recognized when product is shipped to customers. The decrease in product revenue was primarily due to a decrease in the purchase volume of FreeHold Duo and Trio intracorporeal organ retractors. There were no changes to the per unit sale price in either period presented.
Cost of Product Revenue
Cost of product revenue decreased by $10,000, or approximately 19%, to $44,000 in the three months ended June 30, 2024 from $54,000 for the three months ended June 30, 2023. The decrease was primarily due to decreased sales of FreeHold Duo and Trio intracorporeal organ retractors.
Research and Development Expenses
The following table summarizes our research and development expenses for the three months ended June 30, 2024 and 2023 (in thousands):
| | | | | | | | | | | | |
| | | | | | | | Three Months Ended June 30, |
| | | | | | | | 2024 | | 2023 |
Personnel and consulting costs | | | | | | | | $ | 4,909 | | $ | 3,868 |
Non-clinical development costs | | | | | | | | | 4,713 | | | 3,871 |
Clinical development costs | | | | | | | | | 1,504 | | | 760 |
Total research and development expenses | | | | | | | | $ | 11,126 | | $ | 8,499 |
Research and development expenses increased by $2.6 million, or approximately 31%, to $11.1 million for the three months ended June 30, 2024, from $8.5 million for the three months ended June 30, 2023. This is primarily due to an increase in support of ongoing work to advance BackBeat CNT and Virtue SAB into planned pivotal studies during 2024 and included an increase in personnel related expenses of $671,000 due to increased headcount and associated expenses, along with increased stock-based compensation of $370,000, an increase of $842,000 in non-clinical development costs, and an increase of $744,000 in research and development program costs, supplies, and testing.
The total research and development expenses summarized above include $4.0 million for the three months ended June 30, 2024 and $4.4 million for the three months ended June 30, 2023 related to the Terumo Agreement. The decrease of $453,000 is due to decreased expense activity related to the Terumo Agreement during the 2024 period.
Selling, General and Administrative Expenses
Selling, general and administrative expenses increased by $1.2 million, or approximately 22%, to $6.5 million for the three months ended June 30, 2024, from $5.3 million of expense for the three months ended June 30, 2023. The increase was primarily due to an increase in stock-based compensation of $685,000, and an increase of $522,000 of accounting, finance, legal expenses, investor relations and public relations expenses incurred in connection with the overall growth of the business and being a public company.
Interest Income, Net
Interest income, net, decreased by $39,000, or approximately 4%, to $902,000 of income for the three months ended June 30, 2024, from $941,000 of income for the three months ended June 30, 2023. The net interest income in the 2024 period consisted primarily of interest earned from marketable securities while the net interest income in the 2023 period consisted primarily of interest earned from marketable securities offset by monthly interest expense incurred resulting from the 2022 Loan and Security Agreement.
Loss on Fair Value of Strategic Investments
The loss in fair value of strategic investments was $23,000 for the three months ended June 30, 2024, as compared to a loss of $31,000 for the three months ended June 30, 2023. The amounts recognized for the three months ended