Improved tumour targeting of di-Fab ' fragments modified with polyethyleneglycol

DFM, a cross-linked di-Fab' version of the murine antibody A5B7 raised agai nst carcinoembryonic antigen (CEA), was chemically modified with 5 kD and 2 5 kD polyethylene glycol (PEG). This led to changes in the pharmacokinetics by significantly increasing the circulating half-life of DFM, nine-fold fo r 5 kD PEG-DFM and thirteen-fold for 25 kD PEG-DFM at 24 h after injection. As a consequence of a longer residence time, tumour uptake levels were als o increased, particularly of 5 kD PEG-DFM, producing 2-2.5-fold significant increases in % injected doses (%id/g) to the tumour at 24 h, 48 h and 144 h. Tumour uptake levels for 25 kD PEG-DFM were similar to DFM alone between 3 h and 48 h; however, by 144 h the %id/g for 25 kD PEG-DFM was doubled. T here was a 40% decrease in the immunoreactivity of 25 kD PEG-DFM, compared to native DFM, whereas there was no loss in activity of 5 kD PEG-DFM, The l arge increase in circulating activity of (125)iodine labelled 25 kD PEG-DFM led to a 23.8-fold increase in absorbed doses to normal tissues; however, th is was not accompanied by a similar increase in tumour dose, which resul ted in lower tumour to blood ratios. Therefore 25 kD PEG-DFM was not consid ered for radioimmunotherapy in this study. However, 5 kD PEG-DFM showed mor e promise as a therapeutic agent, producing significantly higher tumour lev els and a relatively similar therapeutic response to unmodified DFM, althou gh the MTD for both therapeutic agents was not reached. In addition PEGylat ion of A5B7 DFM reduced the high absorbed radiation dose to the kidneys whi ch was found in a recent clinical study. DFM and therefore 5 kD PEG-DFM app ear to be promising for clinical development.