Pediatric low-grade glioma (pLGG) is the most common type of brain tumor among children. It is characterized by slow-growing tumors that are often curable with complete surgical removal (1-3). Of note, ~2,000 patients suffer from this type of cancer annually in the USA. Symptoms vary depending on the location of the tumor, but can include headaches, vision problems, seizures and balance issues. Treatment options may involve surgery, chemotherapy, or targeted therapy, and the optimal treatment approach is selected based on factors, such as the location of the tumor, molecular characteristics and the possibility of complete surgical removal (4). Common treatments include a combination of thioguanine + procarbazine + CCNU + vincristine (TPCV) and vincristine + carboplatin. Trametinib in combination with dabrafenib is used for the treatment of pediatric patients who are ≥1 year of age with pLGG and with a BRAF V600E mutation who require systemic therapy (1).

The B-RAF signaling pathway is a key component of the RAS-RAF-MEK-ERK [mitogen-activated protein kinase (MAPK)] cascade, one of the most critical intracellular signaling networks regulating cell growth, proliferation, differentiation and survival (Fig. 1). Typically signaling begins at the cell surface receptor, such as a receptor tyrosine kinase (RTK) [e.g., epidermal growth factor receptor (EGFR), fibroblast growth factor receptor (FGFR) and platelet-derived growth factor receptor (PDGFR)]. Upon ligand binding (e.g., growth factors), RTKs undergo dimerization and auto phosphorylation. Growth factor receptor-bound protein 2 (GRB2) and son of sevenless (SOS) are key adaptor proteins that link activated RTKs. GRB2 SH2 binds phospho-tyrosines on RTK and SH3 domains of GRB2 recruit SOS (4-6). GRB2 is a 25-kDa adaptor protein. GRB2/SOS also play roles in activating downstream phosphatidylinositol 3-kinase (PI3K), leading to AKR mouse thymoma (AKT) activation for survival and growth. SOS promotes the guanosine diphosphate (GDP)-guanosine triphosphate (GTP) exchange on rat sarcoma virus (RAS). This recruits and activates RAS, a small GTPase that acts as a molecular switch: RAS-GDP → inactive; RAS-GTP → active. The GDP-GTP exchange is a critical molecular switch mechanism that activates G-proteins (or GTPases) in cell signaling, turning them from an ‘off’ (GDP-bound) to an ‘on’ (GTP-bound) state. Guanine nucleotide exchange factors catalyze this process by releasing bound GDP, allowing the more abundant cellular GTP to bind, which triggers conformational changes that initiate cellular responses (7).

Active RAS-GTP recruits rapidly accelerated fibrosarcoma (RAF) kinases (A-RAF, B-RAF and C-RAF/RAF-1) to the plasma membrane. B-RAF is the most potent activator of the downstream pathway due to its higher basal kinase activity and stronger interaction with MEK1/2. Inactive B-RAF is autoinhibited (the N-terminal regulatory region suppresses kinase activity). Upon binding the of RAS-GTP, conformational changes relieve inhibition. B-RAF can form homo (BRAF-BRAF)- or heterodimers (particularly with C-RAF), leading to MEK phosphorylation. MEK phosphorylates extracellular signal-regulated kinase (ERK). The dysregulation of this pathway, often through mutations in BRAF, is a hallmark of several types of cancer, including melanoma, thyroid, brain cancer and colorectal cancers. Although the majority of driver events are MAPK pathway alterations (BRAF fusions/mutations and NF1), the PI3K/AKT/mammalian target of rapamycin (mTOR) activation is common as a co-operating pathway (PTEN loss or promoter methylation, occasional PIK3CA mutations, upstream RTK activation) (3). PI3K/mTOR activation can influence the progression and treatment resistance of pLGG (5). Tovorafenib is a small-molecule kinase inhibitor, specifically a type II pan-RAF inhibitor. It is orally bioavailable anpenetrates the central nervous system (CNS), which is relevant for its use in brain tumors (8-10). Tovorafenib was approved by the US FDA on April 23, 2024 in patients ≥6 months of age with relapsed or refractory pLGG harboring a BRAF fusion or rearrangement or BRAF V600 mutation (9).

A-RAF is the least well-characterized member of the RAF family and generally exhibits a lower basal kinase activity compared with other isoforms. Structurally, as with all RAF kinases, it contains conserved regions (CR1-CR3), including the Ras-binding domain and kinase domain; however, its regulatory regions confer weaker activation efficiency (10).

B-RAF is the most catalytically active RAF isoform and a driver of MAPK signaling. Its structure favors activation as it possesses constitutive phosphorylation-mimicking residues in its activation segment, reducing dependence on upstream priming events. Upon binding to active Ras, B-RAF readily forms dimers and efficiently phosphorylates MEK. Functionally, B-RAF is a dominant activator of the MAPK cascade, particularly in proliferative signaling. Notably, oncogenic mutations (e.g., V600E) render B-RAF constitutively active, constituting it a major therapeutic target in cancer (9).

C-RAF (also known as RAF-1) has intermediate kinase activity; however, it plays a central regulatory role in RAF signaling. Its activation is more tightly controlled and requires multiple steps: Recruitment by Ras, dephosphorylation of inhibitory sites, phosphorylation at activating residues and dimerization (often with B-RAF). Beyond MEK phosphorylation, C-RAF also has kinase-independent roles, including the regulation of apoptosis through interactions with mitochondrial proteins. This makes it functionally more versatile than B-RAF (9).

pLGGs are frequently driven by BRAF alterations (~50-85% of cases), most commonly the KIAA1549-BRAF fusion and BRAF V600E mutation, which constitutively activate the RAS/MAPK pathway to drive tumor growth. The pathway activates downstream MEK and ERK kinases, promoting cell proliferation and survival (2). Some distinct features are summarized in Table I.

The chemical name of tovorafenib is 6-amino-5-chloro-N-((1R)-1-(5-(((5-chloro-4-(trifluoromethyl)-2-pyridinyl)amino)carbonyl)-2-thiazolyl)ethyl)-4-pyrimidinecarboxamide (; its molecular formula is C₁₇H₁₂Cl₂F₃N₇O₂S. It has a molecular weight of 506.29; LogD (pH 7.4) of 2.79; a polar surface area of 164 Ų; a hydrogen bond donor count of 3; a hydrogen bond acceptor count of 11; a freely rotating bond count of 5; a heavy atom count of 32; and a number of ring count 3(10).

Lipinski's Rule of Five dicts that compounds with a molecular weight <500 Da, <5 hydrogen bond donors, <10 hydrogen bond acceptors, and a LogP value <5 are more likely to exhibit favorable oral absorption and bioavailability. Tovorafenib is considered to generally comply with Lipinski's Rule of Five, with physicochemical properties consistent with oral bioavailability. Its balance of molecular weight, polarity and lipophilicity supports adequate membrane permeability and gastrointestinal absorption, which aligns with its clinical use as an orally administered targeted therapy (11).

The synthesis of tovorafenib begins from compound 1 (Fig. 2). Compound 1 is converted into acid chloride using oxalyl chloride conditions and it is then coupled with compound 3 to provide compound 4. Compound 4 reacts with hydroxylamine to yield oxime, compound 5. Compound 5 is converted into primary amine using Zn/AcOH conditions. The chiral resolution of compound 6 using D-ditoluoyl tartaric acid in isopropanol followed by sodium carbonate basification provides chiral amine, compound 7. Compound 7 is coupled with compound 8 using amide coupling agents to afford the final compound 9, tovorafenib (11,12).

The dosage of tovorafenib based on body surface area is 380 mg/m² orally once weekly (the maximum dosage is 600 mg orally once weekly) with or without food (13).

Tovorafenib is a highly selective, type II, pan-RAF inhibitor that stabilizes RAF kinases in their inactive (DFG-out) conformation (19). Unlike type I BRAF inhibitors (vemurafenib and dabrafenib) that bind the active conformation, the type II profile of tovorafenib results in a distinct binding mode that contributes to its activity against BRAF fusions and BRAF V600 as well as wild-type RAF dimers. From the high resolution BRAF-tovorafenib co-crystal structure (PDB: 8F7O), the bi-substituted pyrimidine ring of tovorafenib (the portion mimicking the ATP site) forms two hydrogen bonds with the hinge region of the kinase, one H-bond with the backbone amide of residue Cys532, and another H-bond with the backbone carbonyl of Cys532 (Fig. 3). On the left-hand side of the molecule, the amide nitrogen forms a H-bonding interaction with the side-chain of Thr529. The central thiazole ring of tovorafenib stays between Thr529 and the catalytic lysine (Lys483), although that is more a positional/steric placement than a hydrogen bond. A carbonyl oxygen (adjacent to the central ring) of tovorafenib forms a hydrogen bonding interaction with the backbone amide of Asp594, a residue in the DFG motif. On the right-hand side of the molecule, an amide nitrogen forms a hydrogen bond with the side-chain of Glu501 located on the αC-helix (19,20).

Therefore, tovorafenib engages BRAF through a network of 4-5 hydrogen bonds and locks BRAF into an inactive DFG-out/αC-in. The trifluoromethyl-substituted pyridine ring of tovorafenib sits a hydrophobic pocket created by the displaced DFG phenylalanine (Phe595), the DFG-out flip opens this pocket, and the trifluoromethyl pyridine moiety nestles deeply there, contributing via hydrophobic interactions (19).

The overall inhibitor spans the ATP-binding cleft, leveraging both hinge binding (via the pyrimidine) and extension into the DFG-out hydrophobic pocket; this dual anchoring (hinge + DFG-pocket) is typical of type II inhibitors and is critical for potency and selectivity. By forming the H-bond with Glu501 (on the αC-helix), tovorafenib helps stabilize αC-in a conformation incompatible with kinase activation. This contributes to its ability to inhibit BRAF both as a monomer and in a dimer (19).

The DFG-out stabilization (via D594 backbone H-bond + hydrophobic insertion into the DFG-pocket) locks the activation loop in an inactive conformation, blocking access to ATP and preventing catalytic activity. As both protomers of BRAF dimer can bind tovorafenib (full occupancy), this helps explain the potency of the inhibitor even against BRAF dimers, which are relevant in a number of oncogenic contexts (e.g. BRAF fusions, RAS-driven dimerization) (20).

Tovorafenib is an orally available pan-RAF kinase inhibitor. The pharmacokinetic parameters described below are typically reported for the tablet or oral-suspension formulations used in clinical trials.

From clinical trials, some treatment-related adverse events in patients have been observed, including rash, dry skin, hair color changes, fatigue, vomiting, nausea and headaches. All are manageable and below grade 3 in severity (10).

Tovorafenib is a type-II pan-RAF inhibitor (binds inactive RAF conformation and exhibits potent activity against CRAF and BRAF and RAF dimers), whereas vemurafenib, dabrafenib and encorafenib are type-I BRAF V600 selective inhibitors that bind the active monomeric mutant BRAF (Table III). This difference explains i) why tovorafenib may be active against tumors with BRAF fusions/rearrangements (common in pediatric LGG); and ii) why it may overcome some resistance mechanisms tied to RAF dimerization (20). Tovorafenib was developed with CNS penetration and has shown responses in pLGG; dabrafenib (particularly when combined with trametinib) also exhibits evidence of intracranial activity for V600E gliomas. Vemurafenib and encorafenib are less commonly used for BRAF-fusion gliomas. In cell free, biochemical assays, tovorafenib inhibits all RAF isoforms and vemurafenib, dabrafenib, encorafenic are more selective BRAF V600 mutant inhibitors (21,22). From clinical trial tovorafenib exhibits greater efficacy compared to other RAF inhibitors (10). The long half-life of tovorafenib helps to lower the dosing interval.

Tovorafenib represents a critical step forward over first-generation type I BRAF inhibitors; however, acquired and intrinsic resistance remain major clinical challenges, particularly in the long-term treatment of pLGGs. The resistance mechanism to RAF inhibition primarily involves the reactivation of alternative survival pathways.

One common mechanism involves the reactivation of the MAPK pathway independent of RAF inhibition, driven by enhanced MEK or ERK signaling. This can occur through the upregulation of RTKs, such as FGFR, PDGFR or EGFR, which are frequently expressed in gliomas and can drive RAS activation in the upstream of RAF. Increased RAS-GTP levels may promote RAF dimerization and stabilize signaling complexes, thereby partially reducing the effectiveness of inhibitors, even for type II pan-RAF inhibitors (23).

Another key escape route is the activation of compensatory PI3K/AKT/mTOR signaling, commonly observed in pLGG due to PTEN loss, epigenetic silencing, or upstream RTK activation. This pathway provides an alternative proliferative and survival signal when MAPK signaling is suppressed, contributing to disease stabilization rather than durable tumor regression (24).

Compared to type I BRAF inhibitors, tovorafenib is less likely to cause unwanted MAPK activation in wild-type cells, as it inhibits RAF dimers and CRAF activity. However, this benefit may only delay adaptive resistance mechanisms driven by pathway rewiring and tumor heterogeneity, rather than completely stop them (14).

Given the flexible nature of MAPK signaling and the long treatment times required for pediatric glioma, combination therapy is a sensible approach to improve the durability of response to tovorafenib. Several combination strategies have strong biological support and increasing clinical or preclinical evidence.

One approach is vertical pathway inhibition. In pLGG, the MAPK pathway can be disrupted at several points, leading to increased cell growth, survival and tumor formation. Combination therapies that include BRAF V600E inhibitors (vemurafenib, dabrafenib), MEK inhibitors (trametinib, pimasertib and selumetinib), and ERK inhibitors (ulixertinib) targeting these alterations have shown promising results in pLGG (25).

Another promising approach is horizontal pathway inhibition, combining tovorafenib with PI3K/AKT/mTOR pathway inhibitors. In pLGG, PI3K signaling is frequently coactivated. Dual inhibition may prevent compensatory survival signaling and enhance tumor control. Preclinical studies suggest that RAF inhibition sensitizes tumors to PI3K or mTOR blockade, providing a strong mechanistic rationale for this approach (26).

Tovorafenib is a next-generation, highly selective, CNS-penetrant type II RAF inhibitor that provides a meaningful therapeutic advance for pLGG driven by BRAF alterations, particularly BRAF fusions and rearrangements. By stabilizing RAF in its inactive conformation and avoiding paradoxical MAPK pathway activation, it provides more durable and safer pathway suppression than first-generation BRAF inhibitors. Its once-weekly oral dosing, favorable tolerability profile and clinically significant, durable responses position it as a valuable targeted therapy for children with recurrent or progressive BRAF-altered LGG. Continued research will clarify its role in combination strategies, long-term outcomes and potential applications in broader RAF- or MAPK-activated other tumors (27,28).

Positioning tovorafenib within the treatment landscape of pLGG benefits from a side-by-side comparison with current standards, followed by a clear proposal for how it could be integrated into evolving care algorithms (Table IV).