An off-duty Los Angeles police officer was injured in Acton early Wednesday during a head-on collision with a vehicle driven by a man suspected of being drunk, authorities said. LAPD Officer Amanda Chapin, 38, underwent surgery for leg fractures, department spokeswoman Officer Karen Smith said. Chapin, a nine-year LAPD veteran, works out of the Devonshire station. Cesar Octavio Contreras, 29, of Lancaster was treated for nonlife-threatening injuries before being booked on suspicion of driving under the influence, Smith said. The collision occurred about 3:45a.m. on Sierra Highway near Acton Canyon Road, said Gary Wise, a dispatcher for the Los Angeles County Fire Department. Sierra Highway was closed for more than an hour as the Highway Patrol investigated. Later in the day, five people were injured – two of them seriously – in a four-vehicle collision on Interstate5 north of Pyramid Lake. The incident was caused by a car that rear-ended a horse trailer, then tried to avoid hitting another car. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set!