Repligen Corp. said Wednesday that its decision to reacquireits anti-HIV monoclonal antibodies from Merck & Co. reflecteddifferences of "scope" between the two companies'development priorities.

Repligen stock (NASDAQ:RGEN) on Wednesday reeled from thenews, dropping $8.75 to close at $20.25. However, Repligen didnot give back all of the $11.63 it had gained since Oct. 18,when Merck announced at a scientific meeting that achimpanzee had been protected against AIDS infection by oneof Repligen's monoclonals.

MedImmune Inc. stock (NASDAQ:MEDI) jumped $11.88 to closeat $47 on a separate announcement Tuesday that Merck will putmore than $13 million into a partnership with the company todevelop similarly targeted antibodies.

"What I think happened," analyst David Stone of Cowan & Co.told BioWorld, "is that Merck found a better antibody atMedImmune."

Repligen stock was up based on the excitement over Merck'sdata on the monoclonal, Stone said, and rumors that Repligenwould buy it back.

"And they did," said Stone. "Then Merck turns around andlicenses with a competing company."

Both MedImmune of Gaithersburg, Md., and Repligen ofCambridge, Mass., have developed monoclonals that aredirected against the V3 region of the HIV coat protein gp120.MedImmune and Merck on Tuesday said that their collaborationwill develop a promising candidate that has neutralized manydifferent strains of HIV in test tubes.

The timing of Repligen's announcement was coincidental to theother events, said Walter Herlihy, Repligen's senior vicepresident for research and development. While there had beendiscussions with Merck about parting ways on the antibody,Herlihy said, "we didn't know until Monday that we'd decide toreacquire."

Herlihy said Merck had wanted to focus the antibodies on post-exposure prophylaxis -- for instance to prevent a needle-stickaccident from causing HIV infection.

Repligen instead will initially focus on rapidly evaluating itslead monoclonal in AIDS patients, and will also pursueprevention of mother-to-newborn transmission of HIV. "It's amatter of scope," Herlihy said.

Asked to respond to Repligen's comments, Merck spokesmanKenneth Keen would only say that "the Merck-MedImmuneagreement speaks for itself."

"Merck did not dump the antibody," Montgomery Securitiesanalyst Brandon Fradd told BioWorld. "Repligen asked toreacquire the rights to develop it more broadly."

The fact that Repligen may disagree with Merck as to thedirection in which to take the monoclonal "implies nothingsinister" to their remaining collaboration on an AIDS vaccine,analyst Stone said. "There's no indication Merck is anything buthappy with the (Repligen) vaccine.

"Repligen has identified the best target to generate a vaccine,"Stone said. "My bet is both (Repligen and MedImmune) willmake money on Merck in the long run."

-- Roberta Friedman, Ph.D. Special to BioWorld

(c) 1997 American Health Consultants. All rights reserved.