July 21 (Reuters) - Gilead Sciences GILD.O and Merck MRK.N said on Tuesday their experimental once-weekly HIV pill kept the virus suppressed in two late-stage trials, supporting regulatory filings for what could become the first regimen of its kind for the disease.

Here are some details:

• The combination of Merck's islatravir and Gilead's lenacapavir was tested as a single-tablet regimen in adults whose HIV was already controlled with daily antiretroviral therapy.

• HIV attacks the body's immune system and, if left untreated, can progress to acquired immunodeficiency syndrome (AIDS), the most advanced stage of infection.

• In one trial, none of the patients who switched to the weekly pill had detectable viral levels at 48 weeks, compared with 0.3% of those who remained on Gilead's daily Biktarvy.

• In a second trial, 0.3% of patients taking the weekly pill had detectable HIV levels or higher at 48 weeks, compared with 1.3% of those who remained on standard daily HIV regimens.

• Investors are closely watching the rollout of lenacapavir, branded as Yeztugo, which was approved last year, as Gilead seeks to strengthen its HIV franchise alongside blockbuster treatment Biktarvy.

• The companies said the weekly treatment was non-inferior to Biktarvy and other daily HIV regimens in the two studies, meaning it performed at least as well by the studies' main measure.

• Side effects were generally similar to the daily treatments studied, and no new safety concerns were identified. The most common treatment-related side effects included headache, nausea and diarrhea.

• Merck's once-daily HIV pill combo Idvynso was approved by the U.S. Food and Drug Administration in April, bringing another treatment option for patients suffering from the condition.