# Novel Drug Combinations in Lung Cancer: New Potential Synergies Between 5-FU and Repurposed Drugs
## 4 Discussion  
This study provides valuable insights into the therapeutic potential of combining 5-FU with Sildenafil, Tezosentan, Levosimendan, and Resveratrol for treating lung cancer, specifically in the A549 cell line. Our findings indicate that while 5-FU is effective against A549 cells, combining it with other compounds may offer new strategies to overcome limitations associated with 5-FU, such as side effects and drug resistance. Our calculated IC\({}_{50}\) value of 5.03 \(\upmu\)M for 5-FU in the A549 cell line aligns with the range of values reported in the literature. The differences observed between studies can be attributed to various factors, including differences in experimental protocols, cell line handling, and assay methods. Such variability highlights the importance of standardizing experimental conditions to achieve more comparable results across studies. The mechanism of action of 5-FU is well-known, involving the inhibition of DNA and RNA synthesis, leading to cell death [23]. However, the clinical efficacy of 5-FU can be limited by severe side effects and the development of resistance [24; 25]. Thus, exploring combinations of 5-FU with other drugs may help address these limitations. Sildenafil alone showed minimal anticancer activity, but modestly reduced cell viability when combined with 5-FU. This suggests that Sildenafil may enhance 5-FU efficacy by improving tumor perfusion and drug delivery, consistent with reports of Sildenafil improving tumor vasculature [37; 38]. Similarly, Tezosentan, an endothelin receptor antagonist, also exhibited limited anticancer activity individually, but showed a moderate reduction in cell viability when combined with 5-FU. While Tezosentan as a monotherapy was not highly effective, its ability to modulate endothelin receptors--key regulators of tumor growth and resistance--suggests that it could contribute to enhancing the efficacy of chemotherapeutic combinations [39]. Levosimendan demonstrated a more pronounced effect when combined with 5-FU. While Levosimendan alone showed limited anticancer activity, its combination with 5-FU resulted in a significant reduction in cell viability, up to 40%, and notably inhibited the clonogenic potential of A549 cells. This enhancement of 5-FU efficacy by Levosimendan is particularly evident in the clonogenic assays, where the reduction in the number and size of colonies suggests that Levosimendan exerts a more substantial effect on cell proliferation rather than merely affecting cell viability. This impact on clonogenicity is crucial, as it can reduce the likelihood of tumor recurrence and improve long-term cancer control [57; 58]. Tumors with high clonogenic potential are more likely to regrow after initial treatment, so Levosimendan's action of hindering this potential is particularly valuable in cancer therapy. This suggests that Levosimendan might offer clinical benefits by preventing tumor relapse and improving patient outcomes, in addition to its role as a cytotoxic agent. Additionally, the fact that Levosimendan did not significantly affect non-cancerous MRC-5 cells supports its favorable safety profile. This selective impact on cancer cells compared to normal cells enhances its potential for clinical use, as it reduces the risk of adverse effects on healthy tissue. The differential effects observed also highlight the potential of Levosimendan as an adjuvant therapy, particularly in complex tumor environments were enhancing the efficacy of established chemotherapeutic agents like 5-FU is crucial. Resveratrol demonstrated a consistent and pronounced reduction in cell viability when combined with 5-FU, surpassing the effect observed with Levosimendan. Resveratrol's anticancer properties, such as inducing apoptosis [43], inhibiting cell proliferation [42], and suppressing angiogenesis [59], likely contribute to this enhanced effect.