Different effects of FK317 on multidrug-resistant tumor in vivo and in vitro

FK317, a novel substituted dihydrobenzoxazine, was examined for antitumor effects on multidrug-resistant (MDR) tumor cells in vitro and in vivo. In nude mice, FK317 markedly inhibited the growth of s.c. implanted KB-V1 vinblastine (VLB)-resistant human epidermal carcinoma KB cells, as well as the parent cells (KB-3-1). However, KB-V1 showed much greater resistance to FK317 than to VLB and adriamycin (ADM) in the in vitro study. This resistance was reversed by the addition of verapamil, whereby intracellular accumulation of FK317 in the KB-V1 cells was also decreased. After incubation of FK317 in human and mouse blood, it was shown to be rapidly metabolized to a monodeacetylated form, and slowly metabolized further to a dideacetylated form. With the removal of the acetyl groups from FK317, resistance indexes in KB-V1 and SBC-3/ADM, ADM-resistant human lung carcinoma, decreased. In addition, photolabeling of P-glycoprotein with [3H]azidopine in KB-V1 plasma membrane was completely inhibited by FK317, but not by the deacetylated metabolites. These results indicate that FK317 is metabolized to deacetylated forms, which do not bind to P-glycoprotein and are incorporated into MDR cells, causing cytotoxic effects.

Cytotoxic mechanisms of FK317, a new class of bioreductive agent with potent antitumor activity

FK317 is a member of a new class of bioreductive agents that exhibit strong cytotoxicity against various human cancer cells. The effect of FK317 was found to be stronger than that of mitomycin C (MMC), adriamycin (ADR) or cisplatin (CDDP). Alkaline elution analysis indicated that FK317 formed interstrand DNA-DNA and DNA-protein cross-links in cells. On the other hand, no DNA single-strand breaks were observed in the cells treated with FK317. In a cell-free system the deacetylated metabolites produced cross-linked DNA under reductive conditions, though FK317 itself did not form DNA-DNA cross-links. In order to elucidate the metabolic activation mechanisms, we established an FK317-resistant subline from human non-small cell lung cancer cells (Lu99) by stepwise and brief exposure (1 h) to FK317. The resistant subline (Lu99/317) showed cross-resistance to MMC and carboquone (CQ), but not to ADR or CDDP. DT-diaphorase, which is one of the activation enzymes of MMC and CQ, was deficient in Lu99/317 cells as determined by enzyme activity assay. However, the levels of NADPH:cytochrome P450 reductase, which is another activation enzyme for MMC and CQ, were comparable in resistant and parent cell lines. Treatment of the cells with dicumarol, an inhibitor of DT-diaphorase, reduced the cytotoxicity of FK317 to Lu99 cells, but not to Lu99/317 cells. These results indicate that deacetylation of FK317 is necessary for its reductive activation, and deacetylated FK317 is reduced by DT-diaphorase to form an active metabolite, which produces DNA-DNA interstrand and DNA-protein cross-links that lead to cell death.