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Results

Among the investigated compounds 1–9, the PSMA inhibitors 5 and 6 showed the highest PSMA affinity (lowest IC50 values) after the introduction of a naphthylalanine modification. The affinity was up to three times higher compared to the reference PSMA I&T. Extended aromatic systems such as the biphenylalanine residue in 4 impaired the interaction with the lipophilic binding pocket of PSMA, resulting in a tenfold lower affinity. The IC50 of DOTAGA-conjugated 10 was slightly increased compared to the acetylated analog; however, efficient PSMA-mediated internalization and 80% plasma protein binding of 68Ga-10 resulted in effective tumor targeting and low uptake in non-target tissues of LNCaP tumor-bearing CD-1 nu/nu mice at 1 h p.i., as determined by small-animal PET imaging and biodistribution studies. For prolonged tumor retention, the plasma protein binding was increased by insertion of 4-iodo-d-phenylalanine resulting in 97% plasma protein binding and 16.1 ± 2.5% ID/g tumor uptake of 177Lu-11 at 24 h p.i.

Conclusions

Higher lipophilicity of the novel PSMA ligands 10 and 11 proved to be beneficial in terms of affinity and internalization and resulted in higher tumor uptake compared to the parent compound. Additional combination with para-iodo-phenylalanine in the spacer of ligand 11 elevated the plasma protein binding and enabled sustained tumor accumulation over 24 h, increasing the tumor uptake almost fourfold compared to 177Lu-PSMA I&T. However, high renal uptake remains a drawback and further studies are necessary to elucidate the responsible mechanism behind it.

Background

In the literature, para-halogenated aromatic systems conjugated to the Nɛ-amino group of KuE seem favorable in terms of PSMA affinity [] and PSMA inhibitors containing a para-iodo-benzoic acid derivative as albumin binder demonstrated an increased tumor uptake in vivo [, ]. In addition, para-halogenated aromatic systems are reported to increase plasma protein binding [–], and a reduced tracer uptake in the kidneys was reported for radiopharmaceuticals modified with an albumin-binding entity []. Therefore, we designed several derivatives of the known PSMA inhibitor PSMA I&T in terms of structural modification of the spacer unit with lipophilic moieties, and additionally, we introduced a para-halogenated aromatic residue, para-iodo-phenylalanine (I-f), into the linker between the binding motif and the spacer unit. Subsequently, all PSMA ligands were evaluated regarding PSMA affinity, and the most promising structure 6 was further used to develop the PSMA inhibitors 10 and 11 (Fig. ). These two ligands were radiolabeled with 68GaIII and 177LuIII and comparatively evaluated with particular respect to internalization in LNCaP cells, lipophilicity, plasma protein binding, and albumin binding. In order to assess the influence of the structural modifications in vivo, both ligands were evaluated in micro-PET imaging and biodistribution studies and compared to PSMA I&T.Fig. 2Chemical structures of the DOTAGA-conjugated PSMA inhibitors PSMA I&T, 10, and 11

Synthesis

Hydrolytic stability was especially important for the synthesis of PSMA inhibitor 11. To address plasma proteins, a PSMA inhibitor with a halogenated aromatic moiety (4-iodo-d-phenylalanine; I-f) was developed on the basis of 10. Substitution of the suberic acid spacer with glutaric acid-(I-f) resulted in DOTAGA-y-nal-k(Glut-(I-f)-KuE) (11). For the synthesis of 11, PfpO-Glut-(I-f)-(OtBu)KuE(OtBu)2 was synthesized in solution from (OtBu)KuE(OtBu)2, Fmoc-d-iodo-phenylalanine, and the bis-pentafluorophenylester of glutaric acid (Glut(OPfp)2) and was purified using RP-HPLC. Glut(OPfp)2 was prepared from glutaric acid using pyridine and pentafluorophenol in 87% yield after flash chromatography. Synthesis yields of PSMA inhibitors 1–11 are summarized in Additional file : Table S1.