The Influence of Statin Use on Chemotherapeutic Efficacy in Studies of Mouse Models: A Systematic Review

Abstract

Background/Aim:

Using statins as antitumor agents is an approach to cancer therapy that has been explored extensively in specific cancer types. Reframing the query to how a statin interacts with the treatment regimen instead might provide new insight. Given that cell-cycle regulation influences tumorigenesis, it is possible that the cell-cycle phase which a given chemotherapy acts on influences the synergistic effects with adjuvant statin use. In this review, we outline the effect of statins in combination with chemotherapeutic drugs in in vivo animal model studies based on the class of chemotherapy and its relation to the cell cycle.

Materials and Methods:

This systematic review was conducted using the Preferred Reporting Items for Systematic reviews and Meta-Analyses for Protocols 2015 with 23 articles deemed eligible to be included.

Results:

Our review suggests that statins influence the success of chemotherapy treatments. Furthermore, enhanced efficacy was demonstrated with chemotherapeutic drugs that act at every phase of the cell cycle.

Conclusion:

This type of compilation departs from the norm of describing statin influence on named cancer subtypes and instead catalogs how statins interact with categorical chemotherapy agents which might be beneficial for broader therapeutic decision-making across cancer subtypes, possibly contributing to pharmaceutical development, and thereby helping to maximize patient outcomes.

In 1976, Japanese researcher Akira Endo discovered a molecule he called ML-236B, a 3-hydroxy-3-methylglutaryl coenzyme (HMG-CoA) reductase inhibitor later known as compactin. Prior studies had described such molecules, but they were not well controlled. He was the first to demonstrate convincingly that this potent molecule acts as a competitive inhibitor through kinetic studies and molecularly resembles melanovate (1). By inhibiting HMG-CoA, a key enzyme of the mevalonate pathway that regulates cholesterol synthesis, this compound was shown to affect bacterial growth. Thus, the first natural statin was discovered (2).

The same year, Goldstein and Brown carried out a study with compactin and human cells showing that HMG-CoA reductase activity in active fibroblasts could be partially suppressed by compactin alone, but the presence of low-density lipoproteins (LDL) and small amounts of melanovate results in complete suppression (3). The team collaborated with Kovanen and Bilheimer to develop lovastatin and show that beagle dogs with functional LDL receptors demonstrated a statin-induced increase in LDL receptor activity and, moreover, a decrease in serum LDL and hepatic cholesterol levels (4). It was further confirmed that statins increase LDL receptor activity by showing that patients with familial homozygous hypercholesterolemia (therefore no LDL receptors) had no response to statins (5). This journey led to lovastatin being commercialized in 1987 as a treatment for hypercholesterolemia, thus bearing the hope of reducing the risk of cardiovascular events.

The statins reaching the market since lovastatin are similar in structure, with some minor variations. Like lovastatin, pravastatin and simvastatin are fungal-derived but pravastatin has an additional hydroxyl group that makes it hydrophilic. Rosuvastatin, atorvastatin, and pitavastatin are synthetic derivatives that each have a fluoride side group. Unlike hydrophilic statins such as pravastatin, lipophilic statins can enter the cell membranes of cells other than hepatocytes (6).

In recent decades, it has been recognized that statins exhibit pleiotropic effects independent of their lipid-lowering pharmacological effects. One of these is an antitumor effect. The mevalonate pathway not only regulates cholesterol synthesis, but it also controls posttranslational modifications of Rho-family GTPases, which are key components of cell migration signaling networks (7), by adding farnesyl pyrophosphate and/or geranylgeranyl pyrophosphate (GGPP) in a process called farnesylation. This has implications in cancer biology because Rho-GTPase and other small monomeric GTPases are responsible for cell signal transduction and ultimately cell synthesis. By inhibiting the formation of mevalonate, statins indirectly prevent farnesylation downstream and this results in the prevention of tumorigenesis (8, 9). It has further been shown that lipophilic statins have higher pro-apoptotic activity and cytotoxic activity than hydrophilic statins, such as pravastatin (10). For example, simvastatin induces DNA fragmentation, a feature of apoptosis, in cancer cells but not in normal cells at a dose of 20 μM for 24-72 hours. On the molecular level, it acts on the B-cell lymphoma 2 (BCL2) pathway to inhibit BCL2, an anti-apoptotic gene that produces proteins which inhibit BCL2-associated X, apoptosis regulator (BAX), and to overexpress BAX, a pro-apoptotic gene that produces proteins which carry out the caspase cascade to accomplish programmed cell death (11). Furthermore, simvastatin has been shown to downregulate survivin, an inhibitor of apoptosis protein that regulates mitosis and apoptosis (12).

Using statins as antitumor agents is an approach to cancer therapy that has been explored extensively in specific cancer types. A systematic review of numerous in vitro studies demonstrated antitumor effects in 19 cancer cells that were treated with a statin (10). Each of those studies consistently aligned on five key conclusions: i) Statins have proapoptotic effects on tumor cells of various origins; ii) antitumor effects were only observed for lipophilic statins; iii) statin types differ in their antitumor potential, likely due to their different physicochemical properties; iv) statin cytotoxicity depends on the tumor type, for example, Dimitroulakos et al. showed various pediatric cancers and squamous cell carcinomas had highly variant sensitivity to lovastatin depending on the cancer type (13, 14); v) statin-induced cytotoxicity is reduced in or absent from normal cells when studies compared both tumor and normal cells side-by-side. The same review by Osmak et al. (10) explored 10 clinical trials that studied the effect of statins alone as cancer treatment. Results were conflicting, bringing into question the potency of the results given limited patient numbers, advanced stage of disease, and low median survival. Nonetheless, they suggest that statins are not a reasonable option for monotherapy in cancer treatment.

While statins may not be promising as monotherapy, their antitumor effects are promising nonetheless and have led to numerous clinical trials using statins as an adjuvant to cancer treatment. Results and listed outcomes have been inconsistent. For example, combined statin and chemotherapy was shown to reduce chemoresistance in three patients with small-cell lung cancer who experienced relapse with first-line chemotherapy (15) but patient-centered outcomes such as survival were not reported. When survival was data were explored, statin use in patients with ovarian cancer undergoing chemotherapy was not associated with improved overall survival (16). To date, there have been numerous in vitro, in vivo, in ovo, and ex vivo studies about the relationship between statin use and chemotherapy but the focus has been on one specific chemotherapy regimen for one specific cancer type. Therefore, it is difficult to derive judgment to other chemotherapy regimens, let alone cancer types, because the synergistic mechanisms at play cannot be further generalized.

Beyond focusing on statins as adjuvant cancer treatment for a certain type of cancer, further investigation should be done to understand if there is pharmacological synergy between statins and specific chemotherapy mechanisms of action. Mixed results from research examining statin interaction per cancer subtype might arise from the heterogeneity of treatment regimens within that subtype. Reframing the query to how a statin interacts with the treatment regimen (i.e. type of chemotherapy) instead might provide new insight. Chemotherapy drugs can be divided into two categories: cell cycle-specific agents (CCSA), which affect a certain phase, and cell cycle non-specific agents (CCNSA), which can act on all phases (17, 18) (Figure 1). Given that cell-cycle regulation influences tumorigenesis, it is possible that the cell-cycle phase which a given chemotherapy acts on influences the synergistic effects with adjuvant statin use. In this review, we outline the effect of statins in combination with chemotherapeutic drugs in in vivo animal model studies based on the class of chemotherapy and the relation to its action on the cell cycle. The purpose of this review was to determine how statins exert antitumor effects when given in conjunction with chemotherapy, regardless of the type of cancer. Identifying the synergistic statin–chemotherapy combinations that enhance efficacy may assist pharmaceutical development or guide clinical practice for maximizing the antitumor effects of chemotherapy.

Figure 1.

Schematic representation of the target cell-cycle phases of cell-cycle specific agents and cell-cycle non-specific agent chemotherapy drugs reviewed. HDAC: Histone deacetylase.

Materials and Methods

This systematic review was conducted using the Preferred Reporting Items for Systematic reviews and Meta-Analyses for Protocols 2015 (PRISMA-P 2015) (19). An electronic search of the PubMed database was made to systematically review studies related to statin and chemotherapy co-treatment in mouse models. The key words used were “mouse model”, “statin”, “cancer” and “chemotherapy”. This generated 53 articles on PubMed, which were screened for eligibility as depicted in Figure 2. There were no duplicate studies in the search. A total of 23 studies met our inclusion criteria and were therefore included in this review.

Figure 2.

Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram of studies identified for the systematic review.

Studies published as full-text articles, conducted in the past 25 years (1998-2023), on mouse models of cancer and the use of statin and chemotherapy co-treatment as point of interest were included. Non-full-text articles, studies not in vivo, and studies with non-chemotherapy and statin co-treatments were excluded.

Results

Only articles from the past 25 years (January 1, 1998, to May 1, 2023) were included. Of the 53 studies found in our initial search, no studies were published prior to 1998. Therefore all 53 studies were screened for eligibility based on the content of the abstract. Three articles were excluded if the abstract did not mention statins at minimum. Full-text articles were also a prerequisite, so two non-full-text articles were excluded. The remaining 48 full-text articles were assessed for eligibility based on relevance to statins and chemotherapy as co-treatment in animal models. Twenty-five studies were excluded: 23 investigated statin co-treatment with a non-chemotherapeutic drug, and two did not study an in vivo animal model. Twenty-three articles remained and were deemed eligible to be included in the review (Figure 2). These 23 studies were organized by the mechanism and cell-cycle phase affected by the chemotherapy (Figure 1). Two studies used antimetabolites, two used topoisomerase inhibitors, four used small-molecule inhibitors, three used histone deacetylase inhibitors, three used microtubule inhibitors, six used anthracyclines, three used platinum compounds, and two used monoclonal antibodies. Of note, two studies (20, 21) each tested two different types of chemotherapy that are reviewed separately, leading to a total of 25 results.

The results of the 23 studies in our systematic review are summarized in Table I.

Table I.

Overview of included studies.

Author (year), Ref	Study design	Animal model and cancer type	Outcomes measured	Findings/ conclusions
Antimetabolites
Liu et al. (2016) (23)	Gemcitabine at 100 mg/kg weekly (starting 2 weeks after cell implantation) vs. atorvastatin at 10 mg/kg daily (start 2 weeks prior to cell implantation) vs. co-treatment vs. control (water) for 24 days	4- to 8-Week-old male C57BL/6J mice with Panc02 pancreatic cancer	Tumor growth curve, inflammatory cell populations, cytokines in spleen and tumor tissue	Atorvastatin weakened the efficacy of gemcitabine as it significantly enhanced the roles of gemcitabine-induced M2 macrophages in peripheral blood, which could be a potential mechanism for secondary drug resistance.
Hwang et al. (2015) (28)	Pemetrexed at 100 mg/kg weekly vs. simvastatin at 20 mg/kg daily vs. co-treatment vs. control (phosphate-buffered saline) for 26 days	5- to 6-Week-old BALB/c athymic nude mice with A549 NSCLC	Tumor volume, tumor weight, body weight of mice, tumor diameter using micro-CT imaging	Pemetrexed and simvastatin co-treatment significantly reduced tumor growth. The body weights of mice on subsequent days after the first injection of the drug were not significantly different.
Topoisomerase inhibitors
Wei et al. (2021) (31)	Irinotecan at 40 mg/kg twice weekly vs. lovastatin at 20 mg/kg daily vs. co-treatment vs. control for 14 days	6- to 8-Week-old female BALB/c or C57BL/6 mice with CRC	Tumor volume, tumor weight, SHP2 activity in colon cancer cells	Irinotecan with lovastatin significantly inhibited tumor growth. This effect was reduced when Shp2 was knocked down, demonstrating that lovastatin sensitized to chemotherapy via SHP2-induced DNA-damage aggregation.
Jiang et al. (2014) (32)	Irinotecan at 0.5 mg/kg daily (5 days on 2 days off) vs. pitavastatin at 0.5 mg/kg vs. co-treatment vs. control (phosphate-buffered saline) for 32 days	Nude mice with glioblastoma	Tumor growth curve, tumor size, tumor weight, body weight, tumor cell proliferation with H&E and Ki-67 staining	Pitavastatin and irinotecan combination significantly reduced tumor growth, overall tumor size and weight, and tumor cell proliferation.
Small-molecule inhibitors
Feng et al. (2020) (33)	Sorafenib at 10 mg/kg daily vs. simvastatin at 10 mg/kg daily vs. co-treatment vs. control for 28 days	4-Week-old nude mice with HCC	Tumor volume, H&E staining of liver, kidney, lung, and heart after treatment (assessing tissue damage), serum liver function and blood lipid levels	Sorafenib plus simvastatin significantly reduced tumor volume and serum lipid levels, indicating co-treatment enhanced the effect of (sensitivity to) sorafenib on HCC by promoting apoptosis and suppressing glycolysis.
Solanes-Casado et al. (2021)(34)	Volasertib at 20 mg/kg twice weekly vs. simvastatin thrice weekly vs. co-treatment vs. vehicle control for 21 days	5- to 6-Week-old female athymic nude-Foxn1nu mice with CRC	Tumor growth, antitumor activity (T/C ratio)	Co-treatment significantly reduced tumor volume; T/C ratio of tumors treated with the combinatorial therapy was 37%, confirming the re-sensitizing effect exerted by simvastatin in the presence of volasertib.
Kim et al. (2009) (29)	Enzastaurin at 112.5 mg/kg daily vs. lovastatin at 25 mg/kg vs. Co-treatment vs. Untreated control for 14 days	4-Week-old male C3H/ He mice with HCC	Tumor volume, tumor growth curve, apoptotic levels using TUNEL staining	Tumor volumes and growth rate constants were significantly reduced, and percentages of TUNEL-positive cells were significantly increased with co-treatment, suggesting a synergistic antitumor effect by enhancing apoptosis and reducing angiogenesis.
Ding et al. (2014) (36)	Tipifarnib at 0.8 μg/g daily vs. atorvastatin at 2 μg/g daily vs. celecoxib (2 μg/g) vs. atorvastatin + celecoxib vs. atorvastatin + tipifarnib vs. celecoxib + tipifarnib vs. triple combo vs. vehicle control (5 μl/g body weight) for 30 days	6- to 7-Week-old female SCID mice with pancreatic cancer	Tumor growth curve, tumor size, tumor mass	Treatment with atorvastatin, celecoxib and tipifarnib had a stronger inhibitory effect on the growth of Panc-1 tumors in these mice than for any drug alone or combination of two drugs.
HDAC inhibitors
Li et al. (2017) (42)	Tacedinaline at 20 mg/kg every 2 days vs. atorvastatin at 10 mg/kg daily vs. co-treatment vs. control (0.5% carboxy-methyl cellulose) for 21 days	5-Week-old nude mice with CAL27 oral squamous cell carcinoma	Relative weight of tumor	The relative weight of tumor in xenografts receiving combinational treatment with tacedinaline and atorvastatin was significantly lower.
Kou et al. (2018) (45)	Panobinostat at 0.5 mg/kg daily vs. simvastatin at 10 mg/kg daily vs. co-treatment vs. control vehicle for length of survival	BALB/c nude mice with TNBC	Tumor growth curve, tumor volume, tumor weight, immuno-blotting analysis of HSP90 clients and apoptotic proteins, survival time	Co-treatment significantly slowed tumor growth, final volume and weight, and significantly prolonged survival.
Lin et al. (2017) (27)	Panobinostat at 0.5 mg/kg daily vs. mevastatin at 10 mg/kg daily vs. Co-treatment vs. Control (phosphate-buffered saline) for 35 days	5- to 6-Week-old BALB/c nude athymic female mice with TNBC	Tumor volume, tumor size, tumor growth curve, % weight loss (implying toxicity), relative protein expression, TUNEL assay (apoptotic levels)	Co-treatment is more effective in inhibiting tumor growth, lowering final tumor size, and yields the highest % apoptosis on TUNEL staining. Autophagy blockade is a potential mechanism for synergistic enhancement.
Microtubule inhibitors
Pan et al. (2020) (43)	Paclitaxel at 0.5 mg/kg thrice weekly vs. simvastatin at 1 mg/kg daily vs. co-treatment vs. control vehicle (DMSO/saline, 20%/ 80%) for 30 days	6-Week-old SCID mice with ViBo cervical cancer (HPV negative)	Tumor growth curve, body weight (to monitor toxicity)	When both drugs were combined, tumor growth was completely inhibited.
Gupta et al. (2018) (35)	Paclitaxel at 10 mg/kg weekly vs. lovastatin at 2 mg/kg daily vs. co-treatment vs. control (saline) for 28 days	3- to 4-Week-old female athymic nude mice with CD133 pancreatic cancer	Tumor volume, tumor growth curve, cell proliferation (Wa Ki-67 staining)	The combination treatment significantly reduced the total tumor volume, rate of tumor growth, and resulted in fewer Ki-67-positive cells, indicating fewer proliferative cells compared to other groups.
Marti et al. (2021) (20)	Paclitaxel at 10 mg/kg every other day (starting day 24) vs. atorvastatin at 10 mg/kg daily (starting day 10) vs. co-treatment vs. control 2% DMSO daily (starting day 10) for 32 days	8-Week-old NOD-SCID gamma female mice with metastatic TNBC	Gross primary tumor formations, spleen area, TUNEL assay, Ki-67 staining, thymidine uptake, EdU staining	The addition of atorvastatin did not significantly reduce spleen area, DNA fragmentation, of cell-cycle progression; co-treatment did reduce tumor cell proliferation, but not at a statistically significant level.
Anthracyclines
Marti et al. (2021) (20)	Doxorubicin at 2 mg/kg every other day (starting day 24) vs. atorvastatin at 10 mg/kg daily (starting day 10) vs. co-treatment vs. control 2% DMSO daily (starting day 10) for 32 days	8-Week-old NOD-SCID gamma female mice with metastatic TNBC	Gross primary tumor formations, spleen area, TUNEL assay, Ki-67 staining, thymidine uptake, EdU staining	The addition of atorvastatin did not significantly reduce spleen area, DNA fragmentation, cell-cycle progression; co-treatment did reduce tumor cell proliferation, but not at a statistically significant level.
Mangelinck et al. (2021) (21)	Doxorubicin at 4 mg/kg weekly vs. simvastatin at 15 mg/kg daily vs. co-treatment vs. control (phosphate-buffered saline) all started on day 11 for 1 month	6-Week-old BALB/c homozygous athymic nude mice with osteosarcoma	Tumor volume, TUNEL assay, H&E stain showing metastatic foci, relative metastatic surface	Co-treatment had the strongest inhibitory effect on tumor volume and strongest pro-apoptotic effect, showing that simvastatin reinforces the effects of doxorubicin.
Montero et al. (2008) (30)	Doxorubicin at 10 mg/kg (one injection on day 14) vs. atorvastatin at 10 mg/kg daily vs. co-treatment vs. control (phosphate buffered saline) for 3 weeks	5- to 6-Week-old BALB/c athymic (nu/nu) nude mice with HCC	Tumor growth, TUNEL assay, micro vessel formation.	There was a greater reduction in tumor growth and higher number of apoptotic cells with co-treatment, highlighting the potential relevance of increased mitochondrial cholesterol in modulating chemotherapy response. There was no difference in micro vessel formation, suggesting cholesterol plays a minor role in tumor vascularization.
Samuels et al. (2014) (22)	Daunorubicin vs. simvastatin at 20 mg/kg daily vs. co-treatment vs. vehicle control for 49 days	NOD/SCID mice with ALL	% Survival	Daunorubicin combined with simvastatin showed no significant delay in cancer progression. Co-treatment was unable to re-sensitize cells and act to overcome resistance.
Ciocca et al. (2003) (46)	Doxorubicin at 1 mg/kg twice weekly vs. lovastatin at 25 mg/kg thrice weekly vs. co-treatment vs. control for 26 days (injection schedule in 3 groups: days 0-4, 7-16, or 5-23)	Inbred adult IIM e/Fm and Fm-m rats with breast carcinoma, sarcoma, or lymphoma	TUNEL assay/area of apoptosis, tumor volume, HSP25 and HSP70 expression	In breast carcinoma, co-treatment was associated with a significant increase in apoptosis and decrease in tumor volume. Sarcomas were most resistant to cell death and had the highest expression of HSP70 and HSP25, suggesting they are the most drug resistant.
Feleszko et al. (2002) (44)	Doxorubicin at 1 mg/kg (on day 1, 5, 9, 13) vs. lovastatin at 5 mg/kg daily vs. co-treatment vs. control (saline) for 14 days	11- to 15-Week-old female B6D2F1 mice with melanoma, lung metastases	Tumor growth curve (melanoma), number of tumor nodules (lung metastases)	In melanoma, co-treatment significantly reduced the rate of tumor growth by a median of 6 days compared to the control and 4 days compared to monotherapy. In lung metastases, a statistically significant inhibition of metastasis formation was observed with co-treatment.
Platinum compounds
Mangelinck et al. (2021) (21)	Cisplatin at 7 mg/kg weekly vs. simvastatin at 15 mg/kg daily vs. co-treatment vs. control (phosphate-buffered saline) all started on day 11 for 1 month	6-Week-old BALB/c homozygous athymic nude mice with osteosarcoma	Not available (experiment discontinued)	Unfortunately, cisplatin administration induced various toxicities leading to loss of weight and ethical discontinuance of experimental procedures.
Chen et al. (2012) (37)	Low-dose combination: Carboplatin at 10 mg/kg twice weekly vs. atorvastatin at 1 mg/kg daily vs. co-treatment vs. control for length of survival High-dose combination: Carboplatin at 50 mg/kg twice weekly vs. atorvastatin at 10 mg/kg daily vs. co-treatment vs. control for length of survival	6-Week-old female nude mice with A549 NSCLC	Tumor growth curve for 25 days, survival time, difference in body weight	Average tumor size was significantly reduced, and survival rate was significantly prolonged in both low-dose and high-dose combination treatment, with the high-dose having a more remarkable antitumor effect overall. There was no significant difference in body weight, thus neither dose was associated with toxicity.
Feleszko et al. (1998) (26)	Experiment 1: Cisplatin at 10 mg/kg (on day 7) vs. lovastatin at 100 μg daily (on days 7-13) vs. co-treatment vs. control Experiment 2: Cisplatin at 10 mg/kg (on days 7 & 14) vs. lovastatin at 100 pg daily (on days 7-11, 14-18) vs. co-treatment vs. control	Female B6D2F1 mice with MmB16 melanoma	Tumor growth curve, % survival	Beginning from day 15, the rate of tumor growth was significantly slowed with co-treatment for both experimental groups. There was a significant increase in the survival time with co-treatment in experiment 2, but not experiment 1.
Monoclonal antibodies
Xu et al. (2021) (39)	Capmatinib at 1 mg/kg daily vs. pitavastatin at 2 mg/kg daily vs. co-treatment vs. control (phosphate-buffered saline) for 24 days	6- to 8-Week-old female SCID mice with OSCC	Tumor growth curve, tumor weight, % phosphor-MET-stained area (quantified by naked eye)	The combination significantly inhibited tumor growth and the phosphorylation of MET. A proposed mechanism is complete suppression of the MET pathway with pitavastatin impairing MET from being transported to the Golgi body for maturation, and capmatinib directly inhibiting MET.
Pereira et al. (2018) (47)	Trastuzumab at 5 mg/kg weekly vs. lovastatin at 4.15 mg/kg 12 h prior and at same time as trastuzumab injection vs. co-treatment vs. vehicle control for 40 days	8- to 10-Week-old nu/nu female mice with NCIN87 gastric, BT474 breast, or gastric cancer PDXs (HER2+ tumors)	Tumor growth curve	Co-treatment was associated with significant inhibition of tumor growth. Lovastatin may deplete CAV1, increasing HER2 half-life and availability at the cell membrane, resulting in improved trastuzumab binding and therapy against HER2− positive tumors.

ALL: Acute lymphoblastic leukemia; CAV1: caveolin 1; CRC: colorectal cancer; CT: computed tomography; DMSO: dimethyl sulfoxide; HER2: human epidermal growth factor receptor 2; HCC: hepatocellular cancer; H&E: hematoxylin and eosin; HSP: heat-shock protein; MET: MET proto-oncogene, receptor tyrosine kinase; NSCLC: non-small cell lung cancer; OSCC;:oral squamous cell carcinoma; PDX: patient-derived xenograft; Shp2: Src homology region 2-containing protein tyrosine phosphatase 2;T/C: The ratio of mean tumor volume of the treated group to that of the control group at day 21; TNBC: triple-negative breast cancer; TUNEL: terminal deoxynucleotidyl transferase dUTP nick-end labeling.

Discussion

Overall, there was a significant increase in efficacy of chemotherapeutic agents when combined with use of a statin in mouse models, and this was exhibited across all chemotherapy mechanisms reviewed, suggesting a synergistic effect. The few exceptions to this were not specific to a certain type of chemotherapy. Both types of chemotherapy assessed in Marti et al. (paclitaxel and doxorubicin, both CCNSAs) found that the addition of atorvastatin did not significantly reduce DNA fragmentation, which was measured as thymidine reuptake, nor cell-cycle progression, which was measured with Ki-67 staining (20). It is worth noting that this was the only study in the review that exclusively used a metastatic tumor cell line. Samuels et al. (22) also failed to show that daunorubicin and simvastatin co-treatment was able to delay tumor progression. However, the only outcome this study assessed was survival while all other studies assessed tumor size at minimum. The study of Liu et al. (23) was the only one to find that the addition of atorvastatin to gemcitabine treatment weakened its efficacy and significantly enhanced the roles of gemcitabine-induced M2 macrophages, also known as tumor-associated macrophages, in peripheral blood (23). Tumor-associated macrophages are usually associated with poor prognosis and drug resistance in cancer (24). This finding contradicts a retrospective study which showed that a history of statin use has a favorable effect on overall survival in patients on gemcitabine-erlotinib chemotherapy (25). Finally, one study (21) assessing co-treatment of cisplatin with simvastatin had to be discontinued because cisplatin administration induced significant weight loss, indicating toxicity. This study used a cisplatin dosage of 7 mg/kg weekly for 1 month while the other study that used cisplatin (26) had two different experimental groups with milder dosages of 10 mg/kg by single injection on day 7 or on days 7 and 14.

We observed improved efficacy of chemotherapy with statin co-treatment in in vivo studies across all types of chemotherapy included in this review, which range across seven different classes, and cover mechanisms acting on all phases of the cell cycle. This suggests that the mechanism by which statins act to assert a synergistic effect is at a level beyond that of a specific cell-cycle phase. The proposed mechanisms are reviewed as follows.

Statins as anti-autophagy inhibitors

Lin et al. (27) showed that mevastatin inhibits autophagic flux by evading autophagosome–lysosome fusion, and this activity is enhanced by panobinostat. Moreover, they demonstrated that mevastatin stimulated apoptosis of triple-negative breast cancer cells, suggesting that autophagy serves as a pro-survival agent in cancer cells. Interestingly, Hwang et al. (28) had the opposite finding and demonstrated that pemetrexed and simvastatin co-treatment induced autophagy in malignant mesothelioma and non-small-cell lung cancer cells. Therefore, autophagy would need to be inhibited to improve further the benefits of statins with pemetrexed.

Statins as anti-angiogenesis agents

Kim et al. (29) demonstrated that enzastaurin and lovastatin co-treatment enhanced antitumor effects in hepatocellular carcinoma (HCC). They propose that because enzastaurin is a protein kinase C beta-selective inhibitor, it is anti-angiogenic but not significantly responsible for apoptosis on its own. However, when PKC is inhibited, the apoptotic effect of lovastatin is enhanced.

Statins aid in chemosensitization and prevention of chemoresistance

There are numerous studies to support the specific relevance of cholesterol in chemoresistance. Montero et al. (30) observed that susceptibility of HCC cells in xenografts to doxorubicin chemotherapy was observed with atorvastatin co-treatment. In other words, resistance to chemotherapy was reduced in HCC cells when cholesterol levels were lowered. Wei et al. (31) demonstrated that lovastatin and irinotecan co-treatment potentiated efficacy in colon cancer cells. They speculated that lovastatin activates Src homology region 2-containing protein tyrosine phosphatase 2, which induces the stimulator of interferon gene (STING) pathway-mediated antitumor immunity and suppresses poly ADP-ribose polymerase 1-mediated DNA repair, leading to sensitization to the chemotherapeutic agent. Jiang et al. (32) showed that pitavastatin enhanced the efficacy of irinotecan, a topoisomerase inhibitor, because it suppressed the glycosylation of ATP-binding cassette subfamily B member 1, which inhibits its function and lets irinotecan accumulate in the cell, leading to increased apoptosis and preventing drug resistance.

Similarly, Feng et al. (33) showed that simvastatin enhanced sorafenib efficacy by suppressing pyruvate kinase muscle isozyme M2-mediated glycolysis, thus increasing apoptosis, and reducing cell proliferation, effectively re-sensitizing HCC cells to sorafenib. Solanes-Casado et al. (34) showed that combination treatment with simvastatin and volasertib, another small-molecule inhibitor, provokes inactivation of ATP-binding cassette subfamily B member 1, which inhibits its function and allows volasertib to accumulate in the cell, triggering apoptosis by inhibiting polo-like kinase 1, which normally induces chemoresistance. Additionally, Gupta et al. (35) observed that cells with increased CD133 expression also had increased cholesterol content, hence pancreatic cancer cells that had increased CD133 were more responsive to lovastatin compared to those that had lower expression. Furthermore, treatment with lovastatin sensitized cells to paclitaxel and reduced metastatic spread. This suggests that the CD133 population in a tumor drives its potential for chemoresistance.

Statins inhibit the protein kinase B/extracellular signal-regulated kinase (AKT/ERK) pathway

Ding et al. (36) showed that atorvastatin, celecoxib and tipifarnib in combination inhibited tumor growth, induced apoptosis, and reduced phosphorylation of AKT and ERK1/2 in cultured Panc-1 cells. AKT and ERK1/2 are ERK1/2 downstream signaling molecules that promote cell survival and proliferation. Interestingly, the combination of atorvastatin and tipifarnib reduced the level of pERK1/2, while atorvastatin or tipifarnib alone had little or no effect on its level.

Chen et al. (37) concurred there was a synergistic mechanism between atorvastatin and carboplatin that caused AKT inhibition, which means tissue inhibitor of metalloproteinases 1 cannot be up regulated. This resulted in a significant reduction in average tumor size and significantly prolonged the survival rate in both low-dose and high-dose combination treatment. They proposed that this effect also prevents chemoresistance because inhibition of invasion by cisplatin, another platinum compound, was accompanied by up-regulation of tissue inhibitor of metalloproteinases 1 (38). On the other hand, Feleszko et al. (26) postulated that statins arrest cells in the G1 phase of the cell cycle and this makes tumor cells more vulnerable to the action of cisplatin because G1-arrested cells have been observed to have an increased sensitivity to cisplatin.

Xu et al. (39) demonstrated that the combination of capmatinib, a direct MET proto-oncogene, receptor tyrosine kinase (MET) inhibitor, and pitavastatin significantly inhibited tumor growth and the phosphorylation of MET. They proposed pitavastatin inhibits cell growth by down-regulating AKT and ERK signals through the inhibition of MET maturation due to dysfunction of the Golgi apparatus but acknowledged that insufficient prenylation of some proteins (Rho family GTPases, RAS type GTPase family, and other small G proteins) caused by pitavastatin might also affect cell proliferation and ultimately tumor growth.

Statins as inhibitors of protein prenylation

Protein prenylation is the process of protein farnesylation and geranylgeranylation using farnesyl pyrophosphate or GGPP, respectively, as substrates in the mevalonate pathway to make posttranslational modifications in small GTP-binding proteins. It was historically associated with the progression of non-alcoholic fatty liver disease (40) but also has implications in oncogenesis because prenylated GTPases are critical in cell proliferation, signaling, and cellular plasticity (41). This aligns with what was also proposed by Gobel et al. (8) and Jiang et al. (9), namely that by inhibiting the formation of mevalonate, statins indirectly prevent farnesylation downstream and this results in the prevention of tumorigenesis.

Li et al. (42) showed that tacedinaline and atorvastatin co-treatment significantly lowered the weight of oral squamous cell carcinoma tumors. They hypothesized that the proliferation of cancer cells was more dependent on G proteins such as Rac family small GTPase 1 and ras homolog family member A, therefore cancer cells have a higher sensitivity than normal cells to the depletion of GGPP through indirect inhibition of farnesylation by statins. Pan et al. (43) agreed with this proposed mechanism, as they demonstrated that simvastatin acted on cervical cancer via depleting GGPP, leading to prenylation inhibition and GTPase deactivation. This effect was enhanced in combination with paclitaxel. Feleszko et al. (44) also observed that combination therapy using lovastatin and doxorubicin resulted in significant retardation of tumor growth; they similarly proposed this synergistic effect to be through lovastatin enhancing the pro-apoptotic effects of doxorubicin by inhibiting geranylgeranylation.

Statins as inhibitors of heat-shock proteins (HSPs)

Kou et al. (45) showed that panobinostat and simvastatin co-treatment enhanced the efficacy of panobinostat by simvastatin acting as an HSP90 inhibitor that specifically targets K292-acetylated HSP90 to interfere in multiple oncogenic pathways. The effects of HSPs should be generalized cautiously, however, as Ciocca et al. (46) further found that among breast carcinoma, sarcoma and lymphoma, each tumor type has unique features regarding the expression of HSP25 and HSP70. These proteins seem to be implicated in drug resistance in sarcomas, whereas HSP25 might have a role in apoptosis in breast carcinoma thus protecting against resistance. Lastly, lymphomas also had a unique feature whereby the expression of HSP25 was lacking in tumor cells, yet present in endothelial cells under treatment with lovastatin and doxorubicin.

Statins as caveolin-1 (CAV1) modulators

Caveolae are CAV1-enriched subdomains of the plasma membrane, which are deregulated in cancer cells and have a high content of cholesterol and sphingolipids. In Pereira et al.’s study (47), trastuzumab co-treatment was associated with significant inhibition of tumor growth in human epidermal growth factor receptor 2 (HER2)-positive tumors. They proposed that lovastatin may deplete CAV1, increasing the HER2 half-life and availability at the cell membrane, resulting in improved trastuzumab binding and therapy against HER2-positive tumors.

Study limitations

One overarching finding that emerged from this review is that variations in the methods conducted in these studies may also have contributed to the outcomes observed. For example, the variable frequency of dosing across studies can influence outcomes. Statins were usually given daily, with the lowest frequency provided by Pereira et al. (47) wherein lovastatin was only given 12 hours prior and at the same time as the trastuzumab weekly. Chemotherapy administration ranged from daily to only once over the course of the experiment (30). Statin doses (5 mg/kg/day) are comparable to those used in the treatment of hypercholesterolemic patients and are much lower than those used in patients with cancer (25-45 mg/kg/day). This suggests that future studies should attempt to make dosing as clinically relevant as possible and strive to apply the findings of these in vivo studies to higher level clinical trials.

Conclusion

Our review suggests that statins influence the success of chemotherapy treatments. We observed no difference between the two types of chemotherapy: CCSAs and CCNSAs. Furthermore, enhanced efficacy was demonstrated with chemotherapeutic drugs acting at every phase of the cell cycle. This contrasts with an abundance of previous reviews that have tended to list studies by the tumor cell/cancer type rather than considering if the outcomes are influenced by the specific type of chemotherapy being used (10). Our review is the first to our knowledge to organize outcomes of co-treatment with a statin by the class of chemotherapy, and then compile proposed mechanisms for improved efficacy. This type of compilation departs from the norm of describing statin influence on named cancer subtypes and instead catalogs how statins interact with categorical chemotherapy agents which might be beneficial for broader therapeutic decision-making across cancer subtypes, possibly contributing to pharmaceutical development, and thereby helping to maximize patient outcomes.

There are numerous endpoints that researchers can use to determine efficacy of co-treatments. Common endpoints included tumor volume/size, tumor weight, tumor growth rate, apoptosis level through terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and cell proliferation using Ki-67 staining. We grouped the studies by chemotherapy mechanism so that researchers and clinicians can have a reference that is organized by agents that are the treatment of choice for specific cancer types. Nevertheless, clinicians should demonstrate caution in translating results from animal studies into practice. We hope this review can serve as a source summarizing the mechanistic drug–drug interactions for investigators and informing clinicians on the potential adjunct role for statins in optimizing chemotherapy.