Tumor Hypoxia and Ozone TherapyAuthor: Kornel Kusznieruk
Affiliation: University of Western Ontario
Date Published: Tuesday October 24th, 2006 @ 23:30:29 EST
Abstract
This paper describes the importance of increasing oxygen content in tumors and provides some possible ways of doing this.
Introduction
Tumors are a rapidly expanding mass originating from one transformed cell. This rapid
expansion does not allow for proper vascularization to occur. At the size of 2-3 mm3 the
tumor would hypothetically stop growing because of its need for a blood supply to
provide oxygen and nutrients (1). Unfortunately, the tumor can coerce the host
endothelial cells to enter the growing mass and provide life support. This is
accomplished, in part, by release of chemoattractant compounds from the growing tumor
or from immune system cells which have entered the growing tumor. These compounds
activate neighboring endothelial cells to migrate to the tumor, to cut through host tissue so that they can arrive at the tumor, and to start proliferating and forming new blood vessels for the tumor. This process is called angiogenesis (2).
Although the host-derived endothelium allows the tumor to grow, its vascular structure is
much different from that of normal tissue. The tumor has no intratumor lymphatics, this
does not permit fluid draining from tumour tissue and as a result high interstitial fluid
pressure develops (3). The high interstitial pressure inhibits drugs from penetrating the
whole tumor tissue, as well as forcing death of some tumor cells. This alteration of
interstitial pressure combined with the rapid rate of tumor cell proliferation ends up
forming a situation where the growing tumor is vascularized, but only to the limited
extend that it needs for it’s own survival. Tumor blood vessels do not contain smooth
muscle lining, are resistant to control by the nervous system, and grow in a disorganized manner compared to vasculature in non-tumor tissue (3). An example of the difference between tumor and non-tumor vasculature is that the former relies on tumor secretion of vascular endothelial growth factor (VEGF) for its survival, whereas the former is insensitive to withdrawl of VEGF (4).
Tumors contain areas of hypoxia (4a). The cause of this is multifactorial and includes poor tumor perfusion by the blood (5), clotting of tumor blood vessels due to activated clotting factors on tumor endothelium (6), and the rapid rate of tumor growth. Hypoxia,and poor perfusion have been shown to negatively correlate with prognosis (7, 8).
Cancer cells under hypoxic environments secrete matrix metalloproteases, which allow
them to metastasize (9). In addition, hypoxia programs cancer cells and macrophages to
secrete VEGF, a protein that stimulates angiogenesis as well as immune suppression (10).
Hypoxia activates hypoxia inducible factor (HIF-1) a nuclear transcription factor which is important in promotion of angiogenesis (11). Besides local hypoxia, late stage cancer patients have lower systemic hemoglobin levels compared to healthy controls, this is due in part to lower renal production of erythropoietin (11a). Lower hemoglobin implies less oxygen transport and therefore reduced tumor oxygenation. Studies aimed at increasing hemoglobin levels by administration of erythropoietin have shown increased efficacy of radiotherapy and chemotherapy (11b).
An important consideration of tumor hypoxia is the role it plays in protection of tumorsfrom anti-tumor defense systems. For example, tumor necrosis factor alpha (TNF-α) is acytokine secreted by activated macrophages and T cells, which as the name implies has antitumor activity. Interestingly, the cytotoxicity of TNF-α to tumor cells is reduced under hypoxic conditions (12). Hypoxia also stimulates production of soluble TNF-α receptors which hypothetically may block systemic activities of this cytokine (13). This may explain the poor efficacy of systemic TNF-α therapy which was attempted in the
1980s (reviewed in 13a). If TNF-α can not kill cancer cells, then it is possible that
immune effectors which use intracellular signalling similar to TNF-α will not be able to work either. Such mediators include the Fas signalling pathway (13b). Since Fas ligand and TNF-α are used by T cells in killing of target cells, the poor efficacy of immunotherapy may be explained by hypoxia.
In accordance with the above point: lymphokine activated killer (LAK) cells are effective in killing certain types of tumor cells in vitro and in vivo but the ability to generate these cells is depressed under conditions of hypoxia. Furthermore, the ability of estabilished LAK cells to kill tumor targets is decreased under conditions of hypoxia similar to those found inside the tumor (14). Proliferation of lymphocytes in response to interleukin-2 is also inhibited during hypoxia (15).
In 1953 the impact of tumor oxygenation on efficacy of radiation therapy was described by Gray et al (16). Since then a great number of studies confirming that tumor sensitivity to radiotherapy positively correlates with tumor oxygenation (reviewed in 17). Several other interventions such as etoposide, doxorubicin, camptothecin and vincristine therapy are oxygen dependent (17a).
The Problem
Tumor hypoxia:
1. Stimulates angiogenesis
2. Suppresses immune function by:
a) Blocking TNF-α toxicity
b) Blocking systemic TNF-α activity by shed receptors
c) Blocking lymphocyte proliferation
d) Blocking LAK cell generation and activity
e) Induces production of the immune-suppressive cytokine VEGF
3. Stimulates resistance to radio- and chemotherapy.
Available Solutions
Hyperbaric Chambers
Methods of increasing tumor oxygenation have been described. One such method
involves exposing a patient to higher oxygen tension by use of a hyperbaric chamber.
This approach has shown marginal increases in oxygenation of some tumors although
clinical efficacy is a matter of debate. A randomized trial of squamous cell sarcoma
patients treated with radiotherapy in the presence of air or hyperbaric oxygen
demonstrated a significantly greater number of patients achieving clinical response in the hyperbaric oxygen group (18). Another randomized study assessing the ability of hyperbaric oxygen to increase efficacy of radiotherapy in cervical carcinoma patients demonstrated no beneficial effects (19). In addition to questionable clinical efficacy hyperbaric oxygen is a costly and sometimes dangerous procedure, which is not commonly used.
Carbogen
Inhalation of carbogen, a mixture of 95% oxygen and 5% carbon dioxide has also been
shown to increase tumor oxygenation both in animal models (20,21) and in the clinical
situation (22-24). An explanation for this effect is that carbon dioxide has vasodilatory functions in this setting which allows for better tumor perfusion of the high concentration of inhaled oxygen (25). Therapeutic benefits of carbogen therapy are mixed although some radiosensitizing effects have been observed alone (26-28), or in combination with nicotinamide in Phase I/II trials (29). Phase III trials are needed.
Proposed Solutions
Hypertonic Saline
Patients suffering from hemorragic shock often incur organ failure due to hypo-perfusion and lose of oxygen. Hypertonic solutions have originally been found useful in the treatment of traumatized patients with hemorrhagic shock. In 1980 Velasco et al demonstrated that an injection of 7.5% NaCl in hemorrhaged dogs restored arterial pressure and cardiac output while increasing survival compared to dogs receiving isotonic saline (30). In the same year, his group published human study in which 12 patients with hypovolaemic shock refractory to volume replacement, corticosteroid and dopamine infusions were administered a bolus of 100-400 ml of 7.5% NaCl. 11 of the 12 patients responded with rise in arterial pressure and the resumption of urine flow, the effects lasting for several hours (31). This treatment has been shown to increase oxygen content of organs in part by causing intratissue fluid to enter blood vessels, and increase blood vessel volume. These effects were postulated to be due to the temporary disruption of osmotic balance, as well, as erythrocytes losing their volume and being able to squeeze through areas which previously have not been perfused. Hypertonic saline therapy has not been associated with toxicity and has been used extensively in the clinical setting (33,34). The hemodynamic-stabilizing effects of saline have even been shown benefitial a hamster model of endotoxemia (35).
This stimulates the question of whether infusions of hypertonic saline may be used in
conjunction or alone for increasing tumor oxygenation. An added benefit to this
approach is immunostimulation! It has been demonstrated in mouse and human that
hypertonic saline therapy increases T cell proliferation and IL-2 production, while
suppressing production of PGE-2 and also suppresses IL-4 production (36-40). IL-4 and
PGE-2 have both been shown to suppress antitumor immune responses (41,42).
Another interesting approach to cancer therapy, which has not been attempted is the
combination of hypertonic saline infustion with systemic interleukin-2. One of the big
problems with clinical administration of IL-2 is induction of a shock-like state called the vascular-leak syndrome (43). Along these lines, a randomized trial was conducted examining crystalloid vs colloid resuscitation. No clear benefitial effect was demonstrated (44).
Although therapy with hypertonic saline seems absurdly simple, we must not forget that cisplatin therapy would have most likely been abandoned had Cvitkovic et al not found that rapid fluid administration can spare the host of this drug’s nephrotoxic effects
(45,46).
Ozone Therapy/Stressed Cell Therapy
Introduction
There are some areas in which ideas practiced in alternative medicine may have clinical
relevance, I believe the area of ozone therapy is one of them.
Ozone therapy is administered through several routes:
1. Directly intravenously,
2. Intrarectally,
3. Exposing patient blood to ozone ex-vivo followed by subsequent re-infusion
(Autohematherapy).
The first method was described by Lacoste in 1951 who used it to treat vascular
insuffiency and gangrene (47). Intravenous administration of ozone is possible since
both ozone and oxygen are very soluble in blood (48), however this technique is
considered out-dated and is hardly used today. Rectal administration has been used with
some efficacy for AIDS associated diarrhea (49), although, by far, the safest and most
widely method of ozone therapy is the third one mention: autohematherapy.
Vasocare therapy, invented by Anthony Bolten of Vasogen, is analogous to
autohematherapy, with the exception that UV irradation and/or heat is added to the
patient cells before reintroduction. In contrast to traditional ozone therapy, the effects of Vasocare have been extensively analysized scientifically with several patents and papers available in the public domain. I will clump together Vasocare and traditional autohematherapy for purposes of this discussion.
Clinical Effects
Reports of ozone therapy are largely anectodal due to it being primarily practiced in
alternative medicine. In this field it is a panacea, irresponsibly claimed by detractors to be a cure for everything from AIDS to diabetes (50,51). Reports such has these have
severely damaged the credibility of ozone therapy, even placing it on the National Cancer
Institute’s list of questionable treatments for cancer (52).
With that said, I will examine the credible literature on this topic. Anacedotally reported, treatment of peripheral limb ischemia has been successful with autohematherapy (53,54).
Recently, Tylicki et al treated 12 patients with lower limb atherosclerotic ischemia. Of these, 11 had prolonged ability to walk on a threadmill post-treatment (55). In another pilot study symptoms of atherosclerotic vasculopathy were improved (56). Utility of Vasocare treatment was demonstrated in peripheral vascular disease in a two centered double-blind study by Baird and Belch in the United Kingdom where the treatment group reported a greater than 50% increase in walking distance compared to controls (57). Vasogen has currently received approval for a Phase III pivotal trial in peripheral arterial disease (58).
Vasogen has also demonstrated efficacy of Vasocare therapy in autoimmune diseases
both in animal models and in humans, unfortunately, these studies have not been
published in the peer reviewed literature but only as patents (59-61). More relevant to our discussion, Vasogen noticed some anti-leukemia effects in its trial on graft versus host disease, this is the basis for an ongoing trial of Vasocare in chronic lymphocytic leukemia (62).
Mechanisms
It is postulated that these effects are mediated in part by the ability of the treatment to increase tissue oxygenation, while at the same time having an anti-inflammatory effect(reviewed in 63). Erythrocytes in patients receiving autohematherapy are postulated to be better oxygen carriers than controls due to an increase in 2,3 diphosphoglycerate (64). A randomized trial comparing autohematherapy to hyperbaric oxygen demonstrated that only autohematherapy decreases blood viscosity while increasing erythrocyte filterability
(65). Both of these changes would hypothetically increase tumor oxygenation. Another
mechanism of ozone therapy may be induction of nitric oxide (NO). NO possesses many
effects, but one of the well known ones is vasodilation (66). In Vasogen mentions in two of their patents, that Vasocare blocks platelet aggregation and induces NO (67,68).
Once again, these effects could hypothetically increase tumor oxygenation.
Ozone therapy may also have direct effects on cancer cells. In the 1920s Warburg
postulated cancer cells preferentially utilize anaerobic metabolism (69). Others after him have claimed that high concentrations of oxygen can specifically kill tumor cells, although this was for the most part proven wrong. Interestingly, ozone can preferentially inhibit various primary human tumor cells while sparing normal tissue, in vitro (70).
Similar results were obtained in leukemic cell lines (71).
Conclusion
Ozone therapy has been used for millions of treatments without toxicity (cited in 72).
Under a different form (Vasocare) ozone therapy is now entering Phase III trials. The
combination of ozone therapy with radio, chemo and immune therapies should increase
efficacy of these treatments by overcoming tumor hypoxia. Future studies in this area are
definitely warranted.
________________________________________
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