Synthesis and Biological Evaluation of Low Molecular Weight Fluorescent Imaging Agents for the Prostate-Specific Membrane Antigen

Abstract

Targeted near-infrared (NIR) optical imaging can be used in vivo to detect specific tissues, including malignant cells. A series of NIR fluorescent ligands targeting the prostate-specific membrane antigen (PSMA) was synthesized and each compound was tested for its ability to image PSMA+ tissues in experimental models of prostate cancer. The agents were prepared by conjugating commercially available active esters of NIR dyes, including IRDye800CW, IRDye800RS, Cy5.5, Cy7, or a derivative of indocyanine green (ICG) to the terminal amine group of (S)-2-(3-((S)-5-amino-1-carboxypentyl)ureido)pentanedioic acid 1, (14S,18S)-1-amino-8,16-dioxo-3,6-dioxa-9,15,17-triazaicosane-14,18,20-tricarboxylic acid 2 and (3S,7S)-26-amino-5,13,20-trioxo-4,6,12,21-tetraaza-hexacosane-1,3,7,22-tetracarboxylic acid 3. The Ki values for the dye-inhibitor conjugates ranged from 1 to 700 pM. All compounds proved capable of imaging PSMA+ tumors selectively to varying degrees depending on the choice of fluorophore and linker. The highest tumor uptake was observed with IRDye800CW employing a poly(ethylene glycol) or lysine-suberate linker, as in 800CW-2 and 800CW-3, while the highest tumor to nontarget tissue ratios were obtained for Cy7 with these same linkers, as in Cy7-2 and Cy7-3. Compounds 2 and 3 provide useful scaffolds for targeting of PSMA+ tissues in vivo and should be useful for preparing NIR dye conjugates designed specifically for clinical intraoperative optical imaging devices.

Development of Targeted Near-Infrared Imaging Agents for Prostate Cancer

“…Other studies where fluorophores of intermediate polarity to IRDye800 and Cy5.5 were conjugated to PSMA-1 showed still different in vivo kinetics (data not shown), suggesting that polarity of the fluorophore might dramatically influence pharmacokinetics of the agents. The phenomenon that a particular NIR dye can affect the pharmacokinetics of a conjugated probe was also reported by Chen and colleagues (30). They observed that more hydrophobic probe has longer retention time in the body, consistent with our findings.…”

“…The first small-molecule–based PSMA targeting imaging agent was reported in 2005 by Humblet and colleagues (20). Since then many small molecular PSMA-targeting imaging agents have been developed for single-photon emission tomography (SPECT), PET, and optical imaging (21–30). Among these imaging agents, the urea-based 18 F-DCFBC (25, 31, 32), 123 I-MIP-1072 (26), and 123 I-MIP-1095 (33) have entered into clinical trials and shown the ability to detect both bone and soft-tissue metastases in patients with prostate cancer.…”

PSMA-targeted contrast agents for intraoperative imaging of prostate cancer,

“…5–7,19–22 When KUE was conjugated directly with ZW800-1, however, no membrane binding was observed on LNCaP cells in vitro and in vivo (data not shown). This result indicates that the conjugated fluorophore agitates the favorable pharmacokinetic property of targeted ligand.…”

“…1,2 Many efforts have been made to detect and treat primary and metastatic prostate cancers using PSMA-based targeting such as computed tomography, nuclear imaging, and magnetic resonance imaging. 3–7 Despite the recent advancement in nuclear medicine-based therapy, surgical resection remains a definitive treatment for prostate cancer. 1,8 Compared to PSMA-targeted nuclear medicine, optical imaging could provide real-time guidance during prostate resection with highly sensitive, rapid, and non-radioactive NIR fluorescence light, and importantly, highlight areas of extracapsular extension or in-transit metastases.…”

“…1,8 Compared to PSMA-targeted nuclear medicine, optical imaging could provide real-time guidance during prostate resection with highly sensitive, rapid, and non-radioactive NIR fluorescence light, and importantly, highlight areas of extracapsular extension or in-transit metastases. 7–9 However, surgery is performed without intraoperative image guidance because there are no clinically available optical contrast agents to help the surgeon ensure clean tumor margins and find small occult metastases in the surgical field. 9–11 …”