Abstract: Pexidartinib (PLX3397) is a novel, orally bioavailable small-molecule tyrosine kinase inhibitor that selectively targets the colony-stimulating factor 1 receptor (CSF1R). In 2019, it became the first systemic therapy approved by the US Food and Drug Administration (FDA) for the treatment of adult patients with symptomatic tenosynovial giant cell tumor (TGCT) associated with severe morbidity or functional limitations not amenable to surgical improvement. This comprehensive literature review explores the pharmacological activity, molecular mechanism of action, structure-activity relationship, current limitations, and future perspectives of pexidartinib in TGCT therapeutics.
1. Introduction
Tenosynovial giant cell tumor (TGCT) is a rare, locally aggressive, typically nonmalignant neoplasm arising from the synovial membranes of joints, bursae, and tendon sheaths [1]. The disease is characterized by the overexpression of colony-stimulating factor 1 (CSF1), often driven by a chromosomal translocation t(1;2)(CSF1;COL6A3), which recruits CSF1R-expressing macrophages to form an inflammatory tumor mass [86]. Historically, surgical resection was the only effective treatment; however, recurrence rates are high, and repeated surgeries can lead to severe joint destruction and morbidity [69]. The development of pexidartinib, a CSF1R inhibitor, marked a significant breakthrough, providing the first FDA-approved systemic treatment option for patients with advanced or inoperable TGCT [21].
2. Pharmacological Activity
Pexidartinib has demonstrated robust clinical efficacy in TGCT. In the pivotal Phase 3 ENLIVEN trial, patients treated with pexidartinib achieved an overall response rate (ORR) of 39% by RECIST v1.1 and 56% by Tumor Volume Score (TVS) at week 25, compared to 0% in the placebo group [2][41]. Long-term follow-up data revealed that ORRs increased to 60% (RECIST) and 68% (TVS), with the median duration of response not reached [23]. Furthermore, pexidartinib significantly improved patient-reported outcomes, including physical function (PROMIS-PF), range of motion, and joint stiffness [46].
Pharmacokinetically, pexidartinib is highly protein-bound (99%) and reaches peak plasma concentration (Tmax) in approximately 1 to 2.5 hours [6][31]. Its absorption is significantly affected by food; a high-fat meal delays Tmax and increases the maximum concentration (Cmax) and area under the curve (AUC) by 100% [31]. It is primarily metabolized by CYP3A4 and UGT1A4, with a half-life of 26.6 hours, and is eliminated mainly via feces [35].
3. Molecular Mechanism of Action
The therapeutic efficacy of pexidartinib in TGCT is rooted in its ability to disrupt the CSF1/CSF1R axis. TGCT is driven by a "landscape effect" where a small population of neoplastic cells overexpresses CSF1, which acts in a paracrine manner to recruit a large mass of non-neoplastic, CSF1R-expressing inflammatory cells (macrophages and monocytes) [69]. Pexidartinib selectively binds to and inhibits CSF1R, preventing the binding of CSF1 and subsequent receptor activation [22]. This blockade halts the proliferation, survival, and recruitment of tumor-associated macrophages (TAMs), effectively shrinking the tumor and reducing localized inflammation [47]. Additionally, pexidartinib inhibits KIT proto-oncogene receptor tyrosine kinase (c-KIT) and FMS-like tyrosine kinase 3 (FLT3) harboring internal tandem duplication (ITD) mutations [13].
4. Structure-Activity Relationship (SAR)
Pexidartinib functions as a highly potent, small-molecule tyrosine kinase inhibitor that binds to the ATP-binding pocket of the CSF1R kinase domain, preventing ligand-induced auto-phosphorylation [40]. It exhibits exceptional biochemical potency, with a half-maximal inhibitory concentration (IC50) of 0.013 μM for CSF1R and 0.027 μM for KIT [40]. This potency is significantly higher than that of conventional type 2 inhibitors like imatinib (which has an IC50 of 0.67 μM for CSF1R) [40]. Furthermore, pexidartinib demonstrates high selectivity, showing limited cross-reactivity with other kinases (IC50 > 0.1 μM for CDK19, FLT3, KDR, LCK, FLT1, and TRK3, and > 1 μM for most other tested kinases) [40].
5. Current Limitations
The primary limitation of pexidartinib is its safety profile, specifically the risk of severe hepatotoxicity. Treatment is associated with dose-dependent elevations in transaminases (AST and ALT) and rare but potentially fatal cases of vanishing bile duct syndrome (mixed/cholestatic hepatotoxicity) [9][80]. Due to these risks, pexidartinib is only available in the United States through a restricted Risk Evaluation and Mitigation Strategy (REMS) program [69], and the European Medicines Agency (EMA) refused its authorization [46]. Other common adverse events include hair color changes (attributed to c-KIT inhibition), fatigue, nausea, and dysgeusia [54]. Additionally, the drug's significant food effect necessitates strict dosing modifications (e.g., taking it with a low-fat meal) to prevent toxic overexposure [16].
6. Future Perspectives
Ongoing research is exploring strategies to optimize pexidartinib therapy. One promising avenue is its use as a neoadjuvant treatment to reduce tumor volume, thereby facilitating limb-salvage surgery in patients with previously inoperable TGCT [87]. Clinical trials are also evaluating treatment discontinuation and rechallenge protocols to minimize long-term toxicity while sustaining disease control [39]. Beyond adult TGCT, pexidartinib is being investigated in pediatric TGCT populations [87] and in combination therapies for other malignancies, including FLT3-ITD mutant acute myeloid leukemia, glioblastoma, and advanced solid tumors, leveraging its ability to deplete immunosuppressive tumor-associated macrophages [47][79].
