Healthcare stocks fell Thursday afternoon with the NYSE Healthcare Index decreasing 0.9% and the State Street Health Care Select Sector SPDR ETF (XLV) declining 0.8%.

The iShares Biotechnology ETF (IBB) shed 0.5%.

In corporate news, Pfizer (PFE) shares fell 2.6% after the company said Chief Financial Officer Dave Denton will step down on Aug. 15 after accepting a position in the consumer-goods sector.

Biogen (BIIB) agreed to acquire private biotechnology company RayThera in a deal worth up to $1 billion. Biogen shares fell 1.9%.

NovoCure (NVCR) shares dropped 19% after the company said a phase 3 trial evaluating its tumor treating fields therapy in patients with newly diagnosed glioblastoma failed to meet its primary endpoint.

Kardigan (KARD) shares surged 28% to $20.50 in their debut on the Nasdaq. The clinical-stage precision therapeutics company priced its initial public offering of 25 million shares at $16 each. Underwriters were granted a 30-day option to purchase an additional 3.75 million shares.