Cytotoxic species

The anticancer activity of complex natural products having a cyclodecenediyne system [for a review see <96MI93>] has prompted the synthesis of 54 (X = CH2 and OCH2) <96CC749> and 55 (R = a-OH and p-OH) <95AG(E)2393> on the basis that such compounds are expected to develop anticancer activity as the P-lactam ring opens. This is because cycloaromatization can only occur in the monocyclic enediyne and the diradical intermediate in the cyclization is thought to be the cytotoxic species. 

However, this will also prevent the formation of other cytotoxic species such as the hydroxyl radical. Further evidence that antioxidants prevent against the development of I/R injury comes from studies that show that previous ischaemic injury in the small intestine of rats results in prevention of further damage on subsequent ischaemic challenge in association with measured increases in mucosal glutathione peroxidase and catalase (Osborne et al., 1992). 

FIGURE 3.3 Scheme of photophysical and photochemical processes involved in the production of cytotoxic species during photoexcitation of photosensitizer (PS adapted from Castano et ai, 2004). 

Dacarbazine is a synthetic compound that functions as an alkylating agent following metabolic activation by liver microsomal enzymes by oxidative N-demethylation to the monomethyl derivative. This metabolite spontaneously decomposes to 5-aminoimidazole-4-carboxamide, which is excreted in the urine, and diazomethane. The diazomethane generates a methyl carbonium ion that is believed to be the likely cytotoxic species. Dacarbazine is administered parenterally and is not schedule-dependent. It produces marked nausea, vomiting, and myelosuppression. Its major applications are in melanoma, Hodgkin s disease, and soft tissue sarcomas. 

However, this reaction is expected to be endothermic due to a more negative potential for the NO, H+/HNO couple (147, 164). Dehydrative dimerization of HNO to N20 [Eq. 6 8 x 106M 1s 1 (106)] is substantially faster then deprotonation to 3NO- [Eq. 10 5 x 104M l s OH ] (106)]. These kinetic constraints indicate that 3NO and subsequent ONOO- formation (Eq. 9) does not occur until the HNO concentration drops to the low nanomolar range, which is below the current observation threshold. Furthermore, the oxidative chemistry from Angeli s salt was found to be pH-independent (167), suggesting that HNO not NO- reacts with O2 to form the oxidant and cytotoxic species. Although the nature of the oxidative intermediate has not been determined, it is apparent that the HNO/O2 reaction is fundamentally different than that of the NO/O2 (and the NO /02) reaction in resultant reactive intermediates and product formation. 

Several drugs used to treat cancer are metabolized by cytochromes P450, and it would be anticipated that if the parent drug were the cytotoxic species, inhibition of its metabolism would enhance cytotoxicity, which could either be beneficial if controlled or cause severe toxicity to bone marrow, the nervous system, etc., if concentrations of the parent drug became too high (100). 

Kratochwil NA, Zabel M, Range K-J, Bednarski PJ. Synthesis and X-ray crystal structure of fra s,cis-[Pt(OAc)2I2(en)] A novel type of cisplatin analog that can be photolyzed by visible light to DNA-binding and cytotoxic species in vitro. J Med Chem 1996 39 2499-507. 

Kratochwil NA, Bednarski PJ, Mrozek H, Vogler A, Nagle JK. Photolysis of an iodoplatinum(IV) diamine complex to cytotoxic species by visible hght. Anti-Cancer Drug Res 1996 11 155-71. 

Peroxynitrite is a cytotoxic species generated by the reaction between superoxide and nitric oxide. Catechin polyphenols could also decrease the peroxynitrite-induced nitration of tyrosine and protect apolipoprotein B-lOO of LDL from peroxynihite-induced modification of critical amino acids, which contribute to its surface charge. ... 

Pryor and coworkers have shown that peroxynitrite-mediated nitrosations and nitrations of phenols are modulated by CO2. The reaction was found to be first order with respect to peroxynitrite and zero order with respect to phenol, showing that an activated intermediate of peroxynitrite, perhaps the peroxynitrite anion-C02 adduct (0=N—OO—C02 ), is involved as the intermediate (equation 57) . At pH higher than 8.0, 4-nitrosophenol is the major product, whereas in acidic media significant amounts of the 2- and 4-nitrophenols were formed. Peroxynitrite also induces biological nitration of tyrosine residues of the proteins. The detection of 3-nitrotyrosine is routinely used as an in vivo marker for the production of the cytotoxic species peroxynitrite (ONOO ). It was shown that nitrite anion (N02 ) formed in situ by the reaction of nitric oxide and hypochlorous acid (HOCl) is similarly able to nitrate phenolic substrates such as tyrosine and 4-hydroxyphenylacetic acid . 

However, these indirect effects of nitric oxide derived products are far more prevalent under pathological conditions such as inflammation, where the production of both NO and by the professional phagocytic cell NADPH oxidase enzyme, and induction of iNOS yields the potent cytotoxic species peroxynitrite. Whilst nitric oxide will react with metal centres (as discussed above) at a rate of 5x 10 M" s and the superoxide anion can be dismutated by SOD at a rate of 2.3x10 M s the combined reaction below (Eq. 9), proceeds at a rate faster than either of these individual reactions ... 

Peroxynitrite (ONOO ) is a cytotoxic species generated by the reaction between snperoxide and nitric oxide (NO), which is a very strong oxidant and can cause oxidation of cell membrane protein and lead to cell damage and diseases. It can also generate hydroxyl radicals and NO2 nnder acidic conditions. Pannala et al." found that the scavenging effect of ECG and EGCG on ONOO was more pronounced than that of EC and EGC. Catechins were also found to protect from peroxynitrite-... 

Nitric oxide can act as a vitamin E-like inhibitor of radical chain propagation reactions via radical-radical reaction with cytotoxic species such as lipid peroxyi (LOO-) and aikoxyl (LO-) radicals. Nitric oxide reacts with yLO- and LOO- at near-diffusion-limited rates (for LOO-, ife = 1.3 X 10 M sec" ) (Padmaja and Huie, 1993). This rate constant... 

L.K. Folkes and P. Wardman, Oxidative activation of indole-3-acetic acids to cytotoxic species - a potential new role for plant auxins in cancer therapy, Biochem. Pharmacol., 61 (2001) 129. 

The first observation of suppression of cancer cell proliferation by photoexcited PSi microparticles in vitro was reported in Ref (Timoshenko et al. 2006) where the number of mouse fibroblasts relative to the control decreased after illumination in the presence of PSi with concentration above 0.5 g/L. The death of 80 % of cells was detected for the concentration of 2 fL. These facts were explained by the effect of photosensitized generation of SO because of PSi microparticles. At the same time, the effect of PSi in darkness was almost negligible over the PSi concentration range of 0.5-2.5 g/L. Further experiments revealed that photoexcited PSi microparticles resulted in an increase of the apoptosis rate for cancer cells (Timoshenko et al. 2007). The death of cancer cells was related mostly to the oxidizing action of photosensitized SO, while other reactive oxygen species (ROS), e.g., superoxide ions 02, could also contribute to the PDT effect. The ROS generation by non-oxidized PSi microparticles in solution was revealed by means of the fluorescent probe and by detection the ROS-induced formation of cytotoxic species in vitro (Low et al. 2010). 

---