Enhancement by O-6-benzyl-N2-acetylguanosine of N'-[2-chloroethyl]-N[2-(methylsulphonyl)ethyl]-N'-nitrosourea therapeutic index on nude mice bearing resistant human melanoma

The exposure of cells to O-6-benzyl-N2-acetylguanosine (BNAG) and several guanine derivatives is known to reduce the activity of O-6-alkylguanine-DNA alkyltransferase (MGMT) and to enhance the sensitivity of Mer(+) (methyl enzyme repair positive) tumour cells to chloroethylnitrosoureas (CENUs) in vitro and in vivo. High water solubility and the pharmacokinetic properties of BNAG make it a candidate for simultaneous administration with CENUs by the i.v. route in human clinical use. In vivo we have shown previously that BNAG significantly increases the efficiency of N'-[2-chloroethyl]-N-[2-(methysulphonyl)ethyl]-N'-nitrosourea (cystemustine) against M4Beu melanoma cells (Mer(+)) through its cytostatic activity by the i.p. route, but also increases its toxicity. To investigate the toxicity of BNAG and cystemustine when administered simultaneously in mice, we compared the maximum tolerated dose and LD50 doses of cystemustine alone or in combination with 40 mg kg(-1) BNAG by the i.p. route. The toxicity of cystemustine was enhanced by a factor of almost 1.44 when combined with BNAG. To compare the therapeutic index of cystemustine alone and the cystemustine/BNAG combination, pharmacological tests were carried out in nude mice bearing Mer(+) M4Beu human melanoma cells. Isotoxic doses were calculated using the 1.44 ratio. The treatments were administered three times by the i.v. route on days 1, 5 and 9 after s.c. inoculation of tumour cells. Although the toxicities of the treatments were equal, SNAG strongly enhanced tumour growth inhibition. These results demonstrate the increase of the therapeutic index of cystemustine by BNAG and justify the use of BNAG to enhance nitrosourea efficiency in vivo by i.v. co-injection.