2019, respectively. We anticipate that a significant portion of our operating expenses will continue to be related to research and development as we continue to advance ON 123300 and our other programs.
In September 2019 we closed on an offering of common stock to certain investors. We issued 2,198,938 shares of common stock and amended warrants for the purchase of 2,198,938 shares of common stock. The investors, who were also holders of our preferred stock warrants issued in February 2018 and/or May 2018, received a warrant amendment under which a certain number of such investors’ preferred stock warrants received a reduction in exercise price and an extension of term. Net proceeds from the sale of common stock and the amendment of preferred stock warrants were approximately $3.3 million. In November 2019, we closed on an offering of units of common stock and warrants. We issued 30,250,000 shares of common stock, pre-funded warrants to purchase 24,750,000 shares of common stock, and common stock warrants to purchase 55,000,000 shares of common stock. Net proceeds were approximately $9.7 million. On December 10, 2019, we closed on an offering of units of common stock and warrants. We issued 14,326,648 shares of common stock and common stock warrants to purchase 7,163,324 shares of common stock. Net proceeds were approximately $4.4 million. On December 19, 2019, we closed on an offering of units of common stock and warrants. We issued 13,878,864 shares of common stock and common stock warrants to purchase 6,939,432 shares of common stock. Net proceeds were approximately $4.4 million. During 2019, pre-funded warrants were exercised for 23,720,784 shares of common stock and net proceeds were $35,000. Also during 2019, common warrants were exercised for 21,014,378 shares of common stock and net proceeds were approximately $4.9 million.
In January 2020, we closed on an offering of common stock. We issued 27,662,518 shares of common stock and net proceeds were approximately $9.0 million. Also during 2020, common warrants were exercised for 45,863,397 shares of common stock and net proceeds were approximately $10.3 million.
In January 2021, we closed on an offering of common stock. We issued 19,551,124 shares of common stock and net proceeds were approximately $8.5 million. In February 2021, we closed on an offering of common stock. We issued 28,750,000 shares of common stock and net proceeds were approximately $26.7 million. In addition, since December 31, 2020 common warrants were exercised for 2,325,000 shares of common stock and net proceeds were approximately $0.5 million. As a result of these transactions, as of February 28, 2021, we have 236,612,391 common shares outstanding.
Our net losses were $25.2million and $21.5 million for the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020, we had an accumulated deficit of $428.6 million. We expect to incur significant expenses and operating losses for the foreseeable future as we continue the development and clinical trials of, and seek regulatory approval for, our product candidates, even if milestones under our license and collaboration agreements may be met.
As of December 31, 2020 we had $19.0 million in cash and cash equivalents. As of February 28, 2021, we had $49.5 million in cash and cash equivalents.
In December 2017, we entered into a license and collaboration agreement with HanX Biopharmaceuticals, Inc. (“HanX”), a company focused on development of novel oncology products, for the further development, registration and commercialization in Greater China of ON 123300. We believe ON 123300 has the potential to overcome limitations of current generation CDK 4/6 inhibitors. Under the terms of the agreement, we received an upfront payment, and will receive regulatory and commercial milestone payments, as well as royalties on Chinese sales. The key feature of the collaboration is that HanX provides all funding required for Chinese IND enabling studies performed for Chinese health authority IND approval. The Chinese IND was approved in January 2020. We and HanX also intended for these studies to comply with the FDA standards. Accordingly, such studies were used by us for an IND filing with the FDA in November 2020. The FDA Study May Proceed letter was received in December 2020 and first patient in the study is anticipated in the first half of 2021. Drug product for the US study was manufactured in North America and stability data was submitted as part of the IND.
In March 2018, we entered into a license agreement with Pint granting an exclusive, royalty-bearing license for the development and commercialization of rigosertib in South and Central America. Pint made an upfront equity investment of $1,250,000 in our common stock. In addition, we could receive a subsequent equity investment and additional regulatory, development and sales-based milestone payments as well as