**5.3. The mechanism of reactive oxygen species production of doxorubicin**

Under the standard physiologic condition, ROS can be by synthesis only 1–5% of oxygen consumption [11]. The most acceptable hypothesis of DOX's toxicity is extensive ROS production [4]. The reason for elevation by DOX is associated with its accepting and donating electrons. DOX contains a hexose sugar with tetracycline having quinone and hydroquinone moieties, which are part of the capture electron, producing semiquinone. A superoxide radical can be provided by semiquinone from an oxygen molecule. A superoxide radical does not have a


DOX's ROS production effect is capable of transferring one electron to oxygen resulting in superoxide radicals. So, DOX oxidizes complex I of ETC [44]. DOX contains a quinine moiety, so it can reduce one electron catalyzed by NADPH, resulting in production of semiquinone free radicals. The semiquinone can undergo oxidation by molecular oxygen to superoxide

Mitochondrial Dysfunction Associated with Doxorubicin http://dx.doi.org/10.5772/intechopen.80284 337

The reason why mitochondrial ROS production is crucial is because it could amplify its detrimental effect by triggering intracellular signal pathways. According to one previous study, mitogen-activated kinases (MAPK) have participated in DOX's cardiotoxicity by ROS production [5]. Research has suggested that cardiotoxicity induced by DOX involves p-JNK, the p-ERK1/2 [61], as well as p38 [5]. Based on our previous studies, the renin–angiotensin system also crosstalks with DOX's toxicity [62, 63]. However, we need to investigate which intracel-

Apoptosis plays a role in developmental and homeostatic mechanisms. So, uncontrolled apoptosis relies on an illness, e.g., cancer [64]. This is why apoptosis, known as programmed cell death [65], has a role in the development of cancer and cancer treatment [2]. Apoptotic pathways start as intrinsic or mitochondrial and extrinsic stimulus stimulated by the cell death receptor [64]. There are many ways to initiate the intrinsic apoptotic path, particularly nutrient deficiency, genotoxic damage induced by cytotoxic chemotherapies, and radiation [64].

Extensive research has been conducted on DOX's apoptotic pathways. The evidence supports a significant role of oxidative stress induced by DOX. The difficulties in determining DOX's apoptotic pathways are related to the drug's dosage, route [26], and duration of treatment [9]. It is almost impossible to explain a single, unique apoptotic pathway induced by DOX [26]. However, literature data have suggested that DOX has been reported to trigger apoptosis in

The pathways are controlled under pre- and proapoptotic factors. Apoptosis can be triggered by proapoptotic factors such as Bax or Bak activation by BH3-only protein, BIM, and also BID. When Bax and Bak become oligomerized, mitochondrial outer membrane permeabilization occurs and results in releasing cytochrome-*c* to the cytosol. Then the apoptosome can be formed by cytochrome-*c* with apoptotic protease-activating factor-1 (APAF-1), resulting in a triggering caspase cascade, including caspase-3 (**Figure 4**). In contrast to proapoptotic factors, e.g., Bcl-2, Bcl-XL can prevent apoptotic pathways maintaining monomeric Bax/Bak or BH3 only proteins [64]. Caspase-8 participates in extrinsic pathways, whereas caspase-3 and -9

Bax activation releases cytochrome-*c* by the mitochondrial permeability transition pore (PMT) activation, resulting in APAF-1 activation [26]. After the apoptosome complex is formed by APAF-1, cytochrome-*c*, dATP, and caspase-9, procaspase-3 can be transformed into its activated form by the apoptosome [67]. Alternatively, DOX can facilitate apoptosis through mitochondrial p53 by depolarizing MMP. Recently published data are suggests that p53 elevation

lular signal pathways are potentially involved in DOX's toxicity.

both pathways, intrinsic or mitochondrial and extrinsic [57, 66].

by DOX treatment influences Bcl-2 decline and Bax expression [26, 67].

have a role in the intrinsic route [2].

**5.4. Apoptotic cell death induced by doxorubicin**

oxygen radicals [30, 38].

**Table 1.** Comparison of some parameters of acute and chronic doxorubicin toxicities.

potentially harmful effect. However, superoxide radicals can be transformed by superoxide dismutase converting to H2 O2 ; this is called a Fenton or Haber–Weiss reaction, and the highly toxic hydrogen radical can be produced from H2 O2 . DOX can be reduced in some intracellular enzymes, e.g., xanthine oxidase and microsomal NADPH-cytochrome P450 reductase that expresses in almost all cells. Mitochondrial NADH dehydrogenase, which mediates to produce ROS when DOX is present, is not present in other tissues, except cardiac tissue. This is why DOX is highly toxic to heart tissue because it causes ROS to elevate [10].

Given more detailed knowledge regarding its structure and radical formation, DOX can be reduced at the C13 position from doxorubicinol. Although DOX can be transformed to doxorubicinone at its daunosamine sugar by acid-catalyzed hydrolysis, doxorubicinol can also undergo the same acid-catalyzed hydrolysis to form doxorubicinolone. Both can then experience protonation at C7, resulting in the formation of 7-deoxydoxorubicinone and 7-deoxydoxorubicinolone, respectively, by deletion of the sugar. After double reducing DOX, a tautomer of C7 deoxyaglycone, that is, C-7-quinone-methide, is produced. C7-quinonemethide can connect to DNA and form free radicals [22].

The drug can form ROS via two pathways: the first is iron dependent, and the second is redox cycling, which is catalyzed by NADPH oxidoreductases [30]. DOX has one of the paths, which produces ROS, and is mediated by iron (Fe). According to the Haber–Weiss reaction, the superoxide radical formed by DOX could be transformed into H2 O2 , and then a hydroxyl radical can be produced by H2 O2 in existing iron. Another way is for DOX to directly interplay, resulting in a ferro (Fe2+) to ferric (Fe3+) form of abundant ROS [28].

Oxidative stress produced by DOX relies on nitric oxide synthase (NOS) and nicotinamide adenine dinucleotide phosphate-oxidase (NOX). NOX and/or NOS can transform DOX to its semiquinone form, causing oxidative stress. When nitric oxide is produced by NOS, peroxynitrite, reactively oxidizing DNA, proteins, and lipids are produced as by-products. Moreover, two isoforms of NOS, namely endothelial NOS and inducible NOS (iNOS), have been reported to play a role in DOX's toxicity to produce RNS. Besides NOS, DOX can synthesize radicals by complexing with iron to produce hydroxyl radicals, which are also very dangerous for cells and can have a detrimental effect on DNA, proteins, and especially lipids [20].

DOX's ROS production effect is capable of transferring one electron to oxygen resulting in superoxide radicals. So, DOX oxidizes complex I of ETC [44]. DOX contains a quinine moiety, so it can reduce one electron catalyzed by NADPH, resulting in production of semiquinone free radicals. The semiquinone can undergo oxidation by molecular oxygen to superoxide oxygen radicals [30, 38].

The reason why mitochondrial ROS production is crucial is because it could amplify its detrimental effect by triggering intracellular signal pathways. According to one previous study, mitogen-activated kinases (MAPK) have participated in DOX's cardiotoxicity by ROS production [5]. Research has suggested that cardiotoxicity induced by DOX involves p-JNK, the p-ERK1/2 [61], as well as p38 [5]. Based on our previous studies, the renin–angiotensin system also crosstalks with DOX's toxicity [62, 63]. However, we need to investigate which intracellular signal pathways are potentially involved in DOX's toxicity.
