Antibodies as models for rational drug design

Modification of methyliminodiacetato-trans-R,R-1,2-diamminocyclohexane platinum(II) pharmacology using a platinum-specific monoclonal antibody

Platinum complexes are extremely active chemotherapeutic agents. A murine monoclonal antibody designated 1C1 was developed that binds to the third-generation platinum complex methyliminodiacetato-trans-R,R-1,2-diamminocyclohexane platinum(II) (MIDP). Competitive enzyme-linked immunosorbent assay (ELISA) shows that antibody 1C1 binds preferentially to the 1,2-diamminocyclohexane (DACH) side-chain of the platinum complex, although non-DACH-containing platinum complexes can compete for binding at high concentrations. When tested against MCF-7 breast carcinoma cells, the 1C1-MIDP complex caused 50% growth inhibition at 0.63 micrograms Pt/ml, whereas MIDP alone caused 50% growth inhibition at a concentration of 0.16 micrograms Pt/ml. Pharmacokinetic studies in rats using [3H]-MIDP showed that the drug was cleared triphasically from plasma, with elimination-phase half-lives (t1/2) of 1.2, 10.2, and 243 min for alpha, beta, and gamma phases, respectively. The MIDP-1C1 complex was cleared with longer half-lives of 5, 26, and 291 min, respectively. The overall clearance rate from plasma of the MIDP-1C1 complex was 10-fold lower than that of MIDP alone (0.37 vs 3.01 ml/kg x min). Tissue concentrations of [3H]-MIDP 3 h after administration showed that 1C1 antibody prevented MIDP distribution to most organs and dramatically reduced [3H] concentration in the intestine, liver, kidney, heart, and skeletal muscles. Studies are under way to determine the relative therapeutic activity of the 1C1 antibody-MIDP complex and assess whether the 1C1 antibody may be useful for antibody-directed delivery of platinum complexes to tumors.

Effects of alpha-1-acid glycoprotein administration on propranolol binding and beta blockade in rats

Alpha-1-acid glycoprotein (AAG), 750 mg/kg, was administered to rats to determine its effect on propranolol binding and beta blockade. Anesthetized rats received [3H]propranolol i.v., followed in 15 min by human AAG or bovine serum albumin, 750 mg/kg. AAG treatment produced a human AAG concentration in serum of 7.76 +/- 1.17 mg/ml, several times higher than the endogenous serum AAG concentration in stressed rats. AAG treatment significantly increased the heart rate response to isoproterenol, compared to albumin (95.4 +/- 19.6 vs 28.3 +/- 16.7% of baseline value, measured 45 min after propranolol, P less than 0.001). AAG-treated rats had greater [3H]propranolol binding in serum (93.0 +/- 3.2 vs 76.7 +/- 3.0%, P less than 0.01) and a lower calculated unbound [3H]propranolol concentration in serum (2.7 +/- 1.3 vs 7.4 +/- 3.1 X 10(6) dpm/ml, P less than 0.001) than albumin-treated rats. These data demonstrate that AAG can alter propranolol pharmacokinetics and pharmacodynamics even when administered after the propranolol effect is evident. Because the reported affinity of propranolol for cardiac beta receptors is 10,000 times greater than its affinity for AAG, these data suggest that AAG acted by altering propranolol disposition rather than by directly competing with beta receptors for drug.

Production and characterization of anti-clonidine antibodies not cross-reacting with catecholamines

Polyclonal antibodies against clonidine were developed, with para-aminoclonidine coupled to bovine serumalbumin or hemocyanine with glutaraldehyde used as antigens. The selected antibody (from rabbits) cross-reacted with high specificity with clonidine and its structurally closely related analogues but it recognized neither catecholamines nor various endogenous imidazole molecules such as histamine, purine, adenine, and adenosine, thus appearing to be specific for the aminoimidazoline structure. An interesting cross-reactivity was observed with the bovine clonidine displacing substance, the probable endogenous ligand for receptors involved in the hypotensive effect of clonidine-type substances. This suggested that this molecule should contain an aminoimidazoline or guanidine moiety.

Attempts to obtain anti(D2 dopamine receptor) antibodies via the anti-idiotypic route

Five stable hybridomas have been obtained that secrete monoclonal antibodies against the D2-dopamine receptor-selective drug spiperone. Each monoclonal antibody has been characterized in terms of its ability to bind a range of dopamine-receptor-selective ligands. One monoclonal antibody has been purified by Protein A affinity chromatography and used to immunize mice. Anti-idiotypic antisera and one hybridoma secreting an anti-idiotypic monoclonal antibody were obtained and shown to inhibit [3H]spiperone binding to the anti-spiperone antibody used for immunization. Neither the antisera nor the anti-idiotypic monoclonal antibody, however, inhibited binding of [3H]spiperone to D2-dopamine receptors.

Immunotherapy in the poisoned patient. Overview of present applications and future trends

Immunotherapy for reversal of toxicity due to poisons and drugs is not new. However, refinements in antibody isolation and purification as well as the advancement of hybridoma technology and recombinant DNA biotechnology has led to a new generation of immunotherapeutic and diagnostic agents. The advent of monoclonal antibody technology in 1975 heralded the new age of immunopharmacology and immunotoxicology. Monoclonal antibodies designed for a specific antigen resolved the problem of polyclonality and cross-reactivity of traditional antibodies. Along with the production and isolation of active antibody fragments from both polyclonal and human monoclonal sources, as well as the ability to tailor-make chimeric antibodies by recombinant biotechnology, the development of novel immunotherapeutic agents has taken place. Two immunotherapeutic modalities, digoxin-specific antibody fragments (Fab) and snake antivenin, have been available for the clinician's armamentarium for years. Along the same lines of anti-digoxin Fab development, application of newer antibody isolation technology has led to a purified IgG(T) antibody for snake venom poisoning which is still in the developmental stages. Potential future developments in immunotherapeutics must overcome the clinical problems of immunogenicity and adverse reactions to the antibodies. Human monoclonal sources, active antibody fragments, and chimeric antibodies from transfectomas are all potential resolutions to these problems.