GNC Holdings (GNC) - Get Report saw shares plummet Thursday morning ahead of market's open on news that Goldman Sachs (GS) - Get Report downgraded the company's stock. 

GNC fell 5.3% ahead of market's open, hitting $10.55 per share. This was after closing down 1.15% Wednesday, at $11.15 apiece. 

Goldman put a sell rating on GNC's stock, and gave the company a price target of $8 per share. The Pittsburgh-based retail chain that sells supplements and vitamins closed temporarily at the end of December for a re-branding of its stores.

Meanwhile, Mallinckrodt (MNK) - Get Report saw its shares regaining ground lost Wednesday ahead of market's open. 

The company's shares fell 5.8% Wednesday after news broke that it agreed to pay a $100 million fine to settle federal charges that one of the company's subsidiaries carried out a 2013 acquisition in part to kill competition for its specialty drug used to treat seizures in infants and other serious medical conditions.

Mallinckrodt's shares bounced back Thursday. They were up 4.6% pre-market, before falling again. As of writing, Mallinckrodt's shares were up 1.16% ahead of market's open, trading at $47.07 apiece. 

Meanwhile Novocure Ltd. (NVCR) - Get Report , a small-cap biotech, saw shares rise 10% on news that doctors at 500 treatment facilities have been trained to use its Optune drug, which can be used to treat recurrent glioblastoma, a type of cancer. 

The company's shares hit $7.10 apiece ahead of market's open. 

And Zyne Pharmaceuticals (ZYNE) - Get Report lost nearly all of the ground it gained Wednesday, falling 13% ahead of market's open. 

The small-cap biotech that focuses on cannabis saw shares boost 15% Wednesday after the announcement of a common stock offering. 

Shares fell to $19.55 apiece ahead of market's open. 

Cytori Therapeutics

(CYTX)

was up 15% to $1.90 after announcing it would acquire assets from privately held Azaya Therapeutic.