Targeting to EGF receptors : Preparation and experimental analysis of end-end coupled mEGF-dextran conjugates

Abstract: Targeted tumor therapy is to deliver cytotoxic agents selectively to tumor cells. By such delivery, normal tissues receive minimal damage while a therapeutical effect is achieved in tumor. A targetingsystem can be constructed by conjugating a tumor-seeking molecule and toxic agents via a carrier. In this thesis, mouse epidermal growth factor, mEGF, was the tumor-seeking molecule for targeting against malignancies that overexpress EGF receptors. Dextran was attached to mEGF by different methods and used as a carrier. Preparation of different mEGF-dextran conjugates, effects of different conjugation methods on cellular intenalization and the biodistribution of the conjugates in vivo were studied in the planning for future clinic trials.Two end-end coupling methods have been developed in this thesis. One is direct reductive amination by which the terminal amino group of mEGF reacts with the aldehyde group on rhe reducting end of dextran. The other end-end coupling methods is glutaraldehyde mediated cross-linking, in which a few glutaraldehyde molecules act as a linker between the amino terminus of mEGF and the end-aminated dextran.The end-end coupled mEGF-dextran conjugates were investigated together with a randomly coupled mEGF-dextran conjugate, for cellular binding ptterns using cultured glioma cells. The time for maximal binding and the intracellular retention of the conjugates were analysed. The intracellular radioactivity associated with free mEGF had a fast turnover whereas all three conjugates gave long half-lives for the intracellular retention.The end-end coupled mEGF-dextran, type A prepared by reductive amination, had a dissociation constant of 7.1 x 10-9 M and free mEGF had a value of 6.6 x 10-10 M. The end-end coupled intermediate was further activated by the cyanopyridinium agent CDAP and tyrosines were introduced to the dextran part of the conjugate. The iodinated conjugate could to a large extent be internalized in test cells and the radioactivity was retained intracellularly better than when the radioactivity only was on the mEGF part of the conjugate.Dextranated mEGF had a slower clearance in blood and liver than mEGF as seen in positron emission tomography on rats. Dextranated mEGF also had a lower uptake in kidney than free mEGF. There was an "unknown mEGF complex" in the blood circulation which was formed after mEGF was injected but this complex did not appear in urine samples.