Gatabolism and Distribution of Gammaglobulin A Preliminary Study with 131I-labelled Gammaglobulin

Potential Sources of Inter-Subject Variability in Monoclonal Antibody Pharmacokinetics

Understanding inter-subject variability in drug pharmacokinetics and pharmacodynamics is important to ensure that all patients attain suitable drug exposure to achieve efficacy and avoid toxicity. Inter-subject variability in the pharmacokinetics of therapeutic monoclonal antibodies (mAbs) is generally moderate to high; however, the factors responsible for the high inter-subject variability have not been comprehensively reviewed. In this review, the extent of inter-subject variability for mAb pharmacokinetics is presented and potential factors contributing to this variability are explored and summarised. Disease status, age, sex, ethnicity, body size, genetic polymorphisms, concomitant medication, co-morbidities, immune status and multiple other patient-specific details have been considered. The inter-subject variability for mAb pharmacokinetics most likely depends on the complex interplay of multiple factors. However, studies aimed at investigating the reasons for the inter-subject variability are sparse. Population pharmacokinetic models and physiologically based pharmacokinetic models are useful tools to identify important covariates, aiding in the understanding of factors contributing to inter-subject variability. Further understanding of inter-subject variability in pharmacokinetics should aid in development of dosing regimens that are more appropriate.

Unusual persistence in healthy volunteers and ill patients of hyperimmune immunoglobulin directed against multiple Pseudomonas O-chain and Klebsiella serotypes after intravenous infusion

Persistence of intravenous (i.v.) hyperimmune immunoglobulin (100 mg/kg) directed against clinically predominant serotypes of Pseudomonas and Klebsiella in ill, febrile patients was compared to healthy volunteers to determine if ill patients have a decreased Ig half-life resulting in an increased immunoglobulin requirement. Type-specific antibodies were measured by ELISA for 83 days in eight healthy volunteers and for 35 days in eight ill patients with surgical complications or hematologic malignancy. Mean values and fold rises of antibody concentrations for the two groups were above preinfusion values at 35 days. The antibody fold rises in patients and in healthy volunteers were similar. Type-specific antibody levels in some patients increased after illness coincident with elevation of total immunoglobulins. We conclude that the duration of potentially therapeutic levels of infused type-specific hyperimmune immunoglobulin may persist for a longer period of time than what has been measured for total immunoglobulin. While the mechanism of this persistence remains to be characterized, the possibility of type-specific antibody synthesis induced by immunoglobulin administration must be considered.

Mechanism and computer simulation of immune complex formation, opsonization, and clearance

A computer simulation of immune complex formation, opsonization, and clearance has been developed (ICMODEL) that uses equations describing the kinetics of known immunologic processes and an additional pathologic process of immune complex-mediated tissue damage and antigen production. ICMODEL was used to (1) compare simulated with reported immune response kinetics, (2) evaluate the relative stability of the immune system described by the simulation, and (3) determine the conditions required to produce high immune complex levels as found in patients with immune complex-mediated disease. ICMODEL simulated primary and secondary immune responses as well as short- and long-term immunity. ICMODEL also depicted a relatively stable immune response system. Under certain conditions, however, the system could be perturbed, resulting in an unstable response. For example, when the rate constant regulating Fc gamma-mediated phagocytosis was decreased and the rate constant regulating immune complex-mediated tissue damage/antigen production was increased, immune complex concentrations oscillated with time and increased exponentially. These data suggest that ICMODEL can be used to define the specific parameters that, when perturbed, will give rise to increased immune complex concentrations. As such, this model has direct implications for studies of immune complex-mediated disease in human patients.

The use of severe combined immunodeficiency mice to study the metabolism of human immunoglobulin G

BACKGROUND

Although the four human immunoglobulin G (IgG) isotypes are similar in structure, they exhibit significant differences in effector function and catabolic half-life. With advent of structurally engineered antibodies, there is the potential to design antibody constructs with desired half-lives; however, it is first necessary to discover the structures and mechanisms that control immunoglobulin metabolism.

METHODS

Radioiodinated chimeric antibodies, consisting of a mouse antidansyl variable region and the four human IgG constant regions, were injected intravenously into Balb/c and severe combined immunodeficiency (SCID) mice, and their half-lives were determined by whole body and whole blood counting. Dependence of the rate of immunoglobulin catabolism on immunoglobulin concentration, a normal regulatory phenomenon specific to IgG, was evaluated by the introduction of large amounts of human gamma-globulin intraperitoneally.

RESULTS

Whole body and blood half-lives were statistically indistinguishable. The four IgG isotypes were eliminated from the whole animals in a predominantly single-phasic manner, with the half-life being dependent on the isotype studied. In Balb/c mice, immune elimination frequently occurred after 6 days, although this was not observed in SCID mice. Relevance of the model was confirmed by the demonstration of the presence of the concentration-catabolism phenomenon, a relationship unique to normal IgG regulation.

CONCLUSIONS

SCID mice provide an adequate initial animal model for the study of human-mouse chimeric antibodies. Further understanding of the factors governing immunoglobulin catabolism can be probed by study of recombinant human constant regions in this animal system.

Influence of barrier-crossing limitations on the amount of macromolecular drug taken up by its target

Macromolecules (substitutive enzymes, polymeric prodrugs, immunotoxins, radiolabeled antibodies, or peptide hormones) are of interest in the treatment of several diseases. To reach the tissues, these macromolecular drugs have to cross the capillary wall, which represents an important transfer limitation. While pharmacokinetics usually studies the changes in drug concentration in different body compartments, analyzing the amount of drug gaining access to its target may be more relevant for assessing the efficiency of macromolecules than for low molecular mass drugs. To determine the influence of different parameters on the fraction of the injected dose gaining access to the pharmacologic target, we constructed pharmacokinetic models where two uptakes, both linear or nonlinear, work either in the same compartment (no transport limitation), or in compartments separated by a transport barrier. Numerical applications were carried out with parameters obtained either experimentally or from the literature. We conclude that it is of little use to increase the affinity (K(uptake)) of a macromolecular drug for its target when a transport limitation and an undesired elimination from the plasma space are both present. Likewise, an increase of the uptake (rate of uptake or maximal velocity) by the target is not very productive because permeability of the capillary wall is the factor limiting access of macromolecules to tissues. Maximal efficiency of therapeutic macromolecules could be achieved by increasing, where feasible, the transport across the barrier between the plasma and the target, and by preventing the undesired eliminations as much as possible.

Immunologic and pharmacologic concepts of monoclonal antibodies

While monoclonal antibodies have solved many of the difficulties of using immunologic reagents for radioimmunodiagnosis and therapy, in the 13 years since their introduction a number of persistent problems remain, most notably a low yield of antibody-producing cells from the fusion process, difficulty in obtaining high-affinity antibodies, and the potential immunogenicity of murine immunoglobulins (Igs). Several solutions are under development, including fusion techniques that enrich for cells producing desired antibodies, production of human-mouse chimeric antibodies by recombinant DNA technology, and the generation of human monoclonal antibodies by promising new approaches. Until these upcoming methodologies are established, and to better direct their development and application, a sound understanding of the pharmacology of presently available native and modified monoclonal antibodies is crucial. Although much has been already determined in this area, a great deal of further clarification remains necessary.

Monozygotic twins discordant for systemic lupus erythematosus

A pair of monozygotic twins discordant for systemic lupus erythematosus(SLE) were studied and no differences noted in their immune respose to tetanus toxoid, keyhole lympet hemocyanin, DNCB, delayed sensitivity, or antibody titers to viruses. Both were noted to have biologically false positive serology at an early age, but only one twon developed SLE. The clinically unaffected twin underwent castration at an early age, suggesting that ovarian hormones may play an important role in the development of SLE.