Multifunctional iron platinum stealth immunomicelles: targeted detection of human prostate cancer cells using both fluorescence and magnetic resonance imaging

Abstract

Superparamagnetic iron oxide nanoparticles (SPIONs) are the most common type of contrast agents used in contrast agent-enhanced magnetic resonance imaging (MRI). Still, there is a great deal of room for improvement, and nanoparticles with increased MRI relaxivities are needed to increase the contrast enhancement in MRI applied to various medical conditions including cancer. We report the synthesis of superparamagnetic iron platinum nanoparticles (SIPPs) and subsequent encapsulation using PEGylated phospholipids to create stealth immunomicelles (DSPE-SIPPs) that can be specifically targeted to human prostate cancer cell lines and detected using both MRI and fluorescence imaging. SIPP cores and DSPE-SIPPs were 8.5 ± 1.6 nm and 42.9 ± 8.2 nm in diameter, respectively, and the SIPPs had a magnetic moment of 120 A m(2)/kg iron. J591, a monoclonal antibody against prostate specific membrane antigen (PSMA), was conjugated to the DSPE-SIPPs (J591-DSPE-SIPPs), and specific targeting of J591-DSPE-SIPPs to PSMA-expressing human prostate cancer cell lines was demonstrated using fluorescence confocal microscopy. The transverse relaxivity of the DSPE-SIPPs, measured at 4.7 Tesla, was 300.6 ± 8.5 s(-1) mM(-1), which is 13-fold better than commercially available SPIONs (23.8 ± 6.9 s(-1) mM(-1)) and ~3-fold better than reported relaxivities for Feridex(®) and Resovist(®). Our data suggest that J591-DSPE-SIPPs specifically target human prostate cancer cells in vitro, are superior contrast agents in T(2)-weighted MRI, and can be detected using fluorescence imaging. To our knowledge, this is the first report on the synthesis of multifunctional SIPP micelles and using SIPPs for the specific detection of prostate cancer.

Fig. 1

TEM and DLS of SIPP Cores and DSPE-SIPPs. TEM images of a SIPP cores and b, c DSPE-SIPPs. Scale bars are 20, 50 and 50 nm, respectively. Arrows denote internal areas of the DSPE-SIPPs where space can be seen between the hydrophobic SIPP cores. d DLS of DSPE-SIPPs in PBS

Fig. 2

SIPP Core and DSPE-SIPP TGA. TGA thermograms of a SIPP cores (solid curve) and ODA (dashed curve) and b DSPE-SIPPs (solid curve) and phospholipids (dashed curve). Vertical dashed line denotes the temperature, reported to the left of the dashed line, at which the a SIPP cores and b DSPE-SIPPs stopped loosing significant mass. % values are the percent mass of each sample remaining at the temperature denoted by the vertical dashed line

Fig. 3

Magnetization of SIPPs. Saturation magnetization curves for the mass magnetization of SIPP cores versus the applied magnetic field from −5 to 5 Tesla. Inset shows the zero-field-cooled (ZFC) and field-cooled (FC) curves. Values of the blocking temperature (T_B) were recorded by determining the peak location in the ZFC curve

Fig. 4

Specific detection of PSMA-expressing Prostate Cancer Cells using J591-DSPE-SIPPs. C4-2 (top row), PSMA-positive, and PC-3 (bottom row), PSMA-negative, cell lines were imaged using phase-contrast light microscopy with a 63X objective (a, e). Cells were incubated for 10 min with either PBS (Mock) (b, f), J591-DSPE-SIPPs (c, g), or IgG-DSPE-SIPPs (d, h) and imaged using confocal microscopy with a 60X oil objective. Blue DAPI nuclear stain and Red Liss-Rhod incorporated in the DSPE-SIPPs

Fig. 5

Magnetic resonance relaxometry of DSPE-SIPPs and MACS^® MicroBeads at 4.7 Tesla. a T₂-weighted MRI of 1% agarose samples containing increasing concentrations of DSPE-SIPPs. Top left sample is agarose that did not contain DSPE-SIPPs. The other samples have increasing concentrations of DSPE-SIPPs going from left to right in the top row images and continuing from left to right in the lower row images. b Transverse relaxation rates (1/T₂) versus iron concentration (mM) for the DSPE-SIPPs (squares) and MACS^® MicroBeads (triangles). Linear regression was used to fit the data (solid lines) and the transverse relaxivities (r_n) of the DSPE-SIPPs and MACS^® MicroBead SPIONs, given as the slope of the resulting line, were 300.8 ± 8.5 and 23.8 ± 6.9 s⁻¹ mM⁻¹, respectively