RESUMO
In a retrospective study we analysed the levels of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) in melanoma metastases in patients receiving dacarbazine (DTIC) either as a single drug or as part of combination chemotherapy regimens, and related the expression levels to the clinical response to treatment. Biopsies of subcutaneous and lymph node metastases obtained before chemotherapy in 65 patients with disseminated malignant melanoma were examined for MGMT protein levels by immunohistochemistry using a monoclonal anti-human MGMT antibody. All patients received chemotherapy with DTIC, given either as a single drug or in combination with vindesine and in some cases cisplatin. DTIC as single agent was given to 44 patients, while 21 received combination chemotherapy. Objective responses to chemotherapy were seen in 12 patients, while 53 patients failed to respond to treatment. The expression of MGMT was determined according to the proportion of antibody-stained tumour cells, using a cut-off level of 50%. In 12 of the patients more than one metastasis was analysed, and in seven of these cases the MGMT expression differed between tumours in the same individual. Among the responders a larger proportion (six out of 12, 50%) had tumours containing less than 50% MGMT-positive tumour cells than among the non-responders (12 out of 53, 23%). These data are consistent with the hypothesis that MGMT contributes to resistance to DTIC-based treatment, although the difference between responders and non-responders with respect to the proportion of MGMT-positive tumour cells was not statistically significant (P = 0.077).
Assuntos
Antineoplásicos Alquilantes/uso terapêutico , Dacarbazina/uso terapêutico , Melanoma/enzimologia , Proteínas de Neoplasias/análise , O(6)-Metilguanina-DNA Metiltransferase/análise , Pró-Fármacos/uso terapêutico , Adulto , Idoso , Anticorpos Monoclonais/imunologia , Antineoplásicos Alquilantes/farmacologia , Protocolos de Quimioterapia Combinada Antineoplásica/uso terapêutico , Cisplatino/administração & dosagem , Dano ao DNA , Metilação de DNA , Reparo do DNA , DNA de Neoplasias/efeitos dos fármacos , DNA de Neoplasias/metabolismo , Dacarbazina/administração & dosagem , Dacarbazina/farmacologia , Resistencia a Medicamentos Antineoplásicos , Feminino , Humanos , Técnicas Imunoenzimáticas , Metástase Linfática , Masculino , Melanoma/tratamento farmacológico , Pessoa de Meia-Idade , Proteínas de Neoplasias/imunologia , Proteínas de Neoplasias/fisiologia , O(6)-Metilguanina-DNA Metiltransferase/imunologia , O(6)-Metilguanina-DNA Metiltransferase/fisiologia , Variações Dependentes do Observador , Pró-Fármacos/administração & dosagem , Vindesina/administração & dosagemRESUMO
O6-methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein, which removes alkyl groups from the O6 atom of guanine residues. Tumour cells which lack MGMT are sensitive to cytostatic drugs such as dacarbazine (DTIC), whose active species bind to this site. To explore whether analyses of MGMT expression can be used as a predictive test for clinical sensitivity to DTIC in melanomas, we developed a method to assay MGMT mRNA levels in cells obtained by fine needle aspiration biopsies of metastases. cDNA was synthesised from mRNA prepared from biopsy material. Polymerase chain reaction was performed using primers complementary to MGMT cDNA and to beta-actin, which served as an internal control. Analyses of 44 biopsies from 35 patients showed a considerable variation in MGMT mRNA, with 15 samples (34%) lacking detectable mRNA. In 6 out of 8 patients in whom more than one tumour was analysed, separate metastases had different levels of MGMT mRNA. There was no correlation between MGTM activity studied by a biochemical assay and MGMT mRNA levels when these were compared in 10 surgical biopsies.
