The Fas Ligand Inhibitor APG101 in Transfusion Dependent Patients with Low Risk MDS: Interim Results from a Phase I Study

Introduction: Myelodysplastic syndromes (MDS) are heterogeneous diseases characterized by clonal, ineffective hematopoiesis resulting in peripheral cytopenia and an increased risk of progression to acute myeloid leukemia. Particularly in low risk MDS a pro-apoptotic milieu has been found with increased levels of apoptosis-promoting factors such as tumor necrosis factor and fas ligand (CD95 ligand). This is thought to be a major pathomechanism in low risk MDS resulting in increased apoptosis of medullary erythroid progenitors leading to 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 expressed on effector cells as well as to functionally active ligand in solution, thus blocking the interaction between CD95 and its ligand. Here, we report on interim results of a phase I study in transfusion dependent low risk MDS patients treated with APG101.

Methods: A total of 15 patients have been enrolled by July 30^th 2014. All patients gave their written informed consent. Patients were treated for 12 weeks with weekly intravenous infusions of 100 mg or 400 mg APG101, respectively. Thereafter the treatment was discontinued and the patients were followed for another 6 months. Bone marrow biopsies were taken prior to the first APG101 dosing (baseline), at the end of treatment (EOT, week 12), at week 25 (follow up) and at week 37 (follow up). Methylcellulose assays on isolated CD34+ cells from the bone marrow aspirates at the different time points were performed. In addition, bone marrow samples at each time point were subjected to amplicon deep sequencing to detect gene mutations prior to APG101 dosing and to follow the mutational allele burden during and after treatment. This was done by sequencing of genomic DNA from ficollized mononuclear bone marrow cells on a Roche 454 GS Junior system.

Results: At data cut-off, 5 patients (3 male) had completed the study. Median age was 77 (range 65-78). WPSS categories were low in 2 patients and intermediate in 3 patients, respectively. According to the exclusion criteria none of the patients showed a medullary blast count ≥ 5%. Two patients received 400 mg weekly while 3 patients received 100 mg APG101 weekly, respectively. All patients stayed on treatment as scheduled and no adverse events were reported so far. Careful evaluation of bone marrow smears as well as immune phenotypic analysis of bone marrow aspirates showed no increase in bone marrow blasts at any time point. Moreover, quantification of the allele burden of pre-existing mutations (mutations of ASXL1 and SF3B1 in one patient and DNMT3A in the other patient) showed no significant expansion of the mutated clones. Ex vivo differentiation analyses revealed an increase in granulocytic colony forming as well as CFU-E/BFU-E forming capacity. With regard to efficacy, 4 of 5 patients showed a decrease in transfusion frequencies, which so far did not fulfill the IWG 2006 criteria for hematologic improvement (HI).

Conclusions: APG101 is a well tolerable compound in transfusion-dependent low risk MDS patients, particularly in elderly patients. None of the patients discontinued treatment due to toxicity. Monitoring did not reveal any signs of progress i.e. increase in medullary blasts in any of the patients. Deep amplicon sequencing did not show any signs of expansion of preexisting clones during therapy, i.e. APG101 did not provide malignant clones with a survival benefit. Although no major or minor HI (IWG 2006) was seen in 5 analyzed patients, the majority of patients showed a slight reduction in transfusion need which makes APG101 an interesting target for further investigation.