UPDATE 1-Halozyme drug delivery technology in doubt after trial halted

* Says partner ViroPharma halts trial after unexpected antibody production

* Company says antibodies produced in reaction to its technology

* Says it is confident in the safety of the technology

* Shares fall as much as 18 percent (Adds details on Halozyme, updates stock movement)

Aug 1 (Reuters) - Halozyme Therapeutics Inc received a major blow after partner ViroPharma Inc said it would stop a mid-stage trial of a drug using the company’s drug delivery technology.

Halozyme’s shares fell as much as 18 percent on concerns about the viability of its mainstay technology, which is also licensed to bigger drugmakers such as Pfizer Inc, Roche Holding AG and Baxter International Inc.

Halozyme and ViroPharma said the stopped trial had shown several instances of antibodies being produced in patients in response to Halozyme’s synthetic version of the enzyme hyaluronidase.

The Halozyme enzyme, called rHuPH20, was being used in the study to facilitate absorption of the ViroPharma drug Cinryze, when injected under the skin.

Cinryze is approved as an intravenous treatment to prevent swelling and painful attacks in teenagers and adults with hereditary angioedema - a rare and potentially fatal genetic disorder caused by the deficiency of a protein in blood.

The U.S. Food and Drug Administration denied approval to a Baxter drug using the technology last August, citing safety issues. The FDA then put a hold on the ViroPharma drug trial before lifting it in September.

Halozyme and ViroPharma said on Thursday the trial was being stopped as a precautionary measure as the antibodies were not associated with any adverse clinical effects and were of unknown clinical significance.

Halozyme said it was confident about the safety of its technology.

ViroPharma said it now expected to conduct a late-stage trial with an alternative, low-volume, subcutaneous formulation of Cinryze, in the same timeframe that it had expected to move forward with the Halozyme combination of the drug.

Halozyme shares were down 17 percent at $7.10 in mid-morning trading on the Nasdaq. ViroPharma shares were down 6 percent at $32.14. (Reporting by Esha Dey in Bangalore; Editing by Joyjeet Das and Ted Kerr)