ALBERT

Description: Purpose: Hypomethylating agents are approved in adults with Acute Myeloid Leukemia (AML) or Myelodysplastic Syndrome (MDS). By contrast, data in pediatric hematologic malignancies are scarce. Herein, we report the off-label administration of Azacitidine (Aza) in a cohort of children, adolescents and young adults (AYA). Methods: Patients from Robert-Debré, Saint-Louis and Armand-Trousseau University Hospitals (Assistance Publique-Hôpitaux de Paris), 25 year-old or younger at initiation of treatment and who received Aza between 2009 and 2019 were included. Data on indication, efficacy and toxicity of the treatment were retrospectively collected. Results: As 3 patients (pts) were treated twice with Aza at different stages of their disease, 35 treatments in 32 patients were analyzed. Median age at diagnosis was 11.7 y (range 0.1-22.4). Diagnosis were: 16 Acute Myeloid Leukemias (AML), 6 Juvenile Myelomonocytic Leukemias (JMML), 7 Myelodysplastic Syndromes (MDS), 2 Mixed Phenotype Acute Leukemias (MPAL) and 1 Interdigitating Dendritic Cells Sarcoma. Aza was administrated after a median of 2 lines of treatment (range 0-6) and front line in 5 out of 32 pts (16%; MDS = 4, JMML = 1). Concomitant therapy was administrated in 15/35 treatments (43%) (Sorafenib = 6, Gemtuzumab Ozogamicin = 2, donor lymphocyte infusions = 2, other = 5). Fourteen out of 35 treatments (40%) were delivered in patients with a history of hematopoietic stem cell transplantation (HSCT). Aza was administrated either intravenously (n = 11) or subcutaneously (n = 24) at a dose of 75mg/m² for 7 (20/35, 57%) or 5 (6/35, 17%) consecutive days every 28 days, i.e. same daily dose as in adults; at reduced doses (≤ 50mg/m²) in 3/35 patients; at a dosing scheme varying from one cycle to another in 6/35 patients. Patients received a median of 3 cycles of Aza per treatment (range 1-31). A total of 137 cycles was delivered. Aza was well tolerated with only 5/137 cycles delayed due to hematological (n = 3) or non-hematological (n = 2) toxicity. Aza was discontinued due to toxicity in 2/137 cycles (1 rectal bleeding, 1 severe sepsis). Anemia was noted in 68/130 cycles (52%), thrombocytopenia in 68/128 cycles (53%), neutropenia in 84/127 cycles (66%) and febrile neutropenia in 25/131 cycles (19%). Two grade 5 side effects were observed: 1 patient died from cerebral hemorrhage in a context of anti-platelet poly-immunization, another from septic shock in a context of neutropenia. Most common non-hematological side effects included: nausea, vomiting, diarrhea, subcutaneous injections lesions (local inflammation or hematoma) and infections. Responses to Aza are detailed in the attached table. Best responses to Aza included: 2 complete remissions (CR) both after 3 cycles, 3 CR with incomplete hematologic recovery (CRi) after a median of 2 cycles (range 1-2), 2 partial remissions (PR) after 2 and 3 cycles, 6 stable diseases (SD) during a median of 2 cycles (range 2-12) and 2 hematologic improvements. Overall response rate (sum of CR, CRi, PRs), was 19% (6/32) (3 non evaluable responses). Three out of the 6 patients treated with Aza combined with Sorafenib (AML = 4, MPAL = 2) obtained a response (CR: 2, CRi: 1). The association led to HSCT for 2 of them; the third patient had a CR maintained almost 3 years without HSCT, before relapsing on therapy. One patient with MDS had a SD for 12 months after palliative treatment with Aza. Overall, Aza treatment allowed to proceed to HSCT in 8 out of 32 patients (25%), 7 out of 8 proceeding to a first HSCT [CR: 1 patient, CRi: 3 patients, PR: 2 patients, SD: 1 patient, progressive disease: 1 patient]. These 8 pts included: 4 pts with AML, alive in CR after a median follow-up of 29.5 months (2 of them received concomitant targeted therapy); 2 pts with JMML (1 alive in CR after a follow-up of 25 months); 2 pts with MDS (both died, 7 and 30 months after Aza initiation). Finally, 24 out of 32 patients died and overall survival at 5 years was 23% (IC95% [11;48]) with no difference according to diagnosis. Conclusion: In a cohort of pediatric and AYA patients with heavily pretreated myeloid malignancies, Aza was well tolerated, and allowed to bridge 25% of patients to HSCT. Further prospective studies are needed to explore combination of Aza with targeted therapy in pediatric patients. Disclosures Boissel: NOVARTIS: Consultancy.

Description: Introduction Acute myeloid leukemia (AML) is an aggressive malignancy caused by the accumulation of multiple oncogenetic mutations occurring in a single lineage of hematopoietic progenitors. AML is rare in children and the mutations found are partially different from those in adults, and for some with a lower frequency. Thus, clonal evolution leading to pediatric AML may be specific, and has not been described yet. Methods To define clonal evolution from diagnosis to relapse, we performed whole exome sequencing in matched trio of specimens (diagnosis, germline and relapse) in a 9-years old girl presenting AML FAB M5a with t(9;11)(p22;q23) MLL-AF9 and trisomy 8. At diagnosis, we focused on 3 non-silent somatic mutations candidate for leukemogenesis process, confirmed by Sanger method: EED (R355*), GSDMC (R40*) and ELK1 (3’ UTR). In the same time, we performed cell cultures from bone marrow mononucleated cells at diagnosis. CD34 and CD38 cells were cultured either in liquid long term culture medium (LTC IC) or methylcellulose medium. Results: A total of 512 colonies were collecte. Our 3 interest mutations and trisomy 8 were tracked by allele-specific PCR, and MLL rearrangement detected by FISH, individually in 267 from the 512 colonies. Exploitable results were found in 164 colonies. Through these results in the different cell populations, we were able to establish the clonal architecture at diagnosis. MLL-AF9 fusion and EED mutation were found together as the first concomitant occurring events in the leukemic clone. Then genotyping of the colonies demonstrated that ELK1 mutation, GSDMC mutation, and trisomy 8 were successively acquired. Additional later mutations such as ASXL1 (frameshift), PTPN11 (E76K), EMP2 (3’UTR) and DGCR14 (P314S) were detected in the relapse sample. Discussion The 3 mutations studied in the colonies may impact the progression of the leukemic clone by dysregulating several cellular pathways and networks. First, EED is an essential non-catalytic subunit of the polycomb repressive complex 2 (PRC2) which mediates gene silencing through catalysis of histone H3K27 methylation. PRC2 is known to be enhanced in solid neoplasms such as prostate cancer. On the contrary, in myeloid malignancies and myelodysplasic syndromes, it has been recently demonstrated that mutations involving PRC2 subunits (EED, SUZ12 and EZH1/2) were hypomorphic. These loss-of-functions mutations were responsible for chromatin relaxation and induced transcription of genes promoting self-renewal such as HOXA9. Nevertheless, recent sh-RNA studies in a murine model of MLL-AF9 leukemia demonstrated that residual PRC2 enzymatic activity after EED mutation is needed to unable leukemia growth. These data are coherent with our finding that EED mutation is an early event in leukemogenesis, in cooperation with MLL-AF9 rearrangement. Secondly, ELK1 is targeted by RAS-MAPK pathway, thus its mutation can confer an increased proliferation potential when acquired by the leukemic clone, after its maturation has been blocked and its self-renewal increased through previous MLL rearrangement and EED mutation. Finally, GSDMC may be implicated in monocyte count regulation, and mutated in other neoplasms such as melanoma. As a consequence, it is likely that its mutation occurs lately in the evolution of the monoblastic leukemic clone of our patient. The latest event in the clonal evolution in our patient at diagnosis is the acquisition of trisomy 8. Conclusion This study highlights the clonal evolution in one pediatric AML, and paves the way for further studies to better understand clonal evolution in children. Elucidating, the succession and the cooperation between driver and secondary mutations, is important for both understanding leukemogenesis and developing innovative therapeutic agents targeting founding anomalies in the leukemic clone at its most precocious stage. Moreover, discovering clonal architecture also unable to find new minimal residual disease markers to assess the therapeutic response and risk stratification. Disclosures No relevant conflicts of interest to declare.