Selpercatinib Expands RET Fusion Testing Across Tumors

I kept a notebook beside me while reading the approval materials. Near the top of the page, I wrote four words: “fusion, not tumor site.” That is the practical shift here, though the details around those words matter.

What the regulatory change establishes

Selpercatinib (Retevmo) now has traditional approval for adults and children age 2 years and older whose locally advanced or metastatic solid tumors contain a qualifying RET fusion. Patients must have progressed on or after prior systemic treatment or have no satisfactory alternative treatment option.

The indication is tumor-agnostic. Eligibility rests mainly on the molecular finding and the patient’s clinical situation, rather than the organ where the cancer began.

Traditional approval means the FDA determined that the requirement for confirming clinical benefit had been met. It does not mean selpercatinib outranks every treatment developed for a particular cancer, and it does not show that patients with every RET fusion-positive histology receive the same benefit. Clinicians still have to consider the disease-specific standard of care, earlier treatment, the pace of the cancer, other health conditions and how much evidence exists for that tumor type.

The age cutoff carries its own weight. The same tumor-agnostic indication now extends from age 2 through adulthood, removing a regulatory divide that could complicate precision treatment for children with rare cancers, although pediatric use still calls for age-appropriate prescribing, formulation and safety monitoring under the current label.

I underlined “age 2 years and older” in the notebook. It is a short phrase carrying a considerable amount of clinical work.

Evidence supporting the tumor-agnostic strategy

The development program includes LIBRETTO-001, a phase 1/2 open-label study of selpercatinib in adolescents and adults with advanced RET-altered cancers. It also includes LIBRETTO-121, a phase 1/2 study involving children and young adults with advanced RET-altered solid tumors or primary central nervous system tumors. The tumor-agnostic cohorts did not have randomized comparison groups.

Earlier FDA evidence summaries described clinically meaningful responses, some of them lasting, across several uncommon cancer types. Those observations supported the biological argument that an oncogenic RET fusion can predict sensitivity even when the cancers began in different organs. The traditional approval is now the controlling regulatory decision, and its accompanying prescribing information defines the covered population and safety requirements.

These figures cannot show that selpercatinib is better than another therapy. There was no control arm. The cohorts were small, enrollment depended on molecular selection, and only a few patients represented some individual tumor types, which makes a broad response percentage useful but leaves much less certainty when a doctor and patient are discussing one uncommon cancer.

The FDA summaries cited here did not specify median follow-up. Duration of response is therefore more informative than a generalized statement about how long participants were observed.

RET testing now has broader treatment consequences

The approval gives RET fusion testing more relevance outside lung and thyroid cancers, where such testing is already better established. RET fusions can occur in pancreatic, colorectal, salivary, breast, ovarian and biliary cancers, among others, but they are generally uncommon. Running one gene test after another can waste limited tissue and still miss other actionable findings.

Comprehensive next-generation sequencing can assess RET while looking for additional biomarkers that might shape treatment. Yet the design of the assay matters, a point that can disappear beneath the reassuring phrase “broad panel,” because RET fusions often have intronic breakpoints that some DNA-only panels do not capture well, while RNA sequencing can identify expressed fusion transcripts and clarify uncertain DNA results.

A combined DNA-and-RNA approach may improve detection when enough tissue is available.

The laboratory report also has to separate a pathogenic RET fusion from a RET point mutation, amplification, rearrangement of uncertain significance or computational finding that has not been verified. They are not interchangeable. RET mutations have a separate role in medullary thyroid cancer; the tumor-agnostic solid-tumor indication discussed here requires a qualifying fusion.

Plasma circulating tumor DNA testing can provide information when tumor tissue is unavailable or obtaining it would be unsafe. A negative result has limits. Tumors with low DNA shedding, including low-volume cancers and cancers in anatomical sites that release little DNA into the bloodstream, can produce false-negative liquid-biopsy findings.

A negative plasma test therefore does not reliably rule out a RET fusion. Tissue or RNA testing may still be reasonable when the clinical setting and access to a specimen support another look.

In the notebook, I drew a box around “negative plasma.” Beside it I added, “not the same as absent.” That distinction is easy to lose after a report reaches the patient portal and everyone is waiting for the next decision.

Context belongs on the report too. When possible, the laboratory can identify the fusion partner, explain assay limitations and classify the clinical significance of the alteration. Molecular tumor boards may help with uncommon partners, conflicting DNA and RNA findings, pediatric cases or cancers that already have a well-supported standard treatment that could reasonably come first.

Translating a positive result into treatment selection

A positive assay establishes molecular eligibility. It does not automatically establish treatment priority.

Under the labeled setting, the patient must have locally advanced or metastatic disease that progressed on or after systemic treatment, or there must be no satisfactory alternative. If a cancer has an effective, potentially curative or strongly supported standard approach, this tumor-agnostic approval should not be read as a reason to set that approach aside without a clinical basis.

Before treatment, the current prescribing information remains the source for administration, drug interactions, dose modification and monitoring. Labeled risks include hepatotoxicity, hypertension, QT-interval prolongation, hemorrhagic events, hypersensitivity, tumor lysis syndrome, impaired wound healing, hypothyroidism and embryo-fetal toxicity.

Pediatric care brings additional questions involving formulation, development and growth, while coordination may span pediatric oncology, pharmacy, genetics and supportive care. The regulatory language may now cover patients from age 2 onward, but a toddler and a 62-year-old with different cancers do not arrive at treatment through the same clinical conversation.

Access sits behind all of this. Broad sequencing changes care only when patients can obtain the test, have the result interpreted and reach the treatment. Coverage policies that reserve molecular profiling for selected common cancers may miss rare RET fusion-positive tumors, including pediatric cancers. Because prevalence is low, multiplex testing generally makes more sense than ordering a stand-alone RET assay across cancers without regard to context.

Important uncertainties remain

The central limitation is still the nonrandomized evidence base. Objective response and response duration offer useful information in rare, molecularly selected groups, but they do not measure comparative survival, quality of life or net benefit against the treatment ordinarily used for a specific cancer.

Early-phase trial enrollment can also narrow the population being observed. Patients with poorer performance status, organ dysfunction or substantial comorbidity may not be represented in the same way, so the published response experience may not map neatly onto the people seen in everyday oncology clinics.

Histologic heterogeneity adds another difficulty. Pooling tumor types supports the idea of a shared biological effect, while at the same time it can hide differences in response probability, resistance and natural history. The pediatric data are particularly limited by sample size, and longer observation is needed to understand late toxicities, growth effects, resistance and what happens after treatment ends.

Traditional approval settles the conditional status of the indication. Questions about sequencing and outcomes in small subgroups remain open, leaving work for longer follow-up, prospective registries and studies focused on individual diseases.

That last sentence filled most of a line in the notebook. The next line stayed blank.

Questions clinicians ask

Which patients meet the tumor-agnostic indication?

The indication covers adults and children age 2 years and older who have a locally advanced or metastatic solid tumor containing a qualifying RET fusion. The cancer must have progressed on or after prior systemic treatment, or the patient must have no satisfactory alternative treatment option under the current FDA-approved labeling.

Is a DNA-only negative result enough to exclude a RET fusion?

Not always. Some DNA assays do not adequately cover the large intronic regions where RET breakpoints can occur, while plasma tests may be negative when the tumor releases little DNA. RNA-based testing or a combined DNA-and-RNA panel may be considered when tissue is available and the first assay had limited ability to detect fusions.

Does every RET alteration qualify for selpercatinib treatment?

No. This tumor-agnostic indication concerns qualifying RET gene fusions, rather than amplification or an unclassified rearrangement. RET point mutations are biologically and clinically different, although separate selpercatinib indications cover certain RET-mutant medullary thyroid cancers. The alteration, cancer type and applicable label have to match.

Does traditional approval make selpercatinib the preferred next therapy?

No. Traditional approval confirms the FDA’s determination for the specified population, but the supporting tumor-agnostic cohorts did not compare selpercatinib with standard treatment in randomized trials. Treatment choice and sequencing still depend on disease-specific options, prior therapy, likely benefit, toxicity, patient priorities and the strength of evidence for that histology.

In the notebook, the word “preferred” remains crossed out.

Questions people ask

Who qualifies for selpercatinib under this tumor-agnostic approval?

The author found that eligibility covers adults and children age 2 and older with a qualifying RET fusion in a locally advanced or metastatic solid tumor. The cancer must have progressed after systemic treatment, or there must be no satisfactory alternative treatment option.

Can a negative DNA or plasma test rule out a RET fusion?

The review found that some DNA-only assays may miss RET fusions and that plasma tests can be falsely negative when tumors shed little DNA. The author learned that tissue-based RNA testing or combined DNA-and-RNA testing can sometimes clarify an uncertain result.

Does traditional approval make selpercatinib the preferred treatment?

No. The author noted that the supporting tumor-agnostic studies lacked randomized comparison groups, so they did not establish superiority over standard treatments. Treatment priority still depends on disease-specific options, prior therapy, risks, patient priorities and the evidence for that cancer type.

References

1. FDA Grants Traditional Approval to Selpercatinib for Locally Advanced or Metastatic RET Fusion-Positive Solid Tumors — US Food and Drug Administration, 2026
2. A Study of Selpercatinib in Participants With Advanced Solid Tumors, RET Fusion-Positive Solid Tumors, and Medullary Thyroid Cancer (LIBRETTO-001) — ClinicalTrials.gov, 2017
3. A Study of Selpercatinib in Pediatric Participants With Advanced Solid or Primary Central Nervous System Tumors (LIBRETTO-121) — ClinicalTrials.gov, 2019
4. Retevmo: FDA Drug Approval Record — US Food and Drug Administration, 2020