For patients with differentiated thyroid cancer, radioactive iodine can serve as both a way to find disease and a way to treat it. The problem becomes considerably more difficult when tumors lose the ability to take up enough iodine for that strategy to work, and researchers in China are now reporting that a new fibroblast activation protein inhibitor, or FAPI, radiopharmaceutical may provide a much clearer picture of metastatic disease in that setting.
In a prospective comparative study published Sept. 5 in the European Journal of Nuclear Medicine and Molecular Imaging, Ga-68 CTR-FAPI dramatically outperformed the widely studied Ga-68 FAPI-04 tracer in patients with radioiodine-refractory differentiated thyroid cancer. The study enrolled 40 patients with distant metastases, each of whom underwent both Ga-68 FAPI-04 PET/CT and Ga-68 CTR-FAPI PET/CT within 24 to 48 hours, with lesions confirmed using conventional imaging, pathology when available and long-term clinical and radiological follow-up.
Across the cohort, researchers identified 769 metastatic lesions. CTR-FAPI detected 763 of them, or 99.2%, while FAPI-04 detected 480, or 62.4%, a difference that was statistically significant.
The overall detection rate is striking, but the location of the missed disease may be even more important. CTR-FAPI detected 99.4% of lung metastases compared with just 37.5% for FAPI-04, while it also detected 99% of pleural or peritoneal metastases compared with 76.4%, 99% of lymph-node metastases compared with 81.7%, and 100% of bone metastases compared with 82.9%.
At the patient level, both tracers identified metastatic lesions in 37 of the 40 patients, but two patients had lesions visible only with CTR-FAPI. Researchers also reported that CTR-FAPI provided superior visualization of disease distribution in seven patients, or 17.5% of the cohort, while producing significantly higher tracer uptake in lung and bone metastases and better tumor-to-background contrast in local recurrences and lymph-node disease. Additional brain and liver metastases were also seen on CTR-FAPI imaging that were not identified with FAPI-04.
For physicians trying to understand the full burden of metastatic disease, those differences could matter because a scan that identifies more lesions across more organs may change how extensively a patient is staged, how treatment response is evaluated and, eventually, whether a molecular target appears sufficiently widespread to support a targeted therapy.
CTR-FAPI is not simply another version of the same imaging agent. Researchers incorporated a sulfur(VI) fluoride exchange chemistry-based linker into the molecule to create what they describe as a covalent targeted radioligand, and preclinical research had suggested that the design could provide stronger tumor binding and longer retention than FAPI-04. The new prospective study provides early clinical evidence that those molecular changes may translate into a substantial difference in what physicians can actually see on PET.
That is important for the broader FAPI field because many of the earliest FAP-targeted radiopharmaceuticals have demonstrated excellent tumor visualization but relatively short tumor retention. Rapid clearance can be advantageous for diagnostic imaging because it reduces background activity, but longer retention becomes particularly important when the same biological target is being explored for radiopharmaceutical therapy.
The study therefore points toward a larger question for FAPI development: whether radiopharmaceutical chemistry can be optimized not only to identify FAP-positive tumors, but eventually to keep a therapeutic isotope attached to those tumors long enough to deliver a meaningful radiation dose.
Differentiated thyroid cancers are usually among the more treatable cancers because thyroid tissue naturally concentrates iodine, allowing radioactive iodine to be used after surgery to identify and destroy remaining thyroid cancer cells. Some tumors, however, gradually lose that ability, and once metastatic differentiated thyroid cancer becomes radioiodine refractory, one of nuclear medicine's most effective natural theranostic systems stops working and treatment increasingly depends on other systemic approaches.
That makes radioiodine-refractory thyroid cancer a particularly interesting setting for new molecular targets. FAP is expressed by cancer-associated fibroblasts in the tumor microenvironment rather than by the cancer cells themselves, allowing FAPI radiopharmaceuticals to target a component that can be present across many different tumor types.
A growing body of research has already shown that FAPI imaging can identify recurrent and metastatic differentiated thyroid cancer. A separate recent study of Ga-68 FAPI-04 in differentiated thyroid cancer found high diagnostic accuracy and demonstrated that the tracer could identify metastases even in some patients whose thyroglobulin testing was negative.
The new CTR-FAPI data suggest the next question may no longer be simply whether FAPI works in thyroid cancer, but whether newer generations of FAP-targeted molecules can perform substantially better than the first wave of tracers.
The results are compelling, but they come with important limitations. The trial was conducted at a single center and involved only 40 patients, all of whom already had distant metastatic radioiodine-refractory disease, so the findings cannot yet be assumed to apply across the broader differentiated thyroid cancer population.
The comparison was also between two FAPI agents rather than against FDG PET/CT or another established imaging standard, and improved lesion detection does not automatically mean that patient management or outcomes will improve. Larger multicenter studies will be needed to determine whether the additional lesions found by CTR-FAPI change clinical decisions and whether the tracer can ultimately support a therapeutic counterpart.
Still, the magnitude of the difference is difficult to ignore. Detecting 99.2% of metastatic lesions compared with 62.4% using another agent aimed at the same biological target suggests that the next stage of radiopharmaceutical development may depend as much on molecular engineering as on identifying entirely new targets.
That could be particularly important for FAPI. The target has already attracted enormous interest across nuclear medicine because of its expression across multiple tumor types, but the field is increasingly moving beyond proving that FAP can be imaged and toward determining which molecular designs provide the tumor uptake, retention and pharmacokinetics needed to turn that target into a practical theranostic platform. In radioiodine-refractory thyroid cancer, CTR-FAPI may be an early indication of what that next generation could look like.
