Animal-to-Human Dose Translation of Obiltoxaximab for Treatment of Inhalational Anthrax Under the US FDA Animal Rule.

Obiltoxaximab, a monoclonal antibody against protective antigen (PA), is approved for treatment of inhalational anthrax under the US Food and Drug Administration's (FDA) Animal Rule. The human dose was selected and justified by comparing observed obiltoxaximab exposures in healthy and infected New Zealand White rabbits and cynomolgus macaques to observed exposures in healthy humans, to simulated exposures in healthy and infected humans, and to serum PA levels in infected animals. In humans, at 16 mg/kg intravenous, obiltoxaximab AUC was >2 times that in animals, while maximum serum concentrations were comparable to those in animals and were maintained in excess of the concentration required for PA neutralization in infected animals for 2-3 weeks. Obiltoxaximab 16 mg/kg in humans provided exposure beyond that of 16 mg/kg in animals, ensuring a sufficient duration of PA neutralization to allow for adaptive immunity development. Our approach to dose translation may be applicable to other agents being developed under the Animal Rule.

A transgenic mouse model for studying the clearance of blood-borne pathogens via human complement receptor 1 (CR1).

Complement receptor 1 (CR1) on the surface of human erythrocytes facilitates intravascular clearance of complement-opsonized pathogens. The need for complement activation can be circumvented by directly coupling the organism to CR1 using a bispecific monoclonal antibody heteropolymer (HP). Lack of a functional homologue to CR1 on mouse erythrocytes has made it difficult to study HP-dependent clearance of pathogens in small animals. We have developed a transgenic mouse that expresses human CR1 on erythrocytes. CR1 antigen is of appropriate size and in a clustered distribution as confirmed by immunoblotting and fluorescence microscopy, respectively. HP that immobilized bacteriophage PhiX174 prototype pathogen to erythrocyte CR1 of the transgenic mice increased the rate of clearance of the virus compared with HP that bound bacteriophage, but not CR1. This transgenic mouse model will allow evaluation of different HPs for their in vivo efficacy and potential as human therapeutics.

IL-4 induces IL-2 receptor p75 beta-chain gene expression and IL-2-dependent proliferation in mouse T lymphocytes.

T and B cells exhibit complex responses to the combination of IL-2 and IL-4, each of which can act as a growth or differentiation factor for lymphocytes under certain circumstances. To characterize better the mechanism by which these cytokines interact, mRNA levels of the signal-transducing p75 beta-chain of the IL-2R were analyzed. These studies show that IL-4 increases expression of the IL-2R beta-chain in mouse splenic B and T cells, and the response of B cells was potentiated by concurrent cross-linking of surface Ig. Kinetic analysis of the IL-2R beta response showed a slow onset but maintenance of peak levels of expression between 10 and 24 h. These data indicate that the pathways involved in the lymphocyte response to IL-4 differ for IL-2R and IL-4R, and that the induction of IL-4R precedes the increase in IL-2R. The effect of IL-4 on IL-2R beta mRNA levels was mediated in part by an increase in the rate of gene transcription, and was associated with increased IL-2 binding in the absence of any change in IL-2R alpha levels. In addition, IL-4 increased the level of IL-2R beta expression in thymocytes. Proliferation assays demonstrated that pretreatment of splenic T cells with IL-4 led to a substantial increase in IL-2-dependent proliferation. These results are consistent with a mechanism by which IL-4 can prime T cells and certain thymocytes for responsiveness to IL-2 by increasing IL-2R p75 chain gene expression, independent of general T cell activation.

Mechanism of interleukin-2 signaling: mediation of different outcomes by a single receptor and transduction pathway.

The T cell lymphokine, interleukin-2 (IL-2), plays a pivotal role in an immune response by stimulating antigen-activated B lymphocytes to progress through the cell cycle and to differentiate into antibody-secreting cells. An IL-2 inducible B lymphoma line, in which the growth and differentiation responses are uncoupled, provides a model system for dissecting the signaling mechanisms operating in each response. This system was used to show that both signals are initiated by IL-2 binding to a single, unifunctional receptor complex. Moreover, both signals are transduced by a pathway that does not involve any known second messenger system and that can be blocked by a second T cell lymphokine, interleukin 4. These findings suggest that the pleiotrophic effects of IL-2 are determined by different translations of the signal in the nucleus.