After Bristol-Myers Squibb (NYSE: BMY) returned the rights to cabozantinib, then called XL184, to Exelixis (Nasdaq: EXEL), shares fell 16% and another 5% the following day. Thursday afternoon, Exelixis filed an 8-K saying that Bristol-Myers had also handed back XL281; shares were up 2% on Friday.

It seems investors are learning. Since regaining full rights to cabozantinib 13 months ago, Exelixis is up 137%. Positive data in multiple cancer types has Bristol's relinquishment looking rather silly, to put it nicely.

Exelixis implied that Bristol is returning XL281 because the drug doesn't fit into the latter's "overall research and development priorities and pipeline products." That's essentially the same reason that Bristol gave for returning cabozantinib.

Unfortunately, it doesn't look as though we'll see XL281 follow the same fate of cabozantinib. Exelixis doesn't plan to develop XL281 further once it gets the drug back. Reading between the lines, we can guess that both companies concluded that XL281 wasn't working well enough to compete with Roche's vemurafenib and compounds being developed by GlaxoSmithKline (NYSE: GSK) and Novartis (NYSE: NVS) that are in the same class.

So why aren't investors freaking out? First and foremost, a majority of Exelixis' value is wrapped up in cabozantinib. XL281 isn't the lead compound, as cabozantinib was when it was handed back.

Developing a second drug is important for the company's long-term future, but Exelixis has plenty in the pipeline beyond XL281. It still has four compounds partnered with Bristol, in addition to partnerships with Glaxo, Sanofi (NYSE: SNY), Roche, and Daiichi-Sankyo.

XL281 doesn't look like it'll be cabozantinib 2.0, but when you already have one cabozantinib, that doesn't come as such a big blow.

Interested in keeping track of Exelixis as it develops cabizantinib? Add it to My Watchlist, which will help you keep track of all our Foolish analysis on Exelixis.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Check out his holdings and a short bio. The Motley Fool owns shares of GlaxoSmithKline and Exelixis. Motley Fool newsletter services have recommended buying shares of Exelixis, GlaxoSmithKline, and Novartis. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.