Immunohistochemical examination of the expression of O6-methylguanine-DNA methyltransferase in human melanoma metastases.

In tumour cell lines, an inverse relationship has been shown between susceptibility to the cytotoxic effects of the O6-alkylating agents and the expression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). One of the most effective single agents in chemotherapy of metastatic melanoma is the O6-alkylating drug, dacarbazine. We therefore examined the distribution of MGMT in 37 skin and lymph node melanoma metastases using rabbit antihuman MGMT antiserum. MGMT expression was undetectable in tumours from 2 out of 34 patients and low in 4 further patients. When present, staining was mainly nuclear and showed a marked variation both among tumour cells within the same metastases, between separate metastases in the same patient and between tumours in different patients. MGMT expression determined by immunohistochemistry showed a relation to MGMT activity measurements, but was not related to the number of proliferating cells, as identified by staining with MIB-1 antibody. Tumour cells with moderate to strong immunostaining with MGMT antiserum were significantly more abundant in metastases excised after dacarbazine-based chemotherapy (n = 8) than in those excised before treatment (n = 29).