Breakthroughs in Preclinical Development of Ataluren (PTC124) As Therapeutic Option for Patients Affected By Shwachman-Diamond Syndrome: Towards the First Clinical Trial

Shwachman-Diamond syndrome (SDS) is one of the more common inherited bone marrow failure syndromes (IBMFS). Almost 90% of patients with SDS present mutations in the Shwachman-Bodian-Diamond syndrome gene (SBDS) which encodes for the homonymous small protein involved in ribogenesis. SDS is a multiple-organ disease mostly characterized by exocrine pancreas insufficiency, bone malformations, and more importantly bone marrow failure. Most patients with SDS present severe neutropenia, whereas thrombocytopenia and anemia are less frequent. Furthermore, 15-20% of patients develop myelodysplastic syndrome with high risk of acute myeloid leukemia (AML). STAT3 pathway is upregulated both in primary SDS leukocytes and immortalized B cells. Being STAT3 a key regulator of interleukin-6 (IL-6), we postulated that STAT3 hyper-activation could lead to a dysregulation of the IL-6 signaling cascade. Increased levels of IL-6 have been found in pediatric patients with AML and it has been associated with poorer outcomes in these patients, highlighting IL-6 as a cytokine potentially involved in the development of AML. Thus, our hypothesis is that STAT3-IL6 axis may contribute to leukemogenesis in SDS.

Almost 55% of patients with SDS carry a specific nonsense mutations, namely the c.183-184TA>CT, which cause a premature termination codon (PTC). Ataluren (PTC124, PTC Therapeutics Inc, NJ) is a small PTC suppressor molecule already approved by the European Medicines Agency as a therapeutic option for Duchenne muscular dystrophy. Interestingly, we recently reported that ataluren can restore SBDS expression in bone marrow progenitors and in peripheral blood mononuclear cells isolated from patients with SDS. Moreover, we have shown that ataluren can reduce mTOR hyper-phosphorylation and excessive apoptotic rate observed in SDS leukocytes. More importantly, we reported that ataluren can improve myeloid differentiation in a small cohort of patients (Bezzerri et al, Am J Hematol 2018).

In this further analysis considering an enlarged cohort of 20 SDS patients carrying nonsense mutations we found the following:

1. Ataluren can significantly improve both myeloid colony-forming unit-granulocyte/macrophage (CFU-GM) and colony-forming unit granulocyte, erythrocyte, monocyte, megakaryocyte (CFU-GEMM) generation from bone marrow mononuclear stem cells obtained from an enlarged cohort of 20 patients with SDS carrying nonsense mutations. Ataluren indeed almost doubled the number of CFU-GM and CFU-GEMM after 7 and 14 days of treatment.

2. Colony-forming unit erythroid (CFU-E) generation was not affected by the treatment.

3. Ataluren induces neutrophil maturation in SDS bone marrow mononuclear stem cells (mean increase of 61% CD16⁺ CD11b⁺ cells over untreated controls) after 24-48 hours of treatment.

4. Consistently with STAT3 hyper-activation observed in SDS cells, here we show that patients with SDS present a significantly increased level of IL-6 in plasma (4.3-fold higher expression than the healthy control group). Also lymphoblastoid cell lines (LCL) and primary bone marrow mesenchymal stromal cells (MSC) obtained from patients with SDS show increased IL-6 release in culture supernatants compared to healthy controls (2.5-fold and 6.8-fold higher levels, respectively).

5. Of note, ataluren can reduce IL-6 expression in SDS cells restoring normal levels both in LCL and MSC.

In conclusion, these new data support the enrollment of patients for the first clinical trial for this drug in SDS. Furthermore, this study could pave the way for the use of ataluren for other nonsense mutation-mediated IBMFS where STAT3-IL6 axis and similar pro-leukemic pathways are involved.