| Magnetic resonance imaging (MRI) is among the best noninvasive methodologies today in clinical medicine for assessing anatomy and function of tissues. The MRI technique offers several advantages such as good contrast in soft tissue, excellent spatial resolution, and the lack of exposure to radiation. However, due to its relatively lower sensitivity, its success in this research field suffers. In clinic, the contrast agent is used to heighten sensitivity, including Gd complexes to enhance the MR signal intensity (positive contrast) and superparamagnetic iron oxide nanoparticles (SPION) to reduce the signal intensity (negative contrast). Compared to Gd contrast agents, SPION can provide better contrast effect. However, for commercially available, dextran coated SPION contrast agent, the sensitivity is still not enough to detect the specific tumor markers with low expression level. In this context, a new contrast agent with higher sensitivity is highly desirable for MR molecular imaging of cancer. Therefore, in this dissertation, I have developed a magnetic nanocluster by assemblying hydrophobic SPION with amphiphilic polymer (PEG-PLA) and explored its potential for early detection of lung cancer. My research mainly invoveled the follow parts:(1) Synthesis of PEG-PLA amphiphilic copolymer. mPEG-PLA or Mal-PEG-PLA was synthesized by melting method.(2) Preparation of hydrophobic SPION with different size. Hydrophobic SPION was prepared by decomposition of Fe(acac)3 in organic solvent at high temperature. The size of SPION can be precisely controlled by seeding groth method.(3) Assembly of hydrophobic SPION with the copolymer. The mechanism for the assembly and the effect of cluster size on the MR sensitivity were also performed and compared with therotical prediction.(4) MR molecular detection of lung cancer with mangtic nano cluster probe. The magnetic nano cluster probe was fabricated by conjugating specific peptide with the cluster through mercaptol-maleimide covalent bond. After incubation of the probe with cancer cells, the specificity of the probe was evaluated by Prussian blue staining, MR imaing of cells and ICP-OES measurement of intracellular iron. The subcellular locoalization of the probe was observed by TEM. For early detection of tumor, MR molecular imging of lung cancer (H1299) at different growth stages were performed.Molecular weights of copolymer 5647, 7100, 7500, 8400 and 9300 were synthesized. The size of SPION was fine tuned from 4.6 nm, 6nm, 7nm to 12nm. By assemblying the 6 nm SPION with copolymers, magnetic nano clusters with the condensed (CSC) or loosen (LSC) structures could be fabricated, which depends on the relatively amount of SPION and copolymer used for assembly. For condensed magnetic nano cluster, the transverse relaxivity much higher than that of loosen cluster and it is consistent with theotetical predication when the size is evaluated at dry state (with TEM). When coupling peptide RGD with PEG-PLA, the cluster could be specificially targeting tumor cells in vitro. Furethermore, after injecting into mice at different stages of lung cancer, tumors with size as small as 4 mm could be detected. Moreover, MR signal intensity decrease of small tumors (45±12 mm~3) was more significant than that of big tumors(263±42 mm~3). Histologically, we found the micro vessel density of small tumor(125.4±17.5/mm~2) and the receptor-expressing level (18.31±0.95%) are much higher than the big one(74.35±10.2/mm2 and 12.38±2.73%,respectively), which may be the intrisinc reason for the difference of signal intensity. The result of incubation with lung cancer cell proves the non-toxicity of probe. The swallon is receptor-mediated, and is positively related to the expression level, the incubation concentration and time. In conlusion, we have developed a type of magnetic nano cluster MR contrast agent with high sensitivity, which can can be used for early detection of nascent tumor and differentiate the different stage of cancers. |
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