Monoclonal Antibody-Coated Magnetite Particles as Contrast Agents for MR Imaging and Laser Therapy of Human Tumors

Abstract

Attempts to improve human tumor detection by nonradioactive magnetic resonance techniques have led several investigators to develop antibody-linked para-magnetic contrast agents. Initial studies focused on gadolinium conjugated to monoclonal antibodies. However, very high levels of this contrast agent were needed to significantly reduce proton relaxation times and obtain improved MR images. The use of magnetite (Fe₃O₄) as an MR contrast agent provides a magnetic moment that is approximately one order of magnitude larger than gadolinium. In this study monoclonal antibodies 44 × 14 (specific for squamous cell carcinoma) and 436G10 (specific for melanoma) were obtained from ammonium sulfate precipitation of tumor ascitic fluid. Equal volumes of magnetite solution (1.87 mg Fe/ml) and antibody solution 44 × 14 (5.24 mg protein/ml) and 436G10 (0.64 mg protein/ml) were mixed and sonicated. The 44 × 14magnetite and 436G10-magnetite solutions were then added to equal volumes of M20 and P3 squamous cell carcinoma cell lines. T1 and T2 values were obtained on a Praxis II NMR spectrometer equipped with a 10 mm probe and 0.25 Tesla permanent magnet.

The T2 relaxation times of the magnetite-antibody-cell mixtures were 31 ms with an R = 0.985 for both experimental samples. Our results demonstrate a significant decrease in T2 by binding of the magnetite-coated antibodies to these melanoma and carcinoma cells in vitro. The possibility of detecting subclinical local and metastatic disease with magnetite linked to monoclonal antibodies followed by MRI guided laser tumor ablation therapy may render this technique clinically attractive for treatment of deep-seated tumors.