Exelixis Pushes Ahead

Drug development is never a smooth process. Last year, Rule Breakers pick Exelixis (Nasdaq: EXEL  ) experienced a setback with its lead developmental compound, potential cancer treatment XL999. Yesterday, Exelixis announced that the FDA has allowed it to resume testing of the drug and that it was initiating a phase 1 trial in patients with non-small cell lung cancer.

XL999 is small molecule inhibitor of several important cancer targets. The drug was in the middle of a 131-person phase 2 study for multiple cancer types last year when a data safety monitoring board noticed an increased prevalence of cardiovascular adverse events in the trial.

Once the possible cardiotoxicity issues with XL999 popped up, Exelixis halted all new patient enrollment in the phase 2 study. It did, however, decide to continue to allow dosing of the drug for the 115 patients already enrolled in the study. Even though the trial was halted, XL999 appeared to have some benefit for at least two of the nine lung cancer participants in the trial.

After undergoing an FDA review, human clinical trials of XL999 have been allowed to resume. Exelixis expects to begin enrolling patients for the lung cancer study in the summer.

This week Exelixis also submitted an application to begin human clinical studies for another cancer-targeting drug candidate called XL765. This is the thirteenth compound that Exelixis has brought into the clinic in barely more than three years.

The drug development process isn't just a numbers game, though. Having more compounds in the clinic is not a guaranteed signal of future success; it's really the quality of the compounds that matter. As all Exelixis' drug candidates are in early stage testing, none has yet to be proven in more rigorous late stage studies. That said, Exelixis has proven -- via its development deals with companies like Genentech (NYSE: DNA  ) , GlaxoSmithKline (NYSE: GSK  ) , and Wyeth (NYSE: WYE  ) -- that its preclinical stage work is top-notch. This should give investors some initial confidence in the quality of its clinical stage compounds.

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GlaxoSmithKline is an Income Investor selection.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.