Array BioPharma Inc. entered a collaboration with Callisto Pharmaceuticals Inc. for the development of a new class of antibiotics, a deal that would mean 50-50 ownership of any resulting drugs.

New York-based Callisto, which is privately held, will provide a cellular-based assay to Array for screening against Array’s lead generation library to identify compounds that will kill or weaken bacteria and eliminate them from the body.

“Our role is to develop a small-molecule drug against their target, and the first thing Array will do is screen our lead generation library of compounds against their target,” said Tricia Haugato, corporate communications manager for Boulder, Colo.-based Array.

Haugato noted that this is the first collaboration toward developing Array’s own internal pipeline.

The target is a master switch that controls critical actions that bacteria need to survive and infect humans, Callisto said.

“We think it’s a very exciting development,” said Yanina Wachtfogel, chief operating officer of Callisto. “This is our first collaboration of its type for antibody development.”

In addition to its responsibility for target development and biological validation, Callisto also will be responsible for developing the cellular assays that Array will use to conduct high-throughput screening, Wachtfogel said. Then, Array will optimize the leads, and Callisto will further develop any hits in animal models.

“This is different from the other collaborations we have announced,” Array’s Haugato said. “Whereas before we got paid for drug discovery and received milestones and royalties, in this case we will be jointly owning and funding the research of the resulting product.

“We get a bigger piece of the pie,” she said.

Array’s Discovery Platform, which will be used in the collaboration, has three components: structural biology, high-throughput screening and predictive informatics. The company has success-based collaborating agreements with Amgen Inc., of Thousand Oaks, Calif.; ICOS Corp., of Bothell, Wash.; Takeda Chemical Industries Ltd., of Japan; Trimeris Inc., of Durham, N.C.; and Vertex Pharmaceuticals Inc., of Cambridge, Mass. Haugato has said that they are the company’s “top revenue-generation collaborations.”

Callisto was founded in 1996 by John Zabrieske of Rockefeller University. In addition to its focus on developing products for the diagnosis and treatment of infectious diseases, Callisto is developing a diagnostic test for early onset autoimmune obsessive-compulsive disorder and other related disorders. The company said there is no laboratory test available to diagnose those disorders.

Wachtfogel said Callisto has identified a molecular marker that is consistently and selectively present on the immune cells of patients with early onset OCD. The marker is a B-cell surface protein, which can be detected with a monoclonal antibody known as D8/17.

Callisto also developed an antibody, Anti-Toxic Shock, or ATS antibody, to treat toxic shock syndrome using a peptide whose sequence is similar to toxins of Gram-positive bacteria. Toxic shock syndrome is caused by Gram-positive bacteria, and about 20,000 cases are reported each year, Callisto said. The same technology can be used to counter bioterrorist attacks, the company said.

In addition, Callisto identified a peptide sequence that is common to various toxins of Streptococcus pyogenes and Staphylococcus aureus that it is developing to treat sepsis. Wachtfogel said an antibody directed against that peptide recognizes the entire family of Streptococcal and Staphylococcal toxins. The antibody has shown in studies to protect rabbits from the lethal effects of a number of different toxins, she said.

Array BioPharma’s stock (NASDAQ:ARRY) fell 20 cents Monday to close at $10.26.