The results of this study provide new insights into the role of redox homeostasis in SCLC patients undergoing chemotherapy. While there is a lack of comprehensive studies directly investigating redox dynamics in SCLC during chemotherapy, our study provides valuable insight into the changes in GSH/GSSG and CyS/CySS ratios during treatment. To our knowledge, this is one of the first studies to investigate these highly significant biomarkers of redox balance in SCLC patients, providing a new perspective on redox homeostasis and response to chemotherapy. Our results show that the redox homeostasis of SCLC patients fluctuates during chemotherapy and that these changes are associated with patient survival. The correlation between GSH/GSSG and CyS/CySS ratios emphasizes the idea of coordinated regulation of redox balance.

An important finding of our study is the significant correlation between the pre-treatment GSH/GSSG ratio and patient survival, which emphasizes the role of a balanced redox environment in increasing treatment efficacy and improving patient survival. Patients with a higher baseline GSH/GSSG ratio (above the 75th percentile) had significantly longer overall survival, emphasizing the prognostic value of this redox marker. This finding is consistent with previous research showing that a balanced redox environment is critical for improving treatment efficacy and survival in cancer patients [14]. Other studies suggest that the GSSG/GSH ratio has the potential of a promising and simple assay that could be further validated as a novel clinical biomarker for cancer diagnosis and monitoring [15, 16]. This is consistent with our hypothesis that the GSSG/GSH ratio could play a similar role in cancer detection and treatment. However, the association observed in the Kaplan-Meier analysis was not confirmed in the Cox regression model. This discrepancy may be due to the limited sample size, reduced statistical power, or differences in how the two methods handle time-to-event data. Further studies with larger cohorts are warranted to validate these findings.

Furthermore, these results raise hypotheses about the tumor microenvironment in SCLC. It is plausible that tumors grow in an atmosphere of high oxidative stress, in which tumor cells could survive by accumulating antioxidants to neutralize the deleterious effects of oxidative stress [17]. Alternatively, the oxidative stress induced by chemotherapy could increase ROS levels to a threshold that exceeds cell survival mechanisms and leads to tumor cell death. In patients with poorer outcomes, it is possible that their tumors accumulate antioxidants more efficiently, thereby neutralizing the effects of chemotherapy-induced oxidative stress.

In this study, we observed significant dynamic changes in redox parameters during chemotherapy. Both GSH/GSSG and CyS/CySS ratios showed a significant decrease after two cycles of chemotherapy, which is consistent with the induction of oxidative stress by chemotherapeutic agents and indicates cumulative oxidative stress during treatment. These results emphasize the marked disruption of redox homeostasis in SCLC patients undergoing chemotherapy. Chemotherapy, especially with agents such as cisplatin and etoposide, is known to induce the production of ROS, which significantly disrupt redox homeostasis [18]. Cisplatin exerts its cytotoxic effect primarily through DNA damage, which triggers cellular oxidative stress through the formation of ROS, especially hydroxyl radicals and hydrogen peroxide [19]. This in turn leads to a depletion of GSH, an important antioxidant that mitigates cellular oxidative damage. A reduction in GSH levels leads to an imbalance between oxidative and antioxidant processes, which further exacerbates cellular stress [20]. Similarly, etoposide, a chemotherapeutic agent used to treat SCLC, generates ROS by stabilizing the topoisomerase II-DNA complex, leading to the formation of DNA breaks and oxidative damage [21]. This also disrupts the GSH/GSSG balance, which promotes the accumulation of oxidized glutathione (GSSG) and increases oxidative stress. In addition, CyS levels are significantly disrupted by chemotherapy-induced oxidative stress, as cisplatin and etoposide promote the formation of ROS, leading to the oxidation of reduced cysteine to cystine and increased consumption of CyS for glutathione synthesis, which ultimately disrupts the CyS/CySS ratio and contributes to a pro-oxidant cellular environment [22]. Our results suggest that the observed decrease of GSH and CyS in the blood may also reflect their redistribution towards the tumors. This could support the hypothesis that in patients with poorer outcomes, antioxidants were preferentially consumed in tumor tissue, decreasing their systemic levels. Such mechanisms could explain the lower GSH/GSSG and CyS/CySS ratios in the blood of these patients. This disruption of the redox balance is thought to contribute to the poor prognosis and treatment resistance observed in SCLC patients undergoing chemotherapy. The GSH/GSSG ratio recovered significantly after four cycles of chemotherapy, indicating the potential for adaptive antioxidant responses in some patients. This recovery may reflect not only the activation of defense mechanisms against ROS, as previously reported in other studies on chemotherapy-induced oxidative stress [23], but also a reduced disease burden due to effective therapeutic response. As oxidative stress has been shown to correlate with tumor burden [15], the normalization of redox ratios in the survivor group may be at least partially attributed to tumor shrinkage during treatment. This possibility should be considered as a limitation in the interpretation of redox dynamics. Similarly, CyS/CySS ratios showed the same recovery trend, suggesting that extracellular redox changes are coordinated with intracellular ones. These results suggest that monitoring both intracellular and extracellular redox markers during chemotherapy could provide valuable insights into patient response and help determine treatment strategies.

In contrast, the patients who died in the first six months of the study had a more stable but consistently lower redox profile, particularly in the GSH/GSSG and CyS/CySS ratios. This pattern suggests that an inability to restore redox balance during chemotherapy may contribute to treatment resistance and poor clinical outcomes. Our study further suggests that maintaining or restoring redox homeostasis during treatment may be crucial for improving outcomes in SCLC patients.

A significant positive correlation was observed between the baseline ratios of GSH/GSSG and CyS/CySS. This correlation is consistent with the expected relationship between these two markers of redox balance. Since both the GSH/GSSG and CyS/CySS ratios reflect the balance between reduced and oxidized thiols, it is plausible that changes in one ratio would affect the other. Higher GSH/GSSG and CyS/CySS ratios indicate a more favorable redox environment and lower oxidative stress. The positive correlation observed in our study supports the idea that both intracellular and extracellular redox status are tightly regulated and that this coordinated regulation is critical for the maintenance of overall redox homeostasis. Furthermore, the finding that the GSH/GSSG ratio is significantly associated with survival outcome suggests that a balanced redox milieu, as indicated by intracellular and extracellular markers, plays an important role in treatment efficacy and prognosis in SCLC.

We also investigated the relationship between redox markers and disease stage. Our subgroup analysis revealed that patients with extensive stage (ES) SCLC had a significantly higher CyS/CySS ratio at baseline than patients with limited stage (LS) disease. Since a higher CyS/CySS ratio indicates a more reduced extracellular redox state, this finding may at first seem contradictory to previous reports associating advanced disease with increased oxidative stress [15, 24]. However, it is possible that the CyS/CySS ratio reflects a compensatory extracellular redox response, rather than directly mirroring intracellular oxidative burden. This may suggest a more complex, compartment-specific redox regulation in SCLC. Alternatively, these findings could indicate that redox remodeling in advanced SCLC involves not only enhanced oxidative stress but also altered antioxidant responses, particularly in the extracellular environment. Therefore, the CyS/CySS ratio might still serve as a prognostic biomarker, reflecting disease-associated redox dynamics beyond classical oxidative stress paradigms.

While this study provides valuable insights into the role of redox status in SCLC, there are several limitations that should be addressed in future research. First, the sample size was relatively small, especially in the subgroup analysis, which may limit the generalizability of our findings. Future studies with larger cohorts are needed to validate these results and further explore the mechanisms by which redox status influences chemotherapy response and survival in SCLC. Furthermore, exploring the relationship between redox markers and specific chemotherapeutic agents could help optimize treatment strategies for individual patients. Additionally, the absence of paired plasma and tumor tissue samples in this study limits the direct interpretation of systemic biomarkers as a precise reflection of intratumoral conditions.