new large private insurer contract that became effective in July 2018, the composition of sales mix by payer and by product in comparison to the prior year and amortization expense related to the assets licensed from Sun Scientific, Inc. in October 2018.
Sales and Marketing Expenses
Sales and marketing expenses increased $4.0 million, or 27%, to $18.4 million in the three months ended June 30, 2019, compared to $14.5 million in the three months ended June 30, 2018. The increase was primarily driven by a $3.0 million increase in personnel-related compensation expense due to increased sales and marketing headcount, including $0.3 million of incremental stock-based compensation expense. In addition, other sales and marketing expenses increased $1.0 million, driven by increased spending associated with field sales meetings, travel and entertainment, recruiting and patient training expenses.
Sales and marketing expenses increased $8.8 million, or 33%, to $35.8 million in the six months ended June 30, 2019, compared to $27.0 million in the six months ended June 30, 2018. The increase was primarily driven by a $6.3 million increase in personnel-related compensation expense due to increased sales and marketing headcount, including $0.8 million of incremental stock-based compensation expense. In addition, other sales and marketing expenses increased $2.5 million, driven by increased spending associated with field sales meetings, travel and entertainment, recruiting and patient training expenses.
Research and Development Expenses
Research and development (“R&D”) expenses decreased $0.1 million, or 4%, to $1.2 million in the three months ended June 30, 2019, compared to $1.3 million in the three months ended June 30, 2018. R&D expenses decreased $0.2 million, or 8%, to $2.5 million in the six months ended June 30, 2019, compared to $2.7 million in the six months ended June 30, 2018. The decreases in both periods were primarily attributable to the timing of clinical studies projects.
Reimbursement, General and Administrative Expenses
Reimbursement, general and administrative expenses increased $1.3 million, or 18%, to $8.8 million in the three months ended June 30, 2019, compared to $7.5 million in the three months ended June 30, 2018. This increase was primarily attributable to a $1.0 million increase in personnel-related compensation expense as a result of increased headcount in our reimbursement operations, payer development and corporate functions, including $0.1 million of incremental stock-based compensation expense. The increase in reimbursement, general and administrative expenses was also attributable to a $0.2 million increase in professional fees, legal expenses, facilities and depreciation, as well as $0.1 million in legal expenses specifically related to the defense of the ongoing lawsuit described in “Legal Proceedings” in this report.
Reimbursement, general, and administrative expenses increased $3.4 million, or 23%, to $18.2 million in the six months ended June 30, 2019, compared to $14.8 million in the six months ended June 30, 2018. This increase was primarily attributable to a $2.2 million increase in personnel-related compensation expense as a result of increased headcount in our reimbursement operations, payer development and corporate functions, including $0.9 million of incremental stock-based compensation expense. The increase in reimbursement, general and administrative expenses was also attributable to a $0.9 million increase in professional fees, legal expenses, facilities and depreciation, as well as $0.2 million in legal expenses specifically related to the defense of the ongoing lawsuit described in “Legal Proceedings” in this report.
Other Income, Net
Other income, net, was $0.2 million and $0.1 million for the three months ended June 30, 2019 and 2018, respectively, and $0.3 million and $0.2 million for the six months ended June 30, 2019 and 2018, respectively. The increases in other income in both periods were primarily due to the interest income realized on marketable securities.