O6-methylguanine-DNA methyltransferase in pretreatment tumour biopsies as a predictor of response to temozolomide in melanoma

Resistance of tumour cells to methylating and monochloroethylating agents in vitro and in vivo has been linked to levels of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). In a clinical trial of temozolomide in advanced malignant melanoma, the relationship between pretreatment MGMT levels in biopsies of cutaneous tumours and involved lymph nodes and clinical response to the drug has been studied. Among 50 evaluable patients, there were three complete responses (CR), four partial responses (PR), six with stable disease (SD) and 37 with progressive disease (PD), with an overall response rate of 14%. In 33 patients in whom MGMT level and clinical response could be evaluated, the tumour MGMT levels (fmol mg(-1) protein) were: CR, 158 +/- 119; PR, 607 +/- 481; NC, 171 +/- 101; PD, 185 +/- 42.3. Thus, measurements of pretreatment levels of MGMT in melanoma did not predict for response to temozolomide.

Cytotoxicity of 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MTIC) on Mer+, Mer+Rem- and Mer- cell lines: differential potentiation by 3-acetamidobenzamide

Mechanisms of resistance to the active metabolite 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MTIC) of the drug 5-(3,3-dimethyl-1-triázeno)imidazole-4-carboxamide (DTIC) were studied in three human cell lines with differing amounts of the repair enzyme O6-alkylguanine-DNA alkyltransferase (O6AT). The lines were HT29 (Mer+Rem+), A549 (Mer+Rem-) and VA13 (Mer-). The ability to repair O6 methyl-guanine was directly related to resistance to MTIC (HT29 ID50 650 mumol l-1, A549 ID50 210 mumol l-1, VA13 ID50 15 mumol l-1. MTIC produced DNA single strand breaks over the range of one log of cell kill, but depletion of cellular NAD levels could not be detected until there was greater than 95% cell kill. Inhibitors of the repair enzyme adenosine diphosphoribosyl transferase (ADPRT) potentiated killing by 2-fold in the Mer+ cell lines but not the Mer- line. The enhancement was directly proportional to an increase in DNA strand breaks but not a change in their half-life. Therefore resistance to the clinically used methylating agent MTIC can be partly overcome by inhibiting ADPRT but a role for ADPRT as a suicide mechanism in response to alkylating agent damage is unlikely.

Invasive properties of histone transformed cells

A technique was developed to study the invasion of cells into mouse kidney cortex in the presence of crude rat liver histone at a medium concentration of 100 μg./ml. A marked increase in the invasiveness of normal cells occurred in the presence of histone. Possible explanations of this phenomenon are discussed. The invasiveness was compared with that of cells previously transformed with polyoma virus.