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To study the fate of a low molecular weight antigen (hapten) in the circulation of animals whose sera contain antibodies specific for that low molecular weight antigen, a single injection of digoxin-(3)H (0.4 mg/kg) was administered intravenously to 18 rabbits. Thirteen animals (nine nonimmunized and four immunized with bovine serum albumin) served as control animals. In five rabbits which had been immunized with a digoxin-bovine serum albumin conjugate and whose sera contained digoxin-specific antibodies, the mean 12-h serum digoxin concentration was 8,300 ng/ml (control: 92 ng/ml) and the mean serum concentration 12 mo after the single injection of digoxin-(3)H was 85 ng/ml. In digoxin-immunized rabbits, less than 10% of the digoxin-(3)H was excreted in the first 10 days (control: 77% recovered in urine and feces) and the mean biological half-life of digoxin, as calculated from serum digoxin-(3)H disappearance curves, was 72 days (control: 3.4 days). In sera of digoxin-immunized rabbits, more than 90% of the circulating digoxin-(3)H was immunoglobulin bound, as determined by the double-antibody and dextran-coated charcoal methods. The serum disappearance rate of (125)I-antidigoxin antibodies was similar in nonimmunized and in immunized animals and in the presence or absence of digoxin. It is concluded that the biological half-life of a hapten may be markedly prolonged when the hapten is bound to specific antibody. The persistence of antibody-hapten complexes in the circulation suggests that these complexes may not be deposited in tissues and raises the possibility that low molecular weight determinants may be capable of preventing or reversing the deposition of immune complexes, containing macromolecular antigens, in the tissues of experimental animals and man.

Single-chain antigen-binding proteins are novel recombinant polypeptides, composed of an antibody variable light-chain amino acid sequence (VL) tethered to a variable heavy-chain sequence (VH) by a designed peptide that links the carboxyl terminus of the VL sequence to the amino terminus of the VH sequence. These proteins have the same specificities and affinities for their antigens as the monoclonal antibodies whose VL and VH sequences were used to construct the recombinant genes that were expressed in Escherichia coli. Three of these proteins, one derived from the sequence for a monoclonal antibody to growth hormone and two derived from the sequences of two different monoclonal antibodies to fluorescein, were designed, constructed, synthesized, purified, and assayed. These proteins are expected to have significant advantages over monoclonal antibodies in a number of applications.