95 The following table summarizes our research and development expenses: Year Ended December 31, 2018 2017 Change (in thousands) ABC Platform external expenses (1)............................................................... $ 1,397 $ 931 $ 466 KSI-301 program external expenses (2) ......................................................... 8,252 14,021 (5,769) Payroll and personnel expenses (3)................................................................. 6,825 3,288 3,537 Other research and development expenses (4)................................................ 2,319 3,782 (1,463) Total research and development expenses............................................... $ 18,793 $ 22,022 $ (3,229) (1) ABC Platform external expenses primarily relates to manufacturing of biopolymer intermediate drug substance which can be used with multiple product candidates. These expenses are primarily for services provided by CMOs and CROs. (2) KSI-301 program external expenses relates to development of KSI-301, including manufacturing and clinical trial costs. These expenses are primarily for services provided by CMOs and CROs. (3) Payroll and personnel expenses includes salaries, benefits and stock-based compensation for our personnel involved in research and development activities. These expenses are separately classified and not allocated to specific programs because these expenses relate to multiple programs. (4) Other research and development expenses includes direct costs related to research and development activities other than those listed above. ABC Platform external expenses increased $0.5 million during the year ended December 31, 2018 as compared to 2017. The increase was primarily driven by biopolymer intermediate drug substance manufacturing runs completed during 2018. KSI-301 program external expenses decreased $5.8 million during the year ended December 31, 2018 as compared to 2017. The decrease was primarily due to a decrease in manufacturing runs for KSI-301 from 2018 to 2017. Payroll and personnel expenses increased $3.5 million during the year ended December 31, 2018 as compared to 2017. The increase was a result of increased headcount. Other research and development expenses decreased $1.5 million during the year ended December 31, 2018 as compared to 2017. The decrease was primarily due to increased focus on KSI-301. Our other research and development expenses may fluctuate in future periods as we elect to develop KSI-501 or other product candidates. General and Administrative Expenses General and administrative expenses increased $4.1 million, or 117%, from the year ended December 31, 2017 to the year ended December 31, 2018. The increase in general and administrative expenses was primarily attributable to an increase of $2.2 million in professional services related to accounting, audit, legal and consulting services in connection with our IPO and additional costs associated with operating as a public company, an increase of $1.8 million in salaries, including stock- based compensation, and an increase of $0.1 million in additional allocated overhead expenses due to increased headcount. Interest Expense Interest expense increased $4.3 million from the year ended December 31, 2017 to the year ended December 31, 2018, which was mainly attributable to interest expense on convertible notes issued in August 2017 and February 2018, including accretion of debt discount and issuance costs. Other Income (Expense), Net Other income (expense), net increased $2.8 million from the year ended December 31, 2017 to the year ended December 31, 2018, which was mainly attributable to the movement in fair value of the redeemable convertible preferred stock warrant liability and derivative instrument related to the convertible notes issued in February 2018, offset by interest income of $0.6 million. Loss on Extinguishment of Debt Loss on extinguishment of the convertible notes issued in February 2018 was $5.5 million for the year ended December 31, 2018.