Here is the interaction caution about this drug as relayed in Health notes (6-10):
Cyclophosphamide requires activation by the liver through a process called oxidation. In theory, antioxidant nutrients (vitamin A, vitamin E, beta-carotene and others) might interfere with the activation of cyclophosphamide.
There is no published research linking antioxidant vitamins to reduced cyclophosphamide effectiveness in cancer treatment.
In a study of mice with vitamin A deficiency, vitamin A supplementation enhanced the anticancer action of cyclophosphamide. Another animal research report indicated that vitamin C may increase the effectiveness of cyclophosphamide without producing new side effects.
Preliminary human research found that adding antioxidants (beta-carotene, vitamin A, and vitamin E) to cyclophosphamide therapy increased the survival of people with small-cell lung cancer treated with cyclophosphamide.
It is too early to know if adding antioxidants to cyclophosphamide for cancer treatment is better than cyclophosphamide alone. Vitamin A can be toxic in high amounts.
Intravenous injections of the antioxidant, glutathione, may protect the bladder from damage caused by cyclophosphamide.
Preliminary evidence suggests, but cannot confirm, a protective action of glutathione in the bladders of people on cyclophosphamide therapy. There is no evidence that glutathione taken by mouth has the same benefits.
As reported here, when laboratory animal and clinical evaluation is the basis for the information, the potential benefits of antioxidants appear. One could say that there has been some backlash at the anti-antioxidant stance promoted by those who quote Dr. Golde and extend his vitamin C research to imply that all antioxidants are problematic for cancer patients.
At the Rush Presbyterian St. Luke’s Medical Center, the suggestion that vitamin E might inhibit radiation effects was discounted. A press report stated (11):
Vitamin E Does Not Protect Cancer Cells Against Radiation
New York. 15 January 2000 (posted 19 March, 2001). Cancer patients who take vitamin E are probably not hindering the desired effects of radiation, according to a laboratory study done by radiation oncologists at Rush-Presbyterian-St. Luke’s Medical Center in Chicago.
Researchers at Rush were concerned that patients who take vitamin E may be inadvertently providing protection for the cancer cells that are the target of radiation therapy. Radiation damage is one form of oxidation, and vitamin E’s antioxidant properties presumably extend to cancer cells.
To determine if this were true, Rush researchers, led by Dr. Ed Blazek, director of radiation biology in the Rush department of radiation oncology, grew cells originating from human breast and prostate tumors in nutrient solutions containing several concentrations of vitamin E. The cells were then irradiated with the same daily doses used for patients.
The Rush team found that the tested concentrations of vitamin E did not interfere with the desired killing of cancer cells by radiation. An important limitation of this study, however, is that the level of vitamin E taken up by the cancer cells in laboratory culture has not yet been measured, and might be smaller than the level taken up by cells of a tumor in the patient’s body. If so, it is still possible that vitamin E might worsen treatment outcomes.
Although no undesirable protection of cancer cells was found, the researchers issued a caution to those taking vitamin E and other alternative therapies. “Any drug that is taken during cancer radiotherapy or chemotherapy should be tested to prove that it does not protect the tumor cells, defeating the intended effect of the treatment,” Blazek said.
Natural extensions of this work would include the addition of the drug pentoxifylline to vitamin E, since this combination has been reported to partially reverse radiation damage to normal tissue, the testing of vitamin C for radioprotection, and the testing of both vitamins E and C for protection from representative cancer chemotherapy drugs.
This research, performed by Drs. Alex Perez and Katherine Baker together with Dr. Blazek, was presented at the annual meeting of the Radiological Society of North America in Chicago.
Then, in a follow-up report from the same hospital, this time including vitamin C (12, 13):
Vitamins C and E Fight Side Effects of Pelvic Radiation for Cancer
March 20, 2001. A small study of 20 men and women suffering from chronic radiation proctitis has shown that daily vitamins E and C substantially reduced or eliminated their symptoms. Proctitis has traditionally been treated with anti-inflammatory agents, without satisfactory results.
Radiation therapy is one treatment option for men with localized prostate cancer and for women with cervix and endometrial cancers. Radiation therapy is effective in killing cancer cells. But the therapy damages also any normal, non-cancerous cells within range of the beam.
Complications are especially common in patients who are treated with older equipment. New, 3D conformal, Intensity Modulated or Proton beam equipment (available in the USA and some other countries) targets the beam much more precisely. Higher doses can be given to tumor with less damage to bladder and rectum.