Preparation and properties of FabIgG, a chimeric univalent antibody designed to attack tumour cells

A standardized conversion of IgG antibody to bispecific form with inversely altered affinities for Fcγ-receptors II and III

This work aims to enhance killing of antibody-coated human tumor cells by altering the antibody's affinity for two effector-cell Fcγ-receptors (FcγR). Affinity for the activating FcγRIII is raised, affinity for the inhibitory FcγRIIB is reduced, with the ratio between the two association constants increasing >1000-fold. We use as a standard tool the Fab'γ from a monoclonal antibody specific for human FcγRIII. This Fab'γ module is bonded to an IgG antibody by a tandem thioether link running between cysteine residues in the hinge vicinity of each protein, thus forming a bispecific FabIgG construct. Simultaneously, effector function of the IgG module is adjusted by leaving its hinge open and adding negative charges. FabIgG constructs derived from the chimeric IgG1 antibody rituximab show the following properties. (1) The titer for antibody-dependent cellular cytotoxicity is enhanced by 12-100-fold, reflecting the affinity of the Fab'γ module for effector-cell FcγRIII. (2) Two functions of the construct's Fcγ, activation of complement and prolonged metabolic survival, are moderately reduced. (3) In contrast, affinities of the Fcγ for all FcγR are severely reduced, with two anticipated consequences. First, attacks by macrophages on antibody-coated cells are favored by reduced engagement of the inhibitory FcγRIIB. Second, reduced engagement of activating FcγR by the Fcγ lowers the probability of untoward crosslinkings of FcγR, which have been shown to provoke toxicity. If the Fab'γ module possesses human constant regions, the linkage strategy requires prior genetic deletion of at least one cysteine residue. With both Fab'γ and IgG modules available, FabIgG can be prepared in 35 h.

Exploiting CD38-mediated endocytosis for immunoliposome internalization

CD38 appears to be a promising candidate in antibody therapy; it is upregulated on cell surfaces in many lymphoid tumors and undergoes rapid internalization after interaction with antibodies. The receptor-mediated endocytosis allows conjugating toxins/drugs that promote suicide only of the malignant cells. Here, we describe the preparation of CD38-immunoliposomes and test their functionality by incubating them with CD38+/- cells. Liposomes were prepared by extrusion of a lipid mixture containing a biotinylated polyethylene glycol-phospholipid and loaded with 5(6)-carboxyfluorescein. The anti-CD38 antibody (IB4) was biotinylated and then linked to streptavidin molecules; streptavidin acts like a bridge between the antibody and the biotinylated lipid of the liposomes. CD38+/- cells were incubated either with liposomes or immunoliposomes and analyzed by fluorescence microscopy and cytofluorimetry. The results indicated a specific mechanism of internalization, owing to CD38-mediated endocytosis, where CD38+ cells incubated with immunoliposomes scored top fluorescence levels. This coupling strategy, based on the use of a streptavidin bridge to prepare immunoliposomes, does not interfere with the cellular functionality and its broad potential use represents a great advantage. Here IB4, a murine monoclonal anti-CD38 antibody, was used to simplify the experiments, but the coupling procedure may be suitable also with human antibodies, against CD38 or other human markers.

Anti-human interleukin 2 receptor monoclonal antibody isotypic switching: chimeric rat-human antibodies

Univalent and bivalent (2Fab'-Fc) chimeric rat-human antibodies were constructed by chemical coupling (thioether bonds) of Fab' fragments from the 33B31 rat anti-human interleukin 2 receptor (alpha chain) monoclonal antibody to human IgG or Fc fragments. The purity of the chimeric antibodies obtained after purification was assessed by the sodium dodecyl sulfide-polyacrylamide gel electrophoresis method. The affinity of chimeric antibodies for the human interleukin 2 receptor was determined. The affinity of the 2Fab'-Fc was better (Kd = 0.87 nM) than that of the Fab'-IgG (Kd = 2.04 nM) or the Fab'-Fc chimera (Kd = 3.93 nM). Binding studies on a T-cell clone showed that the percentage of positive cells recognized by chimeric antibodies was comparable to that obtained with unmodified 33B3.1 IgG2a or its corresponding Fab' fragment. In addition, the inhibition of interleukin 2-induced cell proliferation and allogeneic proliferative response by chimeric antibodies was of the same magnitude as that obtained with the rat IgG2a anti-interleukin 2 receptor monoclonal antibody and Fab' fragments. This study shows the possibility of changing the isotype of monoclonal antibodies without important modification to their binding activity. These reagents may offer an alternative to unmodified monoclonal antibodies for therapeutic application.

Attack on neoplastic cell membranes by therapeutic antibody

Mouse monoclonal antibody is not well fitted to destroying tumour cell targets. Complement and cellular effectors are inefficiently recruited, the cells can undergo antigenic modulation, antigen-negative mutants can arise, and the tumour-bearing subject can amount an immune response against the therapeutic antibody. This paper describes the preparation of two chimeric antibody derivatives designed to cirvumvent some of these problems. The first derivative is FabFc, prepared by linking Fab' gamma from monoclonal antibody to Fc gamma from human IgG. The bismaleimide linking agent forms a thioether bond with an SH group released by reduction of SS bonds in the hinge of each constituent. The second derivative is bisFabFc, formed by a bismaleimide in this case joining two FabFc molecules via a free SH in the Fc hinge of each. As regards antibody activity against target cells bisFabFc can be univalent (one active, one inactive Fab arm), bivalent, or bispecific (with each Fab arm directed against a different cell surface antigen). Its juxtaposed dual Fc regions are designed to promote cooperative binding of effectors. Some preliminary characterization in vitro has employed antibodies of anti-idiotypic specificity directed against guinea-pig L2C leukaemic B lymphocytes. The parent mouse IgG1 antibody failed to invoke complement cytotoxicity or antibody-dependent cellular cytotoxicity, while the chimeric derivatives yielded good killing in both systems. In complement lysis bivalent bicFabFc outperformed univalent, which in turn outperformed the FabFc monomer.

Preparation and analysis of bifunctional immunoconjugates containing monoclonal antibodies OKT3 and BABR1

OKT3 and BABR1 [anti-(breast tumour) antibody] were conjugated using N-succinimidyl-3-(2-pyridy]dithio)propionate (SPDP). The procedure employed mild reducing conditions for SPDP-BABR1 and short conjugation incubations at 37 degrees C. The bifunctional immunoconjugates thus produced were isolated by HPLC gel filtration on a preparative TSK 3000 SW column. Both intact IgG and F(ab')2 fragments have been conjugated. The ratio of SPDP to IgG for the optimal yield of dimeric OKT3-BABR1 heteroconjugates was determined to be 2 for OKT3 and 1-2 for BABR1. The OKT3-BABR1 dimers were shown to be bifunctional heteroconjugates by polyacrylamide gel electrophoresis, isoelectric focusing, radial immunodiffusion, and flow cytometry. The binding specificities of the bifunctional heteroconjugates were identical to the specificities of both parent antibodies.

Immunotoxicotherapy: present status and future trends

Immunotoxicotherapy (ITT) is currently used in humans for the treatment of snake venom and cardiac glycoside poisoning. Other toxins have been studied in animals or in vitro to assess their suitability as candidates for detoxification by specific antibodies. Testing conditions are often empirical suggesting that numerous improvements need to be introduced in ITT. Basic mechanisms in ITT include three phases: sequestration, extraction and elimination. The pharmacokinetics of these three phases depend on the type of antidotal binding site (ABS). IgG or its Fab2, Fab or Fv fragment are the possible choices. The Fab fragment is the most frequently used ABS because of its diffusion properties in the peripheral compartments and its renal excretion by glomerular filtration. Toxicokinetic and pharmacokinetic considerations indicate that the dosage cannot be satisfactorily calculated from stoichiometric principles. Study of the toxin dose-lethality curves shows that ABS dosage can be lowered. Moreover, clinical data reveal that some FAb fragments are directly eliminated without acting on toxin molecules. In order to counteract these drawbacks, a compromise between dosage and duration of infusion is suggested. Other improvements will stem from advances in immunologic methodology. Monoclonal and chimeric antibodies are new tools that will help resolve the clinical problems of immunogenicity and adverse reactions associated with polyclonal ABS.

Problems and prospects in the use of lymphoma idiotypes as therapeutic targets

The infusion of anti-idiotypic antibody in patients with lymphoma has proved a relatively innocuous procedure, but in general had yielded only partial, short-lived remissions of disease. A major problem is that xenogeneic antibody is simply not well-fitted to destroying mammalian cells: complement and cellular effectors (K cells and phagocytes) are not efficiently recruited, and the target cells in any case present some excellent defense mechanisms. Antigenic modulation is particularly prominent among these defenses, and we present evidence here for modulation of idiotype being much more efficient in vivo than in vitro. Two broad types of antibody derivative are under development to improve the killing of neoplastic targets. One type relies on recruiting natural effectors, and is exemplified by univalent chimeric antibody. The other relies on delivering an exogenous effector such as a drug or toxin, and is exemplified by bispecific anti-Id/anti-saporin F(ab'gamma)2 antibody. Both types of derivative have been able to suppress animal lymphoma to the extent that tumor escape occurs largely through the emergence of idiotype-negative mutants.

Therapeutic strategies for B cell malignancies involving idiotype-anti-idiotype interactions

The therapeutic use of unmodified monoclonal anti-idiotypic antibody for human B cell malignancies has met with limited success. Some factors thwarting antibody attack can be identified, such as the presence of extracellular idiotypic immunoglobulin (Ig), and the escape of the target cell by antigenic modulation. Another possibility is a change in idiotypic determinants due to somatic mutation. For direct attack on tumor cells in vivo it might be necessary to use antibody derivatives: univalent antibodies will avoid modulation, and chimeric univalent antibodies consisting of mouse Fab' gamma linked to host Ig of appropriate subclass can be engineered to mediate particular effector functions while reducing immunogenicity. Another approach is to use toxin or isotope-bearing antibodies. However, the final eradication of tumor might involve the natural non-specific, and specific anti-idiotypic mechanisms of the host which should not be damaged by antibody therapy, and which appear to be involved in control of tumor progression in patients in long-term remission. Rapidly growing animal lymphomas presently available as models cannot mimic more than a small fraction of human lymphoma but they provide useful information for design of passive anti-idiotype therapy. However host anti-idiotypic immunity must be induced in such models by pre-immunization with purified idiotype. Such a procedure can generate effective anti-idiotypic immunity which is highly protective in mouse and guinea pig lymphomas. Analysis of mechanisms involved should give insight into the role of the idiotype networks in the behavior of human disease.