Study On The Single Chain Antibody Against Human Colorectal Carcinoma

Colorectal carcinoma is one of the common malignant tumors which have higher incidence, occupying the fourth place in our country and the death rate increasing continuously in recent years. Therefore, efficient diagnosis and therapeutic approaches are important for colorectal carcinoma research. Although in recent years some progress has been made in respect to application of monoclonal antibodies for the therapy and diagnosis of colorectal carcinoma, most mAbs are of murine origin, so that repeated administration can induce human anti - human antibodies ( HAM A) , which can severely limit the efficacy of antibody in clinical utilization, moreover, intact mAbs are generally too large ( Mr 150,000) to penetrate tumor masses. To overcome such deficiencies, antibody gene engineering processes are developed, including human origin antibodies, single chain Fv (scFv) , human -murine chimed antibodies etc. . ScFv, which is comprised of immunoglobulin heavy - and light - chain variable regions that are connected by a short peptide linker, is the gene engineered antibody employed most widely at present, being. The main advantages of scFv over intact and Fab fragment are their small size ( Mr 26,000, amounting to one sixth of intact mAbs) , making them to penetrate a solid tumor mass rapidly and evenly. In addition, the lack of Fc domains in scFv makes them less immunogenic responsive and less capable of binding to Fc receptors distributed on normal cells. These characteristics make scFv potentially useful in tumor diagnosis and therapy as a carrier.The key issue in mAbs application is the full exploitation of its role in targeting against tumors. Antibody -directed enzyme prodrug therapy (ADEPT) is a new tumor therapeutic approach which is designed to overcome main problems in directed tumor therapy such as the lack of tumor selectivity and drug resistance, low uptake of antitumor drugs, etc. . In ADEPT, a monoclonal antibody against a tumor - associated antigen is linked with an active enzyme to produce an antibody - enzyme conjugate which is given i. v. , resulting in selective accumula-tion of the enzyme in the tumor. When the discrimination between tumor and normal tissue enzyme levels is sufficient, a prodrug is given i. v. , which is relatively nontoxic and can be converted to an cytotoxic drug in the tumor, killing the latter specifically. The principal potential advantages of ADEPT are: (1) Significant reduction of drug toxicity. Being of low toxicity or nontoxic, prodrugs do not show cytotoxicity until located on a tumor and activated by an enzyme. The prodrug entering into normal tissue cannot be activated, because of lack of the enzyme, leading to decrease of toxicity. (2) Enhancement of drug concentration in tumor localizations. Because of special mechanisms of enzyme effects, a single enzyme molecule is potentially capable of cleaving many prodrug molecules , providing an amplification effect giving high level of active drugs localized on tumors. (3) Provision of a bystander effect. Because of the low molecular weight, the activated drug can diffuse to adjacent tissues, killing the tumor cells which do not express tumor - associated antigen, efficiently solving the problem of heterogeneity of tumor - associated antigen expression. Currently, ADEPT has shown antitumor activity in animal tumor models of human choriocarcinoma , colonic carcinoma and breasr carcinoma. ADEPT system of carboxypeptidase G2 has been used in phase II clinical trials.ND - 1 is a murine monoclonal antibody against tumor - associated antigen LEA , mainly expressed in human colorectal carcinoma , developed by Professor Song Jindan in 1986, which was obtained by immunizing Balb/c mouse with CC1 -187 human colorectal carcinoma cell line. The histological determination of one thousand pathologic samples showed that ND - 1 can bind specifically to well differentiated and moderate differentiated colorectal carcinoma tissues and its specificity is superior to mAbs against CEA which has been commercialized in the US. The efficacy of...