Spacer length effects on in vitro imaging and surface accessibility of fluorescent inhibitors of prostate specific membrane antigen

Abstract

Prostate-specific membrane antigen (PSMA), a type II transmembrane protein, has been becoming an active target for imaging and therapeutic applications for prostate cancer. Recently, the development of its various chemical inhibitor scaffolds has been explored to serve as carriers for therapeutic or diagnostic payloads targeted to PSMA-positive tumor cells. However, there have been few efforts to definitively determine the optimal length of linker between PSMA inhibitor cores and their payload molecules with regard to the affinity to PSMA and in vitro performance. In our present model study, three spacer-length varied fluorescent inhibitors (FAM-CTT-54, FAM-X-CTT-54 and FAM-PEG8-CTT-54) were synthesized, and further enzymatic inhibition studies displayed linker length-dependent changes in: inhibitory potency (IC50 = 0.41 nM, 0.35 nM, 1.93 nM), modes of binding (reversible, slowly reversible, irreversible), respectively. Furthermore, cell-labeling imaging revealed the spacer length-related change of fluorescence intensity (FAM-X-CTT-54 > FAM-PEG8-CTT-54 > FAM-CTT-54). These results suggest that selection of linkers and their lengths will be important considerations in the development of next-generation prostate tumor-targeted imaging probes and therapeutic agents that specifically home to PSMA on tumor cells.

Targeting prostate cancer cells with a multivalent PSMA inhibitor-guided streptavidin conjugate

“…23, 34, 35 We have observed that a suitable-length spacer installed between the PSMA inhibitor core and its diagnostic or therapeutic payload was necessary for ensuring both inhibitory potency against PSMA and positive in vitro performance. 36 Our previous work also confirmed that small-molecule inhibitor can induce internalization of PSMA-inhibitor complex 9 albeit slower than that reported for antibody-induced rapid internalization. 8 In the present study, we examined the feasibility of a macromolecular tumor-targeting scaffold for prostate cancer by employing a biotin-streptavidin coupling system as a model.…”

PSMA-targeted contrast agents for intraoperative imaging of prostate cancer,

“…PEG spacers can provide sufficient flexibility for a targeting ligand to overcome spatial limitations in order to effectively interact with a corresponding target protein or receptor. 28,29 As shown in Fig. 3b, in vitro PSMA targeting efficiency revealed that PEG2 (10 atoms, ≈11 Å) and PEG4 (16 atoms, ≈18 Å) provided an optimum length for keeping the potent PSMA specificity of KUE.…”

PSMA PET and Radionuclide Therapy in Prostate Cancer

“…DOTA is capable of binding several isotopes including 68 Ga, 177 Lu and 90 Y. The molecule’s binding affinity is remarkably sensitive to small changes in the linker, and changes in the linker also influences the pharmacokinetic properties of the agent [29]. Thus, PSMA DKFZ 617 is a true theranostic inhibitor capable of both imaging and therapy using different radioisotopes.…”