Puma Biotechnology shares ended the day down 20.44 percent Monday.
The stock move comes after news that the biotech company is under investigation for allegedly failing to inform shareholders about changes in its breast cancer drug, Neratinib. The law firm Kahn Swick & Foti is focusing on whether Puma's officers violated federal securities laws or breached their fiduciary duties to Puma's shareholders by making false or misleading statements that were inconsistent with the drug's allegedly real trial results that affected its timeline.
The stock was down as much as 35 percent earlier in the day.
Additionally, an abstract for the breast cancer drug is scheduled to be presented next month at the San Antonio Breast Cancer Symposium. The abstract will present results from its phase 2 trial effects of the drug, specifically that it can cause severe diarrhea.
Jana Partners disclosed in an SEC filing Monday that it took sole share stake of 76,074 shares in the biotech company.
Shares of Puma are down 48.85 percent year to date.
Puma Biotechnology declined to comment to CNBC.