CSF1R inhibitors pexidartinib and sotuletinib induce rapid glial ablation despite their limited brain penetrability

Background

Microglial reactivity, a hallmark of many neurodegenerative diseases, is thought to contribute significantly to disease pathology. In experimental models, colony stimulating factor 1 receptor (CSF1R) inhibitors transiently deplete microglia to resolve inflammation, leading to improved neuropathology. In oncology, CSF1R inhibitors modulate tumor-associated macrophages (TAMs) toward a tumor-suppressive phenotype by silencing CSF1–CSF1R signaling. As for any therapeutic, target engagement depends on effective drug delivery. In the brain a major hurdle is the limited drug delivery caused by the presence of the blood brain barrier (BBB) containing drug efflux transporters. However, the affinity to these transporters of most CSF1R inhibitors is unknown.

Methods

We assessed the brain penetrance of two CSF1R inhibitors, pexidartinib (PLX3397) and sotuletinib (BLZ945), in the absence and presence of drug transporters ABCB1 and ABCG2. We further assessed their impact on peripheral immune populations, tissue-resident macrophages, microglia and oligodendrocyte progenitor cells (OPCs).

Results

Both compounds have a limited brain permeability (brain-to-plasma ratio: 0.1). Sotuletinib was a substrate for both ABCB1 and ABCG2, whereas pexidartinib was transported primarily by ABCB1. Despite low brain exposure, both are able to ablate microglia when given to mice at high doses, accompanied by marked depletion of OPCs and macrophage populations in the liver, intestine, and kidney, as well as non-classical monocytes in blood. Pexidartinib additionally altered splenic immune composition, increasing T cells and neutrophils, and reducing dendritic cells and non-classical monocytes.

Conclusion

These findings highlight that high-dose CSF1R inhibition rapidly depletes microglia, but induces substantial off-target effects. Such systemic impacts, as well as the impact on OPCs, should be considered when interpreting experimental outcomes or translating CSF1R inhibition into clinical contexts where brain targeting is required.

Key messages

• CSF1R inhibitors pexidartinib and sotuletinib show poor brain penetrance (brain-to-plasma ratio 0.1)

• Sotuletinib is a substrate to ABCB1 and ABCG2, pexidartinib is a substrate to ABCB1.

• Both drugs rapidly deplete microglia, despite poor brain penetration

• Microglia depletion is accompanied by loss of OPCs and tissue macrophages