Anti-tumour activity in organoiron compounds was first reported for several ferricenium salts (for example, the picrate 36 and trichloroacetate 37).28 The inactivity of poorly soluble ferrocene compared to the water-soluble ionic ferricenium salts led to the conclusion that the difference in anti-tumour activities can be traced to solubility. The ferricenium salts (but not the ferro-cenes) presumably have the right balance of lipophilicity and hydrophilicity that facilitate permeability. If this is so, then poorly soluble ferrocenium salts should lack anti-tumour activity, and this was corroborated by the absence of activity for the poorly soluble ferricenium heptamolybdate.28 The corollary would be that appropriate substitution of ferrocene with hydrophilic substi-tuents should result in active compounds. This approach was explored in great detail in a series of polyaspartamide ferrocene conjugates with successful
Compound 38 is an example of a ferrocene (Fc)-containing polyaspartamide conjugate with anti-proliferative activity.48 The backbone of 38 is formed from a mix of a- and /3-/D- and L-aspartamide units linked by amide bonds. The presence of ^-peptide and D- configured units prevent premature enzymatic degradation of the polypeptide. The terminal amide group in aspartamide is modified by alkylation. In 38, a pendant N-dimethyaminolpropyl is attached to some amides while 4-ferrocenylbutanoic acid is linked to other amide moieties in the polyaspartamide chain. Like other polyaspartamide conjugates, 38 functions as a water-soluble prodrug, with water solubility conferred by the tertiary amino function which is protonated at physiological pH. The presence of this group may also facilitate the uptake of the conjugate into cells (by adsorptive pinocytosis). The importance of the positively charged amino group is underscored when 38 is compared with its less active analogue 39 which lacks this feature. Once taken up in the cell, the low pH and presence of lysosomal enzymes promote the cleavage of the conjugate to release the active moiety, 4-ferrocenylbutanoic acid.
There has been considerable debate over the identity of the active moiety which exerts the anti-tumour effect. One view is that once internalized in cells, Fe2+ in ferrocene exists in equilibrium with Fe3+. Electron transfer between Fe2+ and Fe3+ occurs rapidly and reversibly.49 The electrochemical potential for this reaction falls within the range of other biological redox couples.50 Thus, it is highly conceivable that electron transfer occurs between Fe2+/Fe3+ and other biological electron donors/acceptors. Free radicals may be formed in the course of these reactions, and these contribute to anti-proliferative activity.
If this postulate holds true, then the oxidation state of iron (Fe2+ or Fe3+) in the metallocene would not be the most important factor determining activity.49 The more important consideration would be the accessibility of the agent in the cells and the ease with which the Fe2+/Fe3+ equilibrium is set up. The latter is determined by the electrochemical potential of the Fe2+/Fe3+ couple, which can be influenced by substituents on the metallocene,51 as well as the conditions in the body compartment.27 The choice of 4-ferrocenylbutanoic acid as the 'active' ferrocene in the polyaspartamide conjugates is traced to its low formal reduction potential (reflecting ease of oxidation).52 Among the various ferrocene polyaspar-tamide conjugates evaluated, anti-proliferative activity was enhanced if structural modifications resulted in a lower reduction potential for the Fe2+/Fe3+ couple.49
Another view holds that only ferricenium cations, not ferrocene or its derivatives, have anti-proliferative activity. Fe3+ in ferrocenium is a stable free radical cation. It can interact with DNA in many ways: covalent bond formation with nucleophilic groups in nucleotides or proteins, electrostatic attraction to negatively charged phosphate residues, charge transfer complex formation and intercalation with nucleotide bases.28,53 In addition, Osella and co-workers showed that the ferricenium cation can generate the highly reactive hydroxyl radical OH' in aqueous solutions.53 Using electron spin resonance (ESR), they demonstrated the formation of a radical adduct when ferroce-nium carboxylic acid hexafluorophosphate was incubated with a spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) in 4-(2-hydroxyethyl)-1-piperazineethane sulphonic acid (HEPES) buffer (pH 7.4). Addition of Ehrlich ascites tumour (EAT) cells attenuated the ESR signals indicating that the cells have reacted with the free radicals generated from the redox activity of the ferrocenium salt. They postulated that these free radicals cause the fragmentation of DNA in EAT cells. In a subsequent investigation, they demonstrated the generation of hydroxyl radicals from another ferricenium salt, decamethylferrocenium tetrafluorborate (DEMFc+), which is cytotoxic towards the human breast adenocarcinoma cells (MCF-7).54 Unlike other ferricenium salts investigated so far, DEMFc+ demonstrates unusual stability in aqueous solutions.
Several investigations have shown that the unsubstituted ferrocene ring has no anti-proliferative activity.28,53,55 However, this does not hold true for substituted ferrocenes. For example, simple ferrocene derivatives like ferrocenyl-acetic acid and 3-ferrocenylbutanoic acid have shown modest antiproliferative activity against murine EMT-6 cancer cells.49 Ong and co-workers noted that the ferrocenyl acridine 40 inhibited the growth of various cancer cell lines (KB, Hela, Colo-205, Hep) with IC50 values of 1-2 mg/ml.56 The mode of action of 40 is proposed to be mediated by the ferrocenium ion which is formed when Fe2+ in ferrocene is oxidized in the biological milieu (for example, by peroxidases in the presence of hydrogen peroxide). The ferrocenium cation would then react with the nucleophilic groups in DNA. The acridine ring which is known to be an effective DNA intercalator helps to position the ferrocene moiety in the vicinity of DNA, and facilitates the DNA ferrocenium reaction. Unfortunately, no experimental evidence was given to support this hypothesis.
The proposal of a ferrocene DNA interaction has been mentioned earlier in relation to the ferrocifens 5 and 6. Whereas Ong and coworkers56 suggested an interaction mediated by the ferrocenium cation, Top etal.26 propose that a Fenton-like reaction involving Fe2+ (Equations 10.1-10.3) would explain the genotoxic effects of these ferrocifens.
While these findings may indicate that ferrocenes owe their anti-proliferative activity to 'proxy' species like the ferrocenium radical ion, and reactive oxygen species like the superoxide and hydroxy free radicals, there is evidence to support a more direct role for ferrocene. Mono- and di-substituted acetyl and carboxaldoxime ferrocenes have been reported to have in vitro activity against five cancer cell lines.55 The acetyl derivatives (41, 42) were less active than the carboxaldoxime derivatives (43, 44). Disubstitution of the ferrocene ring further promotes activity (42 > 41; 44 > 43). These compounds were found to be inhibitors of topoisomerase II (Topo II), an enzyme which plays a critical role in the recombination, segregation and DNA repair processes encountered during cell division and replication. Inhibition was not due to interference with the binding or hydrolysis of ATP which is required for the catalytic activity of Topo II. Instead, the ferrocenes interfered with the formation of the DNA Topo II complex. It was proposed that each of the diacetyl 42 and dicarbox-aldoxime 44 derivatives initially interacts with Topo II in the absence of DNA. DNA combines with the drug Topo II complex and forms a stable ternary complex which promotes the cleavage of DNA and prevents its relegation.
41:R1=-COCH3, R2 = H 42: R1 = R2 = -COCH3 43: R-| =-CH = NOH, R2 = H 44: R1 = R2 = -CH = NOH
Kovjazin and coworkers showed that ferrocene demonstrated an anti-tumour effect when administered intraperitoneally or orally to mice with B-16 melanoma.57 The anti-tumour effect was observed at very low doses (0.05-0.2mg/kg) which was about 2000 times less than the LD50 of ferrocene. They proposed that the anti-tumour effect of ferrocene was mediated by immune stimulation, possibly against a target like p21ras which is activated by redox signalling. In addition, they noted that immune cells from ferrocene-treated tumour-bearing mice were capable of eliciting an anti-tumour reaction in mice not treated with ferrocene.
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