UM171 Is a Novel and Potent Agonist Of Human Hematopoietic Stem Cell Renewal

Abstract Various combinations of soluble factors have been reported to promote the ex vivo proliferation of human HSCs but few of them are capable of promoting self-renewal. Characterization of StemRegenin (SR1), an aryl hydrocarbon receptor (AhR) antagonist that promotes expansion of CD34+ cells ex vivo, provided a proof of principle that low molecular weight (LMW) compounds have the ability to modulate cellular pathways possibly leading to HSC expansion. In an attempt to identify novel putative agonists of HSC self-renewal, we tested a library comprising > 5,000 LMW molecules in a phenotypical screen based on in vitro expansion of CD34+CD45RA- cells. Eight hits were identified, the majority of which were AhR antagonists. Validation studies identified one hit synthesized in our institute which potently expanded human CD34+CD45RA- cord blood cells ex vivo. Importantly this molecule does not antagonize AhR. Structure activity relationship (SAR) generated a compound called UM171 which is 14-25 folds more potent than the starting molecule with an EC50 of 8-15 nM on CD34+ human cord blood (CB). Using the optimal dose of this analog, CD34+, CD34+CD45RA-, and CD34+CD38-CD90+CD45RA-CD49f+ were expanded 115±15, 62±3 and 121±6 fold respectively over the input values after 12 day culture in fed-batch system. The expansion of these phenotypic HSC populations was enhanced, in an additive manner by including SR1. Washout of UM171 led to a rapid loss of the CD34+CD45RA- population suggesting that its effect is reversible. UM171 is not a mitogen and cannot compensate for the absence of cytokines added to the media (Flt3L, SCF, and TPO). Our cell division studies revealed no effect of the compound on division rate compared to control indicating that UM171 cultured cells retain the CD34+CD45RA- phenotype. Apoptosis and cell phonotype analysis showed significantly less cell differentiation and cell death for UM171 cultured cells compared to controls or SR1 supplemented cultures suggesting that the compound acts by anti-differentiation/apoptosis. Similar to SR1, UM171 treated cells showed a 75±20 fold net increase in the numbers of multilineage colony forming cell (CFU-GEMM) and 150±20 fold when both compounds were present in the 12 day culture.To further understand the impact of UM171 on more primitive cell compartments, we transplanted 100, 500, 1000 and 10,000 uncultured CB CD34+ cells and their progeny after a 12-day culture (with or without UM171) in NSG mice and verified the human hematopoietic reconstitution in their bone marrow 20 weeks later. Mice transplanted with UM171 expanded cells showed a much higher level of human CD45 engraftment compared to uncultured or control expanded cells. The NSG Repopulating Cells (NRC) showed a net expansion ex vivo only in the presence of UM171 with values ranging from 3.1 to 5.5 fold increase over input value in 6 independent experiments. Interestingly, the co-treatment with SR1 did not significantly enhance the impact of UM171 on NRC expansion. In these fed-batch conditions SR1 treatment maintained the NRC population at input values. Importantly, we observed a linear relationship between the number of UM171-treated cells transplanted in vivo and level of human CD45 engraftment. Moreover, at limit dilution, these cells were capable of human myeloid and lymphoid differentiation in NSG BM environment suggesting that UM171 expanded cells remain multipotent. Together, our studies have identified a novel compound which truly expands human progenitors while amplifying the more primitive stem cell population. UM171 has successfully passed toxicity studies and is in the final phase of GMP production for a phase I study which will be launched shortly. In addition, preliminary evidence suggests that UM171 strikingly enhances gene transfer efficiency to primitive human CD34 CB cells. Disclosures: No relevant conflicts of interest to declare.