RAPID ELIMINATION OF MOUSE HUMAN CHIMERIC MONOCLONAL-ANTIBODIES IN NUDE-MICE/

At our laboratory we are currently evaluating the suitability of mouse /human chimeric monoclonal antibodies (cmAb) for use in radioimmunothe rapy of patients with head and neck squamous cell carcinoma (HNSCC). W e have developed cmAb containing the human constant IgG1 domain and th e variable domains of murine mAb (mmAb) E48 and U36 respectively. We c onsidered the tumour-bearing nude mouse to be a well-validated model f or a first testing of the targeting capabilities of these cmAb in comp arison with the mmAb. Therefore, 3 mu g cmAb E48 (labelled with I-125) and 3 mu g mmAb E48 (labelled with I-131) were simultaneously injecte d into HNSCC-bearing nude mice and, at various assay times, mAb uptake in blood and other tissues was assessed. Remarkably, while in roughly 50% of the animals the biodistribution of the conjugates was similar, in the other animals cmAb E48 showed a much higher blood clearance th an mmAb E48. This resulted in a lower tumour uptake of cmAb E48 in com parison with mmAb E48. To determine whether this phenomenon was relate d to mAb E48 or to the animal model, other cmAb-mmAb combinations were evaluated in the same way: cmAbs SF-25, 17-1A and U36 (all IgG1) were tested and all showed a rapid elimination in about 50% of the animals . Besides a decrease in blood concentration, an increase of cmAb level s in liver and spleen was observed within 24 h after injection. Isotyp e-specific enzyme-linked immunosorbent assays showed that mice that de monstrated a rapid elimination of cmAb from the blood had much lower e ndogenous IgG1, IgG2b and IgG3 titres than mice showing normal clearan ce. IgG2a levels were low in all mice. Biodistribution experiments wit h 3 mu g chimeric 17-1A isoforms showed high blood clearance in a prop ortion of the mice for IgG1, IgG3 and IgG4, but not for IgG2. Increase of the cmAb dose to 100 mu g resulted in a similar cmAb and mmAb biod istribution in all mice. Moreover, the biodistribution of the F(ab')(2 ) fragment of an IgG1 cmAb was similar for all mice in contrast to tha t of coinjected whole IgG. On the basis of these results it can be hyp othesized that, in mice with low endogenous IgG titres, cmAb with spec ific isotypes are rapidly removed from the blood (and ultimately from the body) by mediation of Fc-binding receptors. Apparently, in mice wi th high endogenous IgG titres or in mice receiving a high cmAb dose, t hese receptors are saturated. Furthermore, the rapid elimination of cm Ab from nude mice, which may occur after injection at a low dose, is a phenomenon related to the nude mouse model.