Safety and Efficacy of the CD95-Ligand Inhibitor APG101 in Transfusion-Dependent Patients with Low Risk MDS: Results from a Phase I Study

Introduction:Particularly in low risk MDS, a pro-apoptotic milieu has been described with increased levels of apoptosis-promoting factors such as tumor necrosis factor and fas ligand (CD95 ligand). Evidence exists that in low risk MDS increased apoptosis of erythroid progenitors mediated via CD95 activation results in peripheral cytopenia. APG101 is a fusion protein consisting of the extracellular domain of human CD95 and the Fc domain of human IgG1. APG101 binds to CD95 ligand on effector cells as well as to soluble ligand, thus blocking the interaction between CD95 and its ligand. Here we report on results of a phase I study in transfusion-dependent low risk MDS patients treated with APG101. Primary objectives were safety and tolerability of APG101. Secondary objectives were hematologic, cytologic and cytogenetic response rates using modified International Working Group (IWG) response criteria (Cheson et al., 2006), incidence and time to leukemic progression as well as overall survival (OS) at 37 weeks. Furthermore, this study was focused on the depiction of molecular and functional changes underlying the stem cell defect in MDS including age related changes and high-throughput molecular profiling to evaluate the biological effect of APG101 in patients with lower risk MDS.

Methods: The study was performed according to ICH-GCP guidelines and the Declaration of Helsinki as an open-label, non-randomized study for up to 37 weeks including an initial 12 weeks treatment phase with APG101 and a follow-up period of 25 weeks. 400 mg of APG101 were given every week as an intravenous (i.v.) infusion to the first 6 patients, 100 mg APG101 were given to all further patients. Bone marrow aspirates were obtained during the screening period (baseline), at the end of treatment (EOT, week 13), at week 25 (follow up) and at week 37 (End of Study) with subsequent comprehensive cytologic, cytogenetic, molecular and immunophenotypic analyses. In order to track clonal development and heterogeneity, methylcellulose assays on isolated CD34⁺cells from bone marrow aspirates at the different time points were performed and subjected to amplicon deep sequencing to detect gene mutations prior to APG101 dosing and to follow the mutational allele burden during and after treatment.

Results: A total of 29 patients were screened for this study. Twenty patients received at least one dose of APG101, 17 patients completed the treatment phase and 15 patients completed the entire study including the follow-up phase. Median age was 74.5 years (range 56-82). WPSS categories were low in 19 patients and intermediate in 1 patient, respectively. Except for two patients with RCMD-RS and RARS, all others were classified as RCMD (WHO 2008). According to the exclusion criteria, none of the patients showed a medullary blast count ≥ 5%. Quantification of the allele burden of pre-existing mutations (mutations of ASXL1, SF3B1, TTBK, del(ETV6) and IDH2, SRSF2, SPEG2, BRCC3, NF1) showed no expansion of the mutated clones during the course of treatment. One patient, however, was reported with leukemic progression and AML transformation. One patient died from sepsis due to pre-existing neutropenia. Ex vivo differentiation analyses revealed an increase in CFU-E/BFU-E forming capacity. With regard to efficacy, an improvement in Hb level was observed in 6 patients at various assessment periods in this study. In 8 patients, a decrease in transfusion frequency from treatment until EOS was observed.

Conclusions: APG101 is a very well tolerable compound in transfusion-dependent lower risk MDS patients comprising mostly of elderly patients. Risk of AML progression was not elevated. Deep amplicon sequencing did not show any signs of expansion of preexisting malignant clones during therapy, i.e. APG101 did not provide these clones with a survival benefit. Analysis of efficacy as secondary objective revealed that APG101 has the potential to improve hematopoiesis in MDS patients. Therefore, APG101 is an interesting compound for further clinical studies.