Thrivent Financial for Lutherans, the Minnesota-based insurer suing the U.S. Labor Department over its fiduciary rule, said Monday it plans to file a preliminary injunction to halt the anti-arbitration clause that is set out in the regulation's best-interest contract exemption.

In a Monday letter to Judge Susan Richard Nelson of the U.S. District Court for the District of Minnesota, the lead attorney for Thrivent, Cozen O'Connor's Andrew Kay, said the insurer would seek an injunction to address concerns the Labor Department won't confront.

Thrivent became the sixth plaintiff to lob a complaint against the fiduciary rule. The insurer filed its suit last September challenging the class-action waiver requirement under the rule's best-interest contract exemption, or BICE.