Introduction

The oral cavity, which is the main entrance for two systems vital to human function and physiology, the gastrointestinal and respiratory systems, consists of the teeth, the buccal, sublingual and gingival mucosa, soft and hard palate, and tongue (Fig. 1). It has a very large and diverse microbiota, harboring numerous microorganisms which include bacteria, fungi, viruses, and protozoa. There is a homeostasis between this microbiota and the host, which forms an environment with specific dynamics, and plays a crucial role in maintaining the oral health as well as the systemic health. Disruption of this balance by various factors will result in dysbiosis, allowing for the survival and establishment of a more virulent polymicrobial community impairing the efficient immune responses. Subsequently, these events can clinically manifest as oral infectious diseases [1]. For the pathogenesis of the oral infections, not only the microbiological aspects but also the immunological host response needs to be considered as crucial elements.

Fig. 1
figure 1

Drug groups and delivery systems for treatment of oral infections

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Oral diseases pose a major health burden for many countries and affect people throughout their lifetime, causing pain, discomfort, disfigurement and even death. These diseases share common risk factors with other major noncommunicable diseases. It is estimated by the World Health Organisation (WHO) that oral diseases affect nearly 3.5 billion people [2]. Oral health is considered as a key indicator of overall health, well-being and quality of life. In 2019, oral health has been included in the Political Declaration on Universal Health Coverage of United Nations Political Declaration [3]. Several systemic diseases manifest in the oral cavity. Vice versa, specific conditions in the oral cavity may create foci of infection that can affect many other vital systems, such as the cardiovascular and renal systems.

Virus-related oral infections

Oral infections, based on their microbial etiology, can be reviewed in three essential groups, which are viral, fungal, and bacterial. Although frequency of oral viral infections is not very high as the bacterial infections and exerts diagnostic challenges, it may be linked to some severe results in the oral cavity [4,5,6].

Many dormant viruses that are present in the human oral cavity can be activated and produce a variety of pathological changes in the oral mucosa in individuals immunocompromised by age, illness, or as a side effect of therapy (e.g., anticancer therapy) [7]. Oral manifestations of general viral infections may be presented as a primary sign of disease, co-symptom of the disease, or the only sign observed in such viral disease. Among the major types of viruses that are responsible for oral infections are Paramyxoviridae, Coxsackieviruses (a subgroup of the RNA enteroviruses), oral papillomas (human Papillomavirus of the Papovavirus family), and human herpesvirus family (Fig. 2). HPV infections have received particular attention in recent years, as persistent strains might increase the risk of experiencing malignant transformation in the oral mucosa. Many other viral infections can affect the oral cavity in humans, either as localized or systemic infections [8, 9]. Diagnosis and early management of viral infections is very critical, as certain viral infections result in serious conditions [4]. Recurrent Herpes Labialis (RHL) is a commonly occurring condition in herpes simplex virus (HSV) infection, characterized as a lesion located on the lips and occasionally the attached gingiva (known as a fever blister or cold sore). Secondary infections from oral bacteria can prolong the healing process. Recurrent intraoral herpes (RIH), which is observed more often in immunocompromised patients, may be difficult to distinguish clinically from other oral mucosal disorders, such as aphthous stomatitis [10]. Topical therapies for oral HSV infections can be categorized as palliative, preventive, and antiviral. Among the palliative topical agents are anesthetics such as benzocaine and lidocaine which are helpful in reducing pain associated with an oral HSV infection. Recurrent herpetic lesions are usually treated with topical antiviral medications such as acyclovir and penciclovir [11]. In immunocompetent patients, oral or parenteral administration has been shown to be more effective [4]. Currently, available antiviral medications or treatments are limited and their efficacy is inadequate.

Fig. 2
figure 2

Pathogens causing oral infections and resulting symptoms/manifestations

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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is a highly transmissible and pathogenic coronavirus, has emerged in late 2019, and it causes acute respiratory disease, named “coronavirus disease 2019” (COVID-19), which threatens human health and public safety. On March 2020, WHO has defined COVID-19 as pandemic [12]. At the time this paper was submitted (January 2020), more than 90 million cases and more than 1.9 million deaths have been reported, with an enormously rapid day-by-day increase in these numbers due to mutations in the virus. SARS-CoV-2 is transmitted from human-to-human by either direct transmission such as cough, sneeze, and droplet inhalation or contact transmission like saliva, contact through mucous membranes of the mouth, nose, and eyes. SARS-CoV-2 uses ACE2 as the receptor and human proteases as entry activators; subsequently, it fuses the viral membrane with the cell membrane and achieves invasion [13, 14]. ACE-2 expression highly occurs in the oral mucosa epithelium, and the expression is more in the dorsum of the tongue [15]. Wang et al. [16] have reported that SARS-CoV-2 might induce acute sialadenitis and associated symptoms, such as pain, discomfort, inflammation, and secretory dysfunction in salivary glands by fusing with them, replicating and lysing the cells. As oral mucosa is the first area infected with SARS- CoV-2 through droplets [17], oral mucosal lesions, painful ulcers, or blisters could be signs of COVID-19, and they should be examined thoroughly [18]. Currently, there is a low certainty of evidence regarding cause–effect relationship between coronavirus infection and the appearance of oral lesions; however, similar to that of HIV infection, COVID-19 patients were reported to develop oral lesions related to immunosuppression. Taste alteration is the most prevalent reported oral manifestation. The multiple clinical aspects suggest coinfections, immunity impairment, and adverse reactions rather than a genuine oral mucosa infection primarily caused by SARS-CoV-2 [19]. Lack of oral hygiene, opportunistic infections, stress, immunosuppression, vasculitis, and hyper-inflammatory response secondary to COVID-19 have been reported as the most important predisposing factors for onset of oral lesions in COVID-19 patients. Among the most common oral manifestations of COVID-19 disease, oral dryness, vesiculobullous lesions, aphthous-like lesions, herpetiform lesions, candidiasis, and oral lesions of Kawasaki-like disease have been reported [18, 20,21,22,23]. Currently, there are no specific medications for COVID-19-related oral manifestations, but progress in investigations is indeed going faster than usual, because it is considered as a “Once-in-a-Century Pandemic”; hence, there would be new effective treatments available for clinical applications in coming months.

Fungi-related oral infections

Candidal and non-candidal fungal infections in oral mucosa occur generally as a result of defects in the immune system. Candidiasis (candidosis) is the most common fungal infection of the oral cavity, whilst the incidence of noncandidal oral fungal infections such as aspergillosis, cryptococcosis, histoplasmosis, blastomycosis, paracoccidioidomycosis, zygomycosis (mucormycosis), oral geotrichosis, Rhodotorula infection, and fusariosis is rather low [24]. The most commonly seen fungal oral infections are summarized in Fig. 1. Fungal infections can be superficial, may cause serious lesions in the oral cavity, or can be indicative of a more serious systemic illness that may even result in mortality [25].

Various systemic and topical agents are used in treatment of candidiasis. Topical delivery has been preferred predominantly in uncomplicated cases, whilst systemic delivery is indicated when topical agents are ineffective or not tolerated in cases such as immunocompromised HIV or patients with cancer. Nystatin, amphotericin B, miconazole, ketoconazole, and clotrimazole are the most commonly used topical antifungal drugs. Other systemic (oral or parenteral) treatment alternatives such as itraconazole, voriconazole, posaconazole, anidulafungin, caspofungin, and isavuconazole have also found applications in treatment [26,27,28].

Bacteria-related oral infections

The oral cavity harbors more than 700 different bacterial species [29]. Oral bacterial infections can occur with intense clinical symptoms, chronic or without apparent symptoms, or clinical findings without impairing the host defenses like disrupting the mucosal barriers. Oral infections commonly originate from an odontogenic (tooth) source in adults and from tonsil and lymphatic sources in children. Odontogenic infections arise from advanced dental caries or periodontal disease. Nonodontogenic oral infections are related to salivary gland infection, lymph node abscess, postoperative infection, chemical, thermal, or trauma injury and may be associated with almost any microorganism. Sexually transmitted pathogens such as herpes simplex, Neisseria gonorrhea, and Treponema pallidum may be considered [30]. These nonodontogenic infections can be potentially life threatening.

The oral mucosal infections can be widespread (stomatitis/mucositis, glossitis, gingivitis) or localized (white lesions, red lesions, ulcers) [31]. The clinical manifestations and bacteria responsible for oral infections are summarized in Fig. 2. Complex biofilms of varying compositions of bacteria can colonize the surfaces of the oral cavity. Dental plaque is the term commonly used for the biofilm formed on teeth; however, this term has now been extended to include biofilms on all oral surfaces. These biofilms consist of complex microbial communities embedded in a matrix of polymers of bacterial and salivary origin, and they are recognized as a virulence factor in many oral infectious diseases, including dental caries, periodontitis, and endodontic infections [32, 33]. Periodontitis, which is a chronic inflammatory disease, may result in progressive destruction of the periodontal ligament and alveolar bone with periodontal pocket formation, gingival recession, or both. Periodontitis can also affect many other vital systems, such as the cardiovascular and renal [34].

On the other hand, severe periodontal inflammation or bleeding may require careful investigation of conditions such as diabetes mellitus, human immunodeficiency virus infection, thrombocytopenia, and leukemia [35]. It is important to diagnose correctly the underlying local or systemic condition of the oral diseases for the right treatment. Examination of the oral cavity should include evaluation for mucosal changes, periodontal inflammation and bleeding, and general condition of the teeth. Oral manifestations of specific systemic conditions are oral lesions (including ulcerative, erosive, or white lesions), swelling, and erythema, mucosal pallor and atrophy, change in mucosal pigmentation, periodontal bleeding, and inflammation [35, 36].

Antibacterial agents such as chlorhexidine, metronidazole, and tetracycline are used topically in the management of these infections; hence, higher concentration of the antibiotic can be available in the affected area and a much lower concentration throughout the rest of the body. By this means, the systemic side effects, as well as the risk of bacterial resistance, are decreased. Furthermore, antibacterial agents have been shown to be effective in the disruption/inhibition of oral biofilm. Nevertheless, current antimicrobial treatments have been reported to treat the problem only provisionally and are not effective at complete elimination of the infections, and the challenge of precisely and continuously eliminating the specific pathogens without disturbing the microbial ecology still exists. Alternate strategies against biofilms such as biofilm-inhibition agents, to prevent the early stages of biofilm formation, or biofilm-dispersal agents to disrupt the biofilm cell community have not been sufficiently efficient in direct treatment and eradication of the established biofilms [37]. Hence, investigation of alternative agents to antibiotics as well as new delivery systems play a key role to improve the efficacy of the antibacterial therapy against oral infections, without necessarily inducing microbial dysbiosis of the oral cavity. In following sections, such approaches will be reviewed with current examples.

Topical drug delivery for treatment of oral infections

Topical drug delivery plays an important role in the management of oral infections. Topical drugs have been extensively used as the first line of therapy in many conditions related to viral, bacterial, and fungal infections. A large number of clinical studies have established the clinical efficacy of topical antimicrobials and antivirals which provides targeted drug delivery options for the treatment of local oral lesions (see Tables 1, 2, and 3) [38]. In general, when compared to systemic delivery, topical drug delivery has a number of advantages such as the ability to deliver drug more selectively to a specific site at higher concentrations, lowering risk of systemic adverse events, and avoiding fluctuations in drug levels, inter- and intra-patient variations, and suitability for self-medication, hence improved patient compliance. The other advantages of topical delivery will be mentioned where appropriate. Nonetheless, there are also some obstacles faced with topical drug delivery into oral cavity such as taste alterations, limited surface area, poor tissue penetration, and rapid removal due to continuous saliva flow and tongue movement and accidental swallowing. The total surface area of the oral mucosa is relatively small (~ 200 cm2). The teeth, keratinized epithelium, and non-keratinized epithelium occupy about 20%, 50%, and 30% of the total surface area, respectively. The average volumes of saliva present in the mouth before and after swallowing were estimated to be 0.77 and 1.07 mL, respectively, and the average thickness of the salivary film in the mouth was calculated as 70 and 100 µm [39]. The permeability to the topical drugs differs significantly in different oral regions, depending on the pattern of epithelial differentiation, such as thickness and the extent of keratinization. Buccal and sublingual regions in the oral cavity are lined by non-keratinized, stratified squamous epithelium, which is 100–200 μm and 8–12 cells thick in the sublingual region, and 500–800 μm and 40–50 cells thick in the buccal region. The loss of the permeability barrier in the oral mucosa, which can be encountered due to oral manifestations, may lead to rapid diffusion of the drug into tissues when compared to the intact mucosa. The oral epithelium is covered by a complex mucus layer with an average thickness of 70–100 μm, which has an impact on the mobility of delivery systems and drug molecules. Mucus forms a protective coating on epithelial surfaces and plays a key role in host defense. Mucins are the primary structural components of mucus that create its viscoelastic properties as well as its protecting functions in oral diseases such as HIV/AIDS, oral candidiasis, and dental caries [40].

Topical drug delivery systems are traditionally formulated as solid dosage forms (e.g., tablets, wafers, films, fibers, and patches), liquid dosage forms (e.g., sprays and drops), and semi-solid dosage forms (e.g., gels, ointments) [41,42,43,44,45]. Conventional topical dosage forms are commonly affected by physiological factors, which can reduce the contact of the formulation with the mucosa and lead to reduced efficacy. Hence, numerous strategies have been proposed in order to overcome these difficulties and improve the retention and permeation of drugs in the oral cavity [32, 46]. In 1950s, antimicrobials were incorporated into dental cements and resins in order to provide local drug release of antimicrobials [47]. Recognition of local antimicrobial delivery systems in the management of bacterial infections in the oral cavity resulted in a shift in treatment modalities of dental diseases [48]. Chlorhexidine chip [49], metronidazole oral gel [50], and minocycline dental gel [51] are amongst the first formulations brought to the market.

For a successful local drug delivery, mucoadhesive delivery systems have been widely utilized to avoid the rapid removal of the system from the side of application due to physiological conditions in the oral cavity [52, 53]. Interaction between mucin and mucoadhesive polymer enables the system to remain attached on the application site and also provides prolonged release of drug. Penetration enhancers are also incorporated into delivery systems to generate improved efficacy for both local and systemic drug delivery. In designing a local delivery system, it is also important to take into consideration the condition of the disease, as each condition may require distinct penetration and drug retention/distribution profiles for an optimized efficacy. In most conditions, the drug is required to penetrate to deeper layers of the epithelium. Consequently, regarding all the requirements mentioned above, various delivery systems (e.g., liposomes, polymeric nanoparticles, lipid nanoparticles, hydrogels, fibers, films) other than conventional formulations have been investigated for an improved treatment of the oral infections [54].

Liposomes

In dentistry, liposomes have been used topically to control the oral biofilm (preventing caries and gingivitis), to treat oral lesions and periodontitis, and in photodynamic therapy. Liposomes are synthetic nano-sized vesicles consisting of one or more phospholipid bilayers, able to accommodate hydrophilic and lipophilic molecules. Liposomes may be formulated with a range of characteristics including different size, charge, and drug retention, which can be customized for a given drug and target site [55, 56]. In early 80 s, Mezei and Gulasekharam [57, 58] have shown the applicability of liposomes as drug carriers for the topical administration using triamcinolone as a model drug. Later, the potential of liposomes as drug carriers to the ulcerated oral mucosa was investigated in vivo in hamsters using radioactive triamcinolone acetonide palmitate [59]. Liposomes were shown to increase local and decrease systemic drug concentration. In addition, the authors suggested that liposomes decrease drug diffusion into neighboring tissues and localize the drug in the area of inflammation. Proteoliposomes with surface-bound succinylated concanavalin A were prepared to deliver triclosan for elimination of Streptococcus sanguis biofilms [60]. It was shown that triclosan delivered in liposome was a more effective growth inhibitor than free triclosan. Further, reactive liposomes were prepared encapsulating the enzymes, glucose oxidase (GO) and GO in combination with horse radish peroxidase (HRP) to eliminate the biofilms of the oral bacterium Streptococcus gordonii [61]. Increased bacterial inhibition was observed with the reactive liposomes. Antibacterial activity in the presence of saliva was also observed with the reactive liposomes.

Immunoliposomes were developed to increase the specificity and affinity of bactericide delivery to a specific model bacterium [62]. Antibacterial immunoliposomes were prepared using covalently bound antibody, extended to the cell surface of the bacterium Streptococcus oralis, and chlorhexidine and triclosan were incorporated as the bactericides. For short exposure times to the biofilms, several times enhanced growth inhibition of S. oralis was obtained with immunoliposomes when compared to free bactericides.

Variable results have been reported in regard to relation between the surface charge of the liposomes and their effect on biofilms, most likely due to the differences between test methods used in the studies. Nguyen et al. [63] have reported that negatively charged liposomes, specifically targeting for the teeth, appeared to be the most suitable for use in the oral cavity because these liposomes were found to be the least reactive with the components of parotid saliva. On the other side, Sugano et al. [64] have investigated the behavior of cationic liposomes on S. mutans in planktonic cells and biofilms and they reported that cationic liposomes have higher affinity not only to oral bacterial cells but also biofilms than conventional liposomes. It was demonstrated microscopically that cationic liposomes interacted with the negative charge on the bacterial surface and penetrated the deep layers of biofilms.

Lectin-conjugated liposomes were prepared using wheat germ agglutinin (WGA) to serve as bioadhesive drug carrier that can rapidly bind to oral epithelial cells within minutes, and stay on the cells to provide sustained, localized drug release for the management of oral ulcerative lesions and other related complications [65]. A significant reduction in oral cell damage was obtained when the bacterially infected cells were treated with amoxicillin-loaded WGA liposomes compared to the untreated controls.

Erjavec et al. [66] have investigated liposome formulations of varying composition and size to identify a suitable carrier for drug delivery to oral mucosal lesions by assessing the effects of a hyperaemic drug on the oral mucosa using in vivo EPR (electron paramagnetic resonance)-oximetry. They have reported that multi-lamellar liposomes made from hydrogenated soy lecithin appeared to be the most appropriate for local drug delivery to oral mucosa.

Liposome formulations have been widely investigated for treatment of periodontitis. It was shown in vivo that local delivery of liposome-encapsulated superoxide dismutase and catalase suppressed periodontal inflammation in experimentally induced periodontitis beagle dogs [67]. As an adjunctive treatment for chronic periodontitis, liposome formulation for an antimicrobial drug, minocycline was developed and investigated in vitro on murine macrophages (ANA-1) [68]. Liposomes were shown to have stronger and longer inhibition effect on LPS-stimulated TNF-α secretion of macrophages cell when compared to that of solution of the drug. This result was attributed to the specificity of liposomes for macrophages and also controlled delivery provided by the liposomes.

pH-responsive quaternary ammonium chitosan (TMC)-liposome formulations loaded with doxycycline were developed for periodontal treatment [69]. The periodontitis healing capacity of the developed formulations was evaluated in rats. The formulations showed antimicrobial activity against P. gingivalis and Prevotella intermedia, strong inhibition on biofilm formation and prevented alveolar bone absorption in vivo.

Periodontal therapy usually requires also local anesthesia. A liposomal lidocaine/prilocaine, thermosetting anesthetic gel formulation delivered into periodontal pocket was investigated for pain control during scaling and root planing (anti-infective periodontal therapy) in 40 volunteers with moderate to severe chronic periodontitis [70]. It was reported that the intra-pocket anesthetic gel would be a good option for anxious patients, or those who have a fear of needles.

Micelles

Micelles are self-assembling colloidal systems obtained by the aggregation of block or graft amphiphilic copolymers [71]. Micelles have found applications in dentistry for a targeted-delivery of antimicrobials to the tooth surface against biofilm formation. Chen et al. [72], have used alendronate terminated Pluronic copolymers to prepare triclosan-loaded tooth-binding micelles and demonstrated that micelles were able to inhibit initial biofilm growth of S. mutans. The use of alendronate as a binding moiety, however, has raised concerns on the safety of these tooth-binding micelles; therefore, the same group has replaced alendronate with diphosphoserine and conjugated it to the chain termini of Pluronic P123 and combined it with another biodegradable tooth-binding moiety, pyrophosphate (PPi) [73]. Tooth-binding potential and binding stability as well as anti-biofilm activity against S. mutans of the developed micelles were found to be significant. Recently, a multifunctional matrix for the treatment of periodontitis and enhancement of regeneration of the periodontal tissue was prepared from vitamin E containing hydrogel made of alginate and gelatin, and doxycycline HCl containing methoxy poly(ethylene glycol)-block-polycaprolactone micelles [74]. A sustained drug release and enhanced antimicrobial activity were observed against E. coli and S. aureus.

Nanoemulsions

Nanoemulsions have been recognized to have unique properties that make them more versatile than other emulsion systems. Nanoemulsions consist of droplets 100 nm or smaller in diameter of immiscible liquids stabilized by surface-active materials [75]. They are distinguished from microemulsions by their stability. Microemulsions are indefinitely stable thermodynamic phases, whereas nanoemulsions are transient, kinetically stable structures. Nanoemulsions can be fabricated with great flexibility to deliver different drug moieties with different characteristics. Several studies have shown the efficacy of nanoemulsions in prevention and treatment of diseases of the oral cavity [76]. Nanoemulsions are reported to provide improved penetration of the drug into deeper layers of the oral mucosa due to their size, thereby resulting in better and complete cure of the disease. Nanoemulsions with high surfactant content have also been shown to exert antimicrobial activity. Karthikeyan et al. [77] have prepared oil-in-water nanoemulsion composed of soybean oil, cetylpyridiniumchloride, and Triton X-100 and demonstrated its antimicrobial activity against cariogenic Streptococcus mutans, suggesting this formulation for prevention of dental caries.

Hydrogels

Hydrogels have a three-dimensional porous and interconnected structures composed of hydrophilic, cross-linked macromolecules that absorb water, aqueous solutions, or physiological fluids, but remain insoluble due to their network structure [78, 79]. They provide a biocompatible microenvironment for cell attachment and proliferation and possess many unique advantages on the targeted delivery systems for hydrophilic and hydrophobic agents and other biomolecules. Localized application is possible with hydrogels, and they can be tailored to release the drug for a long time by controlling the hydrogel architectures, network pores, and gelation mechanisms (physical and chemical gelation). Synthetic (poly(hydroxyethyl methacrylate) (polyHEMA, PHEMA) polyethylene glycol and derivatives, poly(vinyl alcohol), polyvinylpyrrolidone, polyimide, polyacrylate, polyurethane [80], and natural (chitosan, alginate, collagen, gelatin etc.) [81] polymers have been used for preparation of hydrogels. Most of these polymers also exert adhesive properties which enables a longer retention of the system on application site. Hydrogels have found applications in dentistry for regenerative therapies to provide recovery of the function of tissues lost due to oral and dental pathologies of infection as well as traumatic and neoplastic origin [82,83,84]. Furthermore, various hydrogel formulations have been used for treatment of oral lesions and also for delivery of antimicrobials, anesthetics, and antiinflammatory drugs [85,86,87,88,89,90,91]. Our group has investigated gel formulations based on chitosan, which is a material widely investigated in dental field both for its bioactive properties such as wound healing, tissue regeneration, antimicrobial, and as a biocompatible, bioadhesive biopolymer for delivery of drugs, especially the anti-inflammatory and antimicrobial molecules [84, 92]. Chitosan gel itself has been shown to exhibit antimicrobial activity against various dental patogens [93]. Antimicrobial activity was found to depend on the properties of the chitosan used (source—animal or non-animal, molecular weight, solubility, degree of deacetylation etc.) as well as the type of the strains tested. Furthermore, when incorporated with various antimicrobial drugs such as chlorhexidine [87, 94], nystatin [95], moxifloxacin [96], metronidazole [86], and anti-inflammatory drug such as atorvastatin [85, 97], the effect of the drug was found to be enhanced in presence of chitosan, besides the improved retention time and prolonged drug release. Chitosan gel itself has also been shown in vivo in human to be promising for periodontal tissue regeneration [98].

Hydrogels for the anesthetic drug lidocaine hydrochloride were prepared for buccal application using chitosan glutamate, or its binary mixture with glycerin. The anesthetic activity of mucoadhesive hydrogels was assessed in healthy volunteers in comparison to commercial semisolid formulations. Prolonged release of drug, which resulted in local anesthetic activity lasting for 20 to 30 min upon application, was obtained. The developed hydrogels were suggested as potential delivery system reducing the pain symptoms that characterize aphthosis and other mouth diseases [99].

Hydrogels exert appropriate syringeability properties which make it suitable for administration into periodontal pocket [100]. The injectable thermosensitive hydrogels have gained more attention, especially for unapproachable periodontal pockets. An injectable thermogel system for the treatment of oral mucosa-related ulcers was developed by Luo et al. [101]. These thermogels were formed from a series of chitosan-based conjugates, composed of a chitosan backbone and synthetic side chains of thermosensitive poly(N-isopropylacrylamide) (PNIPAAM). Ulcer healing was investigated in vivo in rats and the antibacterial activity against Staphylococcus aureus as well as proliferation promotion; hemostasis effect of the developed formulation was demonstrated. Ji et al. [102] have developed a thermosensitive hydrogel based on chitosan, quaternized chitosan, and β-glycerophosphate loaded with 0.1% w/w chlorhexidine. Higher antimicrobial activity against P. gingivalis and Prevotella intermedia was obtained with gels prepared using quaternized chitosan when compared to that with chitosan.

A thermo-reversible poly-isocyanopeptide (PIC), which is a water-soluble polymer forming a gel at very low polymer concentrations with good injectability properties, and has a sol–gel transition temperature of 15–18 °C [103], was investigated as a hydrogel for delivery of doxycycline and/or lipoxin A 4 for antimicrobial and anti-inflammatory treatment [100]. The PIC hydrogel facilitated the drug for around 4 days in vitro. When applied in dogs, local or systemic adverse effects were observed. The subgingival bacterial load and pro-inflammatory interleukin-8 level were shown to reduce with the hydrogel formulations. Gingival clinical attachment was improved when compared to mechanical debridement.

Dong et al. [104] have incorporated metronidazole loaded microcapsules into a poly(vinyl alcohol) injectable hydrogel by dynamic covalent bonding and ionic interaction through a 4-carboxyphenylboronic acid bridge. The developed formulation exhibited desirable antibacterial activity against P. gingivalis and Fusobacterium nucleatum for 1-week period on the rats.

Hydrogels have been used also to deliver antimicrobial peptides (AMPs), which are one of the most well-studied classes of biofilm eradication agents [105, 106]. AMPs are a diverse group of host-defense molecules that include defensins, cathelicidins, histatins, neuropeptides, peptide hormones, and many other proven and putative peptides. In the oral cavity, the AMPs are produced by the salivary glands and the oral epithelium [107]. AMPs are effective defensive weapons and have been shown to modify cellular functions such as chemotaxis, apoptosis, gene transcription, and cytokine production. Further, they play role in stimulation of wound healing and angiogenesis. Due to their antibacterial, anti-inflammatory, and/or immune modulatory actions, they are used to control oral infections [108,109,110,111,112,113,114]. Sani et al. [89] have developed a hydrogel based on a visible-light-activated naturally derived polymer (gelatin) and an antimicrobial peptide (AMP) for treatment of peri-implant diseases. An enhanced antimicrobial activity against P. gingivalis was obtained with the gels.

In oral mucosal conditions related to immunological pathogenesis, clinical studies have shown that topical immunomodulators such as cyclosporine, tacrolimus, and pimecrolimus are also effective when compared to the steroids which are the conventionally used drugs [115,116,117,118,119,120]. In order to enhance their activities, these immunomodulators were incorporated into bioadhesive gels. For the treatment of oral lichen planus, clobetasol and cyclosporin adhesive gels based on hydroxyethyl cellulose were applied twice a day on dried lesions for 2 months and significant healing was observed with the gels [121].

Currently, there are commercially available products based on hydrogels. A two syringe mixing system (Atridox) is a subgingival controlled-release product composed of the syringe A: Atrigel® Delivery System, which is a bioabsorbable, flowable polymeric formulation composed of 36.7% poly(DLlactide) (PLA) dissolved in 63.3% N-methyl-2-pyrrolidone (NMP) and syringe B containing doxycycline hyclate [122]. Upon contact with the crevicular fluid, the liquid product solidifies and then allows for controlled release of drug for a period of 7 days. In addition, numerous gel formulations of metronidazole are also available on the market.

Nanomaterials and polymeric nanoparticles

Materials in nano-size and drug-incorporated nanoparticles as well as their combination have found wide applications in dentistry for prevention, diagnosis, therapeutic, restoration, and tissue regeneration purposes [54, 123,124,125].

Metallic nanoparticles such as silver, gold, and zinc oxide due to their broad-spectrum antibacterial activity have been used to eliminate the biofilms in the oral cavity [126,127,128,129,130,131]. The large surface area and high charge density of these nanoparticles enable them to interact with the negatively charged surface of bacterial cells to a greater extent resulting in enhanced antimicrobial activity. In order to enhance the antimicrobial activity, these metals have been combined with other antimicrobial agents such as chlorhexidine [132]. Recently, the antimicrobial efficacy of silver and gold nanoparticles with diode laser was investigated against S. mutans in teeth sample, and the greatest reduction in colony-forming units (CFU) was observed with the combination of silver nanoparticles with diode laser group [133].

Metallic nanoparticles combined with polymers or coated onto biomaterial surfaces have been shown to exhibit superior antimicrobial properties in the oral cavity [132, 134, 135]. Besides silver, gold, and zinc oxide, bismuth subsalicylate nanoparticles have also been shown to inhibit the growth of several periodontal pathogens including A. actinomycetemcomitans, C. gingivalis, and P. gingivalis [136].

Further, mesoporous silica nanoparticles, which have a porous structure with large surface area, have been investigated as anti-biofilm agents [137, 138]. When combined with another antimicrobial such as chlorhexidine, antibacterial activity against S. mutans, F. nucleatum, A. actinomycetemcomitans, and P. gingivalis was shown to be enhanced [139].

Recently, graphene family nanomaterials, due to their superior mechanical, chemical, and biological properties, have gained great attention in dentistry. Graphene oxide (GO), as the derivative of graphene, was investigated for its antimicrobial property against various dental pathogens including S. mutans, Fusobacterium nucleatum, and P. gingivalis, and GO nanosheets were reported to be highly effective in inhibiting the growth of dental pathogens [140]. It was also shown by transmission electron microscopy that the cell wall and membrane of bacteria lost their integrity and the intracellular contents leaked out after they were treated by GO. Furthermore, graphene oxide (GO) has been widely investigated as a nanodelivery system for variety of drugs [141, 142], which makes it a promising material for treatment of infections in the oral cavity.

In the past decade, the application of antimicrobial photodynamic therapy (aPDT) on oral infectious diseases has attracted great interest. The bacteria can be killed when induced with light in the presence of a sensitizing agent, by means of generation of cytotoxic, reactive oxygen species (ROS) [143]. There are a number of sensitizers that interact with bacterial cell and generate ROS such as methylene blue, erythrosine, indocyanine green, eosin-Y, psoralen, and toluidine blue ortho [105]. Erythrosine has been applied with white light (500–650 nm) which successfully killed S. mutans and inhibited biofilm formation [144]. A dental light with haematoporphyrin sensitizer was investigated against S. mutans, A. actinomycetemcomitans, and E. faecalis, and it was found that the sensitizer can penetrate gram-positive bacteria cell, whilst by A. actinomycetemcomitans, the sensitizer is taken up in presence of 10% EDTA [145]. Furthermore, dental LEDs with blue light absorbing photosensitizer were demonstrated to disrupt E. faecalis biofilm depending on the concentration of sensitizer [146]. However, due to the hydrophobic characteristics of the photosensitizers, aPDT was not very effective on the viability of biofilms; hence, nanomaterials (metal and metal oxide nanoparticles) or polymeric nanoparticles have been used in order to enhance the antimicrobial performance of aPDT [147, 148].

The photosensitizer indocyanine green (ICG) was incorporated into chitosan nanoparticles, and A. actinomycetemcomitans ATCC 33384 strain was treated with these nanoparticles, which was excited with a diode laser [149]. The expression of rcpA gene which is involved in biofilm formation of A. actinomycetemcomitans was found to be significantly downregulated upon using nanoparticles for aPDT, indicating a promising approach for control of periodontal pathogens. Similarly, indocyanine green was incorporated into PLGA nanoparticles coated with chitosan for aPDT [150]. A significantly higher antibacterial activity against P. gingivalis was observed. De Freitas et al. [151] have investigated the effect of aPDT on human dental plaque bacteria using methylene blue (MB)-loaded poly(lactic-co-glycolic) nanoparticles in a clinical pilot study with 10 adult human subjects with chronic periodontitis. Patients were treated either with ultrasonic scaling and scaling and root planing (US + SRP) or ultrasonic scaling + SRP + aPDT with MB-nanoparticles. The clinical study demonstrated the safety of aPDT. At month three, more profound effect (28.82%) on gingival bleeding index was observed in ultrasonic SRP + aPDT group when compared to ultrasonic SRP.

In literature, there are numerous studies on polymeric nanoparticles used to deliver drugs into oral cavity for treatment of oral infections [152,153,154,155,156]. Due to their versatile characteristics such as surface charge, dimension, and hydrophobicity, it has been possible to prepare tailor-made polymeric nanoparticles for an enhanced local treatment of oral infections (see Tables 1, 2, and 3).

Microparticles

Polymer-based microparticles have also been investigated to maintain therapeutic drug concentrations for longer periods of time for treatment of dental and mucosal infections in the oral cavity [157, 158]. Numerous synthetic (e.g., PLGA) and natural polymers (e.g., chitosan) have been successfully used in preparation of microparticles for drug delivery [159]. Due to its own antimicrobial activity, chitosan alone in microparticle form has also been investigated. Kawatika et al. [160] have compared the effect of chitosan aqueous dispersion and microparticles on mature biofilms of S. mutans and demonstarted that chitosan in microparticle form reduced the bacterial viability and acidogenicity more effectively than the dispersions, thereby was more effective to control the growth of mature biofilms. Moura et al. [161] have investigated the release of locally delivered doxycycline-loaded PLGA microspheres in the periodontal pocket of patients with chronic periodontitis. The microspheres were demonstrated to provide sustained release after local administration, as an adjunct to non-surgical periodontal therapy.

Currently, there is a commercially available subgingival sustained-release product (Arestin®), which consists of minocycline hydrochloride incorporated microspheres prepared using bioresorbable polymer, poly (glycolide-co-dl-lactide). It is used in combination with scaling and root planing procedures to treat patients with adult periodontitis [162].

Strips/fibers

Strips and fibers composed of polymeric matrix have been used to deliver antimicrobials as an adjunct to mechanical treatment of periodontal disease [163]. They can be designed in appropriate dimension which allows practical insertion in periodontal pocket resulting in desirable clinical outcomes [164]. Various polymers and their combinations have been used to prepare strips and fiber for delivery of antimicrobials such as chlorhexidine, doxycycline, tetracycline, minocycline, and metronidazole [165,166,167]. In early years, acrylic polymers have been widely used providing significant improvements in various clinical conditions by effective microbial eradication from the pockets [164]. However, due to several disadvantages such as being non-absorbable, removal required after therapy, which may impair the regenerating tissue at the site, other polymers such as cellulose derivatives (hydroxypropyl cellulose, hydroxypropyl methylcellulose, ethyl cellulose), polycaprolactone, polyhydroxybutyric acid, poly- methylmethacrylate, and PLGA have been preferred over acrylic polymers.

Strips containing tetracycline hydrochloride or metronidazole 25% in polyhydroxybutyric acid as a biodegradable polymer matrix were evaluated in patients suffering from advanced periodontal disease [168]. The greatest response to therapy was observed with tetracycline hydrochloride strips inserted into periodontal pockets at 4-day intervals for 16 days, compared with an untreated control group. Metronidazole strips or root-planing tended not to be as effective. The clinical improvement produced by each treatment was not maintained when treatment was terminated. A commercially available periodontal fiber (Actisite®) for periodontal pocket placement has been developed in 90 s, which consists of a 23-cm monofilament of ethylene/vinyl acetate copolymer, 0.5 mm in diameter, impregnated with 25% tetracycline hydrochloride, providing continuous release of tetracycline for 10 days. It was demonstrated that the local delivery system was more effective than scaling and root planing (SRP) with respect to decreasing probing depth, increasing attachment level, and decreasing bleeding on probing (BOP) [169]. Later, the tetracycline strips were prepared with the identical polymer system to that of the fiber except for its physical shape and method of placement and investigated in patients following administration singly or in multiples in conjunction with root planing, versus root planing alone, or to an untreated control [166]. It was concluded that multiple strips which fill the periodontal pocket were superior to a single strip in reducing BOP and that use of locally delivered tetracycline was superior to SRP alone in decreasing probing depth.

The commercially available biodegradable local delivery system (PerioChip), which contains chlorhexidine gluconate in a biodegradable matrix of hydrolyzed gelatin (cross-linked with glutaraldehyde) developed by Steinberg et al. [170], can also be considered as a strip with rectangular shape. Drug concentration was shown to remain above the minimum inhibitory concentration for more than 99% of periodontal pocket flora for up to nine days.

In recent years, nanostructured polymeric fibers have been developed in order to enhance the efficacy of the drugs at the application site [171]. The advantages of nanofibers involve large surface area, high porosity, and high mechanical strength which make it a potential candidate for the application into the periodontal pocket [172]. Electrospinning technique is the mostly used method for the production of nanofibers, which allows adjustment of fiber size, drug loading, and mechanical properties can be adjusted [173]. In a recent study, the antibacterial activity of chitosan nanofiber, cross-linked with tetracycline comprising polyvinyl alcohol (PVA), was evaluated and enhanced antibacterial activity against a wide range of periodontal pathogens was demonstrated [174]. Mahmoud et al. [175] have developed polymeric electrospun fibers using PLGA, poly(L-lactic acid) (PLLA), and polycaprolactone (PCL) alone or blended with polyethylene oxide (PEO), incorporated with an antimicrobial peptide (BAR), and evaluated their safety and functionality against P. gingivalis/S. gordonii biofilms. The most promising formulation was found to be PLGA:PEO providing a sustained drug release and a dose-dependent inhibition of biofilm formation.

Films and wafers

Films containing antimicrobials and anti-inflammatory drugs have been designed for treatment of oral infections [176, 177]. Film formulations can be applied on the oral mucosa as well as into the periodontal pockets. They are thin and flexible, and particularly, mucoadhesive films can resist to physiological conditions in the oral cavity [178]. Numerous synthetic or natural polymers have been investigated to develop mucoadhesive films with one or more layers, or films based on stimuli responsive hydrogel. Chitosan is one of the most investigated polymers for preparation of films. Due to its bioadhesive properties, it can retain on the application site in prolonged periods of time and also exerts synergetic effect due to it antimicrobial activity [94, 179]. Recently, a two layer-polymeric film was prepared using a polymeric gel-like blend (including chitosan, HPMC, methocel at various ratios), with the basal layer (with lidocaine hydrochloride for a faster release than the apical layer with benzydamine HCl and N-acetyl-cysteine [180]. The single patient study showed that the association of polymers with the addition of analgesics, anti-inflammatories, and mucolytics promoted the reduction of inflammation, tumefaction, and an erythematous halo with significant mucosal regeneration in 30 days.

Exerting similar properties to that of films, wafers have been also investigated for local delivery to the oral cavity. The advantage of wafers of films is reported to be low residual moisture and increased drug loading [181]. Recently, our group has developed monolayer and bilayered mucoadhesive film and wafer formulations as local drug delivery platforms for treatment of oral infections, using chitosan and hydroxypropyl methylcellulose (HPMC) [182]. Cefuroxime axetil (CA) was used as the model drug. Antimicrobial activity was evaluated against E. coli and S. aureus. HPMC-based formulations were found to disintegrate within less than 30 min, whereas chitosan-based formulations remained intact up to 6 h. Significantly higher drug release was obtained with wafer formulations. Antimicrobial activity was found to increase in presence of chitosan, and HPMC was also observed to contribute to antimicrobial activity. Bilayered wafer formulation with adhesive chitosan backing layer and HPMC-based drug-loaded layer is suggested as a promising local delivery system for treatment of the infections in the oral cavity.

Conclusion and future perspectives

The mouth is said to be the gateway to one’s overall health, because the oral cavity may exhibit manifestations of underlying systemic infectious or autoimmune, hematologic, endocrine, and neoplastic-related diseases and serve as an indicator of overall health. On the other hand, oral cavity can act also as the site of origin for dissemination of pathogenic organisms to distant body sites, especially in immunocompromised hosts such as patients suffering from diabetes or rheumatoid arthritis or receiving immunosuppressive treatment. The oral microbiome encompasses a highly diverse microbiota, consisting of over 700 microorganisms, including bacteria, fungi, and viruses. In order to maintain the oral health, it is important to diagnose precisely the underlying local or systemic condition of the oral diseases in order to take the right actions for prevention and treatment. Appropriate drugs and delivery systems are crucial to reduce /eliminate the local as well as the systemic complications related to oral infections. Currently, there are numerous drugs available for preventive and palliative therapies, which reduce the infection incidence but cannot adequately eliminate the infection. Topical use of antivirals and antimicrobials has been successfully used for treatment of oral infections; however, there are still some obstacles to be addressed related to the drugs used as well as the delivery systems. Antimicrobial resistance has emerged as a huge challenge to the effective treatment of infectious diseases with antibiotics. Owing to the challenges faced with discovery of a new drug and the limited number of new classes of antibiotics, researchers have focused more on non-conventional approaches that could serve as alternatives to antibiotics. The alternative approaches to antibiotics include immunomodulators, competitive exclusion of pathogenic bacteria via probiotics and their combination, antimicrobial peptides, and photoactivateable agents. Another widely investigated approach against antimicrobial resistance is use of nanomaterials either as the active agent (e.g., silver or gold nanoparticles, nanographene, zinc oxide) or for drug delivery (antibiotics or above-mentioned alternate compounds). Nano delivery systems have the potential to deliver the antibiotic payload into the infected cells; therefore, enhancing penetration and release of antibiotics inside the infected cells. By this means, they can reduce the antibiotic overuse and help prevent antimicrobial resistance. Furthermore, nano systems have the potential to treat biofilm-forming pathogen infections by acting as a protective coat, shielding against interactions of drug by biofilm compartments and resident enzymes. Nevertheless, oral cavity is a complex environment for local drug delivery due to its distinct characteristics such as different epithelial structure (keratinized or non-keratinized, thickness etc.) in different regions, continuous secretion of saliva, movement of the tongue, and swallowing. Specialized delivery systems are required to deliver the drug in the desired period of time, resisting the physiological conditions of the oral cavity and avoiding the removal of the drug from the application site. Furthermore, for delivery to certain regions in the oral cavity such as periodontal pocket and tooth, systems with certain shape and dimension are required.

Currently, there are numerous products available on the market, which are mostly based on conventional dosage forms and aimed for inhibition of bacterial growth and biofilm formation, for reducing inflammation or for tissue regeneration. On the other side, many promising results have been reported for prevention and treatment of oral infections with the newly developed advanced systems, particularly, with those that apply nanotechnology and/or use novel compounds alternative to those currently used; however, up to date, very few have reached to the market. More studies in human are needed to prove the safety and efficacy of these systems, so that they can be available for routine clinical applications. In regard to manufacturing of these systems, commercial scale-up may pose some problems due to their complexities; however, the delivery market opportunities are plentiful. There are indeed some exciting developments within topical delivery into the oral cavity. For future, the most lacking research area appears to be the development of vaccines for prevention of oral infections. This is indeed an important aspect which the researchers should focus more.

Table 1 Drug delivery systems for treatment of bacterial infection-related conditions in the oral cavity
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Table 2 Drug delivery systems for treatment of viral infection–related conditions in the oral cavity
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Table 3 Drug delivery systems for treatment of fungal infection-related conditions in the oral cavity
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