A Bombesin-Shepherdin Radioconjugate Designed for Combined Extra- and Intracellular Targeting

Abstract

Radiolabeled peptides which target tumor-specific membrane structures of cancer cells represent a promising class of targeted radiopharmaceuticals for the diagnosis and therapy of cancer. A potential drawback of a number of reported radiopeptides is the rapid washout of a substantial fraction of the initially delivered radioactivity from cancer cells and tumors. This renders the initial targeting effort in part futile and results in a lower imaging quality and efficacy of the radiotracer than achievable. We are investigating the combination of internalizing radiopeptides with molecular entities specific for an intracellular target. By enabling intracellular interactions of the radioconjugate, we aim at reducing/decelerating the externalization of radioactivity from cancer cells. Using the “click-to-chelate” approach, the 99mTc-tricarbonyl core as a reporter probe for single-photon emission computed tomography (SPECT) was combined with the binding sequence of bombesin for extracellular targeting of the gastrin-releasing peptide receptor (GRP-r) and peptidic inhibitors of the cytosolic heat shock 90 protein (Hsp90) for intracellular targeting. Receptor-specific uptake of the multifunctional radioconjugate could be confirmed, however, the cellular washout of radioactivity was not improved. We assume that either endosomal trapping or lysosomal degradation of the radioconjugate is accountable for these observations.

Sorbitol as a Polar Pharmacological Modifier to Enhance the Hydrophilicity of 99mTc-Tricarbonyl-Based Radiopharmaceuticals

“…Because this study aims at the investigation of differences in the pharmacokinetics of the radiopeptides (e.g., rate and route of excretion) rather than in tumor uptake, we continued with the evaluation of both compounds in vivo. The receptor affinity (K d ) and maximum receptor occupancy (B max ) were determined by receptor saturation assays using increasing amounts of the radiolabeled peptides ( Figure 3 [18,23,27], yet in this case somewhat surprising as we did not expect the sorbitol moiety to have an influence on these parameters; this in particular because the results of cell binding and internalization experiments were similar for both radiolabeled peptides ( Figure 2). Differences in specific molar activities (MBq/mol) could account for this observation and cannot be entirely excluded even though both 99m Tc-radiopeptides were prepared under identical conditions.…”

“…The receptor affinity (Kd) and maximum receptor occupancy (Bmax) were determined by receptor saturation assays using increasing amounts of the radiolabeled peptides ( Figure 3 (13)] (Kd = 3.7 ± 1.0 nM, Bmax = 0.37 ± 0.02 nM; Table 2) [23]. Some variability in experimentally determined Kd and Bmax of structurally related radiolabeled peptides is not uncommon [18,23,27], yet in this case somewhat surprising as we did not expect the sorbitol moiety to have an influence on these parameters; this in particular because the results of cell binding and internalization experiments were similar for both radiolabeled peptides ( Figure 2). Differences in specific molar activities (MBq/mol) could account for this observation and cannot be entirely excluded even though both 99m Tc-radiopeptides were prepared under identical conditions.…”

A Multifunctional Radiotheranostic Agent for Dual Targeting of Breast Cancer Cells

“…Radioconjugates consisting of two different molecular moieties in the same agent have been designed in order to combine different imaging modalities such as single photon emission computed tomography (SPECT) and near‐infrared (NIR) optical imaging or to associate the therapeutic effect of cytotoxic drugs with imaging ability . Diverse multifunctional radioconjugates have also been prepared to enhance cellular retention and internalization and to potentiate their uptake in specific subcellular localizations such as the nucleus and the mitochondria . The enhancement of nuclear uptake has been particularly important in the design of radioconjugates intended for auger electron therapy (AET).…”

Pharmaceuticals—Special Issue on Radiopharmaceutical Chemistry between Imaging and Endoradiotherapy

“…Unfortunately, while the specificity of the radioconjugates towards the gastrin-releasing peptide receptor (GRPr) could be confirmed, the cellular externalisation of the radioactivity was not improved (Scheme 2) [6]. …”

“…An example of how a radiotracer could be entrapped after binding to the chosen receptor is given by Fischer CA et al with the contribution A Bombesin-Shepherdin Radioconjugate Designed for Combined Extra- and Intracellular Targeting . Unfortunately, while the specificity of the radioconjugates towards the gastrin-releasing peptide receptor (GRPr) could be confirmed, the cellular externalisation of the radioactivity was not improved ( Scheme 2 ) [ 6 ].…”