Final Results of the Fibromet Trial: An Open Label Phase II Study to Evaluate Metformin Effects on Bone Marrow Fibrosis and Disease Progression in Primary Myelofibrosis Patients

Background: Primary myelofibrosis (PMF) is a chronic myeloproliferative neoplasm characterized by myeloid expansion associated with elevation of cytokines involved in fibrosis, angiogenesis, and osteosclerosis, leading to progressive fibrous connective tissue deposition in the bone marrow (BM) and BM failure. Although JAK2, CALR and MPL mutations are frequently seen in PMF, patients' molecular heterogeneity and the lack of an expressive clinical and laboratory response following JAK1/2 inhibitors suggest that other factors, such as unknown protein interactions, additional mutations or epigenetic mechanisms may be involved in the progression of PMF. Metformin (MTF) is an anti-diabetic drug, which has been described to possess anti-cancerous properties through the modulation of the AMPK/TORC1 pathway, thereby causing apoptosis in neoplastic cells. Previous reports demonstrated that MTF significantly reduced Ba/F3 JAK2V617F tumor burden and splenomegaly in Jak2V617F knock-in-induced MPN mice. In this context, our goal was to evaluate the effects of MTF treatment in PMF patients.

Aims: To report final results of an open label phase II trial (FIBROMET), which evaluated outcomes of PMF patients after 24mo on MTF treatment.

Methods: PMF non-diabetic adults were eligible. Patients received MTF in increasing doses until a maximum of 2500mg PO daily, according to tolerance. Primary endpoint was BM fibrosis reversion. Secondary endpoints included reduction of inflammation and downregulation of the JAK-STAT pathway. Samples were collected at the time points: screening (0), 3mo, 6mo, 12mo, 18mo and 24mo. The extent of collagen deposits in BM biopsies was semi-quantitatively assessed with Masson's trichrome stainings, following the recommendations of the European Consensus on grading of BM fibrosis (grades 0, 1, 2 or 3). The levels of CXCL4, sIL-2Ra, IP-10, VEFG-A, MIG, MCP-1, MIP-1b, FGF-2, IL-1RA, IL-5, IL-6, IL-8, IL-15, IL-18, TNFa and TGFb1 were analyzed in BM samples using multiplex assay. Phosphorylation status of intracellular proteins STAT3 and STAT5 was analyzed by flow cytometry and the percentage of cells was recorded using FlowJo software. This trial was approved by the Institutional and National Review Board; written informed consent was obtained from all subjects. REBEC registry number: RBR-52ty66.

Results: 11 patients (aged 40-84y) were included between Aug/2018- Feb/2019. Two subjects had early treatment discontinuation due to non-related causes. One patient had disease progression after 12mo of treatment and was submitted to BM transplantation. The median exposure to MTF was 21 mo (3-24) and the median dose was 2500mg/day (1500-2500mg). The most frequent adverse event was diarrhea. No life threatening event occurred. BM collagen deposits were independently evaluated by two hematopathologists with an overall agreement of 80.64% (grade 0: 100%, grade 1: 62.5%, grade 2: 60.0%, grade 3: 90.0%); discordant cases were submitted to joint review. BM collagen deposits did not change when different time points were compared. After 24mo of MTF treatment, a significant 35.4% reduction in IL-5 levels was observed (p=0.03); a 36% reduction in MCP-1 levels was also observed, however this finding was not statistically significant (p=0.06). Flow cytometry analysis demonstrated a STAT3 phosphorylation decrease when comparing screening samples versus 6, 12, 18 and 24 mo of MTF use (all p<0.05), as well as a STAT5 phosphorylation decrease when comparing screening samples versus 6 and 12 mo (all p<0.05). Mean fluorescence intensity for pSTAT3 was: screening 12.01±2.58, 3mo 7.24±1.19, 6mo 4.40±0.24, 12mo 6.31±0.47, 18 mo 6.89±1.01, 24mo 5.71±1.18; and for pSTAT5: screening 15.32±3.59, 3mo 11.50±3.74, 6mo 4.67±0.45, 12mo 6.25±0.58, 18mo 6.24±0.81, 24mo 4.98±0.72.

Conclusions: Final results of the FIBROMET trial demonstrated that, in our study population, metformin was not capable of reversing established bone marrow fibrosis in PMF patients. However, a significant downregulation of the JAK-STAT pathway and reduction of cytokine secretion was observed. Finally, metformin showed to be a safe and well-tolerated drug. Phase III studies are required to confirm our results and to evaluate whether MTF treatment in early PMF phases could delay the progression of BM fibrosis through the downregulation of the JAK-STAT pathway.