Polyphenols: natural products to treat leukemia

I was asked a question today about polyphenols which reminded me of an editorial I wrote about them a year or so ago. I don't think this has ever appeared on the blog so I reprint it here. It was in response to the Mayo Clinics trial on green tea.

Shanafelt et al. have described the beneficial effects of green tea extracts in four patients with low grade B-cell malignancies [1]. It is one more example of the increasing interest in the possible use of natural products for malignant disease. There is a tendency for scientists to back away from the word “natural” for fear of being contaminated by the beards and sandals brigade flogging organic produce, acupuncture meridians, radio-ionic diagnosis and snake oils of various flavors. We tend to forget that plants have proved a rich source of medicines from digoxin, quinine and aspirin to vincristine and taxotere.

Anyone approaching this subject afresh is met with a bewildering range of names that have no reference point, though they hint at some connection with organic chemistry. Plant products with strong anti-oxidant properties are known generally as polyphenols. Polyphenols are categorized as bioflavonoids, phenolic acids, stilbenes and lignans. Bioflavonoids are the most abundant group—over 6000 have been identified and they provide the color and flavor of fruits, vegetables, berries and flowers. Leukemia Research has recently reviewed the multiple effects of bioflavonoids on gene regulation, cell proliferation and apoptosis [2]. These natural plant pigments are essential for the growth, development and survival. Many are biologically active in mammalian cells as well as plant cells. In a typical study Matsui et al. demonstrated that six individual bioflavonoids, flavone, luteolin, apigenin, genistein, quercetin and fistein could induce apoptosis in leukemia and lymphoma cell lines in a dose-dependent manner [3]. The mechanism of action involved disruption of mitochondrial membranes and activation of caspase-3.

Resveratrol is a stilbene rather than a bioflavonoid, a different type of polyphenol. It too induces apoptosis in B-cell lines and fresh chronic lymphocytic leukemia (CLL) cells, apparently by inhibiting nitric oxide synthetase, nitric oxide being a well recognized inhibitor of caspases. However, the semi-synthetic flavonoid, flavoperidol, has similar actions, suggesting that pro-apoptotic properties may be possessed by a wide range of plant products [4].

Resveratrol is a grape phytoalexin. Phytoalexins are part of the defense mechanism of plants against invading fungi and bacteria. The cruciferous phytoalexins including brassinin, 1-methoxybrassinin, and spirobrassin and related compounds are produced by plants of the Cruciferous family such as broccoli, Brussels sprouts, cabbage and cauliflower. They are not chemically related to resveratrol, being indole based, but were also found to be pro-apoptotic in the Jurkat T-cell line [5].

Many of the products offered by health food shops are not so refined, being mixtures of many phyto-active substances. Propolis is a resinous substance used by bees to maintain their hives. It is a mixture of bioflavonoids, phenolic acids and wax. It has been shown to have a pro-apoptotic effect on a cell line derived from T-cell acute lymphoblastic leukemia by inhibiting the expression of telomerase [6]. Pycnogenol is a commercialized supplement extracted form the bark of the European coastal pine that contains a mixture of bioflavonoids and phenolic acids. It has pro-apoptotic and differentiating effects on HL-60 cells [7].

More familiar plant extracts are also being exploited. The cannabis component tetrahydrocannabinol has been suggested as a new treatment modality for tumors of the lymphoid system. It induces apoptosis in Jurkat cells via the intrinsic pathway [8]. Garlic has been suggested as an anti-cancer agent for 3500 years. Several studies have demonstrated an in vitro effect of the garlic extract, ajoene, in stimulating apoptosis and inhibiting proliferation of leukemia cell lines [9].

Ah! There's the rub. Translation of what happens in the test tube to an effective agent in the patient has been the downfall of many a white hope. Perhaps the best example is the semi-synthetic flavonoid, flavoperidol. In the test tube this drug is able to kill CLL cells, even those refractory to other drugs because of p53 deficiency. In the patients they produced only toxicity. Despite not binding to fetal calf serum, flavoperidol is tightly bound to human serum albumin [10]. It therefore comes as a great relief that the polyphenol extracted from green tea, epigallocatechin-3-gallate (EGCG), has been reported to have a clinical effect [1]. Ever since this same group reported on the in vitro ability of EGCG to induce apoptotic cell death in CLL cells, in part mediated through its effects on the VEGF receptor [11], there can hardly be a CLL patient with access to the internet who has not tried it. Sales of green tea, both caffeinated and decaffeinated, or of EGCG capsules have blossomed. Anecdotally, white counts have risen and fallen. Some swear by it; others decry it. Here, at last, is an objective report. Three patients with CLL and one with follicular lymphoma, of their own volition, took EGCG containing products. Before and after measurements of their disease are available. In three there were partial responses as judged by recognized criteria and in the other a sustained reduction in absolute lymphocyte count. There was little if any toxicity.

Of course, this report can be criticised: it was not a formal trial; we do not know how many patients were taking green tea but failed to respond; we do not know how much EGCG each patient took, or even if the preparation really contained EGCG; the responses may have been spontaneous regressions. Two of the three CLL patients were negative for ZAP-70 (the other was not tested) and it may be that natural dietary supplements have some role in controlling cancers that are principally diseases of accumulation owing to a failure of apoptosis, but will appear much less impressive in diseases with a more prominent proliferative component.

Nevertheless, the Mayo Clinic has initiated a phase I/II clinical trial of green tea in patients with asymptomatic early stage CLL. This trial will have merit in more formally evaluating the activity of green tea and in alleviating what the patients call watch and worry.

References

[1] T.D. Shanafelt, Y.K. Lee, T.G. Call, G.S. Nowakowski, D. Dingli and C.S. Zent et al., Clinical effects of oral green tea extracts in four patients with low grade B-cell malignancies, Leuk Res 30 (2006), pp. 707–712.

[2] P.M. Strissel and R. Strick, Multiple effects of bioflavonoids on gene regulation, cell proliferation and apoptosis: natural compounds move into the lime light of cancer research, Leuk Res 29 (2005), pp. 859–861.

[3] J. Matsui, N. Kiyokawa, H. Takenouchi, T. Taguchi, K. Suzuke and Y. Shiozawa et al., Dietary bioflavonoids induce apoptosis in human leukemia cells, Leuk Res 29 (2005), pp. 573–582.

[4] C. Quincey, D. Dauzonne, C. Kern, J.-D. Fourneron, J.-C. Izard and R.M. Mohammad et al., Flavones and polyphenols inhibit the NO pathway during apoptosis of leukemia B cells, Leuk Res 28 (2004), pp. 851–861.

[5] M. Pilatova, M. Sarissky, P. Kutschy, A. Mirossay, R. Mezencev and Z. Curillova et al., Cruciferous phytoalexins: antiproliferative effects in T-Jurkat leukemic cells, Leuk Res 29 (2005), pp. 415–421.

[6] C. Gunduz, C. Biray, B. Kosova, B. Yilmaz, Z. Eroglu and F. Sahin et al., Evaluation of Manisa propolis effect of leukemia cell line by telomerase activity, Leuk Res 29 (2005), pp. 1343–1346.

[7] W.W. Huang, J.S. Yang, C.F. Lin, W.J. Ho and M.R. Lee, Pycnogenol induces differentiation and apoptosis in human promyeloid leukemia HL60 cells, Leuk Res 29 (2005), pp. 685–692.

[8] C. Lombard, M. Nagarkatti and P.S. Nagarkatti, Targeting cannabinoid receptors to treat leukemia: role of cross-talk between extrinsic and intrinsic pathways in delta 9-tetrahydrocannabinol (THC)-induced apoptosis in Jurkat cells, Leuk Res 29 (2005), pp. 915–922.

[9] H.T. Hassan, Ajoene (natural garlic compound): a new anti-leukemia agent for AML therapy, Leuk Res 28 (2004), pp. 667–671.

[10] Y. Lee, N. Bone, A. Strege, T. Shanafelt, D. Jelinek and N. Kay, VEGF receptor phosphorylation status and apoptosis is modulated by a green tea component, epigallocatechin-3-gallate (EGCG), in B cell chronic lymphocytic leukemia, Blood 104 (2004), pp. 788–794.

[11] I.W. Flinn, J.C. Byrd, N. Bartlett, T. Kipps, J. Gribben and D. Thomas et al., Flavopiridol administered as a 24-hour continuous infusion in chronic lymphocytic leukemia lacks clinical activity, Leuk Res 29 (2005), pp. 1253–1257.