increase of $6.0 million, or 12%, compared to $48.6 million for the three months ended March 31, 2022. These results reflect new patients, high compliance, and broader access.
Collaboration revenues. Collaboration revenues was $6.0 thousand for the three months ended March 31, 2023, a decrease of $1.0 thousand, or 14%, from $7.0 thousand for the three months ended March 31, 2022. The activity for collaboration revenue was immaterial for the three months ended March 31, 2023, and 2022.
Royalty revenue. Royalty revenue was $30.8 million for the three months ended March 31, 2023, an increase of $11.9 million, or 63%, from $18.9 million for the three months ended March 31, 2022. The increase in royalty revenue was due to higher Evrysdi sales in the three months ended March 31, 2023 as compared to the three months ended March 31, 2022. 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 $2.0 million for the three months ended March 31, 2023. The increase is due to the manufacturing services related to the production of plasmid DNA and AAV vectors for gene therapy applications for external customers. No manufacturing revenue was recognized for the three months ended March 31, 2022.
Cost of product sales, excluding amortization of acquired intangible asset. Cost of product sales, excluding amortization of acquired intangible asset, were $14.1 million for the three months ended March 31, 2023, an increase of $4.0 million, or 40%, from $10.1 million for the three months ended March 31, 2022. 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 the increase in net product revenue and royalty revenue.
Amortization of acquired intangible asset. Amortization of our intangible assets was $39.4 million for the three months ended March 31, 2023, an increase of $15.9 million, or 68%, from $23.5 million for the three months ended March 31, 2022. 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.
Research and development expense. Research and development expense was $195.1 million for the three months ended March 31, 2023, an increase of $55.0 million, or 39%, from $140.1 million for the three months ended March 31, 2022. The increase in research and development expenses is primarily related to increased investment in research programs and advancement of the clinical pipeline. The increase in R&D expenses includes the achievement of a $30.0 million success-based development milestone for the completion of enrollment of a Phase 3 clinical trial for sepiapterin for PKU.
Selling, general and administrative expense. Selling, general and administrative expense was $86.9 million for the three months ended March 31, 2023, an increase of $13.6 million, or 19%, from $73.3 million for the three months ended March 31, 2022. 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 loss of $2.4 million for the three months ended March 31, 2023, a change of $14.1 million, or over 100%, from a gain of $11.7 million for the three months ended March 31, 2022. 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 $27.3 million for the three months ended March 31, 2023, an increase of $3.8 million, or 16%, from $23.5 million for the three months ended March 31, 2022. 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 and the Blackstone Credit Agreement.