By Bill Berkrot

Feb 21 (Reuters) - Abbott Laboratories said on Thursday it was working with Johnson & Johnson to see if a genetic diagnostic test made by Abbott would help to identify patients most likely to benefit from a promising experimental leukemia treatment.

The drug, ibrutinib, which is being developed by J&J and Pharmacyclics Inc for chronic lymphocytic leukemia (CLL), recently became one of the first five drugs to receive the new "breakthrough" designation from the U.S. Food and Drug Administration.

The designation is being awarded to experimental drugs the agency views as potentially substantial improvements over existing therapies in a coordinated effort to get the medicines to patients faster. Twelve more drugs are currently under consideration for breakthrough status, the FDA said.

Under the collaboration, Abbott will develop a companion diagnostic test to identify high-risk CLL patients who have an abnormality within a specific chromosome that makes them most likely to respond to treatment with ibrutinib.

The Abbott technology known as FISH - fluorescence in-situ hybridization - can identify whether too many, or too few, copies of a particular gene are present in the body's cells or whether certain genes have rearrangements that play an active role in disease progression.

With advances in genetic screening and better understanding of the molecular biology of cancer, many more drugs are expected to be developed along with companion diagnostics.

Abbott previously worked with Pfizer Inc to develop a companion diagnostic for Pfizer's Xalkori to identify the estimated 4 percent of lung cancer patients with a specific gene mutation called ALK that the drug targets. That drug and the ALK test were approved in 2011.

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