Iodine-131 can increase the overexpression of NADPH oxidase (NOX)1 in thyroid tissue, resulting in numerous ROS
[12]. At the same time, mitochondria are more vulnerable to damage when exposed to iodine radiation. This is due to ultrastructural changes resulting in a decreased antioxidant capacity
[36][37]. In other words, the levels of enzymatic antioxidants, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), and thioredoxin (Trx), as well as non-enzymatic antioxidants such as glutathione (GSH), ascorbic acid, and tocopherol, were reduced in response to
131I
[38][39]. Herein, ferroptosis described a novel form of regulatory cell death that was induced by fatal lipid peroxidation
[40], dependent on iron, which was subsequently induced by an oxidation-damaged phospholipid accumulation and associated with the glutathione-dependent antioxidant defense dysfunction mediated by GPX4 via various pathways. Radiation has been shown to induce ferroptosis
[41]. Iodine-131 likely triggered the declines in the metabolism of the lipid peroxides catalyzed by the GPX4 and GSH levels intracellularly and lead to Fe
2+ oxidizing lipids in a Fenton-like manner, which enhanced ferroptosis and was responsible for thyroid cancer cell death
[42]. Comparatively, a GSH deficiency disrupts the REDOX homeostasis, causing ROS accumulation, which eventually results in cell death. The CAT and SOD enzymes play a key role in free radical management, and their reduced activity contributes to an increase in the accumulation of O
2− and H
2O
2 [43][44][45][46][47][48][49]. Additionally, excessive ROS interact with specific cellular targets to trigger a cascade reaction involving polyunsaturated fatty acid free radicals (lipid peroxidation) on the cell membranes, resulting in an increase in the malondialdehyde (MDA) (marker of lipid peroxidation) levels and a decrease in the CAT, SOD, and GSH activity, resulting in an imbalance between oxidants and antioxidants. The excessive depletion of endogenous antioxidants leads to a decrease in the total antioxidant status (TAS), which ultimately contributes to oxidative stress
[48][50][51]. As a result, RAI in the remaining thyroid tissue may result in significant apoptosis and mitotic cell death
[52].
In contrast, although most radiation from RAI enters the thyroid gland, a small amount of
131I present in the blood and tissues is also capable of causing radiation in other parts of the body
[53], such as lipid peroxidation in the kidney, salivary glands, and erythrocytes, resulting in structural and functional damage to the cells
[22][53]. Specifically, reductions in salivary TAS, SOD, CAT, and uric acid molecules may have long-term cumulative effects on the oral cavity. A study found that
131I treatment decreased SOD activity by 40%. The gastrointestinal tract may be adversely affected as saliva is continuously swallowed after secretion
[16]. Other studies have demonstrated that
131I ionizing radiation can indirectly promote or induce significant changes in the red blood cell oxidative and antioxidant status. In addition, it can alter the appearance of erythrocytes, as well as their characteristics, such as their lifespan, permeability, and microcirculation
[52].
3. Antioxidants Reduce 131I Side Effects
In general, it can be observed that oxidative stress mediates the pathological process of almost all the 131I side effects. Herein, the antioxidants showed a robust effectiveness against their side effects. The antioxidants that have been proven to alleviate the side effects of 131I are shown in Figure 3 and the drug type, drug treatment, subject, dose, side effects, and drug efficacy are summarized in Table 1.
Figure 3. The natural and synthetic antioxidants applied to combat 131I side effects.
Table 1. The applications of various antioxidants to alleviate the side effects of 131I. (8-Epi-prostaglandin F2alpha (8-epi-PGF2α); uptake fraction (UF); uptake index (UI); excretion fraction (EF); excretion ratio (ER); first-minute uptake ratio (FUR); maximum uptake ratio (MUR); hypoxia inducible factor-1α (HIF-1α)).
3.1. Natural Antioxidant
Natural antioxidants, sourced mostly from plants, counteract radiation by neutralizing the free radicals produced in the body when it is exposed to the radiation
[70][71]. The mechanism of action generally involves scavenging free radicals and preventing them from damaging cells, tissues, and DNA. As a result, they are capable of shielding the organism cells from damage and aiding in the prevention of cancer and other health problems associated with exposure to radiation
[72]. One of the advantages of natural antioxidants is that they are safer than synthetic antioxidants and have been utilized in conventional medicine for centuries. Furthermore, natural antioxidants are metabolized by the body into harmless compounds, most of which are excreted through normal metabolic processes and are more easily tolerated
[73][74].
Vitamin C as ascorbic acid regulates the activity of the glutamate receptors, lowering the level of free radicals produced by the glutamate release, and has been proven to reduce the frequency of chromosomal aberrations by approximately 30%, significantly reduces the number of DNA breaks, and has a repairing effect on DNA
[75][76].Vitamin C reacts directly with alkoxyl, hydroxyl, and lipid peroxyl radicals or neutralizes them and converts them into water, alcohols, and hydroperoxylated lipids, respectively. Importantly, studies have indicated that vitamin C has a radioprotective effect against oxidative stress, regardless of the timing of administration before and after RAI treatment
[57]. Vitamin C in plasma leads to an increased resistance to lipid peroxidation and a decrease in DNA, lipid, and protein oxidation. In addition, vitamin C leads to the neutralization of free radicals of other antioxidants in the form of glutathione and vitamin E, as well as their regeneration. Approx. 2 days after RAI (5550 MBq), the MDA levels and CAT activity declined and the GSH levels decreased, while the daily administration of 1500 mg vitamin C starting two days before significantly reduced the MDA levels and not only prevented the reduction in GSH, but also significantly increased its levels after RAI treatment
[22].
Additionally, vitamin E is the collective term for four tocopherols (α-, β-, γ-, and δ-tocopherols) and four tocotrienols (α-, β-, γ-, and δ-tocotrienols) found in food, and is a lipid-soluble antioxidant that protects polyunsaturated fatty acids in the membranes from oxidation, regulates the production of reactive oxygen species and reactive nitrogen species, and modulates the signal transduction
[51]. The significant protective effect of vitamin E on the parotid and submandibular glands after
131I (23 mCi) treatment with DTC has been published
[55][77], which was comparable to the results of Filiz Aydoğan et al.
[78]. RAI (111 MBq/kg) resulted in a significant increase in the tissue TOS, TNF-α, IL-6 levels and a significant decrease in the IL-10 and TAS levels, while vitamin D (200 ng/kg/day) dramatically reversed all these parameters
[56]. Meanwhile, sialogogues such as lemon candy, vitamin E, lemon juice, and lemon slices as well as parotic gland massages may all minimize injury to the salivary glands
[10]. Parotid massages, aromatherapy, vitamin E, selenium, and bethanechol showed a significant reduction in the salivary gland dysfunction induced from the
131I treatment (2960–7890 MBq)
[57]. Additionally, keratinocyte growth factor-1 (KGF-1) (100 μg/1 mL PBS) restored saliva homeostasis and reduced the
131I-induced (0.01 mCi/g) cell apoptosis in the mice
[60]. A marker of lipid peroxidation, 8-Epi-PGF
2α, is the outcome of free radical-mediated arachidonic acid peroxidation, and the effect of high-activity treatment (2960 or 7400 MBq) is significantly higher and longer in length than that of low-activity treatment (185 or 740 MBq), with a dose-dependent oxidative damage in vivo
[79]. In the research of Rosário et al., the 8-epi-PGF
2α concentrations were significantly higher in thyroid cancer patients 2 days before and 7 days after the
131I injection, and the increase (percentage) was significantly larger (mean 112.3% vs. 56.3% compared to the intervention group). Iodine-131 (3.7 GBq) after 2 days of plasma 8-epi-PGF
2α significantly increased, while the daily intake of 2000 mg of vitamin C, 1000 mg of vitamin E, and 400 µg of selenium for 21 days before RAI treatment significantly reduced 8-epi-PGF
2α and inhibited oxidative stress
[54].
In terms of the protection against DNA damage, the use of curcumin and alginate as antioxidants reduced the number of DSBs caused by
131I. At the same time, the radiation protection effect of curcumin exceeded that of trehalose
[59]. Melatonin and Se NPs (as radioprotective agents) reduced the
131I-induced DSBs levels in peripheral lymphocytes
[60]. Vitamins E and C were capable of reducing the DSBs levels by 21.5% and 36.4%, respectively
[23]. The positive results of the Barbados cherry fruit radiation protection may be due in part to its rich content of antioxidant compounds, including vitamins A, B1, B2 and C; carotenoids; anthocyanins; phenols; and flavonoids. The
131I (25 μCi) treatment of Wistar rats with an increased thyroid function and associated vitamins and sugars from the Barbados cherry fruit stimulated a significant increase in the mitotic index in the normal cells of the rat bone marrow. In particular, the Barbados cherry juice (5 mg) may act as an effective scavenger of the reactive oxygen species in acute radiation protection treatment, protecting the cells by neutralizing free radicals before and during treatment. Meanwhile, it may play a role in the healing process of ionizing radiation-induced damage after treatment. Barbados cherry sub-chronic treatment has higher radioprotective activity in terms of trapping free radicals or preventing their formation
[61]. N-acetyl-L-cysteine has also been demonstrated to guard against an increase in ROS and eventual DNA damage in thyroid cells caused by
131I in vivo
[62]. Before, during, and after
131I treatment, β-carotene exerts a significant anti-mutagenic/radioprotective activity, stimulates the DNA repair systems, and minimizes chromosomal aberrations and genetic material damage
[12]. Apart from this, resveratrol had anticancer and antioxidant effects, protected the histopathological pattern of the lacrimal gland from damage, reduced inflammation in the histopathological assessment, and decreased the histocytokine levels, apoptosis, and DNA fragmentation on the lacrimal gland after RAI
[63]. Iodine-131 caused an edema of the duodenum and ileum lamina propria, duodenal ulceration, gastric mucosal erosion, and gastric and colonic mucosal degeneration in the rats, whereas lycopene resulted in a statistically corresponding reduction in the inflammation present
[64].
3.2. Synthetic Antioxidants
Synthetic antioxidants have advantages in radiation protection due to their greater potency, consistency, stability, and application flexibility. Despite the fact that natural substances have been used in traditional medicine for centuries, their variability, lack of specificity, and instability require modifications to their properties
[80][81]. Accordingly, synthetic substances offer a reliable and effective way to protect against the harmful effects of radiation. Thus, further research and development is required to create more effective radiation protection, safer synthetic substances for human consumption, and to determine the safe limits for their applications
[82][83][84]. However, it is important to note that synthetic antioxidants can frequently cause adverse health effects when used in high doses
[85].
Iodine-131 (555–660 MBq) treatment with 200 mg/kg L-carnitine or amifostine for 10 days can provide radiation protection and reduce salivary gland injury
[65]. Amifostine is an organic thiophosphate, which is dephosphorylated to the active metabolite WR-1065 in normal tissues. Once activated in the cells, WR-1065 acts as a free radical scavenger. Additionally, many studies have reported the radiation-proof effect on
131I treatment
[86][87].
Iodine-131 causes transient unstable DNA damage composed of reactive oxygen-induced SSBs, and increased chromosome damage in hypothyroidism patients (mutations in enzymes deputed to DNA repair (DNA-1) or in the enzymes involved in the scavenging of free oxygen radicals (DNA-2)). The rhTSH administration reduced radiation exposure by 27% over 120 h and decreased the genomic instability by maintaining hyperthyroidism and normal renal clearance (Epi-GFR and creatinine values). It significantly induced a reduction in the reactive oxygen metabolites-derived compounds. The patients had less radiation-induced chromosome damage, even though several enzyme mutations were present
[13].
Lin et al. prepared a drug delivery system with
131I-labeled caerin 1.1 peptide (F1) (200 μCi
131I and 8 μg caerin 1.1 peptide). The MTT results showed that 5 μg F1 had an inhibitory effect on the CAL-62 cells cultured in vitro. Interestingly, studies identified weight loss over time in the
131I treatment group in vivo, but not in the
131I-F1 or F1 groups. It is possible that
131I-F1 or F1 was confined to the tumor after injection, while
131I may have entered the microcirculation through the blood vessels within the tumor and then entered the internal circulation. In view of the fact that radiation entering the human body can cause acute injury, the occurrence of acute radiation sickness or syndrome characterized by weight loss suggests that
131I-F1 is safer with fewer side effects
[67].
Additionally, synthetic drugs have been studied for the treatment of other side effects. Treatment with dexmedetomidine (3 μg/kg) significantly decreased the levels of MDA, advanced the oxidized protein products induced by RAI (2 MBq), significantly increased the levels of the total sulfur group and CAT, and reduced histopathological abnormalities, which could be applied as a post-
131I liver protection regimen
[68]. In the case of RAI, a high absorbed dose may be produced in the lung parenchyma, thus causing lung damage
[88]. Montelukast (10 mg/kg/day) significantly reduced the degree of inflammation and pulmonary fibrosis in the Wistar rats treated with
131I (111 MBq/kg). The authors attributed this protective effect in part to the antioxidant effect of montelukast
[69].
3.3. Antioxidant Deficiency
In summary, the application of the above antioxidants will hopefully play an important role in alleviating the side effects of
131I. It is important to highlight that even when the use of antioxidants has been shown to ameliorate the side effects of
131I therapy, there are also reports on the drawbacks of using them. Some antioxidants induce oxidative stress at high concentrations (e.g., β-carotene)
[24]. Meanwhile, it has been reported that an excessive vitamin E intake can affect the absorption and function of other fat-soluble vitamins
[89]. Furthermore, synthetic antioxidants have been reported to cause potential health hazards, including liver damage and cancer
[90][91][92]. Therefore, further investigation is needed at a pre-clinical level to standardize the use of antioxidants as adjuvants for
131I treatment.