Contribution of Statins towards Periodontal Treatment: A Review

The pleiotropic effects of statins have been evaluated to assess their potential benefit in the treatment of various inflammatory and immune-mediated diseases including periodontitis. Herein, the adjunctive use of statins in periodontal therapy in vitro, in vivo, and in clinical trials was reviewed. Statins act through several pathways to modulate inflammation, immune response, bone metabolism, and bacterial clearance. They control periodontal inflammation through inhibition of proinflammatory cytokines and promotion of anti-inflammatory and/or proresolution molecule release, mainly, through the ERK, MAPK, PI3-Akt, and NF-κB pathways. Moreover, they are able to modulate the host response activated by bacterial challenge, to prevent inflammation-mediated bone resorption and to promote bone formation. Furthermore, they reduce bacterial growth, disrupt bacterial membrane stability, and increase bacterial clearance, thus averting the exacerbation of infection. Local statin delivery as adjunct to both nonsurgical and surgical periodontal therapies results in better periodontal treatment outcomes compared to systemic delivery. Moreover, combination of statin therapy with other regenerative agents improves periodontal healing response. Therefore, statins could be proposed as a potential adjuvant to periodontal therapy. However, optimization of the combination of their dose, type, and carrier could be instrumental in achieving the best treatment response.


Introduction
Periodontitis is an inflammatory disease of infectious origin characterized by progressive destruction of periodontal soft and hard tissues leading to tooth loss.The main symptoms comprise gingival inflammation, formation of periodontal pocket, alveolar bone loss, abscess, or tooth mobility [1].The pathogenesis of periodontitis involves a complex interaction of immune and inflammatory cascades initiated by bacteria of the oral biofilm [2].Persistent inflammation and dysbiosis worsen periodontal tissue damage, and the host response plays a vital role in this phenomenon contributing to tissue destruction [3].
The conventional treatment comprising scaling and root planing (SRP) presents limitations in certain cases involving deep periodontal pockets, inaccessible areas, or severe periodontitis [4].Therefore, several adjunctive pharmacological therapeutics have been tested to improve its outcomes.In this context, systemic and local deliveries of drugs such as antibiotics, bisphosphonates, anti-inflammatory drugs, anticytokines, probiotics, and prebiotics have been tested so far to reduce bacterial load and to control inflammation [5][6][7][8][9].Likewise, the use of statins in periodontal treatment has been explored recently [10].Statins, or inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase), are a group of drugs, used primarily to treat hyperlipidemia and to prevent cardiovascular diseases [11].After their discovery in the 70s, they have been widely prescribed worldwide [12].They differ mainly in their ring structure, and these structural differences modify their pharmacological properties including hydrophilicity and lipophilicity.The lactone ring is present in an active form (already hydrolyzed) in all statins except for simvastatin, lovastatin, and mevastatin, in which the lactone ring is activated (hydrolyzed) in the liver.The lactone form of the statins enables their transport, metabolism, and clearance [13] (Table 1).
Apart from their lipid-lowering properties, statins possess pleiotropic effects due to their anti-inflammatory, antioxidative, antibacterial, and immunomodulatory properties [14][15][16][17].Statins have also been reported to have anabolic effects on the bone by augmenting bone morphogenetic protein-2 (BMP-2) expression, thus contributing towards the differentiation and activity of osteoblasts (OBs) [18].In view of their beneficial properties, statins have been presented as new potential candidates for improving periodontal therapy outcomes [19,20].
In several preclinical and clinical studies, statins have exhibited contradictory results [21][22][23] depending on the mode of delivery (local vs systemic), anatomy and severity of the lesions, type of disease, and treatment approach (nonsurgical vs surgical).Therefore, the aim of this literature review was to establish a better understanding of the prophylactic and therapeutic effects of all statin types administered locally or systemically as adjuvant to nonsurgical/surgical periodontal treatment in existing preclinical models and clinical settings and to explore the biological mechanisms underlying these healing and proregenerative effects in the management of periodontitis.

Methods
2.1.Literature Search.Studies published in English language only were included, and the last search was carried out in September 2018.Regarding studies performed on animal models and clinical trials, a systematic literature search was performed in the PubMed/MEDLINE and ScienceDirect databases.A hand search has also been performed after checking references of the identified articles.Concerning in vivo studies, the following keywords were used for the search: periodontitis OR periodontal disease OR alveolar bone loss OR periodontal attachment loss OR periodontal pocket AND simvastatin OR statin OR rosuvastatin OR atorvastatin OR cerivastatin OR mevastatin OR lovastatin OR pravastatin OR Fluvastatin OR pitavastatin OR Hydroxymethylglutaryl-CoA Reductase Inhibitors AND mouse OR dog OR pig OR rat OR rodent OR rabbit OR monkey OR in vivo.A study was considered eligible if it met the following criteria: (1) experimentally induced periodontitis (EIP) and/or acute/chronic periodontal defects (ACP), (2) treatment of EIP and/or ACP with statins (local or systemic or combination) with or without SRP or other periodontal treatment modalities, and (3) at least one periodontal parameter assessed as outcome.Exclusion criteria for in vivo studies were the following: (1) periapical lesions, (2) tooth extraction models, (3) orthodontic movements, (4) calvarial models, (5) long bone defects, and (6) drug-induced gingival enlargement.
Concerning clinical studies, the following keywords were used for the search: periodontitis OR periodontal disease OR alveolar bone loss OR periodontal attachment loss OR periodontal pocket AND simvastatin OR statin OR rosuvastatin OR atorvastatin OR cerivastatin OR mevastatin OR lovastatin OR pravastatin OR Fluvastatin OR pitavastatin OR Hydroxymethylglutaryl-CoA Reductase Inhibitors.A study was considered eligible if it met the following criteria: (1) randomized and controlled clinical trials, (2) cohort clinical studies, (3) longitudinal studies, (4) patients with diagnosis of chronic or aggressive periodontitis, (5) systemic or local administration of statins with nonsurgical or surgical periodontal treatment, and (6) at least one periodontal parameter: pocket depth (PD), clinical attachment level (CAL), bone loss (BL), or tooth loss (TL) assessed as outcome.Exclusion criteria for clinical studies were the following: (1) no follow-up, (2) no periodontal treatment, and (3) reviews, letters, and case reports.

Study
Selection.Titles and abstracts of the studies were screened independently by two reviewers (CP and FB) and categorized as suitable or not for inclusion.Full reports were reviewed independently for studies appearing to meet the inclusion criteria or for which there was insufficient information in the title and abstract to allow a clear decision.Disagreements between the authors were resolved after discussion with a third reviewer (OH).
2.3.Risk of Bias Assessment.Risk of bias was assessed using the Cochrane Collaboration's tool for assessing risk of bias which provided guidelines for the following parameters: sequence generation, allocation concealment method, blinding of the examiner, address of incomplete outcome data, and free of selective outcome reporting.The degree of bias was categorized as follows: low risk if all the criteria were met, moderate risk when only one criterion was missing, and high risk if two or more criteria were missing.Two reviewers (FB and CP) independently performed the quality assessment, and any disagreement was resolved by a third investigator (OH) (Supplemental Table 1).

Effect of Statins on the Inflammatory-Immune
Crosstalk.Localization of periodontium at the interface between the teeth and jaws exposes periodontal tissues to continuous bacterial challenge which could contribute to exacerbation of the immune response during periodontal  Mediators of Inflammation wound healing.Recruitment of inflammatory cells at the periodontal site, including polymorphonuclear (PMN) leukocytes, macrophages, and lymphocytes, is associated to the release of a complex nexus of cytokines.When the inflammatory front migrates toward the alveolar bone, it stimulates osteoclastogenesis and subsequent alveolar bone destruction [24].Therefore, the importance of inflammation control at the soft tissue level cannot be undermined.The effects of statins on the inflammatory-immune crosstalk involved in the periodontal wound healing have been evaluated.Statins decrease the levels of proinflammatory cytokines (interleukin-1 beta (IL-1β), interleukin-8 (IL-8), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α)) and increase the release of anti-inflammatory mediators (IL-10) and chemokines [25,26].There are several pathways implicated in the action of statins, notably suppression of HMG-CoA reductase, thereby inhibiting Rac and p21Ras phosphorylation.As Rac and p21Ras are coupled to the transcription of proinflammatory molecules via MAP kinase (MAPK) pathways, therefore, statins also suppress nuclear factor kappa B (NF-κB) activation, thus reducing the expression of proinflammatory molecules [27] (Figure 1).

Effect of Statins on Proresolution
Molecules.Periodontal wound healing and regeneration involve a constant "tug-of-war" between the proinflammatory and anti-inflammatory/proresolution mediators [36,37].Anti-inflammatory effects of statins enhancing resolution of periodontal inflammation, that is, initiated by several endogenous chemical and lipid mediators, such as the lipoxins (LXs), resolvins (RVs), protectins, and maresins, could possibly explain the positive treatment outcomes [38,39].However, further studies need to explore the exact effect of statins on the proresolution mediators.

Effect of Statins on Host Modulation.
Literature reports contradictory results regarding the effect of statins on different types of immune cells.For instance, in an ACP model, simvastatin did not change circulating white blood cell (WBC) counts in a study [33], whereas leukocyte infiltration was decreased by atorvastatin gavage in an EIP model [40].Similarly, regulatory T (Treg) cells that control adaptive immunity against pathogens and activate other effector 3 Mediators of Inflammation immune cells were reported to be regulated by statins.In this regard, atorvastatin and simvastatin demonstrated an increase in the number of human Treg cells and differentiation of CD4 into Treg in vitro [41,42].
Furthermore, TLRs have an important role in the immune-inflammatory crosstalk with a consequent impact on periodontal wound healing response.In the context of periodontal treatment, targeting TLRs has been proposed as it could enhance antimicrobial properties, suppress adverse inflammation, or activate tissue repair [43].Interestingly, simvastatin inhibited the stimulation of several TLRs (1, 2, 3, 4, 6, 7, and 9) by Aggregatibacter actinomycetemcomitans (A.a) LPS in vivo, reducing its capability to escape innate immune response [33].Hence, statins play an instrumental role in the modulation of inflammatory and immune responses.

Inhibition of Major Histocompatibility Complex Class II (MHC-II) by Statins.
In case of nonresolving periodontal lesions, bacterial antigens are processed and presented by antigen-presenting cells and macrophages.Such process is associated to massive immune cell recruitment implicated in tissular destruction [2].In this regard, statins are able to inhibit MHC-II expression due to inhibition of the inducible promoter IV of the class II transactivator (CIITA) as observed in several cell types, including monocytes and macrophages [44].This effect renders statins to have a potential host-modulating impact on periodontal treatment.

Lymphocyte Function-Associated Antigen-1 LFA1 Site
Binding by Statins.Lymphocyte function-associated antigen-1 (LFA-1), an integrin with its main ligand intercellular adhesion molecule-1 (ICAM-1), is activated on the surface of fibroblasts (FBs) by IFN-γ and represents a critical phase in the early stage of inflammation.ICAM-1 regulates LFA-1-dependent neutrophil transmigration and recruitment to the inflammation site [45].Several studies have demonstrated the inhibition of LFA-1 by statins in many inflammatory and immune diseases other than periodontitis.Statins inhibit ICAM-1 upregulation and chemotaxis of monocytes [46].Lovastatin, simvastatin, and mevastatin, but not pravastatin, were able to inhibit the LFA-1/ICAM-1 interaction in vitro by binding to the L-site of LFA-1 [47].In this way, statins limit the exacerbation of immune-mediated inflammatory response at the lesion site.However, the impact of statins on LFA-1 binding in the context of periodontal wound healing remains unexplored.
3.1.6.Effect of Statins on Nitric Oxide Synthase (NOS).NOS plays an important role in host defence and homeostasis and has been implicated in the pathogenesis of periodontitis, where it is expressed in FBs, epithelial cells, rests of Malassez, macrophages, osteoclasts (OC), and vascular endothelial cells [48,49].In chronic periodontitis, bacterial challenge induces proinflammatory cytokine release and a higher expression of inducible NOS (iNOS) and NOS derived from FBs and WBCs that migrate to the periodontal lesion [50][51][52] leading to inflammation-mediated bone resorption [53].Various studies demonstrated a NOS-inhibiting effect by the use of statins.For instance, in vivo, rosuvastatin significantly reduced inflammation-mediated tissue destruction and gingival iNOS expression [54].
Concerning the underlying mechanism of action, statins attenuate the production of reactive oxygen species (ROS) induced by NADPH oxidase by suppressing Rac's geranylation.Phosphatidylinositol-3 active kinase (PI3-Akt) is a kinase that phosphorylates and stimulates eNOS.Mevalonate is able to inhibit PI3-Akt; therefore, by reducing the concentration of mevalonate, statins upregulate eNOS-derived NO production resulting in vasorelaxation that leads to improved angiogenesis and wound healing response [27].

Effect of Statins on Matrix Metalloproteinases (MMPs).
MMPs degrade extracellular matrix proteins, especially collagen, contributing to the degradation of periodontal tissue including alveolar bone [55].Most statins have been reported to potently inhibit the expression of MMP-1, MMP-8, and MMP-9 upregulated by LPS as demonstrated for simvastatin in mononuclear cells in vitro [56].Moreover, in vivo, a decrease of MMP-1, MMP-2, MMP-8, and MMP-9 was observed by the use of statins [31,[57][58][59].Thus, statins prevent periodontal tissue and alveolar bone destruction by inhibiting the release of MMPs.

Role of Statins in the Promotion of Osteogenesis.
Inhibition of HMG-CoA by statins decreases prenylation of farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GPP) leading to increased levels of BMP-2 and VEGF through the PI3-Akt pathway.Interestingly, both VEGF and BMP-2 regulate OB differentiation and bone formation during bone repair and regeneration [65,66].Concerning BMP, simvastatin and lovastatin increased the levels of BMP-2, consequently, increasing OB activity in vitro [58,63].Statins present a cost-effective option when compared with growth factors such as BMP-2 [67,68].
Cholesterol is an integral component needed by bacteria for maintaining their membrane integrity.Statins can counter bacteria by inhibiting the intermediate in the isoprenoid biosynthesis pathway necessary for membrane stability, which is substituted by cholesterol and protects bacteria from the toxic effect of statins.Statins, therefore, kill bacteria directly and by lowering accessible host cholesterol content for bacterial growth and protection.Such effects may be due to the disruption of teichoic acid structures reducing biofilm formation ( [81]).Statins display antibacterial activity towards anaerobic bacteria, including periodontal pathogens such as A.a and P.g.For instance, low concentration of simvastatin was proven to be effective against A.a and P.g even if A.a was more sensitive (MIC < 1 μg/mL) than P.g (MIC until 2 μg/mL dilution) [82].The hydrophobic nature of simvastatin may explain its antibacterial activity against periodontal pathogens where it disrupts the bacterial membrane in a "soap-like" manner causing its death [83].Nevertheless, not all statins exhibit antibacterial activity.The degree of HMG-CoA reductase inhibition corresponds directly to the cholesterol-lowering capabilities of statins [84] but it does not seem commensurate with their antibacterial potency [85].
Some other mechanisms are modulated by the action of statins on lipoxin A4 (LXA4) production, a proresolving lipid mediator that enhances bacterial clearance, consequently reducing the severity of periodontal disease [86,87].Furthermore, the mechanistic target of rapamycin (mTOR) signaling, regulated principally by TLRs via two major pathways (NF-κB-dependent pathway and a PI3-Akt-dependent pathway), is also involved in bacterial clearance [88].It is known that statins inhibit isoprenoid synthesis, impeding intracellular signaling molecules like Rho or Rac [89].
Therefore, it is plausible that statins possess certain antibacterial properties that could facilitate periodontal treatment.However, since periodontitis is a polymicrobial disease, the susceptibility of various other periodontal pathogens to statins must also be evaluated.

Effects of Statins in Induced Periodontitis Models.
Statins have been tested in several induced periodontitis models to evaluate improvement in periodontal parameters and their underlying biological mechanisms.In vivo, 35 studies were identified based on the inclusion criteria (Figure 4), out of which 16 involved local statin delivery (Table 3), 17 used systemic route (Table 4), and 2 employed a combination of both modes (Table 5).In the studies evaluating local statin application, 8 studies involved the treatment of EIPs while the remaining 8 investigated the treatment of ACP models, one of which was induced by LPS injection of Escherichia coli (E.coli) [90].Concerning the systemic administration of statins (Table 4), 14 out of the total 17 studies treated EIPs, whereas the 3 remaining studies involved ACP models by LPS injections of A.a [32,33] and P.g into the gingiva [76].
Regarding the mode of periodontitis induction, in total, 24 out of 35 studies had EIP with ligatures (cotton, nylon, or silk), whereas 11 used ACP including the 4 studies where periodontitis was induced by bacterial LPS.Studies were mostly performed in rodents (Tables 3, 4, and 5).In ACP models, the surgically created lesions were mainly intrabony defects, fenestration defects, dehiscence defects, furcation class II defects, and 3-walled intrabony defects.
3.5.Clinical Outcomes.The selected studies evaluating the effect of statins in the context of periodontal treatment included 23 controlled and randomized clinical trials, 8 cohort studies, and 1 longitudinal study (Figure 4).Primary outcomes varied between improvement of clinical attachment level (CAL), reduction of pocket depth (PD), tooth loss, radiographic bone defect depth, periodontal inflamed surface area (PISA), and serum and/or GCF proinflammatory cytokines level.Most of the studies focused on the local administration (n = 25) of statins (Table 6), while 7 investigated the impact of systemic route (Table 7).Essentially, effects of statins have been evaluated as an adjunct to both nonsurgical and surgical treatments, mainly in the context of chronic periodontitis in healthy patients.

Statins as a Local Adjunct to Nonsurgical Periodontal
Treatment.The effect of local delivery of statins as an adjunct to nonsurgical periodontal therapy (SRP) was studied in 20 clinical trials (Table 6).Atorvastatin and simvastatin have been the most commonly studied statins.Amongst the identified studies, 13 demonstrated a significant PD reduction, CAL gain, and IBD fill in healthy patients, 2 in well-controlled type II diabetes patients, and 3 in smokers.At contrary, in 2 studies, the test groups using atorvastatin or simvastatin did not show any significant differences when compared with the control [21,100].For instance, with simvastatin, the mean PD gain was 1 23 ± 0 57 mm for the control group versus 1 83 ± 0 07 mm for the test group (p = 0,112) and the mean CAL gain was 2 09 ± 0 08 mm for the control group versus 2 43 ± 0 01 mm for the test group (p = 0 889) after 45 days.Nevertheless, authors found a statistically significant reduction of PI, BOP, IL-6, and IL-8 levels [21].Only 4 studies compared the outcomes obtained with more than one statin; however, contradictory results were observed.For instance, one study did not show any significant difference between atorvastatin and simvastatin [100], whereas better results were obtained with atorvastatin in another study [101].Nevertheless, two studies highlighted greater efficacy with rosuvastatin in comparison with atorvastatin [20,102].
Interestingly, studies that have investigated the effects of statin treatment on the biological markers from GCF showed that simvastatin administration reduced significantly IL-6, IL-8 and increased the anti-inflammatory IL-10 [21, 100, 103].

Statins as a Local Adjunct to Surgical Periodontal
Treatment.Statins have also been inspected for their role in the surgical treatment outcomes.In all identified studies where statins (simvastatin, atorvastatin, and rosuvastatin) were locally administered concomitant to surgical approach (including the use of biomaterials or PRF), a significant reduction of PD, improvement of CAL, and bone defect fill was achieved in the test group in comparison to the control            6).Amongst these studies, the mean difference of PD between the test and control groups ranged from 1 3 ± 0 21 mm to 2 51 ± 0 22 mm (p < 0 001).Thus, the mean difference of CAL between the test and control groups ranged from 1 16 ± 0 09 mm to 2 35 ± 0 08 (p < 0 001).Moreover, the mean difference of bone defect fill between the test and control groups ranged from 1 336 ± 0 714 to 3 08 ± 0 07 (p < 0 001).

Impact of Systemic Administration of Statins on
Nonsurgical Periodontal Treatment Outcomes.The impact of systemic administration of statins on nonsurgical periodontal treatment outcomes was evaluated in a few studies (Table 7).From the 7 studies identified, 4 demonstrated significant improvements regarding reduction of PD, CAL gain, and/or tooth loss in comparison to the control group [56,[109][110][111].At contrary, 3 other studies did not show any significant differences in periodontal outcomes between the statin-treated and control groups [112][113][114].These discrepancies could be due to the very short follow-up of the abovementioned 3 studies (3 months) compared to the other ones (from 3 months to 7 years follow-up).Moreover, one of the studies did not compare the treatment group with a control group [110].

Discussion
Statins exhibit multiple effects, including modulation of inflammatory-immune crosstalk, bone regeneration, and antibacterial activity, to promote periodontal wound healing and regeneration (Figure 5).They act through several closely interrelated pathways highlighting potential therapeutic targets.The hydrophobic or hydrophilic nature of statins determines their efficacy, action on periodontal pathogens, and treatment response and appears to be largely cell and tissue dependent [69,78].Further insight into this may help selecting the best statin.Moreover, the mode of statin delivery also affects the treatment outcomes.Oral systemic administration of statins reduces periodontal inflammation and consequent tooth loss [111] but the low resultant dose available to the tissues after hepatic bypass renders them relatively less efficacious [60].On the other hand, a higher dose to enhance efficacy can manifest systemic side effects such as statin-induced myopathy, hepatotoxicity, nephrotoxicity, pulmonary manifestations, ophthalmological manifestations, gastrointestinal hemorrhage risk, and oral manifestations (dryness, itch, bitterness, and cough) [115,116].Therefore, to avoid these side effects, various local application strategies have been tested that allow site-specific delivery reducing the required dose, frequency of application, and bioavailability in the blood [60,117,118], concomitantly improving patient compliance [119].
The development and selection of an optimal statin delivery carrier are crucial as it enhances the statin retention on the lesion and acts as a scaffold for cell growth and differentiation [120]; therefore, it should be capable to withstand the oral environment, continuous fluid exchange inside the pocket, and salivary influx.
Several studies demonstrate that anti-inflammatory properties of statins vary according to the type and dose of statin used [121].On a cellular level, modulation of macrophage polarization from a proinflammatory M1 to a proresolution M2 phenotype by systemic delivery of immune modulatory drugs resolved persistent inflammation associated with chronic periodontitis [122].In this context, statins' ability to switch M1 to M2 to promote periodontal wound healing and regeneration needs to be explored.Furthermore, The animals included in the studies are healthy unless stated otherwise.Treatment was considered (1) "preventative" when it started at least one day before the start of EIP/ACP induction, (ii) "protective" when it started the same day as that of EIP/ACP induction, and (iii) "therapeutic" when it started at least one day after the start of EIP/ACP induction.17        it is yet to be established if statin-induced reduction in plasma total cholesterol and LDL cholesterol levels in the periodontal space could decrease macrophage recruitment to improve the treatment outcome.

Mediators of Inflammation
Despite the documented anti-inflammatory properties of statins, a local high-dose statin application causes considerable soft tissue inflammation [123].Accordingly, studies determined that reducing the simvastatin dose from 2.2 mg to 0.5 mg reduced inflammation without compromising its bone growth potential [67].A 10 mg/kg/day dose in rats is equivalent to 70 mg/day for humans, so it is a high systemic dose compared to that commonly used in clinical practice (20-40 mg/day) [124].
Most of the review articles have focused on the use of statins as adjunct to the nonsurgical SRP in clinical settings [127][128][129].Here, this review encompasses the use of statins (local, systemic, or combination), alone or in addition to other drugs or scaffolds, in nonsurgical or surgical periodontal treatment in vitro, in vivo, and in clinical trials.However, the potential of statins in surgical periodontal therapy remains relatively less explored except for a few studies where treatment outcomes were improved, primarily, with the combination of some other regenerative agents such as allograft or PRF [105,106].Cognizant of the numerous studies involving statins, not all statin types have been studied so far; thus, exploring all natural and synthetic statins to compare their efficacy and safety could be instrumental.
Notably, 17 out of 32 clinical studies were carried out by the same group of researchers on similar population; therefore, generalizations should be drawn with caution.Additionally, in most studies involving statins, the follow-up period was no longer than 9 months [103,130].Hence, it is imperative to follow clinical studies for periods longer than those commonly investigated so as to achieve a deeper and more genuine insight into their long-term benefits.Discrepancies amongst outcomes between time points are of importance to clearly conclude.For instance, the meta-analysis performed by Sinjab et al. [131] declared the outcomes of the control group of a study [20] to be better by considering the data up to 6 months follow-up, whereas the meta-analysis performed by Ambrósio et al. regarded the treatment group of the same study to have better outcomes as the follow-up data until 9 months was taken into account [132].
Moreover, the studies carried out so far mainly involved hyperlipidemic patients, diabetic patients, or smokers.Systemic diseases, such as obesity or metabolic syndrome, have been linked with periodontitis [133].It has been demonstrated that such conditions modify significantly the host response to periodontal pathogens [134] but also could impaired treatment response.For instance, in a rat model of metabolic syndrome, the effects induced by statins in rats with metabolic syndrome were different in comparison with rats without [32] highlighting the potential modulation of pharmacologic effect due to the systemic condition.Even if clinical trials performed in diabetes patients or exhibiting hyperlipidemia showed promising results when statins were administered concomitantly to nonsurgical periodontal treatment [56,110,113,114], more studies are required to better understand the differential biological mechanisms modulated by statin's administration.It would also be of importance to assess statins' tolerance and efficacy in subjects with different systemic conditions where periodontal treatment response is impaired (e.g., liver diseases, kidney dysfunction, and immunocompromised states).
In clinical trials, the local application of statins with surgical periodontal treatment always showed significant improvements in periodontal parameters [105,106].However, in vivo, statin application in ACP models showed contradictory results [99] which could be explained by the limitations of animal models to simulate conditions identical to human periodontal disease.Nevertheless, as a direct optimization of treatment protocols in humans is not ethically permissible, the utility of preclinical models to get directions and overall assessment of the expected treatment outcomes in clinical scenarios cannot be undermined.
Concerning the systemic administration of statins, a study reported that using a combination of two pharmacokinetically different statins (20 mg/day of atorvastatin plus 40 mg/day of pravastatin) in hyperlipidemic patients for one year improved their lipid profiles compared to those on monotherapies [135].Besides, a case of a hyperlipidemic patient experiencing certain side effects with a high dose of systemic simvastatin who could well tolerate a combination of reduced doses of simvastatin and rosuvastatin instead has also been reported [136].To the best of our knowledge, no two statins have been combined for periodontal treatment so far; nonetheless, combination of two statins could be tested for its impact on periodontal treatment response.
Likewise, the impact of incorporating statins with antimicrobial agents, growth factors, or other proregenerative molecules within a local application system could be studied as adjunct to SRP.Statin integration into gels [21] or dentifrice [137] could enhance ease of application and patient's compliance and could be potentially beneficial in the maintenance phase to counter periodontal breakdown that persists after conventional periodontal treatment.The literature does not report the impact of statins on patients with extremely poor oral hygiene; nonetheless, it could be interesting to explore the impact of statins on oral hygiene indicators.

Conclusion
Statins have been studied in depth in the context of bone regeneration, but soft tissue healing remains relatively less explored.Further research into it could present statins as a potential adjunctive therapeutic strategy with a positive impact on both hard and soft periodontal tissue healing.Furthermore, the impact of statins on proresolution molecules has not been investigated in the context of periodontal wound healing and regeneration.This could unveil new vistas for statins as regenerative therapeutics.Since all available statins have not been tested yet, new studies need to evaluate the impact of other statins on antibacterial, inflammatory, immune, and osteoprogenitor responses.To conclude, choosing an optimum dose of statins, based on the mode of drug delivery and the carrier employed, may enhance the positive impact of statins on the periodontal treatment outcomes.Moreover, combining statins with growth factors or other drugs in an efficient carrier system may be beneficial to promote periodontal regeneration.Experimentally induced periodontitis ACP:

↗Figure 1 :
Figure 1: Effect of statins on the inflammatory-immune crosstalk.Direct LFA1 site binding by lipophilic statins decreases ICAM-1 presentation leading to reduced leukocyte chemotaxis and antigen presentation.Statins inhibit MHC-II induction by IFN-γ leading to decreased T-cell activation.Statins lower mevalonate release, leading to resolution of inflammation via the ERK, MAPK, and PI3K-Akt pathways.

Figure 3 :
Figure 3: Antibacterial effect of statins.Statins arrest bacterial growth and disrupt their membrane stability by decreasing cholesterol.Statins increase bacterial clearance by decreasing NF-κB and ROS signaling (via the PI3K-Akt and NADPH oxidase pathways, respectively) and by enhancing proresolution molecule release.

Figure 4 :
Figure 4: Selection of the studies.

Figure 5 :
Figure 5: Pleiotropic effects of statins in the context of periodontitis management.Statin biological properties might be of interest for the management of periodontitis as they act on each tissular compartment and mechanisms including inflammatory-immune crosstalk, bone metabolism and bacterial clearance.

Table 1 :
Physical properties of different types of statins.

Table 2 :
Representative in vitro studies evaluating the impact of statins on periodontal pathogens.

Table 3 :
In vivo studies evaluating the impact of local statin administration on periodontal wound healing.

Table 4 :
In vivo studies evaluating the impact of systemic statin administration on periodontal wound healing.

Table 5 :
In vivo studies evaluating the impact of a combination of local and systemic statin administration on periodontal wound healing.

Table 6 :
Clinical studies evaluating the impact of local statin administration on periodontal wound healing.

Table 7 :
Clinical studies evaluating impact of systemic statin administration on periodontal wound healing.