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

NEW YORK (Reuters) - Short-seller Andrew Left's Citron Research expanded on his prediction that U.S. drugmaker AbbVie Inc's ABBV.N stock would fall to $60 a share, arguing that new regulations to speed biosimilar drugs to the market and reform rebates will hurt revenue from the company's top-selling drug, Humira.

Left initially tweeted the $60-a-share call last Thursday. The shares fell as much as 7 percent that day before closing 4.7 percent lower. AbbVie shares closed up 2.4 percent at $91.54 on Tuesday.

In a research note published on Tuesday afternoon, Left detailed his skepticism about AbbVie’s prospects. “There finally seems to be changes coming to the system,” he wrote.

AbbVie declined to comment on the report.

Other analysts have said that deals AbbVie has signed with rivals such as Amgen Inc AMGN.O, Biogen Inc BIIB.O and Mylan NV MYL.O to prevent them from launching biosimilar competitors to Humira until 2023 will likely protect the company from any of the moves regulators make in the near-term.

Last week, U.S. Food and Drug Administration Commissioner Scott Gottlieb laid out a plan to increase biosimilar competition for biologic drugs. The Trump administration has proposed a rule that would scale back protections currently in place that allow rebates between drug manufacturers and insurers and pharmacy benefits managers.

Humira, for rheumatoid arthritis and other autoimmune diseases, is the world’s top selling prescription medicine with sales expected to exceed $20 billion this year, according to Thomson Reuters data.