Pharmacokinetic analysis of the microscopic distribution of enzyme-conjugated antibodies and prodrugs: comparison with experimental data.

A mathematical model was developed to improve understanding of the biodistribution and microscopic profiles of drugs and prodrugs in a system using enzyme-conjugated antibodies as part of a two-step method for cancer treatment. The use of monoclonal antibodies alone may lead to heterogeneous uptake within the tumour tissue; the use of a second, low molecular weight agent may provide greater penetration into tumour tissue. This mathematical model was used to describe concentration profiles surrounding individual blood vessels within a tumour. From these profiles the area under the curve and specificity ratios were determined. By integrating these results spatially, average tissue concentrations were determined and compared with experimental results from three different systems in the literature; two using murine antibodies and one using humanised fusion proteins. The maximum enzyme conversion rate (Vmax) and the residual antibody concentration in the plasma and normal tissue were seen to be key determinants of drug concentration and drug-prodrug ratios in the tumour and other organs. Thus, longer time delays between the two injections, clearing the antibody from the blood stream and the use of 'weaker' enzymes (lower Vmax) will be important factors in improving this prodrug approach. Of these, the model found the effective clearance of the antibody outside of the tumour to be the most effective. The use of enzyme-conjugated antibodies may offer the following advantages over the bifunctional antibody-hapten system: (i) more uniform distribution of the active agent; (ii) higher concentrations possible for the active agent; and (iii) greater specificity (therapeutic index).