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The neurotransmitter serotonin is involved in numerous bodily functions via seven different serotonin receptor subfamilies. Serotonin plays a role in gastrointestinal functions like intestinal secretion or peristalsis and neuropsychiatric events like depression or migraine. One of these subtypes has been found on glioblastoma cells, inducing growth promotion. In our study we attempted to target imaging agents to glioblastoma cells via the serotonin receptor. For this we coupled serotonin to the fluorescent dye rhodamine and the magnetic resonance imaging contrast agent gadolinium (Gd)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The cellular uptake, cytotoxicity and detection sensitivity of the conjugates were evaluated by confocal laser scanning microscopy (CLSM), cell growth analysis, flow cytometry and magnetic resonance relaxometry on U373 human glioblastoma cells. Receptor-dependency of the uptake was confirmed by competition experiments with excess of unmarked serotonin. Cellular uptake of the conjugates was found in CLSM, magnetic resonance relaxometry and flow cytometry experiments.CLSM revealed the cytoplasmic character of the uptake. In cell growth analysis experiments no adverse effect of either conjugate on the cells was observed. Competition experiments performed with the conjugates and unmarked serotonin showed decreased conjugate uptake compared to the experiments without competition. In conclusion the neurotransmitter serotonin could be successfully used to target imaging agents into human glioblastoma cells. This makes it of interest for future glioblastoma imaging methods.

The main use of glycopeptide antibiotics is treatment of infections which are resistant to the commonly used β- lactam antibiotics. Antitumor activity has also been reported for some glycopeptide antibiotics like bleomycin. In the present study we investigated the chemotherapeutic and diagnostic potential of two imaging agent derivatives of the glycopeptide antibiotic vancomycin. For the first conjugate, vancomycin was coupled to the fluorescent dye rhodamine, used in confocal laser scanning microscopy (CLSM) and fluorescence-activated cell sorting (FACS). The second conjugate consisted of vancomycin coupled to gadolinium-1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid (GdDOTA), a magnetic resonance imaging (MRI) contrast agent. The cellular uptake, specificity, and the accessibility by imaging methods of the two vancomycin conjugates was evaluated on 8 human cell lines (one benign, 7 malignant) by CLSM, FACS, and MRI experiments. Cytotoxicity of both vancomycin conjugates was determined in the FACS experiments with the annexin test indicating disrupted cell membranes. Some of the malignant cell lines showed clearly stronger uptake than the others and the benign cell line was among the cell lines with the lowest uptake. In the annexin test the cytotoxicity could be correlated to the conjugate uptake for all cell lines. The intracellular uptake of the vancomycin conjugates and the increased uptake into some of the malignant cell lines were interesting findings which should be further pursued.

Magnetic resonance imaging (MRI) is presently the method of choice for detection of brain tumors. However, MRI alone is not conclusive. As the commonly used contrast agents do not bind to the cells and are not taken up into the cells, they generally do accumulate in regions where the blood-brain-barrier is disrupted. While this can be brain tumors (WHO grade II-III and above), it can also be inflammations. A cell-directed contrast agent would be a great asset not only to avoid unnecessary brain biopsies, but also to achieve sharper tumor margins during intraoperative MRI. The gastrin/cholecystockinin receptor found in the brain and the intestinal tract is a potential target for a cell-directed contrast agent. The receptor has already been found in human glioma cell lines and autocrine stimulation has also been demonstrated for the receptor and its ligand gastrin. We coupled the correct and a mutant 17-amino-acid gastrin to gadolinium -1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid (an MRI contrast agent) and rhodamine isothiocyanate (a fluorescent dye). Using confocal laser scanning microscopy and magnetic resonance relaxometry experiments we found cytoplasmic uptake of the correct gastrin conjugate into human U373 glioma cells. Surprisingly, the mutant conjugate was also taken up into the cells in a similar pattern, albeit to a lesser degree. Both conjugates showed no cytotoxicity. These conjugates show potential for future use in magnetic resonance imaging studies of brain tumors after systemic or intraoperative local application. The cytoplasm specificity of the conjugates also makes it a potential building block for the design of future cytoplasmdirected imaging and therapeutic conjugates.

Detection of prostate carcinoma metastases is currently performed either via indirect tests like the prostate specific antigen (PSA) or prostate cancer gene 3 (PCA3) or by biopsies from masses found with medical imaging methods. Our goal was to use an ectopic odorant receptor to target prostate-derived cells throughout the body for imaging by magnetic resonance and fluorescence imaging. We synthesized a conjugate containing undecylic aldehyde (an antagonist of the human olfactory receptor hOR17-4), gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (a common magnetic resonance contrast agent) and fluorescein isothiocyanate (a fluorescent dye). Two different prostate cancer cell lines as well as five other different malignant cell lines and healthy prostate epithelial cells were incubated with this conjugate and evaluated by flow cytometry, confocal laser scanning microscopy and magnetic resonance imaging. The prostate- derived healthy and malignant cells showed stronger fluorescence than the non-prostate cancer cell lines in the flow cytometry and confocal laser scanning microscopy experiments. In the magnetic resonance imaging experiments the T1 relaxation time reduction (higher signal intensity) was also stronger for the prostate-derived cells than for the non-prostate cells. The examined conjugate showed high prostate-cell-specificity. This property makes it of potential value in the diagnosis of prostate cancer lymph node metastases.

The extracellular glycoprotein Tenascin-C (TN-C) is highly upregulated in gliomas. Therefore, many chemotherapies with radiolabeled antibodies against TN-C have been performed. However, TN-Cs binding partner Syndecan-4 did not play any role as a therapeutic or imaging target in gliomas. We constructed an imaging compound containing the magnetic resonance imaging (MRI) contrast agent gadolinium (Gd)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), the fluorescence dye sulforhodamine and a synthetic Syndecan-4-specific 21 amino acid peptide derived from TN-C. Magnetic resonance relaxometry, confocal laser scanning microscopy, and flow cytometry showed that the Syndecan-4-DOTA-Rhodamine conjugate was taken up into the cytoplasm of human U373 glioma cells without any cytotoxic effects. Competition experiments indicate that this uptake was receptor-mediated. This conjugate might be used for future MRI studies of brain tumors after systemic or intraoperative local application.

The interleukin 6 receptor (IL-6R) and its ligand interleukin 6 (IL-6) play a crucial role in glioma growth and development accomplished by autocrine growth promotion and induction of angiogenesis via activation of vascular epithelial growth factor A (VEGF-A). Therefore, IL-6R represents a target for both therapy (preventing VEGF-A activation by blocking the receptor) and imaging (higher receptor density on tumor cells). A short heptapeptide that selectively binds to IL-6R and which inhibits the effect of IL-6 was coupled to the magnetic resonance imaging (MRI) contrast agent gadolinium (Gd)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and the fluorescent dye rhodamine. MRI, confocal laser scanning microscopy, and flow cytometry showed that our IL-6-DOTA-rhodamine conjugate was taken up into the cytoplasm of human U373 glioma cells without any cytotoxic effects. Competition experiments indicate that this uptake was receptor-mediated. This conjugate might be used for future MRI studies of brain tumors after systemic or intraoperative local application. The cytoplasm specificity of the conjugate also makes it a potential building block for the design of future cytoplasm-directed imaging and therapeutic conjugates.

Gadolinium (Gd)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is commonly used as contrast agent in magnetic resonance imaging (MRI), but cannot enter the cytoplasm or cell nucleus. We designed a tetrapeptide carrying fluorescein isothiocyanate (FITC) and Gd-DOTA. This conjugate was coupled to the nuclear localisation sequence (NLS) of the Simian Virus (SV) 40 T antigen elongated by four arginines. In a second conjugate one lysine of the original SV 40 T antigen NLS was replaced by threonine. An FITC-labelled DOTA-tetrapeptide conjugate lacking the NLS peptide served as a negative control. We tried to achieve sequence specific entry of the Gd-DOTA-complex into the cytoplasm and nucleus of human U373 and LN18 glioma cells. Using confocal laser scanning microscopy (CLSM), fluorescence activated cell sorting (FACS), magnetic resonance imaging (MRI) and viability tests we found that both NLS conjugates stained the cell nuclei of U373 and LN18 glioma cells, represented also by a rise in signal intensity compared to the native control in MRI. The majority of stained cells remained viable. All conjugates were also produced without Gd. The Gd-free DOTA-conjugates showed an increase in cellular uptake rate. Conjugate cytotoxicity correlated closely to cellular uptake. Gd-containing DOTA-conjugates directed to the cytoplasm or the nucleus may be the basis for the development of novel diagnostic agents.

The objective of this article is to illustrate both the potential and the limitations of molecular imaging in stroke research. By molecular imaging we mean the visual representation of biological processes at the cellular and molecular level. The use of molecular imaging for stroke diagnosis is still at a very preliminary stage and many of these procedures have only been tested in animals. In rats, stroke therapy using stem cells can be monitored by magnetic resonance imaging (MRI), green fluorescent protein (GFP) or luciferase (LUC) imaging. The migration of macrophages, which take up intravenously administered iron-based contrast agents and then migrate to the area of infarction, can already be observed in stroke patients. With MRI, the new agent Gd-DTPA-sLexA that binds to E- and P-selectin can specifically visualize selectin-mediated early endothelial activation after transient focal ischemia "in vivo". Decreased glial fibrillary acidic protein (GFAP) gene expression can be imaged in vivo by scintigraphy 24 hours after cerebral ischemia using a peptide nucleic acid antisense conjugate labeled with 111In and that hybridizes to the rat GFAP mRNA. Technetium-99m hydrazine nicotinamide-labeled HYNIC-annexin V SPECT can not only detect sites of neuronal injury in stroke patients but also can monitor the effects of neuroprotective therapy with a monoclonal antibody raised against FasLigand (FasL) in rats. Finally, information about cell metabolism in the infarct region can be gained using certain intracellular tracers [e.g. 18F-fluoromisonidazole (FMISO)]. Imaging benzodiazepine receptors with 11C-flumazenil (FMZ) can distinguish between irreversibly damaged and viable penumbra tissue early after stroke.

Molecular imaging is defined as the characterization and measurement of biological processes at the cellular and molecular level. Molecular imaging, therefore, necessitates a sufficient amount of contrast agent within the cell. Consequently, we realized that the intracellular uptake and cell compartment specificity of the commonly used interstitial contrast agent gadolinium (Gd(3+)) with a cell-nucleus directed peptide module could be helpful. This modular molecule is characterized by a Gd(3+)-complex module that is bound to a transmembrane transport unit (TPU) of human origin and further to a nucleus-directed address module (nuclear localization sequence) resulting in a specific cell nucleus-directed nuclear localization sequence-conjugated Gd(3+)-complex (CNN-Gd(3+)-complex). By use of magnetic resonance imaging, Gd(3+) was detected within DU-145 prostate cancer cells after only 10 min. The nuclear localization was confirmed with confocal laser scanning microscopy. The resulting MRI signal enhancement only slightly decreased over the next 48 h compared with an absolute loss of signal enhancement after only 8 h when a random target sequence was used. Therefore, our method seems promising for in vivo application in molecular imaging.