Human Umbilical Cord Derived Mesenchymal Stromal Cells to Treat Steroid-Refractory Acute GvHD III/IV or Overlap Syndrome: Interim Analysis of a Multicenter Phase I/II Study

Background Treatment of patients developing steroid-refractory acute GvHD (SR-aGvHD) after allogeneic hematopoietic cell transplantation (HCT) remains an unmet clinical need and a major therapeutic challenge. Mesenchymal stromal cells (MSC) induce immunosuppression and reduce the inflammatory status initiated by aGvHD. We performed an interim analysis of a multicenter, prospective, non-randomized, phase I/II study, planned to test the safety and activity of human umbilical cord (UC), third-party MSC given sequentially after Pentatostatin for the treatment of SR-aGvHD grade III-IV as well as the overlap syndrome (EUDRACT n. 2012-000582-21), NCT02032446). Patients and Methods Third-party UC MSC were produced under GMP conditions as previously described (Capelli C et al Cytotherapy, 2011, 13, 786-801). The treatment schedule was based on the sequential administration of Pentostatin, given iv at dose of 1 mg/m^2 for 3 consecutive days, followed by 3 MSC infusions given at weekly intervals. Between October 2013 and May 2018 (range 4.6 years) we enrolled 27 allogeneic HCT adult recipients (22 male) with SR-aGvHD (n 25) or overlap syndrome (n 2), median age 47 years (range 19-62) transplanted from HLA-identical sibling donors (6; 22%), haploidentical donors (2; 7%), unrelated donors (18; 67%) or cord blood (1; 4%). Target organ involvement included skin in 11 cases (44%), gastrointestinal tract (GI) in 23 (92%) and liver in 12 (48%). Nine patients had grade III (33%), 16 patients grade IV (60%) and 2 patients had severe overlap syndrome (7%). Most of patients exhibited multiple organ involvement (n 16, 64%). Patients started MSC therapy at median of 22 days (10-2101) after the onset of aGvHD or overlap syndrome. Results The infusions of UC MSC-infusion were always well tolerated without any immediate or late toxic event. Response to MSC therapy, by day 30 after the final MSC dose, was evaluable in 16 of 27 patients: 7 achieved CR (26%), 6 PR (22%) and 3 NR (11%). Ten patients died before reaching the evaluation time of response: 8 patients for aGvHD and 2 for haematological disease. One patient was lost to follow up after 9 days from the last UC-MSC infusion. Remarkably, the subgroup of patients with single organ involvement (8 GI, 1 liver) were all evaluable showing 6 patients in overall response (67%, 4 RC and 2 PR). No response was observed in 2 cases of severe overlap syndrome. Patients achieving CR had improved longer term OS vs patients in PR/NR (80% vs 50% at 1 year). For patients with grade III and IV SR-aGvHD, CR rates at day 30 were 67% and 31% respectively. During tapering of steroid therapy 4 out of 7 patients that obtained CR had aGvHD recurrence, at median time of 87 days. We observed relapse of underlying malignancy in 5 patients and all died with active disease. At last follow-up 8 out of 27 patients were alive with no GVHD in 5 of them; 14 patients died from TRM: aGvHD or overlap syndrome itself (n=10), infections (n= 2), cGvHD (n= 2). Two patients with uncontrolled GvHD had clinical signs of Transplant Associated Microangiopathy (TAM). A total of 17 reported AE and 12 SAE were registered. In 23 out of 27 patients infectious complications were observed, in most cases virus related (n.25; CMV followed by BK virus and EBV), and fungal infections (3 aspergillus pneumoniae). Blood-stream documented infections (5) and pneumoniae (5) were also observed. Conclusions The infusion of cord blood derived MSC proved safe in all cases. As expected in patients with steroid refractory aGvHD, a high rate of infectious complications were observed. When considering that most patients (60 %) had grade IV aGvHD, the response rate and the OS are promising and this warrants not only the completion of this Phase I/II study but also planning a future pivotal trial. Disclosures No relevant conflicts of interest to declare.