Amgen (NASDAQ:AMGN) has made some investors really, really rich. Accounting for stock splits through the years, Amgen has delivered a return of close to 58,000%. An initial investment of $10,000 back in 1983 would now be worth in the ballpark of $5.9 million. And a big chunk of that gain has come in just the past 10 years.

Is Amgen still a stock that can make investors a lot of money in the future? Or is the big biotech past its prime? 

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A different dynamic

My Motley Fool colleague Cory Renauer summed up Amgen's second-quarter results in two words: treading water. Actually, Cory was being generous. The big biotech sank a little. Amgen reported a year-over-year revenue decline of 3%.

The problem is that the drugs that have generated billions of dollars in sales for Amgen in the past are losing steam. Sure, they're still making tons of money. But they're either growing very sluggishly or sales are declining.

Amgen scored a huge court victory recently in defending patents for its top-selling drug, Enbrel. However, the company's lawyers can't help improve the immunology drug's 4.7% year-over-year growth rate notched in the second quarter.

The legal eagles can't do anything about sliding sales for Amgen's other top products. Sales for Sensipar in Q2 plunged nearly 71% from the prior-year period. Neulasta's sales sank 25%. Epogen and Aranesp were down 10% and 8%, respectively.

It's not that Amgen doesn't have some stars in its current lineup, though. Blockbuster osteoporosis drugs Prolia and Xgeva continue to show solid momentum. 

Looking to the newbies

Amgen also has some newer drugs with a lot of potential. Aimovig is off to a good start, racking up sales of $142 million in the first six months of this year as its commercial launch gains traction. Analysts project peak sales for the migraine drug could reach $1.2 billion.

The biotech has high hopes for leukemia drug Blincyto as well. The bispecific T-cell engager (BiTE) antibody is especially gaining sales momentum in international markets. New osteoporosis drug Evenity could rake in over $500 million annually.

But it's fair to say that Amgen has experienced some disappointments with new drugs, too. Cholesterol drug Repatha isn't anywhere close to the blockbuster level the biotech anticipated. Amgen has also made a concerted push to develop biosimilars, but all of its biosimilars combined generated only $137 million in the first half of 2019.

Pipelines are the lifeblood of biotechs. If you look at Amgen's pipeline, though, there's something that will jump out immediately: It's heavily skewed toward early-stage programs. Amgen has only five late-stage programs and three mid-stage programs, compared to 24 programs in phase 1.

The biotech is also betting heavily on oncology, with the majority of those early-stage programs targeting cancer. Unfortunately, cancer drugs in phase 1 testing have the lowest odds of any therapeutic area of eventually winning FDA approval. Out of every 100 early-stage cancer drugs, only five will ultimately gain approval, based on historical data.

Wild cards

It's possible, of course, that one of Amgen's pipeline candidates could become a huge success and change the company's trajectory. There's also another wild card that could shake things up: the possibility of a game-changing acquisition.

Amgen is a prime candidate to go on a buying spree. The company had nearly $21.8 billion in cash, cash equivalents, and marketable securities as of June 30, 2019. It could easily afford to acquire one or more smaller biotechs. Alternatively, the company could use its cash to license promising therapies instead of buying another drugmaker outright.

Acquisitions don't always work as well as intended, though. Amgen spent $10.4 billion in its 2013 buyout of Onyx to get multiple myeloma drug Kyprolis. The company is a long way off from getting a return on that investment.

Is Amgen a buy?

All of this might sound like I'm down on Amgen. Actually, I think the stock is a pretty good fit for income-oriented investors. Its dividend currently yields nearly 3%. Amgen also claims an impressive track record of boosting its dividend, with the payout growing 138% over the past five years.

But is Amgen a buy for most investors? I don't think so -- at least not yet. I wouldn't rule out the possibility of its making investors a lot of money over the long run. However, my view is that it's better to wait and see how Amgen's early-stage pipeline candidates progress. There are plenty of other biotech stocks with more promising growth prospects. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.