Nature of business and basis of presentation	9 Months Ended
Sep. 30, 2017
Nature of business and basis of presentation
Nature of business and basis of presentation	1. Nature of business and basis of presentation Mersana Therapeutics, Inc. (the Company) is a clinical stage company located in Cambridge, Massachusetts. The Company is advancing a proprietary pipeline of targeted oncology therapeutics leveraging its Dolaflexin® antibody drug conjugate (ADC) platform. Mersana's first product candidate, XMT-1522, designed to address a much broader population of patients with HER2-expressing tumors than served by currently approved HER2 therapies, is currently in a Phase 1 dose escalation study. The Company’s second product candidate, XMT-1536, is an ADC targeting NaPi2b, an antigen broadly expressed in certain types of cancer. In October 2017, the Company received FDA clearance of the XMT-1536 IND and we expect to begin dosing patients in early 2018. The Company also has partnerships utilizing the Dolaflexin platform with multiple strategic partners. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development of technological innovations by competitors, reliance on third party manufacturers and ability to transition from pilot-scale production to large-scale manufacturing of products. On July 3, 2017, the Company completed an initial public offering (IPO), in which the Company issued and sold 5,000,000 shares of its common stock at a public offering price of $15.00 per share, for aggregate gross proceeds of $75,000. The Company received $67,420 in net proceeds after deducting $7,580 of underwriting discounts and commissions and offering costs. On August 2, 2017, the Company issued and sold 51,977 shares of common stock at $15.00 per share for gross proceeds of $780 upon the partial exercise of the underwriters’ overallotment option. The Company received net proceeds of $725 after deducting $55 in underwriting discounts and commissions. Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 16,154,671 shares of common stock. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding. In connection with the IPO, the Company amended and restated its certificate of incorporation to change the authorized capital stock to 175,000,000 shares designated as common stock and 25,000,000 shares designated as preferred stock, all with a par value of $0.0001 per share. In connection with preparing for its IPO, the Company effected a 1-for-4.5 reverse stock split of the Company’s common stock. The reverse stock split became effective on June 15, 2017. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. The financial statements have also been retroactively adjusted to reflect adjustments to the conversion ratio for each series of convertible preferred stock effected in connection with the reverse stock split. The Company has incurred net losses since inception. The Company’s net loss was $24,740 for the nine months ended September 30, 2017 and $13,700 for the year ended December 31, 2016. The Company expects to continue to incur operating losses for at least the next several years. As of September 30, 2017, the Company had an accumulated deficit of $83,911. The future success of the Company is dependent on its ability to identify and develop its product candidates, and ultimately upon its ability to attain profitable operations. The Company has devoted substantially all of its financial resources and efforts to research and development and general and administrative expense to support such research and development. The Company’s net losses may fluctuate significantly from quarter to quarter and year to year. Net losses and negative cash flows have had, and will continue to have, an adverse effect on the Company’s stockholders' deficit and working capital. The Company believes that its existing cash, cash equivalents and marketable securities as of September 30, 2017, will enable it to fund its operating plan through at least mid-2019, which the Company expects will allow it to achieve initial clinical data readouts for its two lead development programs. The Company’s unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) and the rules and regulations of the Securities and Exchange Commission (SEC). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASU) of the Financial Accounting Standards Board (FASB). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2016 and notes thereto, included in the Company’s final prospectus for the IPO filed with the Securities and Exchange Commission pursuant to Rule 424(b)(4) on June 29, 2017 (the Prospectus). The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2017, the results of its operations for the three and nine months ended September 30, 2017 and 2016 and cash flows for the nine months ended September 30, 2017 and 2016. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results for the year ending December 31, 2017, or for any future period.

1. Nature of business and basis of presentation

Mersana Therapeutics, Inc. (the Company) is a clinical stage company located in Cambridge, Massachusetts. The Company is advancing a proprietary pipeline of targeted oncology therapeutics leveraging its Dolaflexin® antibody drug conjugate (ADC) platform. Mersana's first product candidate, XMT-1522, designed to address a much broader population of patients with HER2-expressing tumors than served by currently approved HER2 therapies, is currently in a Phase 1 dose escalation study. The Company’s second product candidate, XMT-1536, is an ADC targeting NaPi2b, an antigen broadly expressed in certain types of cancer. In October 2017, the Company received FDA clearance of the XMT-1536 IND and we expect to begin dosing patients in early 2018. The Company also has partnerships utilizing the Dolaflexin platform with multiple strategic partners.

The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development of technological innovations by competitors, reliance on third party manufacturers and ability to transition from pilot-scale production to large-scale manufacturing of products.

On July 3, 2017, the Company completed an initial public offering (IPO), in which the Company issued and sold 5,000,000 shares of its common stock at a public offering price of $15.00 per share, for aggregate gross proceeds of $75,000. The Company received $67,420 in net proceeds after deducting $7,580 of underwriting discounts and commissions and offering costs. On August 2, 2017, the Company issued and sold 51,977 shares of common stock at $15.00 per share for gross proceeds of $780 upon the partial exercise of the underwriters’ overallotment option. The Company received net proceeds of $725 after deducting $55 in underwriting discounts and commissions.

Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 16,154,671 shares of common stock. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding. In connection with the IPO, the Company amended and restated its certificate of incorporation to change the authorized capital stock to 175,000,000 shares designated as common stock and 25,000,000 shares designated as preferred stock, all with a par value of $0.0001 per share.

In connection with preparing for its IPO, the Company effected a 1-for-4.5 reverse stock split of the Company’s common stock. The reverse stock split became effective on June 15, 2017. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. The financial statements have also been retroactively adjusted to reflect adjustments to the conversion ratio for each series of convertible preferred stock effected in connection with the reverse stock split.

The Company has incurred net losses since inception. The Company’s net loss was $24,740 for the nine months ended September 30, 2017 and $13,700 for the year ended December 31, 2016. The Company expects to continue to incur operating losses for at least the next several years. As of September 30, 2017, the Company had an accumulated deficit of $83,911. The future success of the Company is dependent on its ability to identify and develop its product candidates, and ultimately upon its ability to attain profitable operations. The Company has devoted substantially all of its financial resources and efforts to research and development and general and administrative expense to support such research and development. The Company’s net losses may fluctuate significantly from quarter to quarter and year to year. Net losses and negative cash flows have had, and will continue to have, an adverse effect on the Company’s stockholders' deficit and working capital. The Company believes that its existing cash, cash equivalents and marketable securities as of September 30, 2017, will enable it to fund its operating plan through at least mid-2019, which the Company expects will allow it to achieve initial clinical data readouts for its two lead development programs.

The Company’s unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) and the rules and regulations of the Securities and Exchange Commission (SEC). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASU) of the Financial Accounting Standards Board (FASB). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2016 and notes thereto, included in the Company’s final prospectus for the IPO filed with the Securities and Exchange Commission pursuant to Rule 424(b)(4) on June 29, 2017 (the Prospectus).

The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of September 30, 2017, the results of its operations for the three and nine months ended September 30, 2017 and 2016 and cash flows for the nine months ended September 30, 2017 and 2016. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results for the year ending December 31, 2017, or for any future period.