Folate-targeted polymeric micelles loaded with ultrasmall superparamagnetic iron oxide: combined small size and high MRI sensitivity

Figures & data

Table 1 Properties of SPIONs with different formulations

Micelle formulation	Micelle size (nm)	Fe3O4 content (wt%)	r1	r2	r2/r1
Fa-PEG-PCL-SPIONsa	44 ± 3	33.5%	1.7	113	66
PEG-PCL-SPIONs	38 ± 3	37.0%	1.4	110	78
Hydrophobic SPIO	6	77.0%b	nd	nd	nd
WSPIOsc	6		3.2	40	12.5
Resovit	65		7.2	82	11.3

Notes: r₁ and r₂ values are expressed as Fe mM⁻¹ s⁻¹.

a Surface layer was a mixture of Fa-PEG-PCL and PEG-PCL containing 20% Fa-PEG-PCL;

b Tested by TGA;

c Hydrophilic SPIO obtained by small molecular surfactant coating.

Abbreviations: Fa-PEG-PCL-SPIONs, folate-attached poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles; WSPIOs, water-soluble superparamagnetic iron oxide nanoparticles; nd, not determined.

Figure 1 Synthetic approach of folate-PEG-PCL for SPION coating.

Abbreviations: PEG-PCL, poly(ethylene glycol)-poly(ɛ-caprolactone); SPION, superparamagnetic iron oxide nanoparticle.

Figure 2 Synthesis of monodisperse Fe₃O₄ nanoparticles coated with biodegradable diblock copolymer.

Abbreviation: PEG-PCL, poly(ethylene glycol)-poly(ɛ-caprolactone).

Figure 3 Transmission electron microscopy (TEM) images of synthesized 6 nm SPIONs (A) and PEG-PCL-SPIONs (B). The insert (a) shows the synthesized SPIONs were soluble in hexane, and PEG-PCL-SPIONs were easily dispersed in water (b). The selected area electron diffraction (SAED) pattern of 6-nm Fe₃O₄ nanoparticles is shown in (C).

Abbreviation: PEG-PCL-SPIONs, poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles.

Figure 4 DLS profiles of PEG-PCL-SPIONs and blank PEG-PCL micelle.

Abbreviations: DLS, dynamic light scattering; PEG-PCL-SPIONs, poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles.

Figure 5 Hysteresis loops of 6-nm Fe₃O₄ nanoparticles (A) and PEG-PCL-SPIONs (B) measured at 300 K and 10 K.

Abbreviation: PEG-PCL-SPIONs, poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles.

Table 2 Dependence of MRI T₂-weighted signal intensity and signal intensity changes of tumor on the postinjection time

Hour(s)	Fa-PEG-PCL-SPIO		PEG-PCL-SPIO
	MRI signal intensity (&xmacr;±s)	MRI signal intensity changes, ΔSI (%, M)	MRI signal intensity (&xmacr;±s)	MRI signal intensity changes, ΔSI (%, M)
0	1691 ± 35	0	1822 ± 84	0
3	998 ± 30*	−41.2*	1541 ± 81*	−16.4*
6	1145 ± 71*	−32.4*	1576 ± 108*	−12.9*
24	1503 ± 81*	−11.5*	1700 ± 77	−6.43
Statistical value	F= 177.52	χ ^2 = 21.69	F = 12.56	χ ^2 = 19.98

Notes:

* P < 0.05.

At each experimental time point, six mice were scanned for both targeting and nontargeting groups. ΔSI was calculated according to the following equation: ΔSI = (SI_post − SI_pre)/SI_pre × 100%, where SI_pre and SI_post are signal intensities of pre- and postinjection, respectively.

Abbreviation: Fa-PEG-PCL-SPIO, folate-attached poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide.

Figure 6 T₂-weighted MRI images (T_R/T_E, 5000/100 ms) taken at 0 and 3 hours after injecting 5 mg of Fe/kg of PEG-PCL-SPIONs (A and B) and Fa-PEG-PCL-SPIONs (C and D) via tail vein into a nude mouse bearing BEL-7402 tumor (about 1 cm in diameter).

Note: The white arrow indicates the xenograft tumor for determining T₂-weighted signal intensity change.

Abbreviations: MRI, magnetic resonance imaging; T_R, repetition time; T_E, echo time; Fa-PEG-PCL-SPIONs, folate-attached poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles.

Figure 7 Prussian blue staining images of tumor tissues taken from mice at a time point 3 hours after injection of PEG-PCL-SPIONs (A) and Fa-PEG-PCL-SPIONs (B).

Note: Blue stain density reflects the level of SPIO accumulation within tumor.

Abbreviation: Fa-PEG-PCL-SPIONs, folate-attached poly(ethylene glycol)-poly(ɛ-caprolactone) superparamagnetic iron oxide nanoparticles.