One of the things that makes AstraZeneca (AZN -0.25%) a top growth stock is all of the different drugs it has contributing to its top line. It has multiple blockbusters that generate more than $1 billion in annual revenue for its operations. That diversification is important as investors know they aren't putting all their hopes on just one product.

What should get investors even more excited, however, is the prospect that it could have another incredibly promising drug to count on: Enhertu. 

Some promising results

Enhertu is a cancer drug that AstraZeneca is developing and commercializing in partnership with Japanese drug manufacturer Daiichi Sankyo. The drug is still in its early stages and isn't a blockbuster just yet. But earlier this month, the company unveiled some impressive results from a study looking at how effective Enhertu was in treating breast cancer tumors with low levels of HER2, which is a receptor that it targets.

Last year, AstraZeneca released a promising study about the drug's effectiveness in treating cancer with high levels of HER2. In that study, Enhertu showed that it could reduce the risk of death or the cancer spreading by 72% compared to another HER2-focused treatment, trastuzumab emtansine. In May, the Food and Drug Administration approved Enhertu in treating adults with HER2-positive (i.e high levels of HER2) breast cancer if it is unresectable or metastatic and they have previously taken an HER2-targeting treatment.

The phase 3 study AstraZeneca released this month, however, suggests that the drug could go further and also be effective for breast cancer patients with low levels of HER2. In these cases, the drug was able to reduce the risk of death or progression by 49% when compared against chemotherapy. The healthcare company is going to send the data from the phase 3 trial to the Food and Drug Administration in the hopes that the agency expands the scope of approval for the drug, so that it can also be used on patients with low levels of HER2.

The takeaway for investors here is that this will likely open up a much broader pool of patients that the drug could help.

This could mean billions more in revenue

Breast cancer is the most common type of cancer in the U.S. and it claims more than 40,000 lives each year. By being effective for low levels of HER2, that may result in three times as many patients using Enhertu. Analysts at Jefferies already projected billions in revenue for the drug. Now, with Enhertu potentially reaching even more patients, they have increased their forecast for the drug's peak annual sales to $6.6 billion, which would represent an additional $2.5 billion from their earlier projection.

This is great news for AstraZeneca because over the long term, it will mean an even more diverse business. Last year, AstraZeneca acquired rare disease company Alexion Pharmaceuticals in an effort to widen its product mix. Rare disease revenue totaled $1.7 billion for the first three months of 2022 and the segment now accounts for more than 15% of AstraZeneca's total product revenue.

Oncology remains the company's top business, however, with $3.4 billion in revenue from cancer drugs making up 30% of product revenue.  And that's with Enhertu still in its early growth stages. This past quarter, it contributed $11 million in product sales and $86 million total when including collaboration revenue.

There is no shortage of growth opportunities for the business to pursue, which makes it a promising stock to own.

Buy AstraZeneca stock on this news?

Year to date, AstraZeneca has proven to be a fairly stable investment to own, with shares up a modest 3% while the S&P 500 has crashed by 20%. The stock trades at a forward price-to-earnings multiple of 16, which is in line with the average stock on the Health Care Select Sector SPDR Fund. 

Although it's not a bargain-basement type of deal, this can still be a growth stock worth buying and holding right now. Not only does it provide safety and long-term growth, but it also pays a decent dividend yield of 2.3%, which is better than the S&P 500 average of around 1.4%.

Overall, this news makes an already promising stock look like an even better buy.