Selected Imaging Applications In Ddd 731 D1 Target Validation

We will illustrate the role of imaging for target validation with several examples from oncology. Peptide receptors are frequently over-expressed in tumors and therefore constitute attractive targets both for therapeutic interventions and for diagnostic imaging. Extensively studied examples are the membrane-bound somatostatin receptors (SSTRs) that are highly expressed in neuroendocrine tumors. A number of imaging probes based on octreotide, a metabolically stabilized analogue of the endogenous ligand somatostatin (SST-14 or SST28), have been developed, using either radionuclides or fluorescent groups as reporters. Feasibility of visualizing SSTR expression has been demonstrated both in patients suffering from neuroendocrine tumors and in animal models of the human disease. The imaging probes specifically accumulated at the tumor site(s). One of the ligands (111In-pentetreotide, Octreoscan®) has been approved for clinical use as diagnostic SPECT probe. There are several other radiopeptides that are currently being evaluated as tumor-specific imaging agents such as bombesin or cholecystokinin, for example, for detection of prostate cancer. Alternatively fluorescent probes have been developed for experimental studies in animals (Figure 7.4).

Overexpression of the Her-2/neu tyrosine kinase receptor is observed in approximately 25% of human breast cancers and is associated with poor prognosis. Inhibition of Her-2/neu signaling, for example, using a specific antibody (trastumazab) therefore constitutes a therapeutic strategy in these cancer patients. Correspondingly, an imaging probe suited for demonstrating Her-2/neu overexpression would be essential for the selection of patients amenable to therapy. A PET imaging probe based on Her-2 antibody fragments has been used in murine tumor models to study the effects of inhibition of heat shock protein 90, a molecular chaperon, on Her-2/neu levels. A significant reduction of Her-2/neu levels could be observed noninvasively within hours after therapy onset, which later translated into reduced tumor growth.

A third example relates to tumor angiogenesis, a critical step in the formation of a neoplastic lesion. A high degree of neovascularization is commonly associated with rapid tumor proliferation and thus malignancy. Inhibition of angiogenesis is considered an attractive strategy in tumor therapy, in particular in combination with other therapeutic strategies. Critical proangiogenic factors induced are vascular endothelial growth factor (VEGF) and its receptor (VEGF-R). When selecting patients for treatment with VEGF-R inhibitors, the demonstration of high levels of the receptor in tumor tissue would be relevant; this has prompted the development of target-specific assays suitable for in vivo imaging. As VEGF-R is expressed at the endoluminal side of the endothelial cell layer it

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FIGURE 7.4 Structure of octreotide and related imaging ligands for measuring the distribution of SSTRs. Octreotide is a somatostatin analogue that is stabilized against degradation by peptidases through cyclization (disulfide bridge). Structure-activity relationship revealed that the amino acids phenylalanine-tryptophane-lysine-threonine (Phe-Trp-Lys-Thr) are essential for the interaction with the somatostatin receptor 2 (SSTR2). This pharmacophor (shaded area) must be retained when designing a targeted imaging ligand. The SPECT ligand pentetreotide comprises a chelated indium-111 reporter group (g-photon emitter radionuclide) while for the optical probe the fluorescent indocyanine dye Cy5.5 has been used. Both reporter groups are linked to the terminal D-phenylalanine, distant from the pharmacophor group.

FIGURE 7.4 Structure of octreotide and related imaging ligands for measuring the distribution of SSTRs. Octreotide is a somatostatin analogue that is stabilized against degradation by peptidases through cyclization (disulfide bridge). Structure-activity relationship revealed that the amino acids phenylalanine-tryptophane-lysine-threonine (Phe-Trp-Lys-Thr) are essential for the interaction with the somatostatin receptor 2 (SSTR2). This pharmacophor (shaded area) must be retained when designing a targeted imaging ligand. The SPECT ligand pentetreotide comprises a chelated indium-111 reporter group (g-photon emitter radionuclide) while for the optical probe the fluorescent indocyanine dye Cy5.5 has been used. Both reporter groups are linked to the terminal D-phenylalanine, distant from the pharmacophor group.

can easily be reached also by macromolecular probes. VEGF-R expression has been demonstrated using radiolabeled probes that are based on the endogenous ligand VEGF or on VEGF-R targeting antibodies.

The demonstration of altered expression levels of a potential drug target in a pathological condition is an important, yet not sufficient, step in the target-validation process.

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