4. INTANGIBLES

All of our intangible assets are subject to amortization and are amortized using the straight-line method over their estimated useful life.

We acquired license and supply agreements in relation to cytisinicline upon the acquisition of Extab Corporation, or Extab, on May 18, 2015. The agreements were determined to have a fair value of $3.1 million with an estimated useful life of 14 years.

For each of the three months ended March 31, 2024 and 2023, we recorded license agreement amortization expense of $0.1 million. The following table outlines the estimated future amortization expense related to intangible assets held as of March 31, 2024:

We evaluate the carrying amount of intangible assets periodically by taking into account events or circumstances that may warrant revised estimates of useful life or that indicate the asset may be impaired. We conducted an analysis of potential impairment indicators for long lived assets, including the license and supply agreements for the active pharmaceutical ingredient cytisinicline, and concluded that there were no indicators of impairment identified as of March 31, 2024.