It's been six months since U.S. drug regulators refused to review

Northfield Laboratories'

(NFLD)

experimental blood substitute, PolyHeme. But company executives still aren't offering much insight into what's needed to get the product back on track.

Thursday, Northfield President Steven Gould said he was optimistic that the "constructive dialogue" with the Food and Drug Administration would lead the agency to reaccept PolyHeme's approval application for review. But Gould declined to give specifics, nor would he explain why these discussions are dragging on so long. Gould spoke at last week's Deutsche Banc Alex. Brown Health Care Conference.

The FDA issued a refuse-to-file letter for PolyHeme on Nov. 19, sending Northfield shares sharply lower. Since then, company executives have snapped on a virtual

cone of silence about the future of its blood substitute -- trying investor patience.

Northfield shares have sunk about 45% since Nov. 19, to less than $7 a share, as of Friday.

What has investors antsy is the fear that Northfield executives are holding back bad news. It took

ImClone Systems

(IMCL)

only two months after receiving its own refuse-to-file letter to

meet with FDA officials and hammer out a new road map for its cancer drug, Erbitux.

In November, Northfield CEO Richard DeWoskin

told

TheStreet.com

that the major area of dispute between the company and the FDA centered on the design of the PolyHeme clinical trial. Northfield was hoping to convince regulators, in a matter of a few months, that its trial was strong enough for consideration. If not, the company would have to conduct another study -- a time-consuming and expensive process -- before it could resubmit PolyHeme for review.

During Thursday's presentation, Gould wouldn't say, specifically, whether the company has made any progress with the FDA on this issue.

"It is difficult to forecast a definitive time line," he said. "We believe we are making good progress, and we are optimistic that we will achieve consensus on the approval of PolyHeme for use in patients with urgent blood loss."

Expand the Trial?

Historically, Northfield executives have been disinclined to offer a lot of information about PolyHeme's clinical progress. The blood substitute is being developed for use in patients in trauma situations in which large quantities of blood have been lost and replacement blood is unavailable. Northfield claims its clinical trials have proved PolyHeme to be lifesaving and safe, but the actual data from which these conclusions are made have never been given a full public airing.

Thursday, Gould opened the kimono a bit, verifying for the first time that the company's pivotal clinical trial tested PolyHeme on 171 trauma patients. By comparison,

Biopure

(BPUR)

, a competitor, enrolled almost 700 patients in its clinical trial for its blood substitute, Hemopure. Another rival,

Hemosol

(HMSL)

, has also run into roadblocks with the FDA, forcing it to rework some of its clinical trials.

The FDA's tougher stance on blood substitutes increases the odds that Northfield will have to test PolyHeme in more patients, especially because an approval in trauma would allow the product to be used on a wide swath of critically ill patients.

Northfield's DeWoskin and Gould weren't available for comment, but company spokesman Bill Schmidle said to his knowledge, the FDA has not yet asked Northfield to conduct a new clinical trial. He acknowledged that the perceived lack of progress on PolyHeme is difficult for investors to understand.

"For people with knowledge of the product, there was really no new ground plowed there," he said, referring to Gould's presentation at the Deutsche Banc conference. "There just isn't much news to report."

Given the troubles encountered by all the blood substitute firms, Northfield's current situation seems par for the course. But if anything, it underscores just how risky the blood substitute field is for investors.