In vivo stability and metabolism of 99Tcm-labelled anti-CD4 monoclonal antibodies and Fab' fragments.

In a comparative study, the in vivo metabolic profile of 99Tcm-anti-CD4 monoclonal antibodies (MAb) and Fab' fragments (direct thiol-reduction method) was investigated in rats with adjuvant arthritis. Plasma samples were obtained 1, 4 and 15 h following intravenous injection and urine was collected over 15 h. The composition of the radiolabelled molecules was analysed by gel filtration chromatography. The profile of the complete MAb was as follows. (1) Serum: a predominant peak of 150 kDa (complete MAb; approximately 80%); three minor peaks of 125, 100 and 50 kDa (presumably H2L1, H2 and H1 chains; 14%); and two small peaks of approximately 1500 and 300 Da (presumably 99Tcm-glutathione and Cys-99Tcm-Cys; 5%). (2) Urine: minor peaks of 125, 100 and 50 kDa (15%), major peaks of 1500 and 300 Da (50%), and an additional 25-kDa peak (30%). The profile of the Fab' was as follows. (1) Serum: three minor peaks of 150 kDa [conceivably F(ab')2 plus soluble-CD4], 110 kDa [F(ab')2] and 95 kDa (Fab' plus soluble-CD4; together 30%); a predominant peak of 55 kDa (Fab'; 61%); and two minor peaks of 1500 and 300 Da (8%). (2) Urine: predominantly a 55 kDa (16%) and 1500 and 300 Da peaks (74%). Also, the anti-CD4 Fab' showed higher urine excretion and lower plasma levels than the complete anti-CD4 MAb, but still more favourable tissue-to-blood ratios following scintigraphy of the arthritic joints. Thus, the complex metabolic profile of the anti-CD4 Fab' does not appear to interfere with specific targeting of the inflamed synovial membrane.