Pleiotropic effects of statins: A focus on cancer

Abstract

The statin drugs (‘statins’) potently inhibit hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase by competitively blocking the active site of the enzyme. Statins decrease de novo cholesterol biosynthesis and thereby reduce plasma cholesterol levels. Statins exhibit “pleiotropic” properties that are independent of their lipid-lowering effects. For example, preclinical evidence suggests that statins inhibit tumor growth and induce apoptosis in specific cancer cell types. Furthermore, statins show chemo-sensitizing effects by impairing Ras family GTPase signaling. However, whether statins have clinically meaningful anti-cancer effects remains an area of active investigation. Both preclinical and clinical studies on the potential mechanisms of action of statins in several cancers have been reviewed in the literature. Considering the contradictory data on their efficacy, we present an up-to-date summary of the pleiotropic effects of statins in cancer therapy and review their impact on different malignancies. We also discuss the synergistic anti-cancer effects of statins when combined with other more conventional anti-cancer drugs to highlight areas of potential therapeutic development.

1. An introduction to statins

Statins are potent, competitive inhibitors of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR). They directly block the enzyme’s active site and inhibit the conversion of HMG-CoA to mevalonate (MA) (Fig. 1), the proximal and rate-limiting step in cholesterol biosynthesis [1]. The source of plasma cholesterol in humans originates either from dietary intake or cellular de novo biosynthesis. Statins reduce blood plasma cholesterol levels by decreasing de novo cholesterol biosynthesis and by inducing changes in low density lipoprotein (LDL) receptor expression [2]. MA is also converted to isoprenoid lipids, i.e. isopentenyl pyrophosphate (IPP), farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP), upstream of cholesterol. These intermediates are critical in the posttranslational modification of numerous oncoproteins, and in particular the small monomeric GTPases, including the Ras and Rho families, which have been implicated in various aspects of tumor development and progression [3]. In addition, several clinical studies have suggested that targeting the MA pathway might be a promising strategy in cancer therapy and chemoprevention [4].

Fig. 1.

The mevalonate pathway. Statins competitively inhibit HMG-CoA reductase and decrease cholesterol biosynthesis. Inhibition of the mevalonate pathway also reduces the biosynthesis of GGPP and FPP. These intermediates are involved in the isoprenylation of small Rho-GTPases. Interference with the posttranslational modification and activation of these proteins importantly accounts for some of the pleiotropic effects of statins. Abbr. 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), acetoacetyl-CoA (ACAC-CoA), acetyl-CoA (AC-CoA), coenzyme A (CoA), dimethylallyl pyrophosphate (DMAPP), farnesyl pyrophosphate (FPP), geranyl pyrophosphate (GPP), geranylgeranyl pyrophosphate (GGPP), isopentenyl-5-pyrophosphate (IPP), mevalonate (MV), mevalonate-5-phosphate (MV-P), mevalonate-5-pyrophosphate (MV-PP).

The development of statins as “cholesterol-lowering” agents began in the mid-1970s, when they were discovered as a fungal metabolite; the first of these being a natural product called mevastatin (1). Another naturally occurring statin, lovastatin (2), was soon isolated and showed a 2-fold higher inhibitory potency against HMGCR as compared to mevastatin. Later, lovastatin became the first FDA-approved statin for therapeutic use to treat hypercholesterolemia and cardiovascular diseases. Next, two statins were developed based on the structures of these two naturally occurring statins, simvastatin (3) and pravastatin (4), followed by the newer synthetic varieties atorvastatin (5), fluvastatin (6), rosuvastatin (7), cerivastatin (8), and pitavastatin (9) (Fig. 2) [5]. Over (at least) four decades of use, statins have become one of the most widely prescribed drugs in the world [6].

Fig. 2.

Chemical structures of selected statins.

1.1. Structure activity relationship (SAR) studies

Based on X-ray crystallography structures of different statins bound to HMGCR, two structural features are important for binding to the active site: the 3,5-dihydroxypentanoic acid moiety and the bicyclic decalin ring (Fig. 2) [7]. The only difference in chemical structure between the naturally occurring statins, mevastatin (1) and lovastatin (2), is the presence of a methyl group at the bicyclic decalin moiety (Fig. 2 - Site 1). Both possess a lactone (Fig. 2 - Site 2), which is hydrolyzed in the liver into 3,5-dihydroxypentanoic acid [8]. Once lactone is hydrolyzed, the hydroxy groups adopt the erythro-relationship; this intermediate compound mimics the substrate HMG-CoA and inhibits the enzyme.

In the mid-1980s, Stokker et al. prepared a series of HMGCR inhibitors and observed several meaningful structure-activity relationships (SARs) [9]:

1. The stereochemistry on the lactone (Fig. 2 - Site 1) is essential for potent inhibition -only one enantiomer of the open-ring form of lactone possessed the activity;

2. The presence and specific arrangement (erythro-relationship) of the two hydroxy groups, and the presence of a carboxylic acid group are essential for drug activity since it mimics the enzymatic substrate HMG-CoA;

3. Alterations or removal of the linker, a moiety bridging the bicyclic decalin ring and the lactone moiety, results in reduced or even loss of activity (Fig. 2);

4. Minor modifications of the bicyclic decalin ring, including replacement of this moiety with an aromatic substituted ring, are well-tolerated.

Later work showed that the bicyclic decalin moiety (Fig. 2 - Site 1) of statins is responsible for binding to the CoA site of the enzyme [10]. A systematic study of substituents at aromatic rings revealed that 4-fluorophenyl, indole, and pyrimidine rings represent the most optimal structural features to mimic the bicyclic decalin ring system that is present in natural statins and pravastatin [11].

The aforementioned SAR studies led to the discovery of simvastatin (3) (brand name Zocor^®), an analog of lovastatin, which has only one modification at the bicyclic decalin moiety, and, hence, many similar properties to lovastatin (Fig. 2). Like lovastatin, it possesses a lactone moiety, which increases its lipophilicity, and therefore qualifies as a prodrug. After oral administration, the cytochrome P450 (CYP) 3A4 isoenzyme is responsible for first-pass metabolism [12], rendering only 5% or less of the simvastatin bioavailable for therapeutic effects [8]; the elimination half-life amounts to approximately 3 h.

Pravastatin (4) (brand name Pravachol) has an active, open form of the lactone (the 3,5-dihydroxypentanoic acid moiety at ‘Site 2’), and a minor modification on the bicyclic decalin moiety (Fig. 2). As compared to simvastatin, the methyl group at the bicyclic decalin ring is replaced with a polar hydroxyl group. These two modifications contribute to the more hydrophilic properties of pravastatin, causing it to be rapidly absorbed following oral administration and reach maximum drug serum concentration within 1 h. The level of protein binding of pravastatin is about 50%, which is significantly lower than that of other statins, and CYP 3A is mainly responsible for first-pass oxidative metabolism. Pravastatin has a markedly higher oral bioavailability, about 18–20%, but a similar elimination half-life (~2 h) as simvastatin [13].

Replacement of the bicyclic moiety with aromatic rings has yielded several potent HMG-CoA reductase inhibitors, including atorvastatin, fluvastatin, rosuvastatin, and pitavastatin (Fig. 2). Atorvastatin (5) (brand name Lipitor^®) has the 3,5-dihydroxypentanoic acid side chain (the open form of lactone) responsible for the inhibition of HMG-CoA reductase and a penta-substituted pyrrole moiety instead of the bicyclic decalin ring. These bulky, hydrophobic substitutions on the pyrrole ring make atorvastatin similarly lipophilic to simvastatin and much more lipophilic than pravastatin, but still less lipophilic than naturally occurring statins. Atorvastatin has a moderate oral bioavailability of 14% and a much longer elimination half time of approximately 14–15 h. Of note, atorvastatin was the first synthetic statin to enter clinical studies [14].

Similar to pravastatin, fluvastatin (6) (brand name Lescol) features the open lactone form, with the characteristic erythro-relationship of hydroxyl groups and carboxylic acid moiety. Although fluvastatin harbors an indole ring, it is still an acidic drug as the lone electron pair on nitrogen participates in the aromaticity of the pi system. Fluvastatin exhibits elevated binding to plasma proteins and has a much better oral bioavailability (20–30%), but a short elimination half-life (approximately 1 h). Currently, it is not completely clear which enzymes metabolize fluvastatin [15].

Rosuvastatin (7) (brand name Crestor^®) is structurally very similar to atorvastatin possessing a large, hydrophobic 4-fluorophenyl ring attached to the central pyrimidine moiety. Although the pyrimidine ring is weakly basic, the nitrogen atoms are not protonated at physiological pH. Rosuvastatin is the most hydrophilic synthetic statin produced to date and has a moderate oral bioavailability (around 20%) and much longer plasma half-life (t_1/2 = 18–19 h) than other statins [16]. In addition, rosuvastatin is subject to minimal metabolism via the CYP system, which results in less drug-drug interactions as compared to other commonly used statins [17].

Pitavastatin (9) (brand name Livalo^®), approved by the FDA in 2009, contains the typical 3,5-dihydroxypentanoic acid moiety and a tri-substituted quinoline core. It has the highest oral bioavailability (60%) of all currently available statins with an elimination half-life of approximately 11 h [18].

Cerivastatin (8) (brand name Baycol^®) is one of the most potent synthetic statins, with a reported K_i of 1.3 pM and an elimination half-life of 2.5 h [19]. However, cerivastatin was withdrawn from the market in 2001 due to the increased incidence of rhabdomyolysis.

Additional studies demonstrated that anti-cancer activities of statins, which will be discussed in detail later in this review, may be improved by converting the lactone ring or carboxylic group of lovastatin and atorvastatin (Fig. 2 – Site 2), respectively, into hydroxamate [20]. These modifications led to dual inhibition of histone deacetylase (HDAC) and HMGR, both of which are enzymes involved in cancer development and progression [21].

Statins can also positively impact human health independent of their cholesterol-lowering capacity through so-called pleiotropic effects (see Section 2) [22]. They include, but are not limited to, enhancing the stability of atherosclerotic plaques, decreasing profibrotic effects of TGF-β₁ in airway mesenchymal cells, improving endothelial function, decreasing inflammation and oxidative stress, and inhibition of the thrombogenic response as well as extrahepatic effects on the immune system, central nerve system, and bone [23–25].

Considering the abundant data on various effects of statins in several diseases, the purpose of this review is to comprehensively summarize the available literature and discuss our current understanding of the pleiotropic effects of statins specifically in cancer therapy. Detailed mechanisms of action of statins against the different types of cancers, covering both preclinical and clinical studies, will be presented. In addition, the synergistic anti-cancer effects of statins, when combined with other (anti-cancer) drugs, will be discussed in this paper.

2. Pleiotropic properties of statins

“Pleiotropy” originates from the Greek words “pleion” (more) and “tropos” (direction/way), meaning “more than one way”. In pharmacology, this term refers to both beneficial and adverse effects of a pharmacological agent, excluding those for which the drug was specifically developed [26]. Although ample evidence suggests that their main therapeutic effects are associated with the inhibition of cholesterol biosynthesis, statins also exert (therapeutic) actions through mechanisms independent of their cholesterol-lowering capacity [23,27,28]. These pleiotropic effects are diverse in nature and include the improvement of cardiovascular function [29], broad anti-oxidant and anti-inflammatory effects [30,31], anti-fibrotic effects [24,25], enhancement of bone formation [32], and neuro- [33] and renal-protective effects [34] (Table 1).

Table 1.

A summary of pleiotropic effects of statins in different diseases.

Target organ/disease	Mechanism of action	Effects	Therapeutic outcome	Refs.
Cardiovascular system disorders, inflammatory diseases and the immune system, stroke, dementia	Attenuation of signaling pathways associated with small GTP-binding proteins by depleting isoprenoids required for posttranslational modification. Focus on Rho and ROCK, Rac, and PPAR activation	↑ eNOS activity, ↓ ROS, ↓ endothelin-1 expression, ↓ number of inflammatory cells, ↓ CRP, ↓ platelets adhesion/aggregation, ↓ muscle hypertrophy, ↓ profibrotic function mesenchymal cells, ↓ oxidized LDL, ↓ plasminogen activator inhibitor type-1, ↓ MHC II, ↓ thromboxane A2 biosynthesis, ↓ proinflammatory cytokines, ↓ macrophage growth, ↑ mobilization of endothelial progenitor stem cells	↑ endothelial function, ↓ thrombogenic activity, ↓ inflammatory response, ↓ vascular inflammation, ↓ cardiac hypertrophy, ↑ myocardial blood flow, ↓ thromboembolism	[29,40,229–231] [232] [25]
Brain ischemia, stroke	Attenuation of signaling pathways associated with small GTP-binding proteins by inhibiting isoprenylation	↑ eNOS-derived NO, ↓ nNOS/iNOS-derived NO, NADPH oxidase inhibition and ↓ ROS, ↓ lipoprotein-associated phospholipase A2 and CRP, ↓ pro-inflammatory cytokines, ↓ thromboxane A2, ↓ metalloproteinases	↓ stroke risk, ↓ ischemic damage volume, improved functional outcomes	[233,234]
Alzheimer disease	Prevention of Rac and Rho isoprenylation, upregulation of the heme oxygenase-1	↓ oxidative stress and inflammation, ↑ SOD activity, ↑ eNOS, ↓ cortical soluble Aβ, ↑ glutamatergic transporters, ↓ oxidized cholesterol	Improved cognitive function, ↓ senile plaques, ↑ performance in mild-to-moderate AD subjects, ↓ development of AD	[235]
Cardiovascular disorders, CNS disorders, RA, infections	Inhibition of TLR4-Myd88- NF-κB signaling pathway, and prevention of small GTPase protein isoprenylation	↓ pro-inflammatory cytokines, ↓ oxidative stress, ↓ iNOS activity and nitrosative stress, ↓ NLRP3 inflammasome formation	Anti-inflammatory effects, ↓ the progression of atherosclerosis, cardioprotection, neuroprotection, ↓ joint pain and swelling	[236]
Kidney disorders	Cell cycle regulation, inhibition of isoprenylation	↓ monocyte- and macrophage-derived chemokines, ↓ VCAM and ICAM-1, cell-cycle arrest at G1 phase, mesangial cell proliferation, ↑ eNOS	Improved kidney cancer-specific and overall survival, modestly reduced proteinuria and rate of eGFR decline	[237–239]
Liver diseases	Not mentioned	↑ endothelial function, ↓ ROS, antiproliferative and anti-inflammatory effects, ↑ neo-angiogenesis	Antifibrotic effects, ↓ hepatic vein pressure, ↑ survival rate by ↓ variceal bleeding, ↓ infections in cirrhotic patients, ↓ hepatocellular carcinoma	[240]
Chronic periodontitis	Blocking the synthesis of isoprenoid intermediates	↑ bone formation, ↓ osteonecrosis, anti-inflammatory and immunomodulation, ↓ CRP, IL-1β, IL-6 and TNF-α, ↓ matrix metalloproteinases	Improvement in the periodontal parameters	[241,242]

HMG-CoA reductase is responsible for the conversion of HMG-CoA to MA, which is a crucial intermediate molecule in the production of cholesterol and a wide range of non-steroidal isoprenoid intermediates, including GGPP, FPP, and IPP (Fig. 1) [35]. These intermediate compounds are required for the post-translational modification and activation of many key proteins, such as small monomeric GTPases (e.g., Ras, Rho, Rac or Rap) and the γ-subunit of G-protein coupled receptors, involved in numerous important intracellular signaling pathways [36].

Isoprenylation is a post-translational modification that facilitates and regulates the behavior of proteins (e.g., subcellular localization, trafficking, and establishment of covalent attachments). For example, isoprenylation of Rho and Ras proteins through the action of prenyltransferases facilitates their anchoring in plasma membranes, activation and effector association of these signaling molecules, which regulate a broad variety of cellular functions such as endocytosis/exocytosis, differentiation, cytoskeletal rearrangement and contraction, migration, apoptosis, and proliferation, to name a few [37,38]. The inhibition of signaling mediated by Rho and Rho-associated kinases (ROCKs) and the activation of peroxisome proliferator-activated receptors (PPARs) are two examples of mechanisms underpinning the pleiotropic effects of statins [39–41]. The effects of statins on the regulation and activity of monomeric G-proteins will be discussed in more detail in Section 3.1.

Recent studies revealed that the attenuation of ER stress [42], increase in nitric oxide (NO) synthesis and bioavailability [43], and functional modulation of heat shock proteins (HSPs) [44] may also constitute some of the pleiotropic effects of statins. In keeping with the scope of this review, we will primarily highlight the beneficial pleiotropic effects of statins on cancer.

3. Statins and cancer

Cancer is still one of the leading causes of death worldwide, and is the second leading cause of death (after cardiovascular diseases) in the USA [45,46]. Recently developed targeted therapies that destroy neoplastic cells while sparing healthy ones (i.e. vectors delivering anti-cancer substances only to the target) [47], the application of precise radiotherapy, and the implementation of more accurate surgical equipment etc., have not resulted in the anticipated significant decrease in all-cause cancer mortality [48]. Advancements in cancer detection [49] and novel therapies have improved survival in some types of cancers, but in others hardly any progress has been made since the 1970s [49,50].

The anti-neoplastic properties of statins have received a lot of interest since the 1990s [51]. Pre-clinical evidence suggests that statins inhibit tumor growth and induce apoptosis in specific cancer types [52–59]. Due to the well-established safety profile of statins, such studies are less expensive than the development of novel drugs. Interest in the improvement and enhancement of currently used therapies, which might help overcome drug resistance mechanisms (e.g. efflux via multidrug resistance proteins or lysosomes accumulating drugs [60–63]), is a robust and active area of investigation.

The results of several significant epidemiologic studies indicate statins have potential anti-neoplastic properties. A large-scale, 15-year observational study of a Danish subpopulation was one of the most comprehensive of these investigations. Multivariable-adjusted hazard ratios for statin users were 0.85 (95%CI: 0.82–0.87) and 0.85 (95%CI: 0.83–0.87) for death from cancer (reduced mortality for 13 types of cancer included in the study) and from any other cause, respectively, as compared to non-users [64]. While statin use in cancer patients was associated with reduced cancer-related mortality, no dose-dependent relationship for statin use and cancer mortality was observed.

Analysis of the results from another retrospective study enrolling almost 150,000 patients that were followed for 14.6 years (median), led to a similar conclusion. The risk of cancer death was significantly lower in postmenopausal women (HR = 0.78, 95%CI: 0.71–0.86) who used statins as compared to never-users. Interestingly, all-cause mortality was also lower among statin users (HR = 0.8, 95%CI: 0.74–0.88) [65]. Nonetheless, usage of statins in the past was not found to be a factor in lower cancer-related mortality, and any effect on cancer incidence was not reported [65].

Conversely, a meta-analysis of 22 trials on the results of 134,537 patients (median follow-up 4.8 years) revealed that there is no relationship between LDL reduction with a statin and cancer incidence or mortality [66]. Similarly, a meta-analysis of 27 trials (175,000 patients) failed to prove that 5 years of statin therapy influences cancer prevalence or death [67], and a Finnish retrospective study (900,000 subjects, with an average follow up of 8.8 years) found no relationship between cancer and statin use [68]. Considering long-term statin intake (> 5 years), two meta-analyses (133,255 and 90,056 participants) did not report any significant association between long-term statin use and overall and particular cancer incidence [69,70].

The results of various in vitro and in vivo studies have revealed the efficacy of statins against colon, lung, breast, brain, muscle, and hematological cancers [58,59,71–74]. A recently performed meta-analysis on over 1 million cancer patients has shown that statin use could reduce all-cause mortality and cancer-specific mortality by 30% and 40%, respectively [75]. Paradoxically, another meta-analysis showed that the co-treatment with statins and standard anti-cancer drugs did not significantly improve overall survival rate or progression-free survival in patients suffering from advanced cancer with a prognosis of less than two years [56]. Furthermore, some evidence even suggests that statins could potentially increase the risk of some cancer types such as melanoma and non-melanoma skin and prostate cancers [76]. Therefore, findings remain inconclusive regarding the association between statin use and outcomes in cancer patients undergoing cancer therapy.

The inconsistency in published results regarding statin use and cancer prevalence and mortality may be due to marked differences in follow-up periods, as well as other inherent biases in retrospective and other observational study designs. A 5-year follow-up may not be long enough to detect a relationship between statin use and the development of cancer. It is also possible that statins do not influence cancer incidence per se, but significantly reduce mortality after cancer diagnosis [46]. Currently, the fact is that evidence for the anti-neoplastic effects of statins is not incontestable and the inconsistent findings highlight the importance of the need for well-designed randomized trials to conclusively establish the clinical potential of statins as single or combination chemotherapeutic agents in oncology. Clearly, complex issues such as statin type, delivery route, cancer tissue bioavailability, and proper trial design require careful consideration and planning.

3.1. Possible mechanisms underpinning the anti-cancer effects of statins

Due to increased proliferation, the metabolic rate is significantly higher in cancer cells than in healthy functioning cells. Since statins are well-known for their anti-proliferative effects, this phenotypic difference might favor (hyper) proliferative neoplasms as a target for statins [57–59,77–79].

Statins interfere with the MA pathway, important for the modification of Ras superfamily proteins such as Ras and Rho, through inhibition of HMG-CoA reductase and the subsequent depletion of isoprenoids (GGPP and FPP) [38,48,59,78–80]. Since isoprenylation is crucial for the membrane anchoring of small GTPases, the MA pathway is involved in the signal transduction of a multitude of membrane receptors, including GPCRs and growth factor receptors [38], crucial for the transcription of many genes involved in cell proliferation, angiogenesis, apoptosis, and differentiation [78,81–84]. In addition to their direct anti-cancer effects, statins are also chemosensitizing by impairing Ras family protein signaling [83,85], and have recently been used in combination with anti-cancer drugs to enhance therapeutic effects and potential outcomes in human cancer cells [58,79].

Different statins exert a variety of effects depending on their target cells. For example, lovastatin induces apoptosis much more effectively than commonly used pro-apoptotic agents like etoposide or cycloheximide in a prostate cancer cell line (LNCaP) [86]. Simvastatin significantly inhibits growth and induces apoptosis in poorly differentiated cancer cells [87]. In addition, highly aggressive tumors are more prone to growth inhibition by statins (simvastatin) than differentiated mature cells (HCT116 cell, MCF-7 Bcl2 and Erb2 over expressing cells). A similar relationship between effectiveness and tumor grading was observed for other statins (mouse embryonic stem cells) [88–91]. A general overview of well-known anti-neoplastic mechanisms of statins is presented in Fig. 3 [58]. Further details of the molecular mechanisms driving the effects of statins on cell cycle arrest and apoptosis are covered in Section 3.1.3.

Fig. 3.

The anti-neoplastic effects of statins – mechanisms of action. Under normal conditions, high levels of mevalonate are associated with farnesylation and geranylgeranylation of key signaling proteins. Inhibition of mevalonate production and downstream protein isoprenylation results in decreased Bcl-2 protein expression and activation of caspases, ultimately leading to apoptosis. Inhibition of Ras farnesylation and RhoA geranylgeranylation interferes with cell cycle progression from the G1 to S phase. Furthermore, statins interfere with cells adhesion, invasion, and the formation of proteasomes.

3.1.1. Differences between hydrophobic and hydrophilic statins

Statins vary based on differences in physicochemical properties, including hydro- and lipophilicity. Lipophilic statins can passively diffuse through cell plasma membranes to reach their canonical target HMG-CoA reductase. On the other hand, hydrophilic statins often require transmembrane peptide transporters to cross the cell membrane, and commonly have less extra-hepatic distribution as compared to lipophilic statins. Hydrophilic statins require an OATP1B transporter, which is expressed largely in the liver. Other cells and tissues do express other subtypes of OATPs that may be relevant to statin transportation [92]. This explains, at least in part, why hydrophilic statins, such as pravastatin and rosuvastatin, initially accumulate in the liver [93,94].

In vitro cell culture studies showed a lack of beneficial effects of pravastatin when OATP1B was not expressed [87,95–97]. This may suggest that using more lipophilic statins in clinical trials or conducting studies that clarify the pharmacokinetics of (lipophilic) statins in diseases other than hyperlipidemia or cardiovascular diseases are warranted. However, the risk of cancer death in the aforementioned cohort of postmenopausal women was not related to statin hydro- or lipophilicity [65]. Previous clinical studies did not correlate the anti-cancer effects of statins with the dosage required for cholesterol production blockage. Hence, statins may exert anti-cancer effects in vivo at much lower concentrations.

Depending on their chemical structure, statins are likely to have different intracellular effects [98]. For example, lipophilic statins have higher pro-apoptotic activity than hydrophilic statins, which is likely due to differential transmembrane uptake [97]. Because of these differences, lipophilic statins also have a higher cytotoxic potential and, while undesirable in hypercholesterolemia therapy, this may be advantageous in cancer treatment [87].

3.1.2. Potential anti-cancer effects of statins via mevalonate regulation

HMG-CoA reductase inhibition by statins depletes the intracellular pools of MA, IPP, FPP, and GGPP, and is the rate-limiting step in cholesterol biosynthesis. As indicated, this process interferes with the function of Ras and Rho family GTPases [99–104]. Thus, statins inhibit Ras and Rho GTPase activity by preventing their isoprenylation and subsequent membrane anchoring, effector association, and intracellular signaling [79]. In addition to their importance to ‘normal’ cell physiology, these GTPases play a crucial role in malignant cell transformation, cell adhesion, and proliferation [105–107].

The hypothesis that antagonizing the MA cascade is connected to the anti-tumor properties of statins is supported by the results of studies using bisphosphonate drugs [48,108–110], which also inhibit the production of isoprenoids downstream of HMG-CoA reductase. Isoprenoids in cancer cells affect epithelial-to-mesenchymal transition (EMT), which is a crucial stage in cancer cell metastasis, i.e. spreading from the primary site of the tumor to other organs [111]. Interfering with the MA cascade and blocking isoprenoid biosynthesis affects cell polarity, motility, adhesion, growth, membrane integrity, protein synthesis, and cytoskeletal function (cytoskeletal rearrangement and contraction) [38,48,99,102,112–120]. Indeed, lower dolichol, GGPP and FPP concentrations, followed by reduced Ras and Rho isoprenylation, signal transduction, and DNA synthesis [57,88,121–124] are important functional consequences of statin treatment of cancer cells.

Tumors with mutations in Ras are subject to constant activation associated with uncontrolled cellular proliferation, and this is observed in ~30% of all human cancers [51,124]. Inhibition of the farnesylation of these proteins restricts their activity and might impede cancer cell proliferation [125,126]. Similarly, such effects on proliferation and other critical cellular processes important for cancer onset and progression, such as migration, invasion and apoptosis, might also be achieved by interfering with RhoA-mediated signaling pathways through preventing geranylgeranylation [51,121,127]. For example, a recent study demonstrated that the inhibitory effects of simvastatin on proliferation, migration, and invasion of gastric cancer cell lines MKN45 and MGC803 was due to inhibition of β-catenin and YAP signaling, as a consequence of reduced GGPP and RhoA activity [128].

By disrupting the isoprenylation of key regulatory proteins, statins induce the intrinsic apoptotic pathway. Thus, isoprenoid depletion followed by translocation of small GTPases from the cell membrane to the cytosol leads to apoptosis in a variety of cancer cells [58,78]. Specifically, the effects of simvastatin could be mimicked by GGT1–298, but not FT1–277, indicating that the prevention of geranylgeranylation rather than farnesylation is involved in the pro-apoptotic effects of simvastatin in vitro [78,129].

The importance of HMG-CoA reductase in maintaining cell viability was confirmed by the results from studies on several cancer cell lines [82,130–137], indicating that inhibition of this enzyme induced apoptosis and negatively affected cell growth. Furthermore, statins inhibit reprogramming of cancer cells to cancer stem cells - responsible for cancer metastases and resistance - by reducing transcriptional activity of YAP and TAZ via inhibition of GGPP production [138].

3.1.3. Induction of cell cycle arrest and apoptosis

Statins affect the expression and activity of cyclins, cyclin-dependent kinase (CDK), and CDK inhibitors [139]. This leads to cell cycle arrest at two points, G1/S and G2/M [95,136,140–145]. Thus, cell cycle arrest might be induced by the stabilization of CDK inhibitors – p21 and p27 [121]. Furthermore, cell cycle progression is halted by inhibition of Ras farnesylation [121,146] or a reduction in the level of geranylgeranylated proteins, which are essential for cells to enter the S-phase [147,148]. For instance, the small GTPase RhoA is required for G1 to S transition in mammary epithelial cells, both in response to epithelial growth factor (EGF) and oncogenic Ras, which is linked to Rho-mediated p21(CIP1) repression and cyclin D1 induction [149]. MA pathway inhibition decreases the level of geranylgeranylated RhoA for activation, thereby interrupting RhoA-mediated p21 (CIP1) repression [150]. Other proposed mechanisms leading to an increase of p21 levels include the release of promoter-associated histone deacetylase and inhibition of histone deacetylase [151]. Further, the addition of GGPP to cell cultures prevents cell cycle arrest induced by statins, confirming the critical importance of geranylgeranylated proteins in the progression of cell division [121,152,153].

Statins trigger the intrinsic apoptosis pathway and decrease Bcl-2 protein expression [154–156], increase Bax and BIM protein expression [156–159], and activate several caspases [86,96,97,158,160–162] (Fig. 4). Under the right conditions and with a high enough dose, the imbalance between pro- and anti-apoptotic proteins leads to statin-induced cell death. Activation of the extrinsic apoptosis pathway might also be crucial for the anti-neoplastic effects of statins through upregulation of Fas [97,159].

Fig. 4.

Mechanisms of statin-induced apoptosis. Statins decrease Bcl-2 protein expression and activate different caspases via diverse mechanisms. Activated caspase-8 and -10 further promote the conversion of pro-caspase-3 to activated caspase-3, thereby increasing cleaved PARP-1 levels. In addition, decreased Bcl-2 expression leads to decreased inhibitory effects on Bax/Bak. These Bax/Bak dimers prime the release of mitochondrial cytochrome c, which then binds to pro-caspase-9 to create an apoptosome. The apoptosome cleaves pro-caspase-9 to active caspase-9, which further activates effector caspase-3. Depending on the dose and duration of treatment, the imbalance between pro- and anti-apoptotic proteins leads to statin-induced cell death.

Our recent investigations in different tumor cell lines (breast, brain, and lung) showed that simvastatin-induced apoptosis is dependent on decreasing mitochondrial membrane potential and increasing reactive oxygen species (ROS) production [59,78]. We have also shown that statins induce intrinsic apoptosis in primary human lung mesenchymal cells via an Apaf-1-dependent pathway [163]. In addition, simvastatin triggered Smac and Omi release from mitochondria, which subsequently activated caspase-9 without Apaf-1 release [163]. This latter process relied on the translocation of Drp1 and a change in the balance of mitochondrial fission/fusion [163].

3.1.4. Statins inhibit angiogenesis and metastasis

Ample evidence indicates that HMG-CoA reductase inhibition with statins suppresses vascular endothelial-growth-factor receptor (VEGFR) expression and plays an important role in angiogenesis inhibition. This is mainly attributed to a decrease in endothelial locomotion which is due to localization of RhoA from the membrane to the cytoplasm, rendering RhoA inactive, and subsequent actin stress fiber disorganization [116,164–166]. Statins can also inhibit angiogenesis via inhibition of endothelial proliferation (HUVEC cells) and an increase in TNFα receptor activity (NIH-3T3 and HBL100-neo cells) [167–169]. Conversely, simvastatin induces phosphorylation of endothelial nitric oxide synthase, inhibits apoptosis, and promotes angiogenesis in an Akt-dependent fashion in ischemic limbs of normocholesterolemic rabbits [170]; when ischemia is concerned, promotion of angiogenesis by statins might actually be considered a beneficial side-effect. Clearly, the pro- and anti-angiogenic activities of statins are highly dependent on the target tissue, the type and dose of statin, and the animal and disease model used [171–175].

Statins also interfere with cell mobility [121,171,173,176–179] and, accordingly, inhibit the motility of cancer cells and reduce metastasis [180–183]. An accumulating body of evidence suggests that statins inhibit metastasis via the disruption of geranylgeranylation and farnesylation of small GTPases [121,160,176,183]. Simvastatin slows down castration-resistant prostate cancer metastasis and androgen receptor antagonist-resistant cells by decreasing caveolin-1 (Cav-1) expression in LNCaP prostate cancer xenografts-bearing mice [184]. Similarly, atorvastatin induces mitochondrial damage and increases ROS in oral squamous cell carcinoma cell lines with subsequent cytotoxic induction leading to the inhibition of metastasis in these cells [185].

In two Phase III clinical trials, statin use was associated with better survival in metastatic pancreatic cancer patients who were treated with first-line chemotherapy [186]. In a retrospective cohort study (2006–2014), investigators showed that the use of any type of statin was associated with a decrease in mortality in pancreatic cancer patients [187]. Recent work has shown that atorvastatin inhibits EMT in A549 cells in vitro; a similar mechanism could contribute to the statin-mediated reduction of metastasis in non-small cell lung cancer [188]. Also, simvastatin inhibits triple negative breast cancer cells by inhibiting FOXO3a expression in an in vivo model [189], and has been shown to prevent metastasis in renal endometrial cancer cells in vitro via modulation of the MAPK and AKT/mTOR pathways [190,191].

3.2. Anti-cancer effects of statins in vitro

In vitro studies have convincingly demonstrated the anti-proliferative effects of statins on cancer cell lines. In many neoplasms, the effect is based on both the inhibition of proliferation and induction of apoptosis. Statins typically arrest the cell cycle in the G1 phase, as has been shown for prostate cancer [95], lymphoma [140], renal cell carcinoma (RCC) [180], breast cancer cells [121] and multiple myeloma [142]. Regarding apoptosis, statin-mediated depletion of MA pathway isoprenylation intermediates leads to pro-apoptotic effects in colorectal cancer (CRC) cells [88], lung adenocarcinoma, glioblastoma, and breast cancer cell lines [78].

Statins also reduce the invasiveness and metastases of highly invasive tumor cells [88,89]. For example, cerivastatin reduces proliferation, which is related to an increase in p21 (CIP1), and inhibits tissue invasiveness by causing disorganization of actin fibers and disappearance of focal adhesions. In breast cancer cell lines, both these inhibitory actions are reversed by GGPP, implicating statin-mediated RhoA inhibition [121]. Furthermore, by reducing cell migration, statins diminish colony formation of metastatic prostate cancer cells [192]. Also, a combination of lovastatin and all-trans retinoic acid additively reversed radiation-induced metastases via a decrease in E-selectin expression in HUVEC cells [193], indicating a direct effect of lovastatin on tumor cell adhesion.

Previous studies revealed that co-administration of statins with more conventional anti-cancer drugs in various cell lines, including acute myeloid leukemia, GBM, small-cell lung cancer, breast cancer, and ovarian cancer, results in improved therapeutic effects (Table 2) [58,73,194–197]. For example, simvastatin not only inhibits autophagosome and lysosome fusion in GBM cell lines and primary cells, but also sensitizes these cancer cells to temozolomide-induced apoptosis [58], thereby improving its therapeutic efficacy.

Table 2.

The anti-cancer effect of statins in preclinical studies, categorized by neoplasm and type of statin.

Anti-cancer effect	Neoplasm	Statin	Refs.
Direct effect on cancer cell proliferation	Prostate cancer	Lovastatin	[86]
		Simvastatin	[200]
		Atorvastatin	[205]
	Melanoma	Lovastatin	[161]
		Simvastatin	[177]
		Lovastatin	[177]
		Mevastatin	[177]
	Lymphoma	Lovastatin	[140]
	Multiple myeloma	Simvastatin	[142]
	RCC	Fluvastatin	[180]
	Lung cancer	Simvastatin	[73]
	Breast cancer	Simvastatin	[198]
		Lovastatin	[89]
	Glioma	Lovastatin	[147]
Reduction of invasiveness and metastasis	Breast cancer	Cerivastatin	[121]
		Simvastatin, lovastatin	[243]
	Prostate cancer	Atorvastatin, mevastatin, simvastatin, rosuvastatin	[192]
	Melanoma	Atorvastatin, simvastatin, lovastatin, mevastatin	[177]
	RCC	Fluvastatin	[180]
	Lung adenocarcinoma	Simvastatin	[202]

3.3. Anti-cancer effects of statins in vivo

In vivo studies have been conducted in an attempt to verify the observed beneficial effects of statins on cancer cells in vitro. Studies on mice xenografts confirmed the induction of apoptotic cell death by statins in breast [198,199], renal [180], and prostate cancer models [200]. In breast cancer xenografts, simvastatin prevented tumor growth by reducing Akt phosphorylation and BclXL transcription, while simultaneously increasing the transcription of pro-apoptotic/anti-proliferative PTEN (phosphatase and tensin homolog) [198]. In another investigation, intraperitoneal injection of pitavastatin resulted in growth inhibition of subcutaneous glioblastoma tumor cells, which is linked to statin-mediated targeting of the MA pathway in these cells [201]. Further confirming an association between monomeric GTPases and cancer metastasis, atorvastatin inhibited in vivo metastasis of melanoma cells, which overexpress RhoC [183]. In a lung cancer xenograft mouse model, simvastatin significantly inhibits tumor growth and bone metastasis, which occurred concomitantly with a reduction in MAPK/ERK activity [202].

As indicated previously, combined therapies of statins and existing anti-cancer drugs might hold promise for the improvement of therapeutic strategies and have therefore been subject to investigation in several in vivo models. Mutations in the Kirsten rat sarcoma viral oncogene homolog (KRAS) often result in resistance to cetuximab therapy in CRC [203]. In a murine model, simvastatin and cetuximab combination therapy significantly reduced the growth of xenograft KRAS mutated CRC tumors compared to cetuximab single therapy [203]. Furthermore, atorvastatin significantly enhanced the efficacy of temozolomide to inhibit tumor growth in an in vivo GBM model via inhibition of Ras isoprenylation with subsequent decreased activation of Ras, ERK, rS6, and elF4E [204]. In mice, the administration of a combination of celecoxib and atorvastatin was more effective than each individual treatment, and effectively prevented prostate cancer progression from androgen dependent to androgen independent [205]. Lastly, suppression and delay in the relapse of tumor growth were observed in pancreatic cancer xenografts when mice were subjected to gemcitabine and fluvastatin combination therapy [206].

3.4. Potential anti-cancer effects of statins in clinical trials

Given the plethora of promising in vitro and in vivo data on the effectiveness of statin therapy (alone or in combination with other agents) in several different cancer models, multiple clinical trials have tested statin intervention in human populations. In Table 3, we summarize clinical trial results where statins were used either as a single agent or in combination cancer therapy. Most of these studies are focused on prostate cancer, breast cancer, CRC, or GBM. However, these trials are still considered early stage investigations in patients and will likely require larger-scale randomized clinical trials to better elucidate the true clinical efficacy of statins in cancer. For example, while the concurrent use of statins and radiotherapy is believed to potentiate the efficacy of radiotherapy and decrease some of the side-effects [207], overall clinical trial results regarding statin use in cancer have been inconclusive, and no current recommendation exists for the addition of statins to standard chemotherapy. In the following sections, we review some of the disease-specific clinical trials using statins in combination with cancer treatment.

Table 3.

Clinical trials of statins in human cancer.

Neoplasm	Statin (treatment protocol)	Combination agent (treatment protocol)	Population	Results	Ref/CTG.ID
Monotherapy
Advanced breast cancer	Simvastatin (40 mg/day)		60	NR	NCT04418089
Solid tumors	Lovastatin	–	88	Minor response in patient with anaplastic astrocytoma	[211]
Breast cancer	Atorvastatin (80 mg/day for two weeks prior to surgery)	–	50	decreased proliferation via cyclin D1 expression	[244]/NCT00816244
Breast cancer	Simvastatin (40 mg/day for two years)	–	48	NR	[212]
Breast cancer	Lovastatin (40 mg/day for 6 months)	–	30	No biomarker alteration	[213]
Breast cancer	Fluvastatin (80 mg/day for 3–6 weeks before surgery)		35	Reduced tumor proliferation and increased apoptotic activity	[245]/NCT00416403
Childhood cancer	Atorvastatin (40 mg/day for 6 month)		27	Non-significant improvement in endothelial function	[246]/NCT01733953
Early-stage breast cancer	Atorvastatin (80 mg/day for two weeks before surgery)	–	50	No significant biomarker alteration	[214]
Endometrial cancer	Atorvastatin (NR)		24	NR	NCT02767362
Glioblastoma	Lovastatin (NR)	–		One partial response and one minor response	[247]
Melanoma	Lovastatin		80	NR	NCT00462280
Non-Hodgkin’s lymphoma	Atorvastatin (80 mg/day)		25	NR	NCT00185731
Prostate cancer	Simvastatin (40 mg/day for four weeks before prostatectomy)	–	42	No correlation with tumor biomarker status	NCT00572468
Prostate cancer	Fluvastatin (80 mg for 4–12 weeks)	–	33	Promising effects on tumor cell apoptosis	[248]/NCT01992042
Combination therapy
Acute myeloid leukemia/myelodysplastic syndromes	Pravastatin (NR)	Idarubicin (NR) Cytarabine (NR)	24	NR	NCT01831232
Breast cancer	Simvastatin (NR)	Anastrozole (NR)	11	NR	NCT00354640
CRC	Atorvastatin (NR)	Sulindac/dietary fiber	85	No risk reduction	NCT00335504
CRC	Simvastatin (NR)	Capecitabine plus oxaliplatin/bevacizumab	43	NR	NCT02026583
CRC	Simvastatin (80 mg/day every two weeks)	Cetuximab (500 mg/m2) irinotecan (150–180 mg/m2)	52	NR	NCT01281761
Gastric cancer	Simvastatin (40 mg/day until disease progression or unacceptable toxicity)	Capecitabine (1000 mg/m2 twice daily for 14 days) and cisplatin (80 mg/m2 on day 1 every 3 weeks)	207	No significant changes	[249]/NCT01099085
Glioblastoma	Atorvastatin (80 mg/day until disease progression or unacceptable toxicity)	Temozolomide (75 mg/m2/ day during radiotherapy, followed by 150–200 mg/m2/day on days 1–5 for 6 cycles)	32	NR	NCT02029573
Hepatocellular carcinoma (Child-Pugh A)	Pravastatin (40 mg/day)	Sorafenib (800 mg, twice daily)	474	NR	NCT01903694
Hepatocarcinoma	Pravastatin (40 mg/day)	Sorafenib (400 mg/12 h)	216	NR	NCT01418729
Liver cancer	Pravastatin (40 mg/day until death or sever adverse effect)	Sorafenib tosylate (400 mg twice a day)	323	NR	[250]/NCT01075555
Leukemia	Pravastatin (NR)	Cytarabine (NR) Idarubicin (NR)	36	NR	NCT00107523
Metastatic CRC	Simvastatin (NR)	Irinotecan (NR) Fluorouracil (NR) Leucovorin (NR)	50	NR	NCT00313859
Prostate cancer	Atorvastatin (NR)	Celecoxib (NR)	27	NR	NCT01220973
Small cell lung cancer	Simvastatin (40 mg/day until disease progression, unacceptable toxicity)	Irinotecan (65 mg/m2) Cisplatin (30 mg/m2) on day 1 and day 8, repeated every 3 weeks	62	NR	NCT00452634
Small cell lung cancer	Pravastatin (40 mg/day until maximum 2 years)	Carboplatin Cisplatin Etoposide phosphate	846	NR	NCT00433498
Squamous cell carcinoma non-small cell lung cancer	Rosuvastatin (escalating dose levels starting at 1 mg/kg/day until disease progression or unacceptable toxicity)	Erlotinib (150 mg/day)	24	Disease stabilization	[251]/NCT00966472
Non-small cell lung cancer	Simvastatin (40 mg/day)	Gefitinib (250 mg/day)	110	Higher response rate	[252,253]/NCT00452244
Malignant solid tumors	Rosuvastatin (40 mg/day)	Regorafenib (160 mg/day) Digoxin (0.5 mg/day)	42	NR	NCT02106845

CRC: colorectal cancer; NR: not reported.

3.4.1. Prostate cancer

Several preclinical cell- and animal-based investigations support epidemiological evidence for beneficial therapeutic impact of statins by reported. not NR: cancer; CRC: colorectal showing direct effects on prostate cancer development and progression. In fact, the most encouraging results of potential anti-neoplastic effects of statins stem from prostate cancer research, particularly regarding the advanced stage of this disease. Moreover, it has been shown that statins may increase the survival of prostate cancer patients undergoing radiotherapy [208]. Interestingly, the potential anti-neoplastic effect of statins on prostate cancer does not appear to depend on race; statins were shown to have similar effects in prostate cancer patients from Caucasian and African American populations [209].

A large population-based study on newly diagnosed non-metastatic prostate cancer patients revealed that the use of statins after cancer diagnosis is associated with a 24% decrease in cancer-related mortality [210]. In this study, prior use of statins was also associated with beneficial effects for patients as indicated by decreased morbidity [210].

In a Phase 1 clinical trial, none of the 88 prostate cancer patients elicited a response to therapy of 2–45 mg/kg/day and 20–30 mg/kg/day of lovastatin [211]. Further attempts were made to assess the effectiveness of statins as a monotherapy in prostate cancer with studies focusing on statin use before prostatectomy. One study aimed to assess the ability of lovastatin to downregulate MYC (V-myc myelocytomatosis viral oncogene homolog) in localized prostate cancer, but it was discontinued because of a severe adverse effect (rhabdomyolysis) (NCT01478828).

A study on 42 participants receiving 40 mg of simvastatin for four weeks prior to prostatectomy revealed no correlation between statin use and tumor biomarker status. Additionally, serious adverse effects occurred more frequently in the intervention group (NCT00572468). The results of this study suggest that lower doses of simvastatin over a longer period may reduce the risk of adverse effects. However, the statin effect on tumor biomarkers and potential myotoxicity of simvastatin should be considered with prolonged treatment [59].

3.4.2. Breast cancer

A prospective study designed to identify biomarkers of statin responsiveness in (postmenopausal) women at increased risk of breast cancer concluded that simvastatin (40 mg, daily for two years) induces significant changes in lipid profiles, high-sensitivity CRP, as well as a decrease in estrone sulfate levels [212]. Conversely, another study in women at high risk for breast cancer failed to demonstrate significant breast cancer biomarker modulation after lovastatin therapy (40 mg, 6-month therapy) [213].

In recent years, statins have been the subject of trials as a monotherapy in the treatment of late-stage breast cancer. In postmenopausal women with early-stage breast cancer, 80 mg/day of atorvastatin (two weeks before surgery) reduced average tumor proliferation, and, importantly, caused a corresponding and significant decrease of tumor cancer cells. Moreover, an upregulation of the HMG-CoA reductase following atorvastatin treatment was observed in 68% of the paired samples (pre- and post-treatment), probably as a response to functional inhibition of the enzyme [214]. The findings of this ‘window of opportunity’ trial suggest that statins target HMG-CoA reductase in breast cancer cells in vivo and potentially inhibit proliferation of breast cancer cells positive for HMG-CoA reductase [214].

3.4.3. Glioblastoma

Long-term statin use may improve survival in GBM patients treated with temozolomide chemotherapy [215]. Currently, co-treatment of atorvastatin with radiotherapy and temozolomide is being investigated; results have yet to be published (NCT02029573).

3.4.4. Colorectal cancer

Thus far, studies assessing the effects of long-term statin use on CRC risk have yielded contradictory results [216–218]. A meta-analysis of two extensive prospective studies revealed no association between statin use and overall risk of CRC [219]. Moreover, statin use during or after chemotherapy is not associated with improved disease-free-, recurrence-free-, or overall survival in stage II colon cancer patients (n = 842). In a Phase II trial involving 85 subjects with an increased risk for developing neoplasia, treatment for 6-months with atorvastatin, sulindac, or dietary fiber failed to provide sufficient evidence for a significant therapy-induced reduction in CRC risk (NCT00335504). New strategies to target metastatic CRC are currently being pursued, and involve the use of simvastatin in combination with either XELOX (capecitabine plus oxaliplatin) or bevacizumab (NCT02026583), or with irinotecan, fluorouracil, and leucovorin (NCT00313859); outcomes have not yet been reported.

4. Conclusions and future directions

The anti-neoplastic properties of statins have been under investigation for decades. Preclinical studies revealed that statins modulate cell proliferation, differentiation, and apoptosis by depletion of MAMA pathway intermediates, which interferes with the posttranslational modification and activation of small GTPases and their downstream signaling. Expression of cyclins, CDK and CDK inhibitors are among the proteins affected by statins, resulting in cell cycle arrest at G1/S and G2/M. Statins decrease the expression of anti-apoptotic proteins and increase the expression of pro-apoptotic proteins that activate caspases and trigger intrinsic apoptosis. In addition, statins may inhibit EMT - the necessary step for cancer metastasis - since isoprenoids play a crucial role in cell polarity, motility, adhesion, growth, membrane integrity and cytoskeletal function. For now, however, their actual value as anti-cancer therapeutic agents in humans remains an area of active research. In vitro investigations indicate that a number of beneficial statin effects are due to mechanisms that predominantly interfere with tumor growth and invasion, but the results of these studies are not entirely consistent. Furthermore, findings from both in vitro systems and in vivo animal models have not been satisfactorily confirmed by clinical trial results on the application of statins as either a single or combination adjuvant therapy.

Therefore, additional well-designed clinical trials are needed to validate the abundance of published pre-clinical results. In addition, these trials must be carefully designed to account for the differences among statins, including their varied physiochemical and pharmacokinetic properties, which likely affect study outcomes. Furthermore, additional considerations regarding statin bioavailability and drug peripheral tissue distribution are needed. In other words, if statins do not penetrate into the cancer tissue of interest, they may not be as effective in humans as in cells or animal models, where they are generally tested on much smaller tumors.

The anti-cancer effects of statins in animal and cell models are not necessarily reflected by the doses used in clinical trials. Statins were originally developed to treat cardiovascular disease, not cancer, and, thus, the doses used in cancer clinical trials were never optimized or designed for use in cancer. This is a reoccurring limitation in many if not most human studies performed in the field. Highly anticipated results of ongoing trials may address at least some of the discrepancies between experimental data and clinical observations.

Based on the presently available data, statins appear to “potentiate” the effects of currently used cancer therapies when administered in combination, and this may be a more promising strategy than statin monotherapy. For instance, cell cycle arrest induced by statins may be an effective booster of chemo-/radiotherapy [220–223]. Other beneficial therapeutic effects of combination therapy are still on the horizon. A combined bioinformatics-experimental approach to identify biomarkers of statin-responsive cells [46] could be used to optimize therapy for individual patients (customized drug combinations).

Furthermore, with respect to cancer outcomes, epidemiological studies suggest that long-term treatment with statins is significantly better than short-term or temporary interventions. In addition to optimizing therapies for specific cancer patient groups, which includes selection of the right type of statin, and assessing ideal duration and ‘window of opportunity’ of treatment, determining if single or combined therapy is best will be necessary in the design of future randomized clinical trials.

Similar to statins, there are examples of other drug classes that were initially developed for other purposes and show beneficial anticancer effects during sustained and long-term use; these include aspirin (especially in CRC) [224], metformin [225,226], anti-androgens [227], and several others [228]. Although existing ex vivo, animal model and non-randomized data on the anti-cancer effects of statins are promising, long-term clinical trials and results are necessary to evaluate and conclusively establish their therapeutic efficacy.

Abbreviations:

AD

Alzheimer’s disease

atRA

all-trans-retinoic acid

CDK

cyclin-dependent kinase

CNS

central nervous system

COPD

chronic obstructive pulmonary disease

CRC

colorectal cancer cells

CREB

cAMP response element-binding protein

CRP

C-reactive protein

CTG.ID

ClinicalTrials.gov Identifier

CYP

cytochrome P450

eNOS

endothelial nitric oxide synthase

FPP

farnesyl pyrophosphate

GBM

glioblastoma

GGPP

geranylgeranyl pyrophosphate

HCC

hepatocellular carcinoma

HMG-CoA

hydroxymethylglutaryl-coenzyme A

HMGCR

HMG-CoA reductase

HSPs

heat shock proteins

KRAS

Kirsten rat sarcoma viral oncogene homolog

MA

mevalonate

NF-κB

nuclear factor kappa light chain enhancer of activated B cells

PH

pulmonary hypertension

PPARs

peroxisome proliferator-activated receptors

PTEN

phosphatase and tensin homolog

RCC

renal cell carcinoma

ROCK

Rho and Rho-associated kinases

SAR

structure-activity relationships

TLR

toll-like receptor

VEGF

vascular endothelial-growth-factor receptor