Non-irradiation-derived reactive oxygen species (ROS) and cancer: therapeutic implications.

Owing to their chemical reactivity, radicals have cytocidal properties. Destruction of cells by irradiation-induced radical formation is one of the most frequent interventions in cancer therapy. An alternative to irradiation-induced radical formation is in principle drug-induced formation of radicals, and the formation of toxic metabolites by enzyme catalysed reactions. Although these developments are currently still in their infancy, they nevertheless deserve consideration. There are now numerous examples known of conventional anti-cancer drugs that may at least in part exert cytotoxicity by induction of radical formation. Some drugs, such as arsenic trioxide and 2-methoxy-estradiol, were shown to induce programmed cell death due to radical formation. Enzyme-catalysed radical formation has the advantage that cytotoxic products are produced continuously over an extended period of time in the vicinity of tumour cells. Up to now the enzymatic formation of toxic metabolites has nearly exclusively been investigated using bovine serum amine oxidase (BSAO), and spermine as substrate. The metabolites of this reaction, hydrogen peroxide and aldehydes are cytotoxic. The combination of BSAO and spermine is not only able to prevent tumour cell growth, but prevents also tumour growth, particularly well if the enzyme has been conjugated with a biocompatible gel. Since the tumour cells release substrates of BSAO, the administration of spermine is not required. Combination with cytotoxic drugs, and elevation of temperature improves the cytocidal effect of spermine metabolites. The fact that multidrug resistant cells are more sensitive to spermine metabolites than their wild type counterparts makes this new approach especially attractive, since the development of multidrug resistance is one of the major problems of conventional cancer therapy.

How important is the oxidative degradation of spermine?: minireview article.

Spermine is a constituent of most eucaryotic cells, however, it is not of vital importance for the vertebrate organism, as is demonstrated by the existence of transgenic (Gy) mice that lack spermine and spermine synthase. In contrast its degradation appears to be of vital importance, since mice die after chronic administration of N1,N4-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72517). Under this condition spermine accumulates in red blood cells and blood plasma. Lethal toxicity can be avoided by intervals of MDL 72527-free periods. During these periods spermine appears to be directly degraded to spermidine without an intermediary acetylation step within the red blood cells. Since this reaction is of enormous physiological significance, it will be important to characterise the red blood cell spermine oxidase, and it will be particularly important to determine whether this oxidase is identical with the FAD-dependent polyamine oxidase that is considered to be involved in the polyamine interconversion sequence, or whether it is one of the recently characterised spermine oxidase isoenzymes.

Catabolism of polyamines.

Owing to the establishment of cells and transgenic animals which either lack or over-express acetylCoA:spermidine N(1)-acetyltransferase a major progress was made in our understanding of the role of polyamine acetylation. Cloning of polyamine oxidases of mammalian cell origin revealed the existence of several enzymes with different substrate and molecular properties. One appears to be identical with the polyamine oxidase that was postulated to catalyse the conversion of spermidine to putrescine within the interconversion cycle. The other oxidases are presumably spermine oxidases, because they prefer free spermine to its acetyl derivatives as substrate. Transgenic mice and cells which lack spermine synthase revealed that spermine is not of vital importance for the mammalian organism, but its transformation into spermidine is a vitally important reaction, since in the absence of active polyamine oxidase, spermine accumulates in blood and causes lethal toxic effects. Numerous metabolites of putrescine, spermidine and spermine, which are presumably the result of diamine oxidase-catalysed oxidative deaminations, are known as normal constituents of organs of vertebrates and of urine. Reasons for the apparent contradiction that spermine is in vitro a poor substrate of diamine oxidase, but is readily transformed into N(8)-(2-carboxyethyl)spermidine in vivo, will need clarification.Several attempts were made to establish diamine oxidase as a regulatory enzyme of polyamine metabolism. However, diamine oxidase has a slow turnover. This, together with the efficacy of the homeostatic regulation of the polyamines via the interconversion reactions and by transport pathways renders a role of diamine oxidase in the regulation of polyamine concentrations unlikely. 4-Aminobutyric acid, the product of putrescine catabolism has been reported to have antiproliferative properties. Since ornithine decarboxylase and diamine oxidase activities are frequently elevated in tumours, it may be hypothesised that diamine oxidase converts excessive putrescine into 4-aminobutyric acid and thus restricts tumour growth and prevents malignant transformation. This function of diamine oxidase is to be considered as part of a general defence function, of which the prevention of histamine and cadaverine accumulation from the gastrointestinal tract is a well-known aspect.

Polyamine metabolism in primary human colon adenocarcinoma cells (SW480) and their lymph node metastatic derivatives (SW620).

The natural polyamines are multifunctional constituents of all eucaryotic cells. The objective of this work was to compare aspects of polyamine metabolism in two related cell lines with the idea to investigate whether metabolic differences can be attributed to functional differences of the cells. The human colon carcinoma-derived cell lines SW480 and SW620 were chosen as models. SW480 cells were isolated from the primary tumour, SW620 cells from a lymph node of the same patient. SW620 cells grow faster, and the key regulatory enzymes of polyamine biosynthesis (ODC and AdoMetDC) are more active in the metastatic cells. Moreover, their ability to accumulate polyamines from the environment is more important than of SW480 cells. Likewise polyamine concentrations were markedly higher in SW620 cells, although they are much smaller than SW480 cells, and have a particularly small cytoplasmic space. Both cell lines show a striking diminution of ODC and AdoMetDC activities and changes in the polyamine patterns at the transition from exponential to non-exponential growth--most probably as a consequence of high cell density. Depletion of putrescine and spermidine due to inactivation of ODC by DFMO causes accumulation of cells in G1, and a proportional decrease of S-phase cells in both cell lines. Based on morphologic and other criteria SW480 and SW620 cells were typified as poorly differentiated. In agreement with their low grade of differentiation they exhibit a low alkaline phosphatase activity. However, the time-dependent decrease of alkaline phosphatase is not typical of differentiation patterns of other adenocarcinoma-derived cell lines or of normal enterocytes. The high capacity of de novo polyamine biosynthesis and of polyamine uptake is presumably a prerequisite for the rapid growth and invasiveness. The fact that these properties were more accentuated in the case of SW620 cells and paralleled enhanced metastatic properties indicate relationships between basic parameters of polyamine metabolism and malignancy.

Cytotoxic effects of the polyamine oxidase inactivator MDL 72527 to two human colon carcinoma cell lines SW480 and SW620.

N1,N4-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72527) was considered to be a selective inactivator of FAD-dependent tissue polyamine oxidase. Recently MDL 72527 was reported to induce apoptosis in transformed hematopoietic cells through lysosomotropic effects. Since it is the only useful inhibitor of polyamine oxidase available at present, the re-evaluation of its properties seemed important. Human colon carcinoma-derived SW480 cells and their lymph node metastatic derivatives (SW620) were chosen for our study because they differ in various aspects of polyamine metabolism but have similar polyamine oxidase activities. MDL 72527 inhibited cell growth in a concentration-dependent manner, depleted intracellular polyamine pools, and caused the accumulation of N1-acetyl derivatives of spermidine and spermine. SW620 cells were more sensitive to the drug than were SW480 cells. At 150 micromol/L MDL 72527, SW620 cells accumulated in S-phase of the cell cycle, showed decreased polyamine transport rate, and showed no increase of polyamine N1-acetyltransferase activity. In contrast, SW480 cells were not arrested in a particular phase of the cell cycle, showed enhanced polyamine uptake, and showed a mild induction of acetyltransferase. The results suggest that MDL 72527 retains its value as a selective tool in short-term experiments only at concentrations not exceeding those necessary for the inactivation of polyamine oxidase. At concentrations above 50 micromol/L and at exposure times longer than 24 h, it may derange cell functions nonspecifically, and thus blur the results of studies intended to elucidate polyamine oxidase functions.

Dimethylsilane polyamines: cytostatic compounds with potentials as anticancer drugs. II. Uptake and potential cytotoxic mechanisms.

Dimethylsilane tetramines are structural analogues of spermine with a (CH3)2 Si-group incorporated into the central carbon chain. They have potential as anticancer drugs. Their cytotoxic effect was considered to rely mainly on their polyamine antagonist property. In order to obtain new ideas about cellular mechanisms, which are potential targets of the dimethylsilane polyamines, the effects of these compounds on some basic cell functions, such as protein and DNA synthesis, and calmodulin antagonism were studied. In addition, their mode of accumulation in cells was investigated. It became evident that the intracellular accumulation of dimethylsilane polyamines is almost exclusively achieved via the polyamine transport system. However, the exchange of a part of the intracellular natural polyamines against dimethylsilane polyamines has only a small effect on polyamine uptake. Binding to the endoplasmic reticulum and inhibition of protein synthesis are presumably important for the cytotoxic action of bis(11-amino-4,8-diazaundecyl)dimethylsilane, a hexamine, but seem of no importance for the tetramines. Calmodulin antagonism, however, is likely to contribute to their cytotoxic effect.

Resveratrol inhibits intestinal tumorigenesis and modulates host-defense-related gene expression in an animal model of human familial adenomatous polyposis.

We studied the effect of oral administration of resveratrol, a natural constituent of grapes, on tumorigenesis in Min mice. Min mice are congenic mice genetically predisposed to develop intestinal tumors as a result of a mutation of the Apc gene. Resveratrol (0.01% in the drinking water containing 0.4% ethanol) was administered for seven weeks to Min mice starting at five weeks of age. The control group was fed the same diet and received water containing 0.4% ethanol. Resveratrol prevented the formation of colon tumors and reduced the formation of small intestinal tumors by 70%. Comparison of the expression of 588 genes in the small intestinal mucosa showed that resveratrol downregulated genes that are directly involved in cell cycle progression or cell proliferation (cyclins D1 and D2, DP-1 transcription factor, and Y-box binding protein). In addition, resveratrol upregulated several genes that are involved in the recruitment and activation of immune cells (cytotoxic T lymphocyte Ag-4, leukemia inhibitory factor receptor, and monocyte chemotactic protein 3) and in the inhibition of the carcinogenic process and tumor expansion (tumor susceptibility protein TSG101, transforming growth factor-beta, inhibin-beta A subunit, and desmocollin 2). Our data highlight the complexity of the events associated with intestinal tumorigenesis and the multiplicity of the molecular targets of resveratrol. The high potency and efficacy of resveratrol support its use as a chemopreventive agent in the management of intestinal carcinogenesis.

N-Benzylpolyamines as vectors of boron and fluorine for cancer therapy and imaging: synthesis and biological evaluation.

Cancer cells have high-affinity polyamine uptake systems with a low stringency for structural features. Putrescine, spermidine, and spermine have, therefore, been considered as potential vectors for the selective accumulation in tumors of therapeutically or diagnostically useful structures and elements. We envisaged N-benzyl derivatives of the polyamines as vectors of (10)B and (18)F for boron neutron capture therapy (BNCT) and tumor imaging by positron emission tomography (PET), respectively. In the present work, the synthesis, transport characteristics, DNA-binding properties, and cytotoxicity of several N-benzyl derivatives of putrescine and spermidine are described. The fluorinated spermidine derivative N-(3-[(4-aminobutyl)amino]propyl)[(4-fluorophenyl)methyl]amine (N(1)-4-Fbz-spd) may be useful for PET because of its high accumulation in cancer cells via the polyamine transport system. Among the boron-containing benzyl polyamines, N-(4-aminobutyl)([4-(dihydroxyboryl)phenyl]methyl)amine (4-Bbz-put) and N-(3-[(4-aminobutyl)amino]propyl)([4-(dihydroxyboryl)phenyl]methyl)amine (N(1)-4-Bbz-spd) should be suitable for BNCT, because their accumulation in B16 melanoma cells was more efficient than that of borocaptate and borophenylalanine, two reference compounds used in BNCT.

Geraniol, a component of plant essential oils, inhibits growth and polyamine biosynthesis in human colon cancer cells.

Geraniol and other monoterpenes found in essential oils of fruits and herbs have been suggested to represent a new class of agents for cancer chemoprevention. As a first step in clarifying the mode of action of geraniol on colon carcinogenesis, we studied its effects on the growth of a human colon cancer cell line (Caco-2). Geraniol (400 microM) caused a 70% inhibition of cell growth, with cells accumulating in the S transition phase of the cell cycle, and concomitant inhibition of DNA synthesis. No signs of cytotoxicity or apoptosis were detected. Geraniol caused a 50% decrease of ornithine decarboxylase activity, a key enzyme of polyamine biosynthesis, which is enhanced in cancer growth. This led to a 40% reduction of the intracellular pool of putrescine. Geraniol also activated the intracellular catabolism of polyamines, indicated by enhanced polyamine acetylation. These observations indicate that polyamine metabolism is presumably a target in the antiproliferative properties of geraniol.

Rat colon ornithine and arginine metabolism: coordinated effects after proliferative stimuli.

Ornithine decarboxylase (ODC) catalyzes the first step in the polyamine biosynthetic pathway, a highly regulated pathway in which activity increases during rapid growth. Other enzymes also metabolize ornithine, and in hepatomas, rate of growth correlates with decreased activity of these other enzymes, which thus channels more ornithine to polyamine biosynthesis. Ornithine is produced from arginase cleavage of arginine, which also serves as the precursor for nitric oxide production. To study whether short-term coordination of ornithine and arginine metabolism exists in rat colon, ODC, ornithine aminotransferase (OAT), arginase, ornithine, arginine, and polyamine levels were measured after two stimuli (refeeding and/or deoxycholate exposure) known to synergistically induce ODC activity. Increased ODC activity was accompanied by increased putrescine levels, whereas OAT and arginase activity were reduced by either treatment, accompanied by an increase in both arginine and ornithine levels. These results indicate a rapid reciprocal change in ODC, OAT, and arginase activity in response to refeeding or deoxycholate. The accompanying increases in ornithine and arginine concentration are likely to contribute to increased flux through the polyamine and nitric oxide biosynthetic pathways in vivo.

Effect of the polyamine oxidase inactivator MDL 72527 on N(1)-(n-octanesulfonyl)spermine toxicity.

N(1)-(n-octanesulfonyl)spermine (N(1)OSSpm) is a potent calmodulin antagonist. In the present work, its toxicity to DHD/K12/TRb and CaCo-2 cells, two colon carcinoma-derived cell lines, was studied with the aim to identify those properties of the cells, which determine their sensitivity to N(1)OSSpm and related structures. Exposure of the cells to MDL 72527, a compound considered to be a selective inactivator of polyamine oxidase (PAO) increased the cytotoxicity of N(1)OSSpm to both cell lines. In contrast, toxicity of trifluoperazine, a calmodulin antagonist with a polyamine-unrelated structure, was not enhanced by MDL 72527. Combined exposure of cells to 2-(difluoromethyl)ornithine (DFMO) (a selective inactivator of ornithine decarboxylase), MDL 72527 and N(1)OSSpm produced a synergistic cytotoxic effect. Neither the intrinsic PAO activity of the cells (as determined with N(1), N(12)-diacetylspermine as substrate), nor their ability to accumulate the drug was a determinant of the cytotoxic effect of N(1)OSSpm. These data suggest that MDL 72527 has a target unrelated to PAO, which is responsible for the enhancement of N(1)OSSpm (and spermine) toxicity. Identification of this target may be of use if the therapeutic potentials of MDL 72527 are to be exploited.

Spermine cytotoxicity to human colon carcinoma-derived cells (CaCo-2).

Spermine is a constituent of all vertebrate cells. Nevertheless, it exerts toxic effects if it accumulates in cells. Spermine is a natural substrate of the FAD-dependent polyamine oxidase, a constitutive enzyme of many cell types. It has been reported that the toxicity of spermine was enhanced if polyamine oxidase was inhibited. We were interested to examine spermine toxicity to human colon carcinoma-derived CaCo-2 cells because, in contrast to most tumor cell lines, CaCo-2 cells undergo differentiation, which is paralleled by changes in polyamine metabolism. CaCo-2 cells were remarkably resistant to spermine accumulation, presumably because spermine is degraded by polyamine oxidase at a rate sufficient to provide spermidine for the maintenance of growth. Inactivation of polyamine oxidase increased the sensitivity to spermine. A major reason for the enhanced spermine cytotoxicity at low polyamine oxidase activity is presumably the profound depletion of spermidine, and the consequent occupation of spermidine binding sites by spermine. Hydrogen peroxide and the aldehydes 3-aminopropanal and 3-acetamidopropanal, the products of polyamine oxidase-catalyzed splitting of spermine and N1-acetylspermine, contribute little to spermine cytotoxicity. Activation of caspase by spermine was insignificant, and the formation of DNA ladders, another indicator of apoptotic cell death, could not be observed. Thus it appears that cell death due to excessive accumulation of spermine in CaCo-2 cells was mainly nonapoptotic. The content of brush border membranes did not change between days 6 and 8 after seeding, and it was not affected by exposure of the cells to spermine. However, the activities of alkaline phosphatase, sucrase, and aminopeptidase in nontreated cells were considerably enhanced during this period, but remained low if cells were exposed to spermine. These changes appear to indicate that differentiation is prevented by intoxication with spermine, although other explanations cannot be excluded.

Anti-proliferative effect of resveratrol, a natural component of grapes and wine, on human colonic cancer cells.

Resveratrol, a natural polyphenolic phytoalexine present in grapes and wines, has been reported to exert a variety of important pharmacological effects. We investigated the effects of resveratrol on the growth and polyamine metabolism of CaCo-2 human colon cancer cells. Treatment of the CaCo-2 cells with 25 microM resveratrol caused a 70% growth inhibition. The cells accumulated at the S/G2 phase transition of the cell cycle. No signs of cytotoxicity or apoptosis were detected. Resveratrol caused a significant decrease of ornithine decarboxylase (ODC) activity, a key enzyme of polyamine biosynthesis, which is enhanced in cancer growth. ODC inhibition resulted in the reduction of the intracellular putrescine content, indicating that polyamines might represent one of several targets involved in the anti-proliferative effects of resveratrol.

Inhibition of polyamine oxidase enhances the cytotoxicity of polyamine oxidase substrates. A model study with N1-(n-octanesulfonyl)spermine and human colon cancer cells.

N(1)-(n-octanesulfonyl)spermine (N(1) OSSpm) is a substrate of polyamine oxidase. It shares several properties with spermine, such as antagonism of NMDA-type glutamate receptors, calmodulin antagonism, and cytotoxicity, but it is more potent by orders of magnitude in these regards than spermine. The human colon carcinoma-derived cell line CaCo-2 was used as a model to study the toxicity of N(1) OSSpm as a function of polyamine oxidase (PAO) activity and differentiation. If the formation of hydrogen peroxide and aminoaldehyde by the PAO-catalysed reactions was prevented by selective inactivation of the enzyme with MDL 72527, cytotoxicity of N(1)OSSpm was not diminished, but on the contrary, enhanced. Exponentially growing CaCo-2 cells were considerably more sensitive to N(1)OSSpm than differentiating cells. The results suggest that cytotoxic substrates of PAO exhibit enhanced cytotoxicity in cells, if PAO activity is inhibited. Since tumour cells are known to have lower polyamine oxidase activities than their normal counterparts, it will be interesting to explore whether cytotoxic substrates of polyamine oxidase, for which N(1)OSSpm is an example, are suited to preferentially kill tumour cells.

Oxidation of polyamines and brain injury.

Several amine oxidases are involved in the metabolism of the natural polyamines putrescine, spermidine, and spermine, and play a role in the regulation of intracellular concentrations, and the elimination of these amines. Since the products of the amine oxidase-catalyzed reactions -- hydrogen peroxide and aminoaldehydes -- are cytotoxic, oxidative degradations of the polyamines have been considered as a cause of apoptotic cell death, among other things in brain injury. Since a generally accepted, unambiguous nomenclature for amine oxidases is missing, considerable confusion exists with regard to the polyamine oxidizing enzymes. Consequently the role of the different amine oxidases in physiological and pathological processes is frequently misunderstood. In the present overview the reactions, which are catalyzed by the different polyamine-oxidizing enzymes are summarized, and their potential role in brain damage is discussed.

Ornithine aminotransferase, a potential target for the treatment of hyperammonemias.

Ornithine-delta-aminotransferase (OAT) (EC 2.6.1.13) is a pyridoxal-5' phosphate dependent mitochondrial matrix enzyme. It controls the L-ornithine (Orn) level in tissues by catalysing the transfer of the delta-amino group of Orn to 2-oxoglutarate. The products of this reaction are L-glutamate-gamma-semialdehyde and L-glutamate. Among the compounds known to inhibit (or inactivate) OAT, only L-canaline and (SS)-5-(fluoromethyl)ornithine [(SS)-5FMOrn] are selective for OAT. Treatment of laboratory animals with 5FMOrn causes a dramatic accumulation of Orn in most tissues and organs, and the enhanced formation of urea due to saturation of ornithine:carbamoyltransferase with its substrate. The enhancement of urea formation by increased endogenous levels of Orn is comparable with that produced by large doses of Orn and arginine, a treatment known to enhance the detoxification of ammonia. However, protection to lethal doses of ammonium salts by exogenous Orn is rapidly fading. In contrast, inactivation of OAT by a small dose of 5FMOrn renders a long-lasting protective effect against various forms of hyperammonemic states. Among these the reduction of ammonia concentrations in blood and tissues, and the reduction of the pathologic excretion of orotic acid to normal levels in mice with hereditary defects of the urea cycle, were most impressive. In human hereditary OAT deficiency the elevated intraocular concentrations of Orn are considered to be a cause of gyrate atrophy. This is presumably the reason, why OAT has not been considered as a therapeutically useful target. Chronic inactivation of OAT by repeated administration of 5FMOrn, caused elevated intraocular Orn concentrations, but this treatment had no effect on the function and histology of the visual system, or the behaviour of adult mice. The confirmation of this and related observations in higher species will show, whether OAT inactivation has potentials in the treatment of hyperammonemic states.

Aberrations of ammonia metabolism in ornithine carbamoyltransferase-deficient spf-ash mice and their prevention by treatment with urea cycle intermediate amino acids and an ornithine aminotransferase inactivator.

Sparse fur with abnormal skin and hair (spf-ash) mice are deficient in ornithine carbamoyltransferase (OCT) activity, but their OCT protein is kinetically normal. We administered ammonium chloride to spf-ash mice, in order to analyze ammonia metabolism and to find a rationale for the therapy of OCT deficiency. Ammonia concentration in the liver of spf-ash mice increased to a level much higher than in the control. Ammonium chloride injection caused an increase in ornithine (Orn) 5 min after injection and an increase in the sum of Orn, citrulline (Cit) and arginine (Arg) for at least 15 min in the liver of control mice, but no increase in Orn, Cit and Arg in the liver of spf-ash mice. Treatment of spf-ash mice with Arg 5-20 min prior to the injection of ammonium chloride kept the hepatic ammonia concentration at a level comparable to that without the load. A significant reciprocal relationship between ammonia and Orn concentrations in the liver of spf-ash mice 5 min after an ammonium chloride load with or without Arg strongly suggests that ammonia disposal is dependent on the supply of Orn. In spf-ash mice loaded with tryptone as a nitrogen source, Arg supplementation showed a dramatic decrease in urinary orotic acid excretion in a dose-dependent manner. Similar effects were observed with Cit and Orn at the same dose, and a long-lasting effect with an ornithine aminotransferase inactivator, 5-(fluoromethyl)ornithine, at a much lower dose. The rate of urea formation in liver perfused with ammonium chloride was lower in spf-ash mice than in controls, but with the addition of Orn to the medium it increased to a similar level in control and spf-ash mice. These results indicate that OCT is not saturated with Orn in vivo under physiological conditions and that the administration or enrichment of the urea cycle intermediate amino acids enhances the OCT reaction so that the ammonia metabolism of OCT-deficient spf-ash mice is at least partially normalized.

Polyamines in human breast cancer and its relations to classical prognostic features: clinical implications.

Experimental evidence suggest an important role of polyamines in breast cancer development. Polyamines have been determined in tissue and erythrocyte samples from 100 patients with primary invasive breast cancer and 30 patients with fibroadenomas. Statistical analysis was performed in order to determine the prognostic value of the polyamine patterns of tumor tissues and erythrocytes in comparison with clinical and histological prognostic factors. In malignant tissues, polyamine levels were significantly higher than in benign tissues. They correlated with markers of tumor aggressivity (axillary node involvement and especially with markers of high mitotic rate as Ki-67 staining, histological grade). No correlation was found between estrogen and progesterone status, tumor size and polyamine concentrations. Erythrocyte polyamines levels were identical between cancer patients and controls. The knowledge of the polyamine pattern in breast cancer could become useful in clinical practice particularly if polyamine metabolism is targeted as a therapeutic approach.

Polyamine metabolism as target for cancer chemoprevention (review).

The natural polyamines putrescine, spermidine and spermine are intimately involved in growth-related processes. More and more evidence indicates that the excessive accumulation of putrescine and spermidine favors malignant transformation of cells. Selective depletion of putrescine has been shown to restore in some transformed cells the normal phenotype. Inhibition of polyamine formation appears, therefore, a rational target in chemoprevention. Clinical trials with 2-(difluoromethyl)ornithine, a selective inactivator of ornithine decarboxylase, a key enzyme of polyamine biosynthesis, are promising. Structural analogs of the polyamines with polyamine-mimetic or antagonist properties, and calmodulin antagonists are other types of drugs which affect several key reactions of polyamine metabolism, and appear to be candidates for the prevention of carcinogenesis especially of the gastrointestinal tract.