News of the U.S. Food and Drug Administration's "complete response letter" sent shares of Halozyme down 56 percent. Shares of Baxter, which said it plans to amend the drug application, were off by less than 1 percent.

The FDA said patients should no longer be dosed with the experimental immunodeficiency drug, known as HyQ. It also halted trials of a version of ViroPharma Inc's genetic disorder drug, Cinryze, that uses Halozyme's technology for subcutaneous delivery.

Halozyme is developing a number of subcutaneous versions of biological drugs that can now be given only by infusion, including Roche cancer drugs Herceptin and Rituxan. The company is also using its technology to develop a fast acting insulin.

Baxter and Halozyme said on Wednesday that issues raised by the FDA included the possible effects of non-neutralizing antibodies on reproduction, development and fertility.

Halozyme officials said on a conference call with analysts and investors that the FDA's concerns involved proteins derived from blood plasma, not the company's other experimental products.

"Roche has indicated that they do not believe this impacts their programs at this time," Halozyme said in an emailed statement.

The term "non-neutralizing antibodies" refers to antibodies that bind to the targeted proteins, but not to regions that are functionally relevant.

Halozyme shares, which closed at $8.56 on Nasdaq, were trading at $3.80 after hours.

(Reporting By Deena Beasley; Editing by Andre Grenon)

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