Modifiers of cell survival: Oxygen Effect Flashcards
What is the approximate maximum diffusion distance of oxygen from a normally-oxygenated capillary through a typical respiring tissue?
A. 5 nm
B. 15 µm
C. 200 µm
D. 900 µm
E. 2.6 mm
C
In a typical respiring tissue, the approximate distance that oxygen can diffuse from a normally oxygenated capillary before cellular hypoxia is detectable is approximately 70-200 µm. The oxygen diffusion distance will depend on the partial pressure of oxygen in the capillary and on the rate of oxygen consumption by the tissue, and therefore shows some variability. Thomlinson and Gray measured 150 µm in their landmark experiments in 1955. Olive et al. (IJROBP 1992) determined that the maximum oxygen diffusion distance using solid tumor cubes incubated with fluorescent probes and found it to range from 107 um to 192 µm, depending on the cell line. Torres Filho et al. (Proc. Natl. AcadUSA 1994) measured in vivo oxygen concentration in a SCID mouse model and found hypoxia to occur at distances >200 µm.
A dose of 10 Gy of X-rays reduces the tumor cell surviving fraction to 0.001 in an animal irradiated while breathing air, and to 0.1 in an animal irradiated under nitrogen. An estimate of the hypoxic fraction for this tumor in the air-breathing mice would be:
A. 0.0001
B. 0.01
C. 0.25
D. 10
E. 25
B
The fraction of cells in a tumor that are hypoxic can be estimated using the paired survival curve method. This corresponds to the surviving fraction of cells irradiated in normally oxygenated tumors divided by the surviving fraction of cells from a tumor made fully hypoxic by asphyxiating the host with nitrogen immediately prior to irradiation, which is assumed to render all of the tumor cells radiobiologically hypoxic. Thus, the estimate for the fraction of hypoxic cells would be 0.001/0.1 = 0.01.
The Km for radiosensitization by oxygen (the oxygen concentration at which cellular radiosensitivity is halfway between the fully aerobic and fully hypoxic response) corresponds to an oxygen concentration of approximately:
A. 0.02%
B. 0.5%
C. 3%
D. 15%
E. 30%
B
The Km value occurs at an oxygen concentration of roughly 0.5-1% or 3-8 mm Hg
The most dramatic change in radiation sensitivity occurs over which of the following ranges of oxygen tension (in units of mm Hg or Torr)?
A. 0-30
B. 30-60
C. 60-100
D. 100-250
E. 250-760
A
The most dramatic change in radiation sensitivity occurs over an oxygen tension range of 0-30 mm Hg (Torr). Cells irradiated under an oxygen partial pressure at the low end of this range are maximally radioresistant, whereas irradiation at 30 mm Hg oxygen results in near maximum radiosensitization.
Which of the following statements concerning the oxygen effect is TRUE?
A. OER values obtained for high energy protons used in radiotherapy are similar to those measured for X-rays
B. During irradiation, an oxygen partial pressure of about 30% is required to produce full radiosensitization.
C. The OER is defined as the ratio of the surviving fraction of cells irradiated with a particular X-ray dose under hypoxic conditions divided by the surviving fraction of cells irradiated with the same dose under aerated conditions
D. Tumors are thought to contain regions of both acute and chronic hypoxia; however, only chronically hypoxic cells can reoxygenate
E. The oxygen effect is principally a manifestation of the reaction of O2 with sulfhydryl compounds to form SOO
A
Since the high energy protons used in radiotherapy have an LET similar to that of X-rays, their OER values are also similar. However it is important to know that as LET increases, OER values decrease.
An oxygen partial pressure greater than about 2-3% during irradiation will result in essentially full radiosensitization (Answer Choice B).
The OER is defined as the ratio of the radiation dose needed to cause a certain biological effect in hypoxic cells divided by the dose needed to produce the same effect in aerated cells (Answer Choice C).
Both acutely and chronically hypoxia cells can reoxygenate (Answer Choice D).
The increased cell killing resulting from irradiation in the presence of oxygen is thought to be the result of increased radical damage and damage fixation by oxygen. The initial number of ionizations produced by radiation in the aerated and hypoxic cells would be the same (Answer Choice E).
For single, large radiation doses delivered at a high dose rate, the ratio of the OER for X-rays divided by the OER for 15-MeV neutrons is approximately:
A. 0.3
B. 1
C. 2
D. 4
E. 10
C
Since the X-ray OER is typically about 3 and the OER for 15 MeV neutrons is about 1.6, the ratio of the OERs is about 2.
Which of the following statements concerning the effect of oxygen is TRUE?
A. Oxygen acts as a radiosensitizer because it inhibits chemical repair of DNA
B. The OER and RBE both increase with increasing LET
C. Based on pO2 microelectrode measurements, few human tumors contain regions of hypoxia
D. An oxygen partial pressure of about 30 mM Hg results in a radiosensitivity halfway between hypoxic conditions and fully oxygenated conditions.
E. Exposure of cells to hypoxia may stimulate gene transcription
E
Exposure of cells to hypoxia, as in other stress situations, leads to changes in expression of a number of stress genes, many of which are responsive to the transcription factor, hypoxia-inducible factor-1a (HIF-1a) (HIF1A).
Under normoxic conditions, HIF-1a is hydroxylated on proline residues by oxygen-dependent prolyl hydroxylases. The hydroxylated prolines bind to the von Hippel-Lindau (VHL) protein, which is a component of the E3 ubiquitin-protein ligase complex that ubiquitinates HIF-1a and targets it for degradation.
Oxygen acts as a radiosensitizer principally through its ability to “fix” radiation-induced DNA damage but does not inhibit DNA repair (Answer Choice A).
The OER decreases with increasing LET, whereas the RBE increases with LET until reaching a maximum at approximately 100 keV/µm, and then decreases (Answer Choice B).
Measurements with pO2 microelectrodes and bioreductive probes have demonstrated that hypoxic cells are often present in human tumors (Answer Choice C).
The Km of radiosensitivity for cells (i.e., the concentration at which there is 50% radiosensitivity compared to oxic conditions) is about 3mm Hg (about 0.5%), not 30mm Hg.
Which of the following statements is FALSE when describing tumor hypoxia?
A. In rodent tumors, the hypoxic cell fraction is generally within the range of 5-50%
B. Hypoxia is rarely observed in common human solid tumors
C. Oxygen diffusion and delivery is limited in some parts of tumors
D. Hypoxia can enhance tumor progression by means of hypoxia-related changes in gene expression
E. Hypoxia induces gene amplification and mutation
B
Hypoxia in tumors has been detected using both imaging and direct electrode measurements. The other statements are true.
Which chemical or compound CANNOT be used to mitgate hypoxia-related radioresistance?
A. Nicotinamide and carbogen
B. Perfluorocarbon
C. Amifostine
D. Misonidazole
E. Nimorazole
C
Amifostine is a drug whose active metabolite contains a sulfhydrl moiety and acts as a free radical scavenger. It has been studied as a radioprotectant in several clinical and preclinical settings. As a radioprotectant, it does not sensitize hypoxic cells.
Carbogen is a mixture of 95% oxygen and 5% carbon dioxide and has been used to mitigate chronic hypoxia. Nicotinamide, used concurrently with carbogen, is intended to mitigate acute intermittent hypoxia seen in tumor vessels by preventing intermittent vessel closure (Answer Choice A).
Perfluorocarbons such as perflubron have been shown to improve tumor oxygenation in preclinical cancer models but have not yet shown clinical utility
Oxygen enhancement ratio (OER) changes depends on the type of radiation. Which of the following combinations is FALSE?
A. OER 3.0 for x-rays
B. OER 1.6 for neutrons
C. OER 3.0 for protons
D. OER 0.5 for carbon ions
E. OER 1.0 for alpha-particles
D
OER for energized ions should be 1.0. By definition, OER cannot be smaller than 1.0.
Tirapazamine and other hypoxic cytotoxins have been in preclinical and clinical development. Which of the following statements is FALSE when describing the mechanisms and effects of tirapazamine?
A. If it gains one electron in hypoxic conditions, it becomes cytotoxic
B. When two electrons are extracted in aerobic conditions, it becomes less toxic
C. In normoxic conditions, it can also sensitize cells to radiation
D. Its uptake is greater for cells in hypoxic conditions than cells in aerobic conditions
E. The potency of some chemotherapy agents can be enhanced by the presence of this cytotoxin
D
There is no difference in the uptake of the chemical by aerobic and hypoxic cells, but there is an obvious difference in the action of cell kill due to the amount of oxygen available in the cells.
Tirapazamine (TPZ) is a benzotriazine di-N-oxide which is reduced (i.e. gaining an electron) preferentially under hypoxia with the help of an intracellular reductase forming the reactive, TPZ radical as well as the very potent hydroxyl radical downstream. Hence Answer A is correct.
In contrast, under aerobic conditions, the initial TPZ radical is rapidly back-oxidized (i.e. losing electrons) to the parent compound losing its reactive potency, ultimately driving the selectivity for hypoxic cells (answer B is correct). A byproduct of this reversal is superoxide which may be responsible for the muscle cramps seen in patients
Which of the following statements about tirapazamine is FALSE?
A. A trial examining the utility of the drug in the definitive treatment of locally advanced cervical cancer was conducted but failed to fully accrue due to lack of drug availability.
B. Addition of tirapazamine failed to improve 2-year overall survival in head and neck patients treated with cisplatin-based chemoradiation compared to chemoradiation alone without tirapazamine.
C. The use of tirapazamine was not associated with higher rates of esophagitis in limited stage small cell lung cancer patients treated with definitive chemoradiation compared to historical controls.
D. In GOG 219, tirapazamine increased gastrointestinal toxicity while having no effect on progression free survival.
E. In GOG 219, tirapazamine failed to show benefits in progression free survival but led to significant increase in leukopenia and hepatic/renal dysfunction.
C
In the SWOG 0222 trial, tirapazamine was associated with higher rates of esophagitis compared to historical estimates. Tirapazamine was developed as a hypoxic cytotoxin to potentially enhance tumor responses and showed promise in single-arm phase II trials in several disease sites. However, the phase III trials failed to show efficacy above the control arms. These include GOG 219, which examined the efficacy of the drug when added to standard chemoradiation for locally advanced cervical cancer. The trial failed to accrue due to lack of drug availability. Of a planned accrual of 750 patients, only 379 were eligible and evaluable; no difference in overall or progression-free survival was seen, but there was an increase in grade 3+ leukopenia, GI toxicity, and hepatic/renal dysfunction. The TROG examined tirapazamine’s utility and safety in head and neck cancers in the HeadSTART TROG 02.02 Trial. There was no overall survival or failure-free survival benefit of tirapazamine when added to cisplatin-based chemoradiation in this trial. The authors reported more frequent muscle cramps, diarrhea, and skin rash in the experimental arm.
Which of the following is TRUE regarding the use of the hypoxic radiosensitizer nimorazole in treating head and neck cancers with radiotherapy?
A. The DAHANCA 5-85 trial showed improved locoregional control but not overall survival with the addition of nimorazole.
B. Low plasma concentrations of osteopontin were associated with worse outcomes compared to higher concentrations in the DAHANCA 5 trial but was also associated with a higher benefit from the addition of nimorazole.
C. The addition of nimorazole improves locoregional control in p16- positive tumors but not p16-negative tumors.
D. The addition of nimorazole to radiation is associated with increased acute mucositis and late fibrosis compared to placebo.
E. The DAHANCA 5-85 trial showed improved locoregional control but not disease-specific survival with the addition of nimorazole.
A
The DAHANCA 5-85 Trial examined the addition of nimorazole to conventionally fractionated radiotherapy for pharyngeal and supraglottic larynx cancers. The trial demonstrated that nimorazole improved locoregional control and disease-specific survival compared to placebo but did not significantly improve overall survival.
Nimorazole was reasonably well-tolerated in the DAHANCA 5-85 Trial, although only 60% of patients completed the treatment, and it was associated with a higher rate of nausea/vomiting, flushing, dizziness, and skin rash; patients also had trouble swallowing the large capsules of the drug. There was no increase in mucositis or late complications with the addition of nimorazole (Answer Choice D).
Additional subset analyses of the trial showed that high osteopontin concentration was associated with worse disease-specific mortality, but also improved response to nimorazole in terms of locoregional control and disease-specific mortality (Answer Choice B).
The benefit of nimorazole also appears to be isolated to p16-negative tumors, whereas p16-positive tumors did not appear to benefit (Answer Choice C).
