Exelixis has no plans to halt a Phase II trial for its tyrosine kinase inhibitor XL999 or delay moving the compound forward, despite suspending patient enrollment due to cardiovascular events.

Speaking during Exelixis' third quarter earnings call, CEO George Scangos said the trial will be delayed two to three weeks while the firm further evaluates the adverse events. He added that further guidance on next steps will be made in several weeks.

The trial, involving acute myeloid leukemia and ovarian cancer patients, is one of six studies comprising Exelixis' Phase II oncology program for XL999. Other trials are looking at renal cell carcinoma, colon and non-small cell lung cancer.

"I want to emphasize that we're not now discontinuing the trials and we expect to move forward with the compound, which has shown real signs of efficacy," the exec said. "At the same time, we're committed to doing the right thing for patients and we want to move forward responsibly."

XL999 is one of 12 agents that South San Francisco-based Exelixis is developing as part of an agreement with GlaxoSmithKline. Under the arrangement, GSK has the right to choose up to three compounds at proof-of-concept (Phase IIa completion) to further develop, while Exelixis retains the right to those not selected. Milestone payments to Exelixis could reach $275 mil.

According to Exelixis' website, in addition to XL999, three other compounds that GSK has options on, XL784, XL880 and XL647 are all nearing Phase IIa completion.

However, in response to an analyst's question as to whether one of the other three compound might be optioned to GSK before XL999, Scangos said, "That's a little bit of speculation right now and we don't know. We'll have more to say on that in a couple of weeks."

According to Scangos, 10% of the 117 patients who have been dosed with XL999 through the end of September had a cardiovascular event, but he noted there have been no deaths.

"While we monitor these events closely, our interpretation of the data was that the frequency seemed to be consistent with what is seen" with Pfizer's Sutent (sunitinib), he added.

- Jonathan M. Block (j.block@elsevier.com)

Despite Enrollment Suspension, Exelixis Optimistic On XL999

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