Synthesis and evaluation of a peptide targeted small molecular Gd-DOTA monoamide conjugate for MR molecular imaging of prostate cancer

Targeted Contrast Agents for Magnetic Resonance Molecular Imaging of Cancer

Magnetic resonance imaging (MRI) is a clinical imaging modality that provides high-resolution images of soft tissues, including cancerous lesions. Stable gadolinium(III) chelates have been used as contrast agents (CA) in MRI to enhance the contrast between the tissues of interest and surrounding tissues for accurate diagnostic imaging. Magnetic resonance molecular imaging (MRMI) of cancer requires targeted CA to specifically elucidate cancer-associated molecular processes and can provide high-resolution delineation and characterization of cancer for precision medicine. The main challenge for MRMI is the lack of sufficient sensitivity to detect the low concentration of the cellular oncogenic markers. In addition, targeted CA must satisfy regulatory safety requirements prior to clinical development. Up to now, there is no FDA-approved targeted CA for MRMI of cancer.In this Account, we discuss the latest developments in the design and development of clinically translatable targeted CA for MRMI of cancer, with an emphasis on our own research. The primary limitation of MRMI can be overcome by designing small molecular targeted CA to target abundant cancer-specific targets found in the tumor microenvironment (TME). For example, aggressive tumors have a unique extracellular matrix (ECM) composed of oncoproteins, which can be used as targetable markers for MRMI. We have designed and prepared small peptide conjugates of clinical contrast agents, including Gd-DTPA and Gd-DOTA, to target fibrin-fibronectin clots in tumors. These small molecular CA have been effective in enhancing MRMI detection of solid tumors and have demonstrated the ability to detect submillimeter cancer micrometastases in mouse tumor models, exceeding the detection limit of current clinical imaging modalities. We have also identified extradomain B fibronectin (EDB-FN), an oncofetal subtype of fibronectin, as a promising TME target to leverage in the design and development of small peptide targeted CA for clinical translation. The expression level of EDB-FN is correlated with invasiveness of cancer cells and poor patient survival of multiple cancer types. ZD2 peptide with a sequence of seven amino acids (TVRTSAD) was identified to specifically bind to the EDB protein fragment. Several ZD2 conjugates of macrocyclic GBCA, including Gd-DOTA and Gd(HP-DO3A), have been synthesized and tested in mouse tumor models. ZD2-N3-Gd(HP-DO3A) (MT218) with a high r₁ relaxivity was selected as the lead agent for clinical translation. The physicochemical properties and preclinical assessments of MT218 are summarized in this Account. MRMI of EDB-FN with MT218 can effectively detect invasive tumors of multiple cancers with risk-stratification and monitor tumor response to anticancer therapies in mouse models. Currently, MT218 is in clinical trials for precision cancer MRMI. Herein, we will show that using targeted MRI contrast agents specific to abundant TME biomarkers is a pragmatic solution for effective precision cancer imaging in high spatial resolution. And thus, we illustrate a replicable approach for CA development that is vital for cancer MRMI.

Imaging of Dysfunctional Elastogenesis in Atherosclerosis Using an Improved Gadolinium-Based Tetrameric MRI Probe Targeted to Tropoelastin

Dysfunctional elastin turnover plays a major role in the progression of atherosclerotic plaques. Failure of tropoelastin cross-linking into mature elastin leads to the accumulation of tropoelastin within the growing plaque, increasing its instability. Here we present Gd₄-TESMA, an MRI contrast agent specifically designed for molecular imaging of tropoelastin within plaques. Gd₄-TESMA is a tetrameric probe composed of a tropoelastin-binding peptide (the VVGS-peptide) conjugated with four Gd(III)-DOTA-monoamide chelates. It shows a relaxivity per molecule of 34.0 ± 0.8 mM^-1 s^-1 (20 MHz, 298 K, pH 7.2), a good binding affinity to tropoelastin (K_D = 41 ± 12 μM), and a serum half-life longer than 2 h. Gd₄-TESMA accumulates specifically in atherosclerotic plaques in the ApoE^-/- murine model of plaque progression, with 2 h persistence of contrast enhancement. As compared to the monomeric counterpart (Gd-TESMA), the tetrameric Gd₄-TESMA probe shows a clear advantage regarding both sensitivity and imaging time window, allowing for a better characterization of atherosclerotic plaques.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Integrin α6-Targeted Magnetic Resonance Imaging of Hepatocellular Carcinoma in Mice

PURPOSE

Magnetic resonance imaging (MRI) has a high spatial resolution for detecting hepatocellular carcinoma (HCC). Integrin α6 has emerged as a diagnostic and prognostic biomarker of HCC. Here, we developed the MR contrast agent RWY-dL-(Gd-DOTA)₄ based on the integrin α6-targeted RWY peptide that we previously identified to detect HCC.

PROCEDURES

Contrast-enhanced MRI was carried out to evaluate the use of RWY-dL-(Gd-DOTA)₄ to detect HCC lesions in subcutaneous and diethylnitrosamine (DEN)-induced HCC mouse models.

RESULTS

Enhancement MR signals were observed in HCC-LM3 subcutaneous liver tumors in the first 5 min post-injection of RWY-dL-(Gd-DOTA)₄ at a low dose of 0.03 mmol Gd/kg. Moreover, RWY-dL-(Gd-DOTA)₄ generated superior contrast enhancement for liver tumors in chemical-induced HCC mice. Importantly, RWY-dL-(Gd-DOTA)₄ provided complementary enhancement MR signals to the clinical available hepatobiliary MR contrast agent gadoxetate disodium Gd-EOB-DTPA. Additionally, RWY-dL-(Gd-DOTA)₄ showed minimal gadolinium retention in normal tissues and organs at 48 h post-injection.

CONCLUSION

These findings potentiate the use of RWY-dL-(Gd-DOTA)₄ for the MRI of HCC to improve the diagnosis of HCC.

Molecular MR Contrast Agents

Molecular magnetic resonance (MR) imaging utilizes molecular probes to provide added biochemical or cellular information to what can already be achieved with anatomical and functional MR imaging. This review provides an overview of molecular MR and focuses specifically on molecular MR contrast agents that provide contrast by shortening the T1 time. We describe the requirements for a successful molecular MR contrast agent and the challenges for clinical translation. The review highlights work from the last 5 years and places an emphasis on new contrast agents that have been validated in multiple preclinical models. Applications of molecular MR include imaging of inflammation, fibrosis, fibrogenesis, thromboembolic disease, and cancers. Molecular MR is positioned to move beyond detection of disease to the quantitative staging of disease and measurement of treatment response.

Targeting hemostasis-related moieties for tumor treatment

Under normal conditions, the hemostatic system, that includes the involvement of the coagulation response and platelets, is anatomically and functionally inseparable from the vasculature. However, the hemostatic response always occurs in a wide range of tumors because of the high expression of coagulation initiator tissue factor (TF) in many tumor tissues, and due to the leakage of coagulation factors and platelets from the circulation system into the tumor interstitium through abnormal tumor vessels. Therefore, in addition to TF, these coagulation factors, platelets, the central moiety thrombin, the final product fibrin, and fibronectin, which is capable of stabilizing coagulation clots, are also abundant in tumors. These hemostasis-related moieties (HRMs), including TF, thrombin, fibrin, fibronectin, and platelets, are also closely associated with tumor progression, e.g., primary tumor growth and distal metastasis. The hemostatic response only occurs under pathological conditions, such as tumors, thrombosis, and atherosclerosis other than in normal tissues. The HRMs within tumors are also highly specific, establishing functional and therapeutic targets for tumor treatment. Therefore, strategies including active targeting to these moieties, modulation of HRMs deposited in the tumor microenvironment to improve tumor drug delivery, activation of prodrug by the coagulation complex formed during coagulation response, and direct inhibition of the tumor-promoting activity of HRMs could be designed for tumor therapy. In this review, we summarize various strategies that target HRMs for tumor treatment.

Solid phase synthesis in the development of magnetic resonance imaging probes

We review the use of the solid phase synthesis methodology for the preparation of diverse and potent MRI probes.

Effective MR Molecular Imaging of Triple Negative Breast Cancer With an EDB-Fibronectin-Specific Contrast Agent at Reduced Doses

MR molecular imaging (MRMI) of abundant oncogenic biomarkers in tumor microenvironment has the potential to provide precision cancer imaging in high resolution. Extradomain-B fibronectin (EDB-FN) is an oncogenic extracellular matrix protein, highly expressed in aggressive triple negative breast cancer. A targeted macrocyclic gadolinium-based contrast agent (GBCA) ZD2-N3-Gd(HP-DO3A) (MT218), specific to EDB-FN, was developed for MRMI of aggressive breast cancer. The effectiveness of different doses of MT218 for MRMI was tested in MDA-MB-231 and Hs578T human triple negative breast cancer models. At clinical dose of 0.1 and subclinical dose of 0.04 mmol Gd/kg, MT218 rapidly bound to the extracellular matrix EDB-FN and produced robust tumor contrast enhancement in both the tumor models, as early as 1-30 min post-injection. Substantial tumor enhancement was also observed in both the models with MT218 at doses as low as 0.02 mmol Gd/kg, which was significantly better than the clinical agent Gd(HP-DO3A) at 0.1 mmol Gd/kg. Little non-specific enhancement was observed in the normal tissues including liver, spleen, and brain for MT218 at all the tested doses, with renal clearance at 30 min. These results demonstrate that MRMI with reduced doses of MT218 is safe and effective for sensitive and specific imaging of aggressive breast cancers.

A novel plectin/integrin-targeted bispecific molecular probe for magnetic resonance/near-infrared imaging of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest human malignancies with poor patient outcomes often resulting from delayed diagnosis. Therefore, early diagnosis can lead to a better prognosis and improved outcomes. In this study, we have developed a novel conjugate complex of plectin/integrin-targeted bispecific molecular probe, termed Gd-Cy7-PTP/RGD, to be used for magnetic resonance/near-infrared imaging (MRI/NIRF) of pancreatic cancer in vivo. This bispecific molecular probe comprises four parts: Gd(III) for MRI, cyanine 7 (Cy7) for NIRF, the peptide PTP for binding to plectin-1 specifically overexpressed on the surface of PDAC cells, and the peptide RGD for targeting integrin widely expressed on pancreatic duct epithelial cells and angiogenesis. Remarkably, the combination of PTP and RGD greatly increased the targeting efficiency in vitro and in vivo compared to that of either single peptide. Moreover, such bispecific molecular probes target pancreatic neoplasms and angiogenesis simultaneously, producing a "multi-level" targeting effect confirmed by immunofluorescence testing in vitro and in vivo. Under the guidance of MRI/NIRF dual-modality imaging, NIRF-guided delineation of surgical margins during operations was successfully achieved in orthotopic pancreatic cancer. This study promotes further exploration of bispecific molecular probes for clinical application.

Synthesis of High Relaxivity Gadolinium AAZTA Tetramers as Building Blocks for Bioconjugation

Molecular imaging requires the specific accumulation of contrast agents at the target. To exploit the superb resolution of MRI for applications in molecular imaging, gadolinium chelates, as the MRI contrast agents (CA), have to be conjugated to a specific vector able to recognize the epitope of interest. Several Gd(III)-chelates can be chemically linked to the same binding vector in order to deliver multiple copies of the CA (multimers) in a single targeting event thus increasing the sensitivity of the molecular probe. Herein three novel bifunctional agents, carrying one functional group for the bioconjugation to targeting vectors and four Gd(III)-AAZTA chelate functions for MRI contrast enhancement (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid), are reported. The relaxivity in the tetrameric derivatives is 16.4 ± 0.2 mM_Gd^-1 s^-1 at 21.5 MHz and 25 °C, being 2.4-fold higher than that of parent, monomeric Gd(III)-AAZTA. These compounds can be used as versatile building blocks to insert preformed, high relaxivity, and high density Gd-centers to biological targeting vectors. As an example, we describe the use of these bifunctional Gd(III)-chelates to label a fibrin-targeting peptide.

Magnetic resonance molecular imaging of metastatic breast cancer by targeting extradomain-B fibronectin in the tumor microenvironment

PURPOSE

Non-invasive early accurate detection of malignant breast cancer is paramount to the clinical management of the life-threatening disease. Here, we aim to test a small peptide targeted MRI contrast agent, ZD2-Gd(HP-DO3A), specific to an oncoprotein, extradomain-B fibronectin (EDB-FN), in the tumor microenvironment for MR molecular imaging of breast cancer.

METHOD

EDB-FN expression in 4T1 and MDA-MB-231 cancers was analyzed with quantitative real-time PCR and western blot. Primary and metastatic triple negative breast cancer mouse models were developed using 4T1 and MDA-MB-231 cells. Contrast-enhanced MRI was carried out to evaluate the use of ZD2-Gd(HP-DO3A) in detecting 4T1 and MDA-MB-231 primary and metastatic tumors.

RESULTS

EDB-FN was abundantly expressed in the extracellular matrix (ECM) of both the primary and metastatic TNBC tumors. In T₁ -weighted MRI, ZD2-Gd(HP-DO3A) generated superior contrast enhancement in primary TNBC tumors than a nonspecific clinical agent Gd(HP-DO3A), during 30 min after contrast injection. ZD2-Gd(HP-DO3A) also produced a significant increase in contrast-to-noise ratio (CNR) of TNBC metastases, enabling sensitive localization and delineation of metastases that occulted in non-contrast-enhanced or Gd(HP-DO3A)-enhanced MRI.

CONCLUSIONS

These findings potentiate the use of ZD2-Gd(HP-DO3A) for MR molecular imaging of malignant breast cancers to improve the healthcare of breast cancer patients. Magn Reson Med 79:3135-3143, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Molecular imaging of the tumor microenvironment

The tumor microenvironment plays a critical role in tumor initiation, progression, metastasis, and resistance to therapy. It is different from normal tissue in the extracellular matrix, vascular and lymphatic networks, as well as physiologic conditions. Molecular imaging of the tumor microenvironment provides a better understanding of its function in cancer biology, and thus allowing for the design of new diagnostics and therapeutics for early cancer diagnosis and treatment. The clinical translation of cancer molecular imaging is often hampered by the high cost of commercialization of targeted imaging agents as well as the limited clinical applications and small market size of some of the agents. Because many different cancer types share similar tumor microenvironment features, the ability to target these biomarkers has the potential to provide clinically translatable molecular imaging technologies for a spectrum of cancers and broad clinical applications. There has been significant progress in targeting the tumor microenvironment for cancer molecular imaging. In this review, we summarize the principles and strategies of recent advances made in molecular imaging of the tumor microenvironment, using various imaging modalities for early detection and diagnosis of cancer.

Targeting Fibronectin for Cancer Imaging and Therapy

During cancer progression, the extracellular matrix (ECM) undergoes dramatic changes, which promote cancer cell migration and invasion. In the remodeled tumor ECM, fibronectin (FN) level is upregulated to assist tumor growth, progression, and invasion. FN serves as a central organizer of ECM molecules and mediates the crosstalk between the tumor microenvironment and cancer cells. Its upregulation is correlated with angiogenesis, cancer progression, metastasis, and drug resistance. A number of FN-targeting ligands have been developed for cancer imaging and therapy. Thus far, FN-targeting imaging agents have been tested for nuclear imaging, MRI, and fluorescence imaging, for tumor detection and localization. FN-targeting therapeutics, including nuclear medicine, chemotherapy drugs, cytokines, and photothermal moieties, were also developed in cancer therapy. Because of the prevalence of FN overexpression in cancer, FN targeting imaging agents and therapeutics have the promise of broad applications in the diagnosis, treatment, and image-guided interventions of many types of cancers. This review will summarize current understanding on the role of FN in cancer, discuss the design and development of FN-targeting agents, and highlight the applications of these FN-targeting agents in cancer imaging and therapy.

Synthesis and Evaluation of Gd(III) -Based Magnetic Resonance Contrast Agents for Molecular Imaging of Prostate-Specific Membrane Antigen

Magnetic resonance (MR) imaging is advantageous because it concurrently provides anatomic, functional, and molecular information. MR molecular imaging can combine the high spatial resolution of this established clinical modality with molecular profiling in vivo. However, as a result of the intrinsically low sensitivity of MR imaging, high local concentrations of biological targets are required to generate discernable MR contrast. We hypothesize that the prostate-specific membrane antigen (PSMA), an attractive target for imaging and therapy of prostate cancer, could serve as a suitable biomarker for MR-based molecular imaging. We have synthesized three new high-affinity, low-molecular-weight Gd(III) -based PSMA-targeted contrast agents containing one to three Gd(III)  chelates per molecule. We evaluated the relaxometric properties of these agents in solution, in prostate cancer cells, and in an in vivo experimental model to demonstrate the feasibility of PSMA-based MR molecular imaging.

EDB Fibronectin Specific Peptide for Prostate Cancer Targeting

Extradomain-B fibronectin (EDB-FN), one of the oncofetal fibronectin (onfFN) isoforms, is a high-molecular-weight glycoprotein that mediates cell adhesion and migration. The expression of EDB-FN is associated with a number of cancer-related biological processes such as tumorigenesis, angiogenesis, and epithelial-to-mesenchymal transition (EMT). Here, we report the development of a small peptide specific to EDB-FN for targeting prostate cancer. A cyclic nonapeptide, CTVRTSADC (ZD2), was identified using peptide phage display. A ZD2-Cy5 conjugate was synthesized to accomplish molecular imaging of prostate cancer in vitro and in vivo. ZD2-Cy5 demonstrated effective binding to up-regulated EDB-FN secreted by TGF-β-induced PC3 cancer cells following EMT. Following intravenous injections, the targeted fluorescent probe specifically bound to and delineated PC3-GFP prostate tumors in nude mice bearing the tumor xenografts. ZD2-Cy5 also showed stronger binding to human prostate tumor specimens with a higher Gleason score (GS9) compared to those with a lower score (GS 7), with no binding in benign prostatic hyperplasia (BPH). Thus, the ZD2 peptide is a promising strategy for molecular imaging and targeted therapy of prostate cancer.

Peptide targeted high-resolution molecular imaging of prostate cancer with MRI

Non-invasive accurate detection of prostate cancer is critical for clinical management of the disease. Molecular MRI has a potential for accurate detection of prostate cancer with high spatial resolution. Fibronectin is a hallmark of epithelial-mesenchymal transition occurred in aggressive prostate cancer and highly expressed in malignant tumors. A pentapeptide CREKA targeted contrast agent CREKA-dL-(DOTA-Gd)4 was synthesized and evaluated to target fibrin-fibronectin complexes in tumor extracellular matrix for molecular MRI of prostate cancer. The contrast agent was synthesized by solid-phase peptide synthesis. The T1 relaxivity of CREKA-(DOTA-Gd)4 at 1.5 T was 33.2 mM(-1)s(-1) per molecule (8.3 per Gd). The fluorescence imaging showed that CREKA specifically bound to orthotopic PC3 prostate tumor in athymic nude mice. Strong enhancement was observed in the tumor tissue injected with CREKA-(DOTA-Gd)4 in the first 5 minutes post-injection before MR signal became visible in the bladder at a low dose of 0.03 mmol-Gd/kg. The targeted contrast agent exhibited minimal Gd retention in the main organs and tissues 2 days after injection. The peptide targeted contrast agent CREKA-(DOTA-Gd)4 is promising for high-resolution molecular MRI of prostate cancer.

Molecular MRI of atherosclerotic plaque progression in an ApoE(-/-) mouse model with a CLT1 peptide targeted macrocyclic Gd(III) chelate

Molecular imaging of atherosclerotic biomarkers is critical for non-invasive detection and diagnosis of atherosclerotic plaques and therapeutic management. Fibrin and fibronectin accumulate at elevated levels in atherosclerotic plaques and are associated with atherogenesis and disease progression. Molecular imaging of these biomarkers has the potential to non-invasively characterize plaque burden. In this work, we investigated the effectiveness of a peptide-targeted macrocyclic Gd(III) chelate, CLT1-dL-(DOTA-Gd)4, specific to fibrin-fibronectin complexes for molecular MRI of atherosclerosis. Atherosclerotic plaques were induced in Apolipoprotein E-knockout (ApoE(-/-)) mice by feeding with high fat and cholesterol-enriched diet (HFD) for up to 30 weeks. MRI of the vessel wall in the arch aorta was performed at 10, 20 and 30 weeks after the onset of HFD. High spatial-resolution MRI was performed prior and up to 35 minutes after i.v. injection of CLT1-dL-(DOTA-Gd)4 or a nonspecific control agent at a dose of 0.1 mmol-Gd/kg. CLT1-dL-(DOTA-Gd)4 produced stronger enhancement in the atherosclerotic lesions of the aortic wall than the control at all time points in the mice. Cross sectional MR images of the aortic arch revealed progressive thickening of the atherosclerotic vessel wall in the mice on HFD for up to 30 weeks. This progression correlated well to histological staining, as well as fibrin and fibronectin immunochemical stained images. Molecular MRI with CLT1-dL-(DOTA-Gd)4 has a potential for detecting atherosclerosis and non-invasive monitoring of the progression of the plaques.

MR molecular imaging of prostate cancer with a small molecular CLT1 peptide targeted contrast agent

Tumor extracellular matrix has abundance of cancer related proteins that can be used as biomarkers for cancer molecular imaging. In this work, we demonstrated effective MR cancer molecular imaging with a small molecular peptide targeted Gd-DOTA monoamide complex as a targeted MRI contrast agent specific to clotted plasma proteins in tumor stroma. We performed the experiment of evaluating the effectiveness of the agent for non-invasive detection of prostate tumor with MRI in a mouse orthotopic PC-3 prostate cancer model. The targeted contrast agent was effective to produce significant tumor contrast enhancement at a low dose of 0.03 mmol Gd/kg. The peptide targeted MRI contrast agent is promising for MR molecular imaging of prostate tumor.

Translational Molecular Imaging of Prostate Cancer

Prostate cancer is a heterogeneous disease, and its management is now evolving to become more personalized and to incorporate new targeted therapies. With these new changes comes a demand for molecular imaging techniques that can not only detect disease but also assess biology and treatment response. This review article summarizes current molecular imaging approaches in prostate cancer (e.g. 99mTc bone scintigraphy and 18F-fluorodeoxyglucose positron emission tomography) and highlights emerging clinical and preclinical imaging agents, with an emphasis on mechanism and clinical application. Emerging agents at various stages of clinical translation include radiolabeled analogs of lipid, amino acid, and nucleoside metabolism, as well as agents more specifically targeting prostate cancer biomarkers including androgen receptor, prostate-specific membrane antigen and others. We also highlight new techniques and targeted contrast agents for magnetic resonance imaging and spectroscopy. For all these imaging techniques, a growing and important unmet need is for well-designed prospective clinical trials to establish clear indications with clinical benefit in prostate cancer.

Peptide targeted tripod macrocyclic Gd(III) chelates for cancer molecular MRI

Rational design and develop of targeted contrast agents binding to cancer-related proteins will achieve more accurate cancer diagnosis and prognosis by magnetic resonance (MR) imaging. CREKA is a tumor-homing pentapeptide (Cys-Arg-Glu-Lys-Ala) specifically homes to fibrin-fibronectin complexes abundantly expressed in tumor microenvironment. In this study, we developed and evaluated a CREKA peptide targeted multiplexed Gd-MR probe (CREKA-Tris-Gd(DOTA)3) for MR imaging of breast tumors. CREKA and azide bearing Gd(III) was attached to a maleimide-functional trialkyne scaffold via thiol-maleimide and azide-alkyne click chemistry, respectively. CREKA-Tris-Gd(DOTA)3 has a well-defined structure with a molecular weight of 2914 Da. The T1 relaxivity of CREKA-Tris-Gd(DOTA)3 is 8.06 mM(-1) s(-1) per Gd (24.18 mM(-1) s(-1) per molecule) at room temperature and 3 T. Fluorescence imaging showed high binding specificity of CREKA to a 4T1 breast tumor model in mice while it was not found for the scrambled CREKA (CERAK). The CREKA peptide-targeted contrast agent resulted in greater contrast enhancement than the corresponding CERAK agent and the commercialized contrast agent ProHance(®) in tumor at a dose of 0.1 mmol Gd/kg in female athymic mice bearing 4T1 breast carcinoma xenograft. This small molecular contrast agent was easily excreted from body after imaging indicated low toxicity. The targeted MRI contrast agent has a potential for specific cancer molecular imaging with MRI.

Molecular Magnetic Resonance Imaging of Tumors with a PTPµ Targeted Contrast Agent

Molecular magnetic resonance imaging (MRI) of tumors improves the specificity of MRI by using targeted probes conjugated to contrast-generating metals. The limitation of this approach is in the identification of a target molecule present in sufficient concentration for visualization and the development of a labeling reagent that can penetrate tumor tissue with the fast kinetics required for use in a clinical setting. The receptor protein tyrosine phosphatase PTPµ is a transmembrane protein that is continuously proteolyzed in the tumor microenvironment to generate a high concentration of extracellular fragment that can be recognized by the SBK2 probe. We conjugated the SBK2 peptide to a gadolinium chelate [SBK2-Tris-(Gd-DOTA)3] to test whether the SBK2 probe could be developed as an MR molecular imaging probe. When intravenously injected into mice bearing flank tumors of human glioma cells, SBK2-Tris-(Gd-DOTA)3 labeled the tumors within 5 minutes with a high level of contrast for up to 2 hours post-injection. The contrast enhancement of SBK2-Tris-(Gd-DOTA)3 was significantly higher than that observed with a current MRI macrocyclic gadolinium chelate (Gadoteridol, ProHance) alone or a scrambled control. These results demonstrate that SBK2-Tris-(Gd-DOTA)3 labeling of the PTPµ extracellular fragment is a more specific MR molecular imaging probe than ProHance or a scrambled control. Consequently, the SBK2 probe may be more useful than the current gold standard reagent for MRI to identify tumors and to co-register tumor borders during surgical resection.

Tailoring encodable lanthanide-binding tags as MRI contrast agents

Lanthanide-binding tags (LBTs), peptide-based coexpression tags with high affinity for lanthanide ions, have previously been applied as luminescent probes to provide phasing for structure determination in X-ray crystallography and to provide restraints for structural refinement and distance information in NMR. The native affinity of LBTs for Gd(3+) indicates their potential as the basis for engineering of peptide-based MRI agents. However, the lanthanide coordination state that enhances luminescence and affords tightest binding would not be ideal for applications of LBTs as contrast agents, due to the exclusion of water from the inner coordination sphere. Herein, we use structurally defined LBTs as the starting point for re-engineering the first coordination shell of the lanthanide ion to provide for high contrast through direct coordination of water to Gd(3+) (resulting in the single LBT peptide, m-sLBT). The effectiveness of LBTs as MRI contrast agents was examined in vitro through measurement of binding affinity and proton relaxivity. For imaging applications that require targeted observation, fusion to specific protein partners is desirable. However, a fusion protein comprising a concatenated double LBT (dLBT) as an N-terminal tag for the model protein ubiquitin had reduced relaxivity compared with the free dLBT peptide. This limitation was overcome by the use of a construct based on the m-sLBT sequence (q-dLBT-ubiquitin). The structural basis for the enhanced contrast was examined by comparison of the X-ray crystal structure of xq-dLBT-ubiquitin (wherein two tryptophan residues are replaced with serine), to that of dLBT-ubiquitin. The structure shows that the backbone conformational dynamics of the MRI variant may allow enhanced water exchange. This engineered LBT represents a first step in expanding the current base of specificity-targeted agents available.

Promising strategies for Gd-based responsive magnetic resonance imaging contrast agents

Magnetic resonance imaging is a powerful imaging modality that is often coupled with paramagnetic contrast agents based on gadolinium to enhance sensitivity and image quality. Responsive contrast agents are key to furthering the diagnostic potential of MRI, both to provide anatomical information and to discern biochemical activity. Recent design of responsive gadolinium-based T₁ agents has made interesting progress, with the development of novel complexes which sense their chemical environment through changes in the coordination of water molecules, the molecular tumbling time or the number of metal centres. Particular promising design strategies include the use of multimeric systems, and the development of dual imaging probes.