Optimizing PSMA-617-based inhibitors through charged linker modifications: Insights into structure-activity...

Area of Science:

• Radiopharmaceutical Chemistry
• Nuclear Medicine
• Prostate Cancer Research

Background:

• Pluvicto® ([177Lu]Lu-vipivotide tetraxetan) is a key radioligand therapy for PSMA-positive metastatic castration-resistant prostate cancer (mCRPC).
• Dose escalation of [177Lu]Lu-PSMA-617 and alpha-emitters improves efficacy but is limited by toxicity in organs like kidneys and salivary glands.
• Modifications to the linker region of PSMA inhibitors can significantly influence their pharmacokinetic profiles.

Purpose of the Study:

• To synthesize and evaluate novel PSMA-617 variants with histidine and/or glutamic acid modifications in the linker region.
• To investigate the effects of these modifications on pharmacokinetic properties, binding affinity, and cellular uptake.
• To assess the in vivo behavior of these novel compounds using small-animal PET imaging.

Main Methods:

• Eleven novel PSMA-targeting inhibitors were synthesized based on the PSMA-617 structure, incorporating histidine and/or glutamic acid moieties.
• Compounds were radiolabeled with [68Ga]Ga3+ and [177Lu]Lu3+ for stability and property assessments.
• In vitro evaluation included competitive cell binding and internalization assays using LNCaP cells.
• In vivo studies involved dynamic and static small-animal PET imaging with 68Ga-labeled inhibitors in LNCaP xenografts.

Main Results:

• Radiolabeled compounds exhibited high radiochemical purities (>94%) and reduced lipophilicity compared to the reference.
• Modifications significantly affected PSMA affinity (IC50: 10.40 ± 2.94 nM to 78.6 ± 44.1 nM).
• Variants with glutamic acid or histidine-glutamic acid showed enhanced binding, internalization, tumor accumulation (SUV1h: 0.21 ± 0.05 to 1.32 ± 0.08 g/ml), and rapid clearance from kidneys and background tissue within 60 minutes.

Conclusions:

• Histidine and/or glutamic acid modifications in the PSMA-617 linker alter pharmacokinetic properties in vitro and in vivo.
• Some variants demonstrated improved tumor-to-kidney ratios and favorable early excretion profiles.
• Predicting clinical off-targeting effects, such as salivary gland uptake, remains a challenge, necessitating further mechanistic and translational studies to evaluate therapeutic potential.