Second Quarter andYear-to-Date Financial Results
Total revenue represents revenue generated from product sales, development activities and royalties. Total revenue was $28.4 million for the three months ended June 30, 2019, compared to $14.2 million for the comparable period in 2018, a 101% increase. For the six months ended June 30, 2019, total revenue was $51.7 million, compared to $26.9 million for the comparable period in 2018, a 93% increase.
Product sales represent sales of our proprietary products and devices or device components to our partners. Product sales were $20.6 million for the three months ended June 30, 2019, compared to $11.1 million for the comparable period in 2018, an 86% increase. For the six month period ended June 30, 2019, product sales were $38.9 million, compared to $22.0 million for the comparable period in 2018, a 77% increase.
Sales of our proprietary products XYOSTED® and OTREXUP® generated revenue of $9.0 million and $13.8 million for the three and six months ended June 30, 2019, respectively, as compared to $3.8 million and $7.7 million for the three and six months ended June 30, 2018, respectively. The increase in proprietary product sales for the three and six months ended June 30, 2019 as compared to the three and six months ended June 30, 2018 were principally attributable to sales of XYOSTED®, which was launched for commercial sale in late 2018.
We sell devices, components and fully assembled and packaged product to our partners Teva, AMAG and Ferring. Partnered product sales were $11.6 million and $7.3 million for the three months ended June 30, 2019 and 2018, respectively, and $25.2 million and $14.3 million for the six months ended June 30, 2019 and 2018, respectively. The increase in sales of partnered products for the three and six months ended June 30, 2019 as compared to the same period in 2018 is primarily attributable to sales of auto injector devices to Teva for use with their Epinephrine Injection USP, an increase in sales of Sumatriptan Injection USP and teriparatide pen injector devices to Teva, and sales of needle-free devices to Ferring. We will stop supplying needle-free devices after the completion of the Ferring transaction, which is expected to close in the second half of 2019.
Licensing and development revenue includes license fees received from partners for the right to use our intellectual property and amounts earned in joint development arrangements with partners under which we perform development activities or develop new products on their behalf. Licensing and development revenue was $2.2 million and $3.2 million for the three and six month periods ended June 30, 2019, respectively, compared to $1.8 million and $3.0 million for the comparable periods in 2018, respectively. Licensing and development revenue for the three and six month periods was primarily from Teva teriparatide and Pfizer rescue pen development programs.
Royalties are recognized based onin-market sales of products sold by our partners. Royalty revenue was $5.6 million for the three months ended June 30, 2019 compared to $1.3 million for the same period in 2018, a 335% increase. For the six month period ended June 30, 2019, royalty revenue was $9.6 million, compared to $1.8 million for the comparable period in 2018, a 451% increase. The significant increases in royalty revenue for the three and six month periods were primarily attributable to royalties recognized from AMAG on their net sales of the Makena® subcutaneous auto injector and from Teva on their net sales of Epinephrine Injection USP, which was launched in late 2018.
Operating expenses were $17.6 million for the second quarter of 2019 compared to $11.1 million in the comparable period of 2018. Total operating expenses for the six months ended June 30, 2019 were $34.9 million as compared to $22.2 million for the comparable period in 2018. The increase in operating expenses for the three and six month periods of 2019 was primarily attributable to additional sales and marketing expenses associated with the launch of XYOSTED®.
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