the U.S. market.