On June 30, 2021 SELLAS Life Sciences Group, Inc. (NASDAQ: SLS) ("SELLAS" or the "Company"), a late-stage clinical biopharmaceutical company focused on developing novel cancer immunotherapies for a broad range of indications, reported promising updated clinical data and initial immunobiological data from its Phase 1/2 clinical trial with its lead asset, galinpepimut-S (GPS), the Company’s Wilms Tumor-1 (WT1)-targeting peptide immunotherapeutic, in combination with the checkpoint inhibitor pembrolizumab (Keytruda) (Press release, Sellas Life Sciences, JUN 30, 2021, View Source [SID1234584509]).
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Conducted under a Clinical Trial Collaboration and Supply Agreement with Merck & Co., Inc., Kenilworth, N.J. USA (known as MSD outside of the United States and Canada), the study is investigating the combination of GPS and pembrolizumab in treating patients diagnosed with second- or third-line WT1(+) relapsed or refractory platinum-resistant, advanced metastatic ovarian cancer. The WT1 antigen is one of the most widely expressed cancer antigens in multiple malignancies and has been ranked by the National Cancer Institute as the top priority among cancer antigens for immunotherapy.
The study details are as follows:
Eleven patients (median age: 63 years) who received at least three GPS doses, the last of which was combined with pembrolizumab, were evaluated for clinical responses and three of those patients were also evaluated for immune responses.
66.7 percent of evaluable patients were refractory to or had failed their second-line therapies, and 33.3 percent failed third-line therapy or later.
All enrolled patients (100 percent) were resistant to the standard of care platinum-based therapy. Expected overall survival for patients receiving standard of care platinum-based therapy is nine to 12 months.
Median overall survival among the patients in this trial is not yet known as all patients are still alive at the time of the analysis, which period of time exceeds nine months.
Disease Control Rate
An ad hoc analysis of clinical outcomes in the cohort of 11 patients shows a disease control rate (DCR), the sum of overall response rate and rate of stable disease, of 63.6 percent, with a median follow-up of 15.4 weeks. In December 2020, the Company reported initial data showing a DCR of 87.5 percent in eight patients, with a median follow-up of 9.4 weeks. In this very difficult treatment-resistant patient population, at the time of the follow-up analysis, median progression-free survival (PFS) was 11.8 weeks. The landmark PFS rate by log-rank analysis at six months (26 weeks) was 33 percent.
Analysis of the updated data, using a validated immunohistochemistry assay during the eligibility screening period, shows that the rate of WT1 ovarian tumor positivity in this patient population remained high at approximately 63.6 percent. As of the time of this analysis, all patients are alive, and five patients (45.5 percent) are continuing to receive investigational therapy. Enrollment for this study is ongoing, with a target of approximately 20 total evaluable patients.
The safety profile of the GPS-pembrolizumab combination was similar to that seen with pembrolizumab alone, with the addition of only low-grade, temporary local reactions at the GPS injection site, consistent with previously performed clinical studies with GPS.
Immunobiological Data
CD8+ and CD4+ T-lymphocytes were isolated from peripheral blood mononuclear cells from three patients from whom samples had been collected both at baseline and at the time of the sixth GPS dose (i.e., 18 weeks after starting investigational therapy). The T-cells were assayed ex-vivo for immune responses against the pool of the four peptides that comprise GPS using the validated assay intracellular cytokine staining with fluorescence-activated single cell sorting (ICS-FACS) (Scorpion Biological Services, San Antonio, Texas), with appropriate positive and negative controls.
A total of five cytokine "channels" were used for the analysis (i.e., interferon-g, TNF-a, interleukin-2, CD107a and MIP-1b). The peptide re-challenge incubation period was seven days. At the 18-week time point versus pre-vaccination baseline, the assay demonstrated a relative increase in WT1-specific T-lymphocyte frequencies in peripheral blood averaging +242 percent (range: +104 to +385 percent across five cytokines) for CD8+ and +80.5 percent (range: +1 to +174 percent) for CD4+. There was also evidence of polyfunctional T-cell activation (increases in secretion of >2 cytokines) in two out of three patients (66 percent).
"Considering the overall poor prognosis in this particular clinical setting and based on the observed median PFS, overall survival and DCR in this study, combining GPS with the PD1 inhibitor pembrolizumab appears to be clinically promising as compared to bevacizumab-free salvage chemotherapy regimens and without the toxicity burden associated with the latter," said Angelos Stergiou, M.D., Sc.D. h.c., President and CEO, SELLAS. "Patients treated with GPS plus pembrolizumab also appear to maintain a considerable degree of stable disease, as evidenced by the median DCR of 63.6 percent – all evaluable patients are alive. Continuing to review the clinical data will help us determine the fundamental value of the combination approach to fighting this disease. The initial trends are promising, and further maturity of the data and studying additional patients will allow us to draw more definitive conclusions regarding the clinical benefit. We expect to perform another set of similar ad hoc clinical and immunobiological analyses over the next six months as the study progresses."
"Based on this early data, it is encouraging to see the induction of WT1-specific T-cell immune responses with the administration of GPS in combination with pembrolizumab with a validated complex ex-vivo immune response assay on peripheral blood from patients with platinum-refractory metastatic ovarian cancer who had undergone numerous prior therapies," added Jeffrey S. Weber, M.D., Ph.D.; Deputy Director of the Perlmutter Cancer Center at New York University (NYU)-Langone Health; Co-Director of its Melanoma Research Program Center; and Chair of SELLAS’ Scientific Advisory Board. "Expansion of these results with data from additional patients, as well as at time points longer than 18 weeks (when such patient samples become available for testing), will be key in getting a more comprehensive picture of the combination immunotherapy’s biological effect."
About Ovarian Cancer
Ovarian cancer is one of the most common gynecologic malignancies and the fifth most frequent cause of cancer death in women in the United States. Over 22,000 cases are diagnosed annually, and there are an estimated 15,500 deaths per year. The majority of patients have widespread disease at presentation. The five-year survival for the advanced-stage disease remains less than 30 percent. Combining GPS with the checkpoint inhibitor pembrolizumab, which beneficially and profoundly alters the tumor microenvironment (TME), is hypothesized to increase the proportion of patients who develop an immune response against their cancer and potentially improve their clinical outcome over pembrolizumab monotherapy, without the burden of additional toxicities in macroscopically measurable malignancies.