Development of human anti‐murine antibody (HAMA) response in patients

Human anti-mouse antibody (HAMA) response was determined in the serum of 67 patients who received subcutaneously administered radiolabelled murine monoclonal antibodies (MoAb) (50 micrograms-3 mg) for immunolymphoscintigraphy and of 10 patients with advanced colorectal cancer who received murine MoAb-N-acetyl melphalan (MoAb-N-AcMEL) conjugates (amount of MoAb ranged from 120 mg/m2 body surface area to 1000 mg/m2 body surface area) as therapy. A pre-existing low level of apparent human anti-mouse antibody reactivity could be detected in the serum of normal subjects and patients prior to administration of murine MoAb. Subcutaneous administration of low doses of murine MoAb, as used in immunolymphoscintigraphy, was associated with a low incidence (4/67 or 6%) of elevated HAMA response; the use of F(ab')2 fragments was associated with the development of elevated HAMA response in one of three patients. By contrast, therapy with hepatic artery infusion of murine MoAb-N-AcMEL conjugates in three repetitive daily doses (each infusion lasting 2 h) elicited elevated HAMA responses in 10/10 (100%) patients, usually 1-3 weeks after the start of therapy. The HAMA response of patients in the therapy group was higher than those in the immunolymphoscintigraphy study and the use of steroids did not prevent the development of the HAMA response. Further administration of MoAb-N-AcMEL conjugates to a patient, who had already developed HAMA, led to 'serum sickness'-type symptoms and a transient reduction in the HAMA titres. The elevated HAMA response was polyclonal, containing increased levels of both immunoglobulin M and G (IgM and IgG) and was directed against mouse-specific determinant, the isotype (presumed to be the Fc portion), the F(ab')2 and the 'idiotype' of mouse immunoglobulins.