Free speech protections will be at the center of a legal fight early next month as pharmaceutical companies challenge a new federal rule requiring them to disclose drug list prices in television ads.

Merck & Co., Eli Lilly and Co., Amgen Inc., and an advertising trade association filed a lawsuit June 14 in federal court, claiming the Centers for Medicare & Medicaid Services lacks the statutory authority to impose such a requirement.

The final rule came out May 8 and...