Eflornithine (a-Difluoromethylornithine; DFMO) is an irreversible inhibitor of ornithine decarboxylase, a critical rate-limiting enzyme in the de novo biosynthesis of the polyamines putrescine, spermidine, and spermine [15]. Although the precise function of polyamines in cellular proliferation is not well-understood, they seem to play a fundamental role in the synthesis and/or regulation of DNA, RNA, and protein. The implication that polyamines are somehow involved in cell proliferation stems from the fact that high levels of ornithine decarboxylase (ODC) enzyme and polyamines are found in cancer and other cell types that have a high proliferation rate. The observations that polyamines might play a role in tumor development [16-18], led to an immense amount of work done in late 1970s and all through the 1980s on ODC and polyamine metabolism. The cellular inhibition of ODC by eflornithine causes a marked reduction in putrescine and spermidine, and a variable reduction in spermine depending on the length of treatment and the cell type. A number of excellent reviews have been written on ODC regulation and polyamine metabolism [19-22]. Eflornithine binds the active site of ODC as a substrate, and is then decarboxylated just like the natural substrate ornithine, however, during this catalytic process; a reactive intermediate is formed that covalently attaches itself to the enzyme’s active site, causing a permanent deactivation or suicide-inhibition of the enzyme [23,24]. The cellular half-life of ODC of about 30 minutes is one of the shortest of all known enzymes [24]. Thus, in order to achieve a significant antiproliferative effect, the enzyme inhibition must be sustained by a constant inhibitory level of the inhibitor. Among the number of ODC inhibitors that have been synthesized and evaluated, eflornithine by far is the most studied [24].
Exposure of normal as well as a cancerous cell to eflornithine causes a dose-dependent reduction in polyamine levels and results in an anti-proliferative effect on cells. In a comparative study of seven anticancer amino acid analogs using a panel of 60 cultured human tumor cell lines, Ahluwalia et al. found eflornithine to be devoid of the cytotoxic activity that was a common feature for the other six agents evaluated in the study [25]. The antiproliferative effect of eflornithine has therefore been attributed to its cytostatic rather than cytotoxic activity [26]. Probably because of this cytostatic mode of action, eflornithine has not been very effective as a single agent in human anticancer studies [27,28]. However, in the course of anticancer investigations, it was found that eflornithine has a low potential for systemic toxicity at doses that are effective in inhibiting ODC and reducing cellular polyamine levels [25,26,29,30]. Based on this rationale, the therapeutic potential of eflorni – tine was investigated at low doses (< 1.0 gm/m2/day) for chemopreventive activity against colon, bladder, and breast cancers [31-33]. Because of a lack of clear therapeutic end point of chemoprevention studies, target tissue polyamine levels, were used as surrogate markers to assess treatment successes [34].
Several active molecules, synthetic and natural, have been investigated by the National Cancer Institute for their potential chemotherapeutic use. These include, nonsteroidal antiinflammatory drugs (NSAIDs), antioxidents, retinoids and carotenoids, Vitamins (C&E), polyphenols, and eflornithine [32]. Among these, eflornithine has been the most studied and has the most scientific rationale for development as a chemopreventive agent. One of the primary objectives for any chemoprevention study trial is to find a therapeutic drug dose that is either devoid of toxicity or has minor and acceptable level of side effects for chronic and perhaps life-time use. An eflornithine dose of 0.5 gm/m2/day has been indicated in several long-term dose-seeking studies to be the dose without significant toxicity [35,36]. Ototoxicity, in particular, the low-frequency hearing loss has been shown to be the most sensitive measure of the drug-related toxic effects in human. Based on this toxicity outcome, the observed ‘no-effect’ dose of eflornithine was determined to be 0.4 gm/m2/day or 0.74 gm/day for a typical 1.85 m2 person [37].