A bifunctional murine::human chimeric antibody with one antigen-binding arm replaced by bacterial beta-lactamase

Bispecific antibody conjugates in therapeutics

Bispecific monoclonal antibodies have drawn considerable attention from the research community due to their unique structure against two different antigens. The two-arm structure of bsMAb allows researchers to place a therapeutic agent on one arm while allowing the other to specifically target the disease site. The therapeutic agent can be a drug, toxin, enzyme, DNA, radionuclide, etc. Furthermore, bsMAb may redirect the cytotoxicity of immune effector cells towards the diseased cells or induce a systemic immune response against the target. BsMAb holds great promise for numerous therapeutic needs in the light of: (1) recent breakthroughs in recombinant DNA technology, (2) the increased number of identified disease targets as the result of the completion of human genomic map project, and (3) a better understanding of the mechanism of human immune system. This review focuses on therapeutic applications and production of bsMAb while providing the up-to-date clinical trial information.

A new model for intermediate molecular weight recombinant bispecific and trispecific antibodies by efficient heterodimerization of single chain variable domains through fusion to a Fab-chain

Due to their specificity and versatility in use, bispecific antibodies (BsAbs) are promising therapeutic tools in tomorrow's medicine, provided sufficient BsAb can be produced. Expression systems favoring efficient heterodimerization of intermediate-sized bispecific antibodies will significantly improve existing production methods. Recombinant BsAb can be made by fusing single chain variable fragments (scFv) to a heterodimerization domain. We compare the efficiency of the isolated CL and CH1 constant domains with complete Fab chains to drive heterodimerization of BsAbs in mammalian cells. We found that the isolated CL:CH1 domain interaction was inefficient for secretion of heterodimers. However, when the complete Fab chains were used, secretion of a heterodimerized bispecific antibody was successful. Since the Fab chain encodes a binding specificity on its own, bispecific (BsAb) or trispecific (TsAb) antibodies can be made by C-terminal fusion of scFv molecules to the L or Fd Fab chains. This gave rise to disulphide stabilized Fab-scFv BsAb (Bibody)or Fab-(scFv)2 TsAb (Tribody) of intermediate molecular size. Heterodimerization of the L and Fd-containing fusion proteins was very efficient, and up to 90% of all secreted antibody fragments was in the desired heterodimerized format. All building blocks remained functional in the fusion product, and the bispecific character of the molecules as well as the immunological functionality was demonstrated.

Antibody-directed enzyme prodrug therapy (ADEPT): a review

Antibody-directed enzyme prodrug therapy (ADEPT) is a therapeutic strategy which aims to improve the selectivity of anticancer drugs. ADEPT is a two-step antibody targeting system that has benefits over a one-step chemo-, toxin- or radioimmunoconjugate. The basic principles of ADEPT are discussed alongside the requirements of the components: antibodies, enzymes and prodrugs. The design and syntheses of prodrugs are detailed particularly prodrug/drug systems of potential clinical use, the rationale behind their design and the in vitro and in vivo results obtained. The main features of ADEPT, such as targeting of cancer cells by the antibody-enzyme conjugates, enzymic activation of the prodrugs, selection of the prodrug/drug and enzyme/prodrug systems are reviewed.

Production of monoclonal antibodies in COS and CHO cells

Recent advances in the generation of genetically engineered monoclonal antibodies have enhanced the importance of COS cells as expression systems for rapidly producing sufficient quantities of these proteins for preliminary biochemical and biophysical analysis. In order to meet the demand for clinical supplies, a gradual increase has occurred in the usage of dihydrofolate reductase negative (DHFR-) Chinese hamster ovary (CHO) cells for large-scale antibody production. Using a variety of mammalian expression vectors and selection/amplification protocols, CHO cell lines capable of producing monoclonal antibodies at levels exceeding 1 gl-1 can now be obtained in an almost routine fashion. For the applications of monoclonal antibodies to expand into additional therapeutic areas, however, a 5-10-fold increase over current highest expression levels may still need to be achieved.

Prodrugs in cancer chemotherapy

At present, chemotherapy is not very effective against common solid cancers, especially once they have metastasized. However, laboratory experiments and studies on dose intensification in humans have indicated that some anticancer agents might be curative, but only if the dose given was very much higher than that attainable clinically. Prodrugs activated by enzymes expressed at a high level in tumors can deliver at least 50-fold the normal dose and can cure animals with tumors normally resistant to chemotherapy. The approach is not practicable clinically because of the rarity of human tumors expressing a high level of an activating enzyme. However, new therapies have been proposed that overcome this limitation of prodrug therapy. Enzymes that activate prodrugs can be directed to human tumor xenografts by conjugating them to tumor-associated antibodies. After allowing for the conjugate to clear from the blood a prodrug is administered which is normally inert, but which is activated by the enzyme delivered to the tumor. This procedure is referred to as ADEPT (antibody-directed enzyme prodrug therapy). Using different combinations of antibody, enzyme and prodrug, many classes of human tumor xenograft have been shown to be very sensitive to this procedure although in most cases they are quite resistant to conventional chemotherapy. Early clinical trials are promising and indicate that ADEPT may become an effective treatment for all solid cancers for which tumor-associated or tumor-specific antibodies are known. Tumors have also been targeted with the genes encoding for prodrug activating enzymes. This approach has been called virus-directed enzyme prodrug therapy (VDEPT) or more generally GDEPT (gene-directed enzyme prodrug therapy) and has shown good results in laboratory systems. These new therapies may finally realize the potential of prodrugs in cancer chemotherapy.