

	2019							2018
as of December 31 (in millions)	Gross carrying amount		Accumulated amortization			Net carrying amount		Gross carrying amount		Accumulated amortization			Net carrying amount
Definite-lived intangible assets
Developed product rights	$	19,547	$	(6,405	)	$	13,142	$	15,872	$	(5,614	)	$	10,258
License agreements	7,798		(2,291		)	5,507		7,865		(1,810		)	6,055
Total definite-lived intangible assets	27,345		(8,696		)	18,649		23,737		(7,424		)	16,313
Indefinite-lived research and development	—		—			—		4,920		—			4,920
Total intangible assets, net	$	27,345	$	(8,696	)	$	18,649	$	28,657	$	(7,424	)	$	21,233

Indefinite-lived intangible assets represent acquired IPR&D associated with products that have not yet received regulatory approval. The company performs its annual impairment assessment of indefinite-lived intangible assets in the third quarter, or earlier if impairment indicators exist.

In April 2019, the U.S. Food and Drug Administration (FDA) and the European Commission approved SKYRIZI (risankizumab) for the treatment of moderate to severe plaque psoriasis. As a result, AbbVie reclassified $3.9 billion of indefinite-lived intangible assets related to SKYRIZI to developed product rights definite-lived intangible assets. This amount will be amortized over its estimated useful life using the estimated pattern of economic benefit.

During the fourth quarter of 2018, the company made a decision to stop enrollment for the TAHOE trial, a Phase 3 study evaluating rovalpituzumab tesirine (Rova-T) as a second-line therapy for advanced small-cell lung cancer following a recommendation from an Independent Data Monitoring Committee. This decision lowered the probabilities of success of achieving regulatory approval across Rova-T and other early-stage assets and represented a triggering event which required the company to evaluate for impairment the IPR&D assets associated with the Stemcentrx acquisition. The company utilized multi-period excess earnings models of the “income approach” and determined that the fair value was $1.0 billion as of December 31, 2018, which was lower than the carrying value of $6.1 billion and resulted in an impairment charge of $5.1 billion. This impairment charge was recorded to R&D expense in the consolidated statement of earnings for the year ended December 31, 2018. In the third quarter of 2019, following the announcement of the decision to terminate the Rova-T research and development program, the company recorded an impairment charge of $1.0 billion which represented the remaining value of the IPR&D acquired as part of the 2016 Stemcentrx acquisition. This impairment charge was recorded to R&D expense in the consolidated statement of earnings for the year ended December 31, 2019.

Definite-lived intangible assets are amortized over their estimated useful lives, which range between 2 to 16 years with an average of 11 years for both developed product rights and license agreements. Amortization expense was $1.6 billion in 2019, $1.3 billion in 2018 and $1.1 billion in 2017 and was included in cost of products sold in the consolidated statements of earnings. The anticipated annual amortization expense for definite-lived intangible assets recorded as of December 31, 2019 is as follows:

No definite-lived intangible asset impairment charges were recorded in 2019 or 2018. In 2017, an impairment charge of $354 million was recorded related to ZINBRYTA that reduced both the gross carrying amount and net carrying amount of the underlying intangible assets due to lower expected future cash flows for the product. The impairment charge was based on discounted cash flow analyses and was included in cost of products sold in the consolidated statements of earnings.