In vivo studies with the novel anticancer agent mitozolomide (NSC 353451) on Lewis lung carcinoma

Depletion of O6-alkylguanine-DNA alkyltransferase correlates with potentiation of temozolomide and CCNU toxicity in human tumour cells

Temozolomide (8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4-(3H)-one) has shown promising activity in Phase I trials against some brain (glioma) and skin (melanoma, mycosis fungoides) cancers. Temozolomide and lomustine (CCNU) showed parallel toxicity in seven human tumour cell lines and this generally correlated (correlation coefficients 0.87 and 0.92 respectively) with the level of expression of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase, EC 2.1.1.63). Pretreating cells with the ATase inhibitor, O6-benzylguanine (BG), potentiated cytotoxicity to a similar degree with both drugs, but did not sensitise a cell line (ZR-75-1) expressing very low levels of this protein. When BG pretreatment was combined with repeat doses of temozolomide a dramatic potentiation (300 fold) was seen in MAWI cells, which express high levels of ATase, but not in a cell line (U373) expressing lower levels of ATase. [14C]-labelled temozolomide uptake was similar in sensitive and resistant lines. Human ATase-cDNA transfected xeroderma pigmentosum (XP) fibroblasts were more resistant than XP control cells to temozolomide and the related chloroethylating agent mitozolomide and although BG completely suppressed ATase activity in these cells, resistance was still greater than in control cells.

Cell cycle analysis of p53-induced cell death in murine erythroleukemia cells

A temperature-sensitive mutant of murine p53 (p53Val-135) was transfected by electroporation into murine erythroleukemia cells (DP16-1) lacking endogenous expression of p53. While the transfected cells grew normally in the presence of mutant p53 (37.5 degrees C), wild-type p53 (32.5 degrees C) was associated with a rapid loss of cell viability. Genomic DNA extracted at 32.5 degrees C was seen to be fragmented into a characteristic ladder consistent with cell death due to apoptosis. Following synchronization by density arrest, transfected cells released into G1 at 32.5 degrees C were found to lose viability more rapidly than did randomly growing cultures. Following release into G1, cells became irreversibly committed to cell death after 4 h at 32.5 degrees C. Commitment to cell death correlated with the first appearance of fragmented DNA. Synchronized cells allowed to pass out of G1 prior to being placed at 32.5 degrees C continued to cycle until subsequently arrested in G1; loss of viability occurred following G1 arrest. In contrast to cells in G1, cells cultured at 32.5 degrees C for prolonged periods during S phase and G2/M, and then returned to 37.5 degrees C, did not become committed to cell death. G1 arrest at 37.5 degrees C, utilizing either mimosine or isoleucine deprivation, does not lead to rapid cell death. Upon transfer to 32.5 degrees C, these G1 synchronized cell populations quickly lost viability. Cells that were kept density arrested at 32.5 degrees C (G0) lost viability at a much slower rate than did cells released into G1. Taken together, these results indicate that wild-type p53 induces cell death in murine erythroleukemia cells and that this effect occurs predominantly in the G1 phase of actively cycling cells.

Cell cycle analysis of p53-induced cell death in murine erythroleukemia cells

A temperature-sensitive mutant of murine p53 (p53Val-135) was transfected by electroporation into murine erythroleukemia cells (DP16-1) lacking endogenous expression of p53. While the transfected cells grew normally in the presence of mutant p53 (37.5 degrees C), wild-type p53 (32.5 degrees C) was associated with a rapid loss of cell viability. Genomic DNA extracted at 32.5 degrees C was seen to be fragmented into a characteristic ladder consistent with cell death due to apoptosis. Following synchronization by density arrest, transfected cells released into G1 at 32.5 degrees C were found to lose viability more rapidly than did randomly growing cultures. Following release into G1, cells became irreversibly committed to cell death after 4 h at 32.5 degrees C. Commitment to cell death correlated with the first appearance of fragmented DNA. Synchronized cells allowed to pass out of G1 prior to being placed at 32.5 degrees C continued to cycle until subsequently arrested in G1; loss of viability occurred following G1 arrest. In contrast to cells in G1, cells cultured at 32.5 degrees C for prolonged periods during S phase and G2/M, and then returned to 37.5 degrees C, did not become committed to cell death. G1 arrest at 37.5 degrees C, utilizing either mimosine or isoleucine deprivation, does not lead to rapid cell death. Upon transfer to 32.5 degrees C, these G1 synchronized cell populations quickly lost viability. Cells that were kept density arrested at 32.5 degrees C (G0) lost viability at a much slower rate than did cells released into G1. Taken together, these results indicate that wild-type p53 induces cell death in murine erythroleukemia cells and that this effect occurs predominantly in the G1 phase of actively cycling cells.

Antitumour imidazotetrazines--XVI. Macromolecular alkylation by 3-substituted imidazotetrazinones

The extent of macromolecular alkylation by three imidazotetrazinones, 8-carbamoyl-3-(2-chloroethyl)imidazo[5,1-d]-1,2,3,5-tetrazin-4-(3H )-one (mitozolomide) and the 3-methyl CCRG 81045) and 3-ethyl (CCRG 82019) analogues has been studied both in intact cells and with isolated DNA, RNA and protein. Towards isolated DNA and RNA CCRG 81045 was about twice as reactive as mitozolomide and 5-10-fold more reactive than CCRG 82019. Two cell lines were chosen to study macromolecular alkylation, GM892A and Raji, the latter being 10-20-fold less sensitive to mitozolomide and CCRG 81045 than the former, but only one-and-a-half-fold less sensitive to CCRG 82019. Drug uptake into both cell lines was shown to be by a rapid diffusion process with a cell medium distribution ratio not far from unity. For all three agents intracellular radioactivity became associated with macromolecules, and the level found at any time is a balance between the rate of alkylation and the rate of alkyl group removal by repair processes. Both CCRG 81045 and CCRG 82019 produced approximately the same level of alkyl groups bound to DNA, RNA and protein over a 24-hr period, whereas mitozolomide produced a greater extent of alkylation. All three agents left more alkyl groups bound to DNA and RNA in GM892A than in Raji cells, but there was no difference in the level of alkyl groups remaining bound to proteins. However, in GM892A cells the overall level of alkylation of DNA by CCRG 81045 exceeded that of CCRG 82019 only after 24 hr of drug incubation despite the twenty-fold difference in potency of these agents. These results suggest that specific base alkylations rather than total macromolecular alkylation may be more important in determining relative cytotoxicity.

Mitozolomide activity on human cancer cells in vitro

The growth inhibitory effects, the reduction of [3H]-TdR incorporation and the perturbation of the cell cycle induced by the new agent mitozolomide on the M14 human melanoma cell line and on the SW626 human ovarian cancer cell line were compared to those produced by BCNU. Flow cytometry showed an interesting difference: at the high concentration mitozolomide induced an accumulation of cells in S middle and S late-G2-M phase of the cell cycle whereas BCNU caused only a block in S late-G2-M. Further studies were aimed at investigating the susceptibility of freshly isolated human ovarian cancer cells to pharmacologically reasonable mitozolomide concentrations. Only in one out of 16 primary cultures of human ovarian cancers was mitozolomide able to induce cell cycle perturbation, suggesting that ovarian carcinoma cells may not be sensitive to this drug.