Radiolabeled Peptides in PET and SPECT Imaging
Peptide-Based Imaging
3 key components
Every radiolabeled peptide tracer has three parts: a receptor-targeting peptide, a chelator that holds the radioactive atom, and a radionuclide that emits the signal the scanner detects.
Aloj et al., Journal of Peptide Science, 2024
Aloj et al., Journal of Peptide Science, 2024
View as imageRadiolabeled peptides are short amino acid chains attached to radioactive atoms that light up specific targets inside the body. A peptide engineered to bind somatostatin receptors, for example, will travel through the bloodstream, find every tumor cell that overexpresses those receptors, and emit radiation that a PET or SPECT scanner converts into a three-dimensional image. The result is a molecular map of receptor expression across the entire body, produced in a single scan. This approach has transformed the diagnosis of neuroendocrine tumors and is expanding to prostate cancer, breast cancer, insulinomas, and gliomas. For the foundational imaging agent that started this field, see OctreoScan: The Original Somatostatin Receptor Imaging Agent.
Key Takeaways
- Radiolabeled peptide tracers consist of three components: a receptor-targeting peptide, a chelator (DOTA, NOTA, or HYNIC), and a radionuclide (68Ga for PET or 99mTc/111In for SPECT)[1]
- 68Ga-DOTATATE PET/CT has replaced OctreoScan (111In-pentetreotide SPECT) as the gold standard for neuroendocrine tumor imaging, with higher sensitivity and spatial resolution[5]
- GLP-1 receptor PET using 68Ga-exendin-4 detected insulinomas with higher accuracy than conventional imaging and 68Ga-DOTATATE combined[8]
- Novel 68Ga-labeled GRPR-targeted PET tracers are being developed for prostate and breast cancer imaging[3]
- Computational peptide design is accelerating radiotracer development: a 2026 study used ligand-based computational methods to design a novel cyclic peptide radiotracer[12]
- Interim PET/CT during PRRT treatment provides prognostic information that can guide therapy modifications[10]
The Three-Part Architecture
Every radiolabeled peptide tracer is built from three components. Understanding this architecture explains how tracers are designed and why different tracers work for different applications.
The targeting peptide determines where the tracer goes. It is a short amino acid sequence (typically 5-15 residues) engineered to bind a specific receptor with high affinity and specificity. Somatostatin analogs (octreotide, TATE, TOC) target somatostatin receptor type 2 (SSTR2). Bombesin-derived peptides target the gastrin-releasing peptide receptor (GRPR). Exendin-4 targets the GLP-1 receptor. The peptide must survive long enough in the bloodstream to reach its target, so modifications for protease resistance (D-amino acid substitutions, cyclization) are common.
The chelator is a molecular cage that holds the radioactive atom. The choice of chelator determines which radionuclides can be used. DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) is the most versatile, stably binding gallium-68, lutetium-177, yttrium-90, and indium-111. NOTA is preferred for gallium-68 in some applications because it forms complexes at room temperature. HYNIC is used for technetium-99m labeling in SPECT. Chen and colleagues (2026) published work on optimizing 68Ga-DOTA radiolabeling conditions for multicyclic peptides, demonstrating that even established chelation chemistry continues to be refined.[2]
The radionuclide provides the signal. For PET imaging, the radionuclide emits positrons (gallium-68, fluorine-18, copper-64). For SPECT imaging, it emits gamma rays (technetium-99m, indium-111). The choice of radionuclide determines image quality, scanning time, and radiation dose to the patient. Aloj and colleagues (2024) reviewed the expanding role of radiolabeled peptides in clinical imaging and targeted therapy, noting that gallium-68 has become the dominant PET isotope for peptide tracers because of its generator-based production (no cyclotron needed), 68-minute half-life (long enough for imaging, short enough to limit patient dose), and compatibility with DOTA chelation.[1]
PET vs SPECT: Why PET Won for Peptide Imaging
PET (positron emission tomography) and SPECT (single-photon emission computed tomography) are both nuclear medicine techniques that detect gamma radiation emitted from within the body. But their performance differs in ways that matter for peptide-based imaging.
SPECT uses radionuclides that directly emit gamma rays. Technetium-99m (99mTc) is the workhorse, with a 6-hour half-life and low cost. Indium-111 (111In) was used in the original OctreoScan. SPECT scanners use collimators to determine the direction of incoming gamma rays, which limits spatial resolution to approximately 8-12 mm.
PET uses radionuclides that emit positrons, which annihilate with nearby electrons to produce two 511 keV gamma rays traveling in opposite directions. By detecting these coincident photon pairs, PET achieves spatial resolution of 4-6 mm (and better with newer scanners), higher sensitivity, and the ability to quantify tracer uptake in standardized uptake values (SUV).
The practical consequence: a PET scan with 68Ga-DOTATATE detects smaller lesions and provides more accurate quantification than a SPECT scan with 111In-octreotide (OctreoScan). Alonzo and colleagues (2025) confirmed in a systematic review that somatostatin receptor PET/CT has become the first-line molecular imaging modality for neuroendocrine tumors, largely replacing SPECT-based approaches.[5] For more on the specific tracer, see 68Ga-DOTATATE PET: The Gold Standard for Neuroendocrine Tumor Imaging.
Clinical Applications by Receptor Target
Somatostatin receptor imaging (SSTR2)
The most established application. 68Ga-DOTATATE, 68Ga-DOTATOC, and 68Ga-DOTANOC target SSTR2 (and to varying degrees SSTR3 and SSTR5) on neuroendocrine tumor cells. These tracers are FDA-approved for PET/CT imaging and are used for diagnosis, staging, restaging, and selection of patients for peptide receptor radionuclide therapy (PRRT). Bekkhoucha and colleagues (2026) reported a case of lung metastases from meningioma diagnosed on whole-body 68Ga-DOTATOC PET/CT and successfully treated with PRRT, illustrating how peptide imaging extends beyond its original neuroendocrine indication.[4]
Dubash and colleagues (2024) compared a newer 18F-labeled somatostatin peptide tracer ([18F]FET-betaAG-TOCA) with 68Ga-DOTA-peptide PET/CT, exploring whether fluorine-18's longer half-life (110 minutes vs. 68 minutes for gallium-68) could provide practical advantages in clinical workflow.[9] For more on 18F-labeled peptide tracers, see 18F-Labeled Peptide PET Tracers: Longer Half-Life, Different Applications.
GLP-1 receptor imaging (insulinoma)
Insulinomas are small, often occult pancreatic tumors that overexpress GLP-1 receptors. 68Ga-exendin-4 (a radiolabeled GLP-1 receptor agonist) PET/CT can locate insulinomas that conventional CT, MRI, and even 68Ga-DOTATATE PET miss. Yu and colleagues (2025) published a head-to-head comparison showing that 68Ga-NOTA-exendin-4 PET/CT outperformed 68Ga-DOTATATE, 18F-FDG, and conventional imaging for insulinoma detection.[7]
Zhang and colleagues (2025) reported real-world data on GLP-1 receptor PET/CT with 68Ga-exendin-4 for localizing insulinomas, confirming its clinical utility in patients who had negative conventional imaging.[8]
GRPR imaging (prostate and breast cancer)
The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer and breast cancer. Jozi and colleagues (2026) reported the synthesis and evaluation of novel 68Ga-labeled GRPR-targeted PET tracers derived from bombesin analogs, achieving high tumor uptake and favorable pharmacokinetics in preclinical models.[3] GRPR-targeted PET may complement PSMA-targeted imaging in prostate cancer, particularly in tumors with low PSMA expression.
Emerging targets
Xue and colleagues (2026) developed an FGF2-based cyclic peptide PET tracer for noninvasive detection of FGFR1 expression in non-small cell lung cancer. This demonstrates how the radiolabeled peptide platform can be adapted to virtually any receptor target by swapping the targeting peptide while keeping the chelator-radionuclide architecture intact.[6]
Wang and colleagues (2026) used ligand-based computational design to create a novel cyclic peptide radiotracer, illustrating how AI and computational chemistry are accelerating the discovery of new imaging agents.[12]
Imaging During Treatment: Prognostic Value
Radiolabeled peptide imaging is not limited to diagnosis. It also provides prognostic information during treatment.
Shin and colleagues (2025) showed that interim 68Ga-DOTA-TOC PET/CT performed during PRRT provides prognostic value, with changes in tumor uptake predicting treatment outcomes and potentially guiding therapy modifications.[10]
Mamulashvili and colleagues (2025) demonstrated organ-specific response assessment using 68Ga PET/CT after 177Lu-DOTATATE PRRT, showing that different metastatic sites can respond differently to the same treatment, information that is invisible to conventional imaging.[11]
For how peptide imaging feeds directly into therapy selection, see Gallium-68 Labeled Peptides: Why This Isotope Revolutionized PET Imaging.
The Bottom Line
Radiolabeled peptides are three-component molecular probes (targeting peptide + chelator + radionuclide) that visualize receptor expression across the entire body in a single PET or SPECT scan. Gallium-68 DOTA-peptide PET/CT has become the standard for neuroendocrine tumor imaging, replacing older SPECT-based approaches. New peptide tracers targeting GLP-1R, GRPR, FGFR1, and other receptors are expanding the approach to insulinomas, prostate cancer, breast cancer, and lung cancer. Computational peptide design is accelerating the development pipeline.