A
B16 melanoma line was repeatedly transplanted subcutaneously in C57BL/6 mice. On day 4 after every transplant, the animals were treated with
doxorubicin (DXR), 10 mg/kg i.p. The aim of the work was to develop an in-vivo model of resistance to the antiblastic in order to analyze some possible mechanistic aspects of the process in the course of time. After 16 transplants and treatments the
melanoma completely lost its sensitivity to the antiproliferative effects of maximal tolerated doses of DXR and showed over-expression of
P-glycoprotein. Compared to the parental line, the in vitro resistance index was 4.6. After 27 transplants and treatments the
melanoma did not increase its in vitro resistance to DXR further, and this resistance was completely reversed by
verapamil. The behavior of the
antioxidant defenses (
superoxide dismutase,
catalase,
glutathione peroxidase,
glutathione transferase,
glutathione reductase and
glutathione) was evaluated after 4, 16 and 27 transplants and treatments with DXR. At no stage did the treated
melanoma show any variation in the
antioxidant enzymes. Compared to the parental counterpart its
glutathione levels were elevated after four treatments (+80%), when, however, the line was still sensitive to the in vivo effects of DXR, and after 16 treatments (+30%). Instead, no variation of the
glutathione content was seen after 27 treatments with DXR. These results seem to exclude the possibility that the
antioxidant defenses play a major role in the resistance of this
B16 melanoma line to DXR. On the other hand, the low but, however, 'clinically' significant resistance of the
tumor to the antiblastic seems mainly related to the mechanisms linked to the
P-glycoprotein over-expression.