Theranostic Peptides in Cancer

Peptide Theranostics

2 FDA-approved agents

Lutetium-177-DOTATATE (Lutathera) for neuroendocrine tumors and lutetium-177-PSMA-617 (Pluvicto) for prostate cancer are the first two FDA-approved peptide theranostic agents, with dozens more in clinical trials targeting different tumor receptors.

Hofland et al., JCEM, 2022

Hofland et al., JCEM, 2022

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The word "theranostic" combines "therapeutic" and "diagnostic" into a single concept: using the same targeting molecule for both purposes. In peptide theranostics, a tumor-seeking peptide is labeled with a gamma- or positron-emitting radioisotope for imaging, then the same peptide is labeled with a beta- or alpha-emitting radioisotope for treatment. The peptide finds the tumor through receptor binding; the isotope determines whether the result is an image or cell death. This approach was pioneered with radiolabeled somatostatin analogs targeting neuroendocrine tumors and has since expanded to prostate cancer (via PSMA-targeting peptides), gastrin-releasing peptide receptor (GRPR)-expressing tumors, and multiple other receptor systems.^[1] Two peptide theranostic agents have reached FDA approval: lutetium-177-DOTATATE (Lutathera, 2018) for somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors, and lutetium-177-PSMA-617 (Pluvicto, 2022) for PSMA-positive metastatic castration-resistant prostate cancer. This article covers the theranostic principle, the isotope pairs used, the approved agents and their clinical evidence, emerging receptor targets, and the limitations of current approaches. For how somatostatin analogs became the foundation of this field, see How Somatostatin Analogs Became the Gold Standard for Theranostics. For the imaging-first treatment paradigm, see Personalized Peptide Theranostics.

The Theranostic Principle: Same Peptide, Different Isotope

The core logic of peptide theranostics rests on receptor overexpression. Many tumors express specific cell-surface receptors at densities 10- to 100-fold higher than surrounding normal tissue. A peptide engineered to bind one of these receptors will accumulate preferentially in tumor tissue. Attach a diagnostic radioisotope, and you can see the tumor on a PET or SPECT scan. Attach a therapeutic radioisotope, and the peptide delivers targeted radiation directly to the tumor cells while largely sparing healthy tissue.

The diagnostic step comes first and serves a gatekeeping function. If a patient's tumor does not take up the diagnostic tracer on imaging, that patient will not benefit from the therapeutic version of the same peptide, because the peptide cannot reach the tumor in sufficient quantity. This "see it, then treat it" paradigm is what makes theranostics fundamentally different from conventional chemotherapy or external beam radiation: therapy is only offered to patients whose tumors have been proven to express the target receptor.

The Isotope Pairs

For diagnostic imaging, the most common isotope is gallium-68 (68Ga), a positron emitter with a 68-minute half-life used for PET/CT scanning. Fluorine-18 (18F) is used for some newer tracers, with a 110-minute half-life that allows more flexible logistics.

For therapy, the workhorse isotope is lutetium-177 (177Lu), a beta emitter with a 6.6-day half-life and tissue penetration of approximately 2 mm. This range is sufficient to irradiate small tumor cell clusters while limiting damage to distant normal cells. For tumors resistant to beta radiation, alpha emitters like actinium-225 (225Ac) are being investigated. Alpha particles have much shorter range (50-80 micrometers, roughly 2-3 cell diameters) but deposit 400 times more energy per unit distance, making them effective against microscopic disease and radioresistant tumors.^[2]

The typical theranostic pair uses 68Ga for PET imaging and 177Lu for therapy on the same peptide scaffold: 68Ga-DOTATATE for imaging plus 177Lu-DOTATATE for treatment in neuroendocrine tumors, or 68Ga-PSMA-11 for imaging plus 177Lu-PSMA-617 for treatment in prostate cancer.

Why Peptides Are Ideal Theranostic Carriers

Peptides occupy a pharmacological sweet spot for theranostics. They are large enough to achieve receptor-specific binding with high affinity (typically nanomolar range) but small enough for rapid tissue penetration and blood clearance. Their molecular weight (typically 1,000-5,000 daltons) gives them faster tumor penetration than antibodies (150,000 daltons), which can take days to distribute into solid tumors. At the same time, peptides clear from the blood within hours, reducing background radiation and improving tumor-to-blood ratios for both imaging contrast and therapeutic dose delivery.

Peptides can be synthesized with precise chemical modifications: chelators like DOTA or NOTA are attached to one end of the peptide to hold the radiometal ion, while the receptor-binding pharmacophore is preserved at the other end. Modifications to the peptide backbone (D-amino acid substitutions, cyclization, PEGylation) can tune the metabolic stability and pharmacokinetics without disrupting receptor binding. This chemical tractability makes peptides highly engineerable for theranostic optimization.

The limitation is renal accumulation. Small peptides are filtered by the glomerulus and reabsorbed in the proximal tubule via megalin-mediated endocytosis. This concentrates radiolabeled peptides in the kidney cortex, making the kidney the dose-limiting organ for most peptide-based PRRT protocols. Co-infusion of positively charged amino acids (lysine and arginine) competitively inhibits tubular reabsorption and reduces kidney dose by approximately 40%, but renal toxicity remains the primary safety constraint.

Somatostatin Receptor PRRT: The Proven Paradigm

Peptide receptor radionuclide therapy targeting somatostatin receptor subtype 2 (SSTR2) on neuroendocrine tumors was the first clinical theranostic success and remains the most extensively studied. The somatostatin analog octreotide, modified with a DOTA chelator and labeled with 177Lu to create 177Lu-DOTATATE, binds SSTR2 on tumor cells, is internalized, and delivers beta radiation from within the cell.

The landmark NETTER-1 trial compared 177Lu-DOTATATE plus octreotide LAR against octreotide LAR alone in patients with progressive, well-differentiated midgut neuroendocrine tumors. After 20 months, progression-free survival was 65.2% in the PRRT group versus 10.8% in the control group. This trial led to FDA approval of 177Lu-DOTATATE (Lutathera) in January 2018 and EMA approval the same year.^[3]

PRRT has since been incorporated into NCCN, ENETS, and other major oncology guidelines. A 2024 review of the current PRRT landscape noted that the NETTER-2 trial is evaluating first-line PRRT in grade 2 and grade 3 NET patients, potentially expanding the eligible population beyond the progressive, pretreated setting of NETTER-1.^[4] A 2024 comparative study (SeqEveR trial) examined the optimal sequencing of PRRT versus everolimus in metastatic NETs, addressing whether PRRT should be used before or after targeted molecular therapy.^[5]

The theranostic approach has expanded beyond gastroenteropancreatic NETs. A 2026 systematic review documented PRRT use across lung NETs, meningiomas, pheochromocytomas, paragangliomas, and medullary thyroid carcinomas, all tumors that can express somatostatin receptors.^[6]

For how somatostatin receptor biology underpins this therapy, and the role of different receptor subtypes, see the somatostatin pillar article. For the original somatostatin receptor imaging agent that preceded PET-based theranostics, see OctreoScan.

PSMA-Targeted Radioligand Therapy: The Prostate Cancer Application

Prostate-specific membrane antigen (PSMA) is overexpressed on most prostate cancer cells and provides a second major theranostic target. While PSMA-617 is technically a small molecule urea-based inhibitor rather than a peptide, it operates on the same theranostic principle and its development was directly inspired by the success of peptide-based PRRT for NETs. The broader radioligand therapy field that includes PSMA agents was built on the receptor-targeting and isotope-labeling infrastructure originally developed for peptide theranostics.

The VISION trial demonstrated that 177Lu-PSMA-617 (Pluvicto) plus standard of care prolonged both progression-free survival (8.7 vs 3.4 months) and overall survival (15.3 vs 11.3 months) compared to standard of care alone in patients with PSMA-positive metastatic castration-resistant prostate cancer. This led to FDA approval in March 2022.

The gatekeeping imaging step is critical: patients undergo 68Ga-PSMA-11 PET/CT to confirm that their tumors express PSMA before receiving 177Lu-PSMA-617 therapy. Approximately 87% of screened patients have sufficient PSMA expression for therapy, but the 13% who do not are spared an ineffective treatment. This selectivity is a defining feature of the theranostic approach. Conventional chemotherapy is administered to all eligible patients regardless of whether the drug can reach their specific tumor biology. Theranostics inverts this: proof of target engagement comes before therapy, not after.

The expansion from NETs to prostate cancer doubled the patient population eligible for theranostic treatments virtually overnight. Prostate cancer is the second most common cancer in men worldwide, and metastatic castration-resistant prostate cancer has historically had limited treatment options. The approval of Pluvicto demonstrated that the theranostic concept is generalizable beyond its origin in rare neuroendocrine tumors.

Emerging Peptide Theranostic Targets

The success of SSTR2 and PSMA theranostics has driven research into additional receptor targets. Each new target follows the same development pattern: identify a receptor overexpressed on tumors, develop a peptide that binds it with high affinity, label it with 68Ga for imaging, then 177Lu or 225Ac for therapy.

Gastrin-Releasing Peptide Receptor (GRPR/Bombesin)

GRPR is overexpressed in prostate, breast, and gastrointestinal cancers. Bombesin analogs labeled with 68Ga and 177Lu are in clinical evaluation. A 2026 study explored two metabolically stable GRPR-targeting peptides designed for improved theranostic potential, addressing the rapid enzymatic degradation that has limited earlier bombesin-based tracers.^[7]

CXCR4 (Chemokine Receptor)

CXCR4 is overexpressed in hematologic malignancies including multiple myeloma, lymphoma, and leukemia. A 2026 study developed an optimized CXCR4-targeting theranostic peptide pair that showed improved tumor uptake and imaging quality compared to earlier-generation compounds, bringing CXCR4 theranostics closer to clinical viability.^[8]

LHRH and FSH Receptors

Luteinizing hormone-releasing hormone (LHRH) and follicle-stimulating hormone (FSH) receptors are overexpressed in breast, prostate, and ovarian cancers. A 2025 systematic review evaluated radiolabeled LHRH and FSH analogs as cancer theranostic agents, finding preclinical evidence supporting receptor-targeted imaging and therapy but limited clinical data to date.^[9]

Additional Targets Under Investigation

Several other receptor-peptide pairs are in preclinical or early clinical development:

• CCK2R (cholecystokinin-2 receptor): overexpressed in medullary thyroid carcinoma, using minigastrin analogs
• NPY1R (neuropeptide Y receptor 1): overexpressed in gliomas, with 68Ga/211At-labeled peptide probes in preclinical evaluation
• GLP-1R (glucagon-like peptide-1 receptor): overexpressed in insulinomas and benign insulin-secreting tumors, using exendin-4 analogs for localization
• Fibroblast activation protein (FAP): overexpressed in the stroma of many solid tumors, using peptide-based FAP inhibitors

For how peptide-drug conjugates offer an alternative delivery mechanism to radionuclides, see the dedicated article. For the broader landscape of tumor-targeting peptides, see the targeting library article.

Agonists Versus Antagonists: A Paradigm Shift

First-generation somatostatin-based theranostics used receptor agonists like DOTATATE and DOTATOC. These agonists bind SSTR2, trigger receptor internalization, and are carried into the cell along with their radioactive payload. Internalization was initially considered essential for delivering radiation close to the cell nucleus.

However, receptor antagonists have emerged as potentially superior theranostic agents. Antagonists do not trigger internalization, yet they bind more receptor sites on the cell surface (agonists only bind the active conformation, while antagonists bind both active and inactive receptor states). This means antagonists achieve higher total tumor uptake despite not being internalized.

A 2023 review examined the comparative advantages of agonist versus antagonist radioligands across multiple peptide receptor systems, noting that antagonists consistently showed higher tumor-to-background ratios in preclinical models.^[10] A 2026 clinical case report demonstrated this principle in practice: a patient with metastatic small bowel NET showed insufficient tumor uptake on standard agonist-based 68Ga-DOTATOC imaging, precluding conventional PRRT. Switching to an antagonist-based radioligand produced dramatically improved tumor targeting, enabling therapy that would not have been possible with agonists alone.^[11]

This agonist-to-antagonist transition is being explored across multiple receptor systems and could expand the eligible patient population for theranostic therapy by rescuing patients whose tumors show insufficient uptake on agonist-based imaging.

The mechanistic explanation for antagonist superiority involves receptor biology. Agonists bind only the G-protein-coupled "active" conformation of the receptor, which represents a fraction of total surface receptors at any given time. Antagonists bind the receptor regardless of its conformational state, accessing the full pool of surface receptors. Additionally, agonist-induced internalization removes receptors from the cell surface during therapy, progressively reducing the binding sites available for subsequent radiolabeled peptide molecules. Antagonists leave receptors on the surface, maintaining a high density of binding sites throughout the treatment period. These factors combine to produce higher tumor-to-background ratios and potentially greater therapeutic efficacy with antagonist-based approaches.

TANDEM Therapy: Combining Alpha and Beta Emitters

A frontier approach in peptide theranostics is TANDEM therapy, which combines two different therapeutic radioisotopes on the same peptide to exploit the complementary physics of beta and alpha radiation. Beta particles (from 177Lu) have longer range (2 mm) and are effective against larger tumor deposits. Alpha particles (from 225Ac) have much shorter range (50-80 micrometers) but cause dense ionization damage that is difficult for cells to repair.

A 2024 case report described a patient with rapidly progressive, therapy-refractory grade 3 neuroendocrine pancreatic tumor with extensive liver, bone, and lymph node metastases who received TANDEM PRRT combining 177Lu and 225Ac on the somatostatin antagonist DOTA-LM3. The response was described as "impressive," with marked tumor regression in a patient who had failed all prior therapies.^[12]

Alpha-emitter PRRT is also being evaluated as a standalone approach. A 2024 review summarized the growing evidence for 225Ac-labeled peptides in neuroendocrine tumors, noting higher response rates compared to 177Lu-only therapy but also higher rates of xerostomia (dry mouth) due to salivary gland uptake.^[13] For a detailed analysis of alpha-emitter PRRT, see Alpha-Emitter PRRT.

Enhancing Theranostic Effectiveness

Several strategies are being investigated to improve PRRT outcomes:

Epigenetic upregulation of target receptors. A 2022 study showed that the DNA hypomethylating agent guadecitabine increased SSTR2 expression by 150% in neuroendocrine tumor models, potentially making SSTR2-low tumors amenable to PRRT.^[14]

Combination with chemotherapy. Capecitabine and temozolomide (CAPTEM) combined with PRRT have shown improved response rates compared to PRRT alone in retrospective analyses, though prospective randomized data remain limited.

Radiosensitization. Pre-treating tumors with agents that increase radiation sensitivity may amplify the cell-killing effect of PRRT without increasing the administered dose of radiopharmaceutical.

Intra-arterial delivery. Delivering radiolabeled peptides directly into hepatic arteries for liver-dominant NET metastases increases local tumor dose while reducing systemic exposure and nephrotoxicity.

Long-acting formulations. Optimizing the pharmacokinetics of radiolabeled peptides to increase tumor residence time while reducing renal accumulation is an active area of research. Modified peptide scaffolds with extended circulation half-lives could improve the therapeutic ratio by allowing more radioactive decays to occur within tumor tissue rather than in the kidneys or blood pool.

The Dosimetry Challenge

One of the more complex aspects of peptide theranostics is dosimetry: calculating the radiation dose delivered to tumors and normal organs. Unlike external beam radiation, where the dose is precisely controlled by the machine, internal radiation from PRRT depends on the biodistribution and kinetics of the radiolabeled peptide in each individual patient. Two patients receiving the same administered activity of 177Lu-DOTATATE may receive different tumor doses depending on their tumor receptor density, blood clearance rate, and renal function.

Post-treatment imaging with the gamma emissions from 177Lu allows estimation of absorbed doses, but this adds complexity to treatment planning. Dosimetry-guided PRRT, where subsequent treatment cycles are adjusted based on measured organ doses from prior cycles, is being adopted at specialized centers but is not yet standard practice. The balance between standardized fixed-dose protocols (simpler to implement) and individualized dosimetry-guided protocols (potentially safer and more effective) remains an active debate in nuclear medicine. The molecular imaging step that precedes therapy provides some individualization by default, since patients whose tumors show low uptake on diagnostic imaging are excluded from therapy. But refining the therapeutic dose based on measured kinetics could improve outcomes for the patients who do proceed to treatment.

Limitations and Toxicities

Peptide theranostics face several constraints. Renal toxicity is the primary dose-limiting factor for 177Lu-DOTATATE therapy, because radiolabeled peptides are filtered by the kidney and reabsorbed in the proximal tubules, exposing renal tissue to radiation. A 2026 review characterized PRRT-induced radiation nephropathy as an emerging clinical concern, particularly with retreatment or alpha-emitter use, and noted that current mitigation strategies (amino acid co-infusion, dose fractionation) reduce but do not eliminate renal risk.^[15]

Hematologic toxicity (cytopenias, rare myelodysplastic syndrome) occurs in a small percentage of patients due to bone marrow irradiation. The long-term risk of myelodysplastic syndrome or acute leukemia after PRRT is estimated at 2-3% but remains a concern for younger patients with long life expectancy.

Tumor heterogeneity is a biological limitation. Not all tumor cells within a single lesion express the target receptor uniformly, and receptor expression can change over time or after prior therapy. A tumor that images well on 68Ga-DOTATATE PET may contain SSTR2-negative subclones that survive PRRT and drive recurrence.

Response assessment is technically challenging. Standard RECIST criteria (measuring tumor size) may not capture the biological response to PRRT, since tumors can become necrotic and remain large despite effective treatment. A 2025 study compared RECIST 1.1, mRECIST, and PERCIST criteria for assessing PRRT response and found significant discordance between the methods.^[16]

For the biochemical markers used alongside imaging to monitor treatment response, see Chromogranin A.

The Bottom Line

Peptide theranostics use the same tumor-targeting peptide for both diagnostic imaging and therapeutic radiation delivery. The field was established with somatostatin receptor-targeted PRRT for neuroendocrine tumors (177Lu-DOTATATE, approved 2018) and expanded to prostate cancer (177Lu-PSMA-617, approved 2022). New receptor targets including GRPR, CXCR4, LHRH, and CCK2R are in development. The shift from agonist to antagonist radioligands is improving tumor uptake, and TANDEM therapy combining beta and alpha emitters is being explored for refractory disease. Renal toxicity, hematologic risk, and tumor heterogeneity remain the primary limitations.

Frequently Asked Questions

What are theranostic peptides?

Theranostic peptides are tumor-targeting peptides used for both diagnosis and treatment of cancer. The same peptide is labeled with a diagnostic radioisotope (like gallium-68) for PET/CT imaging, then with a therapeutic radioisotope (like lutetium-177) for targeted radiation therapy. The peptide binds to receptors overexpressed on tumor cells, delivering radiation specifically to cancer tissue while sparing normal organs.^[1]

What is peptide receptor radionuclide therapy PRRT?

PRRT is a targeted cancer treatment that uses radiolabeled peptides to deliver radiation directly to tumor cells expressing specific receptors. The most established form uses 177Lu-DOTATATE (Lutathera), which targets somatostatin receptor subtype 2 on neuroendocrine tumors. In the NETTER-1 trial, PRRT achieved 65.2% progression-free survival at 20 months compared to 10.8% for standard therapy alone.^[3]

How does Lutathera work for neuroendocrine tumors?

Lutathera (177Lu-DOTATATE) is a somatostatin analog peptide attached to the radioactive isotope lutetium-177. It binds to somatostatin receptor subtype 2, which is overexpressed on neuroendocrine tumor cells. After binding, the receptor-peptide complex is internalized, and lutetium-177 emits beta radiation that damages tumor cell DNA from within. Patients first undergo gallium-68-DOTATATE PET imaging to confirm their tumors express the target receptor before receiving therapy.

What is the difference between agonist and antagonist radioligands?

Agonist radioligands (like DOTATATE) activate the target receptor and are internalized into tumor cells, delivering radiation from inside. Antagonist radioligands do not trigger internalization but bind more receptor sites on the cell surface, often achieving higher total tumor uptake. A 2026 study showed that switching from agonist to antagonist imaging rescued patients whose tumors showed insufficient uptake on standard agonist-based scans.^[11]

What cancers can be treated with peptide theranostics?

Currently FDA-approved peptide theranostic agents treat somatostatin receptor-positive neuroendocrine tumors (Lutathera) and PSMA-positive metastatic prostate cancer (Pluvicto). Research is expanding to breast cancer (via GRPR/bombesin), hematologic malignancies (via CXCR4), gliomas (via NPY1R), medullary thyroid carcinoma (via CCK2R), and insulinomas (via GLP-1R). Any tumor that overexpresses a targetable receptor is a potential candidate.

What are the side effects of PRRT?

The primary side effects of PRRT include renal toxicity from radiolabeled peptide reabsorption in the kidneys, hematologic toxicity (low blood counts, rare myelodysplastic syndrome at 2-3% long-term risk), nausea, and fatigue during treatment. Alpha-emitter PRRT using actinium-225 can cause xerostomia (dry mouth) due to salivary gland uptake. Amino acid co-infusion during treatment helps reduce kidney radiation exposure.^[15]