Four-hourly scheduling of temozolomide improves tumour growth delay but not therapeutic index in A375M melanoma xenografts

Purpose: To establish whether temozolomide is more effective against A375M human melanoma xenografts if given every 4 h rather than every 24 h, in ord er to exploit depletion of the DNA repair protein O-6-alkylguanine-DNA alky ltransferase (ATase) by prior doses of the drug. Methods: ATase depletion i n A375M human melanoma xenografts was determined over 24 h after a single d ose of temozolomide. The effect of different drug schedules tall of total d ose 500 mg/kg) in delaying the growth of the xenografts was tested, and ATa se depletion and DNA methylation damage assessed in tumour and normal tissu e. Results: Maximal depletion of ATase in tumour, to 2.52 +/- 0.23% of pret reatment levels, occurred 4-8 h after a single 100 mg/kg i.p. dose of temoz olomide, with 23.0% recovery of protein levels at 24 h. Scheduling of temoz olomide every 4 h increased tumour growth delay (33.6 +/- 1.39 days with te mozolomide 100 mg/kg 4-hourly x5 versus 23.2 +/- 1.43 days with temozolomid e 100 mg/kg once daily x5; P < 0.0001) at the expense of increased toxicity (17.4 +/- 1.55% animal weight loss versus 10.6 +/- 1.27%, respectively). T emozolomide every 4 h did not increase ATase depletion compared with the 5- day schedule, but resulted in greater DNA O-6-guanine methylation (29.0% mo re in tumour, 20.8% in liver and 56.0% in brain, comparing areas under the methylation-time curve). Conclusions: The 4-hourly schedule of temozolomide delayed tumour growth significantly more than the once-daily and 12-hourly schedules, probably asa result of greater DNA damage inflicted, but also i ncreased toxicity. It remains to be seen if this regimen confers a net bene fit over the standard schedule.