By Mariam Sunny and Sneha S K

Sept 9 (Reuters) - Relay Therapeutics ( RLAY ) said on

Monday interim data from an early-stage study of its

experimental combination treatment for breast cancer showed it

meaningfully extended the time patients lived without their

disease worsening.

The main goal of the trial, which enrolled 118 heavily

pre-treated patients with PI3K mutant, advanced breast cancer,

was to test the safety and tolerability of the combination

therapy.

The treatment, RLY-2608, in combination with AstraZeneca's ( AZN )

Faslodex, was generally well tolerated across all doses,

the company said.

It helped 52 patients live for an average of 9.2 months

without the disease worsening after treatment in the study.

But two patients out of 64, who received a 600 milligram

dose of the combination, discontinued the treatment due to

related side effects, including itching and nausea, while one

experienced severe hyperglycemia or high blood sugar.

Breast cancer is the second-most common cancer and the

second-leading cause of cancer death among women in the United

States, according to American Cancer Society.

The therapy selectively targets a type of protein called

PI3K, a mutation of which affects more than one in three people

with breast cancer in the U.S., according to the company.

Current treatment options include Novartis' Piqray

and AstraZeneca's ( AZN ) Truqap. Roche is testing its

experimental drug, inavolisib, in late-stage studies.

Piqray is "quite toxic and has a lot of side effects and

it's very poorly tolerated by patients", Guggenheim analyst

Michael Schmidt had said ahead of the interim data.

"At the end of the day, we look for ... better safety and

better activity compared to, for example, drugs like Novartis'

Piqray," Schmidt added.

If successful, Schmidt estimates potential U.S. peak sales

of about $1.5 billion for the drug in second-line treatment of

breast cancer.

Astra's combination drug Truqap had helped improve the time

patients with gene-altered tumors lived without their disease

worsening by 7.3 months.

Relay plans to start a late-stage trial for the combination

next year, pending regulatory discussions.