## 4 Discussion
FTS was also administered orally to 30 patients with _KRAS_ mutant NSCLC in phase II clinical trial; unfortunately, no significant antitumor effect was observed [41]. This lack of FTS effectiveness in vivo might be caused by alternative resistance mechanisms (i.e., _KRAS_ gene amplification) or off-target effects and requires further research [42]. KRAS signaling can also be suppressed by interrupting the interaction between KRAS and PDE8. One of the molecules binding the prenyl-binding pocket of PDE8 is deltarasin, which downregulates the RAS protein [33]. In our study, deltarasin downregulated _HRAS_ mRNA levels in the A549 cell line after 48 h of incubation and showed growth inhibitory effects on A549 cancer cells with IC50 2.5 uM after 24 h of incubation. [PERSON] et al. indicatedthat deltarasin inhibits lung cancer cell lines' A549 and H358 growth by inducing both apoptosis and autophagy. They showed deltarasin significantly reduced cell viability in A549 in a dose-dependent manner, with IC50 values of 5.29 \(\pm\) 0.07 [15]. Similar results were obtained by [PERSON] et al., with IC50 6.90 \(\pm\) 0.96 for A549 cells treated with deltarasin after 72 h incubation [20]. Moreover, incubation of A549 cells with deltarasin significantly reduced the amount of GTP-RAS observed compared to the control cells [15], which could explain the decrease in _HRAS_ expression level in NSCLC observed in our experiment. In another study, [PERSON] et al. confirmed that PDE\(\delta\) disruption effectively disrupted KRAS protein plasma membrane localization [14]. These findings suggest that deltarasin plays a role in suppressing the RAS downstream signaling pathways in lung cancer cells and seems to be an interesting therapeutic agent. RAS proteins were previously thought to be \"undruggable\" due to the absence of apparent drug-accepting pockets in their structures. However, [PERSON] et al. found Kobe0065, a compound that inhibits the binding of HRAS GTP to c-RAF-1, to inhibit HRAS and RAF interactions, and thus inhibit the growth and induce apoptosis of HRASG12V-transformed NIH 3T3 cells [21]. Following on, [PERSON] et al. designed 21 analogues (TKR01-TKR21) with a structure of urea or thiourea on the basis of Kobe0065 and assessed their cytotoxic effects on A549 cell line with the aim of blocking KRAS-effector protein interactions. Among these inhibitors, 1,3-bis(4-(Trifluoromethyl)phenyl)thiourea, named TKR15, significantly inhibited A549 cell growth, inducing 30% to 80% apoptosis in comparison with controls [37]. Unfortunately, no similar results were achieved in the present study. No significant decrease in viability was observed for cells treated with different concentrations of Kobe0065, and no significant changes in _KRAS_ expression level were found after incubation with the selected RAS-dependent signaling inhibitors in vitro. This could be explained by the fact that the A549 cell line is a human lung adenocarcinoma cell line with _KRAS_ G12S mutation [14] and, hence, may demonstrate initially impaired _KRAS_ expression. There is also some evidence that in vitro drug screens might not be enough for RAS inhibitor examination because full RAS inhibitor effectiveness require complete inflammatory tumor-to-host interactions and might be observed only in vivo [20]. Moreover, single agent therapy could not be effective for all RAS-dependent cancers; therefore, future studies should investigate the use of combinations of several inhibitors or modified existing compounds [36]. Our findings regarding the _KRAS_ and _HRAS_ genes in lung cancer are preliminary and require further research to better understand their expression and clinical significance. The authors are aware of the limitations of results presented in _KRAS_ and _HRAS_ mRNA expression in A549 NSCLC cell line; however, these results may contribute to further research on RAS inhibitors in non-small cell lung cancer and be a prelude to future research plans.