Rational linker design to accelerate excretion and reduce background uptake of peptidomimetic PSMA-targeting hybrid molecules

Abstract

The evolution of peptidomimetic hybrid molecules for preoperative imaging and guided surgery targeting the prostate-specific membrane antigen (PSMA) significantly progressed over the past years and some approaches are currently evaluated for further clinical translation. However, accumulation in non-malignant tissue such as kidney, bladder, spleen or liver might limit tumor-to-background contrast for precise lesion delineation particularly in a surgical setting. To overcome these limitations a rational linker design aims at the development of a second generation of PSMA-11 based hybrid molecules with enhanced pharmacokinetic profile and improved imaging contrasts. Methods: A selection of rational designed linkers was introduced to the PSMA-targeting hybrid molecule Glu-urea-Lys-HBED-CC-IRDye800CW resulting in a second generation peptidomimetic hybrid molecule library. The biological properties were investigated in cell-based assays. In a preclinical proof-of-concept study with the radionuclide 68Ga, the impact of the modifications was evaluated by determination of specific tumor uptake, pharmacokinetics and fluorescence-imaging in tumor-bearing mice. Results: The modified hybrid molecules carrying various selected linkers revealed high PSMA-specific binding affinity and effective internalization. The highest tumor-to-background contrast of all modifications investigated was identified for the introduction of a histidine (H) and glutamic acid (E) containing linker ((HE)3-linker) between PSMA-binding motif and chelator. In comparison to the parental core structure, uptake in non-malignant tissue was significantly reduced to a minimum exemplified by an 11-fold reduced spleen uptake from 38.12±14.62 %ID/g to 3.47±1.39 %ID/g (1 h p.i.). The specific tumor uptake of this compound (7.59±0.95 %ID/g, 1 h p.i.) was detected to be significantly higher as compared to the parental tracer PSMA-11. These findings confirmed by PET and fluorescence imaging are accompanied by an enhanced pharmacokinetic profile with accelerated background clearance at early time points post injection. Conclusion: The novel generation of PSMA-targeting hybrid molecules reveal fast elimination, reduced background organ enrichment and high PSMA-specific tumor uptake meeting the key demands for potent tracers in Nuclear Medicine and fluorescence-guided surgery. The approach's efficacy of improving the pharmacokinetic profile highlights the strengths of rational linker design as a powerful tool in strategic hybrid molecule development.