Internalization and subcellular fate of free
doxorubicin or its polymeric conjugates based on poly
N-(2-hydroxypropyl)methacrylamide (pHPMA), either non-targeted or targeted with anti-Thy1.2 or anti-CD71
monoclonal antibody was tested on EL-4 mouse
T-cell lymphoma, SW620 human
colorectal carcinoma and OVCAR-3 human ovarian
adenocarcinoma.
Doxorubicin fluorescence allowed us to follow the internalization and intracellular distribution of tested conjugates by
laser scanning confocal microscopy and/or by fluorescent microscopy. Whereas free
doxorubicin was always detectable only in the nuclei of treated cells, detectable fluorescence of
doxorubicin bound to a polymeric carrier, targeted or non-targeted, was detectable up to 3 days of incubation only in the cytoplasmatic structures. While free
doxorubicin causes apoptosis in the populations of tested
cancer cell lines, significant number of apoptotic cells was never found in cell cultures exposed to targeted or non-targeted polymeric conjugates. In contrast to free
doxorubicin, which is a strong inducer of p53 expression, increased p53 expression was never observed after the treatment with the polymeric
drug. High-performance liquid chromatographic analysis shows that the percentage of cleaved
doxorubicin is very low even after 48 h of incubation of tested cells with the polymeric conjugate, and cannot be the only reason for the toxicity of the conjugate. We suggest that: (a) after the treatment with pHPMA-bound
drug, the cells die by
necrosis and (b) the toxicity of pHPMA-based conjugates is a combination of the toxic effect of released
doxorubicin and the toxic effect of
doxorubicin in
polymer-bound form directed against cell membranes.