How Different Albumin-Binders Drive Probe Distribution of Fluorescent RGD Mimetics

Abstract

The biodistribution of medical imaging probes depends on the chemical nature of the probe and the preferred metabolization and excretion routes. Especially targeted probes, which have to reach a certain (sub)cellular destination, have to be guided to the tissue of interest. Therefore, small molecular probes need to exhibit a well-balanced polarity and lipophilicity to maintain an advantageous bioavailability. Labelled antibodies circulate for several days due to their size. To alter the biodistribution behavior of probes, different strategies have been pursued, including utilizing serum albumin as an inherent transport mechanism for small molecules. We describe here the modification of an existing fluorescent RGD mimetic probe targeted to integrin αvβ3 with three different albumin binding moieties (ABMs): a diphenylcyclohexyl (DPCH) group, a p-iodophenyl butyric acid (IPBA) and a fatty acid (FA) group with the purpose to identify an optimal ABM for molecular imaging applications. All three modifications result in transient albumin binding and a preservation of the target binding capability. Spectrophotometric measurements applying variable amounts of bovine serum albumin (BSA) reveal considerable differences between the compounds concerning their absorption and emission characteristics and hence their BSA binding mode. In vivo the modified probes were investigated in a murine U87MG glioblastoma xenograft model over the course of 1 wk by fluorescence reflectance imaging (FRI) and fluorescence mediated tomography (FMT). While the unmodified probe was excreted rapidly, the albumin-binding probes were accumulating in tumor tissue for at least 5 days. Considerable differences between the three probes in biodistribution and excretion characteristics were proved, with the DPCH-modified probe showing the highest overall signal intensities, while the FA-modified probe exhibits a low but more specific fluorescent signal. In conclusion, the modification of small molecular RGD mimetics with ABMs can precisely fine-tune probe distribution and offers potential for future clinical applications.

“Clickable” Albumin Binders for Modulating the Tumor Uptake of Targeted Radiopharmaceuticals

“…Probably the most intriguing examples come from drugs for the treatment of diabetes. − Furthermore, the concept of intentional binding to albumin has also entered the design of chemotherapeutic drugs − and more recently targeted radiopharmaceuticals for the treatment of cancer. , Regarding the latter application, binding to albumin aims at increasing the time-integral uptake of radioactivity in (and thus the total radiation dose delivered to) the tumor and at decreasing the uptake in normal organs such as kidneys. This increase in tumor uptake is often attributed to the prolonged blood circulation time of the albumin-bound radioligand and a presumed higher affinity of the radioligand to the actual target protein than to albumin. ,− However, the explanation seems to be misleading considering the “free drug hypothesis” mentioned above with the underlying interplay between the concentration of unbound radioligand in the blood and clearance rate. Accordingly, the time profile of total radioligand is altered in the blood by albumin binding, but the area under the curve (AUC) for the free fraction of radioligand (not bound to albumin) is unaffected.…”

Detection of Early Endothelial Dysfunction by Optoacoustic Tomography

“…A small molecular RGD-mimetic compound previously developed was labeled with the NHS ester of IRDye800cw (LI-COR Biosciences, Bad Homburg, Germany) to meet the requirements of the MSOT system [19,40]. The compound was purified with HPLC (Shimadzu Prominence gradient RP-HPLC system with diode array detection, Shimadzu Deutschland GmbH, Duisburg, Germany), acetonitrile, and purified water containing 0.1% trifluoro acetic acid as mobile phases, and Nucleosil ® 100-5 C18 RP columns.…”