DESCRIPTION OF BUSINESS	9 Months Ended
Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	DESCRIPTION OF BUSINESS Sesen Bio, Inc. ("Sesen" or the “Company”), a Delaware corporation, is a late-stage clinical company developing targeted fusion protein therapeutics ("TFPTs") composed of an anti-cancer antibody fragment tethered to a protein toxin for the treatment of cancer. The Company genetically fuses the cancer-targeting antibody fragment and the cytotoxic protein payload into a single molecule which is produced through the Company's proprietary one-step, microbial manufacturing process. The Company targets tumor cell surface antigens with limited expression on normal cells. Binding of the target antigen by the TFPT allows for rapid internalization into the targeted cancer cell. The Company has designed its targeted fusion proteins to overcome the fundamental efficacy and safety challenges inherent in existing antibody-drug conjugates ("ADCs"), where a payload is chemically attached to a targeting antibody. The Company’s most advanced product candidate, VB4-845, also known as Vicinium®, is a locally-administered targeted fusion protein composed of an anti-EpCAM, or epithelial cell adhesion molecule, antibody fragment tethered to a truncated form of Pseudomonas exotoxin A for the treatment of high-risk, non-muscle invasive bladder cancer ("NMIBC"). The Company has an ongoing single-arm, multi-center, open-label Phase 3 clinical trial of Vicinium as a monotherapy in patients with high-risk, bacillus Calmette-Guérin ("BCG") unresponsive NMIBC, called the VISTA Trial. The VISTA Trial completed enrollment in April 2018 with a total of 133 patients, and the Company is continuing to collect and assess the trial data. Liquidity and Going Concern As of September 30, 2019, the Company had cash and cash equivalents of $57.9 million, net working capital of $50.3 million and an accumulated deficit of $260.0 million. The Company incurred negative cash flows from operating activities of $22.8 million for the year ended December 31, 2018 and $23.7 million for the nine months ended September 30, 2019. Since its inception, the Company has received no revenue from sales of its products currently under development, and management anticipates that operating losses will continue for the foreseeable future as the Company continues its ongoing Phase 3 clinical trial for Vicinium and seeks marketing approval from the United States Food and Drug Administration ("FDA"). The Company has financed its operations to date primarily through private placements of its common stock, preferred stock and convertible bridge notes, venture debt borrowings, its initial public offering ("IPO"), follow-on public offerings, sales effected in an "at-the-market" ("ATM") offering and a License Agreement with F. Hoffmann-La Roche Ltd and Hoffman-La Roche Inc. (collectively, "Roche") (the "License Agreement"). See “Note 8. Stockholders’ Equity” below for information regarding the Company’s recently completed equity financings. Under Accounting Standards Codification ("ASC") Topic 205-40, Presentation of Financial Statements - Going Concern, management is required at each reporting period to evaluate whether there are conditions and events, considered in the aggregate, that raise substantial doubt about an entity's ability to continue as a going concern within one year after the date that the financial statements are issued. This evaluation initially does not take into consideration the potential mitigating effect of management's plans that have not been fully implemented as of the date the financial statements are issued. When substantial doubt exists, management evaluates whether the mitigating effect of its plans sufficiently alleviates the substantial doubt about the Company's ability to continue as a going concern. The mitigating effect of management's plans, however, is only considered if both (i) it is probable that the plans will be effectively implemented within one year after the date that the financial statements are issued and (ii) it is probable that the plans, when implemented, will mitigate the relevant conditions or events that raise substantial doubt about the entity's ability to continue as a going concern within one year after the date that the financial statements are issued. Generally, to be considered probable of being effectively implemented, the plans must have been approved by the Company's Board of Directors before the date that the financial statements are issued. The Company's future success is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. In order to commercialize its product candidates, the Company needs to complete clinical development and comply with comprehensive regulatory requirements. The Company is subject to a number of risks similar to other late-stage clinical companies, including, but not limited to, successful discovery and development of its product candidates, raising additional capital, development by its competitors of new technological innovations, protection of proprietary technology and market acceptance of its products. The successful discovery and development of product candidates requires substantial working capital, and management expects to seek additional funds through equity or debt financings or through additional collaboration, licensing transactions or other sources. The Company may be unable to obtain equity or debt financings or enter into additional collaboration or licensing transactions at favorable terms, or at all. To the extent that the Company raises additional capital through the sale of equity or convertible debt securities, the ownership interests of existing stockholders will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect the rights of existing stockholders. Debt financing, if available, may involve agreements that include liens or other restrictive covenants limiting the Company's ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends. If the Company raises additional funds through government or other third-party funding, strategic collaborations and alliances or licensing arrangements, it may have to relinquish valuable rights to its technologies, future revenue streams, research programs or product candidates or grant licenses on terms that may not be favorable. If the Company is unable to raise additional funds when needed, it may be required to implement cost reduction strategies and delay, limit, reduce or terminate its product development or future commercialization efforts or grant rights to develop and market products or product candidates that management would otherwise prefer to develop and market. The Company's management does not believe that its cash and cash equivalents of $57.9 million as of September 30, 2019 is sufficient to fund the Company's current operating plan for at least twelve months after the issuance of these condensed consolidated financial statements. The history of significant losses, the negative cash flows from operations, the limited cash resources currently on hand and the dependence by the Company on its ability - about which there can be no certainty - to obtain additional financing to fund its operations after the current cash resources are exhausted raises substantial doubt about the Company's ability to continue as a going concern. These condensed consolidated financial statements were prepared under the assumption that the Company will continue as a going concern and do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty.

DESCRIPTION OF BUSINESS

Sesen Bio, Inc. ("Sesen" or the “Company”), a Delaware corporation, is a late-stage clinical company developing targeted fusion protein therapeutics ("TFPTs") composed of an anti-cancer antibody fragment tethered to a protein toxin for the treatment of cancer. The Company genetically fuses the cancer-targeting antibody fragment and the cytotoxic protein payload into a single molecule which is produced through the Company's proprietary one-step, microbial manufacturing process. The Company targets tumor cell surface antigens with limited expression on normal cells. Binding of the target antigen by the TFPT allows for rapid internalization into the targeted cancer cell. The Company has designed its targeted fusion proteins to overcome the fundamental efficacy and safety challenges inherent in existing antibody-drug conjugates ("ADCs"), where a payload is chemically attached to a targeting antibody.

The Company’s most advanced product candidate, VB4-845, also known as Vicinium®, is a locally-administered targeted fusion protein composed of an anti-EpCAM, or epithelial cell adhesion molecule, antibody fragment tethered to a truncated form of Pseudomonas exotoxin A for the treatment of high-risk, non-muscle invasive bladder cancer ("NMIBC"). The Company has an ongoing single-arm, multi-center, open-label Phase 3 clinical trial of Vicinium as a monotherapy in patients with high-risk, bacillus Calmette-Guérin ("BCG") unresponsive NMIBC, called the VISTA Trial. The VISTA Trial completed enrollment in April 2018 with a total of 133 patients, and the Company is continuing to collect and assess the trial data.

Liquidity and Going Concern

As of September 30, 2019, the Company had cash and cash equivalents of $57.9 million, net working capital of $50.3 million and an accumulated deficit of $260.0 million. The Company incurred negative cash flows from operating activities of $22.8 million for the year ended December 31, 2018 and $23.7 million for the nine months ended September 30, 2019. Since its inception, the Company has received no revenue from sales of its products currently under development, and management anticipates that operating losses will continue for the foreseeable future as the Company continues its ongoing Phase 3 clinical trial for Vicinium and seeks marketing approval from the United States Food and Drug Administration ("FDA"). The Company has financed its operations to date primarily through private placements of its common stock, preferred stock and convertible bridge notes, venture debt borrowings, its initial public offering ("IPO"), follow-on public offerings, sales effected in an "at-the-market" ("ATM") offering and a License Agreement with F. Hoffmann-La Roche Ltd and Hoffman-La Roche Inc. (collectively, "Roche") (the "License Agreement"). See “Note 8. Stockholders’ Equity” below for information regarding the Company’s recently completed equity financings.

Under Accounting Standards Codification ("ASC") Topic 205-40, Presentation of Financial Statements - Going Concern, management is required at each reporting period to evaluate whether there are conditions and events, considered in the aggregate, that raise substantial doubt about an entity's ability to continue as a going concern within one year after the date that the financial statements are issued. This evaluation initially does not take into consideration the potential mitigating effect of management's plans that have not been fully implemented as of the date the financial statements are issued. When substantial doubt exists, management evaluates whether the mitigating effect of its plans sufficiently alleviates the substantial doubt about the Company's ability to continue as a going concern. The mitigating effect of management's plans, however, is only considered if both (i) it is probable that the plans will be effectively implemented within one year after the date that the financial statements are issued and (ii) it is probable that the plans, when implemented, will mitigate the relevant conditions or events that raise substantial doubt about the entity's ability to continue as a going concern within one year after the date that the financial statements are issued. Generally, to be considered probable of being effectively implemented, the plans must have been approved by the Company's Board of Directors before the date that the financial statements are issued.

The Company's future success is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. In order to commercialize its product candidates, the Company needs to complete clinical development and comply with comprehensive regulatory requirements. The Company is subject to a number of risks similar to other late-stage clinical companies, including, but not limited to, successful discovery and development of its product candidates, raising additional capital, development by its competitors of new technological innovations, protection of proprietary technology and market acceptance of its products. The successful discovery and development of product candidates requires substantial working capital, and management expects to seek additional funds through equity or debt financings or through additional collaboration, licensing transactions or other sources. The Company may be unable to obtain equity or debt financings or enter into additional collaboration or licensing transactions at favorable terms, or at all. To the extent that the Company raises additional capital through the sale of equity or convertible debt securities, the ownership interests of existing stockholders will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect the rights of existing stockholders. Debt financing, if available, may involve agreements that include liens or other restrictive covenants limiting the Company's ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends. If the Company raises additional funds through government or other third-party funding, strategic collaborations and alliances or licensing arrangements, it may have to relinquish valuable rights to its technologies, future revenue streams, research programs or product candidates or grant licenses on terms that may not be favorable. If the Company is unable to raise additional funds when needed, it may be required to implement cost reduction strategies and delay, limit, reduce or terminate its product development or future commercialization efforts or grant rights to develop and market products or product candidates that management would otherwise prefer to develop and market.

The Company's management does not believe that its cash and cash equivalents of $57.9 million as of September 30, 2019 is sufficient to fund the Company's current operating plan for at least twelve months after the issuance of these condensed consolidated financial statements. The history of significant losses, the negative cash flows from operations, the limited cash resources currently on hand and the dependence by the Company on its ability - about which there can be no certainty - to obtain additional financing to fund its operations after the current cash resources are exhausted raises substantial doubt about the Company's ability to continue as a going concern. These condensed consolidated financial statements were prepared under the assumption that the Company will continue as a going concern and do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty.