million in stock-based compensation expense, partially offset by a decrease of $1.5 million in other personnel-related expenses.
General and Administrative Expenses
General and administrative expenses increased $0.3 million, or 1%, from $25.1 million for the nine months ended September 30, 2022 to $25.4 million for the nine months ended September 30, 2023 due primarily to an increase in stock-based compensation expense during the current year period, partially offset by one-time costs incurred during the first quarter of 2022.
We had 27 and 23 full-time equivalent general and administrative employees as of September 30, 2023 and 2022, respectively.
Interest Income
Interest income increased $8.8 million from $1.8 million for the nine months ended September 30, 2022 to $10.6 million for the nine months ended September 30, 2023. This increase was due primarily to higher invested balances as well as higher yields on invested balances during a period of increasing interest rates compared to the prior year period.
Liquidity and Capital Resources
Sources of Liquidity
Historically, we have funded our operations primarily from net proceeds from the sale of shares of our common stock and the receipt of payments under collaboration agreements.
Proceeds from Sales of Our Common Stock
In April 2023, we completed an underwritten public offering of 5,000,000 shares of our common stock at a public offering price of $20.00 per share and issued an additional 750,000 shares of common stock at a price of $20.00 per share following the underwriters’ exercise of their option to purchase additional shares. Net proceeds, after deducting underwriting commissions and offering costs paid by us, were approximately $107.8 million.
In August 2022, we entered into an Open Market Sale AgreementSM (the “Sales Agreement”), pursuant to which we may offer and sell up to $100.0 million shares of our common stock from time to time in “at-the-market” offerings (the “2022 ATM Facility”). There were no sales under the 2022 ATM Facility during the year ended December 31, 2022. During the three months ended March 31, 2023, we sold 1,749,199 shares of our common stock under the 2022 ATM Facility for net proceeds of $24.3 million, after deducting issuance costs. There were no sales of our common stock under the 2022 ATM Facility during the three months ended June 30, 2023 and September 30, 2023.
In August 2018, we entered into a Securities Purchase Agreement with certain accredited investors (each, an “Investor” and, collectively, the “Investors”), pursuant to which we sold an aggregate of 2,750,000 shares of our common stock at a price of $8.00 per share, for aggregate net proceeds of $21.7 million, after deducting offering expenses payable by us. In a concurrent private placement, we issued the Investors warrants to purchase an aggregate of 2,750,000 shares of our common stock (each, a “Warrant” and, collectively, the “Warrants”). Each Warrant was exercisable from August 8, 2018 through August 8, 2023. Warrants to purchase 1,375,000 shares of our common stock had an exercise price of $10.00 per share and Warrants to purchase 1,375,000 shares of our common stock had an exercise price of $15.00 per share. The exercise price and number of shares of our common stock issuable upon the exercise of the Warrants (the “Warrant Shares”) were subject to adjustment in the event of any stock dividends and splits, reverse stock split, recapitalization, reorganization or similar transaction, as described in the Warrants. Under certain circumstances, the Warrants were exercisable on a “cashless” basis. In connection with the issuance and sale of the common stock and Warrants, we granted the Investors certain registration rights with respect to the Warrants and the Warrant Shares. The common stock and Warrants were classified as equity in accordance with Accounting