How Larotrectinib and Entrectinib Work: TRK Fusion Inhibitors
Larotrectinib and entrectinib are oral inhibitors of the TRK family of kinases (TRKA, TRKB, TRKC), encoded by NTRK1, NTRK2 and NTRK3. They are approved on a tumour-agnostic basis: eligibility depends on the tumour carrying an NTRK gene fusion, not on where the cancer started.
Quick Answer
Larotrectinib and entrectinib are oral inhibitors of the TRK family of kinases (TRKA, TRKB, TRKC), encoded by NTRK1, NTRK2 and NTRK3. They are approved on a tumour-agnostic basis: eligibility depends on the tumour carrying an NTRK gene fusion, not on where the cancer started.
Side-by-side comparison
The two first-generation TRK inhibitors share the NTRK-fusion indication but differ in target range, CNS design and side-effect emphasis.
| Feature | Larotrectinib | Entrectinib |
|---|---|---|
| Kinase targets | TRKA, TRKB, TRKC only | TRK plus ROS1 and ALK |
| CNS design | Active intracranially | Explicitly designed for CNS penetration |
| Also approved for | NTRK fusions, any tumour type | NTRK fusions and ROS1-positive lung cancer |
| Emphasised toxicity | Dizziness, raised transaminases, weight gain | Dizziness, weight gain, oedema, QT, reduced cardiac function |
What an NTRK Fusion Produces
An NTRK gene fusion joins the kinase-coding part of NTRK1, NTRK2 or NTRK3 to an unrelated partner gene. The partner typically provides a dimerisation domain and a constantly active promoter, so the resulting TRK fusion protein signals continuously through pathways including RAS-MAPK, PI3K-AKT and PLCγ.
When present, such a fusion is usually the dominant driver of that tumour, which is why blocking it can produce responses regardless of tissue of origin.
How the Drugs Inhibit It
Both larotrectinib and entrectinib bind the ATP pocket of the TRK kinase domain and stop it phosphorylating downstream targets. Larotrectinib is highly selective for the three TRK kinases. Entrectinib also inhibits ROS1 and ALK and was designed to penetrate the central nervous system.
Reported overall response rates across fusion-positive tumours are high (roughly 55-75%), with responses often durable, but the trials pooled many rare tumour types and numbers per type are small.
Testing Is the Gate
NTRK fusions are rare in common cancers (well under 1%) and near-universal in a few rare ones (infantile fibrosarcoma, secretory carcinoma). Detection uses RNA-based sequencing, DNA panels that cover intronic breakpoints, or pan-TRK immunohistochemistry as a screen. Each method has blind spots, and confirming a true fusion (not just an NTRK point mutation or amplification) matters.
A tumour-agnostic label does not mean every fusion-positive tumour behaves identically.
Resistance
On-target resistance occurs through mutations in the TRK kinase domain — solvent-front, gatekeeper and xDFG mutations — that block drug binding. Next-generation TRK inhibitors (selitrectinib, repotrectinib) were designed to overcome these.
Off-target resistance through activation of a bypass pathway, such as MET or KRAS, also occurs.
Dosing and Class Side Effects
Both are oral and taken continuously until progression or intolerance, with entrectinib once daily and larotrectinib twice daily; weight-based dosing and paediatric formulations exist because these fusions are common in some childhood tumours. Neither is a cytotoxic drug, and marrow suppression is not a prominent feature.
Class effects come partly from blocking normal TRK signalling in the nervous system: dizziness, unsteady gait, tingling and cognitive or mood changes, which are dose-related and often improve with reduction. Weight gain and raised liver enzymes are monitored across the class, and entrectinib adds QT prolongation and fluid retention with reduced cardiac function. Stopping abruptly can cause a temporary pain flare.
Key Takeaways
- ·NTRK fusions create a constantly active TRK kinase that is usually the tumour's main driver.
- ·Larotrectinib and entrectinib block that kinase and are approved by fusion status, not tumour type.
- ·Kinase-domain mutations cause on-target resistance; next-generation TRK inhibitors target them.
Frequently asked questions
What does a tumour-agnostic approval mean here?
Eligibility depends on the tumour carrying an NTRK gene fusion, not on the organ where the cancer started. The same drug can be used in a salivary, thyroid, lung or soft-tissue cancer if the fusion is present.
How is an NTRK fusion detected?
By RNA-based sequencing, DNA panels that cover the relevant introns, or pan-TRK immunohistochemistry as a screen. Each method has blind spots, and a true fusion must be distinguished from an NTRK point mutation or amplification, which do not predict response.
What happens when these drugs stop working?
On-target kinase-domain mutations (solvent-front, gatekeeper, xDFG) are the classic route; next-generation TRK inhibitors such as selitrectinib and repotrectinib were designed to cover them. Bypass-pathway activation also occurs.
References
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