Liu T, Toriyabe Y, Kazak M, Berkman CE. performance. The polyethylene glycol (PEG) spacer has been widely applied in nanotechnology to covalently couple small-molecule ligands or antibodies onto the surfaces of nanoparticles for targeted imaging or drug delivery.27C30 In general, PEG spacers can CL 316243 disodium salt lead to improved plasma circulation and biocompatibility of nanoparticles due to their enhanced hydrophilicity, and provide sufficient flexibility for a targeting molecule to overcome spatial limitations in order to effectively interact with a corresponding target protein or receptor.28 In the present study, we examined the effect of the spacer length between a representative phosphoramidate PSMA inhibitor core (CTT-54) and fluorescein-based dye (Fig. 1) upon both the inhibitory potency against PSMA and the cell-labeling of PSMA+ cells. The preparation of both the phosphate PMSA inhibitor and its fluorescein conjugates is provided in the Supplementary data. Open in a separate window Figure 1 Structures of PSMA inhibitor core CTT-54, and its fluorescein conjugates: FAM-CTT-54, FAM-X-CTT-54, and FAM-PEG8-CTT-54. In this study, a series of fluorescent PSMA inhibitor conjugates (FAM-CTT-54, FAM-X-CTT-54 and FAM-PEG8-CTT-54) were synthesized according to a previously reported method.18 As shown in the Supplementary data (Fig. S1, S2), the absorption spectra (400 ~ 800 nm) and CL 316243 disodium salt fluorescence emission (at ~520 nm) of free fluorescein dye and the fluorescent PSMA inhibitor conjugates displayed similar absorbance spectra and fluorescence intensity. These data suggest that that conjugation of CTT-54 through various spacer lengths had little impact on the spectral properties. In Figure S3ACD, PSMA inhibition studies confirmed that conjugation of CTT-5419, 20, 31 to fluorescein-based dyes through various spacer lengths (FAM-CTT-54, IC50 = 0.41 nM; FAM-X-CTT-54, IC50 = 0.35 nM;19 FAM-PEG8-CTT-54, IC50 = 1.93 nM) had no adverse effect upon the inhibitory potency of the parent inhibitor core CTT-54 (IC50 = 14 nM).19 To understand the impact of spacer length on the fluorescent inhibitor conjugates, we examined the enzymatic activity recovery profiles for PSMA inhibition by CTT-54, FAM-CTT-54, FAM-X-CTT-54 and FAM-PEG8-CTT-54, according to our previously reported method.16, 18 Both CTT-54 and FAM-PEG8-CTT-54 were shown to be irreversible inhibitors, while FAM-CTT-54 was completely irreversible and FAM-X-CTT-54 exhibited characteristics of slowly reversible inhibitors (Fig. 2). These data suggest that the placement of the fluorophore too close to the PSMA active may prevent essential conformational changes necessary for irreversible inhibition. Open in a separate window Figure 2 The enzymatic activity recovery profiles for PSMA inhibited by FAM-CTT-54, FAM-XCTT-54, and FAM-PEG8-CTT-54 and CTT-54. On the basis of recovery profiles, CTT-54 and FAMPEG8-CTT-54 are irreversible; FAM-CTT-54 is completely reversible and FAM-X-CTT-54 is slowly reversible. Uninhibited PSMA served as a control. To determine whether CL 316243 disodium salt the spacer length would affect the imaging of PSMA-positive prostate cancer cells (LNCaP), these cells were treated with each of the fluorescent inhibitor conjugates in the presences of 0.2% NaN3 Rabbit Polyclonal to DDX3Y to block energy-dependent PSMA internalization.19, 32 Confocal microscopy revealed that the surfaces of LNCaP cells treated with FAM-X-CTT-54 and FAM-PEG8-CTT-54 were considerably more fluorescent than cells treated with FAM-CTT-54 (Fig. 3). This data suggested quenching of the fluorophore when bound deeper into PSMA due to the absence of a spacer to link the fluorophore and the inhibitor core. Open in a separate window Figure 3 Direct fluorescent labeling of PSMA-positive cells with fluorescent inhibitors. Live LNCaP cells were labeled with 5 M each of fluorescent inhibitors (green) for 30 min at 37 C: (A) FAM-CTT-54, (B) FAM-X-CTT-54, and (C) FAM-PEG8-CTT-54. All cells were fixed and nuclei stained with Hoechst 33342 (blue). Distance scale is 20 m. An anti-fluorescein antibody-coupled to AlexaFluor 594 (red) was used to probe the surface accessibility of the fluorophore on the fluorescent inhibitor conjugates when bound to PSMA on LNCaP cells (Fig. 4). Red fluorescence was only observed on the surface of LNCaP cells treated first with FAM-PEG8-CTT-54. This data suggested that unlike the shorter spacers, a spacer length such as PEG8 would allow the fluorophore to be sufficiently remote from the PSMA surface and accessible to its antibody binding. This data were consistent with the finding above indicating that with the shorter linker, the fluorophore was likely buried in the PSMA binding cavity resulting in fluorescence quenching. Open in a separate window Figure.