Comparison of the Three and Six Months ended June 30, 2019 and 2018
Revenue increased $0.4 million, or 1%, to $26.7 million during the three months ended June 30, 2019, compared to $26.3 million during the three months ended June 30, 2018, and increased $2.3 million, or 5%, to $53.3 million during the six months ended June 30, 2019, compared to $51.0 million during the six months ended June 30, 2018. The growth in revenue was attributable to an increase in the average selling price of our PROPEL family of products and initial sales of SINUVA, which contributed approximately 4% to revenue during the three and six months ended June 30, 2019.
Cost of Sales and Gross Margin
Cost of sales decreased $0.6 million, or 9%, to $5.0 million during the three months ended June 30, 2019, compared to $5.6 million during the three months ended June 30, 2018, and decreased $1.3 million, or 12%, to $9.7 million during the six months ended June 30, 2019, compared to $11.0 million during the six months ended June 30, 2018. The decrease in cost of sales for the three months ended June 30, 2019 was primarily attributable to the favorable impact of a lower cost structure during the three months ended June 30, 2019. The decrease in cost of sales for the six months ended June 30, 2019 was primarily attributable to charges during the six months ended June 30, 2018 related to our decision not to commercialize the initial SINUVA production output and the favorable impact of a lower cost structure during the six months ended June 30, 2019.
Gross margin for the three months ended June 30, 2019, increased to 81%, compared to 79% for the three months ended June 30, 2018, and for the six months ended June 30, 2019, increased to 82% compared to 78% for the six months ended June 30, 2018. The increase in gross margin for the three months ended June 30, 2019 was primarily attributable to the favorable impact of lower manufacturing costs and a higher average selling price. The increase in gross margin for the six months ended June 30, 2019 was primarily attributable to charges during the six months ended June 30, 2018 related to our decision not to commercialize the initial SINUVA production output, which accounted for approximately 2% of the increase in gross margin, and the favorable impact of lower manufacturing cost and a higher average selling price during the six months ended June 30, 2019. The per unit allocation of our manufacturing overhead costs may increase and our gross margin may decline as production volume decreases.
Selling, General and Administrative Expenses
SG&A expenses increased $6.6 million, or 31%, to $27.6 million during the three months ended June 30, 2019, compared to $21.0 million during the three months ended June 30, 2018, and increased $12.3 million, or 29%, to $54.8 million during the six months ended June 30, 2019, compared to $42.5 million during the six months ended June 30, 2018. The increase in SG&A expenses was primarily due to the incremental stock-based compensation associated with the leadership change as well as an increase in headcount and related expenses to support the commercial launch of SINUVA, which was launched in March 2018, and the ongoing commercialization of our PROPEL family of products.
Research and Development Expenses
R&D expenses increased $1.6 million, or 38%, to $6.0 million during the three months ended June 30, 2019, compared to $4.4 million during the three months ended June 30, 2018, and increased $3.7 million, or 42%, to $12.3 million during the six months ended June 30, 2019, compared to $8.6 million during the six months ended June 30, 2018. The increase in R&D expenses was primarily due to an increase in headcount and related expenses, and development of our investigational ASCEND drug-coated sinus balloon.
Interest Income and Other, Net
Interest income and other, net, increased $0.2 million to $0.7 million during the three months ended June 30, 2019, compared to $0.5 million during the three months ended June 30, 2018, and increased $0.4 million to $1.3 million during the six months ended June 30, 2019, compared to $0.9 million during the six months ended June 30, 2018. The increase in interest income and other, net, was primarily attributable to higher interest rates earned on our investments.
Liquidity and Capital Resources
As of June 30, 2019, we had cash, cash equivalents and short-term investments of $93.5 million, compared to cash, cash equivalents and short-term investments of $100.8 million as of December 31, 2018.