The distribution of porphyrins with different tumour localising ability among human plasma proteins.

The distribution among the main fractions of human plasma lipoproteins of a number of porphyrins with different tumour localising ability has been determined by means of ultracentrifugation. A main trend is that the fraction of the dyes that are bound to low density lipoprotein (LDL) increases, and the fraction bound to HSA decreases with decreasing polarity of the dyes. An asymmetric charge distribution, such as in TPPS2a, favours LDL-binding more than expected on the basis of lipophilicity. No correlation between the known tumour localising ability of the drugs tested in the present work and their relative affinity for LDL was found. One of the best tumour localisers reported in the literature, TPPS4, hardly binds to LDL, while Hp and Pp, which are commonly considered inefficient tumour localisers, do have a significant affinity for LDL. On the other hand, the LDL binding capacity for a drug is suggested to be a good index for cellular uptake. Such an index does not necessarily imply that the actual uptake occurs by the LDL pathway.