Develop a drug; get bought by a big pharma. It's the biotech way.

Only a few biotechs have survived long enough to develop multiple drugs. And as Genzyme showed, even those hardy veterans aren't immune to being taken out.

While any biotech, at any point, can technically be called a takeover target, the potential really starts heating up when it has a drug that's close to bringing in revenue. As patent cliffs for its current stable of blockbuster drugs approach, pharmaceuticals' lust for new late-stage assets has increased dramatically. Here are five potential takeover targets among biotech contenders.

Big potential, waiting for data
Bloomberg reported last week that Exelixis (Nasdaq: EXEL) had hired Goldman Sachs to shop the development-stage drugmaker around to the highest bidder. With an asset like XL184, there should be someone interested -- if the reports are actually true, of course.

XL184 is in a phase 3 trial for medullary thyroid cancer, but its phase 2 data in ovarian and prostate cancer makes the drug a potential blockbuster. Medullary thyroid cancer may get XL184 on the market, but those larger markets could truly propel it to blockbuster status.

The biggest knock against Exelixis as a takeover target is that it has numerous partners -- GlaxoSmithKline, Bristol-Myers Squibb (NYSE: BMY), Roche, Pfizer (NYSE: PFE), and others -- which might deter someone from swooping in and buying the company. Fortunately most of those partners are in full control of the drugs they licensed from Exelixis, so an acquirer wouldn't really need to deal with Exelixis's partners. They'd simply need to cash the milestone and royalty checks if those partnered drugs prove successful.

Clinical trial success, waiting for approval
In the less-risky category, we have Amarin (Nasdaq: AMRN) and Vertex Pharmaceuticals (Nasdaq: VRTX), which both have multiple successful phase 3 trials under their belts.

Amarin is developing AMR101to treat patients with high triglyceride levels. The company owns worldwide rights to AMR101 and has no other drugs in its pipeline, so it's about as clean a purchase as you can get. It would be a good fit at Pfizer to replace Lipitor sales, but I could also see AstraZeneca or Merck coming in to market it alongside Crestor or Vytorin, respectively.

Vertex has already licensed its hepatitis C drug telaprevir to Johnson & Johnson, but that represents a clean division of labor between countries, with Vertex retaining the rights to North America. Thus, I don't consider that partnership a major obstacle for any company keen to buy Vertex.

The bigger issue for a potential Vertex buyer is price. With a market cap of $10 billion, there's a heck of a lot of sales already baked into its shares. I suspect potential buyers will probably wait to confirm that Vertex gets telaprevir approved, then bump it down into the next category while they confirm that the peak sales everyone expects are actually possible.

FDA approval, waiting for sales
Human Genome Sciences' (Nasdaq: HGSI) and Dendreon's (Nasdaq: DNDN) lead drugs have cleared their FDA hurdles, but they haven't been bought out -- yet. Big pharma is most likely waiting to see exactly how well the drugs will sell.

Human Genome's lupus drug Benlysta was approved last month, so we're still months away from knowing how well the launch is going.

The company also shares the drug with marketing partner Glaxo, which makes it a little less attractive to any potential acquirer not named "Glaxo." Keep in mind, though, that Eli Lilly bought ImClone Systems knowing that it would have to deal with Bristol-Myers to market Erbitux, so this reservation's still not a dealbreaker.

Dendreon is farther along in its sales of prostate cancer treatment Provenge, but the ramp has been slow, because Dendreon needed to increase manufacturing capabilities. Because of the complex nature of the immunotherapy, I suspect big pharma wants to wait until it's sure about the potential for the peak sales of Provenge before jumping onboard.

Waiting, waiting, waiting
There's always something that big pharma can wait for -- clinical trial results, regulatory approval, more sales -- that might deter it from buying a biotech right now. Investing in the hopes of gaining a quick buyout is often a recipe for disappointment.

If on the other hand, you're convinced that the next value-building milestone will fall in the company's favor, go ahead and buy. If you're right, you'll profit whether the company gets taken out or not.

Looking for more stock ideas? Here are six stocks that David and Tom Gardner think you should be watching.