AVID200, a Potent Trap for TGF-β Ligands Inhibits TGF-β1 Signaling in Human Myelofibrosis

Myelofibrosis (MF) is caused by driver mutations which upregulate JAK/STAT signaling. The only curative treatment for MF is hematopoietic stem cell transplant. Ruxolitinib alleviates many of the symptoms in MF but does not significantly alter survival. There is, therefore, an urgent need for additional rational therapies for MF. Bone marrow fibrosis and collagen deposition are hallmarks of MF which have been attributed to megakaryocyte (MK) derived TGFβ, which also plays a role in myelo-proliferation. There are three isoforms of TGFβ (TGFβ1, β2, and β3). AVID200, which was constructed by fusing TGFβR ectodomains to IgG Fc regions, is a potent TGFβ trap with pM potency against two of the three TGFβ ligands, TGFβ1 and β3 (IC₅₀ values of ~ 3 pM ). AVID200's IC₅₀ for TGFβ2 is ~4,000-fold higher indicating that it has minimal activity against TGFβ2, which is desirable since TGFβ2 is a positive regulator of hematopoiesis.

We explored the therapeutic potential of AVID200 by culturing MF or normal donor (ND) mononuclear cells (MNCs) first in the presence of stem cell factor and thrombopoietin (TPO) and then TPO alone in order to generate MK-enriched populations. Although the percentage of mature MKs from ND and MF MNCs was similar, the absolute number of CD41⁺/CD42⁺ MKs generated from MF MNCs was two-fold greater than ND MNCs.

To determine the levels of TGFβ secreted by the MKs we screened MF and ND MNC conditioned media (CM). We observed significantly higher levels of TGFβ1 but not TGFβ2 and TGFβ3 in MF MK CM. The effects of AVID200 on MKs were then evaluated by measuring the levels of phosphorylated SMAD2. Treatment with 0.001 - 0.1 nM AVID200 decreased phosphorylation of SMAD2, suggesting that AVID200 blocks autocrine MK TGFβ signaling.

The increased levels of TGFβ in MF patients promote the proliferation and deposition of collagen by mesenchymal stem cells (MSCs). Cellular proliferation of MSCs was evaluated following treatment with either recombinant TGFβ1 or ND/MF CM in the presence or absence of AVID200. In the absence of AVID200, both recombinant TGFβ1 and MK-derived CM increased the proliferation of MSCs by 1.4- and 1.6-fold respectively, which returned to basal levels with the addition of increasing concentrations of AVID200. These data indicate that AVID200 directly blocks the effect of TGFβ1 on MSCs.

MF stroma is characterized by an increase in Type I collagen. We therefore examined if treatment with AVID200 interferes with the ability of TGFβ1 to induce collagen expression by MSCs. MSCs were cultured in presence of recombinant TGFβ1 alone or in combination with varying concentrations of AVID200 for 72 hours. Recombinant TGFβ1 alone induced an increase in COL1A1 mRNA expression as compared to untreated controls (p<0.01). Addition of AVID200 eliminated the TGFβ-mediated increase in COL1A1 expression in a dose dependent manner. ND and MF MK-derived CM also increased COL1A1 expression by MSCs as compared to un-treated controls (p<0.01) and that effect was eliminated by AVID200 treatment (p<0.01). We next demonstrated that TGFβ1 activated pSMAD2 in MSCs without affecting total SMAD2/3 expression and that SMAD2 phosphorylation was reduced by adding AVID200. Furthermore, AVID200 treatment decreased pSTAT3 which is associated with the ability of TGFβ to induce fibrosis.

We next investigated the effect of AVID200 on MF hematopoiesis. Briefly, MNCs (which produce TGFβ) from two JAK2V617F⁺ MF patients were incubated with or without 50 nM of AVID200 and plated in semi-solid media. Treatment with AVID200 did not affect the overall number of colonies generated, but reduced the numbers of JAKV617F⁺ colonies while increasing the numbers of WT colonies: for PT1, there were 32% JAKV617F⁺ CFUs in untreated cultures (11 JAKV617F⁺/34 total colonies) versus 16% JAKV617F⁺ CFUs (7 JAKV617F⁺/42 total CFUs) in AVID200 treated cultures; for PT2 there were 100% JAKV617F⁺ CFUs in untreated cultures (37 JAKV617F⁺/37 total CFUs) versus 94% JAKV617F⁺ CFUs (49 JAK2V617F⁺/52 total CFUs) in AVID200 treated cultures. The in vivo effects of AVID200 on the development of MF in GATA1 ^low mice will be presented at the meeting.

These data indicate that AVID200 selectively suppresses TGFβ1 signaling associated with the proliferation of MSCs and type I collagen synthesis, and depletes MF MNCs of JAK2V617F⁺progenitor cells. We conclude that AVID200 is a promising agent for treating MF patients which will be evaluated in a phase 1 clinical trial.