Washington Editor

Cerus Corp. and partner Baxter Healthcare Corp. stopped a Phase III trial of their pathogen-inactivated red blood cell program.

The companies reached the decision after two study patients developed antibodies to red blood cells treated with S-303, a compound used in the companies' investigational pathogen-inactivation system for red blood cells, Concord, Calif.-based Cerus said in a prepared statement.

Cerus' stock (NASDAQ:CERS) closed Thursday at $5.29, down $2.42, or 31.4 percent.

Cerus and Baxter, of Deerfield, Ill., have worked together for a decade developing the Intercept Blood System for platelets, plasma and red blood cells.

Stephen Isaacs, Cerus' president and CEO, told BioWorld Today the companies have not determined the source of the antibodies in the red blood cell area, cautioning that the findings have nothing to do with the platelet and plasma programs.

"The data is being analyzed right now," he said. "Patient safety is our highest priority, so we felt it was appropriate to voluntarily halt the trial until we get a better understanding of what's going on. We did this as a precautionary action."

He added that none of the trial patients demonstrated clinical adverse events after transfusion with the S-303-treated red blood cells. S-303 is activated by the change in pH created by the introduction of red blood cells into the disposable set.

At this point, Isaacs said, it's too early to say when the trial will resume.

"We had to do a lot of work leading up to the Phase III trial. First, we did a very extensive analysis in animals showing no generation of any type of antibody or response to our treatment process. We did a Phase Ib study in normal volunteers where people were transfused four or five times and there was absolutely no trace of antibodies generated," he said. "Quite frankly, this was an event which the protocol anticipated in terms of detecting it, but we were very surprised to see it happen."

The antibody response was discovered using a standard blood test that was part of the protocol.

The platelet and plasma programs use amotosalen (S-59), which is activated by an ultraviolet light. The platelet program is on the market in Europe and the subject of studies in the U.S.