Optimizing therapeutic strategies to inhibit circulating soluble target molecules with monoclonal antibodies: example of the soluble IL‐6 receptors

Therapeutic targeting of soluble molecules such as cytokines can be achieved with monoclonal antibodies (mAb). Anti‐IL‐6 mAb have been shown to form circulating complexes, resulting in the increase of the half‐life of the cytokine in vivo. In IL‐6‐related diseases, the soluble human IL‐6 receptors (shIL‐6R), which have been shown to possess strong agonist activity, circulate in the plasma at a high concentration and must be neutralized. Their clearance was studied in mice that had been made to express circulating shIL‐6R after i.p. grafting of mouse thymoma cells transfected with a gene coding for shIL‐6R, treated with various anti‐shIL‐6R mAb recognizing different epitopes of the molecule. Injection of one anti‐hIL‐6R mAb stabilized the short‐lived hIL‐6R and led to their accumulation. The same result was observed when two mAb directed against two different epitopes of the hIL‐6R were used. Clearance of the receptors was only achieved when three mAb specific for three different epitopes were injected. A permanent clearing of the hIL‐6R could be obtained by repeated injections of the clearing mixture. No correlation was found between the ability of the mAb to clear the sIL‐6R and to immunoprecipitate them in agarose gel. The F(ab')2 fragments lost the clearing ability of the intact mAb. These results clearly show that therapeutic clearance of sIL‐6R by mAb need at least three mAb directed against three different epitopes of the molecule, a conclusion which is likely to apply for clearing any soluble target molecule.