An important consideration in the field of leukemia stem cells (LSCs) is to define culture condition that best mimic the in vivo microenvironment. LSCs rapidly differentiate when introduced in culture. This hampers our ability to conduct cell-based high-throughput lethality screens and partly explains the lack of success in drug development for this disease.

To overcome these constraints we conducted a high-throughput screening and identified small molecules that inhibit differentiation and support LSC activity in vitro. The first class of compounds identified, represented by SR1, inhibits the aryl hydrocarbon receptor (AhR) pathway. Interestingly this pathway is rapidly upregulated when cells are introduced in culture with obvious and rapid cell differentiation. A second compound, UM729, in combination with SR1 was shown to better maintain LSC activity in vitro. UM729 lacks AhR suppressor activity, which indicates that at least 2 different pathways are involved. These 2 compounds define the best culture conditions for improved ex vivo culture of primary human AML cells (Pabst et al., Nature Methods 2014).

Unfortunately, approximately 30% of specimens respond poorly to these molecules suggesting that a 3rd pathway is operational in securing LSC self-renewal divisions. That is why we continue to screen new molecules and investigate our best hits.