Entrectinib: Uses, Side Effects, and Efficacy

Entrectinib, sold under the brand name Rozlytrek, is a targeted cancer drug approved to treat solid tumors driven by specific genetic changes known as NTRK gene fusions, as well as a form of lung cancer fueled by ROS1 gene rearrangements. Unlike traditional chemotherapy, which attacks rapidly dividing cells broadly, entrectinib zeroes in on the abnormal proteins produced by these gene fusions, and it has a distinctive ability to cross into the brain, making it relevant for patients whose cancer has spread there. The drug has been approved in the United States, Europe, and other regions, and the evidence behind it spans multiple tumor types in both adults and children.

What Entrectinib Treats

The FDA approved entrectinib in August 2019 for two distinct groups of patients. The first is adults and children aged 12 and older with solid tumors that harbor an NTRK gene fusion, provided the cancer is metastatic or cannot be surgically removed without severe harm, and there is no satisfactory alternative treatment. This approval is “tissue agnostic,” meaning it applies regardless of where in the body the tumor originated. The second approved use is for adults with a type of non-small-cell lung cancer (NSCLC) that carries a ROS1 rearrangement.¹

NTRK fusions are rare across cancer as a whole, but they turn up in a wide range of tumor types. In the trials that led to approval, patients had ten different kinds of cancer, including sarcomas, lung cancers, salivary gland tumors, thyroid cancers, breast cancers, and colorectal cancers, among others. This breadth is what makes the tissue-agnostic label meaningful: if your tumor has the right molecular signature, the drug can be considered regardless of the organ involved.

ROS1-positive NSCLC is also uncommon, affecting roughly 1–2% of lung cancer patients. But for those patients, entrectinib offers a targeted oral therapy that has shown strong and durable responses, including in the brain.

How It Works

Entrectinib is a small-molecule inhibitor that blocks a family of enzymes called tyrosine kinases. Specifically, it targets three TRK proteins (TRKA, TRKB, and TRKC, which are encoded by the NTRK1, NTRK2, and NTRK3 genes), the ROS1 protein, and ALK.² When a gene fusion occurs, two genes that are normally separate get joined together, producing an abnormal protein that is permanently switched on and drives cancer growth. Entrectinib works by competing for the site on these proteins where the energy molecule ATP normally binds, effectively shutting the protein down.³

One of the drug’s most clinically important features is its ability to cross the blood-brain barrier. Many cancer drugs struggle to reach meaningful concentrations in the brain because a protein pump called P-glycoprotein actively pushes them back out. Entrectinib interacts only weakly with this pump, which translates into much higher drug levels in cerebrospinal fluid compared with some related drugs. In preclinical testing, entrectinib achieved a ratio of cerebrospinal fluid concentration to unbound plasma concentration of about 0.22, roughly seven times higher than what was measured for crizotinib and larotrectinib under similar conditions.⁴ This property matters because brain metastases are a common and serious complication, particularly in lung cancer.

Efficacy in NTRK Fusion-Positive Tumors

The evidence for entrectinib in NTRK-positive cancers comes from pooled analyses of three single-arm clinical trials. In the original FDA review, 54 patients with ten different tumor types were evaluated, and about 57% had a confirmed objective response, meaning their tumors shrank substantially. About 7% achieved a complete response, with the remainder showing partial responses.¹

An updated analysis with a larger pool of 121 patients showed improvement over time: about 61% responded, with roughly 16% achieving complete responses. Responses lasted a median of 20 months.⁵ The response rate was notably higher in patients who had not received prior systemic therapy for metastatic disease, at about 81%, compared with roughly 52% in those who had already been treated with other regimens.⁶ That gap suggests entrectinib may work best as an early-line treatment when fewer resistance mechanisms have had a chance to develop.

For patients with brain metastases, the drug showed strong intracranial activity. Among 16 patients with baseline brain involvement, half had their brain tumors respond. In a subset of 8 patients with measurable brain lesions, the intracranial response rate reached about 63%. Only 3 out of 74 patients in the broader evaluation showed confirmed progression of disease in the brain, raising the possibility that entrectinib may also help prevent new brain metastases from taking hold.⁷

Efficacy in ROS1-Positive Lung Cancer

The story for ROS1-positive NSCLC is similarly encouraging. An integrated analysis of 161 evaluable patients found an overall response rate of about 67%, with responses lasting a median of roughly 16 months. The 12-month overall survival rate was 81%.⁸

Longer follow-up data from a larger cohort of 168 patients showed a response rate of 68%, including a 13% complete response rate. Median duration of response climbed to about 20.5 months, and median overall survival reached nearly 48 months. More than half of patients were alive without disease progression at the 12-month mark.⁹ These numbers are impressive for a population with advanced lung cancer, and they improved as more data accumulated.

The brain was again a notable area of activity. Among 24 patients with measurable brain metastases at baseline, about 79% had their intracranial tumors respond, with a median intracranial duration of response of nearly 13 months.⁸ A confirmatory phase 2/3 trial (the BFAST study) found that the response rate exceeded 70%, meeting its primary endpoint and aligning with earlier data.¹⁰

Use in Children

NTRK fusions turn up more frequently in certain pediatric cancers, including a type of brain tumor called infantile glioma. Entrectinib has been studied in children through the STARTRK-NG trial, which enrolled patients with solid or central nervous system tumors harboring NTRK, ROS1, or ALK fusions. In patients with confirmed gene fusions, the response rate was about 58%, and the median duration of response had not been reached at the time of analysis, suggesting that many responders were still benefiting from treatment.¹¹

A more recent analysis with additional follow-up evaluated 64 pediatric patients and found stronger results. Children with NTRK fusions had a confirmed response rate of about 73%, while those with ROS1 fusions responded at a rate of 65%. Median duration of response was not reached in either group, again pointing to lasting benefit for many patients. The median age was 4 years for the NTRK group and 7.5 for the ROS1 group, reflecting that these fusions often appear in very young children.¹² One property that makes entrectinib especially relevant in pediatric care is its ability to penetrate the central nervous system, since brain tumors are disproportionately common in children. Cerebrospinal fluid sampling in treated pediatric patients has confirmed that the drug does reach the brain compartment.¹³

Side Effects

Entrectinib is generally considered manageable from a side-effect standpoint, but it does produce a recognizable list of adverse events. The most frequently reported effects in clinical trials include altered taste, fatigue, dizziness, constipation, diarrhea, nausea, weight gain, tingling or numbness in the extremities, elevated creatinine levels, muscle pain, swelling in the limbs, vomiting, joint pain, anemia, and elevated liver enzymes.¹⁴ Many of these effects are mild to moderate and can be handled with dose adjustments or supportive care.

Some side effects tie directly to the drug’s mechanism. Because TRK proteins have normal roles in sensation and balance, blocking them can produce altered taste and problems with coordination. Weight gain is thought to relate to the role of TRKB in appetite regulation, and cognitive side effects have also been reported, consistent with what is seen with other drugs in this class.¹⁵ In pediatric patients, the most common treatment-related adverse events were weight gain (about 35%) and anemia (about 32%).¹²

There are also less common but more serious concerns. Entrectinib has been associated with cognitive and vision problems, congestive heart failure, prolongation of the QTc interval on an electrocardiogram (a marker of heart rhythm disturbance), and bone fractures.¹⁶ These are uncommon, but oncologists monitor for them, particularly in older patients or those with pre-existing heart conditions. Dose reductions and temporary treatment pauses are the standard tools for managing side effects that become bothersome or clinically significant.

How Quality of Life Holds Up During Treatment

One concern with any cancer drug is whether the side effects undercut the clinical benefit. Patient-reported outcome data from the STARTRK-2 trial paint a relatively favorable picture for entrectinib. Patients started treatment with moderate-to-high self-reported overall health scores, and those scores held steady during therapy. Physical and role functioning either stayed the same or trended toward improvement. Symptom burden was low to moderate at baseline and generally stayed that way or got better. Symptoms often linked to cancer treatment, like nausea and fatigue, remained absent or mild for most patients.¹⁷

For patients with NTRK-positive tumors specifically, pain and role functioning showed clinically meaningful improvement while on entrectinib. Patients with ROS1-positive lung cancer reported meaningful improvement in cough, one of the most bothersome symptoms in lung cancer.¹⁸ These findings suggest that for most patients, entrectinib does not significantly erode daily life, and in some areas actively improves it.

Drug Interactions Worth Knowing About

Entrectinib is broken down in the body primarily by an enzyme called CYP3A4. That matters because many common medications either speed up or slow down this enzyme, potentially altering how much entrectinib ends up in your bloodstream. In studies with healthy volunteers, the strong CYP3A4 inhibitor itraconazole (an antifungal) increased entrectinib blood levels by about fivefold. On the other end, rifampin (an antibiotic used for tuberculosis) cut entrectinib levels by about 77%.¹⁹

In practice, this means that certain antifungals, some antibiotics, grapefruit juice, and several other medications can raise entrectinib to potentially toxic levels, while drugs like some anti-seizure medications and the herbal supplement St. John’s wort can lower it to the point where it stops working effectively. Your oncologist will review your full medication list before starting entrectinib and may adjust doses or switch interacting drugs.

Finding Out if You Are a Candidate

Entrectinib only works when the tumor carries the specific gene fusion it targets, so molecular testing is a prerequisite. Several laboratory methods can detect NTRK fusions, each with trade-offs. A technique based on sequencing the RNA in tumor tissue (RNA-based next-generation sequencing) is considered the gold standard because it directly detects the fusion transcript. Alternative approaches are available in settings where this sequencing is not accessible.²⁰ ROS1 fusions are typically identified through similar molecular panels or through a preliminary screening test called immunohistochemistry followed by a confirmatory assay.

Because NTRK fusions are rare in common cancers but more frequent in certain uncommon tumor types, guidelines recommend broad genomic profiling for patients with advanced solid tumors, particularly when standard treatments have been exhausted or when the tumor’s molecular profile is unknown. The tissue-agnostic nature of the approval means that a positive NTRK fusion result is actionable regardless of where the cancer started, which makes testing especially valuable in rare or hard-to-treat tumor types where few standard options exist.

When Entrectinib Stops Working

Like most targeted cancer drugs, entrectinib can lose its effectiveness over time as tumors develop resistance mutations. In ROS1-positive cancers, two mutations have been identified as recurring culprits. One, called ROS1 F2004C, makes the cancer about 13-fold less sensitive to entrectinib. The other, ROS1 G2032R, is far more problematic, reducing sensitivity roughly 76-fold.²¹ The G2032R mutation is a challenge across several drugs in this class, not just entrectinib.

Part of what makes resistance predictable is the way entrectinib fits into its target. Structural modeling suggests it binds to the protein in a manner that makes it susceptible to both major classes of resistance mutations, giving tumors more than one escape route.²² When resistance develops, oncologists may switch to next-generation inhibitors designed to overcome specific mutations, or explore combination strategies. Re-biopsy of the tumor or analysis of circulating tumor DNA at the time of progression can identify the specific mutation driving resistance and guide the next treatment choice.

Entrectinib Versus Other TRK and ROS1 Inhibitors

Entrectinib is not the only drug that targets NTRK fusions. Larotrectinib, approved around the same time, hits the same TRK proteins but does not target ROS1 or ALK. Both drugs produce high response rates in NTRK-positive tumors, and no head-to-head trial has directly compared them. The most concrete difference lies in how well they get into the brain. As noted, entrectinib achieves cerebrospinal fluid concentrations about seven times higher than larotrectinib in preclinical models, driven largely by a weaker interaction with the P-glycoprotein efflux pump.⁴ This makes entrectinib a natural consideration when brain involvement is present or is a realistic risk.

For ROS1-positive lung cancer, the competitive landscape includes crizotinib (the first ROS1-targeted drug approved) and newer agents like lorlatinib and repotrectinib. Crizotinib shares the same limitation of poor brain penetration. Lorlatinib and repotrectinib are designed to overcome some resistance mutations that defeat entrectinib, making them potential options when entrectinib stops working. The choice among these agents often comes down to the patient’s specific clinical picture: whether brain metastases are present, which (if any) prior therapies have been tried, and what mutations are driving resistance.