U.S. drug regulators completed their review of a blood cancer drug from Spectrum Pharmaceuticals ( SPPI) Friday but delayed approval until the company submitted more data.

Spectrum said it plans to respond to the Food and Drug Administration request by Wednesday, but final approval of its drug Zevalin probably will be delayed by two months.

"This is clearly not the result we were looking for, but we're still very confident about Zevalin's approval," said Spectrum spokesman Paul Arndt, in an interview Sunday night.

Spectrum is seeking FDA approval to market Zevalin as a first-line treatment option for patients with non-Hodgkin's lymphoma (NHL). The drug is already approved for more advanced NHL patients who have been treated previously with other drugs.

Zevalin was first approved in 2001, but the drug has been a commercial disappointment. Spectrum is hoping that the expanded approval will spur doctors to treat more of their NHL patients with the drug.

FDA asked Spectrum to supply more data to support a medical claim the company is seeking for Zevalin's label, said Arndt. Spectrum will be able to respond to FDA's request with data from its existing clinical trial; and FDA has not asked the company to conduct any new Zevalin studies, added Arndt.

"This is not a circle-the-wagons type of request," said Arndt of the FDA's complete response letter. "We're getting the information together as quickly as possible and hopefully, the FDA will respond in kind."

The FDA is expected to classify the new Zevalin data as a so-called "class 1 response," which gives the agency 60 days to review and issue a new approval decision.

Spectrum shares fell about 20% to $5.27 in light and early pre-market trading Monday, not unexpected given the recent run-up in the stock and the high hopes for Zevalin's speedy approval.

As I discussed in Friday's Biotech Stock Mailbag, Spectrum's share price has jumped from $2.50 to as high as $8 since May -- an increase that was likely a bit premature given the uncertainty over Zevalin's future sales.
Adam Feuerstein writes regularly for TheStreet.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.