Source: Flickr user sboneham.

I tend to be more of a growth investor than a value investor, and as a result I often lean toward buying shares of biotechs that are working on intriguing late-stage medicines. However, not all biotech stocks are created equal -- some are just too risky even for aggressive investors like me. For example, there are just too many questions about these three biotech stocks for me to buy into them.

No. 1: MannKind Corp. (NASDAQ:56400P706)
MannKind is a polarizing stock with just one drug on the market. Investors tend to either love or hate the company, but personally, I'm agnostic on it.

Sure, the potential to offer diabetics an inhalable insulin alternative to injections is intriguing, but the verdict is still out on whether the dosing benefit of MannKind's inhalable Afrezza can overcome the hurdle of periodic testing requirements tied to the drug's lung safety risks.

Although it has a high-profile marketing partner in Sanofi, the maker of blockbuster long-lasting insulin Lantus, MannKind did not line up that deal until after the Food and Drug Administration approved Afrezza last summer. As a result, MannKind is saddled with debt.

Even with Sanofi's marketing might behind it, Afrezza's early sales have been more a trickle than a flood. After launching in February, Afrezza revenue totaled just 1 million euros in the first quarter, according to Sanofi. Since Afrezza's sales ramp is slow and MannKind's balance sheet is questionable, I'm content to wait on the sidelines until the inhalable insulin can confirm its blockbuster potential and validate the company's Technosphere platform for potentially developing more inhalable drugs.

Source: Arena Pharmaceuticals,

No. 2: Arena Pharmaceuticals (NASDAQ:ARNA)
Arena Pharmaceuticals could be the poster child for companies with big promise that have yet to deliver profit.

The company markets the obesity pill Belviq. Given that obesity rates globally are soaring, and spending on obesity eclipsed $147 billion in the United States alone in 2008, investor optimism following Belviq's launch was sky high.

However, sales have fallen far short of forecasts. The company's revenue totaled just $12.3 million last quarter, including $6.6 million tied to Belviq prescriptions. Meanwhile, the company forked out $22 million in research and development costs and another $8.4 million on administrative expenses during the quarter.

Granted, Arena Pharmaceuticals' plans include a long-lasting version of Belviq that could reduce patient pill burden, a potential refiling for approval in Europe, and some intriguing drug candidates in its pipeline. But with anemic Belviq sales, new competitors on the market, and the unlikelihood of any dramatic spending cuts, I'm staying away from Arena.

No. 3: Amarin Corporation Plc (NASDAQ:AMRN)
Sales of Amarin's triglyceride-lowering medicine Vascepa are climbing, but they remain far south of bullish forecasts leading up to its approval in 2012. Vascepa's revenue potential is being weighed down while the FDA waits for cardiovascular outcome data on the drug before considering whether to expand its addressable patient population.

Currently, Vascepa is used to lower trigylcerides in patients with ultra-high levels that are north of 500 mg/dl. That's a narrow pool of patients. Furthermore, Amarin estimates just 3.8% of the 3.5 million people with triglyceride levels over 500 mg/dl receive treatment.

With such a small patient population, it's probably not surprising that Vascepa's sales totaled just $15.6 million in the first quarter. Those sales fell far shy of the combined $24.7 million in sales, general, and administrative costs and $12.6 million in research and development expenses that Amarin racked up last quarter.

Although interim data from the highly-anticipated cardiovascular trial should be available next year, complete results are not expected to be released until 2018. That means we're still years away from any potential label expansion that could significantly move the revenue needle. Given that costs are outstripping income and we might not get an FDA review of the cardiovascular outcomes data until 2018 or 2019, Amarin is just too risky for me.

Tying it together
MannKind, Arena Pharmaceuticals, and Amarin all market drugs that were once believed to have blockbuster potential, but each company risks falling far short of pre-launch estimates.

These companies serve as an important reminder that investors should approach all biotech stocks with a healthy dose of caution because there are no guarantees for sales success -- even if a product makes its way past regulators. Although each of these companies could turn the corner someday, I think they're too risky to make such a bullish bet yet, and that's why I just can't bring myself to buy them.