Waiting for a drugmaker's clinical trial results can be one of the toughest aspects of biotech investing. Exelixis' (NASDAQ:EXEL) investors now know how long their latest wait will be, after yesterday's announcement of a phase 3 study for one of the company's lead drugs.

XL184 is Exelixis' proposed small-molecule pill to treat a form of late-stage thyroid cancer. Although XL184 is not Exelixis' first phase 3 drug candidate -- its bile duct tumor treatment XL119 failed in testing two years ago -- it's now Exelixis' only compound in active phase 3 development.

Anti-cancer drug studies usually take a long time to produce clinical trial results, but a quick look at clinicaltrials.gov shows that Exelixis expects the first phase 3 data from XL184 around next December. If it's positive, the progression-free survival (PFS) data expected then could be enough for Exelixis to have XL184 approved and on the market by late 2010.

At best, this is a rough guess at XL184's timeline; it can be very tough to predict when data from cancer clinical trials will come out. The data may face several months' delay if either the placebo group or XL184-treated patients end up surviving longer than expected. Also, if the PFS data next year is not good, XL184 won't necessarily be dead in development. The study will look at other measures of efficacy, like overall survival, that could still get the drug approved -- albeit at a much later date.

There are currently no targeted therapies on the market specifically approved to treat the form of thyroid cancer that XL184 attacks. Other drugmakers like AstraZeneca (NYSE:AZN) with Zactima, Onyx Pharmaceuticals (NASDAQ:ONXX) with Nexavar, and Pfizer's (NYSE:PFE) Sutent have also set their sights on treating thyroid cancer, though.

GlaxoSmithKline (NYSE:GSK) still has the option to take control over XL184 if it wants to. If it doesn't decide to do so -- it should make up its mind by this fall -- Exelixis will be free to partner the drug with others. Whatever level of control Exelixis has over XL184, at least we won't have to wait too much longer to finally get some pivotal phase 3 data on the compound.