MODULATING NITROSOUREA RESPONSE IN TUMORS AND NORMAL TISSUE - APPLICATION TO FOTEMUSTINE

Fotemustine (Muphoran(R)) is a chloroethylnitrosourea (CNU) and, by an alogy with other CNUs, its principal mechanism of cell killing is like ly to be via the formation of a chloroethylating species that generate s C-G interstrand cross-links in DNA following an initial chloroethyla tion reaction at the O-6-position of guanine residues. CNU-mediated DN A cross-link formation can be prevented by the action of the DNA repai r protein, O-6-alkylguanine-DNA alkyltransferase (ATase), on the initi al O-6-chloroethylguanine monoadduct. Several groups have shown that o verexpression of ATase in mammalian cells in culture following transfe ction of pro- or eukaryotic ATase-encoding expression vectors protects against the toxic effects of CNUs, including fotemustine, demonstrati ng that this is indeed the principal mechanism of resistance to such a gents, although other mechanisms, including the action of DNA glycosyl ases, cannot be discounted. Many of the tumor types that we have immun ostained with anti-human ATase antibodies express high levels of this protein, but regional variations in immunostaining within a tumor and different levels of immunostaining in different melanoma metastases fr om the same patient have also been seen. On the other hand, bone marro w, the principal site of dose-limiting toxicity for most nitrosoureas, generally expresses relatively little ATase activity. ATase in tumor cells can be depleted by the administration of O-6-alkylguanines that act as substrates for the protein, and this increases the growth inhib itory effect of CNUs and related agents in several human tumor xenogra fts grown in nude mice. Increasing the ATase levels in murine bone mar row by the ex vivo transduction of retroviruses encoding human ATase i nto bone marrow cells decreases their sensitivity to the toxic effects of this class of agents. Similar modulation of ATase levels might, th erefore, be expected to increase the therapeutic index of fotemustine and other CNUs when used clinically.