THE LOADING RATE DETERMINES TUMOR TARGETING PROPERTIES OF METHOTREXATE-ALBUMIN CONJUGATES IN RATS

Albumin dominates the plasma proteins in man. Following our observatio n that albumin turnover in rodent tumors is markedly increased, we wil l present evidence that albumin can be employed as an efficient carrie r for targeting cytostatic agents like methotrexate (MTX) into tumors. The considerable discrepancy in the molecular weight of MTX (454 Da) and albumin (about 67 000 Da) tempted researchers to load multiple dru g molecules on one carrier molecule. It was supposed that the optimal therapeutic efficacy of MTX protein conjugates could be achieved by in creasing the number of the molecules of MTX attached to the carrier, i n this paper we will show that only loading rates of close to 1 mol of the cytostatic drug MTX/mol of albumin offer optimal conditions for t argeting MTX-albumin conjugates into rodent tumors. Conjugates bearing 5, 7, 10 and 20 molecules of MTX on average showed considerable alter ations in the HPLC profiles of the conjugates compared to albumin. Con jugates carrying 5-20 mol MTX, tagged with a residualizing radiolabel, were efficiently trapped by the liver before reaching the tumor. The tumor uptake rates of these conjugates declined dramatically with an i ncreasing molecular load of the cytotoxic drug linked to albumin. Comp etition experiments with maleylated bovine serum albumin and fucoidan revealed that scavenger receptors present on the cells of the liver mo nocyte macrophage system were involved in this process. For further pr eclinical and clinical studies, we chose MTX-albumin conjugates, deriv atized at a molar ratio of 1:1. These conjugates enjoy the same favora ble tumor targeting properties like albumin, e.g. high tumor uptake ra tes, low liver uptake rates and a very long biological half-life.