Want high-growth potential, but without the risk that small-cap stocks bring to the table? Then mid-cap growth stocks could be just what the doctor ordered. Mid-cap stocks, which are traditionally defined as having a market cap of between $2 billion and $10 billion, generally offer higher growth potential than more mature companies, but they usually also come with less hit-or-miss risk present with small-cap stocks since their businesses tend to be more time-tested. 

With this in mind, we asked three of our Fool contributors to name one mid-cap stock they believe could be worth buying in October. Rising to the top of the list are cancer drug developers Exelixis (NASDAQ:EXEL) and Puma Biotechnology (NASDAQ:PBYI), as well as database platform company MongoDB (NASDAQ:MDB).

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High growth for an exceptionally inexpensive price 

Sean Williams (Exelixis): Though it's a company I've suggested investors consider buying on numerous occasions in recent months, as well as a longtime holding in my own portfolio, mid-cap cancer drug developer Exelixis looks to be as undervalued as they come.

Make no mistake about it, the Exelixis story is all about Cabometyx. This is a drug that was the first to hit a "trifecta" in treating second-line advanced renal cell carcinoma (RCC). In other words, it provided a statistically significant improvement in objective response rate, progression-free survival, and overall survival. It also dazzled in the mid-stage Cabosun trial that led to an expanded label into first-line RCC.

In recent months, there's been concern that Exelixis could face increased competition from Bristol-Myers Squibb's (NYSE:BMY) immunotherapy combination of Opdivo and Yervoy in first-line RCC. But what Wall Street might be overlooking is Exelixis' first-to-launch status in RCC, relative to the Bristol-Myers combination, as well as its stellar results in Cabosun and for its second-line RCC approval. Exelixis and Bristol-Myers are also working together on RCC combination studies of their own.

There's also a very good chance that Cabometyx's label is expanded on or before Jan. 14, 2019. This is when the Food and Drug Administration is expected to decide on whether Cabometyx has done enough via the Celestial trial to expand its label to treat advanced hepatocellular carcinoma. Cabometyx easily met its primary endpoint, with an improvement in median overall survival to 10.2 months from 8 months for patients in the placebo arm, and a better than doubling in progression-free survival to 5.2 months from 1.9 months for the placebo group. 

Exelixis looks to be on track to generate $1.72 in annual EPS by 2021, according to Wall Street's consensus, placing it at a price-to-earnings ratio of less than 10, assuming its share price didn't budge. With most high-growth biotechs valued at a significantly more aggressive P/E ratio, I believe there's ample room for upside.

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Reinventing the database

Keith Speights (MongoDB): There's one mid-cap stock that I really have my eye on right now -- MongoDB. The database company's market cap currently stands at close to $4.4 billion, but I think that number could grow much larger over the next few years.

Most of the databases used today are based on decades-old technology. These databases are built only for structured data and are designed to be housed in on-premise data centers. But this approach doesn't meet the needs of today, where massive amounts of unstructured and structured data need to be stored and accessed, preferably in the cloud because of its scalability and lower costs.

That's where MongoDB comes in. The company developed its database around 10 years ago to directly address the areas where the leading databases fell short. To say that MongoDB has become wildly popular is an understatement. The company, which uses a "freemium" model where the basic database is free with premium features and services costing extra, has seen more than 30 million downloads of its MongoDB Community Server database. A survey last year found that more developers want to work with MongoDB than any other database.

Despite giving its basic database away for free, MongoDB's revenue has skyrocketed by a compound annual growth rate (CAGR) of more than 50%. With the global database market continuing to grow robustly, I think that MongoDB is poised to be a market disruptor that could generate huge returns for investors over the long run.

A red-and-silver cancer drug capsule punching and destroying a red cancer cell.

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A recent approval and upcoming data could reward investors

Todd Campbell (Puma Biotechnology): Shares in Puma Biotech got hammered in 2015 after higher-than-expected rates of diarrhea were observed in the trials of its breast cancer drug, Nerlynx. However, Nerlynx still went on to win FDA approval in 2017, and its sales growth since then has me thinking this stock may be underappreciated.  

Approved to delay the return of cancer in HER2+ early-stage breast cancer patients following Herceptin, Nerlynx sales were $20 million in Q4 2017, $36 million in Q1 2018, and $51 million in Q2 2018. That growth has management forecasting sales of between $175 million to $200 million in 2018.  

Revenue could head even higher next year. Puma Biotech secured European Union approval in September and that should increase Nerlynx's addressable patient population significantly. Management hasn't partnered up with anyone for that market yet, so it will benefit 100% from any Nerlynx sales in the region.

Data from key trials could move the needle soon, too. Results from a phase 3 trial for its use in third-line metastatic breast cancer are expected in either Q4 or early in 2019. A phase 2 study evaluating treatment approaches to reduce diarrhea rates is expected in Q4, and some data from a trial evaluating Nerlynx in solid tumor cancers should be available soon as well.  

Because breast cancer is common, and there's an important need for additional treatment options, I think Puma Biotech's opportunity in the U.S. and EU justifies buying it, especially if the data from these trials is good.