**5.4 Lignans**

Within the class of lignans, (-)syringaresinol possesses anti-leukaemic behaviour. This is because it induces G0/G1 HL-60 cell cycle arrest in a manner that is both dose and time dependent. This is accompanied by the activation of both caspase-3 and caspase-9, DNA fragmentation and the release of cytochrome c [97].

#### **5.5 Tannins**

Tannins such as woodfordin C, cuphiin D1, cuphiin D2 and oenothein B have been found to possess cytotoxic behaviour on HL-60 cells [145, 168]. Tannic acid also induces apoptosis in HL-60 in both a time and dose dependent manner. The apoptotic mechanism was noted to involve the activation of caspases, PARP cleavage and cytochrome c release. Interestingly, tannic acid enhanced the cytotoxic effect of arsenic trioxide on HL-60 cells. This finding suggests the potential use of tannic acid in combination with arsenic trioxide [98].

#### **5.6 Coumarins**

Apoptotic activity on HL-60 cells was also recorded following treatment with 4-substituted coumarins, as well as furanone-coumarins, with an enhanced activity of caspases -3 and 9 also being recorded [99, 100]. Moreover, interestingly, coumarin was also found to induce cell death in drug resistant HL-60 cells when combined with doxorubicin [101]. This combination has great potential in overcoming the issue of drug resistance.

#### **5.7 Phenolic alcohols and secoiridoids**

While most of the effects reported referred to the inhibitory effect of phenolics, some studies have focused on their differentiating activity. Such studies have focused mainly on HL-60 cells, while other cell lines have been overlooked. Polyphenols from pomegranates and green tea, proanthocyanidins from barley and ellagic acid from fruits such as blackberries, pomegranates and strawberries have been found to induce differentiation HL-60 differentiation [159, 169–171]. Another three studies have focused on phenols from olive oil and the use of an olive leaf extract [102–104]. Two of these studies further confirm that phenolic compounds are capable of inhibiting cell proliferation and inducing differentiation in HL-60 cells. For the olive leaf extract study, the differentiation inducing compounds were found to be oleuropein and apigenin 7-glucoside [103]. The results from the study using olive oil on HL-60 cells show that dialdehydic compounds of elenoic acid with tyrosol and hydroxytyrosol are capable of inducing apoptosis and differentiation. It was reported that the effect of these two compounds was only a minor percentage of the total effect seen using the crude phenol extract [102]. Results from another study using an olive leaf extract with oleuropein as the major constituent show that the extract is capable of inducing both apoptosis as well as differentiation in K562 cells, along the monocyte/macrophage lineage [104].

The apoptotic effects recorded for phenolic compounds on leukaemia cell lines are potentially more similar to those of antitumour antibiotics as opposed to microtubule inhibitors and alkylating agents. This is beneficial as the latter two categories are highly unspecific as they target cells by mitotic spindle inhibition or DNA adduct formation respectively.

## **6. Phenolic compounds and leukaemia:** *In vivo* **studies**

In addition to the *in vitro* effects of phenolic compounds, some *in vivo* studies have also been conducted. These studies focus on the use of gallic acid, curcumin, and resveratrol, and will be discussed in this section.

Using AML xenograft tumour NOD/SCID mice models injected with MV411 leukaemia cells, the effect of gallic acid in combination with daunorubicin and cytarabine was investigated. The results show that when gallic acid was used in combination with such drugs, tumour inhibition was observed when compared to the use of the drugs alone as single agents [91].

Interestingly, both gallic acid and curcumin were found to inhibit WEHI-3 leukaemia cells *in vivo*. Using BALB/c mice injected with WEHI-3 cells, both gallic acid and curcumin caused a reduction in the weights of the livers and spleens of such mice. For gallic acid, it has been postulated that this effect occurs through the increase in macrophage phagocytosis. This finding is particularly interesting in that it contrasts highly with the enlarged spleen associated with WEHI-3 leukaemia. Moreover, both phenolics caused a reduction in the Mac-3 marker (macrophage precursor) percentage [84, 92].

For curcumin, an inhibition of CML was recorded using CML xenograft SCID mice and mice treated with curcumin had smaller tumours. Moreover, plasma exosomes of treated mice were found to contain higher levels of miR-21 [85]. Curcumin *Phenolic Compounds - An Emerging Group of Natural Compounds against Leukaemia… DOI: http://dx.doi.org/10.5772/intechopen.98935*

also inhibits the growth of SHI-1 leukaemia cells in SHI-1 injected SCID mice. The mechanism involves signaling of NF-kB and ERK pathways, and an activation of JNK and p38 [86].

Using mice treated with L1210 cells, resveratrol was found to increase the life span of such mice, as well as the activity of NK cells, which is an important mechanism for eradication of a tumour. Furthermore, lymphocyte proliferation and the humoral immune response were found to be enhanced following resveratrol treatment [172].

### **7. Phenolic compounds and leukaemia: clinical trials**

In addition to *in vitro* and *in vivo* studies, the beneficial effects of phenolic compounds have also been made evident through clinical trials. The main sources of phenolics investigated in such studies have been olive oil, pomegranate juice, *Curcuma longa* and green tea. A daily short-term consumption of olive oil has been found to affect a number of biomarkers related to oxidative stress, with an increase in high density lipoprotein cholesterol and a decrease plasma oxidized low density lipoprotein being observed dose-dependently according to the phenolic content of olive oil [173, 174]. For cancer, clinical trials have shown phenolics to be effective against prostate and colorectal cancer [175, 176].

For leukaemia, the clinical trials that have been performed to date have focused on CLL and utilized olive oil and a green tea extract as the polyphenolic sources. For one study, an olive oil rich in oleocanthal and oleacin at concentrations of 416 mg/kg and 284 mg/kg respectively, was selected. For this trial, performed in 2019, a cohort of 21 patients with CLL Rai stage 0 to II were chosen, who were not receiving any treatment. The effect of daily ingestion of 40 mL of olive oil per day for a period of six months was tested through the analysis of a number of molecular, haematological and biochemical markers at different time points. Such tests included liver function, kidney function, glucose profile, lipidemic profile and an analysis of apoptotic markers CCK18, Apo1-Fas and anti-apoptotic protein survivin. The glucose and lipidemic profiles of such patients were found to improve, the levels of the apoptotic markers CCK18 and Apo1-Fas increased and survivin decreased [177].

Similarly, on Rai stage 0 to II CLL patients, phase I (33 patients) and phase II (42 patients) clinical trials were conducted using a green tea extract (Polyphenon E), containing a standardized dose of EGCG. The results from the phase I clinical trial showed a good toleration of the extract in patients at doses ranging from 400 mg to 2000 mg twice daily, as well as a decline in both the absolute lymphocyte count as well as in lymphadenopathy. The same positive results were obtained in the phase II clinical trial, this time with a twice daily dose of 2000 mg. The side effects were reported to include nausea, transaminitis, and abdominal pain [87, 88].

### **8. Future perspective**

The anti-leukaemic potential of phenolic compounds has been well documented through both *in vivo* and *in vitro* studies. While a number of clinical trials also show the promise of such compounds as treatments for a variety of cancers, more clinical trials on leukaemia are needed in order to ensure that the findings from *in vitro* and *in vivo* studies are confirmed, as well as determine the safety and efficacy of such treatments.
