Amgen Inc. will lay off 172 employees in December from its headquarters facility in Thousand Oaks. The biotech giant notified the state Employment Development Department of the matter in October, specifying that the facility will remain open. Many of the affected workers are field-based employees located throughout the United States, the company said. Layoffs are likely to be reduced if employees apply for other positions within Amgen. Affected employees are not represented by a labor union, the company said in its notice to the state.