Purpose: To establish whether temozolomide is more effective against A375M human melanoma xenografts if given every 4 h rather than every 24 h, in order to exploit depletion of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase) by prior doses of the drug.
Methods: ATase depletion in A375M human melanoma xenografts was determined over 24 h after a single dose of temozolomide. The effect of different drug schedules (all of total dose 500 mg/kg) in delaying the growth of the xenografts was tested, and ATase depletion and DNA methylation damage assessed in tumour and normal tissue.
Results: Maximal depletion of ATase in tumour, to 2.52 +/- 0.23% of pretreatment levels, occurred 4-8 h after a single 100 mg/kg i.p. dose of temozolomide, with 23.0% recovery of protein levels at 24 h. Scheduling of temozolomide every 4 h increased tumour growth delay (33.6 +/- 1.39 days with temozolomide 100 mg/kg 4-hourly x versus 23.2 +/- 1.43 days with temozolomide 100 mg/kg once daily x 5; P < 0.0001) at the expense of increased toxicity (17.4 +/- 1.55% animal weight loss versus 10.6 +/- 1.27%. respectively). Temozolomide every 4 h did not increase ATase depletion compared with the 5-day schedule, but resulted in greater DNA 06-guanine methylation (29.0% more 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 as a result of greater DNA damage inflicted, but also increased toxicity. It remains to be seen if this regimen confers a net benefit over the standard schedule.