Novel, cysteine-modified chelation strategy for the incorporation of [M(I)(CO)(3)](+) (M = Re, (99m)Tc) in an α-MSH peptide

MC1R and melanin-based molecular probes for theranostic of melanoma and beyond

Malignant melanoma is accounting for most of skin cancer-associated mortality. The incidence of melanoma increased every year worldwide especially in western countries. Treatment efficiency is highly related to the stage of melanoma. Therefore, accurate staging and restaging play a pivotal role in the management of melanoma patients. Though 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography (PET) has been widely used in imaging of tumor metastases, novel radioactive probes for specific targeted imaging of both primary and metastasized melanoma are still desired. Melanocortin receptor 1 (MC1R) and melanin are two promising biomarkers specifically for melanoma, and numerous research groups including us have been actively developing a plethora of radioactive probes based on targeting of MC1R or melanin for over two decades. In this review, some of the MC1R-targeted tracers and melanin-associated molecular imaging probes developed in our research and others have been briefly summarized, and it provides a quick glance of melanoma-targeted probe design and may contribute to further developing novel molecular probes for cancer theranostics.

Metal Peptide Conjugates in Cell and Tissue Imaging and Biosensing

Metal complex luminophores have seen dramatic expansion in application as imaging probes over the past decade. This has been enabled by growing understanding of methods to promote their cell permeation and intracellular targeting. Amongst the successful approaches that have been applied in this regard is peptide-facilitated delivery. Cell-permeating or signal peptides can be readily conjugated to metal complex luminophores and have shown excellent response in carrying such cargo through the cell membrane. In this article, we describe the rationale behind applying metal complexes as probes and sensors in cell imaging and outline the advantages to be gained by applying peptides as the carrier for complex luminophores. We describe some of the progress that has been made in applying peptides in metal complex peptide-driven conjugates as a strategy for cell permeation and targeting of transition metal luminophores. Finally, we provide key examples of their application and outline areas for future progress.

Design of Rhenium Compounds in Targeted Anticancer Therapeutics

Background:

Many rhenium (Re) complexes with potential anticancer properties have been synthesized in the recent years with the aim to overcome the clinical limitations of platinum agents. Re(I) tricarbonyl complexes are the most common but Re compounds with higher oxidation states have also been investigated, as well as hetero-metallic complexes and Re-loaded self-assembling devices. Many of these compounds display promising cytotoxic and phototoxic properties against malignant cells but all Re compounds are still at the stage of preclinical studies.

Methods:

The present review focused on the rhenium based cancer drugs that were in preclinical and clinical trials were examined critically. The detailed targeted interactions and experimental evidences of Re compounds reported by the patentable and non-patentable research findings used to write this review.

Results:

In the present review, we described the most recent and promising rhenium compounds focusing on their potential mechanism of action including, phototoxicity, DNA binding, mitochondrial effects, oxidative stress regulation or enzyme inhibition. Many ligands have been described that modulating the lipophilicity, the luminescent properties, the cellular uptake, the biodistribution, and the cytotoxicity, the pharmacological and toxicological profile.

Conclusion:

Re-based anticancer drugs can also be used in targeted therapies by coupling to a variety of biologically relevant targeting molecules. On the other hand, combination with conventional cytotoxic molecules, such as doxorubicin, allowed to take into profit the targeting properties of Re for example toward mitochondria. Through the example of the diseleno-Re complex, we showed that the main target could be the oxidative status, with a down-stream regulation of signaling pathways, and further on selective cell death of cancer cells versus normal cells.

Evaluation of a Novel Pb-203-Labeled Lactam-Cyclized Alpha-Melanocyte-Stimulating Hormone Peptide for Melanoma Targeting

The purpose of this study is to examine the melanocortin-1 receptor (MC1R) targeting and specificity of 203Pb-DOTA-GGNle-CycMSHhex in melanoma cells and tumors to facilitate its potential therapeutic application when labeled with 212Pb. The MC1R-specific targeting and imaging properties of 203Pb-DOTA-GGNle-CycMSHhex were determined on B16/F1 and B16/F10 murine melanoma cells and in B16/F1 flank melanoma-, B16/F10 flank melanoma-, and B16/F10 pulmonary metastatic melanoma-bearing C57 mice. 203Pb-DOTA-GGNle-CycMSHhex displayed MC1R-specific binding on B16/F1 and B16/F10 melanoma cells and tumors. B16/F1 flank melanoma, B16/F10 flank melanoma, and B16/F10 pulmonary metastatic melanoma lesions could be clearly imaged by single photon emission computed tomography (SPECT) using 203Pb-DOTA-GGNle-CycMSHhex as an imaging probe. The favorable melanoma targeting and imaging properties highlighted the potential of 203Pb-DOTA-GGNle-CycMSHhex as a MC1R-targeting melanoma imaging probe and warranted the evaluation of 212Pb-DOTA-GGNle-CycMSHhex for melanoma therapy in future studies.

68Ga-DOTA-GGNle-CycMSHhex targets the melanocortin-1 receptor for melanoma imaging

Melanocortin-1 receptor (MC1R) is a molecular target for melanoma imaging and therapy because of its overexpression on rodent and human melanoma cells. Here, we evaluated the MC1R targeting and specificity of 68Ga-DOTA-GGNle-CycMSHhex and Cy5.5-GGNle-CycMSHhex using murine and human melanoma cells, and murine and xenografted tumors. 68Ga-DOTA-GGNle-CycMSHhex was used first in human as an imaging probe to evaluate the possibility of radionuclide therapy in patients with advanced-stage melanoma. 68Ga-DOTA-GGNle-CycMSHhex and Cy5.5-GGNle-CycMSHhex displayed MC1R-specific targeting properties in murine and human melanoma cells, as well as in murine melanoma and human melanoma-xenografted tumors. Both B16/F10 and M21 melanoma lesions could be easily imaged by positron emission tomography using 68Ga-DOTA-GGNle-CycMSHhex The first-in-human images of melanoma brain metastases in patients demonstrated the clinical relevance of MC1R as a molecular target for melanoma imaging, highlighting the potential of 68Ga-DOTA-GGNle-CycMSHhex as an MC1R-targeting melanoma imaging probe and underscoring the need to develop MC1R-targeting therapeutic agents for treating patients with metastatic melanoma.

Metastatic melanoma imaging using a novel Tc-99m-labeled lactam-cyclized alpha-MSH peptide

The purpose of this study was to determine the metastatic melanoma imaging property of ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex {hydrazinonicotinamide-8-aminooctanoic acid-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂}. HYNIC-Aoc-Nle-CycMSH_hex was synthesized using fluorenylmethyloxy carbonyl (Fmoc) chemistry. The IC₅₀ value of HYNIC-Aoc-Nle-CycMSH_hex was 0.78 ± 0.13 nM for B16/F10 melanoma cells. ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex displayed significantly higher uptake (14.26 ± 2.74 and 10.45 ± 2.31% ID/g) in B16/F10 metastatic melanoma-bearing lung than that in normal lung (0.90 ± 0.15 and 0.53 ± 0.14% ID/g) at 2 and 4 h post-injection, respectively. B16/F10 pulmonary metastatic melanoma lesions were clearly visualized by SPECT/CT using ^99mTc(EDDA)-HYNIC-Aoc-Nle-CycMSH_hex as an imaging probe at 2 h post-injection, underscoring its potential as an imaging probe for metastatic melanoma detection.

Technetium(I) Complexes of Bathophenanthrolinedisulfonic Acid

Bathophenanthrolinedisulfonate (BPS) complexes of technetium(I) of the type [Tc(CO)₃(BPS)(L)]ⁿ (L = imidazole derivatives) were synthesized and evaluated both in vitro and in vivo. [^99mTc(CO)₃(BPS)(MeIm)]^- (MeIm = 1-methyl-1H-imidazole) was prepared in near-quantitative yield using a convenient two-step, one-pot labeling procedure. A targeted analogue capable of binding regions of calcium turnover associated with bone metabolism was also prepared. Here, a bisphosphonate was linked to the metal through an imidazole ligand to give [^99mTc(CO)₃(BPS)(ImAln)]^2- (ImAln = an imidazole-alendronate ligand) in high yield. The technetium(I) complexes were stable in vitro, and in biodistribution studies, [^99mTc(CO)₃(BPS)(ImAln)]^2- exhibited rapid clearance from nontarget tissues and significant accumulation in the shoulder (7.9 ± 0.2% ID/g) and knees (15.1 ± 0.9% ID/g) by 6 h, with the residence time in the skeleton reaching 24 h. A rhenium analogue, which is luminescent and has the same structure, was also prepared and used for fluorescence labeling of cells in vitro. The data reported demonstrate the potential of this class of compounds for use in creating isostructural optical and nuclear probes.

Preparation of tetrazine-containing [2 + 1] complexes of 99mTc and in vivo targeting using bioorthogonal inverse electron demand Diels–Alder chemistry

A new ^99mTc-labelled tetrazine for targeted imaging using bioorthogonal chemistry was developed and evaluated in vivo using a trans-cyclooctene derived bisphosphonate targeting regions of high bone turnover and bone lesions.

Imaging human melanoma using a novel Tc-99m-labeled lactam bridge-cyclized alpha-MSH peptide

In this study, the human melanoma targeting property of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex {hydrazinonicotinamide-8-aminooctanoic acid-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2} was determined in M21 human melanoma-xenografts to demonstrate its potential for human melanoma imaging. The IC50 value of HYNIC-AocNle-CycMSHhex was 0.48±0.01nM in M21 human melanoma cells (1281receptors/cell). The M21 human melanoma uptake of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex was 4.03±1.25, 3.26±1.23 and 3.36±1.48%ID/g at 0.5, 2 and 4h post-injection, respectively. Approximately 92% of injected dose cleared out the body via urinary system at 2h post-injection. (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex showed high tumor/blood, tumor/muscle and tumor/skin uptake ratios after 2h post-injection. The M21 human melanoma-xenografted tumor lesions were clearly visualized by SPECT/CT using (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex as an imaging probe at 2h post-injection. Overall, (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex exhibited favorable human melanoma imaging property, highlighting its potential as an imaging probe for human metastatic melanoma detection.

Isothiocyanate-Functionalized Bifunctional Chelates and fac-[M(I)(CO)3](+) (M = Re, (99m)Tc) Complexes for Targeting uPAR in Prostate Cancer

Developing new strategies to rapidly incorporate the fac-[M(I)(CO)3](+) (M = Re, (99m)Tc) core into biological targeting vectors in radiopharmaceuticals continues to expand as molecules become more complex and as efforts to minimize nonspecific binding increase. This work examines a novel isothiocyanate-functionalized bifunctional chelate based on 2,2'-dipicolylamine (DPA) specifically designed for complexing the fac-[M(I)(CO)3](+) core. Two strategies (postlabeling and prelabeling) were explored using the isothiocyanate-functionalized DPA to determine the effectiveness of assembly on the overall yield and purity of the complex with amine containing biomolecules. A model amino acid (lysine) examined (1) amine conjugation of isothiocyanate-functionalized DPA followed by complexation with fac-[M(I)(CO)3](+) (postlabeling) and (2) complexation of fac-[M(I)(CO)3](+) with isothiocyanate-functionalized DPA followed by amine conjugation (prelabeling). Conducted with stable Re and radioactive (99m)Tc analogs, both strategies formed the product in good to excellent yields under macroscopic and radiotracer concentrations. A synthetic peptide (AE105) which targets an emerging biomarker in CaP prognosis, urokinase-type plasminogen activator receptor (uPAR), was also explored using the isothiocyanate-functionalized DPA strategy. In vitro PC-3 (uPAR+) cell uptake assays with the (99m)Tc-labeled peptide (8a) showed 4.2 ± 0.5% uptake at 4 h. In a murine model bearing PC-3 tumor xenografts, in vivo biodistribution of 8a led to favorable tumor uptake (3.7 ± 0.7% ID/g) at 4 h p.i. with relatively low accumulation (<2% ID/g) in normal organs not associated with normal peptide excretion. These results illustrate the promise of the isothiocyanate-functionalized approach for labeling amine containing biological targeting vectors with fac-[M(I)(CO)3](+).

Influence of functionalized pyridine ligands on the radio/chemical behavior of [M(I)(CO)3](+) (M = Re and (99m)Tc) 2 + 1 complexes

While a number of chelate strategies have been developed for the organometallic precursor fac-[M(I)(OH2)3(CO)3](+) (M = Re, (99m)Tc), a unique challenge has been to improve the overall function and performance of these complexes for in vivo and in vitro applications. Since its discovery, fac-[M(I)(OH2)3(CO)3](+) has served as an essential scaffold for the development of new targeted (99m)Tc based radiopharmaceuticals due to its labile aquo ligands. However, the lipophilic nature of the fac-[M(I)(CO)3](+) core can influence the in vivo pharmacokinetics and biodistribution of the complexes. In an effort to understand and improve this behavior, monosubstituted pyridine ligands were used to assess the impact of donor nitrogen basicity on binding strength and stability of fac-[M(I)(CO)3](+) in a 2 + 1 labeling strategy. A series of Re and (99m)Tc complexes were synthesized with picolinic acid as a bidentate ligand and 4-substituted pyridine ligands. These complexes were designed to probe the effect of pKa from the monodentate pyridine ligand both at the macro scale and radiochemical concentrations. Comparison of X-ray structural data and radiochemical solution experiments clearly indicate an increase in overall yield and stability as pyridine basicity increased.

Peptide self-assembly triggered by metal ions

This review summarizes the recent development of structures, functions, as well as strategies of a peptide self-assembly induced by metal ions.

Introduction of an 8-aminooctanoic acid linker enhances uptake of 99mTc-labeled lactam bridge-cyclized α-MSH peptide in melanoma

The purpose of this study was to examine the effects of amino acid, hydrocarbon, and polyethylene glycol (PEG) linkers on the melanoma targeting and imaging properties of (99m)Tc-labeled lactam bridge-cyclized HYNIC-linker-Nle-CycMSHhex (hydrazinonicotinamide-linker-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2) peptides.

METHODS

Four novel peptides (HYNIC-GGGNle-CycMSHhex, HYNIC-GSGNle-CycMSHhex, HYNIC-PEG2Nle-CycMSHhex, and HYNIC-AocNle-CycMSHhex) were designed and synthesized. The melanocortin-1 receptor binding affinities of the peptides were determined in B16/F1 melanoma cells. The biodistribution of (99m)Tc(ethylenediaminediacetic acid [EDDA])-HYNIC-GGGNle-CycMSHhex, (99m)Tc(EDDA)-HYNIC-GSGNle-CycMSHhex, (99m)Tc(EDDA)-HYNIC-PEG2Nle-CycMSHhex, and (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were determined in B16/F1 melanoma-bearing C57 mice at 2 h after injection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were further examined because of its high melanoma uptake.

RESULTS

The inhibitory concentrations of 50% (IC50) for HYNIC-GGGNle-CycMSHhex, HYNIC-GSGNle-CycMSHhex, HYNIC-PEG2Nle-CycMSHhex, and HYNIC-AocNle-CycMSHhex were 0.7 ± 0.1, 0.8 ± 0.09, 0.4 ± 0.08, and 0.3 ± 0.06 nM, respectively, in B16/F1 melanoma cells. Among these four (99m)Tc-labeled peptides, (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex displayed the highest melanoma uptake (22.3 ± 1.72 percentage injected dose/g) at 2 h after injection. (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex exhibited high tumor-to-normal-organ uptake ratios except for the kidneys. The tumor-to-kidney uptake ratios of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were 3.29, 3.63, and 6.78 at 2, 4, and 24 h, respectively, after injection. The melanoma lesions were clearly visualized by SPECT/CT using (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex as an imaging probe at 2 h after injection.

CONCLUSION

High melanoma uptake and fast urinary clearance of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex highlighted its potential for metastatic melanoma detection in the future.

Advances in imaging probes and optical microendoscopic imaging techniques for early in vivo cancer assessment

A new chapter in the history of medical diagnosis happened when the first X-ray technology was invented in the late 1800s. Since then, many non-invasive and minimally invasive imaging techniques have been invented for clinical diagnosis to research in cellular biology, drug discovery, and disease monitoring. These imaging modalities have leveraged the benefits of significant advances in computer, electronics, and information technology and, more recently, targeted molecular imaging. The development of targeted contrast agents such as fluorescent and nanoparticle probes coupled with optical imaging techniques has made it possible to selectively view specific biological events and processes in both in vivo and ex vivo systems with great sensitivity and selectivity. Thus, the combination of targeted molecular imaging probes and optical imaging techniques have become a mainstay in modern medicinal and biological research. Many promising results have demonstrated great potentials to translate to clinical applications. In this review, we describe a discussion of employing imaging probes and optical microendoscopic imaging techniques for cancer diagnosis.

Theranostics of malignant melanoma with 64CuCl2

Human copper transporter 1 (CTR1) is overexpressed in a variety of cancers. This study aimed to evaluate the use of (64)CuCl2 as a theranostic agent for PET and radionuclide therapy of malignant melanoma.

METHODS

CTR1 expression levels were detected by Western blot analysis of a group of tumor cell lines. Two melanoma cell lines (B16F10 and A375M) that highly expressed CTR1 were then selected to study the uptake and efflux of (64)CuCl2. Mice bearing B16F10 or A375M tumors (n = 4 for each group) were subjected to 5 min of static whole-body PET scans at different time points after intravenous injection of (64)CuCl2. Dynamic scans were also obtained for B16F10 tumor-bearing mice. All mice were sacrificed at 72 h after injection of (64)CuCl2, and biodistribution studies were performed. Mice bearing B16F10 or A375M tumors were further subjected to (64)CuCl2 radionuclide therapy. Specifically, when the tumor size reached 0.5-0.8 cm in diameter, tumor-bearing mice were systemically administered (64)CuCl2 (74 MBq) or phosphate-buffered saline, and tumor sizes were monitored over the treatment period.

RESULTS

CTR1 was found to be overexpressed in the cancer cell lines tested at different levels, and high expression levels in melanoma cells and tissues were observed (melanotic B16F10 and amelanotic A375M). (64)CuCl2 displayed high and specific uptake in B16F10 and A375M cells. In vivo (64)CuCl2 PET imaging demonstrated that both B16F10 and A375M tumors were clearly visualized. Radionuclide treatment studies showed that the tumor growth in both the B16F10 and the A375M models under (64)CuCl2 treatment were much slower than that of the control group.

CONCLUSION

Both melanotic and amelanotic melanomas (B16F10 and A375M) tested were found to overexpress CTR1. The tumors can be successfully visualized by (64)CuCl2 PET and further treated by (64)CuCl2, highlighting the high potential of using (64)CuCl2 as a theranostic agent for the management of melanoma.

Clickable, hydrophilic ligand for fac-[M(I)(CO)3](+) (M = Re/(99m)Tc) applied in an S-functionalized α-MSH peptide

The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (1 and 3) for fac-[M^I(CO)₃]⁺ (M = Re/^99mTc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (3) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of fac-[^99mTc^I(CO)₃]⁺ complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The fac-[Re^I(CO)₃(3)] complex (4) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between 3 and fac-[^99mTc^I(OH₂)₃(CO)₃]⁺. The corresponding ^99mTc complex (4a) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log P analysis compared to an analogous complex with nonfunctionalized pyridine rings (2a). An α-MSH peptide functionalized with an azide was labeled with fac-[M^I(CO)₃]⁺ using both click, then chelate (CuAAC reaction with 1 or 3 followed by metal complexation) and chelate, then click (metal complexation of 1 and 3 followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on fac-[M^I(CO)₃]⁺ complexation within a peptide framework. The peptides from the click, then chelate strategy had different HPLC t_R’s and in vitro stabilities compared to those from the chelate, then click strategy, suggesting nonspecific coordination of fac-[M^I(CO)₃]⁺ using this synthetic route. The fac-[M^I(CO)₃]⁺-complexed peptides from the chelate, then click strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC₅₀ analyses, and led to moderately high uptake in B16F10 melanoma cells. Log P analysis of the ^99mTc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (10a′) compared to the peptide with the unmodified DPA chelate (9a′). In vivo biodistribution analysis of 9a′ and 10a′ showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for 10a′ compared to predominantly hepatic accumulation for 9a′. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted fac-[M^I(CO)₃]⁺ complexes in future studies.

99mTc-labeled cystine knot peptide targeting integrin αvβ6 for tumor SPECT imaging

Integrin α_vβ₆ is overexpressed in a variety of cancers, and its expression is often associated with poor prognosis. Therefore, there is a need to develop affinity reagents for noninvasive imaging of integrin α_vβ₆ expression since it may provide early cancer diagnosis, more accurate prognosis, and better treatment planning. We recently engineered and validated highly stable cystine knot peptides that selectively bind integrin α_vβ₆ with no cross-reactivity to integrins α_vβ₅, α₅β₁, or α_vβ₃, also known to be overexpressed in many cancers. Here, we developed a single photon emission computed tomography (SPECT) probe for imaging integrin α_vβ₆ positive tumors. Cystine knot peptide, S₀2, was first conjugated with a single amino acid chelate (SAAC) and labeled with [^99mTc(H₂O)₃(CO)₃]⁺. The resulting probe, ^99mTc-SAAC-S₀2, was then evaluated by in vitro cell uptake studies using two α_vβ₆ positive cell lines (human lung adenocarcinoma cell line HCC4006 and pancreatic cancer cell line BxPC-3) and two α_vβ₆ negative cell lines (human lung adenocarcinoma cell line H838 and human embryonic kidney cell line 293T). Next, SPECT/CT and biodistribution studies were performed in nude mice bearing HCC4006 and H838 tumor xenografts to evaluate the in vivo performance of ^99mTc-SAAC-S₀2. Significant differences in the uptake of ^99mTc-SAAC-S₀2 were observed in α_vβ₆ positive vs negative cells (P < 0.05). Biodistribution and small animal SPECT/CT studies revealed that ^99mTc-SAAC-S₀2 accumulated to moderate levels in antigen positive tumors (∼2% ID/g at 1 and 6 h postinjection, n = 3 or 4/group). Moreover, the probe demonstrated tumor-to-background tissue ratios of 6.81 ± 2.32 (tumor-to-muscle) and 1.63 ± 0.18 (tumor-to-blood) at 6 h postinjection in α_vβ₆ positive tumor xenografts. Co-incubation of the probe with excess amount of unlabeled S₀2 as a blocking agent demonstrated significantly reduced tumor uptake, which is consistent with specific binding to the target. Renal filtration was the main route of clearance. In conclusion, knottin peptides are excellent scaffolds for which to develop highly stable imaging probes for a variety of oncological targets. ^99mTc-SAAC-S₀2 demonstrates promise for use as a SPECT agent to image integrin α_vβ₆ expression in living systems.