(Reuters) -Eli Lilly said on Thursday it would appeal against a UK agency's recommendation to not reimburse the cost of Alzheimer's drug Kisunla, blocking access for patients seeking treatment under the country's state-run health service.

The U.S.-based drugmaker plans to appeal on the grounds that the National Institute for Health and Care Excellence's (NICE) recommendation was unreasonable based on the evidence the company, clinical experts and patient groups have submitted.

Kisunla was approved in the UK last year to treat mild cognitive impairment and mild dementia caused by Alzheimer's disease. NICE, which determines the cost-effectiveness of treatments paid for by public funds, had at the time deemed Lilly's drug too expensive for wide use.

Leqembi, a rival drug from Eisai and Biogen, was also deemed too costly for UK's state-run health service in August last year.

NICE's reimbursement recommendation marks another hurdle for Lilly's drug, after the EU medicines regulator in March rejected Kisunla saying the treatment's ability to slow cognitive decline was not significant enough to outweigh the risk of serious brain swelling in patients.

Both Kisunla and Leqembi are designed to clear sticky clumps of a protein called amyloid beta in the brain, believed to be a hallmark of Alzheimer's.

They entered the market after decades of failed attempts to combat the mind-wasting disease.

Adoption of the drugs has been slow in the United States due to the complexities involved with their use, such as the need for extra diagnostic tests and regular brain scans to monitor for potentially serious side effects.

(Reporting by Sriparna Roy in Bengaluru; Editing by Devika Syamnath)