Let's take a quick look at four stocks -- Herbalife (NYSE:HLF), Exact Sciences (NASDAQ:EXAS), AstraZeneca (NYSE:AZN), and Johnson & Johnson (NYSE:JNJ) -- which could loom large in health care headlines this Tuesday morning.

Herbalife investors brace for another Ackman assault
Herbalife is down more than 3% in pre-market trading, following yesterday's 11% plunge after Bill Ackman's latest assault on the nutritional products company.

During an interview on "Bottom Line" yesterday, Ackman announced that he would show how Herbalife's Nutrition Clubs were equivalent to Enron's fake trading rooms. Ackman also confirmed that his hedge fund, Pershing Square Capital Management, had spent $50 million of investors' money to prove that Herbalife is a "pyramid scheme". Ackman's webcast is scheduled for 10:00 AM EDT at this dedicated website.

Ackman has been shorting Herbalife since May 2012, eventually loading up a $1 billion bet against the company by December of that year. Yet Herbalife has survived Ackman's assault, thanks to support from Carl Icahn's Icahn Capital, which held 17% of Herbalife's outstanding shares as of March 31.

Herbalife claims that there is nothing nefarious about its salesperson recruiting techniques, which is similar to other multi-level marketing operations like Avon, Nu Skin, and Mary Kay. But Ackman asserts that Herbalife intentionally targets lower-income individuals with unrealistic promises, with some salespeople ending up with monthly losses after deducting expenses. Ackman's core argument -- a familiar one used against multi-level marketing firms -- is that Herbalife's base will eventually crumble when no new recruits can be found.

Exact Sciences posts an in-line loss
Meanwhile, Exact Sciences just reported its second quarter earnings. The company, which specializes in molecular diagnostics testing, reported no revenues for the quarter, but its net loss of $0.24 per share was in-line with analyst estimates. The company's main product, the Cologuard test, is a non-invasive, stool-based DNA test designed to detect defective DNA markers. Cologuard has not been approved by the FDA, but an FDA advisory committee unanimously recommended its approval in March. If approved, Cologuard is expected to generate peak sales between $1 billion to $2 billion.

For the quarter, Exact reported operating expenses of $19.6 million. Research and development expenses climbed 11% year-over-year to $7.2 million, while sales, general, and adminstrative expenses rose 78.9% to $12.4 million, mainly due to the hiring of an 80-person sales force for the anticipated Cologuard launch.

Shares of Exact remain unchanged in pre-market trading after the earnings report. The stock has climbed 15% over the past 12 months.

Advaxis and AstraZeneca's MedImmune start new clinical trials
AstraZeneca's MedImmune and Advaxis have announced a clinical trial collaboration to evaluate MedImmune's investigational anti-PD-L1 immune checkpoint inhibitor, MEDI4736, when combined with Advaxis' cancer immunotherapy vaccine ADXS-HPV as a treatment of HPV-associated cervical, head, and neck cancers.

The phase 1 trial will establish the recommended dosages, while the phase 2 trial will evaluate the safety and efficacy of the combinational regimen. Advaxis will fund and conduct both trials, but MedImmune has the first right of negotiation for future development of the combination. The partnership is a positive development for AstraZeneca's immunotherapy pipeline -- one of the main reasons Pfizer tried to acquire the company earlier this year.

MEDI4736 blocks a tumor's signal which helps it evade immune detection, making it easier for the patients' immune system to "see". ADXS-HPV enhances the ability of immune cells to attack the tumor. MedImmune signed a similar collaboration with Incyte in May, which will test MEDI4736 with Incyte's INCB24360 to study the combination's efficacy in treating solid tumors such as skin, lung, head, neck, and pancreatic cancers.

J&J announces a $5 billion stock buyback
Last but not least, Johnson & Johnson announced a $5 billion stock buyback after the market close yesterday. The program has no time limit and can be discontinued at any time. J&J currently has about 2.8 billion shares outstanding.

Investors should note that J&J topped analyst estimates on both the top and bottom lines last week. Earnings rose 12.2% year-over-year as revenue climbed 9.1%, thanks to strong performance from its pharmaceuticals segment. Shares of J&J have rallied 10% over the past 12 months.