Exceedingly Small Gadolinium Oxide Nanoparticles with Remarkable Relaxivities for Magnetic Resonance Imaging of Tumors

Gd chelates have occupied most of the market of magnetic resonance imaging (MRI) contrast agents for decades. However, there have been some problems (nephrotoxicity, non-specificity, and low r(1)) that limit their applications. Herein, a wet-chemical method is proposed for facile synthesis of poly(acrylic acid) (PAA) stabilized exceedingly small gadolinium oxide nanoparticles (ES-GON-PAA) with an excellent water dispersibility and a size smaller than 2.0 nm, which is a powerful T-1-weighted MRI contrast agent for diagnosis of diseases due to its remarkable relaxivities (r(1) = 70.2 +/- 1.8 mM(-1) s(-1), and r(2)/r(1) = 1.02 +/- 0.03, at 1.5 T). The r(1) is much higher and the r(2)/r(1) is lower than that of the commercial Gd chelates and reported gadolinium oxide nanoparticles (GONs). Further ES-GON-PAA is developed with conjugation of RGD2 (RGD dimer) (i.e., ES-GON-PAA@RGD2) for T-1-weighted MRI of tumors that overexpress RGD receptors (i.e., integrin alpha(v)beta(3)). The maximum signal enhancement (Delta SNR) for T-1-weighted MRI of tumors reaches up to 372 +/- 56% at 2 h post-injection of ES-GON-PAA@RGD2, which is much higher than commercial Gd-chelates (<80%). Due to the high biocompatibility and high tumor accumulation, ES-GON-PAA@RGD2 with remarkable relaxivities is a promising and powerful T-1-weighted MRI contrast agent.