million, or 5%, compared to $54.6 million for the three months ended March 31, 2023. These results were driven by new patient starts and high compliance. Translarna net product revenues were $103.6 million for the three months ended March 31, 2024, a decrease of $11.5 million, or 10%, compared to $115.1 million for the three months ended March 31, 2023, driven by new patients in existing geographies and continued geographic expansion. The decrease was due to the timing of bulk patient orders.
Collaboration revenues. Collaboration revenues were $0.0 million for the three months ended March 31, 2024, a decrease of $6.0 thousand, or 100%, from $6.0 thousand for the three months ended March 31, 2023. The activity for collaboration revenue was immaterial for the three months ended March 31, 2024 and the three months ended March 31, 2023.
Royalty revenue. Royalty revenue was $31.2 million for the three months ended March 31, 2024, an increase of $0.3 million, or 1%, from $30.8 million for the three months ended March 31, 2023. The increase in royalty revenue was due to higher Evrysdi sales in the three months ended March 31, 2024 as compared to the three months ended March 31, 2023. In accordance with the SMA License Agreement, we are entitled to royalties on worldwide annual net sales of the product.
Manufacturing revenue. Manufacturing revenues were $1.4 million for the three months ended March 31, 2024, a decrease of $0.6 million, or 32%, from $2.0 million for the three months ended March 31, 2023. The decrease is due to less manufacturing services related to the production of plasmid DNA and AAV vectors for gene therapy applications for external customers being performed in the three months ended March 31, 2024 as compared to the three months ended March 31, 2023.
Cost of product sales, excluding amortization of acquired intangible asset. Cost of product sales, excluding amortization of acquired intangible asset, were $14.7 million for the three months ended March 31, 2024, an increase of $0.6 million, or 4%, from $14.1 million for the three months ended March 31, 2023. Cost of product sales consist primarily of royalty payments associated with Emflaza, Translarna, and Upstaza net product sales, excluding contingent payments to Marathon Pharmaceuticals, LLC (now known as Complete Pharma Holdings, LLC), or Marathon, costs associated with Emflaza, Translarna, and Upstaza products sold during the period, and royalty expense related to royalty revenues and collaboration milestone revenues. The increase in cost of product sales, excluding amortization of acquired intangible asset, is primarily due to increases in Translarna and Upstaza inventory reserves, and increases in royalty costs driven by the full amortization of the Emflaza intangible asset during the three months ended March 31, 2024. All Emflaza milestone payments are now recorded on the consolidated statement of operations within cost of product sales, excluding amortization of acquired intangible assets.
Amortization of acquired intangible asset. Amortization of our intangible assets was $51.5 million for the three months ended March 31, 2024, an increase of $12.1 million, or 31%, from $39.4 million for the three months ended March 31, 2023. These amounts are related to the Emflaza rights acquisition, as well as the Waylivra, Tegsedi, and Upstaza intangible assets, which are all being amortized on a straight-line basis over their estimated useful lives. The amortization increase is primarily related to additional Marathon contingent payments for Emflaza. The Emflaza intangible asset was fully amortized during the three months ended March 31, 2024.
Research and development expense. Research and development expense was $116.1 million for the three months ended March 31, 2024, a decrease of $79.0 million, or 40%, from $195.1 million for the three months ended March 31, 2023. The decrease in research and development expenses reflects strategic portfolio prioritization as we continue to focus our resources on our differentiated, high potential research and development programs.
Selling, general and administrative expense. Selling, general and administrative expense was $73.3 million for the three months ended March 31, 2024, a decrease of $13.6 million, or 16%, from $86.9 million for the three months ended March 31, 2023. The decrease reflects lower employee costs as a result of the reduction in workforce in 2023.
Change in the fair value of contingent consideration. The change in the fair value of contingent consideration was a gain of $0.1 million for the three months ended March 31, 2024, a change of $2.5 million, or over 100%, from a loss of $2.4 million for the three months ended March 31, 2023. 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