Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 13, Issue 1, 2013

Vitamin D deficiency is a re-emerging global health problem, which is primarily due to inadequate vitamin D synthesis in the skin. Supplement use is an effective measure to improve vitamin D status. However, some safety issues have to be considered, which are highlighted in this review article: The concept of vitamin D safety consists of two models, the safe tolerable upper intake level (UL) method, and the idea of adequate circulating 25-hydroxyvitamin D (25[OH]D) levels. Oral vitamin D intakes up to 250 μg/d have not been associated with harm. Hypercalcemia, the hallmark of vitamin D intoxication, may only occur if circulating 25(OH)D levels are consistently above 375-500 nmol/l. However, some observational studies indicate that already circulating 25(OH)D levels > 125 nmol/l are related to an increased morbidity and mortality risk. Therefore, the Institute of Medicine has set the UL for adults at 100 μg/d, and the adequate circulating 25(OH)D level at 50 to 125 nmol/l. In clinical practice, oral vitamin D dosing has to consider that the increment in circulating 25(OH)D depends on baseline 25(OH)D levels and the person's body weight. It is reasonable to assess 25(OH)D before and 3-6 months after initiation of oral vitamin D administration and to adjust the dose, if necessary. In future, two issues have to be clarified: First, would it be more appropriate to define instead of a fixed UL a variable UL, based on the individual's body weight? Second, what are the underlying mechanisms, if any, for potentially harmful vitamin D effects at circulating 25(OH)D levels between 125 and 375 nmol/l.

Garland and Garland first hypothesized that better vitamin D status lowered risk of colorectal cancer in 1980. Subsequently, the relation between vitamin D status and colorectal cancer risk has been investigated in epidemiologic studies. Various approaches have been used to estimate vitamin D status, including direct measures of circulating 25(OH)vitamin D levels, surrogates or determinants of vitamin D (including region of residence, intake, and sun exposure estimates, or a combination of these). These measures of vitamin D status have been studied in relation to colorectal adenoma, precursors of cancer, and colorectal cancer incidence and mortality. In general, all lines of inquiry from observational studies indicate that better vitamin D status is associated with lower colorectal cancer risk. While most of the studies have examined vitamin D status in relation to risk of incident colorectal cancer, some evidence suggests that vitamin D may be additionally important for colorectal cancer progression and mortality. Although the influence of confounding factors cannot be entirely excluded, the consistency of the association using various approaches to measure vitamin D, for diverse endpoints and in diverse populations shows high consistency and is strongly suggestive of a causal association. Thus, improving vitamin D status could be potentially beneficial against colorectal cancer incidence and mortality.

Vitamin D deficiency and low calcium intake are considered risk factors for several cancers. Vitamin D, synthesized in the skin or ingested through the diet, is transformed through two hydroxylation steps to the active metabolite, 1α,25-dihydroxyvitamin D3 (1,25-D3). 25-hydroxylases in the liver are responsible for the first hydroxylation step. The ultimate activation is performed by the renal 25-hydroxyvitamin D 1α-hydroxylase (CYP27B1), while the 1,25-dihydroxyvitamin D 24-hydroxylase (CYP24A1) in the kidneys degrades the active metabolite. These two renal vitamin D hydroxylases control the endocrine serum 1,25-D3 levels, and are responsible for maintaining mineral homeostasis. In addition, the active vitamin D hormone 1,25-D3 regulates cellular proliferation, differentiation, and apoptosis in multiple tissues in a paracrine/autocrine manner. Interestingly, it is the low serum level of the precursor 25- hydroxyvitamin D3 (25-D3) that predisposes to numerous cancers and other chronic diseases, and not the serum concentration of the active vitamin D hormone. The extra-renal autocrine/paracrine vitamin D system is able to synthesize and degrade locally the active 1,25- D3 necessary to maintain normal cell growth and to counteract mitogenic stimuli. Thus, vitamin D hydroxylases play a prominent role in this process. The present review describes the role of the vitamin D hydroxylases in cancer pathogenesis and the cross-talk between the extra-renal autocrine/paracrine vitamin D system and calcium in cancer prevention.

There is a growing interest for vitamin D in the medical and scientific community as well as in the public medias as illustrated by a huge number of publications. Most experts claim that vitamin D deficiency/insufficiency is widespread with potential important public health consequences. It may seem surprising for many persons that a deficiency in a vitamin may be so frequent in countries where food is so diversified and easily available. In fact, vitamin D is not a vitamin stricto sensu as it is mainly synthesized in the skin under the action of UVB rays, while its food sources are scarce. Furthermore, UVB rays are absent during a marked part of the year at latitudes greater than 35-40°, while pollution and cloud cover reduce the number of UVB reaching the earth, and many factors such as age, skin pigmentation, covering clothes, sun creams reduce the capacity of the skin to synthesize vitamin D3. Vitamin D must be hydroxylated to form 1,25dihydroxyvitamin D (1,25OH2D), the active metabolite. As 1,25OH2D is released into the bloodstream and binds to a receptor present in several distant tissues, it may be considered as a hormone, vitamin D being thus a pre-prohormone. In the present article, we review briefly the metabolism and various effects of vitamin D as well as the vitamin D assays and vitamin D treatments. We define vitamin D deficiency/insufficiency considering separately the population and the patient level and propose our opinion about which patients may benefit from vitamin D testing.

The hypothesis that vitamin D deficiency increases the risk of clinical prostate cancer has stimulated an extensive body of research. Ecologic studies have shown that mortality rates from prostate cancer are inversely correlated with levels of ultraviolet radiation, the principal source of vitamin D. Human prostate cells express receptors for 1,25-Dihydroxyvitamin D which exerts pleitropic anticancer effects on these cells in vitro and in animal models. Moreover, normal prostate cells synthesize 1,25-Dihydroxyvitamin D from circulating levels of 25-OHD, whose levels are dependent on exposure to ultraviolet light. Analytic epidemiologic studies of vitamin D and prostate cancer have focused on polymorphisms in the vitamin D receptor (VDR), on serum vitamin D levels, and on solar exposure. A role for VDR polymorphisms in prostate cancer risk and progression is established. Prospective studies of serum 25(OH)D do not support a protective role for higher levels of 25(OH)D on prostate cancer risk overall, but a role for vitamin D deficiency is supported by several studies. Conversely, a growing body of evidence implicates low levels of 25-OHD with an increased risk of fatal prostate cancer. The results of most epidemiologic studies of sunlight exposure are consistent with a protective effect of exposure to ultraviolet radiation. The discrepancy between the results of studies of solar exposure and studies of serum 25-OHD may be related to methodological differences and to uncertainties regarding the critical period for vitamin D exposure. Additionally, both high dietary intake of calcium and high levels of calcium in serum are positively associated with prostate cancer risk. The relationship between serum 25(OH)D levels and risk of prostate cancer may differ by calcium intake.

Cancer patients with advanced disease frequently feel weak and fatigued, and have an increased risk of fracture. At the same time several reports describe the high prevalence of vitamin D deficiency in these patients. This review will summarize the impact of vitamin D deficiency on muscle weakness and fracture risk. While larger clinical trials of vitamin D supplementation are lacking in cancer patients, the evidence from clinical trials among older adults of the general population support a significant benefit of vitamin D on muscle strength, and fall and fracture reduction. Mechanistic evidence regarding the presence of the specific vitamin D receptor in muscle tissue and muscle biopsy abnormalities observed with deficiency will be reviewed, as well as molecular and non-molecular effects of vitamin D in muscle tissue. At the clinical level, the evidence from randomized controlled trials of vitamin D supplementation on functional improvement and fall reduction will be summarized. Finally, trial and epidemiological data will be reviewed to assess desirable serum 25-hydroxyvitamin D levels for optimal muscle health.

Cancer is a broad term for many disparate diseases with different etiologies, commonly classified by affected organ site. This review summarizes the published evidence from prospective cohort studies examining the associations between vitamin D, measured as serum 25-hydroxyvitamin D (25OHD) concentrations, and the risk of rarer cancer sites including pancreatic, non-Hodgkin lymphoma, ovarian, endometrial, kidney, gastric and esophageal cancer. Overall, evidence from prospective cohort studies provides little support for a protective association between adequate or higher serum 25OHD concentrations and risk of these rarer cancer sites. Additionally, controversy persists concerning a potential increased risk of pancreatic cancer associated with serum 25OHD levels >100 nmol/L due to conflicting results reported by two large prospective pooling projects.

It has been more than 100 years when it was first appreciated that increased sun exposure reduced risk of dying of cancer. The most beneficial effect of sun exposure is the production of vitamin D in the skin. Recent evidence suggests that most cells in the body not only have a vitamin D receptor but also have the capacity to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D. Once formed 1,25-dihydroxyvitamin D can inhibit cellular proliferation, induce cellular maturation, inhibit angiogenesis and ultimately cause apoptosis to prevent malignancy. A multitude of studies have associated improved vitamin D status with decreased risk for developing several deadly cancers including colon, breast, pancreatic and ovarian cancers. Patients with cancer are at high risk for vitamin D deficiency. Sensible sun exposure, vitamin D fortification and vitamin D supplementation should be encouraged to improve the vitamin D status of children and adults not only for bone health but for reducing risk of developing and dying of cancer. The goal is to achieve a blood level of 25-hydroxyvitamin D of 40-60 ng/mL. This can be accomplished by children taking 600-1000 and adults 1500-2000 international units (IU) vitamin D daily from diet and supplements along with sensible sun exposure when the sun is capable of producing vitamin D in the skin.

Today, there is a controversial debate in many scientific and public communities on how much sunlight is appropriate to balance between the positive and negative effects of solar UV-exposure. UV exposure undoubtedly causes DNA damage of skin cells and is a major environmental risk factor for all types of skin cancers. In geographic terms, living in parts of the world with increased erythemal UV or high average annual bright sun results in increased risks of skin cancers, with the greatest increased risk for squamous cell carcinoma, followed by basal cell carcinoma and then melanoma. On the other hand, sunlight exerts positive effects on human health, that are mediated in part via UV-B-mediated cutaneous photosynthesis of vitamin D. It has been estimated that at present, approximately 1 billion people worldwide are vitamin D-deficient or –insufficient. This epidemic causes serious health problems that are still widely under-recognized. Vitamin D deficiency leads to well documented problems for bone and muscle function. There are also associations between vitamin D-deficiency and increased incidence of and/or unfavourable outcome for a broad variety of independent diseases, including various types of malignancies (e.g. colon-, skin-, and breast cancer), autoimmune diseases, infectious diseases, and cardiovascular diseases. In this review, the present literature is analyzed to summarize our present knowledge about the important relationship of sunlight, vitamin D and skin cancer.

Based on preclinical studies and early clinical observations, an association between vitamin D status and breast cancer incidence and outcome has been proposed. Against this background, information on vitamin D and breast cancer was reviewed with focused attention on emerging clinical studies in this area. Prospective cohort studies do not associate 25-hydroxyvitamin D levels with breast cancer incidence. While case-control studies of this question are positive, they may be confounded by reverse causality as 25- hydroxyvitamin D levels are influenced by breast cancer presence and stage. Studies of 25-hydroxyvitamin D and subsequent breast cancer recurrence provide mixed results but strongest associations were seen in analyses uncontrolled for prognostic variables, cancer therapy, BMI and physical activity. The one full-scale randomized, placebo-controlled trial evaluating calcium (1000 mg elemental calcium per day) and vitamin D supplementation (400 IU D3 per day) with 36,282 participants failed to demonstrate a supplement effect on lowering breast cancer incidence. Breast cancer patients not uncommonly have vitamin D deficiency but limited control populations in available reports preclude precise prevalence estimates. As breast cancer patients are at risk for bone loss and musculoskeletal complaints from cancer or associated therapies, monitoring 25-hydroxyvitamin D levels and vitamin D3 supplementation in moderate dose (1,000- 1,500 IU D3 per day) can be recommended with expectation of mainly bone benefit. In women with breast cancer, future vitamin D supplementation studies need to be appropriately designed and powered to provide definitive assessments. However, a full-scale randomized trial evaluating the influence of vitamin D supplementation on breast cancer recurrence is likely not feasible.

Accumulating evidence from experimental and epidemiological studies suggests that vitamin D deficiency might be a causal risk factor for cancer and therewith associated mortality. We performed a systematic review in Medline up to February 2012 to identify prospective studies on 25-hydroxyvitamin D (25[OH]D) and cancer mortality as well as on 25(OH)D and survival in cancer patients. Our search retrieved 13 studies on cancer-specific mortality and 20 studies on overall mortality in cancer patients. Data on 25(OH)D and cancer mortality were mainly derived from general populations. The results were inconsistent and yielded either no, inverse or positive associations. By contrast, the majority of studies in cancer patients showed that patients with higher 25(OH)D levels had a decreased risk of mortality. This relationship was particularly evident in cohorts of colorectal cancer patients. In contrast, there was no indication for increased mortality risk with higher vitamin D levels in any cancer cohort. In conclusion, the relationship of vitamin D status and cancerspecific mortality is still unclear and warrants further studies. Our results provide a strong rationale to perform prospective randomized controlled studies to document a potential effect of vitamin D supplementation on survival in cancer patients.

Data from experimental studies suggest that vitamin D receptor activation exerts anti-cancer effects on virtually all steps of carcinogenesis. Epidemiological data support an inverse association of vitamin D serum levels and vitamin D receptor polymorphisms with cancer incidence and mortality. Based on this promising rationale for use of vitamin D and its analogues in cancer prevention and treatment, several interventional studies have been initiated and partially published. Trials with vitamin D were mainly organized for the prevention of fracture in elderly people, usually in association with calcium supplements. Prevention studies with vitamin D have rarely been done in the context of vitamin D to evaluate a protective effect on cancer. Findings from prospective cohort studies on colorectal cancer risk and on mortality constitute pieces of evidence strong enough to consider that previous randomized controlled trials (RCTs) of vitamin D use and cancer may not have correctly addressed the question, and that new randomized trials should be organized. The reasons are due to several unsolved issues including selection of the effective dose, varying baseline levels of subjects before randomization, compliance with the intervention, contamination of the placebo group (i.e., intake of vitamin D supplements by subjects allocated to the placebo group) and unknown effective lag time between start of the intervention and disease onset. The present review summarizes the existing knowledge on vitamin D RCTs and cancer. In addition we also briefly describe the design of some ongoing trials on vitamin D supplementation and cancer.

Vitamin D3 is biologically inert. To become active, it requires two successive hydroxylation steps catalyzed by two cytochrome P450 enzymes, first to synthesize the pro-hormone 25-hydroxyvitamin D3 [25(OH)D3] and then the active hormone 1α,25- dihydroxyvitamin D3 [1α,25(OH)2D3]. 1α,25(OH)2D3 has high affinity for the vitamin D receptor (VDR), a transcription factor and a member of the steroid receptor superfamily. Through VDR, 1α,25(OH)2D3 regulates more than 200 genes in mammals, including those involved in the calcium and phosphorus homeostasis, immune function, reproduction, cardiovascular, central nerve system, inflammation, angiogenesis, and cellular proliferation, differentiation and apoptosis. Due to its versatile roles in maintaining and regulating normal cellular phenotypes and functions, 1α,25(OH)2D3 has been implicated as an anti-cancer agent. In fact, ecological and epidemiologic data have linked vitamin D deficiency with the incidence and mortality of many types of cancer. More importantly, in vitro and in vivo animal model studies have clearly demonstrated the anti-tumor effects of vitamin D. In this review, we describe the anticancer actions of vitamin D, with special emphasis on different pathways underlying the VDR-mediated genomic as well as less-defined non-genomic actions of vitamin D.

The ultraviolet-B (UVB)–vitamin D–cancer hypothesis was proposed in 1980 yet has not been fully accepted. Ecological studies based on geographical variations of cancer rates with respect to solar UVB doses have supported the hypothesis for about 20 cancers. This paper reviews the evidence from studies of personal or group UVB irradiance. Studies have associated personal UVB irradiance with reduced risk for breast, colon, endometrial, prostate, and renal cancer, as well as non-Hodgkin's lymphoma (NHL). However, some studies have also found increased risk of NHL from UV irradiance, probably due to immunosuppression by UVA near 370 nm. Several related approaches have also been used to study the hypothesis. Studies in Norway and the UK found that diagnosis in summer or fall is associated with increased survival rates for breast, colon, lung, and prostate cancer, as well as Hodgkin's lymphoma. Diagnosis of nonmelanoma skin cancer is associated with reduced risk of several cancers in sunny countries, but not often in highlatitude countries. Living at higher surface elevation is associated with reduced risk of some cancers. In a recent analyzed study of cancer rates for 54 occupations in Nordic countries, a UVB index based on standardized incidence ratios of lip cancer less those for lung cancer was inversely correlated with 15 types of cancer for males, but only four types for females. This ecological study provides additional evidence that UVB doses at high latitudes are adequate to reduce the risk of cancer, but requires considerable time outside to produce sufficient vitamin D. Because only vitamin D production has been proposed to explain the UVB–cancer link, studies reviewed in this paper should be considered strong evidence for the hypothesis.

Drugs that target microtubules are a successful class of anti-cancer agents that have been in clinical use for over two decades. Acquired resistance to these drugs, however, remains a serious problem. Microtubule alterations, such as tubulin mutations and altered β- tubulin isotype expression, are prominent factors in development of resistance. Changes in actin and intermediate filament proteins can also mediate sensitivity to microtubule-targeting drugs. This review focuses on the mechanisms by which alterations in cytoskeletal proteins lead to drug resistance. This information will be helpful for improving the targeting of microtubule toxins.

Multidrug resistance (MDR) of cancer tissue is a phenomenon in which cancer cells exhibit reduced sensitivity to a large group of unrelated drugs with different mechanisms of pharmacological activity. Mechanisms that reduce cell sensitivity to damage induced by a variety of chemicals were found to be caused by diverse, albeit well-defined, phenotypic alterations. The molecular basis of MDR commonly involves overexpression of the plasma membrane drug efflux pump – P-glycoprotein (P-gp). This glycoprotein is an ABCB1 member of the ABC transporter family. Cells that develop MDR of this type express massive amounts of P-gp that can induce a drug resistance of more than 100 times higher than normal cells to several drugs, which are substrates of P-gp. Expression of P-gp could be inherent to cancer cells with regard to the specialized tissues from which the cells originated. This is often designated as intrinsic Pgp- mediated MDR. However, overexpression of P-gp may be induced by selection and/or adaptation of cells during exposure to anticancer drugs; this particular example is known as acquired P-gp-mediated MDR. Drugs that are potential inducers of P-gp are often substrates of this transporter. However, several substances that have been proven to not be transportable by P-gp (such as cisplatin or alltrans retinoic acid) could induce minor improvements in P-gp overexpression. It is generally accepted that the drug efflux activity of Pgp is a major cause of reduced cell sensitivity to several compounds. However, P-gp may have side effects that are independent of its drug efflux activity. Several authors have described a direct influence of P-gp on the function of proteins involved in regulatory pathways, including apoptotic progression (such as p53, caspase-3 and Pokemon). Moreover, alterations of cell regulatory pathways, including protein expression, glycosylation and phosphorylation, have been demonstrated in cells overexpressing P-gp, which may consequently induce changes in cell sensitivity to substances that are not P-gp substrates or modulators. We recently reported that P-gppositive L1210 cells exhibit reduced sensitivity to cisplatin, concanavalin A, thapsigargin and tunicamycin. Thus, P-gp-mediated MDR represents a more complex process than was expected, and the unintended effects of P-gp overexpression should be considered when describing this phenotype. The present review aims to provide the most current informations about P-gp-mediated MDR while paying particular attention to the possible dual function of this protein as a drug efflux pump and a regulatory protein that influences diverse cell processes. From a clinical standpoint, overexpression of P-gp in cancer cells represents a real obstacle to effective chemotherapy for malignant diseases. Therefore, this protein should be considered as a viable target for pharmaceutical design.

The potential role of carbon nanotubes (CNTs) in the diagnosis and treatment of malignant melanoma (MM) is still an emerging area of research. To date, there is strong evidence for the efficiency of CNTs in this therapeutic area, despite their unique physical, mechanical and biological properties. In this review, the application of CNTs in cancer diagnostics and treatment is reviewed, and consideration is given to the toxicity issues associated with their use.

Azidothymidine (AZT) is an antiretroviral drug that affects cell proliferation, apoptosis, and the NF-κB pathway. As multiple myeloma (MM) presents with constitutive activation of NF-κB, we analyzed the effect of AZT on human MM cell lines. We evaluated the cytotoxic effect of AZT in human MM cell lines sensitive (8226/S) or resistant to doxorubicin (8226/DX5) and human T cell lymphoblast-like cells, uterine sarcoma cells, and HUVEC using MTT assay. Cytotoxicity was also evaluated in vivo in nude mice xenografted with 8226/S tumor. The effect of AZT on the expression of genes involved in cell proliferation, apoptosis, angiogenesis, and the NF-κB pathway was analyzed in the xenografts using real-time polymerase chain reaction. AZT was effective against both 8226/S and 8226/DX5 cells in a dose and time-dependent manner (p = 0.02) in vitro and promoted cell cycle arrest in S phase in these cells. The tumor volume was lower in mice treated with AZT compared to untreated mice (p = 0.0003). AZT down-regulated the pro-proliferative genes encoding AKT1, MYC, STAT1, MAPK8, MAPK9, CCL-3, Bcl-3, and cyclin D2; pro-angiogenenic genes encoding VEGF and IL8; and genes involved in cell adhesion (ICAM1 and FN1) and the NF-κB pathway. AZT up-regulated the expression of tumor suppressor gene FOXP1 and the pro-apoptotic genes encoding BID, Bcl-10, and caspase-8. Thus, we demonstrated the cytotoxic effect of AZT in human MM cell lines for the first time. Our data may provide the rationale for future clinical trials of AZT for treating MM.