Specific Targeting Of Folate-labeled MRI Contrast Agents To The High Affinity Folate Receptor Expressed In Pulmonary Tumor Xenografts

Background The development of new and improved tumor selective MRI contrast agents is clinically desirable as a means of: (i) detecting and/or confirming the presence and location of primary and metastatic lesions; (ii) probing biochemical features of neoplastic tissue that have implications for tumor staging and/or treatment planning; and (iii) monitoring tumor response to treatment. The folate receptor, or folate binding protein (FBP), is attractive as a potential molecular target for MRI contrast agents delivery, The receptor for folic acid constitutes a useful target for tumor-specific drug delivery, primarily because: (1) it is upregulated in many human cancers, including malignancies of the ovary, lung, kidney, breast, myeloid cells and brain, (2) access to the folate receptor in those normal tissues that express it can be severely limited due to its location on the apical (externally-facing) membrane of polarized epithelia, and (3) folate receptor density appears to increase as the stage /grade of the cancer worsens.Objective This study presents a compound of folate-conjugated PL-Gd-DTPAtargeting to tumor cells via folate receptor as MR targeting contrast agent, and observes MR signal variations and imaging feature of pulmonary tumor xenografts in nude mice after the use of MRI targeting contrast agents, to evaluate feasibility and effectiveness.Materials and methodsI Preparation of Folate-Poly-L-Lysine -Gd-DTPA and EvaluationUsing Poly-L-Lysine as linker, after Poly-L-Lysine was tethered with caDTPA, GdCl₃ was added to label DTPA-Poly-L-Lysine, then Gd-DTPA- Poly-L-Lysine was conjugated to folate, a specific MR contrast agent (folate-PL- Gd-DTPA), was thusprepared. Using HPLC to evaluate the conjugate purity, ICP-AES to Gd3+contnet. And it's activity evaluated by competitive binding folate receptor with folic acid.II In vivo Magnetic resonance imaging specificity studiesLung tumors were induced in athymic nude mice 5-6 weeks of age by subcutaneously injecting, human lung tumor cells expressing the hFR, H460 , in the subscapular area on the dorsal surface, as experimental model. Folate-PL-Gd-DTPA was injected into caudal vein of the mice in the study(n=3). The signal features of the pulmonary tumor xenografts, liver and thigh muscles before and after the injection were observed on T1WI.DI Specific and non-specific MRI contrast agents in pulmonary tumorxenografts Magnetic resonance imagingPulmonary tumor xenografts in nude mice were used as experimental model. The mice were divided into study group(n=6) and control group(n=4). Gd-DTPA-folate was injected into caudal vein of the mice in the study, while the mice in the control group received Gd-DTPA. The signal features of the pulmonary tumor xenografts before and after the injection were observed on Tl WI.Results Folate-conjuated Gd-DTPA-Poly-L-Lysine was successfully achieved. Ithas high affinity for folate receptor and high content of Gd3+, where the -amino groups of poly-1-lysine were conjugated to approximately 65 Gd-DTPA molecules. In vivo studies, Pulmonary tumor xenografts average signal intensity increase of 126.0% at 48 hours after contrast; liver maximum signal intensity increase at 6 hours after injection; Thigh muscle signal intensity at any time-point after injection showed no statistically difference with that observed before injection. And specific and non-specific contrast agents MRI studies indicated: (1) folate-PL-Gd-DTPA had an excellent tumor selectivity in pulmonary tumor xenografts in the animal model. After injection, the tumor signal intensity in the study group was significantly higher than that observed before injection; An average intensity increase of 125.4% was observed from pre-contrast to post-contrast images of the tumor, which was observed at 24-48 hours after injection; The muscle signal intensity at any time-point afterinjection showed no statistically difference with that observed before injection. (2) In control group, the tumor signal intensity showed statistically difference with that observed before injection at 0.5 hour and 3 hours, the biggest difference appeared at 0.5 hour; The muscle signal intensity at 0.5 hour time-point showed statistically difference with that observed before injection.Conlusions Folate-PL- Gd-DTPA, as macromolecular targeting contrast agent,could be binded to tumor cells via folate receptor-mediated and significantly targeted to tumor cells with rich folate teceptors for MR imaging.