Single‐Chain Antibodies in Pancreatic Cancer

Abstract

Pancreatic cancer is a therapeutic challenge for surgical and medical oncology. Development of specific molecular tracers for the diagnosis and treatment of this lethal cancer has been one of our major goals. Monoclonal antibodies (MAbs) have been successfully used as selective carriers for delivering radionuclides, toxins or cytotoxic drugs to malignant cell populations; therefore, monoclonal antibody technology has led to a significant amount of research into optimizing targeted therapy. This targeted therapy results in the selective concentration of cytotoxic agents or radionuclides in tumors and should lessen the toxicity to normal tissues, which would normally limit the dosage and effectiveness of systemically administered drugs. The MAb CC49 reacts with a unique disaccharide, Sialyl-Tn, present on tumor-associated mucin (TAG-72) expressed by a majority of human adenocarcinomas. The unique Sialyl-Tn epitope has provided a potential target for immunotherapy of cancer. A single chain Fv (scFv) recombinant protein from CC49 MAb was prepared by engineering the DNA fragments for coding heavy-chain and light-chain variable regions with an appropriate oligonucleotide linker. scFv molecules, when compared to intact MAbs and the more conventional enzymatically derived F(ab')2 and Fab' fragments, offer several advantages as carriers for the selective delivery of radionuclides to tumors. The divalent antibody fragments (sc(Fv)2 or (scFv)2) display an affinity constant similar to that of the intact CC49 IgG and are stable with storage, and after radiolabeling. In preclinical studies, both the covalent and the non-covalent dimeric scFvs exhibit excellent tumor targeting properties with characteristics similar to those of the monomer, e.g., the rapid blood clearance, low kidney uptake and small size suitable for rapid penetration through tumor tissue. Increased tumor targeting of the dimers are probably due to their increased functional affinity attributable to valency, coupled with their higher molecular weight and fewer interactions with normal organs. These properties make these constructs superior to monovalent CC49 scFv. The relatively high tumor uptake, the in vitro and in vivo targeting specificity, and the stability in storage demonstrated by the dimeric CC49 sc(Fv)2 makes it a promising delivery vehicle for therapeutic applications in pancreatic cancer.

The Cosmc connection to the Tn antigen in cancer

“…Welinder et al [183] reported that the murine anti-Tn antibody displayed anti-tumor activity in xenograft SCID mice. Human tumor-associated glycoprotein-72 (TAG-72) is a carcinoma mucin molecule expressed in colon [184,185], breast [186], pancreatic [187], ovarian [188], lung, and gastric cancers [189]. Epitopes of TAG-72 recognized by the mAbs B72.3 and CC49 are STn and Sialyl T antigens, respectively [190–192].…”

Antibody constructs for radioimmunodiagnosis and treatment of human pancreatic cancer

“…Indeed these divalent scFvs revealed improved antigen affinity in vitro when compared to the monovalent scFvs. In animal models, divalent scFvs exhibited a significant improvement in tumor-targeting over monovalent species like scFv and Fab due to higher avidity and also because of an increase in size causing a longer availability of scFvs for tumor uptake [56,65,[72][73][74]. Wu et al [72] added a human Ig hinge region followed by the human CH 3 domain connected by a 10 amino acid linker (Flex minibody).…”

A physiologically based pharmacokinetic (PBPK) model to characterize and predict the disposition of monoclonal antibody CC49 and its single chain Fv constructs

“…CC49 is among the most extensively studied MAbs for cancer therapy and reacts against tumorassociated antigen TAG-72 which is expressed by a majority of mucinous adenocarcinomas [5]. Further, we have applied the PBPK model to simulate the pharmacokinetics and biodistribution of di-and tetravalent scFv constructs of CC49 in mice.…”

“…In an effort to reduce their size and to circumvent some of the problems associated with the use of intact MAbs, immunoglobulins (IgGs) have been engineered to retain only the domains involved with antigen binding and/or mediating the effector functions of the MAb [4]. One such engineering strategy is the use of single-chain antibody fragments (scFvs) comprised of only the variable regions of the heavy and light chains of IgG, covalently connected by a flexible peptide linker [5,6]. Such fragments exhibit greater systemic clearances than intact IgG [7] and have yielded improvements in radioimmunodiagnosis and radioimmunotherapy by offering shorter imaging times and correspondingly rapid tumor penetration resulting in increased tumorto-background concentration ratios [8].…”