Synthesis and evaluation of a multifunctional probe with a high affinity for prostate-specific membrane antigen (PSMA) and bone

Development of a Radiogallium-Labeled Heterodivalent Imaging Probe Targeting Negative Charges and Integrin on the Surface of Cancer Cell Membranes

Radiopharmaceuticals targeting tumor-specific environments are powerful tools for cancer diagnosis and treatment. We previously demonstrated the considerable high tumor uptake of the cationic amphiphilic peptide, 67Ga-NOTA-KV6, in vivo. However, because this radioligand shows a relatively rapid clearance from the tumor over time, further structural optimization is necessary. In this study, to enhance tumor accumulation and retention, we synthesized and evaluated a heterobivalent radiogallium-labeled radiotracer, [67Ga]Ga-DOTA-KV6-Mal-c(RGDyK) ([67Ga]6a), fusing the KV6 peptide targeting negatively charged sites on the cancer cell membrane and cyclic RGD peptide targeting integrin αvβ3 on the cancer cell membrane. Cellular uptake study revealed high accumulation of [67Ga]6a in integrin αvβ3-expressing U-87MG cancer cells, but uptake was significantly inhibited in the presence of an excess of the cyclic RGD peptide, c(RGDyK) (1). Peptide 6a exhibited integrin αvβ3-binding affinity comparable to those of RGD peptides 1 and DOTA-Mal-c(RGDyK) (8). In vivo biodistribution studies of U-87MG tumor-bearing mice revealed that [67Ga]6a exhibited better accumulation and retention in tumor tissues than [67Ga]Ga-DOTA-KV6-Mal-Et ([67Ga]6b; without the RGD peptide motif) and [67Ga]Ga-DOTA-Mal-c(RGDyK) ([67Ga]9; without the KV6 peptide motif). Single-photon emission computed tomography analysis also revealed high signals of [67Ga]6a in tumor tissues, which were significantly blocked in the presence of excess peptide 1. Although reducing radiotracer accumulation in nontumor tissues, such as the kidneys, remains a challenge, our developed approach exhibits potential to enhance the selectivity and retention of radiopharmaceuticals in tumor tissues.

Future Prospect of Low-Molecular-Weight Prostate-Specific Membrane Antigen Radioisotopes Labeled as Theranostic Agents for Metastatic Castration-Resistant Prostate Cancer

Prostate cancer ranks as the fourth most common cancer among men, with approximately 1.47 million new cases reported annually. The emergence of prostate-specific membrane antigen (PSMA) as a critical biomarker has revolutionized the diagnosis and treatment of prostate cancer. Recent advancements in low-molecular-weight PSMA inhibitors, with their diverse chemical structures and binding properties, have opened new avenues for research and therapeutic applications in prostate cancer management. These novel agents exhibit enhanced tumor targeting and specificity due to their small size, facilitating rapid uptake and localization at the target site while minimizing the retention in non-target tissues. The primary aim of this study is to evaluate the potential of low-molecular-weight PSMA inhibitors labeled with radioisotopes as theranostic agents for prostate cancer. This includes assessing their efficacy in targeted imaging and therapy and understanding their pharmacokinetic properties and mechanisms of action. This study is a literature review focusing on in vitro and clinical research data. The in vitro studies utilize PSMA-targeted radioligands labeled with radioisotopes to assess their binding affinity, specificity, and internalization in prostate cancer cell lines. Additionally, the clinical studies evaluate the safety, effectiveness, and biodistribution of radiolabeled PSMA ligands in patients with advanced prostate cancer. The findings indicate promising outcomes regarding the safety and efficacy of PSMA-targeted radiopharmaceuticals in clinical settings. The specific accumulation of these agents in prostate tumor lesions suggests their potential for various applications, including imaging and therapy. This research underscores the promise of radiopharmaceuticals targeting PSMA in advancing the diagnosis and treatment of prostate cancer. These agents improve diagnostic accuracy and patients' outcomes by enhancing imaging capabilities and enabling personalized treatment strategies.

Differences in the Renal Accumulation of Radiogallium-Labeled (Glu)14 Peptides Containing Different Optical Isomers of Glutamic Acid

Acidic amino acid peptides have a high affinity for bone. Previously, we demonstrated that radiogallium complex-conjugated oligo-acidic amino acids possess promising properties as bone-seeking radiopharmaceuticals. Here, to elucidate the effect of stereoisomers of Glu in Glu-containing peptides [(Glu)14] on their accumulation in the kidney, the biodistributions of [67Ga]Ga-N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid-conjugated (l-Glu)14 ([67Ga]Ga-HBED-CC-(l-Glu)14), [67Ga]Ga-HBED-CC-(d-Glu)14, [67Ga]Ga-HBED-CC-(dl-Glu)14, and [67Ga]Ga-HBED-CC-(d-Glu-l-Glu)7 were compared. Although the accumulation of these compounds in the bone was comparable, their kidney accumulation and retention were strikingly different, with [67Ga]Ga-HBED-CC-(d-Glu-l-Glu)7 exhibiting the lowest level of kidney accumulation among these compounds. Repeated d- and l-peptides may be a useful method for reducing renal accumulation in some cases.

Synthesis and Evaluation of Radiogallium Labeled Bone-Imaging Probes Using Oligo-γ-Carboxy Glutamic Acid Peptides as Carriers to Bone

We investigated the importance of the carboxy group density in bone affinity during the development of peptide-based bone-seeking radiopharmaceuticals and carriers. Oligo-γ-carboxy glutamic acid peptides [(Gla)n] with higher carboxy group density than oligo-glutamic acid peptides [(Glu)n] and oligo-aspartic acid peptides [(Asp)n] were chosen. Using the radiogallium chelator N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC), we synthesized [67Ga]Ga-HBED-CC-(Gla)n (n = 1, 2, 5, 8, 11, or 14) with high yields. Hydroxyapatite-binding assays, biodistribution, and SPECT imaging showed higher affinity and bone accumulation for [67Ga]Ga-HBED-CC-(Gla)n compared to [67Ga]Ga-HBED-CC-(Glu)n. Notably, [67Ga]Ga-HBED-CC-(Gla)8 and [67Ga]Ga-HBED-CC-(Gla)11 exhibited superior bone accumulation and rapid blood clearance. SPECT/CT imaging with [67Ga]Ga-HBED-CC-(Gla)8 exclusively visualized the bone tissue. These findings support the potential use of [67Ga]Ga-HBED-CC-(Gla)n as excellent bone-imaging PET probes, suggesting (Gla)n peptides are superior bone-seeking carriers.