Design of PSMA ligands with modifications at the inhibitor part: an approach to reduce the salivary gland uptake of radiolabeled PSMA inhibitors?

AimTo investigate whether modifications of prostate-specific membrane antigen (PSMA)-targeted radiolabeled urea-based inhibitors could reduce salivary gland uptake and thus improve tumor-to-salivary gland ratios, several analogs of a high affinity PSMA ligand were synthesized and evaluated in in vitro and in vivo studies.MethodsBinding motifs were synthesized 'on-resin' or, when not practicable, in solution. Peptide chain elongations were performed according to optimized standard protocols via solid-phase peptide synthesis. In vitro experiments were performed using PSMA(+) LNCaP cells. In vivo studies as well as mu SPECT/CT scans were conducted with male LNCaP tumor xenograft-bearing CB17-SCID mice.ResultsPSMA ligands with A) modifications within the central Zn2+-binding unit, B) proinhibitor motifs and C) substituents & bioisosteres of the P1-gamma-carboxylic acid were synthesized and evaluated. Modifications within the central Zn2+-binding unit of PSMA-10 (Glu-urea-Glu) provided three compounds. Thereof, only Lu-nat-carbamate I (Lu-nat-3) exhibited high affinity (IC50=7.1 +/- 0.7nM), but low tumor uptake (5.31 +/- 0.94% ID/g, 1h p.i. and 1.20 +/- 0.55% ID/g, 24h p.i.). All proinhibitor motif-based ligands (three in total) exhibited low binding affinities (>1 mu M), no notable internalization and very low tumor uptake (<0.50% ID/g). In addition, four compounds with P1 '-gamma-carboxylate substituents were developed and evaluated. Thereof, only tetrazole derivative Lu-nat-11 revealed high affinity (IC50=16.4 +/- 3.8nM), but also this inhibitor showed low tumor uptake (3.40 +/- 0.63% ID/g, 1h p.i. and 0.68 +/- 0.16% ID/g, 24h p.i.). Salivary gland uptake in mice remained at an equally low level for all compounds (between 0.02 +/- 0.00% ID/g and 0.09 +/- 0.03% ID/g), wherefore apparent tumor-to-submandibular gland and tumor-to-parotid gland ratios for the modified peptides were distinctly lower (factor 8-45) than for [Lu-177]Lu-PSMA-10 at 24h p.i.Conclusions The investigated compounds could not compete with the in vivo characteristics of the EuE-based PSMA inhibitor [Lu-177]Lu-PSMA-10. Although two derivatives (3 and 11) were found to exhibit high affinities towards LNCaP cells, tumor uptake at 24h p.i. was considerably low, while uptake in salivary glands remained unaffected. Optimization of the established animal model should be envisaged to enable a clear identification of PSMA-targeting radioligands with improved tumor-to-salivary gland ratios in future studies.