Vitrakvi (Larotrectinib): What It Is and What to Expect

Most cancer drugs are matched to a place. Breast, lung, colon. Larotrectinib is matched to a genetic accident instead.

The accident is called an NTRK gene fusion. A piece of an NTRK gene gets joined to a different gene. The result is a TRK protein that stays switched on. That signal drives growth.

Larotrectinib blocks TRK proteins. The label calls it a kinase inhibitor.

The fusion is rare. But it turns up in tumors starting almost anywhere in the body. That is why the approval ignores the organ. The brand name is Vitrakvi. The generic name is larotrectinib.

This is the part that gets summarized too loosely. The label sets three conditions, and all three apply at once.

The tumor must carry an NTRK gene fusion without a known acquired resistance mutation. That last clause matters. A resistance change already known to be present takes a patient outside the label.

The tumor must be metastatic, or in a place where surgery would likely cause severe harm.

And there must be no satisfactory alternative treatment, or the disease must have progressed after treatment.

The label also says to select patients using an FDA-approved test. There is one carve-out. In secretory breast cancer, mammary analogue secretory carcinoma, congenital mesoblastic nephroma and infantile fibrosarcoma, treatment can be considered without confirming the rearrangement first.

The dose depends on size, not on age.

For adults and children with a body surface area of one square meter or more, it is 100 mg by mouth twice daily. With food or without. It continues until the disease progresses or toxicity becomes unacceptable.

For children under one square meter, the amount is scaled to body surface area, up to that same twice-daily ceiling.

Those are the label's figures, and they are here so the capsules or the syringe of solution in front of you make sense. The amount to give is whatever the oncology pharmacy has written on your own bottle — it changes for liver problems and for interacting medicines, and for a child it is recalculated as they grow.

The drug comes as 25 mg and 100 mg capsules, and as an oral solution at 20 mg per milliliter. The solution exists because many of the patients are very small children.

There are separate dose rules for liver impairment, and for people taking strong CYP3A4 inhibitors or strong and moderate CYP3A4 inducers. Those are worth raising with a pharmacist, because they cover common medicines.

Hepatotoxicity is one of the label's Warnings and Precautions, and it comes with a timetable rather than a vague instruction.

Liver function tests — ALT, AST, alkaline phosphatase and bilirubin — are obtained before treatment starts, frequently through the first two months, and then regularly for as long as the drug continues. Your team sets the exact dates; what matters on your side is keeping them, because the schedule is how a liver problem gets caught while it is still reversible.

Depending on severity, the drug may be temporarily withheld, reduced, or stopped for good.

Two other warnings sit alongside it.

The first is central nervous system effects. The label names dizziness, cognitive impairment, mood disorders and sleep disturbance. It advises against driving or operating hazardous machinery while these are happening.

The second is skeletal fractures. The instruction is short: evaluate promptly anyone with signs or symptoms of a fracture.

Embryo-fetal toxicity is the fourth warning. The drug can harm a fetus, and effective contraception is advised.

The most common reactions, at 20% or more, include raised AST and ALT, anemia, low albumin, musculoskeletal pain, raised alkaline phosphatase, low white cells, low calcium, fatigue, vomiting, cough, constipation, fever, diarrhea, nausea, abdominal pain, dizziness and rash. The Contraindications section says none.

Efficacy came from 339 patients with NTRK fusion tumors across three trials. Median age was 38 years, with a range from 18 days to 90 years. Thirty-nine percent were under 18.

The overall response rate was 60%, with a 95% confidence interval of 55% to 65%. Twenty-four percent were complete responses and 36% partial.

The average hides a wide spread. Infantile fibrosarcoma reached 94%. Salivary gland tumors 84%. Soft tissue sarcoma and lung both 70%. Differentiated thyroid cancer 78%, against 14% for non-differentiated. Colorectal 46%. Primary CNS tumors 27%.

The fusion was found in local laboratories, by next generation sequencing in 89% of cases, and otherwise by FISH or RT-PCR. Asking which method was used is a fair question.

None of this happens without a test that looks for it.

A standard pathology report does not report NTRK fusions. They are found on a molecular test, usually a next generation sequencing panel run on tumor tissue. In the trials behind this label, 89% of fusions were identified that way. The rest came from FISH or RT-PCR.

Fourteen patients in the efficacy set were included on inference rather than direct evidence. They had a documented ETV6 or NTRK3 translocation found by FISH, and the fusion was inferred from it.

There is one more wrinkle worth knowing. Not every panel reports fusions. Some sequence DNA only, and fusions are easier to catch on RNA. Asking whether the panel that was run covers RNA fusions is a reasonable question when a result comes back with nothing actionable.

Two honest limits.

The first is that this is a control drug, not a cure. Treatment continues until progression or unacceptable toxicity. That phrasing is in the dosing section, and it says something about the expected shape of the course.

The second is resistance. The label excludes tumors with a known acquired resistance mutation, which exists as a category because these tumors do find ways around the drug. A response that lasts years and a response that lasts months are both inside the 60% figure.

For how this fits with other genotype-directed drugs, see selpercatinib and the wider picture in targeted therapy. For how a plan gets assembled around a result like this, see understanding your treatment plan.

• Call 911 or go to an emergency department if a fall or knock leaves a limb painful, misshapen or unable to take weight, or if a child treated with this drug stops using an arm or leg.
• Call your care team the same day if your eyes or skin turn yellow, your urine goes dark, or you have pain under the right ribs with nausea. Liver blood tests are on a fixed schedule for this reason.
• Call your care team within a day or two if you feel dizzy, muddled, low in mood or unable to sleep. Do not drive or use machinery while any of that is happening.

• DailyMed — VITRAKVI (larotrectinib) capsules and oral solution, FDA-approved label
• NCI — Larotrectinib Sulfate