In 1976, Sporn has defined chemoprevention as “the use of pharmacologic or natural agents that inhibit the development of invasive breast cancer either by blocking the DNA damage that initiates carcinogenesis, or by arresting or reversing the progression of premalignant cells in which such damage has already occurred.” Although the precise mechanism or mechanisms that promote a breast cancer are not completely established, the success of several recent clinical trials in preventive settings in selected high-risk populations suggests that chemoprevention is a rational and an appealing strategy. Breast cancer chemoprevention has focused heavily on endocrine intervention using selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs). Achieving much success in this particular setting and new approaches as low-dose administration are actually under investigations in several topics. Unfortunately, these drugs are active in prevention of endocrine responsive lesions only and have no effect in reducing the risk of estrogen-negative breast cancer. Thus, recently new pathways, biomarkers, and agents likely are to be effective in this subgroup of cancers and were put under investigation. Moreover, the identification of new potential molecular targets and the development of agents aimed at these targets within cancer have already had a significant impact on advanced cancer therapy and provide a wealth of opportunities for chemoprevention. This paper will highlight current clinical research in both ER-positive and ER-negative breast cancer chemoprevention, explaining the biologic effect of the various agents on carcinogenesis and precancerous lesions, and finally presenting an excursus on the state-of-the-art about new molecular targets under investigations in breast cancer settings.
While decreases in both breast cancer incidence and mortality have been apparent in recent years, the societal and economic impact of this malignancy continues to be enormous [
Breast cancer chemoprevention history.
Model of human carcinogenesis.
Although the precise mechanism that causes breast cancer is not fully established itis recognized that hormones play a significant role in almost 70% of cases [
Estrogen is well established as a promoter of cell division in the breast, where it causes proliferation of both normal and malignant cells [
This class of drugs includes in particular Tamoxifen (TAM) and Raloxifene, acting as both estrogen agonist and antagonists. Tamoxifen citrate is the first generation of SERMs that competes with circulating estrogen for binding the estrogen receptor (ER) [
TAM has been in clinical use for breast cancer treatment for more than 30 years to reduce the risk of both recurrence and contralateral neoplasia, 42% and 47%, respectively [
The BCPT NSABP-1 [
In addition, 3 European tamoxifen prevention trials have been completed and have reported long-term follow-up data of the effect of this agent in BC incidence: The Italian Tamoxifen Prevention Study, The Royal Marsden Hospital tamoxifen randomized chemoprevention trial, and the International Breast Cancer Intervention Study (IBIS)-1 [
ER+: odds ratios for developing an estrogen receptor-positive invasive breast cancer among women involved in tamoxifen prevention trials.
ER−: odds ratios for developing an estrogen receptor-negative invasive breast cancer among women involved in tamoxifen prevention trials.
The data from the NSABP P-1 trial, which showed a reduction in both invasive and noninvasive breast cancers, led to the 1998 US Food and Drug Administration (FDA) approval of TAM for reduction of breast cancer incidence in high-risk women. However, the adverse effects of this drug have hampered its uptake by women at increased risk. When TAM’s benefits are balanced against its major toxicities, younger women at very high risk and possibly hysterectomized postmenopausal women appear to be the best candidates for preventive TAM.
A simple and economic approach to retain tamoxifen efficacy while reducing the risks may be a dose reduction. In a study conducted by our group, standard dose of tamoxifen (20 mg/die) and two differ lower doses (10 mg/die and 10 mg on alternate days) were administered for 2 months in a cohort of more than 120 healthy women [
These fundamental data provide a strong rationale for the formal assessment of low-dose TAM in preventive setting, and for these reasons we started two phase III randomized placebo trials (actually ongoing in our institute) in order to assess the efficacy of 5 mg/die of TAM in high-risk women as current HRT (HOT study) and with breast intraepithelial neoplasia (IEN).
Raloxifene, a second generation of SERMs has reduced the incidence of breast cancer in preclinical models and several clinical trials evaluated it for the prevention of osteoporosis and heart disease [
Results of MORE and CORE trials led researchers to conduct a comparative, randomized phase III study of raloxifene versus tamoxifen in more than 19000 postmenopausal women at increased risk for breast cancer [
A mechanism to explain the difference in noninvasive breast cancer incidence is unknown, but long-term follow-up results for the STAR trial may result in additional information regarding this issue. Regarding the side effects, more uterine malignancies occurred in the Tamoxifen arm and no statistically significant differences were noted between the 2 groups relative to the incidence of any cardiovascular events.
More recently, results from the Raloxifene Use for the Heart (RUTH) study affirmed the benefit of raloxifene with regard to reduced risk of breast cancer. This trial, designed to focus on heart disease, randomized more than 10,000 postmenopausal women with coronary health disease or multiple coronary health disease risk factors to receive either raloxifene 60 mg per day or placebo [
Data from the STAR trial and the other raloxifene/placebo trial resulted in the approval of raloxifene by the US Food and Drug Administration for a reduction in the risk of invasive breast cancer in postmenopausal women with osteoporosis and reduction in the risk of invasive breast cancer in postmenopausal women at high risk of invasive breast cancer. No data are currently available on the use of raloxifene in patients with BRCA1 or BRCA2 mutations, nor was raloxifene approved for women with a previous invasive breast cancer or for the treatment of invasive breast cancer. However, the approval of raloxifene gives an important new option to postmenopausal women beyond that of tamoxifen, one that avoids an excess of endometrial cancers and reduces the risk of thromboembolic events.
High circulatory estrogen levels, as well as high aromatase levels in breast tissue, have been known to increase breast cancer risk. Thus, inhibition of aromatase would be expected to decrease estrogen production and ultimately estrogen-related breast carcinogenesis.
In adjuvant setting, third generation of AIs (anastrozole, letrozole, and exemestane) has been found to superior to tamoxifen and be able to reduce the incidence of contralateral breast cancers by 37 to 55% [
The International Breast Cancer Intervention (IBIS)-II prevention trial [
Another prevention trial with AIs (MAP3) is actually underway with exemestane (EXE). Authors are comparing placebo or EXE, or EXE plus celecoxib for 5 years in more than 5000 high-risk postmenopausal women. In September 2004 the disclosure of an excess of adverse cardiovascular events in the COX-2 inhibitor arm has recommended authors to revised the study design. They modified it in two different arms (exemestane vs placebo) and a new simple size of 4.560. Despite this, the MAP3 study was reopened to accrual in March 2005 with a revised sample size of 4,560 and two arms, EXE 25 mg/d alone and placebo. The primary endpoint is the incidence of breast cancer specifically to determine if EXE is able to reduce invasive breast cancer by 65% compared to placebo. Secondary endpoints regard also safety and incidence of noninvasive breast cancer.
These data obtained by adjuvant trial provide a rational for exploring AIs in prevention setting. They are superior to tamoxifen, and we hypothesized that the major of ER-positive breast cancer (but not for ER negative) can be prevented by these drugs. Moreover, they are also well tolerated than tamoxifen without uterine and thrombotic effects, but they do lead to bone mineral loss. These effects should be contrasted by the use of bisphosphonates.
Although a number of antiestrogenic agents are being extensively tested in clinical trials, all these agents affect the endocrine pathway and suppress only the development, of estrogen receptor (ER)-positive breast cancer. They have no effect in reducing the risk of ER-negative breast cancer, which accounts for 20–30% of breast cancers and has a poor prognosis [
Thus, it is worth identifying new pathways, biomarkers, and agents that are effective in the treatment and prevention of these subtypes. With the accumulating knowledge in understanding the biology of cancer development several classes of a new generation of chemopreventive agents modulating the nonendocrine biochemical pathways have been developed and many of these are still currently under investigation.
These agents include retinoids, epidermal growth factor receptor (EGFR), tyrosine kinase inhibitors (TKIs), cyclooxygenase-2 (COX-2) inhibitors, bisphosphonates, vitamin D receptor (VDR), statins, peroxisome proliferator- activated receptor (PPAR), and others. A complete summary of involved agents, with their specific pathways, is shown in Table
Class, specific pathways, and agents actually involved in the treatment and prevention of ER-breast cancer.
Class | Targets | Drugs or agents |
---|---|---|
Nuclear receptors | Retinoid acid receptor RXR | Fenretinide (4-HPR) 9 cis-retinoic acid (Targretin) |
VDR | VIT D3 analogues | |
|
Troglitazone, rosiglitazone, pioglitazone | |
| ||
Membrane receptors and signal transduction | HMG-CoA | Statins |
Tyrosine kinase | Gefitinib (Iressa) | |
HER-1, HER-2 | Trastuzumab (Herceptin), lapatinib, gefitinib, erlotinib | |
IGF-R, IGF-1, IGFBP3 | Metformin | |
| ||
Anti-inflammatory and antioxidant | COX-2 | celecoxib, rofecoxib, NSAIDs |
| ||
Angiogenesis | VEGF | Bevacizumab |
| ||
DNA modulation | BRCA1-BRCA2 | PARP inhibitors |
4-HPR: N-(4-hydroxyphenyl) retinamide; COX: cyclooxygenase; ER: oestrogen receptor; HMGCoA: 3 hydroxy-3 methylglutaryl coenzyme A; NSAIDs: nonsteroidal anti-inflammatory drugs; PARP: poly (ADP-ribose) polymerases; PPAR: peroxisome proliferator-activated receptor; RXr: retinoid X receptors, VDR: vitamin D receptor.
Retinoids are natural and synthetic derivative of Vitamin A (Retinol) that have profound effects on development, metabolism, differentiation, and cell growth.
The retinoid, the most widely studied in chemoprevention clinical trials, is the synthetic amide of retinoic acid
Recently, also a new RXR-selective retinoid, commonly named as rexinoids, has been studied as cancer preventative agent. Preclinical studies in fact have demonstrated that this compound is able to maintain the chemopreventive efficacy of the retinoids, also in ER-negative setting, but with substantial minor toxicity [
The EGFR is one of a family of four closely related receptors (EGFR or erbB-1, HER-2/neu or erbB-2, HER-3 or erbB-3, and HER-4 or erbB-4) that uses tyrosine kinase activity and contributes to a large number of processes involved in tumour survival and growth, including cell proliferation and inhibition of apoptosis, angiogenesis, and metastasis [
In particular, the researchers focused their attention to EGFR-HER-1 and HER-2 pathways, because the mechanism of resistance to antioestrogen therapy is usually associated with an increased expression of HER-1 and HER-2 receptors. Inhibition of tyrosine kinase activity, with TKIs, involved in the EGFR signaling cascade could be the right pathway for the treatment and prevention of ER-independent breast.
There are two different and concomitant strategies able to inhibit erbB activity. One involves blockade of this activity with monoclonal antibodies (trastuzumab), whereas the second involves the TKIs. The two strategies differ in several pharmacological properties [
Amplification of the HER2 gene and overexpression of it's related protein have been found in almost 30% of human breast cancer and it is generally correlated with poorer outcomes compared with tumors HER2 negative [
Trastuzumab has largely showed its benefit in adjuvant therapy; in particular, it is able to increase the clinical benefit of first-line chemotherapy in metastatic HER-2 breast cancer [
Apart from the monoclonal antibodies directed against the extracellular receptor domain of HER-2, there is another way to contest erbB activity. As previously explained, the use of small molecules inhibit intracellular tyrosine kinase activity, named TKIs. TKIs have several advantages over monoclonal antibodies such as oral bioavailability, potentially less toxicity, and ability to inhibit truncated forms of EGF receptors [
The inducible isoenzyme COX-2 is expressed in invasive and in situ breast cancers cells [
Celecoxib, a selective COX-2 inhibitor, reduced the incidence and multiplicity of DMBA-induced mammary tumors in rat models by 68 and 86%, respectively [
Bisphosphonates, the drugs of choice for the treatment of osteoporosis, act on the mevalonate pathway [
Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes that play a key role in the repair of DNA damage [
A key role for PARP-1 and PARP-2 is maintaining genomic integrity, in particular, repair of single strand DNA lesions and breaks using the base excision repair (BER) pathway. The inhibition of these enzymes leads to accumulation of DNA single-strand breaks, which can lead to DNA double-strand breaks at replication forks [
The efficacy of a particular risk group as the mutation carriers and the relative good tolerability make these agents well suited for cancer prevention. Further investigations should be proposed in BRCA mutation carriers to assess the ability of this class of agents to prevent cancer, evaluate the safety profile, and reduce the incidence of breast cancer.
There is increasing evidence that presence of hyperinsulinemia and insulin resistance increased breast cancer risk, worsen, the prognosised and partly explained the obesity-breast cancer risk association in postmenopausal women [
Metformin, a biguanide derivative, is the most commonly used drug worldwide to treat type II diabetes. It is generally well tolerated with low toxicity and a very low cost. Epidemiological studies have shown a significant risk reduction in cancer incidence and mortality in diabetic patients on metformin, relative to other antidiabetic drugs, including positive results specifically in breast cancer [
Metformin anticancer mechanism of action.
Several preclinical studies have confirmed these effects of metformin in vitro and in vivo and showed a significant reduction of both breast epithelial cell proliferation and protein synthesis [
Many molecularly pathways and the correlated targeted drugs are actually in development for advanced cancer therapy, and they have potential activity and tolerability also in cancer chemoprevention setting. The identification of new potential molecular targets and the development of agents aimed at these targets within cancer have already had a significant impact on advanced cancer therapy and provide a wealth of opportunities for chemoprevention.
There is substantial evidence that together with the epithelial cells alterations the microenvironment dysfunction is crucial for carcinogenesis, and this makes the microenvironment an interesting target for breast cancer chemoprevention. In particular, there are many excellent publications which consider microenvironment as a good target for cancer therapy, but the application of chemoprevention to control the tumour microenvironment during the early stages of carcinogenesis is not yet adequately analyzed. We will briefly explain a recent progress that indicates that the effects of chemopreventive agents on the microenvironment are an important aspect of their preventive action and that many classes of agents, which showed to have significant chemopreventive actions on epithelia, also have similar useful actions on the microenvironment.
Many molecular targets inside the microenvironment with the correlated drugs are summarized in the Table
Molecular targets and chemopreventive agents in the microenvironment.
Molecular targets | Chemopreventive agents |
---|---|
Oestrogen receptors | Tamoxifen; raloxifene; aroxifene |
Akt and NF |
Curcumin; N-acetyl cysteine; silibinin; xanthohumol; deguelin; EGCG; resveratrol |
NRF2-KEAP1 | Sulphoraphane; oltipraz |
COX2 | Rofecoxib; celecoxib; EGCG |
COX1/2 | Aspirin and other NSAIDs |
Histone deacetylases | Sulphoraphane |
TGF |
CDDO-Imidazolide |
HIF1 |
WGCG; resveratrol; apigenin; sulphoraphane |
STATs | CDDO-Imidazolide |
VEGF | Sulphoraphane; EGCG; fenretinide |
Some of the specific targets in the microenvironment and specific agents that interact with these targets: CDDO: 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid; COX2: cyclooxygenase 2; EGCG: epigallocatechin-3-gallate; HIF1
These transcription factors and their associated regulatory proteins are an ideal target of chemoprevention, and in particular three attractive pathways as the nuclear factor
Nuclear factor-
Experimental studies have shown that natural antioxidant compounds including isoflavones, indole-3-carbinol (I3C), 3,3′-diindolylmethane (DIM), curcumin, epigallocatechin-3-gallate (EGCG), rosveratrol, curcumin and others seems to be able to inhibit the activity of NF-kappaB and the growth of cancer cells and also to induce apoptosis, suggesting that NF-kappaB could be a target for cancer prevention [
Similarly, HIF-1—a master regulator in the control of tissue homeostasis, crucial in adaptive responses to tissue oxygenation including energy status, glucose, and iron metabolism as well as growth factor signaling [
Recent experimental evidence in fact suggests that HIF-1 is a key player in carcinogenesis. Interest in the role of HIF-1 in cancer has grown exponentially over the last two decades, as this factor activates the transcription of many genes that code for proteins involved in several pathways intimately related to cancer [
Tumors are invariably less well oxygenated than the normal tissues from which they arise. Hypoxia-inducible factor-1 (HIF-1) plays a central role in the adaptation of tumor cells to hypoxia by activating the transcription of genes, which regulate several biological processes.
For these reasons, HIF-1 is considered a potential target for cancer therapy, and, recently, many efforts to develop new HIF-1-targeting agents have been made [
The mammalian target of rapamycin (mTOR) is a signaling kinase of the phosphatidylinositol 3-kinase/protein kinase B or PI3K pathway that mediates cell growth and metabolism and coordinates cell cycle progression in response to genetic, epigenetic, and environmental conditions. Pathways involved in mTOR signaling are dysregulated in precancerous human tissues, including breast cancer, and is associated with the development of resistance to endocrine therapy [
Rapamycin and its analogues, the “rapalogues,” decrease tumor growth in many xenograft models, including those with breast cancer cell lines [
The involvement of mTOR pathways in precancerous lesions makes the mTOR signaling an intriguing target for chemopreventive intervention. Thus, several recent preclinical studies explored also the possibility of a chemopreventive action through the mTOR inhibition. In one of this, rapamycin showed chemopreventive activity against mammary gland tumors in transgenic mice, bearing activated ErbB2 (HER-2/neu) receptor either alone (NeuYD) or with VEGF expression [
These results seem to suggest the mTOR inhibition as a possible chemopreventive strategy against metachronous tumors or recurrence in high-risk patients, whose primary tumors overexpressed ErbB-2, or in patients showing dysregulation of the PI3K/AKT/mTOR signaling pathway.
Another recent preclinical study evaluated chemopreventive effects of rapamycin in a transgenic mouse model of human breast carcinogenesis [
Finally, some natural products, such as epigallocatechin gallate (EGCG), caffeine, curcumin, and resveratrol, have been found to inhibit mTOR as well and are actually under investigations in this setting.
In conclusion, the success of chemopreventive approach depends on a tumor-specific risk model for identifying high-risk subjects, increasing preclinical drug test over the development novel and more safety chemopreventative agents, and identifying new surrogate endpoint using molecular pathways and new targets of drugs activity.
Safety is a very important point to take into account, because several large randomized prevention trials in several cancers have shown that major adverse events can prevent widespread public acceptance of active chemoprevention agents.
Despite the success of action showed for example in endocrine intervention is a promising starting point in order to continue to evolve with the rapid integration of molecular approaches into research and clinical practice. It is urgent to find active agents in other fields as nonhormone-responsive lesions. The personalized approaches in advanced cancer therapy and the evolution of molecularly targeted will streamline chemoprevention research and facilitate the development of rational, effective, and safe preventive drugs, involving different pathways and with the ability to modify carcinogenesis in early phases.