Worldwide, breast cancer (BC) is the leading cause of death in women due to malignant neoplasms. In Mexico, the incidence of this disease reported in 2008 was 13,939 new cases per 100,000 inhabitants (1). In two decades (1990–2010), the annual risk increased from 2 to 5%, being more frequent in the north and central part of the country (2). In 2010, the National Institute of Statistics, Geography and Informatics of Mexico (INEGI) reported that 19 of every 100 women enrolled into a medical service, presented with a mammary neoplasm, and 47% of BC deaths were in women with ages ranging from 45 to 64 (3). There are many risk factors for the development of breast cancer. For the Mexican population, the principal risk factors include reproductive factors (e.g. age at menarche and menopause, nulliparity or late pregnancy and non-breastfeeding women) and genetics (BRCA1 and BRCA2 genes) (3). In contrast, a diet rich in fruits and vegetables, low alcohol consumption and no smoking habit, significantly reduce the probability to develop this disease (2).

The treatment for BC in the primary stages (I and II) generally consists of surgical procedures, followed by chemotherapy (mainly anthracycline-type drugs) and/or radiotherapy (depending on the stage of the cancer), offering successful and even curative results in the majority of cases (4). Notwithstanding, the cardiotoxicity of anthracyclines is well documented; the secondary effects are mainly cardiomyopathy and cardiac syncope (5). Epidemiologic studies indicate that 50% of patients exposed to anthracyclines develop cardiac abnormalities after 10 to 20 years of chemotherapy; 40% of patients present with arrhythmia and 5% with heart failure (6). In addition, patients in advanced stages or with metastasis present a survival rate of 22% after 10 years of treatment (7).

In search for new alternatives to the conventional treatment and/or for the improvement of known therapies, aspirin has been one of the most attractive prevention strategy proposals. There is a long literature record, of over 20 years, concerning the study of aspirin and its effect on BC risk. An example of this is the meta-analysis performed by Khuder and Mutgi (2001), where evidence gathered from 1980 to 2000 was analyzed (8). In this work, the results highlighted that women with estrogen receptor (ER)-positive (ER⁺) tumors and who regularly consumed non-steroidal anti-inflammatory drugs (NSAIDs), presented a 22% reduction in BC risk [relative risk (RR) of 0.82; 95% confidence interval (CI), 0.75, 0.89]. In the case of cohort studies, they reported an RR of 0.78 (95% CI, 0.62–0.99), and for case-control studies an RR of 0.87 (95% CI, 0.84–0.91).

More recently, Agrawal and Fentiman (2008) reported that the use of NSAIDs decreased the risk of developing BC by 20% (9). The authors proposed that NSAIDs could be employed as coadjuvant or palliative treatment, together with hormonal therapies, in women diagnosed with BC. Nevertheless, a consensus regarding the ideal drug, the dosage, and administration time has not been reached.

Due to the controversial use of NSAIDs, particularly aspirin, and the risk of developing BC, in this review we carried out a descriptive revision of the free access epidemiologic studies from 2000 to 2012 listed in PubMed, using the following search key words: ‘aspirin or acetylsalicylic acid and breast cancer’, ‘non-steroidal anti-inflammatory drugs’, ‘clinical studies’ and ‘epidemiological reports’. The main objective of this article was to analyze whether aspirin reduces the risk of BC in women, and the optimal conditions required to benefit from the use of this drug. In order to organize the article, it was divided into three main sections: i) epidemiologic studies, ii) aspirin as an inhibitor of metastasis and iii)aspirin, mechanism of action. Table I summarizes the articles consulted in this review.

In this section, 20 studies are reviewed, 12 of them are cohort studies and the rest correspond to case-control studies. Both categories comprise studies in which BC was assessed exclusively and cancer studies where BC information was included. These works are presented in a chronological fashion, from 2000 to 2012.

Aspirin has not only been studied as a chemopreventive drug, but also as an inhibitor of metastasis. It is known that patients with BC have a 50% likelihood to develop metastasis after treatment (chemotherapy, radiation and/or surgery). The maspin protein (mammary serpin) and its mRNA are highly produced in normal breast epithelial cells (35), whereas their expression levels diminish in the presence of BC (36). Maspin protein is a tumor-suppressor serine protease, which is expressed in several tissues including mammary epithelia, prostate, epidermis, lung, and in the stromal cells of the cornea (35,36,61,62). This protein is of special interest as it is able to inhibit cellular invasion and metastasis and to act as a tumor suppressor (37,60).

Girish and colleagues proposed that aspirin corrects the production levels of maspin in human breast cells in vitro and in patients, regulating maspin to normal levels through the stimulation of nitric oxide (NO) synthesis independent of the insulin receptor (37). An epidemiologic example of this previously mentioned concept, is a study carried out in India, where the effect of aspirin and the incidence of metastasis in BC patients was determined (38). In this work, 35 women participated, and the age range was from 41 to 64 years. All of the women were diagnosed with BC and had been previously treated (chemotherapy, radiation and/or surgery). The patients consumed a dose of aspirin of 75 mg/70 kg body weight daily for 3 years. During this period of time, plasmatic levels of NO and maspin were measured, and a monthly surveillance of metastasis was performed by an oncologist and occasionally by biopsy. The control group consisted of 35 healthy volunteers whose plasma maspin levels were measured. The results showed that maspin levels were higher after 24 h of aspirin administration; such levels were maintained during the 3 years of treatment (initially maspin levels were 0.95±0.04 nM increasing to 4.63±0.05 nM at the end of treatment). Six patients developed metastasis, and the rest of the patients were, apparently, free of metastasis after 3 years. Researchers suggest that daily intake of aspirin in patients with previously treated BC may reduce the incidence of metastasis independent of the disease stage.

Recently, a large study performed in the UK with 17,285 patients was carried out in order to elucidate the preventive effect of aspirin on distant metastasis comparing adenocarcinomas vs. other cancer types (39). Patients were divided into four groups according to the metastasis target: i) metastasis to distant tissues (secondary tumors, particularly to the liver, lung, bone, brain or other tissues distant to the primary tumor); ii) metastasis to any specific region; iii) local invasion; and iv) patients presenting rapid progression of disease and not enough clinical information recovered. After data analysis, the authors concluded that aspirin may help reduce the risk of distant metastasis in certain types of cancers from 30 to 40%, and in 50% of metastatic adenocarcinoma, using a low concentration (≤5 mg daily) and long delivery formulation. Additionally, it was observed that aspirin had a major impact on individuals with adenocarcinomas, particularly in those cases where curative surgery was performed (as in BC). The authors proposed that the anti-metastatic activity of aspirin is mediated by platelets, based on the knowledge that they participate in cancer development and metastasis (40), protecting tumoral cells during their travel through the bloodstream. Moreover, platelets activate a coagulation system allowing microthrombosis formation, thus facilitating the hosting of tumoral cells in target tissues (41). In addition, it is known that thrombocytosis is a common disease in many cancer types and it is used as a poor prognostic indicator (42).

Regarding aspirin and breast cancer metastasis, a research group at Brigham and Women’s Hospital together with the Harvard Medical School, performed a prospective observation study in order to evaluate whether aspirin could decrease the risk of death from BC (43). The study consisted of 4,164 women (enrolled between 1976 and 2002; followed up until death or June 2006). According to their results, the use of aspirin reduced the risk of metastasis and BC death. The adjusted relative risks (stage of cancer, menopausal status, BMI, hormone receptor status) for distant recurrence were: for 1 day/week, RR=0.91 (95% CI, 0.62–1.33); 2–5 days/week, RR=0.40 (95% CI, 0.24–0.65); and for 6–7 days/week, RR=0.57 (95% CI, 0.39–0.82).

Evidence to date regarding the use of aspirin as an inhibitor of metastasis in patients diagnosed with cancer, suggests that aspirin delays the invasion of cancer cells to other tissues and in some cases prevents metastasis formation. The information indicates that regular consumption of aspirin and long periods of administration are required for a positive result.

NSAIDs can be classified depending on their mechanisms of action on the COX enzyme: classical NSAIDs (including aspirin) and COX-2 inhibitors (celecoxib and rofecoxib are mainly used). The molecular mechanism by which aspirin and other classical NSAIDs are capable of inhibiting COX-1 and COX-2, is by competing with arachidonic acid for binding to the active site of cyclooxygenase; this is a rapid and an irreversible union, followed by the covalent acetylation of serine 530 in COX-1, hampering the oxidation of arachidonic acid (Fig. 1) (44). After the same modification, COX-2 can still convert arachidonic acid into 15R-hydroxyeicosatertraenoic acid (HETE), instead of PGG2 (45). On the other hand, it is believed that specific inhibitors of COX-2 bind to valine 523, whereas in COX-1 access to this binding site is blocked by the isoleucine residue at the same position (46). This difference explains the different degrees of inhibition towards COX-1 and COX-2 that aspirin possesses.

As known, there are two major isoforms of the COX enzyme, COX-1 that is expressed constitutively, and COX-2, which is induced locally as part of an inflammatory cascade and during early tumor development (46,47). COX-1 performs positive homeostatic functions. It is antithrombotic when released to vascular endothelium and is cytoprotective when produced by the gastric mucous. Moreover, it provokes platelet aggregation and prevents inappropriate bleeding when it is thromboxane A2-dependent (48). On the other hand, COX-2, in addition to its action in tissular damage response, could be constitutively expressed in the brain, kidney, cells of the pancreatic islet, ovary, gestational uterus and intestine (49,50). COX-2 is present in several cancer cell lines and is implicated in carcinogenesis, tumor growth, apoptosis and angiogenesis (51,52). As these isoforms are encoded in independent genes with different chromosomal localizations, their expression and regulation patterns differ. More recently, a third isoform, named COX-3, was identified as a COX-1 splice variant that may play an important role in fever and pain processes (53).

These enzymes are responsible for catalyzing the conversion of arachidonic acid to prostaglandin endoperoxide (prostaglandin H2) and thromboxanes; both COX-1 and COX-2 participate in maintaining physiological regulation in the stomach, platelets, kidneys and intestine (46). The principal side effects of aspirin and NSAIDs such as gastropathy, renal insufficiency and impaired vascular homeostasis among others, are due to a reduction in the appropriate prostaglandins in these organs (46). Prostaglandins are short-lived compounds acting as local mediators of continuous importance in normal cellular reactions, but they appear to be increased in several pathological conditions, particularly inflammation (46), and it has also been shown that the level of prostaglandins in various tumors is greater than that of normal tissues (54).

Reports from the past 15 years, have shown that COX-1 is localized in stromal cells adjacent to the tumor but not in tumor cells. In contrast, high levels of COX-2 are localized primarily in tumor cells but also appear in stromal cells (54). Such is the case of certain malignant human breast tumors that produce more prostaglandin-like material than do benign tumors or normal breast tissues (54).

Additional experimental data suggest that COX-2 can be induced by a mutation of the tumor-suppressor gene APC, subsequently increasing the expression of the nuclear transcription factor PPAR-δ (55). Induction of COX-2 expression in tumors can also occur by lipopolysaccharide through the mitogen-activated protein kinase (MAPK) and protein kinase C-(PKC) pathways (56). Ceramide-stimulated activation of MAPK can also activate c-Jun N-terminal kinase (JNK), which in turn can lead to increased COX-2 gene expression in human mammary epithelial cells (57). In breast tissue, induction of COX-2 expression can trigger prostaglandin production, which indirectly stimulates cellular proliferation increasing the local biosynthesis of estrogen and increasing the expression of the aromatase gene (8,58). In humans, high levels of prostaglandins have been related with metastatic potential and reduced survival of patients (22). Such a process could be reversible when COX-2 is inhibited using NSAIDs with the purpose of reducing prostaglandin synthesis. For those women who present with tumors with positive hormone receptors, the use of aspirin could contribute to the suppression of aromatase activity, reducing the intra-mammary prostaglandin production, and thus, estrogen production (59). In particular, it has been shown that E2 prostaglandin (PGE₂) stimulates the transcription of aromatase, increasing the estrogen levels and, moreover, contributing to the progression of estrogen-dependent BC (60).

The induction and overexpression of COX-2 and its main product PGE₂ in human mammary tissue, have also been associated with aromatase-catalyzed estrogen biosynthesis (61). In addition, some epidemiologic studies have reported that hormone-receptor-positive breast tumors are more responsive to aspirin (62). The ability of aspirin and other NSAIDs to protect against breast cancer might vary according to hormone receptor status. For instance, in 2004, a case-control study, reported a reduction in risk only in women with hormone receptor-positive tumors, but not in women with hormone receptor-negative tumors (9). Moreover, COX-2 is related to the activation of carcinogens, mutagenesis, angiogenesis, inhibition of apoptosis and metastasis (53,63,64). In contrast, COX-2 inhibition may reverse these processes; aspirin and salicylates may also suppress NF-κB-related survival signaling by inhibiting IKKα activation, leading to apoptosis (45).

Molecular studies have also revealed a strong correlation between overexpression of COX-2 and other oncogenes, such as HER-2/Neu, in malignant breast tumors (65,66). A linkage between the expression of COX-2 and MDR1/Pgp 170 was shown by immunohistochemical analyses in human breast tumor specimens, and it is well known that overexpression of P-glycoprotein contributes to primary chemotherapy resistance (66,67).

Regarding the possible anti-metastatic effect of aspirin, it was demonstrated that the daily ingestion of aspirin in patients previously treated with standard therapies indeed reduces metastasis, assigning this effect to the maspin protein (38). Maspin is a serine protease inhibitor of 42 kDa synthesized abundantly in normal mammary epithelial cells, and it has been shown that the expression of this protein is reduced in patients with breast cancer. Thus, the lack of expression of maspin in breast cancer has been suggested to indicate the presence of an aggressive and metastatic tumor (36). Studies have revealed that ingestion of aspirin increases the levels of serum nitric oxide (NO) and maspin, both of which inhibited the growth of breast cancer cells in vitro, as well as invasion and metastatic processes in an animal model (9,38). This finding suggests that aspirin participates in the restoration of maspin synthesis at any stage of the disease, and also that maspin presents beneficial effects at any stage of tumoral progression. It has been observed that maspin is reduced in BC, or even absent in invasive cancer; hence, there is a correlation between the protein synthesis and its reduction according to cancer type and stage (67). There is also evidence of maspin as an inhibitor of angiogenesis (68).

On the other hand, in 2008, Burnett and collaborators investigated the relationship between inflammation and angiogenesis and how it is regulated by aspirin. They proposed that aspirin alters macrophage regulation by inducing expression of IL-10 in MCF-7 cells, and suggested aspirin as a therapeutic strategy for modification of tumor-associated macrophages (TAMs), which can initiate both angiogenesis and invasion (69).

Finally, regarding the mechanisms of aspirin as an antitumor drug (70), Spitz et al discuss the use of acetylsalicylic acid (ASA) and salicylic acid (SA) in tumor cell viability and glucose metabolism. Both drugs modulate an important glycolysis-regulatory intracellular enzyme (6-phosphofructo-1-kinase, PFK) in a dose-dependent manner, and the inhibition occurs due to the modulation of the enzyme quaternary structure. The authors demonstrated that ASA, as well as its precursor SA, decreased the viability of the human breast tumor cell line MCF-7, diminishing its glucose consumption and PFK activity.

One of the most significant discoveries along the quest for novel cancer treatments and preventive therapies has been the COX-2 inhibitors. These drugs have been demonstrated to interfere directly with the carcinogenesis process, restoring apoptosis. The most representative medicine of the last 100 years is probably aspirin, a drug that is still currently under investigation. The COX-2 gene is overexpressed in breast cancer; furthermore, the presence of the COX-2 enzyme in tumoral tissue is directly proportional to the density of cancer cells. Moreover, some studies have revealed that high levels of prostaglandins are linked to unfavorable patient characteristics, such as the risk to develop metastasis and the reduction in survival rate, being good indicators for the correct diagnosis of disease stage.

In respect to the epidemiologic studies reported here, it is of great importance to homologate criteria in this type of research, for example, the time of administration of the drug, in order to truly assess the chemopreventive and anti-metastatic effect of aspirin. This issue should be considered because of the great variability existing in studies.

Evidence of the chemopreventive effect of aspirin cited in this review, suggests that it is likely to lower BC risk when administrated at doses greater than 100 mg/day for 3 or more years. Such information suggests that aspirin may start being favorable after a long period of regular use. However, this dosage poses adverse reactions, such as, an increase in gastric ulcer risk and gastrointestinal bleeding. Cancer is a multifactorial disease, and its behavior is constantly being investigated toward a greater understand and, at the same time, to propose less aggressive and more effective therapies or, when possible, chemopreventive alternatives. Aspirin could yield beneficial results for a specific group of risk patients, but who should be monitored carefully if there are any contraindications to the medicine. In conclusion, aspirin is a suitable alternative in this non-stop search for cancer prevention and treatment.