The potential use of vitamin D analogs in the treatment of cancer

The Role of Dietary Supplements during Cancer Therapy

This guide was compiled after recommendations by the American Institute for Cancer Research (AICR) Cancer Resource Advisory Council. It encompasses the AICR position on current issues in nutrition for cancer survivors during treatment and is intended to provide advice about dietary supplements for cancer survivors who are still being treated. Current scientific findings about the safety and effectiveness of some commonly used dietary antioxidants and nonantioxidant supplements during chemotherapy are presented and assessed. Use of dietary supplements during cancer treatment remains controversial. Patients are cautioned that vitamin and mineral supplements as therapies are not substitutes for established medicine. The current recommendation for cancer patients is to only take moderate doses of supplements because evidence from human clinical studies that confirm their safety and benefits is limited. A daily multivitamin containing supplements at the levels of the Dietary Reference Intakes can be used safely as part of a program of healthy nutrition. In addition, the AICR Cancer Resource Advisory Council concluded that further scientific research is needed to provide a set of firm guidelines for the use of vitamin and mineral supplements by cancer patients during treatment.

Calcium and vitamin D. Their potential roles in colon and breast cancer prevention

The geographic distribution of colon cancer is similar to the historical geographic distribution of rickets. The highest death rates from colon cancer occur in areas that had high prevalence rates of rickets--regions with winter ultraviolet radiation deficiency, generally due to a combination of high or moderately high latitude, high-sulfur content air pollution (acid haze), higher than average stratospheric ozone thickness, and persistently thick winter cloud cover. The geographic distribution of colon cancer mortality rates reveals significantly low death rates at low latitudes in the United States and significantly high rates in the industrialized Northeast. The Northeast has a combination of latitude, climate, and air pollution that prevents any synthesis of vitamin D during a five-month vitamin D winter. Breast cancer death rates in white women also rise with distance from the equator and are highest in areas with long vitamin D winters. Colon cancer incidence rates also have been shown to be inversely proportional to intake of calcium. These findings, which are consistent with laboratory results, indicate that most cases of colon cancer may be prevented with regular intake of calcium in the range of 1,800 mg per day, in a dietary context that includes 800 IU per day (20 micrograms) of vitamin D3. (In women, an intake of approximately 1,000 mg of calcium per 1,000 kcal of energy with 800 IU of vitamin D would be sufficient.) In observational studies, the source of approximately 90% of the calcium intake was vitamin D-fortified milk. Vitamin D may also be obtained from fatty fish. In addition to reduction of incidence and mortality rates from colon cancer, epidemiological data suggest that intake of 800 IU/day of vitamin D may be associated with enhanced survival rates among breast cancer cases.

Recent developments in the use of vitamin D analogues

The non-classical effects of 1alpha,25-dihydroxyvitamin D(3) (1alpha, 25(OH)(2)D(3)) create possible therapeutic applications for immune modulation (e.g., autoimmune diseases and graft rejection), inhibition of cell proliferation (e.g., psoriasis, cancer) and induction of cell differentiation (e.g., cancer). The major drawback related to the use of 1alpha,25(OH)(2)D(3) is its calcaemic effect, which prevents the application of pharmacological concentrations. Intensive research has led to the development of analogues of 1(2)D(3) characterised by a clear dissociation of the antiproliferative and prodifferentiating capacity from the calcaemic effects. Due to this dissociation, these analogues can be used not only for the treatment of bone disorders but also for non-classical applications. In the present review, a summary is given on the use of the 1alpha,25(OH)(2)D(3) analogues for the treatment of cancer, skin and immune disorders and for the prevention of graft rejection. Moreover a brief overview is given on the use of analogues for secondary hyperparathyroidism.

Recent developments in the use of vitamin D analogues

The activated form of vitamin D3, 1 alpha,25(OH)2D3, not only plays a central role in bone and calcium metabolism, but also has potent antiproliferative and prodifferentiating effects. Moreover, the combined presence of 25(OH)D3-1 alpha-hydroxylase, as well as the vitamin D receptor in several tissues introduced the idea of a paracrine role for 1 alpha,25(OH)2D3. By introducing chemical modifications into the flexible molecule 1 alpha,25(OH)2D3, a whole generation of vitamin D analogues was created. Due to a clear dissociation of the antiproliferative and prodifferentiating effects from calcaemic effects, these analogues can be used not only for the treatment of bone disorders but also for non-classical applications. In the present review, a summary is given on the use of the 1 alpha,25(OH)2D3 analogues for the treatment of psoriasis, cancer and immune disorders together with new insights in the mechanism of action of these analogues.

Vitamin D and regulation of gene expression

The function of 1,25-dihydroxyvitamin D3, the biologically active form of vitamin D, extends from bone and mineral homeostasis to the control of cell growth and differentiation in a variety of tissues. Most of these actions are mediated by activation of the nuclear vitamin D receptor, which regulates the transcription of vitamin D target genes. Considerable progress has been made in the understanding of vitamin D receptor function (especially regarding its interaction with coactivators), as well as in the identification of novel vitamin D responsive genes related to cell growth, differentiation and cytokine production.