Exhibit 99.1 SELLAS Life Sciences Reports First Quarter 2024 Financial Results and Provides Corporate Update - Announced Independent Data Monitoring Committee's (IDMC) Recommendation to Continue the Phase 3 REGAL Study in Patients with Acute Myeloid Leukemia (AML) Without Modifications: IDMC to Reconvene in June - - Completed Enrollment in Phase 3 REGAL Study of Galinpepimut-S - - Announced Positive Phase 2 Data of SLS009 Demonstrating 100% Response Rate in r/r AML Patients with ASXL1 Mutation at 30 mg BIW ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2024 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware 20- ...

[SELLAS Life Sciences 2023 Full Year Financial Results and Corporate Update](index=1&type=section&id=SELLAS%20Life%20Sciences%20Reports%20Full%20Year%202023%20Financial%20Results%20and%20Provides%20Corporate%20Update) [CEO Statement and Strategic Outlook](index=1&type=section&id=CEO%20Statement%20and%20Strategic%20Outlook) CEO highlights strong early 2024 pipeline progress, including Phase 3 REGAL enrollment and promising SLS009 Phase 2a data - Key achievements highlighted for early 2024 include **promising Phase 2a data for SLS009 in r/r AML**, **completion of enrollment for the Phase 3 REGAL study**, and receiving **FDA Fast Track Designation for SLS009**[4](index=4&type=chunk) - The company's priorities for 2024 are to **report data from the Phase 3 REGAL trial (GPS in AML)**, **additional data from the Phase 2a study of SLS009 in r/r AML**, and **topline data from the Phase 1b/2 study of SLS009 in Peripheral T-cell Lymphoma (PTCL)**[4](index=4&type=chunk) [Corporate and Clinical Development Update](index=1&type=section&id=Corporate%20and%20Clinical%20Development%20Update) [Recent Highlights (Q1 2024)](index=1&type=section&id=Recent%20Highlights%20(Q1%202024)) Early 2024 highlights include completed Phase 3 REGAL enrollment, strong SLS009 Phase 2a AML data, and a $20.0 million capital raise - Completed enrollment in the **Phase 3 REGAL study for GPS in AML**. The Steering Committee review suggests an interim analysis may be imminent, with the next IDMC meeting scheduled for **late April 2024**[5](index=5&type=chunk)[7](index=7&type=chunk) SLS009 Phase 2a Study in r/r AML - Response Rates (as of March 15, 2024) | Dose Cohort | Response Rate | Response Rate (with Biomarkers) | | :--- | :--- | :--- | | 45 mg (safety) | 10% | 57% (across all levels) | | 60 mg QW | 20% | 57% (across all levels) | | 60 mg (30 mg BIW - optimal) | 50% | 100% | - The U.S. FDA granted **Fast Track Designation to SLS009 for the treatment of relapsed/refractory AML**, aiming to facilitate its development and review[12](index=12&type=chunk) - Secured **$20.0 million** in gross proceeds from a registered direct offering on **March 19, 2024**, to be used for R&D, working capital, and general corporate purposes[9](index=9&type=chunk) [2024 Milestones](index=2&type=section&id=2024%20Milestones) Key 2024 milestones include interim and final analyses for GPS Phase 3 REGAL study and further SLS009 data from AML and PTCL trials - **Galinpepimut-S (GPS):** - **Phase 3 REGAL study (AML):** **Interim analysis is anticipated in 1H 2024**, with the **final analysis expected by the end of 2024**[13](index=13&type=chunk) - **SLS009 (CDK9 inhibitor):** - **Phase 2a (r/r AML):** **Additional data expected in 1H 2024** - **Phase 1b/2 (r/r PTCL):** **Interim analysis projected to occur in 1H 2024** - **Phase 1b/2 (r/r DLBCL):** **First patient was dosed in March 2024** in a combination trial with Brukinsa® (zanubrutinib) - **NCI PIVOT Program (pediatric cancers):** **Initial safety and efficacy data expected throughout 2H 2024**[17](index=17&type=chunk) [2023 Highlights](index=3&type=section&id=2023%20Highlights) In 2023, SELLAS advanced GPS with positive trial data and favorable FDA CMC meeting, while SLS009 completed Phase 1, initiated new trials, and received key FDA designations - **Galinpepimut-S (GPS):** - Presented **positive data from a Phase 1/2 trial in combination with Keytruda® in ovarian cancer** - Reported **positive follow-up data from a Phase 1 trial with Opdivo® in mesothelioma** - Concluded a **favorable Type C meeting with the FDA regarding Chemistry, Manufacturing, and Controls (CMC) for a potential BLA submission**[17](index=17&type=chunk) - **SLS009:** - Completed the Phase 1 trial, establishing the **recommended Phase 2 dose at 60 mg for AML and 100 mg for lymphomas** - **Initiated a Phase 2a study in AML and a potentially registrational Phase 1b/II trial in r/r PTCL**[16](index=16&type=chunk)[18](index=18&type=chunk)[21](index=21&type=chunk) - SLS009 received multiple FDA regulatory designations in 2023: - **Orphan Drug Designation (ODD) for AML** - **ODD for PTCL** - **Fast Track Designation for r/r PTCL**[21](index=21&type=chunk) [Financial Results for the Full Year 2023](index=4&type=section&id=Financial%20Results%20for%20the%20Full%20Year%202023) [Statement of Operations Analysis](index=4&type=section&id=Statement%20of%20Operations%20Analysis) Full year 2023 net loss decreased to $37.3 million from $41.3 million in 2022, primarily due to the absence of a $10.0 million acquired R&D expense Full Year 2023 vs. 2022 Financial Performance | Metric | 2023 | 2022 | Change Driver | | :--- | :--- | :--- | :--- | | R&D Expenses | $24.0 million | $20.3 million | Increased clinical trial expenses for Phase 3 REGAL and SLS009 trials. | | G&A Expenses | $13.9 million | $12.6 million | Increase in personnel and intellectual property expenses. | | Acquired In-Process R&D | $0 | $10.0 million | One-time expense for in-licensing of SLS009 in 2022. | | Net Loss | ($37.3 million) | ($41.3 million) | Reduced due to absence of acquired IPR&D, offset by higher operating expenses. | | Net Loss Per Share | ($1.34) | ($2.13) | Improvement reflects lower net loss and higher weighted average shares. | [Financial Position and Liquidity](index=5&type=section&id=Financial%20Position%20and%20Liquidity) The company ended 2023 with $2.5 million in cash, subsequently raising $29.0 million in gross proceeds through two offerings in early 2024 - Cash and cash equivalents totaled approximately **$2.5 million** as of **December 31, 2023**[24](index=24&type=chunk) - Subsequent to year-end, the company significantly improved its liquidity by raising capital: - **January 8, 2024:** Received **$9.0 million** in gross proceeds from a public offering - **March 19, 2024:** Received **$20.0 million** in gross proceeds from a registered direct offering[24](index=24&type=chunk) [Consolidated Financial Statements](index=7&type=section&id=Consolidated%20Financial%20Statements) [Consolidated Statements of Operations](index=7&type=section&id=CONSOLIDATED%20STATEMENTS%20OF%20OPERATIONS) For 2023, total operating expenses were $37.9 million and net loss was $37.3 million, an improvement from $43.0 million and $41.3 million respectively in 2022 Consolidated Statements of Operations (Amounts in thousands) | Account | Year Ended Dec 31, 2023 (in thousands) | Year Ended Dec 31, 2022 (in thousands) | | :--- | :--- | :--- | | Licensing revenue | $0 | $1,000 | | Research and development | $24,007 | $20,268 | | General and administrative | $13,862 | $12,582 | | Acquired in-process R&D | $0 | $10,000 | | **Total operating expenses** | **$37,869** | **$42,950** | | **Loss from operations** | **($37,869)** | **($41,950)** | | **Net loss** | **($37,340)** | **($41,301)** | | Net loss per share | ($1.34) | ($2.13) | [Consolidated Balance Sheets](index=8&type=section&id=CONSOLIDATED%20BALANCE%20SHEETS) As of December 31, 2023, total assets were $6.2 million, down from $20.9 million in 2022, with a stockholders' deficit of $8.0 million Consolidated Balance Sheets (Amounts in thousands) | Account | December 31, 2023 (in thousands) | December 31, 2022 (in thousands) | | :--- | :--- | :--- | | Cash and cash equivalents | $2,530 | $17,125 | | **Total current assets** | **$3,172** | **$17,756** | | **Total assets** | **$6,219** | **$20,943** | | **Total current liabilities** | **$13,735** | **$15,515** | | **Total liabilities** | **$14,195** | **$16,092** | | **Total stockholders' (deficit) equity** | **($7,976)** | **$4,851** |

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-K ________________________________ (Mark One) ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2023 or ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from _________ to _________ Commission File Number: 001-33958 SELLAS Life Sciences G ...

PART I - FINANCIAL INFORMATION [Financial Statements](index=7&type=section&id=Item%201.%20Financial%20Statements) This section presents the unaudited consolidated financial statements, highlighting a significant decrease in cash and total assets, a shift to a stockholders' deficit, and a net loss of $29.2 million for the nine-month period [Unaudited Consolidated Balance Sheets](index=7&type=section&id=Unaudited%20Consolidated%20Balance%20Sheets) Consolidated Balance Sheet Highlights (in thousands) | Account | Sep 30, 2023 | Dec 31, 2022 | | :--- | :--- | :--- | | **Assets** | | | | Cash and cash equivalents | $3,969 | $17,125 | | Total current assets | $5,203 | $17,756 | | Total assets | $8,086 | $20,943 | | **Liabilities & Equity** | | | | Total current liabilities | $11,735 | $15,515 | | Total liabilities | $11,913 | $16,092 | | Total stockholders' (deficit) equity | $(3,827) | $4,851 | [Unaudited Consolidated Statements of Operations](index=8&type=section&id=Unaudited%20Consolidated%20Statements%20of%20Operations) Operating Results (in thousands, except per share data) | Metric | Q3 2023 | Q3 2022 | 9 Months 2023 | 9 Months 2022 | | :--- | :--- | :--- | :--- | :--- | | Licensing revenue | $0 | $0 | $0 | $1,000 | | Research and development | $5,813 | $4,282 | $18,910 | $14,422 | | General and administrative | $3,548 | $2,864 | $10,782 | $8,982 | | Operating loss | $(9,361) | $(7,146) | $(29,692) | $(32,504) | | Net loss | $(9,267) | $(7,022) | $(29,204) | $(32,180) | | Net loss per share | $(0.33) | $(0.34) | $(1.09) | $(1.70) | [Unaudited Consolidated Statements of Stockholders' (Deficit) Equity](index=9&type=section&id=Unaudited%20Consolidated%20Statements%20of%20Stockholders'%20(Deficit)%20Equity) - Stockholders' equity transitioned to a deficit of **$3.8 million** as of September 30, 2023, from an equity position of **$4.9 million** at the end of 2022. This was primarily driven by a net loss of **$29.2 million** for the nine-month period, partially offset by **$18.6 million** raised from the issuance of common stock[20](index=20&type=chunk) [Unaudited Consolidated Statements of Cash Flows](index=10&type=section&id=Unaudited%20Consolidated%20Statements%20of%20Cash%20Flows) Cash Flow Summary for Nine Months Ended Sep 30 (in thousands) | Activity | 2023 | 2022 | | :--- | :--- | :--- | | Net cash used in operating activities | $(26,585) | $(18,657) | | Net cash used in investing activities | $(5,500) | $(4,500) | | Net cash provided by financing activities | $18,929 | $23,150 | | **Net decrease in cash** | **$(13,156)** | **$(7)** | [Notes to Consolidated Financial Statements](index=11&type=section&id=Unaudited%20Notes%20to%20Consolidated%20Financial%20Statements) - The company is a late-stage clinical biopharmaceutical firm focused on cancer therapeutics, with lead candidates GPS and SLS009[24](index=24&type=chunk) - The company has incurred recurring losses, with an accumulated deficit of **$209.1 million** as of September 30, 2023. Management has substantial doubt about the company's ability to continue as a going concern, as existing cash is not sufficient to fund operations for the next twelve months[25](index=25&type=chunk)[32](index=32&type=chunk) - In February 2023, the company raised net proceeds of approximately **$18.5 million** through an underwritten public offering. Subsequently, in October 2023, it raised an additional **$4.0 million** in gross proceeds from a registered direct offering[26](index=26&type=chunk)[27](index=27&type=chunk) - Under the 3DMed License Agreement, **$191.5 million** in potential future development, regulatory, and sales milestones remain as of September 30, 2023. These are considered variable and not under the company's control[31](index=31&type=chunk)[75](index=75&type=chunk) [Management's Discussion and Analysis of Financial Condition and Results of Operations](index=22&type=section&id=Item%202.%20Management's%20Discussion%20and%20Analysis%20of%20Financial%20Condition%20and%20Results%20of%20Operations) Management discusses the company's late-stage clinical candidates, GPS and SLS009, highlighting increased R&D and G&A expenses, and addresses liquidity concerns with insufficient funds for the next twelve months - The Phase 3 REGAL study for GPS is expected to complete enrollment (excluding China) in November 2023, with an interim analysis anticipated by late 2023 or early 2024[98](index=98&type=chunk) - The SLS009 program is advancing with a Phase 2a trial in AML and has received FDA Orphan Drug Designation for AML and Fast Track Designation for PTCL[106](index=106&type=chunk)[110](index=110&type=chunk) Change in Operating Expenses (Nine Months Ended Sep 30, 2023 vs 2022, in millions) | Expense Category | 2023 | 2022 | Change | | :--- | :--- | :--- | :--- | | Research and development | $18.9 | $14.4 | +$4.5 | | General and administrative | $10.8 | $9.0 | +$1.8 | | Acquired in-process R&D | $0.0 | $10.0 | -$10.0 | - The company's cash and cash equivalents of **$4.0 million** as of September 30, 2023, supplemented by a **$4.0 million** offering in October 2023, are not sufficient to fund planned operations for at least the next twelve months, raising substantial doubt about its ability to continue as a going concern[141](index=141&type=chunk) [Quantitative and Qualitative Disclosures About Market Risk](index=32&type=section&id=Item%203.%20Quantitative%20and%20Qualitative%20Disclosures%20About%20Market%20Risk) The company has determined that quantitative and qualitative disclosures about market risk are not applicable for this reporting period - The company has determined that quantitative and qualitative disclosures about market risk are not applicable[153](index=153&type=chunk) [Controls and Procedures](index=33&type=section&id=Item%204.%20Controls%20and%20Procedures) The company's principal officers concluded that disclosure controls and procedures were effective, with no material changes to internal control over financial reporting during the quarter - The Certifying Officers concluded that the company's disclosure controls and procedures were effective as of the end of the period[155](index=155&type=chunk) - There were no changes in internal control over financial reporting during the quarter that materially affected, or are reasonably likely to materially affect, these controls[156](index=156&type=chunk) PART II - OTHER INFORMATION [Legal Proceedings](index=34&type=section&id=Item%201.%20Legal%20Proceedings) The company reports no pending or threatened litigation as of September 30, 2023 - As of September 30, 2023, there was no pending or threatened litigation against the company[56](index=56&type=chunk)[159](index=159&type=chunk) [Risk Factors](index=34&type=section&id=Item%201A.%20Risk%20Factors) This section refers investors to the comprehensive risk factors detailed in the company's 2022 Annual Report on Form 10-K - The company refers readers to the risk factors discussed in its 2022 Annual Report on Form 10-K[161](index=161&type=chunk) [Unregistered Sales of Equity Securities and Use of Proceeds](index=34&type=section&id=Item%202.%20Unregistered%20Sales%20of%20Equity%20Securities%20and%20Use%20of%20Proceeds) The company reported no unregistered sales of equity securities or use of proceeds from such sales during the period - None reported for the period[162](index=162&type=chunk) [Other Information](index=34&type=section&id=Item%205.%20Other%20Information) The company did not report any other information required to be disclosed under this item for the period - None reported for the period[165](index=165&type=chunk) [Exhibits](index=35&type=section&id=Item%206.%20Exhibits) This section lists all exhibits filed with the Form 10-Q, including corporate governance documents and required officer certifications - The filing includes certifications from the Principal Executive Officer and Principal Financial Officer as required by the Sarbanes-Oxley Act[166](index=166&type=chunk)[167](index=167&type=chunk)

PART I - FINANCIAL INFORMATION [Financial Statements](index=7&type=section&id=Item%201.%20Financial%20Statements) This section presents the unaudited consolidated financial statements for the period ended June 30, 2023, detailing a net loss of $19.9 million and a going concern warning Consolidated Balance Sheets (in thousands) | | June 30, 2023 (in thousands) | December 31, 2022 (in thousands) | | :--- | :--- | :--- | | **Assets** | | | | Cash and cash equivalents | $13,807 | $17,125 | | Total current assets | $15,581 | $17,756 | | Total assets | $18,566 | $20,943 | | **Liabilities & Equity** | | | | Total current liabilities | $13,421 | $15,515 | | Total liabilities | $13,723 | $16,092 | | Total stockholders' equity | $4,843 | $4,851 | Consolidated Statements of Operations (in thousands) | | Three Months Ended June 30, | Six Months Ended June 30, | | :--- | :--- | :--- | | | **2023** | **2022** | **2023** | **2022** | | **Licensing revenue** | $— | $— | $— | $1,000 | | **Research and development** | $5,923 | $5,529 | $13,097 | $10,140 | | **General and administrative** | $3,127 | $3,094 | $7,234 | $6,118 | | **Acquired in-process R&D** | $— | $— | $— | $10,000 | | **Operating loss** | $(9,050) | $(8,623) | $(20,331) | $(25,358) | | **Net loss** | $(8,840) | $(8,414) | $(19,937) | $(25,158) | | **Net loss per share** | $(0.31) | $(0.41) | $(0.77) | $(1.39) | Consolidated Statements of Cash Flows (in thousands) | | For the Six Months Ended June 30, | | :--- | :--- | :--- | | (in thousands) | **2023** | **2022** | | Net cash used in operating activities | $(19,693) | $(12,911) | | Net cash used in investing activities | $(2,500) | $(4,500) | | Net cash provided by financing activities | $18,875 | $23,043 | | Net (decrease) increase in cash | $(3,318) | $5,632 | - The company is a late-stage clinical biopharmaceutical company focused on cancer therapeutics, with lead candidates GPS and SLS009, having incurred recurring losses since inception, with an accumulated deficit of **$199.8 million** as of June 30, 2023[25](index=25&type=chunk)[26](index=26&type=chunk) - Management has substantial doubt about the company's ability to continue as a going concern for at least one year, as current cash and cash equivalents are not sufficient to fund planned operations, requiring substantial additional financing[31](index=31&type=chunk)[32](index=32&type=chunk) [Management's Discussion and Analysis of Financial Condition and Results of Operations](index=22&type=section&id=Item%202.%20Management%27s%20Discussion%20and%20Analysis%20of%20Financial%20Condition%20and%20Results%20of%20Operations) Management discusses the progress of lead product candidates GPS and SLS009, analyzes increased R&D and G&A expenses, and highlights critical liquidity concerns leading to a going concern warning - The company's lead product candidate, GPS, is in a Phase 3 REGAL study for AML, with an interim analysis expected by late 2023 or early 2024, while the second candidate, SLS009, began a Phase 2a clinical trial in June 2023 for AML, with topline data expected in Q4 2023[94](index=94&type=chunk)[103](index=103&type=chunk) - The company expects to receive **$13.0 million** in development milestone payments from its partner 3D Medicines in the third quarter of 2023, related to 3D Medicines' participation in the REGAL study[95](index=95&type=chunk) Key Expense Changes (Six Months Ended June 30, 2023 vs 2022, in millions) | Expense Category | Change (in millions) | Reason | | :--- | :--- | :--- | | Research & Development | +$3.0 | Increased clinical trial expenses for GPS and SLS009, and higher personnel costs | | General & Administrative | +$1.1 | Increased personnel costs, legal fees, and office expenses | | Acquired In-Process R&D | -$10.0 | A $10.0M expense for licensing SLS009 was recognized in 2022, with no similar expense in 2023 | - As of June 30, 2023, the company had **$13.8 million** in cash and cash equivalents, which management states is insufficient to fund operations for the next twelve months, raising substantial doubt about its ability to continue as a going concern[133](index=133&type=chunk) - To address liquidity needs, the company raised approximately **$18.5 million** in net proceeds from a public offering in February 2023 and has approximately **$39.2 million** available for future sales under its Controlled Equity Offering Sales Agreement[131](index=131&type=chunk)[132](index=132&type=chunk) [Quantitative and Qualitative Disclosures About Market Risk](index=31&type=section&id=Item%203.%20Quantitative%20and%20Qualitative%20Disclosures%20About%20Market%20Risk) This section is marked 'Not applicable' as the company, a smaller reporting entity, is not required to provide this information and has no material market risk exposure - The company has stated that this item is not applicable[144](index=144&type=chunk) [Controls and Procedures](index=32&type=section&id=Item%204.%20Controls%20and%20Procedures) Management concluded that disclosure controls and procedures were effective as of June 30, 2023, with no material changes to internal control over financial reporting identified during the quarter - The company's principal executive and financial officers concluded that disclosure controls and procedures were effective as of the end of the period covered by the report[146](index=146&type=chunk) - There were no changes in the company's internal control over financial reporting during the quarter ended June 30, 2023, that have materially affected, or are reasonably likely to materially affect, these controls[147](index=147&type=chunk) PART II - OTHER INFORMATION [Legal Proceedings](index=33&type=section&id=Item%201.%20Legal%20Proceedings) The company reported no pending or threatened litigation as of June 30, 2023, with this information incorporated by reference from the financial statements - As of June 30, 2023, the company had no pending or threatened litigation[56](index=56&type=chunk)[150](index=150&type=chunk) [Risk Factors](index=33&type=section&id=Item%201A.%20Risk%20Factors) This section refers readers to the risk factors previously disclosed in the company's 2022 Annual Report on Form 10-K, with no new or materially changed risks presented - The company refers to the risk factors discussed in its 2022 Annual Report and states that those risks, among others, could materially and adversely affect its business[152](index=152&type=chunk) [Unregistered Sales of Equity Securities and Use of Proceeds](index=33&type=section&id=Item%202.%20Unregistered%20Sales%20of%20Equity%20Securities%20and%20Use%20of%20Proceeds) The company reported no unregistered sales of equity securities during the reporting period - The company reported 'None' for this item[153](index=153&type=chunk) [Defaults Upon Senior Securities](index=33&type=section&id=Item%203.%20Defaults%20Upon%20Senior%20Securities) The company reported no defaults upon its senior securities - The company reported 'None' for this item[154](index=154&type=chunk) [Mine Safety Disclosures](index=33&type=section&id=Item%204.%20Mine%20Safety%20Disclosures) This item is not applicable to the company's business operations - The company reported 'Not applicable' for this item[155](index=155&type=chunk) [Other Information](index=33&type=section&id=Item%205.%20Other%20Information) The company reported no other information to disclose for the period - The company reported 'None' for this item[156](index=156&type=chunk) [Exhibits](index=34&type=section&id=Item%206.%20Exhibits) This section lists all exhibits filed with the Form 10-Q, including the 2023 Equity Incentive Plan, an addendum with 3DMedicines Inc., and required officer certifications - The report includes several exhibits, such as the 2023 Amended and Restated Equity Incentive Plan, an addendum to the Side Letter Agreement with 3DMedicines Inc., and officer certifications required by the Sarbanes-Oxley Act[158](index=158&type=chunk)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2023 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware 20- ...

(Mark One) Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-K ________________________________ ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2022 or ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from _________ to _________ Commission File Number: 001-33958 SELLAS Life Sciences G ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ________________________________ FORM 10-Q ________________________________ ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended September 30, 2022 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 001-33958 SELLAS Life Sciences Group, Inc. (Exact name of registrant as specified in its charter) Delaware ...

ZELLAS CELLAS | --- | --- | --- | --- | --- | --- | --- | |----------------|------------------|-------|-------|-------|-------|-------| | | | | | | | | | | | | | | | | | | | | | | | | | | Company Overview | | | | | | | NASDAQ: SLS | | | | | | | | | | | | | | | | September 2022 | | | | | | | | | | | | | | | Forward Looking Statements This presentation contains forward-looking statements. Such forward-looking statements can be identified by the use of the words "expect," "believe," "will," "anticipate," "esti ...