Abstract: Pexidartinib (PLX3397) is a potent, orally bioavailable small-molecule inhibitor of the colony-stimulating factor 1 receptor (CSF1R). Originally developed and FDA-approved for the treatment of tenosynovial giant cell tumors, it has recently emerged as a pivotal pharmacological tool in neuroscience for its ability to induce rapid and extensive microglial depletion. In the context of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD), microglia often undergo a phenotypic shift from homeostatic to detrimental, pro-inflammatory states. By blocking the CSF-1/CSF-1R signaling axis, Pexidartinib eliminates these aberrant microglia, thereby mitigating neuroinflammation, reducing pathological protein accumulation (such as tau and amyloid-beta), and preventing neuronal loss. This review synthesizes the current literature on Pexidartinib, detailing its pharmacological activity, molecular mechanism of action, structure-activity relationships, and current limitations including hepatotoxicity and off-target effects. Finally, it explores future perspectives, highlighting the therapeutic potential of transient microglial depletion and subsequent repopulation as a cellular "reset" strategy for neuroinflammatory disorders.
1. Introduction
Neuroinflammation is a central driver in the pathogenesis of chronic neurodegenerative disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD). This inflammatory response is primarily mediated by microglia, the resident immune cells of the central nervous system (CNS) [1]. Under physiological conditions, microglia regulate tissue repair, synaptic pruning, and immune surveillance. However, in chronic disease states, they can shift from protective roles to aberrant, pro-inflammatory functions that exacerbate neurodegeneration [1][2].
Microglial survival, proliferation, and differentiation are critically dependent on the colony-stimulating factor 1 receptor (CSF1R) [1]. Pexidartinib (PLX3397) is a potent tyrosine kinase inhibitor that targets CSF1R. Initially investigated as an antitumor agent and subsequently FDA-approved (under the trade name TURALIO®) for the treatment of tenosynovial giant cell tumors (TGCT) [1][4], Pexidartinib has been widely repurposed in preclinical research. Its ability to cross the blood-brain barrier and eliminate up to 99% of microglia has made it an invaluable tool for investigating the role of microglia in neuroinflammation and exploring microglial depletion as a therapeutic strategy for AD and PD [1][2].
2. Pharmacological Activity
Pexidartinib exhibits highly favorable pharmacokinetic and pharmacodynamic properties for CNS targeting. It is orally bioavailable and effectively crosses the blood-brain barrier (BBB) [2]. In murine models, PLX3397 has a half-life of approximately 2.6 hours and a clearance rate of 2.1 mL/min/kg, whereas in humans, the half-life extends to 26.6 hours with a clearance of 5.1 L/h [2]. In preclinical studies, it is typically administered via rodent chow (ranging from 275 to 1,000 mg/kg) or oral gavage (30 to 50 mg/kg), achieving near-complete microglial depletion within 7 to 21 days depending on the dosage and duration [1][2].
In models of AD, Pexidartinib-mediated microglial depletion has been shown to halt tau propagation, reduce neurodegeneration, and prevent the downregulation of synaptic genes in the hippocampus [1][2]. In PD models, it reduces dopaminergic neuronal death and α-synuclein accumulation, particularly when administered before or during disease onset [1]. Beyond microglia, Pexidartinib also impacts other glial populations. For instance, microglial depletion can lead to increased astrocyte reactivity, likely as a compensatory response to phagocytose accumulating microglial debris [2]. Furthermore, Pexidartinib has demonstrated pharmacological activity in oncology, reducing tumor-associated macrophages (TAMs) and restoring the sensitivity of glioma cells to other therapies [5].
3. Molecular Mechanism of Action
The primary mechanism of action of Pexidartinib is the competitive inhibition of the CSF1R tyrosine kinase [2][4]. The CSF-1/CSF-1R signaling axis is the main effector of homeostasis in the erythromyeloid lineage, responsible for the proliferation, differentiation, and survival of mononuclear cells, including microglia [2]. By binding to CSF1R, Pexidartinib blocks the tyrosine kinase-dependent activation of the receptor, cutting off essential survival signals.
Consequently, pharmacological inhibition of CSF1R by Pexidartinib induces rapid microglial apoptosis. Studies have shown that following treatment, microglia become positive for active caspase-3 (a marker of apoptosis) and propidium iodide (indicative of cell death) [1]. The drug acts continuously on microglia; therefore, sustained administration is required to maintain depletion. Upon withdrawal of Pexidartinib, the surviving microglial progenitor cells rapidly proliferate through mitosis, allowing the brain to be fully repopulated with microglia within 5 to 7 days [1][3].
4. Structure-Activity Relationship (SAR)
Pexidartinib was prospectively designed to engage the juxtamembrane region of the CSF1 receptor, which contributes to its high potency [4]. It is a highly potent inhibitor, demonstrating a biochemical half-maximal inhibitory concentration (IC50) for CSF1R of approximately 0.017 to 0.02 µM [2][4].
However, Pexidartinib is considered a dual inhibitor because it also exhibits strong affinity for the c-Kit receptor tyrosine kinase, with an IC50 of 0.01 to 0.012 µM [2][4]. In a comprehensive screen of 226 different kinases, only five other kinases were significantly inhibited by Pexidartinib, and their IC50 values were at least 8-fold higher than those for CSF1R or c-Kit [4]. Additionally, Pexidartinib effectively inhibits FLT3 signaling, showing particular efficacy in overriding structural resistance in cells harboring the FLT3-ITD mutation (IC50 = 0.018 µM) compared to wild-type FLT3 (IC50 = 1.8 µM) [4]. Its specific structural engagement makes it significantly more potent than earlier generation inhibitors such as imatinib [4].
5. Current Limitations
Despite its efficacy in preclinical models, the clinical translation of Pexidartinib faces significant hurdles. The most critical limitation is hepatotoxicity. The US FDA labeling for Pexidartinib carries a boxed warning for serious and potentially fatal liver injury. Consequently, it is only available through a restricted Risk Evaluation and Mitigation Strategy (REMS) program, requiring rigorous liver function monitoring [4].
Pharmacologically, Pexidartinib's dual inhibition of CSF1R and c-Kit leads to off-target peripheral effects. At doses required for microglial depletion, it significantly affects peripheral hematopoiesis, reducing circulating monocytes and bone marrow-derived macrophages [2]. Furthermore, prolonged microglial depletion (e.g., greater than 3 to 6 months) has been associated with detrimental outcomes, such as increased α-synuclein expression and the emergence of PLX-resistant, pro-inflammatory microglia [1].
Preclinical studies also highlight significant sex-specific differences in response to Pexidartinib. Male mice generally exhibit higher levels of microglial depletion and more pronounced behavioral improvements compared to female mice under identical dosing regimens, complicating the standardization of depletion protocols [1].
6. Future Perspectives
Given the limitations of continuous, long-term microglial depletion, future therapeutic strategies are pivoting toward transient or intermittent depletion followed by repopulation. Evidence suggests that repopulated microglia adopt a non-inflammatory, homeostatic phenotype. This repopulation acts as a cellular "reset," restoring normal morphology and promoting anti-inflammatory profiles that have shown neuroprotective effects in both AD and PD models [1].
Additionally, there is a push for the development of more selective CSF1R inhibitors. Molecules like PLX5622, which has a significantly lower inhibitory potential on c-Kit (IC50 = 0.86 µM) compared to Pexidartinib, offer a more targeted approach with fewer peripheral side effects [2]. Researchers are also exploring non-brain-penetrant CSF1R inhibitors (such as PLX73086) to distinguish between the central and peripheral roles of macrophages in neurodegeneration [1]. Finally, cutting-edge approaches, such as engineering inhibitor-resistant human CSF1R variants for microglia replacement therapy, hold promise for selectively replacing diseased microglia while maintaining essential homeostatic functions [2].
7. References
