## 3 Results and Discussion
### Determination of Antitumor Activity In Vitro  
Cancer is a major public health concern, with estimates of increasing incidence, prevalence and death rates in the coming years [124]. Within this perspective, the search for new molecules capable of acting in anticancer therapy has been one of the main targets of the pharmaceutical industry, especially those of natural origin [125]. Considering the period from 1981 to 2019, about 25% of all anticancer medications were based on natural products [126], which reinforces the importance of studies with propolis within this perspective. The extracts of red propolis have presented a great relevance in recent years in relation to their antiproliferative action against tumor lines [127]. The MTT viability test was applied to determine the cytotoxic activity of the ten red propolis extracts on distinct tumor cell lines (HL-60 leukemia, HCT-116 colon carcinoma, SF295 and SNB19 glioblastoma, as well as PC3 prostate carcinoma). The results presented here show that the majority of the extracts (tested at a 50 \(\upmu\)g\(\cdot\)mL\({}^{-1}\) concentration) changed the cell viability of the tested lines (Figure 4), with a significant decrease in the cell concentration (\(p<0.05\)). The percentage of inhibition varied according to the tumor lineage and the extract analyzed, where for the HCT-116 line the range was between 1.73 and 100% for samples E:UT and E:50\(-\)20, respectively; for the PC3 line the range found was from 2.16 to 100.00% for samples E:UT and E:25\(-\)20, respectively; for the SF295 line the range was from 5.21 to 100.00%, for samples E:25\(-\)30 and E:25\(-\)20, respectively; and from 67.89 to 97.52%, samples E:25\(-\)10 and E:25\(-\)20, respectively, for line SNB19, with significant differences among samples (\(p>0.05\)). For the HL-60 line, regardless of the conditions, all the extracts presented a percent inhibition that was higher than 99%, without showing significant differences between the analyzed samples (\(p<0.05\)) (Figure 4a-e). Figure 4: Percent growth inhibition of tumor cell lines (**a**) HL-60 (leukemia), (**b**) HCT-116 (colon carcinoma), (**c**) PC3 (prostate carcinoma), (**d**) SF295 (glioblastoma), and (**e**) SNB19 (glioblastoma) by red propolis extracts obtained under different conditions. Values with the same letter in the same analysis do not show significant differences (\(p>0.05\)) under [PERSON]’s test at 95% confidence. Average of analyses obtained from triplicates (\(n=3\)). In general, the extracts with the highest concentrations of commonetin and kaempfer (E:50\(-\)10, E:50\(-\)20 and E:50\(-\)30) were those presenting higher cell growth inhibition for the tested lines, thus suggesting that these compounds may be associated with the strong cytotoxic action of red propolis against the five neoplasm lines tested in this study. Therefore, it was observed that the extracts obtained at a temperature of 50 \({}^{\circ}\)C were those that showed the greatest cell proliferation inhibition, thus showing the importance of using an adequate temperature, together with the addition of ultrasound, to find extracts with higher biological potential. Different authors have highlighted the antitumor activity of red propolis by comparing it with other types of propolis, as it has been shown to have superior cytotoxic action against different tumor cell lines [19; 41; 48]. [PERSON] et al. [128] evaluated red propolis extracts and their fractions that were rich in formonentin in the inhibition of hematological tumor cells, and they observed the strong antitumor activity due to the presence of this flavonoid. [PERSON] et al. [21] and [PERSON] et al. [84] associated the biological properties of the red propolis extracts with the presence of formonentin, whereas other studies showed that formonentin, in synergy with xanthocimol, confers strong antitumor action to this matrix [20; 129]. Some studies indicate that the antiproliferative action of this type of propolis may be associated to the mechanism of cell cycle arrest and apoptosis [130; 131]. [PERSON] et al.