Biotech stocks' notorious pop-and-drop nature makes them among the riskiest you can own; however, pipeline news can lead to substantial returns for investors willing to take on the risk. For example, in the past three months, these three biotech stocks have soared by more than 100% since offering up insight into their pipelines. Clearly, investors think these stocks have running room, but that doesn't necessarily mean you should be chasing them higher. Let's take a closer look at them and see if any of them are worth buying for long-haul portfolios.

No. 1: Sarepta Therapeutics (NASDAQ:SRPT), up 136%.

In 2013, optimism that Sarepta Therapeutics could be among the first to notch FDA approval for a drug designed to treat Duchene's muscular dystrophy, or DMD, had its shares eclipsing $50. However, worries that eteplirsen would fail to win over regulators led to a 76% drubbing shortly thereafter.

The company's manic 2013 has been followed up by an equally nausea-inducing 2014, and this year, with shares having doubled in the past three months following a rolling FDA filing for approval of eteplirsen, Sarepta Therapeutics investors are probably wondering if a drop is looming.

They're right to be nervous.

Eterplirsen is an interesting drug, but it's far from the only DMD drug advancing through clinical trials and the FDA. Other companies, including BioMarin and PTC Therapeutics, are in the hunt, too. Additionally, eteplirsen is only designed to help 13% of DMD patients who possess a particular genetic makeup, and that means that its commercial opportunity is limited. Because of Sarepta Therapeutics' past stumbles and the competitive marketplace,  this stock just doesn't seem to me to be worth chasing higher.

No. 2: Heron Therapeutics (NASDAQ:HRTX), up 130%
Shares in Heron Therapeutics have been on a tear following news that its lead drug candidate, Sustol, delivered positive results in a phase 3 study.

The positive data is welcome news for Heron Therapeutics investors, given Sustol's shaky history. Sustol, an extended-release version of therapies commonly used to control vomiting and nausea in chemotherapy patients, has already failed to win FDA approval twice.

The company hopes that its most recent study results will prove that the third time is a charm. In that study, adding Sustol to two current standards of care further reduced vomiting and nausea versus standard of care alone. Overall, 64.7% of patients in the 900 person study had their vomiting controlled by the regimen that included Sustol, versus 56.6% for the standard-of-care regimen.

Obviously, there's a significant need to improve the quality of life for chemotherapy patients that Sustol could address, but I'm just not convinced yet that Heron Therapeutics can win over the FDA or that Sustol will be game-changing enough to warrant its widespread use. Because of that, this company is too risky for me to buy at these prices.

Source: Affimed NV.

No. 3: Affimed NV (NASDAQ:AFMD), up 123%
Affimed is a clinical-stage company that's working on immuno-oncology therapies that allow a patient's immune system to find and kill cancer cells more easily. The potential to reshape patient treatment and outcomes in this manner is endlessly intriguing, and for that reason, Affimed's shares are jumping.

Initially, Affimed is focusing its research efforts on modifying the immune system's natural killer cells and T-cells. The company's most advanced therapy in trials is AFM13, a drug that's in phase 2 trials for Hodgkin's lymphoma. AFM13 modifies natural killer cells to identify, bind to, and destroy CD30, a protein commonly expressed in Hodgkin's lymphoma patients, as well as other lymphomas. Overall, Affimed estimates that between 6,000 and 8,000 relapsing or refractory patients with CD30-expressing cancers could benefit from AFM13.

In a small phase 1 trial, 77% of 13 evaluable patients had their disease under control after treatment with AFM13. That finding led the Leukemia and Lymphoma Society to commit $4.4 million to help fund ongoing phase 2a research.

Although Affimed's early-stage success is encouraging, 90% of cancer drugs entering phase 1 fail to make it to market, so there's a lot that can go wrong from here. Having said that, there are two reasons I'm still interested in buying Affimed.

Firstly, I think the company's $500 million market cap is too low, based on its addressable market and the valuation awarded to its peers. Secondly, Affimed has an intriguing collaboration and equity investment in Amphivena, a private equity-funded biotech company that Johnson & Johnson has an option to acquire if Amphivena accomplishes certain milestones, including advancing a drug into human trials. If J&J eventually acquires Amphivena, it could validate Affimed's research and justify a higher valuation. 

Looking ahead
All three of these companies are moving higher based on their pipeline and their potential, rather than their sales or earnings. In fact, none of them generates any revenue or profit from product sales, and because of that, investors should approach these, and all clinical-stage biotech stocks, cautiously. 

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.