Fore Biotherapeutics’ Phase 2 lead: an oral small-molecule BRAF “paradox breaker.” Below, side by side: the computed 3D structure of the molecule, and the real experimental cryo-EM complex of it bound in BRAF V600E.
BRAF class 1 & 2 inhibitorPhase 2 · FORTE master protocolBreakthrough Therapy (HGG)Paradox breaker
1 · Plixorafenib — 3D conformer computed
Carbon grey · N blue · O red · F pale-green · S yellow — drag to rotate.
Molecular make-up
A compact (542.5 g/mol) aryl-sulfonamide BRAF inhibitor: a 7-azaindole hinge-binder, a fluoropyrrolidine sulfonamide, a pyrimidine–cyclopropyl arm, and three fluorines.
RDKit ETKDG v3 + MMFF94 model, from the public SMILES — a computed conformer, not a bound pose.
2 · BRAF·plixorafenib complex experimental
■ BRAF V600E (chain A) ■ 14-3-3 dimer (B/C) ■ plixorafenib (ATP pocket) — drag to rotate.
What you are looking at
The real experimental structure (cryo-EM, PDB 8VYS) of a BRAF V600E monomer with plixorafenib bound — not a docked model. The drug occupies the kinase domain’s ATP pocket, holds the αC helix inactive, and restores the autoinhibited BRAF:14-3-3 state (grey partner). This directly shows target engagement and the monomer-inactivation half of the mechanism. It is the V600E point-mutant monomer — it does not itself depict the dimer disruption that drives BRAF-fusion activity (that mechanism is discussed below).
8VYS
PDB (cryo-EM)
V600E
BRAF variant
A1AEN
ligand code
3
chains (BRAF+14-3-3×2)
Coordinates from RCSB PDB 8VYS. Hydrogens omitted; rendered directly from the deposited experimental model.
💡 Why both structures — a computed model beside experimental ground truth
Panel 1 is a predicted conformer built from the public SMILES; Panel 2 is the experimental cryo-EM structure (PDB 8VYS) of that same molecule captured inside BRAF V600E. Showing them together is deliberate, and it matters for diligence in two ways.
1 · It anchors computation against reality. A model is only as trustworthy as the evidence behind it. Placing the predicted structure next to the deposited experimental one lets a reviewer confirm the computational chemistry is consistent with how the drug is actually observed to bind — not an unverifiable in-silico claim.
2 · It turns a claim into evidence. You can see it directly, not just assert it: plixorafenib bound in the kinase-domain ATP pocket, holding the αC helix inactive and restoring the autoinhibited (off) state of a V600E monomer. Experimentally-confirmed target engagement at atomic resolution is among the strongest structural de-risking signals for a clinical-stage asset. (This structure evidences target engagement and monomer inactivation; the dimer-disruption relevant to fusions is a separate, established property — see the BRAF-fusions section.)
1
Phase 2 registrational lead
Plixorafenib is the compound in Fore’s global Phase 2 FORTE basket trial across BRAF-altered cancers — the company’s central clinical asset.
2
First-in-class mechanism
A paradox breaker: it inhibits BRAF V600 monomers and disrupts the RAF dimer interface, avoiding the paradoxical MAPK activation that limits first-generation RAF inhibitors.
3
Regulatory momentum
FDA Breakthrough Therapy Designation in high-grade glioma, plus Fast Track and Orphan Drug designations for CNS tumors.
🧩 Mechanism — a BRAF “paradox breaker”
First-generation RAF inhibitors can paradoxically activate the MAPK pathway in cells with wild-type BRAF by promoting RAF dimerization — a driver of both toxicity and resistance. Plixorafenib is designed to selectively inhibit BRAF V600 monomers while disrupting the RAF dimer interface, so it targets BRAF V600 point mutations and BRAF fusions/dimers without triggering paradoxical activation.
Why it matters for diligence. The paradox-breaker mechanism is the differentiator versus earlier RAF inhibitors: it opens BRAF class 2 (dimer/fusion) alterations as addressable targets and is positioned to address acquired resistance to first-generation agents. The experimental complex above (PDB 8VYS) confirms the drug engages the BRAF kinase domain and locks a V600E monomer inactive; the dimer-disrupting action — the part relevant to fusions — is established pharmacologically (Nature Medicine, 2018), not depicted by that particular monomer structure.
🧩 From mechanism to patient selection — BRAF fusions
The paradox-breaker mechanism predicts which BRAF alterations should respond. Beyond V600 point mutants, a key opportunity is BRAF fusions — and it is structure, not the fusion partner, that governs responsiveness.
V600E vs. fusion — the distinction (they are different alterations).V600E is a point mutation that switches BRAF on as a single active unit (a monomer) — that is what the experimental structure above captures. A BRAF fusion is a gene rearrangement: BRAF’s kinase half is joined to a partner gene, switching it on as an active dimer. The drug binds the same ATP pocket in both — the BRAF kinase domain, and thus its drug-binding site, is identical by sequence in V600E and in fusions — but its job differs: inactivate the monomer (V600E) versus break the dimer (fusion). (Pocket identity is by kinase-domain sequence conservation; no plixorafenib–fusion co-structure has been deposited.)
Structural selection rule. A BRAF fusion is expected to respond when it (1) retains the intact BRAF kinase domain, (2) loses the N-terminal autoinhibitory region, and (3) gains a partner-contributed dimerization domain — producing an active dimer. Plixorafenib engages the same kinase-domain ATP pocket seen in the structure above, but its distinguishing action here is to disrupt that dimer (its established dimer-breaker mechanism; Nature Medicine 2018). Response is therefore largely partner-agnostic once the BRAF kinase domain is retained.
That makes patient selection tractable from fusion architecture rather than a long partner list. The canonical high-confidence example is KIAA1549–BRAF (the single most common alteration in pediatric low-grade glioma, and the majority alteration in pilocytic astrocytoma); partners such as TRIM24, AGK, SND1, FAM131B and AKAP9 follow the same logic. RAF1/CRAF fusions are de-prioritized — they lack the BRAF kinase domain the drug engages.
Honest caveat. There is no published plixorafenib clinical response in a confirmed fusion patient yet. This thesis rests on the structural mechanism, plixorafenib preclinical data, and class-validation from tovorafenib (FIREFLY-1). FORTE sub-protocol D is the expected clinical readout.
📊 Physicochemical envelope
542.5
MW g/mol
4.11
cLogP
121
TPSA Ų
2
H-bond donors
6
H-bond acceptors
7
rotatable bonds
0.28
Fsp3
6
rings
Computed with RDKit from the disclosed structure. Descriptors sit inside oral small-molecule (Lipinski/Veber) space: MW < 550, HBD/HBA low, RotB 7, TPSA 121 Ų.
Structure & descriptors computed with RDKit from the public disclosed SMILES (PubChem CID 90116675). Experimental complex from RCSB PDB 8VYS. Mechanism and clinical context from Fore Biotherapeutics public disclosures. Reference page — not investment or medical advice. Generated 2026-07-08.