SELLAS Life Sciences Reports Positive Data in Completed Phase 1 Study of Galinpepimut-S Combined with Opdivo® in Advanced Malignant Pleural Mesothelioma

Primary Endpoint of Safety and Efficacy Endpoint Both Met with Clinical Activity and Increased Survival Observed

70.3 Weeks Median Overall Survival for Patients Treated with Combination Therapy with 40% of Patients Alive as of Last Follow-Up; Median Overall Survival in Relapsed/Refractory Patients Treated with Standard of Care is Approximately 28 Weeks

30% Disease Control Rate per RECIST Criteria

No Dose Limiting Toxicities Observed

NEW YORK, June 28, 2023 (GLOBE NEWSWIRE) — SELLAS Life Sciences Group, Inc. (NASDAQ: SLS) (“SELLAS” or the “Company”), a late-stage clinical biopharmaceutical company focused on developing novel therapies for a broad range of cancer indications, today announced topline clinical data from a Phase 1 investigator-sponsored clinical trial of its lead clinical candidate, galinpepimut-S (GPS), combined with the checkpoint inhibitor nivolumab (Opdivo®) in patients with malignant pleural mesothelioma (MPM) who were either refractory to or relapsed after at least one line of the standard of care therapy.

Nine of the ten patients enrolled received at least three doses of GPS, with the third GPS dose given in combination with nivolumab. All enrolled patients had either received and progressed with or were refractory to frontline pemetrexed-based chemotherapy. Additional immune response data from all ten patients is expected by Q4 2023.

The study details are as follows:

• 70.3 weeks (17.6 months) median overall survival (OS) in patients who received the combination therapy (9/10 patients) and 54.1 weeks (13.5 months) for all ten patients (nine patients with combination therapy and one GPS only patient). Median overall survival for patients who entered the study as Stage IV patients was 62.3 weeks (15.6 months). OS was calculated as the time from cessation of the most recent previous therapy until confirmed death or most recent data update for patients who are still alive (40 percent of patients)
• 11.9 weeks median progression-free survival (PFS) for all patients
• 30% disease control rate (DCR) with three patients achieving stable disease per RECIST criteria with the tumor volume decrease of up to 17%. DCR is the sum of overall response rate and rate of stable disease.
• As expected in this high-risk advanced cancer population, all patients experienced adverse events, unrelated and related. Seven out of ten patients (70%) had treatment related toxicities and six (60%) had nivolumab related toxicities. Grade 3 (G3) and higher toxicities were observed in three patients (30%). None of the G3 and higher toxicities were related to GPS. GPS related toxicities were observed in three patients (30%), all were Grade 1 (G1) and included G1 skin induration at the site of injection/injection site reaction and/or fatigue in two patients and G1 dizziness and non-cardiac chest pain, each in one patient.
• Of the ten evaluable patients, eight were male and two were female, with a median age of 69 years. Sixty percent of entered the study as Stage III or IV patients. Initial tumor stages were I (one patient), II (three patients), III (two patients) and IV (four patients).
• All patients had MPM epithelioid and/or sarcomatoid variant, a tumor which universally expresses Wilms Tumor 1 (WT1), one of the most widely expressed cancer antigens, ranked by the National Cancer Institute as the top priority among cancer antigens for immunotherapy.

“In our GPS trials completed in previous years we have observed and reported increased survival in the maintenance therapy setting, which is the primary setting for our GPS therapy. This year, we have seen in two studies where GPS appears to increase the survival benefit in active disease when combined with checkpoint blockade drugs, one with relapsed/refractory WT1 positive ovarian cancer and now in relapsed/refractory WT1 positive mesothelioma. We believe that these observed survival benefits in the active disease setting further confirms strong biological effect of GPS in even the most challenging settings,” said Angelos Stergiou, M.D., Sc.D. h.c., President and CEO, SELLAS. “This is demonstrated by a very promising disease control rate where GPS seems to contribute to stopping progression of extremely aggressive cancers.”

“It is important to note that the positive survival outcomes seen in this study are accompanied with a safety profile which is similar to that of the checkpoint inhibitor alone,” commented Dragan Cicic, MD, SVP Clinical Development, SELLAS. “It is rare for a drug used to treat advanced cancers to result in no toxicities higher than Grade 1. Interestingly, patients in this trial in whom low toxicities were observed all had survival at the median or higher.”

About MPM
With approximately 3,300 cases in the United States each year, accompanied by a rising incidence in developing countries, MPM is notoriously difficult to treat and can lead to poor clinical outcomes with respect to both OS and PFS, especially for those patients with the sarcomatoid variant who show a median OS of approximately 4.0 to 5.0 months. In relapsed and refractory patients who progressed after the first line standard of care pemetrexed, a similar patient population to that in the GPS nivolumab combination trial, the common treatment regimen is vinorelbine and OS in those patients is reported to be between 4.5 and 6.2 months. In patients treated with other chemotherapy regimens, such as carboplatin and irinotecan, median OS is reported to be approximately 7.0 months.

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.

Opdivo® is a registered trademark of Bristol Myers Squibb, and is not a trademark of SELLAS. The manufacturer of this brand is not affiliated with and does not endorse SELLAS or its products.

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 clinical development program, including clinical data of GPS and the potential for GPS as a drug development candidate. 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.

Investor Contact  
Bruce Mackle
Managing Director
LifeSci Advisors, LLC
SELLAS@lifesciadvisors.com