SELLAS Life Sciences Provides Business Update and Reports First Quarter 2023 Financial Results Phase 1 Study of Novel, Highly Selective CDK9 Inhibitor GFH009 Successfully Completed for Patients with Acute Myeloid Leukemi

SELLAS Life Sciences Provides Business Update and Reports First Quarter 2023 Financial Results

Phase 1 Study of Novel, Highly Selective CDK9 Inhibitor GFH009 Successfully Completed for Patients with Acute Myeloid Leukemia 94% of Patients Enrolled to Date Alive and Durable Remission in Patient with Acute Myeloid Leukemia 6 months

Phase 3 REGAL Study of Galinpepimut-S in Patients with Acute Myeloid Leukemia on Track for Interim Analysis by Late 2023 Early 2024

NEW YORK, May 11, 2023 (GLOBE NEWSWIRE) - SELLAS Life Sciences Group, Inc. (NASDAQ SLS) ("SELLAS'' or the "Company"), a late-stage clinical biopharmaceutical company focused on the development of novel therapies for a broad range of cancer indications, today provided a business update and reported its financial results for the quarter ended March 31, 2023.

"We are pleased with the continued advancement of both our galinpepimut-S (GPS) and GFH009 clinical programs during the first quarter of 2023, which positions us for near-term execution of several key value-driving milestones," said Angelos Stergiou, MD, ScD h.c., President and Chief Executive Officer of SELLAS. "Positive topline data for the patient group with acute myeloid leukemia (AML) from the GFH009 Phase 1 dose-escalation trial in relapsed refractory myeloid malignancies suggests strong evidence of anti-tumor activity and survival benefit in this group with no significant safety issues. This supports advancement to a Phase 2a trial in patients with AML in combination with azacitidine and venetoclax, which we are initiating this quarter, with topline data expected in Q4 2023. Last month, the Independent Data Monitoring Committee (IDMC) also recommended that our GPS Phase 3 REGAL study continue without any modifications and is expected to meet again in Q3 to provide further guidance."

Dr. Stergiou continued "The key objectives for us this quarter and next, among others, will include

the initiation of the GFH009 Phase 2a study in AML patients in combination with azacitadine and venetoclax

completion of, and report of, data from the lymphoma patient group from the GFH009 Phase 1 study

report of topline data from the GPS investigator-sponsored trial (IST) in malignant pleural mesothelioma (MPM)

abstract acceptance of our GPS ovarian cancer data from a major medical gynecological conference

3D Medicines participation in the REGAL study and

meeting of, and feedback from, the IDMC for the REGAL study."

"It was a great pleasure meeting our SELLAS colleagues last month at our Shanghai headquarters where we had a chance to discuss and further strengthen our strategic partnership," said John Gong, M.D., Ph.D., Chairman and Chief Executive Officer of 3D Medicines. "The blinded pooled analysis of GPS in AML conducted at the end of last year is indeed very exciting and may have the potential to improve AML patients' quality of life and

extend survival. We eagerly anticipate joining the REGAL trial very soon and to enroll about 20 patients in China to satisfy Chinese regulatory requirements towards a potential market approval. In addition, we will work together to explore solid tumor indications for GPS by combination with our subcutaneous anti-PD-L1, envafolimab, and other products."

Galinpepimut-S (GPS) Wilms Tumor-1 (WT1) targeting immunotherapeutic

Phase 3 REGAL study in AML Enrollment continued in the global Phase 3 REGAL registrational clinical trial in patients with AML who have achieved complete remission following second-line salvage therapy (CR2 patients), with interim analysis continuing to be expected to occur in late 2023 or early 2024. The participation of 3D Medicines in the REGAL study through the enrollment of patients from the Greater China territory will trigger milestone payments totaling $13.0 million to SELLAS, which the Company expects to receive by the end of the third quarter of 2023. The IDMC performed a routine, prespecified risk-benefit assessment of unblinded data from the REGAL study in April 2023 and recommended that the trial continue without modifications. The IDMC endorsed all clinical trial initiatives undertaken in REGAL to advance GPS, including the addition of clinical sites in China. The next routine IDMC meeting is scheduled for the third quarter of 2023.

3D Medicines Phase 1 clinical trial in China 3D Medicines continued enrollment in China for their open-label, single-arm, multi-center Phase 1 clinical trial in patients with AML, multiple myeloma, non-Hodgkin's lymphoma, or higher-risk myelodysplastic syndrome.

Phase 1 2 Study in combination with pembrolizumab (Keytruda ) in ovarian cancer Top-line data released in November 2022 showed clinical benefit of GPS in combination with pembrolizumab anti-PD-1 therapy in WT1 positive relapsed or refractory platinum resistant advanced metastatic ovarian cancer patients. The study was conducted under a Clinical Trial Collaboration and Supply Agreement with Merck Co., Inc., Rahway, N.J., USA (known as MSD outside the United States and Canada), and SELLAS plans to present final data at a medical conference during the fourth quarter of 2023.

Phase 1 Study in combination with nivolumab (Opdivo ) for MPM Patient enrollment was completed at the end of 2022 for an investigator sponsored open-label Phase 1 trial for GPS combination therapy with checkpoint inhibitor nivolumab (Opdivo ) for treatment of MPM in patients who were either refractory to or relapsed after at least one line of the standard of care therapy. SELLAS expects to report topline data during the first half of 2023.

Phase 1 Study in combination with nivolumab (Opdivo ) in ovarian cancer Final analysis was published in the peer-reviewed journal Cancers on previously reported data from the Phase 1 clinical trial showing clinical benefit of GPS combination with anti-PD-1 antibody nivolumab (Opdivo ) in patients with relapsed WT1-expressing ovarian cancers (NCT02737787).

GFH009 highly selective CDK9 inhibitor

Phase 1 clinical trial in hematological malignancies In the Phase 1 trial of GFH009, dose escalation was successfully completed in the group of patients with AML, while dose escalation continues in the lymphoma group at the highest dose level for that group (75 mg). Positive

topline data for the group of patients with AML showed evidence of anti-tumor activity increasing with higher doses and no significant safety issues even at the highest dose levels. The recommended Phase 2 dose (RP2D) has been established for AML. SELLAS plans to commence a Phase 2a trial with GFH009 in combination with venetoclax and azacitidine (aza ven) in patients with AML during the second quarter of 2023 with topline data expected in the fourth quarter of 2023.

Underwritten Public Offering On February 28, 2023, the Company closed an underwritten public offering providing gross proceeds of $20.0 million, before deducting underwriting discounts and commissions and offering expenses.

Appointment of Vice President, Head of Regulatory Affairs Andrew Elnatan joined the leadership team in January 2023 as Vice President, Regulatory Affairs, CMC and Quality. He brings nearly three decades of global regulatory experience with successful breakthrough therapy designation and global drug approvals.

Financial Results for the First Quarter 2023

Licensing Revenue There was no licensing revenue for the first quarter of 2023, as compared to $1.0 million for the same period in 2022.

Cost of Revenue There was no cost of revenue for the first quarter of 2023, as compared to $0.1 million for the same period in 2022.

R D Expenses Research and development expenses for the first quarter of 2023 were $7.2 million, compared to $4.6 million for the same period in 2022. The increase was primarily due to the ongoing Phase 3 REGAL clinical trial of GPS in AML patients and the Phase 1 clinical trial of GFH009 in hematological malignancies.

G A Expenses General and administrative expenses for the first quarter of 2023 were $4.1 million, as compared to $3.0 million for the same period in 2022. The increase was primarily due to personnel-related expenses due to increased headcount.

Acquired In-Process Research and Development There was no acquired in-process research and development for the first quarter of 2023, compared to $10.0 million for the same period in 2022 from the in-licensing of GFH009.

Net Loss Net loss was $11.1 million for the first quarter of 2023, or a basic and diluted loss per share of $0.47, compared to a net loss of $16.7 million for the same period in 2022, or a basic and diluted loss per share of $1.05.

Cash Position As of March 31, 2023, cash and cash equivalents totaled approximately $23.9 million.

Keytruda is a registered trademark of Merck Sharp Dohme LLC, a subsidiary of Merck Co., Inc., Rahway, NJ, USA and is not a trademark of SELLAS. Opdivo is a registered trademark of Bristol-Myers Squibb Company, New York, NY, USA and is not a trademark of SELLAS. The

manufacturers of these brands are not affiliated with and do not endorse SELLAS or its products.

About SELLAS Life Sciences Group, Inc.

SELLAS is a late-stage clinical biopharmaceutical company focused on the development of novel therapeutics for a broad range of cancer indications. SELLAS' lead product candidate, GPS, is licensed from Memorial Sloan Kettering Cancer Center and targets the WT1 protein, which is present in an array of tumor types. GPS has potential as a monotherapy and combination with other therapies to address a broad spectrum of hematologic malignancies and solid tumor indications. The Company is also developing GFH009, a small molecule, highly selective CDK9 inhibitor, which is licensed from GenFleet Therapeutics (Shanghai), Inc., for all therapeutic and diagnostic uses in the world outside of Greater China. For more information on SELLAS, please visit www.sellaslifesciences.com.

Forward-Looking Statements

This press release contains forward-looking statements. All statements other than statements of historical facts are "forward-looking statements," including those relating to future events. In some cases, forward-looking statements can be identified by terminology such as "plan," "expect," "anticipate," "may," "might," "will," "should," "project," "believe," "estimate," "predict," "potential," "intend," or "continue" and other words or terms of similar meaning. These statements include, without limitation, statements related to the GPS and GFH009 clinical development programs, clinical data for both GPS and GFH009, plans for further clinical development of GFH009 and the potential for GPS and GFH009 as drug development candidates. These forward-looking statements are based on current plans, objectives, estimates, expectations and intentions, and inherently involve significant risks and uncertainties. Actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks and uncertainties with oncology product development and clinical success thereof, the uncertainty of regulatory approval, and other risks and uncertainties affecting SELLAS and its development programs as set forth under the caption "Risk Factors" in SELLAS' Annual Report on Form 10-K filed on March 16, 2023 and in its other SEC filings. Other risks and uncertainties of which SELLAS is not currently aware may also affect SELLAS' forward-looking statements and may cause actual results and the timing of events to differ materially from those anticipated. The forward-looking statements herein are made only as of the date hereof. SELLAS undertakes no obligation to update or supplement any forward-looking statements to reflect actual results, new information, future events, changes in its expectations or other circumstances that exist after the date as of which the forward-looking statements were made.

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SELLAS lifesciadvisors.com

SELLAS LIFE SCIENCES GROUP, INC.

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