older. The first commercial sale of Evrysdi in the United States was made in August 2020. This event triggered a $20.0 million milestone payment to us from Roche. In August 2020, the EMA accepted the MAA filed by Roche for Evrysdi for the treatment of SMA, which triggered a $15.0 million milestone payment to us from Roche. No milestones were triggered in the three months ended September 30, 2019. Revenues in the three months ended September 30, 2019 were related to our ongoing collaboration agreements.
Royalty revenue. Royalty revenue was $0.7 million for the three months ended September 30, 2020, an increase of $0.7 million, or 100%, from $0.0 million for the three months ended September 30, 2019. The increase in royalty revenue was due to the FDA approval of Evrysdi in August 2020. In accordance with our License Agreement with Roche, we are entitled to royalties on worldwide annual net sales of the product.
Cost of product sales, excluding amortization of acquired intangible asset. Cost of product sales, excluding amortization of acquired intangible asset, were $4.7 million for the three months ended September 30, 2020, an increase of $1.7 million, or 55%, from $3.0 million for the three months ended September 30, 2019. Cost of product sales consist primarily of royalty payments associated with Emflaza and Translarna net product sales, excluding contingent payments to Marathon Pharmaceuticals, LLC (now known as Complete Pharma Holdings, LLC), or Marathon, and costs associated with Emflaza and Translarna product sold during the period. The increase in cost of product sales, excluding amortization of acquired intangible asset, is primarily due to the increase in net product revenue.
Amortization of acquired intangible asset. Amortization of our intangible assets was $9.6 million for the three months ended September 30, 2020, an increase of $2.6 million, or 37%, from $7.0 million for the three months ended September 30, 2019. These amounts are related to the acquisition of all rights to Emflaza acquired in May 2017, Marathon contingent payments, and our 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 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.
Research and development expense. Research and development expense was $93.0 million for the three months ended September 30, 2020, an increase of $29.9 million, or 47%, from $63.1 million for the three months ended September 30, 2019. The increase reflects additional costs associated with advancing the gene therapy and Bio-e platforms and increased investment in research programs as well as advancement of the clinical pipeline.
Selling, general and administrative expense. Selling, general and administrative expense was $57.8 million for the three months ended September 30, 2020, an increase of $8.6 million, or 17%, from $49.3 million for the three months ended September 30, 2019. The increase was primarily due to 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 $8.4 million for the three months ended September 30, 2020, a decrease of $1.1 million, or 12%, from $9.5 million for the three months ended September 30, 2019. 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 $21.0 million for the three months ended September 30, 2020, an increase of $18.4 million, or over 100%, from $2.7 million for the three months ended September 30, 2019. The increase in interest expense, net was primarily due to interest expense recorded from the liability for the sale of future royalties related to the Royalty Purchase Agreement, interest expense recorded from the 2022 and 2026 Convertible Notes and the Credit Agreement, partially offset by interest income from our investments.
Other income, net. Other income, net was $28.8 million for the three months ended September 30, 2020, an increase of $26.0 million, or over 100%, from other income of $2.8 million for the three months ended September 30, 2019. The