Selective Estrogen Receptor Modulators. Antonio Cano

Chapter 9. Cardiovascular Disease and SERMs

Antonio Cano

A wealth of epidemiological, clinical, and experimental studies link estrogens with cardiovascular disease (CVD). This evidence has promoted CVD as a key area within the extragenital effects of estrogens. The question is of interest because it directly affects the wide clinical use of estrogens as contraceptive agents or as principal constituents of hormonal therapy (HT) formulations in postmenopausal women. The significance of the subject is further reinforced by the relevance of CVD as a cause of mortality and morbidity in both women and men.

CVD is a generic denomination mainly integrated by coronary heart disease (CHD) and stroke. Although not considered as a form of CVD in some instances, venous thromboembolic disease (VTED) shares with the other forms of CVD the territorial assignment, the vascular tree, although clear differences exist in the main pathophysiological mechanisms. In most CVD forms, however, thrombus formation plays a crucial role.

All forms of CVD are serious disorders, although CHD is the most prevalent and lethal. Global data in the USA attribute 54% of deaths from CVD to C HD and 18% to stroke (http://www.americanheart.org). Figures for Europe are similar, where about half of deaths from CVD correspond to CHD and nearly one third to stroke (British Heart Foundation 2000). The burden of the disease is also shared by developing countries where it is estimated that CVD will be the leading cause of death by 2010 (World Health Organization Web site www.who.int/ncd/cvd). Specific gender patterns have been detected for both the prevalence as well as the behavior of CHD, further suggesting a relevant role for reproductive hormones. Fewer differences have been found for stroke.

VTED has a lower prevalence (approximately 1/1000 persons per year), but it rises exponentially with age from < 5 cases per 100,000 persons < 15 years old to = 500 cases per 100,000 (0.5%) at age 80 years (White 2003). Against the strong gender differences found in CHD statistics, no convincing difference between men and women have been detected for VTED (Silverstein et al. 1998).

The observation that, compared with men, women maintain some level of protection against CHD has nourished the debate about a possible favorable effect associated with exposure to estrogens. Furthermore, most of the information gathered in the latter years has confirmed the association of estrogens with many benefits both in experimental as well as in clinical models at the level of intermediate indicators. Consequently, HT was proposed not only for control of symptoms but also for primary and secondary CHD prevention in postmenopausal women. Contrary to the beneficial effects found in the first, observational studies, three more recent randomized controlled trials have found that HT is neutral (Hulley et al. 1998; Anderson et al. 2004) or even prejudicial (Rossouw et al. 2002) when administered to women for the purpose of CVD prevention. The data yielded by these studies affect not only CHD but also stroke and VTED, where HT has been shown to be detrimental, too. Following this evidence, scientific societies, such as the American Heart Association, have advised against the use ofHT for the prevention ofCVD in women (Mosca et al. 2004). The mixture of protective effects, mainly at the level of risk factors and of data gathered from experimental models, as well as of neutral or prejudicial clinical outcomes defines the present picture. It is one where the confirmation of estrogens as important regulators of CVD pathophysiology emerges as a main conclusion. Consequently, selective estrogen receptor modulators (SERMs) offer a unique opportunity to achieve cardiovascular outcome profiles that might improve those attained by conventional HT.

This chapter will analyze some specific traits of CVD that will be used to review the principal variables that have exhibited sensitivity to estrogen agonism. Then, current information on the particular actions of SERMs will be presented.

9.1

The Focal Phenotype of CVD

The vascular tree is divided into two main, well-demarcated areas composed of the arterial and the venous trees. They both define different microenvironments that create the conditions for the development of focal episodes determining the occurrence of obstructive phenomena that are at the base of CVD. Arterial episodes (CHD and stroke) occur at sites of inflamed arteries, while VTED or venous stroke episodes develop as a result of thrombus formation at discrete locations in the venous tree.

Atherosclerosis, a disease of the vascular wall, is the substrate for the arterial forms of CVD. Atherosclerotic plaques exhibit a focal distribution along the arterial tree as a consequence of local conditions that favor their initiation and progression. Low or reversed shear stress, for example, contributes to plaque development, a process in which the regulation of several genes may be involved (Resnick and Gimbrone 1995).

Thrombosis is the other phenomenon that crucially contributes to both the arterial and the venous forms of CVD, although the type of thrombus,

Table 9.1. Risk factors for coronary heart disease and venous thromboembolic disease [adapted from Friedewald (1996) and Rosendaal (1999)]

Coronary heart disease

Venous thromboembolic disease

Unmodifiable

Modifiable

Genetic

Acquired

Mixed genetic and acquired

Age

Cigarette smoking

Protein C deficiency

Immobilization

Increased concentration of prothrombin

Male gender

High blood pressure

Protein S deficiency

Surgery

Increased concentration of factor VIII

Family history of premature disease

High blood cholesterol

Antithrombin deficiency

Trauma

Hyper-homo- cystinemia

 

Physical inactivity Diabetes

Factor V Leiden

Prothrombin 20210A

Pregnancy

Puerperium

 
 

Overweight

Psychological conditions

Increased concentration of factor IX

Lupus anticoagulant Malignant disease Female hormones

 
     

Diseases affecting liver, endothelium, or other organs producing clotting factors

 
     

Abnormal dietary intake of substrates or vitamins (vitamin K)

 

its biological determinants, and consequently the corresponding risk factors differ for each form (Table 9.1). There is a list of factors that are involved in the increased thrombotic risk within the arterial tree (Table 9.2). Some of them directly depend on the altered focal environment, while others are systemic. The interaction between platelets and the arterial wall is one critical step. Platelet adhesion and deposition is strongly determined by local wall phenomena, essentially inflammation, with or without the substrate of an atheromatous plaque. The activation of platelets is followed by the induction of platelet coagulation activity, a process in which there is close collaboration with leukocytes adhering to the initial plug (Cano and Van Baal 2001). Liberation of tissue factor is another mechanism that participates in the generation of a thrombus.

Table 9.2. Factors affecting thrombogenicity in coronary heart disease [from Badimon et al. (1999)]

Local factors

Systemic factors

Degree of plaque disruption (i.e., erosion, ulcer)

Cholesterol, Lp(a)

Degree of stenosis

Catecholamines

(i.e., change in geometry)

(i.e., smoking, stress, cocaine)

Tissue substrate (i.e., lipid-rich plaque)

Fibrinogen, impaired fibrinolysis (i.e., PMI-1), activated platelets and clotting

(i.e., factor VII, thrombin generation, F1 + 2)

Surface of residual thrombus

Infections

(i.e., recurrence)

(C. Pneumoniae, CMV, H. Pylori)

Vasoconstriction

Diabetes

(i.e., platelets, thrombin)

 

Hypercoagulable states, in turn, have been traditionally associated with venous thrombosis. Consequently, attention has been paid to alterations of the hemostatic balance. Although this is a systemic variable, focality is favored due to the contribution of decreased blood flow, as confirmed by the preferential development of venous thrombi at the level of valves, an area of stasis where low-velocity flow is moderately turbulent.

In conclusion, hemostasia intervenes in distinct critical steps of both the arterial and venous forms of CVD. The particulars, however, differ in each case, as confirmed by the different array of risk factors for CHD and VTED. The participation of the vascular wall is pivotal in explaining the focality of these processes. Within the vascular wall, the role of the endothelium is critical given its involvement in the origin of atherosclerosis and its influence on the development of VTED (for review see Cano and Van Baal 2001; Cano 2003).

9.1.1

The Crucial Role of the Endothelium

The vascular wall is an organ composed of an endothelium, smooth muscle, and fibroblasts. The endothelium has a privileged position to act as both a sensor and an effector. The endothelium governs remodeling by releasing growth factors and vasoactive substances that regulate cellular growth and apoptosis. The key role of endothelium in the plasticity of the vascular wall helps to better understand the modern hypotheses that root the initiation and development of atherosclerosis in endothelial dysfunction (Ross 1999). The rupture of the coordinated equilibrium of checks and balances that is at the base of endothelial homeostasis is followed by a well-described sequence of events starting with the increase of adhesiveness of the endothelium to leukocytes or platelets and leading to an atherosclerotic plaque (Ross 1999) (Fig. 1). The progression of the plaque follows several steps represented by lesions I to IV (Fuster et al. 1992a,b).

Fig. 9.1. A dysfunctional or injured endothelium is at the basis for initiation of and progression to atherosclerosis. Several mechanisms, such as adhesion molecules or liberation of von Willebrand factor (vWf, upper panel), determine a series of phenomena, including platelet activation and aggregation. This participation of platelets involves the implication of molecules like glycoprotein IIb/IIIa, fibrinogen, and von Willebrand factor. The endothelium also acts as a source of signals that regulate local functions, including VSMCs (lower panel). A list of the most relevant messengers produced by a functional and a dysfunctonal endothelium is presented in the lower panel

A very innovative area of research has focused on the determinants of plaque stability. An important role seems to be played by enzymes involved in the degradation of the extracellular matrix. The rupture of unstable plaques induces platelet activation, too. Acute thrombus formation under these conditions seems fundamental to the onset of acute ischemic events.

A key concept inferred from the ideas discussed above is the difference between the development of conditions that favor the clinical eruption of any form of CVD (i.e., atherosclerosis) and the proper occurrence of the clinical event, since the inductors do not necessarily have to be the same. Furthermore, the possibility exists that a concrete factor may be protective at several stages of the silent form of the disease, but once it is sufficiently advanced, it may act as a trigger. This distinction is pivotal when considering the role of hormones, which have been shown to differentially regulate atherosclerosis and proper clinical events.

9.2

Estrogen Agonism and CVD

Some crucial steps in the biology of CVD have demonstrated sensitivity to estrogen agonists. Some of these actions have shown to be mediated by the classical pathway of estrogen receptors (ERs), though in other cases the involved mechanisms seem more complex and require the consideration of alternative options (Mendelsohn 2002). The available evidence concentrates on actions on lipids or on direct actions on the vascular wall.

9.2.1

Lipids

There is plenty of information concerning lipid changes as a result of estrogen agonistic effects. Most of the data come from studies with either synthetic or natural estrogens.

A protective lipid profile, with reduction of total cholesterol and LDL and a modest increase in high-density lipoprotein (HDL), has been associated with oral estrogen therapy (Writing Group for the PEPI Trial 1995). This effect, however, has been considered negligible when compared with the benefits traditionally ascribed to estrogens (Marsh et al. 1999).

More interest has been generated by the potential effects of estrogens as modulators of LDL oxidation, a mechanism considered to be the authentic mediator of the detrimental action of LDL particles in atherosclerosis. Oxidized LDL becomes trapped in an artery and is then internalized by macrophages (Steinberg 1997; Navab et al. 1996; Morel et al. 1983; Griendling and Alexander 1997). This internalization leads to the formation of lipid peroxides resulting in the formation of foam cells. Additionally, oxidized LDL is an agent that by itself promotes vasoconstriction and platelet activation (Kugiyama et al. 1990; Chin et al. 1992; Chen et al. 1996).

Estrogens have been shown to limit LDL susceptibility to oxidation, although this action is under discussion at present. Only supraphysiological doses have demonstrated this effect in the laboratory (Hermenegildo et al. 2001; Santanam et al. 1998). Some indirect evidence favoring protection, such as the reduction of antibodies to oxidized LDL, has been proposed (Hoogerbrugge et al. 1998), but, again, there is no general consensus on the subject (Uint et al. 2003; Heikkinen et al. 1998). More recent research has found that estrogens reduce the production of F2a-isoprostanes, a product of a nonenzymatic, free radical catalyzed peroxidation of arachidonic acid (Liu et al. 1998) that has been recognized as a stable, good biomarker of in vivo oxidative stress (de Zwart et al. 1999; Pratico 1999). Moreover, increased Fisoprostane levels have been found in human atherosclerotic lesions (Oguogho et al. 2001) and are being considered as a reliable biomarker of both atherosclerosis (Gross et al. 2005) and coronary events (Vassalle et al. 2003) (Fig. 9.2).

9.2.2

Vascular Wall

The vascular wall is a target for sexual hormones. In the particular case of estrogens, specific receptors have been found in both endothelium and vascular smooth muscle cells (VSMC) (Venkov et al. 1996; Karas et al. 1994). The trophic effects ofestrogens on the endothelium have been advocated as crucial against initiation and promotion ofatherosclerosis. Thus, cellular and animal models, as well as clinical observation with doppler techniques, confirm that estrogens promote vasodilation. This effect is maintained for years in menopausal women subjected to HT (Jokela et al. 2003). Nitric oxide (NO) and prostacyclin (PGI), two main locally produced antiaggregant and vasodilatory mediators, are the principal agents in this myorelaxant effect of estrogens (Couzin 2004). In agreement with current concepts, their effects have been demonstrated as protective against atherosclerosis in animal models (Perrault et al. 2003; Niebauer et al. 2003; Todaka et al. 1999). Together with the protection associated with these mediators, the inhibition of TNF-alpha-induced endothelial cell apoptosis in a dose-dependent manner has been an additional beneficial effect linked with estrogens (Spyridopoulos et al. 1997).

Proliferation and migration ofVSMC follows endothelial dysfunction. Limitation of this activity in VSMC has been understood to be protective against atherosclerosis. The effect ofestrogens on VSMC proliferation is controversial. Some studies have reported a reduced proliferative capacity by estrogens in a dose-dependent manner (Bhalla et al. 1997; Moraghan et al. 1996; Espinosa etal. 1996; Akishita et al. 1997) and through activation of ER (Vargas et al. 1996). In contrast, other investigators have found induction of VSMC proliferation with estrogens (Ricciardelli et al. 1994; Song et al. 1998).

Experiments in monkeys have shown that estrogens alone, or in association with progestogens, protect against diet-induced atherosclerosis (Clarkson 1994). There has been some discussion on whether or not this is the case in humans, although HT was unable to have a significant effect on the progression of the disease in women with established atherosclerosis (Hodis et al. 2003).

9.3

SERMs as an Alternative to Estrogens in CVD

The expectations created for estrogens faded as a result of the publication of randomized clinical studies, which failed to show any protection against any of the CVD forms in postmenopausal women receiving hormones (Hulley et al. 1998; Rossouw et al. 2002). The clear opposition between these trials and most ofthe experimental and previous clinical studies has raised much discussion in the literature (Speroff 2002). Despite the many criticisms against distinct details of the discrepant studies, there is consensus on the appreciable regulatory effects ofthe hormone on the vascular wall. This conception, together with the significant advances experienced by the knowledge on the molecular details of estrogen action, has created a great opportunity for investigating alternative agonists with a potentially better profile than estrogens themselves.

In one a priori analysis the versatility of ER modulation offers a wide array of options. These include the selective activation of either the ERa or the ERβ isoform, or the use of compounds sufficiently similar to estrogens so as to bind to the ER, yet different enough to generate a ligand-receptor complex with a 3D conformation capable of activating cell functions with a profile distinct to estrogens (Fig. 9.3). The interesting observation that ERβ can interact with ERa, together with the varied distribution of each ER isoform in different tissues, has raised attractive possibilities associated with selective binding to one or another isoform. Despite the recent availability of compounds with selective agonism for either ERa or ERβ (Harrington et al. 2003; Muthyala et al. 2003; Ghosh et al. 2003), there is no clear proof to date associating the selective action of any of the available SERMs in the vascular system with preferential binding to one of the two ER isoforms. Consequently, the most consistent data on cardiovascular effects of SERMs have been obtained in studies with compounds that have been approved for use in patients, particularly tamoxifen, raloxifene, and toremifene.

Another important point to keep in mind when reviewing the cardiovascular effects of SERMs is that, in the absence of clinical studies of consistency comparable to estrogens, most of the available evidence has been obtained in experimental models. The work has concentrated on the selective areas of vascular physiology that have shown susceptibility to ER activation and, therefore, has followed steps that often overlap with those taken in research with estrogens.

Fig. 9.3. Several mechanisms underlie the functional versatility of the ER. The different distributions of the alpha and beta isotypes of ER conditions a first step that warrants distinct functional profiles depending on the higher or lower affinity of the ligand for one or another isotype (upperpanel). Then, different ligands generate distinct 3D conformations in the ligand-receptor complex that condition different interaction profiles with the promoters of target genes (lower panel)

9.4

Actions of SERMs

9.4.1

Arterial Disease

Most of the forms of arterial disease result from atherosclerosis and its complications. The evidence against protection refers not only to CHD but also against stroke (Hulley et al. 1998; Rossouw et al. 2002; Bath and Gray 2005).

9.4.1.1

Lipids and Lipid Peroxidation

Changes in the lipid profile, which exhibits small differences from that associated with oral estrogens, have been described for tamoxifen, toremifene, and raloxifene. One common finding has been the decrease in the circulating concentration of cholesterol and LDL cholesterol, an effect with a magnitude that seems directly related to pretreatment levels (Walsh et al. 1998; Delmas et al. 1997; Saarto et al. 1996; Decensi et al. 2003; Joensuu et al. 2000; Herrington et al. 2000). Contrary to the increase in triglycerides described for estrogens, a more beneficial neutral response appears associated with SERMs. Slight increases in triglycerides, however, have been found in women treated with raloxifene (Mosca et al. 2001b; Reid et al. 2004), and cases of acute triglyceridemia have been associated with tamoxifen (Hozumi et al. 1997; Kanel et al. 1997). Only toremifene has achieved increases in the levels of high-density lipoprotein (HDL) (Saarto et al. 1996).

More refined analyses have focused on changes in the ratio of serum concentration of apolipoprotein B, the common constituent in all lipoproteins comprising non-HDL cholesterol, to apoliprotein A, the apolipoprotein associated with HDL. Raloxifene was equivalent to HT in lowering the apolipopro- tein B/apolipoprotein A ratio in one study (Anderson et al. 2001).

Because of the similarity, it is difficult to conclude whether the lipid changes induced by SERMs offer any advantage over the profile determined by HT. Triglyceride levels have been proposed as an independent risk factor for CVD in postmenopausal women (Miller 1998). Further, there are some indications that increases in triglycerides may favor the reduction in the size of LDL particles. Smaller LDL particles are more susceptible to oxidation and have been associated with a higher risk potential (Austin et al. 1988), but whether this observation confers any clinical prejudice to hypertriglyceridemia has not been proven at present.

There is fragmentary information concerning the behavior of some more recent SERMs. Whereas ospemifene showed a neutral effect (Ylikorkala et al.

Fig. 9.4. One pure antiestrogen, ICI 182780, increased the resistance of LDL particles to oxidation. Isolated LDL particles were subjected to oxidation by copper, and the lag time to oxidation, as measured by changes in optical density, increased as a function of the concentration of ICI 182780 (upper panel). The increase in the lag time (min) determined by the different concentrations of ICI 182780 is shown in the lower panel 2003), HMR 3339, a newly designed molecule that binds to human recombinant ER and shows selective agonistic and antagonistic activity in vitro and in vivo, rapidly decreased cholesterol and LDL in a dose-dependent manner (Vogelvang et al. 2004). It seems, therefore, that the decrease in non-HDL cholesterol is a hepatic effect quite accessible to compounds that, despite differences in chemical structure, are capable of exerting some type of SERM activity.

The relevance attributed to oxidized lipids, and particularly oxidized LDL, in atherogenesis has precipitated interest in the ability of SERMs to this regard. Ex vivo experiments have confirmed that both tamoxifen and raloxifene exert some protection against the oxidation of LDL particles (Arteaga et al. 2003; Zuckerman and Bryan 1996) and that, interestingly, raloxifene is a more powerful antioxidant than tamoxifen or estradiol. It seems that this antioxidant effect is not mediated by the activation of the ER since pure antiestrogens like ICI 182780 and other SERMs like EM 652 have proven to have similar protective effects on LDL (Hermenegildo et al. 2002) (Fig. 9.4).

Little evidence exists concerning alternative actions on oxidative stress, such as modulation of the circulating levels of isoprostanes or myeloperoxidase. Some interference with the actions of myeloperoxidase, however, was found in one study (Zuckerman and Bryan 1996). Antioxidant properties have been described for other types of SERMs, thus confirming the wide extension of this potential in the different families of these compounds (Baumer et al. 2001).

9.4.1.2

Direct Actions on Vascular Wall

9.4.1.2.1

Endothelium

The idea that the endothelium is a target organ for estrogens derives from more than just the identification of both isoforms of ER in this tissue (Mendelsohn 2000). There is ample evidence showing rapid responses that are compatible with mechanisms distinct from the classical pathway for estrogen action. A species of membrane ER that determines a rapid activation of nitric oxide synthase (NOS) has been described recently in immortalized human endothelial cells (Li et al. 2003). Both genomic and nongenomic actions have been proposed to explain the estrogenic regulation of endothelial functions.

Among the local mediators directly produced by endothelium, NO and PGI emerge as two principal regulators of vascular tone and platelet aggregation. Both are sensitive to estrogenic stimuli, and, as mentioned in a previous section, their role is crucial in atherogenesis. How their production is modulated by SERMs is, consequently, an important test of vascular protection.

Much of the data concerning the effects of SERMs on these endothelial mediators refer to raloxifene, given its wide therapeutical use in women free of malignancies. Raloxifene has demonstrated the induction of NOS and NO production in endothelial cells in culture. Furthermore, this effect occurs in seconds and involves nongenomic mechanisms where NOS is phosphorylated in a process implicating Akt and extracellular signal-regulated protein kinase with the participation of ER alpha and reduction of oxidative stress (Simoncini and Genazzani 2000; Wassmann et al. 2002). In agreement with this agonistic action, experiments on the same cellular model have confirmed an activation of cyclooxygenase-1 and -2 at both the protein and the gene level, leading to increased prostacyclin production (Oviedo et al. 2004, 2005). Selective blockade of both isoforms of ER has confirmed the involvement of both ERα and ERβ as well as the likely participation of a mechanism distinct to the classical ER-dependent pathway.

Experiments with isolated vessels have confirmed the enhancing effect of raloxifene on endothelial NOS (Rahimian et al. 2002) with a similar behavior for tamoxifen (Hutchison et al. 2001).

The data with cells and isolated organs have been corroborated in animal models. An increase in endothelial NOS expression and activity was observed in spontaneously hypertensive rats (Wassmann et al. 2002), whereas in ovariectomized ewes the vasodilating effect of raloxifene surpassed that of estrogens (Gaynor et al. 2000). Endothelium-dependent vasodilation was observed for rabbit coronary arteries in vitro, an effect that agrees with some vascular relaxing properties described for toremifene, tamoxifen, idoxifene, and EM 652 in rat vessels (Gonzalez-Perez and Crespo 2003; Thorin et al. 2003; Figtree et al. 2000; Christopher et al. 2002; Tatchum-Talom et al. 2003).

Mixed evidence, however, has been described in women. Raloxifene improved flow-mediated, endothelium-dependent vasodilation in postmenopausal women (Sarrel et al. 2003) to an extent similar to that of HT (Colacurci et al. 2003; Saitta et al. 2001). Other investigators, however, have been unable to detect any effect of raloxifene (Ceresini et al. 2003; Griffiths et al. 2003). Flow-mediated vasodilation has been described for droloxifene (Herrington et al. 2000), while a neutral effect on vascular reactivity has been described for ospemifene, a more recent SERM (Ylikorkala et al. 2003).

One early sign of endothelial dysfunction consists of the expression of cell adhesion molecules at the endothelial surface. These molecules facilitate leukocyte and platelet binding. Further, endothelial permeability is dependent on interendothelial junctions, where the participation of cadherin, a transmembrane, endothelium-specific glycoprotein, exerts an important level of control (Bobryshev et al. 1999; Fulimoto et al. 1998). Once expressed, adhesion molecules may be shed from the endothelial surface. An increase in adhesion molecules in plasma, therefore, is understood as a sign of endothelial dysfunction and permeability. Furthermore, raised levels of cell adhesion molecules in blood have been associated with increased risk for CHD (Blankenberg et al. 2001; Hwang et al. 1997). A well-established effect of estrogens has been the reduction of the circulating concentration of cell adhesion molecules (Koh et al. 1997), an effect paralleled by raloxifene (Blum et al. 2000; Sbarouni et al. 2003; Colacurci et al. 2003). Different actions have been found for other SERMs in the sparse literature available. Tamoxifen had a neutral effect in one study (Simoncini et al. 1999), whereas in another study droloxifene had a mixed effect, with a decrease in E-selectin and an increase in vascular cell-adhesion molecule-1 (VCAM-1) (Herrington et al. 2001).

Finally, a new area of research has concentrated on monocyte chemotactic protein-1 (MCP-1), a 76-amino acid peptide that is one of the best-studied members of the C-C chemokine subfamily. Recent human and animal studies indicate that the recruitment of macrophages to the arterial lesion is predom inantly mediated by MCP-1. There are preliminary data showing that both tamoxifen and raloxifene parallel estradiol in reducing the expression of MCP- 1 in a model of endothelial cells in culture (Seli et al. 2002).

9.4.1.2.2

VSMC

Indirect evidence suggests that the blockade of VSMC proliferation is associated with ER agonism (Lavigne et al. 1999). The data obtained with SERMs are still sparse and mainly restricted to raloxifene. In experiments in vitro, raloxifene exhibited an effect similar to estrogens in inducing arrest and apoptosis in VSMC (Takahashi et al. 2003; Mori-Abe et al. 2003). Consistent with this observation, raloxifene was equivalent to estradiol in limiting intimal thickening in a model of ovariectomized senile ewes (Selzman et al. 2002). Some evidence favors a similar protective effect for other SERMs, like idoxifene (Yue et al. 2000) and tamoxifen (Dubey et al. 1999; Somjen et al. 1998; Grainger et al. 1993).

9.4.1.2.3

Atherosclerotic Plaque

The biological effects that have been described above, i.e., reduction of LDL and its oxidation, the protection of endothelial function, and the limitation of VSMC proliferation, globally suggest a protective effect against atherosclerosis. This hypothesis has been assayed with the use of distinct animal models with diet-induced atherosclerosis. Some experiments have been carried out in genetically modified mice that have been subjected to targeted inactivation of the apolipoprotein E (apo E) and LDL receptor (LDLR) genes. These animals respond to moderate amounts of dietary cholesterol with severe hypercholesterolemia and develop lipid-rich vascular lesions resembling human atherosclerotic plaques. An atheroprotective effect has been confirmed for estrogens in rabbits (Haines et al. 1999; Haarbo et al. 1991; Bjarnason et al. 1997, 2001; Hough and Zilversmit 1986) and monkeys (Adams et al. 1990; Wagner et al. 1991) subjected to an atherogenic diet. Experiments with LDLR- and apoE-null mice further confirmed that the extent of atheroprotection by estradiol was greater than could be explained solely by the change in lipid levels (Hodgin et al. 2001; Tangirala et al. 1995; Elhage et al. 1997; Marsh et al. 1999).

The data obtained with SERMs are more mixed. Tamoxifen attenuated atheroma development in apoE-null mice, an effect that correlated with changes in the lipoprotein profile and with elevated levels of transforming growth factor-β (Reckless et al. 1997). The accumulation of cholesterol in atherosclerotic lesions (Bjarnason et al. 1997) in the aorta was limited by

raloxifene in a model of cholesterol-fed rabbits. Subsequent experiments with the same model confirmed that raloxifene reduced atherosclerosis (Bjarnason et al. 2000), an effect similar to that of estrogens in another study where progression of advanced atherosclerosis was limited (Bjarnason et al. 2001) (Fig. 9.5). In a primate model, however, in which a tête-a-tête comparison between estrogens and raloxifene was carried out, only estrogens, but not raloxifene, effectively limited atherosclerosis (Clarkson et al. 1998). Protection against VSMC proliferation in culture as well as in experimental models of atherogenesis in rats has been described for idoxifene (Yue et al. 2000). A more active reendothelialization was observed in treated animals in the same study.

Only fragmentary information exists in the human. In a study on 27 postmenopausal women with breast cancer, tamoxifen slightly slowed the progression of atherosclerosis as assessed by changes in carotid intima-media thickness (Stamatelopoulos et al. 2004).

Fig. 9.5. Protection by SERMs against atherosclerosis has been researched in animals. In a model of ovariectomized rabbits, raloxifene reduced the cholesterol content in the inner part of the aorta more than placebo did (upper panel). This effect was more intense in animals treated with estradiol (Bjarnason et al. 1997). In contrast, in a different model of oophorectomized monkeys (lower panel), estradiol, and not raloxifene at two different dosages, significantly decreased the size of atherosclerotic plaques (Clarkson et al. 1998)

9.4.1.3

Inflammatory Markers and Mediators

The results of both the WHI and HERS studies have contributed decisively to clarifying the difference between atherogenesis itself and the rupture of one atheromatous plaque as the concrete phenomenon leading to an occlusive vascular event. Although necessarily interrelated, the slow progression of a stable plaque, with its consequent reduction of the arterial lumen, may have its ischemic effects limited by the adaptive response including the concurrent development of collateral circulation (Fuster et al. 1992a,b). The concatenation of acute thrombosis as a result of either plaque disruption or severe erosion of the endothelial surface is, however, at the base of most acute coronary syndromes. This concept defines the support of the most widely accepted hypothesis on the action of hormones. As a result of this conception, much interest has arisen in the study of inflammatory mechanisms that underlie disruption of the cap of a lipid-rich plaque, the characteristic form of so-called unstable plaques. It has been shown that estrogens may modify local inflammatory processes and promote the expression and activity of metalloproteinases, a group of enzymes active in the digestion of the matrix (Zanger et al. 2000).

In this new scenario much attention is being paid to the investigation of a series of markers of inflammation as reliable indicators of coronary risk. Their value is stressed by the observation that up to one third of events occurs in subjects without traditional risk factors. The C-reactive protein (CRP) seems to provide the strongest risk prediction for CHD in women (Albert 2000; Ridker 2001), although homocysteine, interleukin-6 (IL-6), and lipoprotein (a) [Lp(a)], among others, have each been independently associated with increased risk for CHD in women (for a review see Davison and Davis 2003; Rader 2000).

As for lipids, the effects of SERMs do not overlap exactly those of HT. Oral estrogens increase the circulating levels of CRP (Writing Group for the PEPI Trial 1995), while this is not the case for raloxifene (Walsh et al. 2000). A better profile was observed for droloxifene as well as for tamoxifen, which achieved a diminution of CRP (Herrington et al. 2001; Cushman et al. 2001).

Slight, yet similar, range decreases were observed for oral estrogens and raloxifene when studied for changes in homocysteine (Walsh et al. 2000; Smolders et al. 2002; Mijatovic et al. 1998; De Leo et al. 2001), a molecule that may have damaging effects on endothelium. A reduction was found for Lp(a), too, although in this case the decrease achieved for estrogens was of a higher magnitude in one study (- 19% vs. - 7%) (Walsh et al. 2001). Droloxifene, however, was more efficient than estrogens in reducing Lp(a) levels (Herrington et al.

2000).

IL-6 participates in both atherogenesis and inflammatory processes. In one interesting mouse model that was double deficient at the apoE and IL-6 loci, animals displayed similar hypercholesterolemia compared to apoE-null mice, but disclosed larger and more calcified lesions at 1 year of age (Klinge 2001). Thus, IL-6 appears to be involved at the fibrous plaque stage of the atherosclerotic process. Moreover, IL-6 is a key factor in the generation of the hepatic acute-phase response and so increases the levels of CRP, fibrinogen, platelet number and activity, and blood viscosity. Only raloxifene has been tested for IL-6, which did not change in one study (Walsh et al. 2001) and decreased by 35% in another study after 24 months of treatment (Gianni et al. 2004). Tumor necrosis factor a (TNFa) is another cytokine associated with cardiovascular risk in epidemiological studies (Ridker et al. 2000). Similar decreases for TNFa have been found in a study comparing HT and raloxifene (Walsh et al. 2001).

Table 9.3. Effects of SERMs on inflammatory markers in postmenopausal women

In conclusion, SERMs exhibit changes in inflammatory markers that do not match those found with oral HT. Some variability exists within HT itself, depending on the compound (estrogens or tibolone) and on the administration route (oral vs. transdermal). There is sufficient background to hold the value of inflammatory markers as strong indicators of coronary risk, but whether interventions modifying their circulating levels have an influence in risk is still uncertain. A summary of the effects of SERMs on inflammatory markers may be found in Table 9.3.

9.4.1.4

Hemostasia

The apparent protection conferred to the endothelium by estrogens in healthy women operates in favor of platelet stabilization. This interpretation agrees with studies on platelet aggregation that is diminished in response to different stimulants while under exposure to estrogens (Bar et al. 1993, 2000; Nakano et al. 1998; Chen et al. 1998). Nonetheless, in women with advanced atherosclerosis and unstable plaques the picture may be different. Furthermore, little is known about the mechanisms involved in platelet activation, and the sparse evidence is not always favorable to estrogens (Garci'a-Marti'nez et al. 2004). Whether a different profile is imposed by SERMs is not totally clear. Recent work has demonstrated that raloxifene shares with estradiol some protective effects on platelet aggregation induced by ovariectomy (Jayachandran et al.

2005). In a flow chamber model tamoxifen has shown no effect on platelet aggregation (Miller et al. 1994), an effect that agrees with experiments on platelets subjected to different endocrine environments since, unlike hormonal contraceptives, tamoxifen reduced intracellular calcium and release (Miller et al. 1995).

Additionally, attention has been focused on some factors that, operating in the hemostatic balance, have been attributed the role of risk markers of clinical events. Thus, increased plasma concentration of factor VII, fibrinogen, plasminogen activator inhibitor type 1 (PAI-1), and the already mentioned Lp(a) have been associated with the occurrence of CHD. Much work has been done on the modulation of these factors by HT (for a review see Cano and Van Baal 2001), and both similarities and differences have been found in the sparse literature on SERM action. Raloxifene and droloxifene decrease fibrinogen more actively than does HT (Walsh et al. 1998; Herrington et al. 2000). In contrast, the effective reduction demonstrated for PAI-1 with oral HT was not confirmed for raloxifene or droloxifene (Walsh et al. 1998; de Valk-de Roo et al. 1999; Herrington et al. 2000).

9.4.1.5

Clinical Data

There are no randomized clinical trials on the efficacy of SERMs in either the primary or the secondary prevention of CHD. The Raloxifene Use for the Heart ;RUTH) study is a trial specifically designed to clarify the effect of raloxifene on the risk of CHD. The study had included 10,101 women from 26 countries it the closure of the inclusion period, August 2000 (Mosca et al. 2001a). Results rom the trial remain to be reported.

Indirect evidence favoring protection has been obtained from a post hoc analysis of the data from the Multiple Outcomes of Raloxifene Evaluation (MORE) study in the subgroup of women who were at increased risk. Using the same scoring system as in the RUTH study to stratify women, a total of 1035 women were assessed as being at significant coronary risk (Barrett- Connor et al. 2002). When women within the group that had been randomized to raloxifene were separated from those randomized to placebo it came up that treatment was associated with protection against new clinical events, and that the higher the score, the greater the protection (Fig. 9.6).

Fig. 9.6. Relative risk (±95% confidence intervals) for any cardiovascular event in the group treated with raloxifene or placebo. The information was obtained from the subgroup of women at increased cardiovascular risk in the MORE study. The overall data seem to favor raloxifene, but this effect is clearer when women were grouped according to their risk as assessed by the previously defined severity score (from Barrett-Connor et al. 2002)

The effects of tamoxifen in women with and without CHD have been ana­lyzed in the National Surgical Adjuvant Breast and Bowel Project Breast Can­cer Prevention Trial (BCPT). This randomized, placebo-controlled study in­cluded 13,388 women at increased risk for breast cancer. The conclusions of the trial are somewhat limited by the fact that it was designed to inves­tigate the effect of tamoxifen as a chemopreventive for breast cancer, and not its effect on CVD risk. There was no indication that tamoxifen would modify the risk of CHD in women with or without heart disease (Reis et al. 2001).

9.4.2

VTED

A consistent observation linked with estrogen agonism has been the increased risk of VTED. Both hormonal contraceptives and HT determine increased risk oscillating from 2- to 11-fold for contraceptives (Hannaford and Owen- Smith 1998) and from 2- to 4-fold for HT (Daly et al. 1996; Jick et al. 1996; Grodstein et al. 1996; Pérez-Gutthan et al. 1997; Varas-Lorenzo et al. 1998). The risk has been associated with estrogens, but particularly in the case of some third-generation molecules used in contraception, also with the progestogenic component (Vandenbroucke et al. 2001). Interestingly, and despite intensive research, there is not a sufficiently clear understanding of the mechanisms set in motion by hormones to promote risk (Cano and Van Baal 2001). Venous thrombogenesis seems influenced by both hypercoagulable states and flow disturbances, including the independent or collaborative effects of decreased flow and local turbulence (Cano 2003). Much of the research has focused on the inhibitory action that some studies have detected for hormones on the natural anticoagulant system. It is intriguing, however, that increased risk associated with exogenous hormones is not reproduced by endogenous hormones. As mentioned above, age and not gender determines the increase in risk in the general population. Some data find an even slightly higher risk for men during aging (Silverstein et al. 1998).

It is remarkable that most of the data collected from the available SERMs are unanimous in reproducing an estrogen agonistic profile in venous throm- bogenesis. The vast clinical experience acquired with tamoxifen confirms an augmented risk for both deep venous thrombosis and pulmonary embolism. This increase, however, did not presuppose increased mortality in the overview of randomized trials of adjuvant tamoxifen for early breast cancer, where the one extra death per 5000 woman-years of tamoxifen attributed to pulmonary embolus was not statistically significant (Early Breast Cancer Trialists’ Collaborative Group 1998).

The main source of data for raloxifene derives from the MORE study. A twofold increased risk for VTED was observed through 4 years of followup (Delmas et al. 2002), and, as for HT and tamoxifen, an accumulation of events occurred during the first year.

There is discussion on the adequacy of tests to identify the hypercoagula- ble states underlying susceptibility to VTED. The complexity of factors and interactions involved in the hemostatic equilibrium has favored the use of functional tests. Among the several options available the measurement of fragments 1 + 2 (F1 + 2), the amino terminus fragment split during the activation of prothrombin has been widely considered the test of choice. The sparse information available for SERMs, however, is unclear. Raloxifene did not modifyF1 + 2 fragments in one study where HT was also neutral (Walsh et al. 1998). Other investigators, however, detected slight increases in F1 + 2 fragments for HT in another direct comparison with raloxifene (de Valk-de Roo et al. 1999).

9.5

Conclusion and Outlook for the Future

The different profiles of the diseases integrated within CVD make their sensitivity to the modulation of ER or, in a more general view encompassing other alternative agonistic pathways, of estrogen action rather variable. There is a clear gender influence on CHD only, but, and of interest, the administration of hormones affects the risk for other forms of CVD, like VTED or stroke. This reality, together with the vast amount of experimental data confirming the action of estrogens on several mechanisms crucial in the pathogenesis of each form of CVD, has reinforced the concept of the important regulatory potential of estrogens. Advances in the knowledge of estrogen action have opened up the field of SERMs, which in one a priori analysis should accomplish a peculiar profile of actions. The data obtained to date confirm this assumption.

The greatest amount of information has been compiled for CHD. The most widely used SERMs, like tamoxifen and raloxifene, seem to behave acceptably concerning the mechanisms underlying the disruption of atherosclerotic plaques. This maybe an advantage over estrogens, and some preliminary clinical data seem to favor this interpretation. In contrast, it seems estrogens might perform better in protecting against atherosclerosis development. There is very little information on whether SERMs may offer advantages against arterial stroke, although the increase associated with estrogens in recent randomized, placebo-controlled clinical trials have not been detected for SERMs. Although it only offered data on mortality and did not clearly separate the distinct CVD forms, the important overview of randomized trials of adjuvant tamoxifen could not find increased mortality for the aggregate of all cardiac or vascular deaths (Early Breast Cancer Trialists’ Collaborative Group 1998).

Venous thrombosis defines a field where there is a strong parallel performance of estrogens and of the SERMs presently developed. This adds to the still mysterious mechanisms underlying the increase in risk that has been found. There is plenty of evidence in favor of an antagonism of hormones on the anticoagulant pathway of hemostatic equilibrium, but very poor data have been obtained with functional tests of coagulation. The dearth of information on the mechanisms by which estrogens/SERMs interfere with anticoagulation further impairs the finding of successful research options.

In conclusion, we are at a very preliminary step on what is probably a long but promising path. The modulation of estrogen action seems a powerful mechanism in the control of CVD risk. Additional advances in the knowledge of estrogen action as well as in the improvement in the design process of new SERMs should offer substantial progress in this area. The concomitant acquisition of clinical data, as is expected from the RUTH study, will consolidate research developments.

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