What happened

Gilead Sciences (GILD -0.84%) stock was off to a good start this week, rising by more than 5% on Monday against the modest (0.4%) gain of the S&P 500 index. The prominent pharmaceutical company had some good news to impart about an important cancer drug.

So what

Gilead announced that morning that its breast cancer treatment Trodelvy recorded "statistically significant and clinically meaningful" results in a phase 3 clinical trial. Trodelvy, which already holds Food and Drug Administration (FDA) approval to treat specific forms of breast and urothelial cancer, was being evaluated as a combination therapy to treat patients with HR+/HER2-metastatic breast cancer.

The company added that the safety profile of the drug was consistent with earlier studies. Gilead said that it has submitted a supplemental Biologics License Application (sBLA) to the FDA for the indication.

In its press release on the matter, Gilead quoted its chief medical officer Merdad Parsey as saying that the trial's results "are important for the breast cancer community, and we are encouraged by the potential this may have in helping patients who otherwise have limited alternatives."

Now what

Gilead wasn't the only publicly traded pharmaceutical company reporting fresh results from a breast cancer drug on Monday. Rival AstraZeneca (AZN -0.75%) announced that its Enhertu -- developed in partnership with Japan's Daiichi Sankyo -- performed similarly well in a phase 3 trial for a new indication. 

According to AstraZeneca, its treatment also demonstrated "a statistically significant and clinically meaningful improvement," in this case for progression-free survival in patients previously administered another cancer medication, Kadcyla, developed by Genentech.

Of those two pieces of news, investors were clearly more excited about Gilead -- they bid up AstraZeneca's share price only by only 0.3% on the day.