By Deena Beasley

July 30 (Reuters) - Eisai 4523.T and Biogen's BIIB.O Alzheimer's drug Leqembi continued to slow progress of the disease with no new safety issues four years into treatment, according to new data presented at a medical meeting on Wednesday.

An injectable version of the drug, currently given by intravenous infusion, is under U.S. regulatory review.

The best results were seen in people who started treatment while in the earliest stages of the brain-wasting disease.

In a pivotal trial of patients with early-stage Alzheimer's, Leqembi was shown to slow cognitive decline by 27% compared to a placebo after 18 months - data that supported the drug's U.S. approval in 2023.

The companies continued to follow about 95% of patients enrolled in that trial. The latest results show that after four years, Leqembi slowed cognitive decline by 34% compared to what would be expected in similar patients who did not receive treatment.

Leqembi targets protofibrils - toxic building blocks that eventually form clumps in the brain known as amyloid plaques, a hallmark of Alzheimer's disease.

There were no new safety findings over the four-year period. Brain swelling and bleeding associated with drugs that work by removing amyloid plaque from the brain largely occurred within the first six months of treatment, according to data presented at the Alzheimer's Association International Conference in Toronto.

More than 50% of patients who started treatment in the earlier stages of Alzheimer's continued to show improvement in clinical scores after four years on Leqembi.

Eisai is conducting a separate study of Leqembi in pre-symptomatic Alzheimer's patients that is due to conclude in late 2027. Eli Lilly LLY.N is also studying its Alzheimer's drug Kisunla in people who have detectable disease pathology, but show no noticeable cognitive decline.

The U.S. Food and Drug Administration, where recent job cuts have raised questions about review times, is slated to decide by August 31 whether to approve an injectable version of Leqembi that could be given to patients at home or at medical facilities.

Lynn Kramer, Eisai's chief clinical officer, said interactions with the FDA "have been right on schedule. They have been communicating with us all the time in an expected manner."

He said the new formulation "will be very helpful to starting new patients" on the drug, which is continuing to see "escalating usage."

Eisai will report its latest quarterly results next week, while Biogen will report results on Thursday.

(Reporting By Deena Beasley in Los AngelesEditing by Bill Berkrot)

((deena.beasley@thomsonreuters.com; 213 955 6746; Reuters Messaging: deena.beasley.thomsonreuters.com@reuters.net))