Amortization of acquired intangible asset. Amortization of our intangible assets was $80.8 million for the nine months ended September 30, 2022, an increase of $42.4 million, or over 100%, from $38.4 million for the nine months ended September 30, 2021. These amounts are related to the acquisition of all rights to Emflaza acquired in May 2017, Marathon contingent payments, and our Upstaza, Waylivra and Tegsedi intangible assets. The increase is primarily related to additional Marathon contingent payments. The amount allocated to the Emflaza intangible asset is amortized on a straight-line basis over its estimated useful life of approximately seven years from the date of the completion of the acquisition of all rights to Emflaza, the period of estimated future cash flows. The Marathon contingent payments, including a $50.0 million contingent milestone payment made in the nine months ended September 30, 2022, are amortized prospectively as incurred, straight-line, over the remaining useful life of the Emflaza intangible asset. The Waylivra and Tegsedi assets are amortized on a straight-line basis over their estimated useful life of approximately ten years, respectively. For Upstaza, $89.5 million was reclassified from indefinite lived intangible assets to definite lived intangible assets, and this amount will be amortized over its expected useful life of 12 years on a straight-line basis.
Research and development expense. Research and development expense was $462.8 million for the nine months ended September 30, 2022, an increase of $72.0 million, or 18%, from $390.8 million for the nine months ended September 30, 2021. The increase in research and development expenses is primarily related to increased investment in research programs and advancement of the clinical pipeline.
Selling, general and administrative expense. Selling, general and administrative expense was $233.3 million for the nine months ended September 30, 2022, an increase of $34.1 million, or 17%, from $199.2 million for the nine months ended September 30, 2021. The increase reflects our continued investment to support our commercial activities including our expanding commercial portfolio
Change in the fair value of deferred and contingent consideration. The change in the fair value of deferred and contingent consideration was a gain of $32.2 million for the nine months ended September 30, 2022, a change of $43.8 million, or over 100%, from a loss of $11.6 million for the nine months ended September 30, 2021. The change is related to the fair valuation of the potential future consideration to be paid to former equityholders of Agilis as a result of our merger with Agilis which closed in August 2018. Changes in the fair value were due to the re-calculation of discounted cash flows for the passage of time and changes to certain other estimated assumptions.
Interest expense, net. Interest expense, net was $66.4 million for the nine months ended September 30, 2022, an increase of $2.9 million, or 4%, from $63.5 million for the nine months ended September 30, 2021. The increase in interest expense, net was primarily due to additional interest expense recorded from the 2026 Convertible Notes and interest income from our investments.
Other expense, net. Other expense, net was $84.4 million for the nine months ended September 30, 2022, an increase of $57.9 million, or over 100%, from other expense, net of $26.5 million for the nine months ended September 30, 2021. The increase in other expense, net resulted primarily from unrealized foreign currency transaction losses and unrealized losses on our equity investments and convertible debt security in ClearPoint Neuro, Inc. of $1.1 million and $2.4 million, respectively, offset by an unrealized foreign exchange gain from the remeasurement of our intercompany loan,
Income tax benefit (expense). Income tax benefit was $9.7 million for the nine months ended September 30, 2022, a change of $10.6 million, or over 100%, compared to income tax expense of $0.9 million for the nine months ended September 30, 2021. We incurred an income tax benefit in the U.S. relating to the partial amortization of an intangible which had previously been classified as an indefinite lived intangible, and the subsequent release of a portion of the valuation allowance associated with this asset. We also incurred income tax expense in various foreign jurisdictions, and our foreign tax liabilities are largely dependent upon the distribution of pre-tax earnings among these different jurisdictions.
Liquidity and capital resources
Sources of liquidity
Since inception, we have incurred significant operating losses.