WASHINGTON (Reuters) - About a dozen advocacy groups and unions, including Public Citizen and the American Federation of Teachers, wrote the Federal Trade Commission on Thursday urging it to consider blocking drugmaker AbbVie Inc's ABBV.N planned $63 billion purchase of Botox maker Allergan Plc AGN.N.

FILE PHOTO: A screen displays the share price for drugmaker AbbVie on the floor of the New York Stock Exchange July 18, 2014. REUTERS/Brendan McDermid/File Photo

The combination of two of the world’s biggest pharmaceutical companies, which was announced in June, would feature a portfolio that includes the world’s top-selling medicine - AbbVie’s rheumatoid arthritis drug Humira - as well as its cancer drugs and Allergan’s blockbuster wrinkle treatment, Botox, and big-selling dry eye drug Restasis.

In the letter seen by Reuters, Consumer Action and others, working with the Service Employees International Union (SEIU) and American Federation of State, County, & Municipal Employees (AFSCME), noted the deal would create the fourth largest pharmaceutical company with strong markets for many drugs at a time when rising prices of many medicines have become a hot political issue.

For example, a four-week supply of Humira has a list price of about $5,174, amounting to more than $60,000 for a year. Humira sales reached $20 billion in 2018.

“We request that the Commission investigate this proposed merger thoroughly and take all necessary action, includ(ing) blocking the merger, to prevent further harm to consumers,” the groups said in the letter.

The FTC declined comment. Neither of the companies immediately responded to a request for comment.

AbbVie has previously said that it would work with the FTC to discuss asset sales, which are sometimes needed to make deals conform with antitrust law.

The organizations urged the FTC to go further than simply identifying where there are product overlaps between the two companies, which is a big part of a traditional antitrust review of a merger of drug companies.

They argued in particular the merger could lead to a broader use of volume-based rebates or other incentives to insurers or pharmacy benefit managers. Drugmakers have said that their need to provide discounts and rebates to payers is one reason they have to keep prices high.

The group also criticized Allergan for practices like attempting to transfer patents for Restasis to a Native American tribe to protect it from patent challenges and delay market entry of cheaper generic versions.

“These practices should receive careful attention as part of the Commission’s investigation into the effects of this proposed merger and if an adequate remedy can not be found, the Commission should challenge this acquisition,” the group said in their letter.