Tumor Targeting Effect of Triphenylphosphonium Cations and Folic Acid Coated with Zr-89-Labeled Silica Nanoparticles

Abstract

In this study, we investigated the tumor targeting effect in cancer cells using triphenylphosphonium (TPP) cations, which are accumulated by differences in membrane potential, and folic acid (FA), which is selectively bound to overexpressed receptors on various cancer cells. We used Food and Drug Administration (FDA)-approved silica nanoparticles (SNPs) as drug carriers, and SNPs conjugated with TPP and FA (STFs) samples were prepared by introducing different amounts of TPP and FA onto the nanoparticle surfaces. STF-1, 2, 3, 4 and 5 are named according to the combination ratio of TPP and FA on the particle surface. To confirm the tumor targeting effect, ⁸⁹Zr (t_1/2 = 3.3 days) was coordinated directly to the silanol group of SNP surfaces without chelators. It was shown that the radiochemical yield was 69% and radiochemical purity was >99%. In the cellular uptake evaluation, SNPs with the most TPP (SFT-5) and FA (SFT-1) attached indicated similar uptake tendencies for mouse colon cancer cells (CT-26). However, the results of the cell internalization assay and measurement of positron emission tomography (PET) images showed that SFT-5 had more affinity for the CT-26 tumor than other samples the TPP ratio of which was lower. Consequently, we confirmed that TPP ligands affect target cancer cells more than FA, which means that cell membrane potential is significantly effective for tumor targeting.

Keywords: PET imaging; Zr-89; folic acid; silica nanoparticles; surface modification; triphenylphosphonium cation.

Scheme 1

Total scheme of biological evaluation for ⁸⁹Zr-labeled STF-1 to 5.

Figure 1

(a) FT-IR spectrum of triphenylphosphonium (TPP)- and folic acid (FA)-modified silica nanoparticles (SNPs); (b) silica nanoparticle; (c) extension spectrum (a) at 1450 to 1900 cm⁻¹.

Figure 2

Scanning electron microscope (SEM) images of (a) SNPs, (b) STF-1, (c) STF-2, (d) STF-3, (e) STF-4, and (f) STF-5.

Figure 3

Dynamic light scattering (DLS) analysis and spectroscopic analysis of nanoparticles. (a) Hydrodynamic diameter of SNPs in distilled water. (b) Dynamic light scattering studies and Log P value of STF-1 to 5. (c) UV absorbance of 0.1 mg/mL STFs in distilled water. (d) The amount of FA and TPP on SNP surfaces (μmol/mg).

Figure 4

(a) The cytotoxicity study using STF and SNP was conducted on CT-26 for 24 h and (b) In-vitro stability evaluation of ⁸⁹Zr-STF: Stability in human serum and saline was monitored by radio TLC. The ⁸⁹Zr -STF was incubated with 1.0 mL of human serum and saline at 37 °C for 7 days and then analyzed.

Figure 5

Results of (a) cellular uptake for STF-1~5 and (b) cell internalization for STF 1 and 5.

Figure 6

Positron emission tomography (PET) study results of CT-26-bearing 5-week-old female Balb/c mice. CT-26 xenograft mice were scanned for a 5-min static image at time points after intravenous injection of (a) ⁸⁹Zr-STF-1 and ⁸⁹Zr-STF-5; (b) ⁸⁹Zr-SNP; (c) ⁸⁹Zr-oxalate; T: tumor, L: liver, S: spleen, B: bladder; (d) ROI analysis of STF-1 and STF-5 one day after injection.