Divergent oriented synthesis for the design of reagents for protein conjugation

uPAR-targeted multimodal tracer for pre- and intraoperative imaging in cancer surgery

Pre- and intraoperative diagnostic techniques facilitating tumor staging are of paramount importance in colorectal cancer surgery. The urokinase receptor (uPAR) plays an important role in the development of cancer, tumor invasion, angiogenesis, and metastasis and over-expression is found in the majority of carcinomas. This study aims to develop the first clinically relevant anti-uPAR antibody-based imaging agent that combines nuclear (¹¹¹In) and real-time near-infrared (NIR) fluorescent imaging (ZW800-1). Conjugation and binding capacities were investigated and validated in vitro using spectrophotometry and cell-based assays. In vivo, three human colorectal xenograft models were used including an orthotopic peritoneal carcinomatosis model to image small tumors. Nuclear and NIR fluorescent signals showed clear tumor delineation between 24h and 72h post-injection, with highest tumor-to-background ratios of 5.0 ± 1.3 at 72h using fluorescence and 4.2 ± 0.1 at 24h with radioactivity. 1-2 mm sized tumors could be clearly recognized by their fluorescent rim. This study showed the feasibility of an uPAR-recognizing multimodal agent to visualize tumors during image-guided resections using NIR fluorescence, whereas its nuclear component assisted in the pre-operative non-invasive recognition of tumors using SPECT imaging. This strategy can assist in surgical planning and subsequent precision surgery to reduce the number of incomplete resections.

High efficiency diffusion molecular retention tumor targeting

Here we introduce diffusion molecular retention (DMR) tumor targeting, a technique that employs PEG-fluorochrome shielded probes that, after a peritumoral (PT) injection, undergo slow vascular uptake and extensive interstitial diffusion, with tumor retention only through integrin molecular recognition. To demonstrate DMR, RGD (integrin binding) and RAD (control) probes were synthesized bearing DOTA (for (111) In(3+)), a NIR fluorochrome, and 5 kDa PEG that endows probes with a protein-like volume of 25 kDa and decreases non-specific interactions. With a GFP-BT-20 breast carcinoma model, tumor targeting by the DMR or i.v. methods was assessed by surface fluorescence, biodistribution of [(111)In] RGD and [(111)In] RAD probes, and whole animal SPECT. After a PT injection, both probes rapidly diffused through the normal and tumor interstitium, with retention of the RGD probe due to integrin interactions. With PT injection and the [(111)In] RGD probe, SPECT indicated a highly tumor specific uptake at 24 h post injection, with 352%ID/g tumor obtained by DMR (vs 4.14%ID/g by i.v.). The high efficiency molecular targeting of DMR employed low probe doses (e.g. 25 ng as RGD peptide), which minimizes toxicity risks and facilitates clinical translation. DMR applications include the delivery of fluorochromes for intraoperative tumor margin delineation, the delivery of radioisotopes (e.g. toxic, short range alpha emitters) for radiotherapy, or the delivery of photosensitizers to tumors accessible to light.

The PEG-fluorochrome shielding approach for targeted probe design

We provide a new approach for fluorescent probe design termed "PEG-fluorochrome shielding", where PEGylation enhances quantum yields while blocking troublesome interactions between fluorochromes and biomolecules. To demonstrate PEG-fluorochrome shielding, fluorochrome-bearing peptide probes were synthesized, three without PEG and three with a 5 kDa PEG functional group. In vitro, PEG blocked the interactions of fluorochrome-labeled peptide probes with each other (absorption spectra, self-quenching) and reduced nonspecific interactions with cells (by FACS). In vivo, PEG blocked interactions with biomolecules that lead to probe retention (by surface fluorescence). Integrin targeting in vivo was obtained as the differential uptake of an (111)In-labeled, fluorochrome-shielded, integrin-binding RGD probe and a control RAD. Using PEG to block fluorochrome-mediated interactions, rather than synthesizing de novo fluorochromes, can yield new approaches for the design of actively or passively targeted near-infrared fluorescent probes.

Imaging agents for the chemokine receptor 4 (CXCR4)

The interaction between the chemokine receptor 4 (CXCR4) and stromal cell-derived factor-1 (SDF-1, also known as CXCL12) is a natural regulatory process in the human body. However, CXCR4 over-expression is also found in diseases such as cancer, where it plays a role in, among others, the metastatic spread. For this reason it is an interesting biomarker for the field of diagnostic oncology, and therefore, it is gaining increasing interest for applications in molecular imaging. Especially "small-molecule" imaging agents based on T140, FC131 and AMD3100 have been extensively studied. SDF-1, antibodies, pepducins and bioluminescence have also been used to visualize CXCR4. In this critical review reported CXCR4 targeting imaging agents are described based on their affinity, specificity and biodistribution. The level wherein CXCR4 is up-regulated in cancer patients and its relation to the different cell lines and animal models used to evaluate the efficacy of the imaging agents is also discussed (221 references).

Dual-labeling strategies for nuclear and fluorescence molecular imaging: a review and analysis

Molecular imaging is used for the detection of biochemical processes through the development of target-specific contrast agents. Separately, modalities such as nuclear and near-infrared fluorescence (NIRF) imaging have been shown to non-invasively monitor disease. More recently, merging of these modalities has shown promise owing to their comparable detection sensitivity and benefited from the development of dual-labeled imaging agents. Dual-labeled agents hold promise for whole-body and intraoperative imaging and could bridge the gap between surgical planning and image-guided resection with a single, molecularly targeted agent. In this review, we summarized the literature for dual-labeled antibodies and peptides that have been developed and have highlighted key considerations for incorporating NIRF dyes into nuclear labeling strategies. We also summarized our findings on several commercially available NIRF dyes and offer perspectives for developing a toolkit to select the optimal NIRF dye and radiometal combination for multimodality imaging.

Multimodal interventional molecular imaging of tumor margins and distant metastases by targeting αvβ3 integrin

α(v)β(3) integrin is involved in (tumor-induced) angiogenesis and is a promising candidate for the specific visualization of both primary tumors and of their distant metastases. Combination of radioactive and fluorescent imaging labels in a single multimodal, or rather hybrid, RGD-based imaging agent enables integration of pre-, intra-, and postoperative angiogenesis imaging. A hybrid imaging agent targeting the α(v)β(3) integrin--(111)In-MSAP-RGD (MSAP = multifunctional single-attachment-point reagent), which contains a targeting moiety, a pentetic acid (DTPA) chelate, and a cyanine dye--was evaluated for its potential value in combined lesion detection and interventional molecular imaging in a 4T1 mouse breast cancer model. SPECT/CT and fluorescence imaging were used to visualize the tumor in vivo. Tracer distribution was evaluated ex vivo down to the microscopic level. The properties of (111)In-MSAP-RGD were compared with those of (111)In-DTPA-RGD. Biodistribution studies revealed a prolonged retention and increased tumor accumulation of (111)In-MSAP-RGD relative to (111)In-DTPA-RGD. With (111)In-MSAP-RGD, identical features could be visualized preoperatively (SPECT/CT) and intraoperatively (fluorescence imaging). As well as the primary tumor, (111)In-MSAP-RGD also enabled detection and accurate excision of distant metastases in the head and neck region of the mice. Therefore, the hybrid RGD derivative (111)In-MSAP-RGD shows potential in preoperative planning and fluorescence-based surgical intervention.

Single reporter for targeted multimodal in vivo imaging

We have developed a multifaceted, highly specific reporter for multimodal in vivo imaging and applied it for detection of brain tumors. A metabolically biotinylated, membrane-bound form of Gaussia luciferase was synthesized, termed mbGluc-biotin. We engineered glioma cells to express this reporter and showed that brain tumor formation can be temporally imaged by bioluminescence following systemic administration of coelenterazine. Brain tumors expressing this reporter had high sensitivity for detection by magnetic resonance and fluorescence tomographic imaging upon injection of streptavidin conjugated to magnetic nanoparticles or fluorophore, respectively. Moreover, single photon emission computed tomography showed enhanced imaging of these tumors upon injection with streptavidin complexed to (111)In-DTPA-biotin. This work shows for the first time a single small reporter (∼40 kDa) which can be monitored with most available molecular imaging modalities and can be extended for single cell imaging using intravital microscopy, allowing real-time tracking of any cell expressing it in vivo.

Hybrid peptide dendrimers for imaging of chemokine receptor 4 (CXCR4) expression

The chemokine receptor 4 (CXCR4), which is overexpressed in many types of cancer, is an emerging target in the field of molecular imaging and therapeutics. The CXCR4 binding of several peptides, including the cyclic Ac-TZ14011, has already been validated. In this study mono-, di- and tetrameric Ac-TZ14011-containing dendrimers were prepared and functionalized with a multimodal (hybrid) label, consisting of a Cy5.5-like fluorophore and a DTPA chelate. Confocal microscopy revealed that all three dendrimers were membrane bound at 4 °C, consistent with CXCR4 binding in vitro. The unlabeled dimer and tetramer had a somewhat lower affinity for CXCR4 than the unlabeled monomer. However, when labeled with the multimodal label the CXCR4 affinity of the dimer and tetramer was considerably higher compared to that of the labeled monomer. On top of that, biodistribution studies revealed that the additional peptides in the dimer and tetramer reduced nonspecific muscle uptake. Thus, multimerization of the cyclic Ac-TZ14011 peptide reduces the negative influence of the multimodal label on the receptor affinity and the biodistribution.

Synthesis and evaluation of a bimodal CXCR4 antagonistic peptide

The antagonistic Ac-TZ14011 peptide, which binds to the chemokine receptor 4, has been labeled with a multifunctional single attachment point reagent that contains a DTPA chelate and a fluorescent dye with Cy5.5 spectral properties. Flow cytometry and confocal microscopy showed that the bimodal labeled peptide gave a specific receptor binding that is similar to monofunctionalized peptide derivatives. Therefore, the newly developed bimodal peptide derivative can be used in multimodal imaging applications.

Multimodal tumor-targeting peptides functionalized with both a radio- and a fluorescent label

The use of monolabeled tumor-targeting peptides for molecular imaging is widespread. However, it is often desirable to use the same compound for different clinical applications, e.g., combined pre- and intraoperative tumor detection. On the basis of their detection sensitivity, the combination of radioactivity and fluorescence is probably the most valuable in multimodal molecular imaging. In this review, we compare multimodal peptide derivatives and discuss the influence of the diagnostic labels on receptor affinity and biodistribution. On the basis of the described constructs, we propose improvements for the design of future multimodal tumor-targeting peptide derivatives.

An intein-mediated site-specific click conjugation strategy for improved tumor targeting of nanoparticle systems

The ability to modify and directly target nanoparticulate carriers has greatly increased their applicability in diagnostic and therapeutic studies. Generally essential to the targeting of nanoparticles is the bioconjugation of targeting ligands to the agent's surface. While bioconjugation techniques have steadily improved in recent years, the field is still plagued with inefficient conjugations reactions and/or the lack of site-specific coupling. To overcome these limitations, click chemistry and expressed protein ligation (EPL) are combined to produce a highly efficient, site-specific reaction. This new EPL-click conjugation strategy is applied to create superparamagnetic iron oxide nanoparticles (SPIO) labeled with HER2/neu affibodies. These HER2-SPIO nanoparticles prove to be highly potent and receptor-specific in both in vitro cell studies and murine tumor models. Moreover, when EPL-click-derived HER2-SPIO are compared with SPIO that had been labeled with HER2 affibodies using other popular bioconjugation methods, they produce a statistically significant improvement in contrast enhancement upon cell binding. The EPL-click system is also successfully extended to other nanoparticle platforms (i.e., liposomes and dendrimers) highlighting the versatility of the approach.