A Novel Lysyl Oxidase Inhibitor Attenuates Bone Marrow Fibrosis and Decreases Spleen Size in a Mouse Model of Primary Myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm (MPN) that arises from a clonal proliferation of hematopoietic stem cells leading to bone marrow (BM) fibrosis with generally poor prognosis. PMF usually presents with constitutional symptoms and splenomegaly. The only known curative treatment for PMF is allogenic hematopoietic stem cell transplant and other treatment modalities are to palliate symptoms thereby, necessitating novel therapeutics to treat this disorder. One novel target in the treatment of the fibrotic phenotype in PMF is lysyl oxidase (LOX). LOX, a matrix building and collagen cross-linking enzyme, is highly expressed by megakaryocytes (MKs) in PMF and has been shown to be important in progression of BM fibrosis (Eliades et. al. J Bio Chem 2011, 286(31):27630-8). We assessed the effect of LOX inhibition via a novel, mechanism-based LOX inhibitor (PXS-5446) on BM fibrosis in GATA-1low mice, a well-studied mouse model of PMF. Fifteen to sixteen week old GATA-1low male and female mice received intra-peritoneal injection of either vehicle (olive oil, n = 9) or PXS-5446 at a dose of 15 mg/kg (n = 8) four times a week for 10 weeks, sacrificed and spleen and femurs harvested for histology and analysis. Under these conditions, the drug was well tolerated. Mice in the PXS-5446 group had significantly lower spleen weights compared to vehicle group (242.25 ± 18 mg vs 305.11 ± 22.4 mg, p < 0.05). There was no difference in pre-treatment hematologic parameters, however, PXS-5446 mice had significantly lower platelet count compared to vehicle mice post-treatment (77.5 ± 4.4 K/uL vs 106 ± 12, p < 0.05). Fibrosis of BM and spleen was quantified as previously published (Lucero et. al. J Biol Methods 2016). BM fibrosis was significantly decreased in PXS-5446 mice compared to vehicle (5.01 ± 0.4 Int Den vs 11.88 ± 0.62. p < 0.0001). Likewise, splenic fibrosis was also decreased in PXS-5446 group vs vehicle (13.17 ± 0.53 vs 16.33 ± 0.5, p < 0.001). Morphologic BM MKs on H&E stain were counted and were decreased in PXS-5446 group compared to vehicle (24.65 ± 0.6 per 20x field vs 32.91 ± 0.71, p < 0.001) and confirmed by count for CD31 positive MKs, using immunostaining. The mechanism of this effect on megakaryocytes might involve an influence of LOX on PDGF signaling. Our pre-clinical data demonstrate that in the GATA-1low mouse model of PMF a novel LOX inhibitor decreases fibrosis in BM and spleen size. Our findings suggest that inhibition of LOX is a potential therapeutic option for patients with PMF.