Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

Good morning, fellow Foolish investors! Let's check in on four stocks -- Exelixis (EXEL 1.80%), Cell Therapeutics (CTIC), Isis Pharmaceuticals (IONS 1.46%), and Amarin (AMRN -0.53%) -- that are making health care headlines this morning.

Exelixis initiates phase 2 trial for prostate cancer drug
Exelixis, a biotech company focused on the development of small molecule therapies for cancer, announced today that it had initiated a phase 2 trial for cabozantinib, a treatment for castration-resistant prostate cancer that had metastasized to the bone and had not been previously treated with chemotherapy.

The new trial will test cabozantinib with abiraterone, another treatment for prostate cancer, and prednisone, a steroid used to treat allergies and certain types of cancers, against a combination treatment of abiraterone and prednisone.

Cabozantinib attempts to inhibit three key proteins -- MET, VEGFR2, and RET -- which all assist in tumor growth. Exelixis expects to enroll 280 chemotherapy-naive CRPC patients in the trial, which will be conducted across 50 sites in North America. During phase 1 trials, the drug's most common side effects -- reported in equal to or less than 25% of patients -- included gastrointestinal problems, the inflammation of the mouth, hand-foot syndrome, weight loss, hypertension, fatigue, and oral pain.

Exelixis currently has one approved drug on the market, Cometriq, a treatment for medullary thyroid cancer that has metastasized to other parts of the body. Exelixis launched Cometriq in the U.S. in late January, and it has generated $10.7 million in revenue through Sept. 30. Peak annual sales estimates are quite low, at less than $15 million, due to the limited size of the market for MTC treatments.

Another milestone payment for Isis Pharmaceuticals
Isis Pharmaceuticals also announced today that it had received a $2 million milestone payment from GlaxoSmithKline related to its ongoing phase 2/3 study of ISIS-TTR Rx in patients with familial amyloid polyneuropathy, a rare group of diseases that affect the peripheral nerve and heart tissues.

This milestone payment brings Isis' cumulative total upfront and milestone payments from ISIS-TTR Rx to $20 million. The $2 million payment announced today is the second milestone payment Isis has earned toward the $50 million in total milestone payments that the company is eligible to receive as the phase 2/3 study progresses.

Shares of Isis have climbed more than 300% over the past 12 months, fueled by similar upfront and milestone payments. Isis currently has one marketed product, Kynamro, which was approved by the FDA in January as an additional treatment for a rare type of high cholesterol known as homozygous familial hypercholesterolemia.

Another positive development for Cell Therapeutics
Meanwhile, Cell Therapeutics announced that it had just received a $5 million milestone payment from Teva Pharmaceutical for achieving a sales milestone for Trisenox, an arsenic trioxide injection for the management of acute leukemia.

Under the agreement, Cell Therapeutics is entitled to an additional $95 million in future payments based on sales and development milestones at Teva. Cell Therapeutics originally sold Trisenox to Cephalon, which was in turn acquired by Teva for $6.8 billion in 2011.

Although this deal won't make a huge impact on Cell Therapeutics' revenue growth, it's the second piece of good news that its shareholders have received over the past month. In mid-November, shares of Cell Therapeutics soared after it announced a deal with Baxter International to jointly develop and market its bone marrow disorder drug, pacritinib, which is currently in a phase 3 trial. The deal, worth up to $362 million, lets Cell Therapeutics and Baxter jointly market the drug in the United States and grants Baxter marketing rights in overseas markets.

Two new patent allowances for Amarin
Last but not least, Amarin -- which plunged off a cliff in October after the FDA recommended that the company not pursue the approval of its fish oil-based treatment for patients with elevated levels of triglycerides (levels equal to or exceeding 200 mg/DL but lower than 500 mg/DL) -- announced two new patent allowances in the U.S. today.

The two new patent allowances are related to lowering triglyceride levels in patients with severe hypertriglyceridemia (levels equal to or exceeding 500 mg/DL) and in patients with hypertriglyceridemia on statin therapy. The patents won't expire until 2030.

Over the past year, Amarin investors had hoped that its only treatment, Vascepa, would win an FDA panel recommendation for the approval of the drug as a treatment for patients with elevated levels of triglycerides -- which would have helped it reach 20% of the U.S. population and allowed it to escape competition from GlaxoSmithKline's Lovaza, a similar fish oil treatment that will soon face generic competition from Par Pharmaceuticals and Teva.

It's unclear if these patent allowances mean anything, since Amarin only recently announced that its appeal to the FDA for a new panel review had been rejected. However, Amarin is still worth keeping an eye on as a possible acquisition target or long-term comeback play.