P-glycoprotein expression in primary and metastatic malignant melanoma.

Metastatic malignant melanoma is notoriously resistant to chemotherapeutic agents, but the exact mechanisms involved in this drug resistance are unknown. One recently defined major mechanism of multidrug resistance involves the overexpression of P-glycoprotein on cell membranes. In order to evaluate the significance of this putative drug efflux pump for chemoresistance of malignant melanoma, five different antibodies were employed to examine P-glycoprotein expression on tissue from 33 primary malignant melanomas and 35 metastases, before and after chemotherapy, using immunohistological techniques. The expression of P-glycoprotein was low on primary cutaneous melanomas (three of 33), and on metastases (one of 35). Normal tissue in and around the melanoma showed reactivity of endothelial cells, stromal cells and eccrine sweat glands with several antibodies tested. Chemotherapy with drugs commonly used in metastatic melanoma, including agents known to induce P-glycoprotein expression in other tumours (vindesine, cisplatin) had no effect on P-glycoprotein expression in human melanoma metastases. The high chemoresistance of human melanoma cells in vitro and in vivo is probably not mediated via P-glycoprotein, and other possible mechanisms involved will have to be explored in future studies.