Targeting the Oxytocin Receptor for Breast Cancer Management: A Niche for Peptide Tracers

Abstract

Breast cancer is a leading cause of death in women, and its management highly depends on early disease diagnosis and monitoring. This remains challenging due to breast cancer’s heterogeneity and a scarcity of specific biomarkers that could predict responses to therapy and enable personalized treatment. This Perspective describes the diagnostic landscape for breast cancer management, molecular strategies targeting receptors overexpressed in tumors, the theranostic potential of the oxytocin receptor (OTR) as an emerging breast cancer target, and the development of OTR-specific optical and nuclear tracers to study, visualize, and treat tumors. A special focus is on the chemistry and pharmacology underpinning OTR tracer development, preclinical in vitro and in vivo studies, challenges, and future directions. The use of peptide-based tracers targeting upregulated receptors in cancer is a highly promising strategy complementing current diagnostics and therapies and providing new opportunities to improve cancer management and patient survival.

Significance

Receptors overexpressed in tumors but not healthy cells are promising targets for theranostics. The peptide hormone oxytocin receptor (OTR) is one such emerging target in breast cancer. Peptide-based optical and nuclear tracers are being developed that target OTR in breast cancer to validate OTR’s role and theranostic potential in breast cancer as well as to develop more effective, safer, and more personalized treatment options.

1. Breast Cancer—A Heterogeneous Disease

Breast cancer is the second most common type of cancer diagnosed in women after non-melanoma skin cancers.¹ The global cancer statistics reported 2.3 million newly diagnosed breast cancer cases and 684,996 deaths (responsible for 15.5% of total mortalities in females) in 2020.^2,3 Despite having a favorable prognosis,⁴ breast cancer remains the leading cause of death in women worldwide.² Efficient diagnosis and classification of breast cancer subtypes facilitate the management of the disease, but the heterogeneity of the malignant cells present in the mammary epithelial tissue often hinders accurate diagnosis.⁵ Current breast cancer diagnosis relies primarily on the assessment of molecular markers, such as the estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2),⁶ and Ki67, a cancer antigen used as a marker for cell proliferation and now an accepted prognostic factor to differentiate between ER-positive (ER⁺) tumor subtypes (Figure 1).^7,8 Based on the expression of hormonal receptors, breast tumors can be classified into the following four subtypes: luminal A, luminal B, HER2-positive (HER2⁺), and triple-negative breast cancer (TNBC).⁹ Luminal A tumors have the highest incidence (∼50%) among women but also have the best therapeutic outcome; by contrast, luminal B tumors have a lower incidence (∼15%) but worse prognosis.⁹ ER-negative (ER^–) breast tumors overexpressing HER2 (incidence of ∼20%) initially had poor therapy outcomes,⁹ which have improved in recent years due to the introduction of combination therapies.¹⁰ FDA-approved anti-HER2 agents include monoclonal antibodies Pertuzumab, Trastuzumab, and Ado-Trastuzumab emtansine (T-DM1);¹¹ tyrosine kinase inhibitors Lapatinib (TYKERB),^11,12 Neratinib (Nerlynx),¹² and Tucatinib (Tukysa);¹² and the antibody–drug conjugate fam-trastuzumab deruxtecan-nxki (DS-8201a, T-DXd, ENHERTU).¹³ Finally, tumors that are negative for the biomarkers mentioned above (ER^–, PR^–, HER2^–) fall in the TNBC category. TNBC includes basal (neoplastic cells constitutively expressing markers, such as cytokeratins or EGFR)¹⁴ and non-basal tumors,¹⁵ with most TNBCs expressing basal markers.¹⁴ TNBC patients have poor overall survival due to the tumors’ aggressiveness and increased risks of relapse^16,17 and have an incidence rate of ∼15%.^9,18 Pembrolizumab, a humanized monoclonal anti-programmed cell death protein 1 (PD1) antibody, is used as a neoadjuvant and adjuvant for treating patients with high-risk early-stage TNBC.^19,20

Figure 1.

Intrinsic subtypes of breast cancer. They are represented as an accumulation of tumor cells in orange (luminal A), purple (luminal B), green (HER2), and dark pink (TNBC). The average incidence is presented in the pie chart, and the relative treatment outcome prognosis for the different tumor subtypes is shown in the bottom triangle (data on molecular profiles adopted from refs (22−24)). Ki67 is a nuclear protein associated with cell proliferation. A high fraction of Ki67-positive tumor cells suggests a high proliferation rate and is often indicative of more aggressive tumors.

According to the “5-year relative survival percentage” reported by the U.S. National Cancer Institute, the highest survival pattern was observed in women with luminal A subtype (94.4%), followed by the luminal B subtype (90.7%), HER2 subtype (84.8%), and finally TNBC (77.1%).⁹ Although mortality has decreased due to early detection and increasing therapeutic options, almost 30% of patients diagnosed with early stages of breast cancer still develop recurrent or metastatic diseases,⁹ with 5-year survival rates in those patients of only 27%.²¹

2. Diagnostic Tools for Breast Cancer Management

In most countries, manual breast palpation is the first screening method to detect breast tumors,²⁵ followed by visualizing abnormalities through different breast imaging techniques. An overview of current technologies for breast cancer detection is presented in Table 1, including their advantages and limitations.

Table 1. Current Breast Cancer Detection and Diagnostic Methods, Including Their Advantages and Limitations^a.

^aA visual overview of the three main limitations is represented with colored circles. “Sensitivity”, in a clinical context, refers to the amount of correctly diagnosed patients (i.e., true positives), as opposed to the term “specificity”, which alludes to the number of healthy people correctly diagnosed as not having the disease (i.e., true negatives).²⁹ Stage I, tumor less than 2 cm; stage II, tumor 2–5 cm.

Mammography utilizes a low dose of X-rays to visualize the internal structure of the breast.²⁶⁻²⁹ It is widely used to identify the early onset of breast cancer before the manifestation of physical symptoms.^26,27 It has, however, low sensitivity in young women due to higher breast tissue density and higher tumor growth rate than in older women.²⁸ Even though mammography is the gold standard used for breast cancer screening,³⁰ other techniques such as ultrasonography³¹ and magnetic resonance imaging (MRI)³² are used to identify tumors that are not detectable in mammograms, as well as to determine tumor size more accurately. Each of these modalities has advantages and limitations, mostly due to their low specificity (i.e., high rate of false positives).^33,34 Other techniques, such as sentinel lymph node biopsy (SLNB)³⁵ or molecular profiling,³⁶ aid in categorizing the stage of breast tumors.

Nuclear imaging techniques such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are used as auxiliary imaging tools to better characterize breast cancer. SPECT and PET can detect tumors using radiotracers (small molecules, peptides, antibodies, affibodies, or nanobodies labeled with radioisotopes) directed to receptors, transporters, or enzymes overexpressed in cancer cells.³⁷⁻⁴¹ The clinical use of SPECT and PET remains limited due to the high costs (especially in the case of PET tracers that have short half-lives and require in-house cyclotron production) and scarcity of radiotracers. However, considering the ongoing technological advances, we expect to see a broader utilization of these technologies, particularly due to their high sensitivity and accuracy.⁴²

SPECT uses radiotracers that emit γ-rays captured with a γ camera to acquire multiple 2D projections from different angles to produce a full 3D body image.⁵⁶ SPECT-CT permits accurate 3D localization of primary and/or metastatic tumors, yet with a limited spatial resolution (∼10 mm).⁴⁹ Typical radiopharmaceuticals used in SPECT imaging of breast cancer include [²⁰¹Tl]thalluimchloride, [^99mTc]technetium methoxyisobutylisonitrile ([^99mTc]Tc-MIBI, [^99mTc]Tc-sestamibi), and [^99mTc]technetium diphosphonates (Figure 2).⁵⁷ As an example, SPECT-CT with [^99mTc]Tc-MIBI is used for finding proliferative tumoral tissue around a breast implant, in the remaining breast parenchyma, or on the chest wall after surgery.⁵⁸

Figure 2.

Radiopharmaceuticals used in SPECT imaging.

PET imaging is based on detecting annihilation photons produced by the disintegration of positron-emitting radiotracers.⁵⁹ The most commonly used radiotracer for the visualization of tumor distribution is [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG), a radiolabeled glucose analogue that cancer cells absorb in greater amounts than normal cells due to their increased metabolic activity.⁴⁷ The different pharmacokinetic (PK) and pharmacodynamic (PD) rates are measured throughout the body with high spatial resolution, allowing the visualization of cancer metastases far away from the breast. A limitation of PET is its deficient detection rate for non-invasive breast cancers and small breast carcinomas.^60,61 Sensitivity of PET highly depends on tumor size,⁵⁰ varying from 95% for tumors larger than 1 cm to 25% for tumors smaller than 1 cm (resolution limit in modern clinical PET scanners is 4 mm).⁶² This fact renders the detection of early-stage breast cancer (stage I, tumors 1–19 mm) challenging.^60,63 Moreover, the variability of glucose uptake in primary breast tumors differs in terms of tissue integrity and vascular density, which can result in false negatives that are difficult to differentiate from real signals.⁶⁴

Both SPECT and PET can assess the presence and extent of disease as well as provide unique information about tumor biological characteristics, such as the rate of proliferation and metabolic activity.⁵⁷ These methods are important and powerful techniques to complement traditional imaging modalities in breast cancer diagnosis and monitoring. The therapeutic potential of these techniques is leveraged when targeted approaches are employed, directing a radiopharmaceutical to the tissue of interest and differentiating it from healthy tissue.

3. Receptor-Targeted Approaches

The discovery that several cell surface receptors are overexpressed in tumor tissues compared to healthy tissue enables tumor-targeting strategies.⁶⁵ Targeting entities include small molecules, peptides, antibodies, or antibody fragments, each having their advantages and limitations. Small molecules, for example, offer good bioavailability, stability, and tumor tissue penetration but are often not target-specific due to their small size and limited chirality.⁶⁶ By contrast, antibodies provide high target specificity and affinity but have to be injected and have poor tumor penetration due to their large size.⁶⁷ Peptide ligands display a good balance in biochemical properties between small molecules and antibodies, having higher tissue penetration than antibodies and better target specificity and affinity than small molecules.⁶⁷⁻⁷⁰ While some peptides have short half-lives, it is relatively easy to tune their metabolism and clearance rate, e.g., using fatty acid modifications for serum albumin binding.⁷¹ Moreover, peptides can be equipped easily with reporter tags compatible with optical (fluorescent tracers) or nuclear (radiotracers) imaging.^68,72 Such peptide tracers, therefore, hold promising potential as diagnostic and therapeutic tools in oncology.^72,73

3.1. Tracers for Optical Imaging

Optical imaging tracers are typically equipped with fluorophores to visualize membrane receptors expressed in cancer cells. While fluorescent ligands can be used in vitro, tissue autofluorescence needs to be considered for in vivo imaging since mammalian tissues are opaque to light in the visible spectrum (400–700 nm).⁷⁴ In these cases, near-infrared (NIR) fluorophores can be used that function in wavelength regions above 700 nm.⁷⁵ NIR fluorophores reduce light-scattering effects, enabling better tissue penetration (>1 cm). NIR imaging is applied in superficial tumor detection, integrated as part of endoscopies and open-surgery procedures, which assists surgeons in removing cancerous tissue;^72,76 indocyanine green (ICG) and methylene blue (MB) are so far the only two NIR fluorophores approved by the FDA.^75,77 NIR peptide tracers have been developed targeting overexpressed EGFR in glioblastomas⁷⁸ and the integrin αvβ3 receptor expressed on sprouting tumor vasculatures.⁷⁹ Most NIR tracer research is still preclinical, focusing on improving optical properties and toxicity profiles,^80,81 with the exception of BLZ-100 (Tozuleristide, Tumor Paint; chlorotoxin peptide conjugated to ICG), which is expected to receive FDA approval for its use in visualizing pediatric brain cancer cells during tumor-removing surgery and has also been applied to breast cancer.^82,83

3.2. Tracers for Nuclear Imaging

Nuclear imaging tracers use radioisotopes to visualize cancer cells. Particularly SPECT and PET techniques are employed to provide 3D positional images of targeted tumors in the body;⁴⁷ this relates to whole-body as well as focused breast imaging, although the latter offers better diagnostic accuracy.^84,85 Radiotracers can be used in the clinic for disease scanning, tumor characterization (staging), and treatment response monitoring.³⁷ The design and synthesis of radiotracers for breast cancer (Figure 3) were initially based on the labeling of identified biomarkers (ER, PR, and HER2). The [¹⁸F]FES PET radiotracer has been used in ER⁺ tumors for preclinical evaluation of a therapeutic response and clinical visualization of breast cancer with an average of 90% specificity and 85% sensitivity.⁸⁶⁻⁹² In the case of PR, [¹⁸F]FFNP was used in a clinical study, identifying ∼94% of PR⁺ breast tumors using PET.⁹³ To visualize HER2⁺ breast tumors, both PET and SPECT radiopharmaceuticals based on the monoclonal antibody trastuzumab were tested,^14−18,94 and the efficacy of ⁶⁴Cu-labeled trastuzumab to detect HER2-positive breast cancer was confirmed.⁹⁵

Figure 3.

Tracers used for nuclear imaging of different breast cancer types. Ligands, linkers, and chelators are colored in black, radionuclides in purple, and antibodies in mauve. Abu, 4-aminobutyric acid; Sta, statine ((3S,4S)-4-amino-3-hydroxy-6-methylheptanoic acid).

Less studied targets include the gastrin-releasing peptide receptor (GRPR) and vasoactive intestinal polypeptide receptor 1 (VIP-R1), yet their expression in breast cancer is documented (GRPR⁹⁶⁻⁹⁹ and VIP-R1^100,101), and they were investigated clinically for the detection of breast malignancies.^102−104 Radiotracers used for the detection of GRPR were [⁶⁸Ga]Ga-SB3, achieving 50% tumor detection (n = 8),¹⁰² and [⁶⁸Ga]Ga-RM2, reaching 72% breast cancer visualization (n = 18),¹⁰³ both GRPR peptide antagonists. In a clinical study to detect VIP-R1, radiotracer [⁶⁴Cu]Cu-TP3805 was used, achieving 100% detection of breast tumors (n = 20).¹⁰⁴

Expression of target receptors often depends on breast cancer subtype and might be influenced by factors such as treatment or disease progression.³⁷ TNBCs currently lack validated target receptors or biomarkers,¹⁰⁵ and targeting metastatic disease is challenging, with a poor 5-year survival (<30%).¹⁰⁶ It remains therefore important to identify and validate new receptor targets for breast cancer management, especially for TNBC. The peptide hormone oxytocin receptor (OTR) is such a new target.^107,108

4. The Oxytocin Receptor: An Emerging Target for Breast Cancer Management

OTR belongs to the class I family (rhodopsin-like) of G protein-coupled receptors (GPCRs) and is activated by its endogenous peptide hormone OT.¹⁰⁹ OT is a nonapeptide comprising a six-residue macrocycle linked by a disulfide bond between positions 1 and 6 and a three-residue tail with a C-terminal amide (Figure 4a). OTR’s structure has recently been resolved via X-ray diffraction, with antagonist retosiban bound to an inactive conformation,¹¹⁰ and via cryo-EM, with OT bound to an active conformation (Figure 4b,c).^111,112 OTR can be upregulated to play important roles in the reproductive, cardiovascular, endocrine, and central nervous systems.¹⁰⁹ OTR becomes upregulated in the female breast during pregnancy when the mammary glands develop in preparation for breastfeeding.^107,109 During breastfeeding, OT is synthesized in the magnocellular neurons of the hypothalamus, transported to the posterior pituitary (neurohypophysis),^109,113 from which it is released into the bloodstream to bind to OTR in the mammary glands, inducing contractions of the myoepithelial cells resulting in milk ejection.¹¹⁴ OTR is also expressed in breast tumors,^107,115 as well as leiomyoma,¹¹⁶ neuroblastoma and glioma,¹¹⁷ adenocarcinoma of the endometrium,¹¹⁸ ovarian carcinoma,¹¹⁹ prostate cancer,^120−122 small-cell lung carcinoma,¹²³ trophoblast,¹²⁴ choriocarcinoma,¹²⁴ and osteosarcoma.^125,126 Peptide radiotracers have been developed successfully targeting OTR in breast malignancies (mouse models),^127,128 supporting sufficient receptor density in tumors for diagnosis and treatment. OTR mRNA was detected in up to 95% of human breast cancer cell lines (n = 60)^129−134 and tissues (n = 57).^130,131 OTR was detected at the protein level in 91% of such tissue samples,^130,133 although immunohistochemistry results need to be considered cautiously due to potential problems related to OTR antibody specificity and lack of appropriate controls. Proliferation assays using human epithelial triple-negative MDA-MB-231 and ER⁺ MCF7¹³⁵ breast cancer cell lines demonstrated increasing antiproliferative effects when OT was administered in a dose-dependent manner (1–100 nM), consistent with previous data.¹³⁶

Figure 4.

OT chemical structure and the OT-OTR complex. (a) Molecular structure of OT (blue) and the recommended labeling position 8 indicated in purple. (b) Side view of OT bound to OTR. (c) Top view of OT bound to OTR, as determined via single-particle cryo-electron microscopy of OTR in complex with OT (adapted from ref (112)). For a detailed map of molecular interactions, please refer to Waltenspühl et al.¹¹²

Administration of OT and OT analogues in different rodent breast cancer models reduced breast tumors substantially (44–82%).^{107,115,137−139} For example, OT or atosiban (biased OTR ligand) was administered via osmotic pumps into xenograft models of BALB/c mice (n = 43) and Fisher rats (n = 22) bearing mammary carcinomas TS/A and D-R3230AC, respectively, for 14 days.¹¹⁵ This resulted in up to 72% tumor reduction in animals treated with OT or atosiban compared to controls. Tumor reduction by atosiban was unexpected since it was thought to be an OTR antagonist; further studies, however, demonstrated atosiban to be a biased ligand, selectively activating G_i signaling while blocking G_q signaling.¹⁴⁰ In another study that used BALB/c mice bearing MC4-L2 mouse mammary adenocarcinomas, treatment with OT for 42 days reduced tumor growth rate and tumor size (∼82%).¹³⁷ Treatment with atosiban was also studied; however, no significant changes in tumor growth or size were observed. Using the same xenograft system, BALB/c mice bearing MC4-L2 mouse mammary adenocarcinomas (n = 56), a 44% tumor reduction upon OT treatment and a 12% increase in tumor volume with atosiban were observed.¹³⁸ In another study, OT was administered for 21 days in BALB/C mice bearing mammary carcinomas TS/A, which led to a tumor reduction of ∼50%.^107,139 While tumor reduction upon OT treatment seems fairly consistent among studies, atosiban displayed varying effects (reduction/proliferation of tumors or no effect in tumor growth), warranting more systematic studies. OT also seems to be released upon physical activity, acting protectively against breast cancer:¹⁴¹ mice bearing breast tumors were assigned to a treadmill for 5 days, increasing OT concentration in plasma. Since an endogenous OT increase was also observed in parallel to tumor growth inhibition effects, a different group of animals was externally administered with OT (no exercise), confirming OT-dependent tumor reduction (tumor volume 42 days after tumor transplantation, tumor + exercise training, and tumor + OT was 1.10, 0.81, and 0.56 cm³, respectively).¹³⁸ All of the used breast cancer cell lines express OTR. The MCF7 cell line originates from a patient with metastatic breast cancer and is the most studied human breast cancer cell line in the world,¹⁴² while TS/A originated from a spontaneous mammary tumor of an inbred BALB/c female mouse exhibiting features typical of human breast cancer.¹⁴³ MC4-L2 xenograft models are experimental models of mammary carcinogenesis in which the administration of medroxyprogesterone acetate to female BALB/c mice induces progestin-dependent ductal metastatic mammary tumors with high levels of both ER and PR.¹⁴⁴ Despite the overall beneficial effects of OT in these animal models, more studies are required to investigate the effects of OT and optimized/selective OTR drug candidates on human xenograft models across a variety of different subtypes. OT itself is considered a poor drug candidate due to its short circulation half-life and activity at closely related vasopressin receptors (VPRs).

In a clinical study, endogenous OT increased up to 9-fold in the peripheral blood from tumor tissue of breast cancer patients (n = 40) compared to contralateral (healthy) breast; this effect increased in the advanced stages of the disease (p ≤ 0.004).¹⁴⁵ Interestingly, OTR gene and protein expression were up to 11-fold downregulated.¹⁴⁵

The relationship between breastfeeding and breast cancer has been studied since the 1950s.¹⁴⁶ The consensus is that breastfeeding reduces the risk of developing breast cancer.^147−155 The most comprehensive study with the highest statistical significance to date was published in 2002, a worldwide collaborative analysis comprising 47 studies which included population-based, case-control, and follow-up analyses in parous women of different ages and ethnic origins in developed and developing countries, and with a wide range of reproductive and breastfeeding patterns.¹⁴⁸ The relative risk of breast cancer development in this study was reduced by 4.3% per year in women who breastfed for at least 12 months. Another study with women in Sri Lanka (n = 19,755) demonstrated a striking reduction of 87–94% in the risk of developing breast cancer among women who breastfed for more extended periods (24–47 months).¹⁵⁵ The exact mechanisms underpinning these protective effects require further investigation; however, evidence points to an involvement of the OT/OTR signaling system.

The exact expression profiles of OTR in breast tumors of different subtypes and stages remain an active research question; however, OTR-specific nuclear imaging tracers accumulated in breast malignancies in vivo, suggesting that OTR expression is sufficient for tumor-specific targeting, at least in the rodent models employed.^127,128,156 More systematic studies investigating OTR expression across a large panel of breast cancer patient samples will be critical in revealing cancer subtypes that display robust OTR overexpression to support tumor imaging and targeted radiotherapy.

5. Oxytocin Receptor Tracer Development

A major bottleneck in OTR ligand development is OTR’s similarity to the closely related VPRs, V_1aR, V_1bR, and V₂R.^157−159 Considerable efforts have been invested in structure–activity relationship (SAR) studies and medicinal chemistry approaches, with thousands of ligands synthesized and pharmacologically characterized.^{157,158,160−164} This resulted in several approved peptide drugs acting via OTR (e.g., OT, demoxytocin, atosiban, carbetocin); however, none of these ligands is OTR-selective,¹⁵⁷ with selectivity defined as a 100-fold affinity preference for one receptor over the others.¹⁶⁵Table 2 lists a range of agonists and antagonists with selectivity/preference for OTR. Models of ligand–receptor interactions originally suggested that the OT ring fits deeply into the transmembrane (TM) core, interacting with a cluster of residues located in TM3, TM5, and TM6, whereas the tail interacts with the upper part of the first TM helix and the second extracellular domain of the receptor (Figure 4b,c).^166−168 Recently elucidated OT-bound OTR structure via single-particle cryo-EM revealed that all 9 amino acids of the OT participate in OTR binding with the cyclic part (residues 1–6) being deeply buried inside of the pocket while the C-terminal tripeptide (7–9) is situated toward extracellular loops (Figure 4b,c).¹¹² Leu⁸ is oriented toward the extracellular space, explaining why position 8 is best suited for attaching fluorophores or other modifications in OT.¹¹² Position 8 of OT (Figure 4a, purple) has been replaced or modified with several moieties without substantially affecting OTR binding or activation. Particularly replacement of Leu⁸ with lysine or ornithine provides a suitable handle for OT-like tracer production.^{127,169−172} This modification is often combined with removing the N-terminal amine, rendering the peptide less susceptible to aminopeptidases and more hydrophobic with a better binding pocket fit and enhanced potency.¹⁷³ Several OTR tracers have been developed¹⁶⁹ and are listed and depicted in Figure 5, along with their pharmacological profiles across OTR and VPRs and their applications.

Table 2. OT Analogues with OTR Preference or Selectivity over VPRs.

^aRSR, receptor selectivity ratio: calculated dividing highest receptor selectivity among receptors by OTR affinity.

^bDisulfide bridge formed with substituted dibromoxylene (m-xylene bridged between residues 1 and 6).

^cthio = S–S bridge was substituted for CH₂–S, with CH₂ in position 6.

^dBuG, N-(n-butyl)glycine.

^ethP = trans-4-hydroxyproline.

^fL(Me) = γ-methylleucine.

^gthio = S–S bridge was substituted for CH₂–S, with CH₂ in position 1.

^hX′ amino acids used to form a lactam bridge.

ⁱzG = azaglycine.

^jG(2-ThiMe) = N-alkylated glycines with 2-thiophenylmethyl.

^kMeBzlG = N-(3methylbenzyl)glycine.

^lFBzlG = N-(4-fluorobenzyl)glycine.

^mC₈ = octanoic acid.

ⁿBiphasic fitting (Hill coefficient for both phases was fixed to −1; “EC₅₀ hi” refers to the high-affinity site and “EC₅₀ lo” to the low-affinity site.¹⁷⁸ All assay values were obtained from pharmacological testing on human isoforms of OTR, V_1aR, V_1bR, and V₂R. *Analogues are based on OT sequence (indicates C-terminal amide). **K_i values are presented in white, EC₅₀ values in gray. n.d. = not determined; “d” = for desamino (no N-terminal amine); d(CH₂)₅ = 1-[β-mercapto-β,β-cyclopentamethylene]propionic acid.

Figure 5.

Overview of developed OTR tracers with information on OTR and VPR pharmacology, OTR selectivity, application, and chemical structure. (a) OTR-targeting fluorescent tracers and radiotracers with their sequence information, the OTR and VPR pharmacology, the OTR selectivity ratio (RSR), and the application information. (b) Chemical structures of all listed OTR tracers. Residues that differ from endogenous OT are highlighted in blue and labeling groups (fluorophores, radionuclides, and metal chelators) in purple.

Some of these OTR radiotracers have been used to target breast cancer.^127,128 ([K([¹¹¹In]In-DOTA)]⁸-OT) displayed high affinity for OTR (MCF7 breast cancer cells) and tumor uptake (BALB/c mice bearing OTR⁺ TS/A tumor) with a tumor-to-blood ratio of 2.67.¹⁵⁶ The desamino version of this tracer, [K([¹¹¹In]In-DOTA)]⁸-dOT, was evaluated in a preclinical study aiming to determine the amount and specificity of the receptor-mediated uptake using an experimental model of OTR⁺ TS/A tumors growing in Balb/c mice.¹²⁷ This biodistribution study demonstrated higher tracer uptake in tumors than in blood or liver (tumor/blood and tumor/liver uptake ratios were 7.58 and 1.42, respectively) but lower than in kidneys (tumor/kidney uptake ratio was 0.06), highlighting a rapid clearing process.¹²⁷ OTR-specificity was confirmed by administering 50 μg of OT 30 min before radiotracer administration, resulting in a 3-fold-reduced tumor uptake.¹²⁷ [K([¹¹¹In]In-DOTA)]⁸-dOT internalized with OTR within 5 min, supporting tracer tumor accumulation, which could be beneficial for tumor imaging or targeted radiotherapy.¹²⁷

[C([^99mTc]Tc-EDDA/HYNIC)]¹-OT is another radiotracer developed to target OTR.¹²⁸ Here, 2-hydrazinonicotinic acid (HYNIC) chelates technetium-99m, a γ-emitting radionuclide with a half-life of 6 h commonly used for SPECT.¹⁸¹ As such complexes may exist in numerous isomeric forms, adding ethylenediaminediacetic acid (EDDA) as co-ligand helps prepare complexes of higher stability and symmetry, resulting in fewer coordination isomers.¹⁸² Interestingly, the addition of the chelator HYNIC to the N-terminal amine of OT did not affect OTR binding,¹⁸³ while introducing [^99mTc]Tc-EDDA/HYNIC to position 8 reduced binding affinity, as determined by a radioimmunocompetition assay (IC₅₀ values of 0.2 and 1 nM, respectively). Both Lys⁸- and Cys¹-labeled tracers were internalized in OTR-expressing MCF7 cells. In vivo, breast tumors were induced in athymic male mice by subcutaneous injection of MCF7 cells. From the two studied labeling positions, only [C([^99mTc]Tc-EDDA/HYNIC)]¹-OT achieved tumor uptake, besides the typical non-specific uptake in kidneys and liver.¹²⁸

6. OTR Tracers for Breast Cancer—Opportunities and Challenges

OTR radiotracers have shown promise in breast cancer mouse models with OTR and tumor-specific uptake.^127,128,156 Rational design to advance OTR tracer development is feasible, particularly considering the many SAR studies with OT analogues along with the well-established OTR/VPR pharmacology. Such tracer development will provide novel imaging tools, if not theranostic leads, that should be beneficial not only for tumor imaging and cancer management but also for fundamental research investigating OTR’s role in health and disease.

While discoveries linking OTR to breast cancer are promising, only a single study evaluated OTR tracers in animals bearing human breast tumors, however, with poor pharmacological tracer characterization (no VPR data) and limited biodistribution info (no tumor/background tracer uptake ratios).¹²⁷ Further in vivo studies using xenograft tumor models with different human breast cancer subtypes are needed to assess translational and clinical perspectives. OTR levels need to be systematically profiled across different subtypes to provide a clearer picture of OTR’s potential as a molecular target for imaging and therapy. OTR quantification at the protein level remains challenging due to the high extracellular homology of OTR with VPRs (V_1aR, V_1bR, and V₂R) and a lack of specific antibodies.^{157,159,184,185} OTR tracers may be able to help in this task, given sufficient signal/brightness and selectivity and affinity to reliably quantify OTR in cells and tissue.

Peptide-based fluorescent tracers and radiotracers have certain opportunities and advantages over antibodies. Indeed, attempts to develop small molecules and biologics targeting OTR for clinical use have so far failed.¹⁵⁸ First, peptides can be rapidly designed and chemically produced at reduced costs compared to antibodies.¹⁸⁶ Peptides have good biocompatibility and low immunogenicity and can be modified to enhance the in vivo stability and pharmacokinetics. For instance, blood circulation of peptides can be increased through conjugation to polyethylene glycol (PEG) chains or serum albumin binders,¹⁸⁷ which might enhance tumor accumulation. Since peptides are smaller than antibodies (∼3 kDa peptides vs ∼150 kDa antibodies), they have better tumor penetration; additionally, they can induce ligand–receptor-mediated internalization, which could lead to enhanced tumor uptake and visualization.¹⁸⁸ This may be particularly important for targeted radiotherapy, where therapeutic radionuclides, e.g., ⁹⁰Y or ¹⁷⁷Lu, or chemotherapeutic agents would accumulate closer to the cancer cell nuclei than without internalization.^73,189,190 Such targeted treatment is desirable for cancer subtypes that do not yet have a targeted therapy option (e.g., TNBC) and where systemic chemotherapy with its severe side effects remains the first-line treatment.¹⁹¹

This so-called peptide receptor radionucleotide therapy (PRRT) has already been successfully applied to neuroendocrine tumors (NETs) targeting the peptide hormone somatostatin receptor (SSTR).^192−194 [¹⁷⁷Lu]Lu-DOTA-TATE (Lutathera) was approved by the EMA in 2017 and the FDA in 2018 for treating SSTR-positive gastroenteropancreatic NETs.¹⁹⁵ The success of somatostatin radiotracers paved the way for other peptide-based radiotracers, e.g., those based on the RGD peptide, vasoactive intestinal peptide (VIP), cholecystokinin (CCK)/gastrin peptide, α-melanocyte-stimulating hormone (α-MSH), neurotensin (NT), T140, exendin-4, neuropeptide Y (NPY), substance P, and tumor molecular targeted peptide 1 (TMTP1).^65,196,197 For breast cancer management, chemerin-based [⁶⁸Ga]Ga-DOTA peptide conjugates targeting the chemokine-like receptor 1 (CMKLR1) with high specificity and affinity could visualize CMKLR1-positive breast cancer xenografts via PET/MRI.¹⁹⁸ [⁶⁸Ga]Ga-NeoBOMB1, a DOTA-coupled GRPR antagonist, was tested in a study in four patients diagnosed with prostate cancer, where it rapidly localized in pathologic lesions, achieving high-contrast imaging during PET/CT.¹⁹⁹ Breast cancer patients with GRPR-positive tumors are potential candidates for treatment with ¹⁷⁷Lu-labeled NeoBOMB1.²⁰⁰ Taken together, these clinical trials support the safety and efficacy of peptide-based radiotracers and the theranostic opportunities of PRRT, providing a strong foundation for a thriving biotech industry that is already embracing these approaches and seeing the therapeutic potential of peptide-based theranostics.^201,202

7. Conclusions and Perspectives

The landscape of breast cancer management has evolved significantly with the advent of targeted therapies, representing a marked departure from the era of indiscriminate chemotherapy or radiation treatments. This transformation translates into tangible benefits for patient survival, overall care, and the emergence of personalized treatment strategies. To further elevate these advancements, the focus must shift toward achieving earlier and more precise diagnoses, leveraging predictive biomarkers, and refining tumor-targeted therapies to mitigate side effects. Overcoming diagnostic challenges posed by the high heterogeneity of breast tumors and dense breast tissue requires an integrated approach, combining mammography with highly sensitive nuclear imaging techniques. Among these, PET stands out as a particularly promising modality, especially when it is coupled with cancer-specific radiotracers.

A notable stride in targeted therapies is the growing recognition of PRRT in the pharmaceutical realm, offering treatments that are both less toxic and more efficient, with recent approvals marking a pivotal milestone. PRRT underscores the advantages of employing peptide radiotracers in cancer treatment, characterized by their high target specificity, excellent tumor penetration, and swift clearance. Within this evolving landscape, OTR emerges as a novel target for breast cancer diagnosis and targeted therapy, particularly relevant in the context of TNBC. Despite its promise, more systematic studies are imperative to validate OTR in human breast cancer xenograft models and assess OTR tumor expression across diverse patient populations.

The development of OTR-specific ligands and tracers plays a critical role in advancing these studies, serving as molecular tools for investigating OTR in breast cancer and beyond. Recent advancements in OTR structural elucidation through X-ray and cryo-EM will enhance the rational designs of OTR-specific radiotracers, and previous studies highlight position 8 of OT as the optimal site for attaching tracer-related tags. It is important to underscore that, akin to successful PRRT approaches targeting overexpressed receptors in cancers (e.g., SSTR, GRPR), the efficacy and feasibility of OTR-targeted radiotherapy hinge on significant OTR overexpression in tumors and the OTR-selectivity of the radiotracers. Such OTR-selective radiotracers are poised to contribute substantially to the expanding frontier of enhancing cancer care—ranging from improved disease diagnosis and staging to precision therapy monitoring and tumor-targeted interventions with fewer side effects.

Glossary

Abbreviations Used

α-MSH

α-melanocyte-stimulating hormone

CCK

cholecystokinin

CMKLR1

chemokine-like receptor 1

EDDA

ethylenediaminediacetic acid

ER

estrogen receptor

GPCR

G protein-coupled receptor

GRPR

gastrin-releasing peptide receptor

HER2

human epidermal growth factor receptor 2

ICG

indocyanine green

MRI

magnetic resonance imaging

NET

neuroendocrine tumor

NIR

near-infrared

NPY

neuropeptide Y

NT

neurotensin

OT

oxytocin

OTR

oxytocin receptor

PD

pharmacodynamic

PD1

programmed cell death protein 1

PEG

polyethylene glycol

PET

positron emission tomography

PK

pharmacokinetic

PR

progesterone receptor

PRRT

peptide receptor radionucleotide therapy

SLNB

sentinel lymph node biopsy

SPECT

single-photon emission computed tomography

SSTR

somatostatin receptor

T-DM1

trastuzumab and ado-trastuzumab emtansine

T-Dxd

fam-trastuzumab deruxtecan-nxki

TM

transmembrane

TMTP1

tumor molecular targeted peptide 1

TNBC

triple-negative breast cancer

TYKERB

tyrosine kinase inhibitor lapatinib

VIP

vasoactive intestinal peptide

VIP-R1

vasoactive intestinal polypeptide receptor 1

VPR

vasopressin receptor

Biographies

Predrag Kalaba is a medicinal chemist with a strong interest in multidisciplinary research, especially in the fields of GPCRs and neuroscience. His core interests include the synthesis and characterization of small molecules and neuropeptides, structure–activity relationship studies, and the development of molecular tools to study biological processes such as learning and memory and cognitive decline and enhancement. His background in biochemistry and medicinal chemistry allows him to tackle these problems from different angles and to develop innovative therapeutic strategies for diseases with unmet medical needs. He is also passionate about teaching and mentoring students. He obtained a Ph.D. degree in pharmaceutical sciences in 2018 from the University of Vienna and currently holds a senior post-doc position at the Muttenthaler lab, University of Vienna.

Cristina Sanchez de la Rosa obtained her Ph.D. degree from the University of Queensland in 2021. During her doctoral studies, she focused on peptide tracer development targeting the oxytocin and vasopressin receptor systems. Prior to her doctoral studies, she worked at the Gambus lab at the Medical School of the University of Birmingham, UK, where she focused on the regulation of DNA replication, and at the Biomedicine Department of the University of Cádiz, Spain, where she focused on Δ6 FAD and Elovl 5 activities through heterologous expression in yeast. She also holds a B.Sc. degree in biotechnology from the University of Cádiz, Spain.

Andreas Möller is a trained biochemist and cancer biologist with >20 years of experience in cancer research. He has a strong background in cancer cell biology, exosome biology, hypoxia research, cancer metastasis, and immunology. His research program focuses on novel approaches to understanding cancer metastasis and how the composition of the tumor microenvironment is coordinated, with the aim of translating the findings into clinical applications. He is an internationally recognized expert in cancer metastasis, extracellular vesicles, and cancer immune responses.

Paul F. Alewood is a pioneer in the fields of venom-based molecular discovery and solid-phase peptide synthesis, where his approaches have been adopted by many researchers in academia and industry. Prof. Alewood has helped develop the highly efficient venomics approach that spans high-throughput sequencing by integrating transcriptomics and proteomics, rapid solid-phase peptide synthesis, including high-throughput strategies to access cysteine-rich toxins, and peptide drug development of lead molecules with commercial potential. One exciting source of bioactive peptides is the marine predatory cone snail, on which Prof. Alewood and collaborators have published over 100 papers describing their identification, synthesis, structures, receptor target, and mode of action.

Markus Muttenthaler is a medicinal chemist working at the interface of chemistry and biology with a strong passion for translational research. His research focuses on neuropeptides and exploring nature’s biodiversity to develop molecular tools, diagnostics, and therapeutics. His background in drug discovery and development, as well as his interdisciplinary training in the fields of chemistry, molecular biology, and pharmacology, assist him in characterizing these often highly potent and selective compounds and studying their interactions with human physiology for medical innovations in pain, cancer, gut disorders, and neurodegenerative diseases.

Author Contributions

^# P.K. and C.S.R. contributed equally to this work as co-first authors.

The authors declare no competing financial interest.