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Investing in small biotech stocks is terribly risky, but once in a while a few stocks produce astounding gains. An $8,000 investment in AxoGen (NASDAQ:AXGN) 10 years ago, for example, would be worth about $258,400 today.

The biggest long-term biotech winner of the past decade, though, is Pfizer's latest splurge, Medivation (NASDAQ:MDVN). Eight grand invested in the developer of prostate cancer therapy Xtandi would be worth about $620,800 today.

Let's look at what's caused these stocks to put up outstanding gains, and measure their chances of further success.

AxoGen: on the periphery

Each year in the U.S. alone, an estimated 1.4 million people suffer peripheral nerve damage, resulting in roughly 700,000 surgical procedures. AxoGen's products have become increasingly popular by addressing drawbacks to traditional surgical nerve repair methods. For example, to bridge long gaps between severed nerves, surgeons generally harvest nerve tissue from a second site on the patient, frequently causing the loss of use of the secondary site. The company's Avance Nerve Graft is an off-the-shelf solution that makes second site harvesting, and the complications it can lead to, unnecessary.

AXGN Chart

AXGN data by YCharts

Last month the company reported record quarterly revenue of $10.4 million, marking its 22nd consecutive quarter of double-digit revenue growth compared to previous year periods. The company's bottom line hasn't followed the same trajectory as the top, but with an estimated market about $1.6 billion in the U.S. alone, it's easy to see why investors are willing to absorb years of losses as the medical community becomes more familiar with the company's products.

Considering the size of the space it operates in, and its rapid growth, the stock's recent price of about 7.5 times trailing sales isn't outrageous, but long-term success is far from certain. The Avance Nerve Graft is derived from donated human cadavers, which brings its ability to rapidly increase supply to meet growing demand into question.

Another potential hurdle is Avance Nerve Graft's odd regulatory status. It's a biological product, but AxoGen hasn't submitted a biologic license application to the FDA yet. The Agency currently permits its distribution while AxoGen conducts a large clinical trial comparing outcomes among patients treated with its Avance Nerve Graft to nerve cuffs derived from cows.

The trial is still enrolling patients, with results expected in December 2018. If successful, the stock could continue soaring. If not, however, AxoGen's less popular AxoGuard products probably won't carry the company to profitability on their own. This would make repaying a $25 million loan due in 2020 extremely difficult.

Medivation: to the highest bidder

Xtandi, Medivation's only commercial-stage product, earned its first FDA approval in 2012. It is used to treat which treats advanced prostate cancer and has become a popular therapy for this large patient population. In the first half of the year, worldwide sales rose 35% over the previous year period, putting it on pace to reach about $2.28 billion this year.

If Medivation were entitled to all of that Xtandi revenue, Pfizer's $14 billion buyout offer would seem like a great value, but it isn't. Medivation has to split half of U.S. profits with Astellas Pharma, and receives a royalty percentage in the low teens to the low twenties on ex-U.S. sales. 

Until recently, collaboration revenue from Astellas was enough to outpace Medivation's operating expenses. In 2015 Medivation acquired worldwide rights to talazoparib from BioMarin, and a recent reevaluation of potential milestone and royalty payments related to the promissing oncology candidate tacked a non-cash charge of $674.0 million to Medivation's operating expenses in the second quarter.

MDVN Chart

MDVN data by YCharts

In April, an investigator-sponsored phase 1 trial with talazoparib in a group of heavily pre-treated patients suggests the PARP inhibitor has interesting potential. Among 40 patients with a variety of cancers (a majority of whom had progressed after six or more different treatments), 23 showed a clinical benefit.

Pfizer has a long track record of making big acquisitions, without delivering much bottom-line growth. Pfizer's earnings over the past twelve months of $1.13 per share is less than half of 2006's reported earnings of $2.66 per share.

Although Xtandi should add $0.05 per share to Pfizer's bottom-line immediately following the acquisition of Medivation, I'm afraid this stock rocket is out of fuel. The offer of $81.50 per share, expected to close before the end of the year, might pique the interest of an M&A arbitrageur, but buying Pfizer stock in hopes of Medivation-esque gains would be ill advised.

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.