Role of Permeation Enhancers in Buccal Drug Delivery of Poorly Absorbed Drugs: A Comprehensive Review

The oral route of drug administration, despite being the most widely adopted and patient-preferred delivery pathway, presents pharmacokinetic constraints that compromise therapeutic efficacy for a substantial fraction of clinically used drug molecules. Hepatic first-pass metabolism, whereby orally administered drugs are extracted by the liver prior to reaching systemic circulation, significantly reduces the bioavailability of numerous high-clearance drugs including nitro-glycerine, testosterone, propranolol, verapamil, and a diverse range of peptide-based therapeutics [1,2]. The hostile environment of the gastrointestinal (GI) tract characterised by wide pH fluctuations (pH 1–8 from stomach to colon), proteolytic and lipolytic enzymes, efflux transporters (P-glycoprotein, MRP2, BCRP), and phase I/II metabolizing enzymes in the enterocytes leads to degradation and inactivation of acid-labile, enzyme-sensitive, and efflux-susceptible compounds, particularly large-molecular-weight biologics such as insulin, calcitonin, glucagon-like peptide-1 (GLP-1) analogues, growth hormone, and monoclonal antibody fragments [3,4].

These limitations have catalysed extensive research into alternative routes of drug administration. Mucosal drug delivery routes including buccal, sublingual, nasal, rectal, and vaginal offer the distinct advantage of delivering drugs directly to systemic circulation while circumventing both GI degradation and the hepatic portal system. The buccal route, involving drug administration via the mucosa lining the inner cheek wall, has attracted particular scientific and commercial interest owing to its accessibility during routine activity, reasonable patient acceptability, relatively large absorptive surface area (~50 cm2 bilaterally), rich submucosal blood supply draining directly into the jugular vein, and a comparatively tolerant immunological environment that does not trigger the robust immune responses associated with nasal or pulmonary delivery [5,6].

Buccal drug delivery is particularly advantageous for: (i) drugs with high hepatic extraction ratios where first-pass metabolism severely limits oral bioavailability; (ii) drugs that are chemically or enzymatically unstable in the GI environment; (iii) drugs requiring sustained systemic plasma levels with avoidance of peak-trough fluctuations; (iv) patients with swallowing difficulties (dysphagia), nausea, vomiting, or unconsciousness; and (v) drugs in which rapid local mucosal absorption is clinically desirable, as in acute pain management or seizure rescue therapy [7,8].

Despite these compelling advantages, the translocation of drug molecules across the buccal epithelium remains a formidable challenge. The non-keratinized stratified squamous epithelium of the buccal mucosa approximately 40–50 cell layers thick functions as a selective permeability barrier through its intercellular lipid lamellae, tight junction architecture, enzymatic environment, and the continuous saliva-mediated washout effect that dilutes and mechanically dislodges buccal formulations. Poorly absorbed drugs, broadly encompassing BCS Class III (high solubility, low permeability) and Class IV (low solubility, low permeability) compounds, are particularly disadvantaged, as their physicochemical properties inherently limit passive transcellular diffusion across lipophilic biological membranes [9,10].

Permeation enhancers also termed absorption promoters or penetration enhancers are pharmacologically inactive excipients that facilitate drug transport across biological membranes by transiently and reversibly altering barrier properties without causing permanent tissue damage. They represent one of the most extensively investigated strategies in buccal drug delivery research, encompassing chemically diverse agents acting through multiple complementary mechanisms. The scientific literature on buccal permeation enhancement has grown substantially in the past two decades, driven by the urgent clinical need for non-injectable delivery of biologics, the commercial opportunities in the CNS and endocrine drug markets, and technological advances in nanomedicine and material science [11,12].

This comprehensive review systematically examines the role of permeation enhancers in buccal drug delivery with a specific focus on poorly absorbed drugs. Following a detailed analysis of buccal mucosal anatomy, physiology, and barrier characteristics, the review provides an evidence-based mechanistic classification of permeation enhancers, examines their applications across major therapeutic drug categories, evaluates formulation strategies and evaluation methodologies, discusses safety and regulatory considerations, and critically appraises recent advances and future prospects. It is intended to serve as a definitive reference for pharmaceutical scientists, formulation developers, regulatory scientists, and clinical pharmacologists working at the frontier of mucosal drug delivery research.

2. ANATOMY AND PHYSIOLOGY OF THE BUCCAL MUCOSA:

2.1 Structural Organization

The oral mucosa constitutes the epithelial lining of the entire oral cavity and is functionally categorized into masticatory mucosa (hard palate, gingiva), specialized mucosa (dorsal tongue), and lining mucosa (buccal, labial, and alveolar regions). The buccal mucosa, forming a significant portion of the lining mucosa, comprises a non-keratinized stratified squamous epithelium resting upon a dense fibrous connective tissue layer the lamina propria beneath which lies the submucosa containing blood vessels, lymphatics, nerves, and accessory salivary glands. The basement membrane, a thin electron-dense basal lamina, separates the epithelium from the lamina propria and plays a critical role in regulating molecular passage between compartments [13,14].

The buccal epithelium is approximately 40–50 cell layers thick with a total thickness ranging from 500 to 800 µm. Unlike the keratinized epithelium of the skin or hard palate, the buccal epithelium lacks a cornified outer layer (stratum corneum equivalent), rendering it generally more permeable to drug molecules. This structural characteristic, combined with underlying vascularity, distinguishes the buccal mucosa as a preferred site for mucosal drug delivery compared to the palate, gingiva, and skin [15].

Figure 1. Cross-sectional Structure of Buccal Mucosa

2.2 Histological Features Relevant to Drug Absorption

The stratified squamous epithelium comprises three major cell populations progressing from the basement membrane to the surface: the basal layer (stratum basale), the spinous layer (stratum spinosum), and the superficial layer (stratum superficiale). Basal columnar or cuboidal cells undergo continuous proliferation, migrating apically while undergoing progressive flattening, glycogen accumulation, and cytoskeletal remodelling before eventual desquamation [16].

Of primary pharmacological significance are the ntercellular spaces between epithelial cells, which contain membrane-coating granule-derived lipids organized as lamellar structures within the intercellular channels. These lipids comprising ceramides (acylceramides, glucosylceramides), cholesterol, cholesterol esters, free fatty acids, and phospholipids occlude intercellular channels and constitute the principal hydrophobic barrier to paracellular drug transport. This lipid composition is qualitatively similar to, but quantitatively less restrictive than, the epidermal lipid barrier of skin, accounting for the greater permeability of the buccal mucosa relative to cutaneous sites [17].

Figure 2. Histological Organization of Oral/Buccal Mucosa

Figure 3. Histological Section of Human Buccal Mucosa

2.3 Blood Supply and Lymphatic Drainage

The buccal mucosa receives arterial supply from branches of the facial artery and buccal branches of the maxillary artery. Venous drainage via the facial vein into the internal jugular vein and then the superior vena cava provides the fundamental pharmacokinetic rationale for buccal drug delivery drugs absorbed transcellularly across the buccal epithelium enter systemic circulation without traversing the hepatic portal system, effectively avoiding hepatic first-pass metabolism. This characteristic can confer substantial bioavailability advantages for high-extraction drugs such as testosterone (oral bioavailability ~1%) and nitro-glycerine (oral bioavailability <1%) [18].

Lymphatic drainage from the buccal region occurs via the submandibular and deep cervical nodes. While contributing minimally to overall drug bioavailability from conventional formulations, lymphatic absorption is relevant for particulate drug carriers (nanoparticles, liposomes) and for immunological surveillance of antigens and adjuvants, making the buccal route of potential interest for mucosal vaccine delivery [19].

2.4 Buccal Fluid Composition and Salivary Dynamics

The buccal environment is continuously bathed by saliva complex biological fluid secreted by the parotid (25%), submandibular (60%), sublingual (5%), and numerous minor salivary glands (~750 dispersed glands) at an average unstimulated flow rate of 0.3–0.5 mL/min, rising to 2–3 mL/min upon gustatory or masticatory stimulation. Salivary pH ranges from approximately 6.2–7.6 depending on flow rate, directly influencing drug ionization state and consequently lipophilicity and membrane partitioning [20].

Saliva contains salivary mucins (MUC5B, MUC7), alpha-amylase, lysozyme, lactoferrin, secretory immunoglobulin A (sIgA), carbonic anhydrase, peroxidases, cystatins, histatins, and a diverse electrolyte profile (Na+, K+, Ca2+, Cl-, HCO3-). Mucinshigh-molecular-weight glycoproteins with a rigid polypeptide backbone and dense carbohydrate side chainsadsorb onto the epithelial surface forming a continuous mucus gel layer 50–450 µm thick that serves as both a lubricant and a selective diffusional barrier. The enzymatic content, including salivary proteases (kallikrein, aminopeptidase A and N, carboxypeptidase), contributes to the metabolic degradation of peptide and protein drugs within the buccal cavity before absorption is complete [21].

2.5 Comparative Permeability: Mucosal Region Hierarchy

Permeability across oral mucosal regions follows a well-characterized hierarchy reflecting differences in epithelial thickness, keratinization degree, and lipid composition: sublingual > buccal > palatal. The sublingual mucosa (~100–200 µm thick, non-keratinized) is the most permeable oral mucosal region, facilitating rapid drug absorption with onset times of minutes exemplified by sublingual nitro-glycerine achieving therapeutic plasma levels within 2–3 minutes. The buccal mucosa, while less permeable, offers 5–6 times greater surface area than the sublingual region (~50 vs ~9 cm2), better mechanical accessibility for sustained dosage form placement, and superior resistance to dislodgement during normal oral activity [22].

The palatal mucosa is the least permeable oral region due to its keratinized nature histologically resembling skin more than typical mucosal tissue. This permeability hierarchy has important formulation design implications: sublingual formulations are preferred for rapid-onset drug delivery (nitro-glycerine, fentanyl), while the buccal route better accommodates controlled-release, sustained, and mucoadhesive formulation approaches requiring prolonged mucosal residence for adequate drug exposure [23].

3. BARRIERS TO BUCCAL DRUG PERMEATION:

3.1 Physical Barriers

The stratified squamous epithelium, despite being non-keratinized, represents a substantial physical barrier to drug absorption through three principal structural features. First, the epithelial thickness of 500–800 µm provides a considerable diffusion path length that impedes passive drug transport. Second, the intercellular lipid lamellae multilamellar lipid structures deposited in the intercellular spaces during epithelial maturation create a highly tortuous, lipophilic barrier that severely restricts paracellular movement of hydrophilic compounds and macromolecules. The tortuosity factor of these intercellular channels, estimated to be 4.5–5.5 for the buccal epithelium, substantially increases the effective diffusion path length beyond the geometric thickness [24].

Third, the mucus layer coating the epithelial surface adds a viscoelastic barrier to drug diffusion. The mucus gel network, with pore sizes of approximately 200–500 nm, effectively excludes larger drug particles and colloidal carriers from reaching the epithelial surface. Negatively charged mucin fibres (due to sialic acid and sulfate residues) can interact electrostatically with cationic drug molecules, forming drug-mucin complexes that retard diffusion. The mucoadhesive interaction between drugs/formulations and the mucin network, while exploited therapeutically for prolonged retention, can simultaneously impair the movement of formulation components toward the epithelial surface [25].

3.2 Biochemical Barriers

The buccal epithelium and salivary environment contain a diverse array of enzymes capable of degrading drug molecules. Salivary and mucosal proteases and peptidases including aminopeptidase A and N, carboxypeptidases, endopeptidases, dipeptidyl peptidase IV (DPP-IV), and neutral endopeptidase (NEP, neprilysin) are particularly deleterious for peptide and protein therapeutics, rapidly hydrolysing peptide bonds to render biologics such as insulin, calcitonin, GLP-1 analogues, and vasopressin biologically inactive before absorption [26,27].

Cytochrome P450 enzymes notably CYP3A4, CYP3A5, and CYP2C9expressed in buccal epithelial cells constitute a pre-systemic metabolic barrier, catalysing phase I metabolic reactions. Phase II enzymes including sulfotransferases, UDP-glucuronosyltransferases, and glutathione S-transferases are also present. Additionally, efflux transporters P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2)are expressed on buccal epithelial cells and actively export absorbed drug molecules back into the mucosal lumen, significantly reducing net transmembrane flux of substrates including paclitaxel, vinca alkaloids, cyclosporine, and numerous other drugs [28,29].

3.3 Physicochemical Barriers

The physicochemical properties of drug molecules themselves impose fundamental constraints on buccal permeation. Passive transcellular diffusion the dominant absorption mechanisms governed by lipophilicity (log P or log D at buccal pH), molecular weight, ionization state (pKa relative to buccal pH 6.5–7.5), hydrogen bond donor/acceptor count, and molecular geometry. Based on Lipinski's rule-of-five framework adapted for mucosal permeation, drugs with log P between 1–3, molecular weight below 500 Da, fewer than 5 hydrogen bond donors, and fewer than 10 hydrogen bond acceptors are optimal candidates for passive buccal absorption [30].

BCS Class III drugs (e.g., metformin, atenolol, acyclovir, cimetidine) suffer from low permeability despite high aqueous solubility due to their hydrophilic nature, high ionization at buccal pH, or significant hydrogen bonding capacity. BCS Class IV drugs (e.g., cyclosporine, ritonavir, curcumin, paclitaxel) face simultaneous solubility and permeability challenges, requiring dual pharmaceutical interventions. High molecular weight biologics such as insulin (5808 Da) and calcitonin (3432 Da) are essentially impermeable by passive diffusion through the intact buccal barrier without specialized enhancement strategies [31].

3.4 Salivary Washout and Dilution

Continuous salivary secretion and swallowing present a dynamic pharmacokinetic challenge to buccal drug delivery. At the unstimulated flow rate of 0.3–0.5 mL/min, buccal formulations are subjected to constant dilution, reducing drug concentration at the absorption site and mechanically eroding or dislodging dosage forms. The average residence time of conventional liquid or lozenge formulations in the buccal cavity typically 15–30 minutes substantially limits the dose deliverable via simple non-mucoadhesive formulations [32].

Swallowing reflex further reduces buccal drug availability, as saliva containing dissolved drug is periodically cleared to the GI tract, where the drug may be subject to the same first-pass metabolism and degradation that the buccal route aims to circumvent. Mucoadhesive formulations employing bioadhesive polymers (carbopol, HPMC, chitosan, sodium alginate) substantially mitigate salivary washout by anchoring the dosage form to the mucosal surface and maintaining drug concentration at the absorption interface [33].

3.5 Patient-Related Variables

Inter- and intra-individual variability in buccal drug absorption is clinically significant. Age-related changes including reduced salivary flow (xerostomia, affecting up to 30% of elderly patients), thinning of the epithelium, and altered mucin glycosylation modify barrier properties and can either increase or decrease drug absorption unpredictably. Xerostomia, common among patients undergoing head-and-neck radiotherapy, taking anticholinergic or diuretic medications, or with Sjögren's syndrome, impairs dosage form dissolution and reduces the lubricating film essential for mucoadhesive formulation performance [34].

Oral mucosal diseases  aphthous ulcers, oral lichen planus, pemphigus vulgaris, and radiation-induced oral mucositis alter epithelial integrity, alter inflammatory mediator profiles, and change local blood flow, potentially modifying drug absorption in unpredictable directions. Tobacco smoking induces mucosal thickening and focal keratinization that reduces permeability. Oral microbiome composition influences local enzymatic activity; dysbiosis may alter the enzymatic barrier. These sources of biological variability represent a key challenge for buccal formulation predictability and must be considered in clinical trial design [35].

4. PATHWAYS OF DRUG ABSORPTION ACROSS BUCCAL MUCOSA:

4.1 Transcellular Lipophilic Pathway

The transcellular pathway involves the sequential partitioning of drug molecules from the mucosal surface into the apical cell membrane, traversal through the cytoplasmic compartment, exit via the basolateral membrane, and transit across the subepithelial connective tissue to reach the vasculature. This route predominates for lipophilic, uncharged molecules with intermediate molecular weight (200–500 Da) and log P values of 1–3. Drugs such as fentanyl (log P 4.05), buprenorphine (log P 4.98), nicotine (log P 1.17), testosterone (log P 3.32), and estradiol (log P 4.01) traverse the buccal mucosa predominantly via this pathway. The rate-limiting step is partitioning into and across the lipid-rich intercellular lamellae [36].

4.2 Paracellular Aqueous Pathway

The paracellular route involves drug transport through the aqueous channels and intercellular junctions connecting adjacent epithelial cells. While theoretically providing a direct aqueous conduit for hydrophilic drugs, this route is restricted by tight junction complexes (zonula occludens) at the apical epithelial surface, which regulate paracellular permeability with high specificity. The tight junction proteins claudins (1, 4, 7, 23), occludin, junctional adhesion molecules (JAMs), and ZO-1/ZO-2 scaffolding proteinsform a continuous intercellular seal that limits paracellular transport to small molecules below approximately 200 Da and discriminates based on charge (cation vs. anion selectivity) [37].

The paracellular route is the primary target for modulation by permeation enhancers such as chitosan, EDTA, zonula occludens toxin (ZOT) analogues, and certain fatty acids that act on tight junction proteins to transiently widen paracellular channels, enabling the paracellular transport of larger hydrophilic molecules [38].

4.3 Carrier-Mediated and Endocytic Pathways

Active and facilitated transport mechanisms involving membrane-bound carrier proteins contribute to the buccal absorption of structurally specific drugs. Peptide transporters (PEPT1, PEPT2), organic anion transporters (OATs), organic cation transporters (OCTs), and amino acid transporters have been identified on buccal epithelial cells, though their quantitative contribution to drug absorption is generally modest compared to passive routes. Endocytic pathwaysclathrin-mediated endocytosis, caveolae-mediated endocytosis, and micro pinocytosis facilitate the uptake and transcytosis of nanoparticulate carriers, macromolecules, and surface-functionalized drug delivery systems. Transcytosis provides a mechanism for the buccal delivery of large molecules such as antibody fragments, recombinant proteins, and nanoparticle-drug conjugates [39,40].

5. CLASSIFICATION AND SELECTION CRITERIA FOR PERMEATION ENHANCERS:

5.1 Definition and Ideal Properties

Permeation enhancers are defined as pharmacologically inactive excipients that, upon incorporation into a drug delivery system or co-administration with a drug, facilitate drug transport across biological membranes by modifying barrier properties transiently and reversibly, without causing permanent tissue damage or systemic pharmacological effects at the concentrations employed. An ideal buccal permeation enhancer should satisfy the following criteria: (i) pharmacologically inert with no intrinsic receptor activity; (ii) capable of producing rapid, reproducible, and concentration-dependent enhancement; (iii) fully reversible in action with complete mucosal integrity restoration; (iv) non-toxic, non-irritating, and non-sensitizing; (v) compatible with all formulation components; (vi) chemically and physically stable over the intended shelf life; and (vii) acceptable organoleptic properties including taste, odour, and texture for patient compliance [41].

In practice, no single enhancer satisfies all criteria optimally. The selection process requires careful evaluation of mechanism of action, efficacy at safe concentrations, mucosal safety profile, compatibility with the drug and formulation matrix, and regulatory acceptability. Combination strategies using two or more agents with complementary mechanisms at sub-toxic individual concentrations often represent the optimal practical approach [42].

5.2 Classification by Chemical Nature

Table 1: Classification of Buccal Permeation Enhancers: Representative Agents, Mechanisms, and Pharmaceutical Applications

5.3 Mechanistic Classification

From a mechanistic perspective, permeation enhancers can be categorised into: (i) transcellular enhancers that act on the lipid bilayers of the epithelial cell membranes and intercellular lipid domains; (ii) paracellular enhancers that modulate tight junction proteins to widen aqueous intercellular channels; (iii) enzymatic inhibitors that reduce pre-systemic drug metabolism within the buccal mucosa; (iv) efflux pump inhibitors that prevent active export of absorbed drug molecules; and (v) mucoadhesion-enhancing agents that prolong formulation contact with the mucosal surface. Many agents operate through more than one of these mechanisms simultaneously, and the dominant mechanism may shift with concentration or formulation context [43].

5.4 Selection Framework

The selection of a permeation enhancer is guided by the drug's physicochemical properties and target absorption pathway. Hydrophilic, high-molecular-weight drugs that require paracellular transport augmentation are best served by cationic polymers (chitosan), divalent cation chelators (EDTA, citric acid), or tight junction modulators (ZOT analogues). Lipophilic drugs requiring transcellular enhancement benefit from fatty acids, terpenes, and bile salts that disorder the intercellular lipid lamellae. The formulation platform (patch, gel, tablet, nanoparticle) determines compatible enhancer physicochemical properties and affects enhancer distribution and contact time with the mucosa [44].

6. MECHANISMS OF ACTION OF PERMEATION ENHANCERS:

6.1 Disruption of Intercellular Lipid Bilayers

Disruption of the intercellular lipid lamellae is the most mechanistically studied and pharmacologically significant pathway by which permeation enhancers augment buccal drug transport. Fatty acids (oleic acid, capric acid, lauric acid), bile salts (sodium deoxycholate, sodium glycocholate), non-ionic surfactants, and terpenes interact with the hydrophobic interior of lipid bilayers to increase membrane fluidity, reduce the degree of lipid chain order, and disrupt the lamellar organization. This is detectable by ATR-FTIR spectroscopy as a shift in the CH2 symmetric stretching wavenumber from approximately 2848 to 2854 cm-1, indicating transition from a gel-phase to liquid-crystalline phase [45].

Oleic acid, a cis-monounsaturated C18 fatty acid, is among the most extensively studied transcellular enhancers for buccal delivery. Its kinked hydrocarbon chain inserts into the lipid bilayer, disrupting the regular packing geometry of saturated lipid chains and creating permanent defects or fluid domains that facilitate drug diffusion. Enhancement ratios (ER) of 3–15-fold have been reported for oleic acid across diverse drug substrates depending on concentration (typically 0.5–5% w/w), drug physicochemical properties, and formulation vehicle. Medium-chain fatty acids (C8–C12), particularly capric acid (C10) and lauric acid (C12), demonstrate comparable transcellular enhancement with generally superior safety profiles compared to oleic acid at equivalent concentrations [46,47].

6.2 Modulation of Tight Junctions

Tight junction (TJ) modulation represents the primary mechanism for augmenting paracellular drug transport across the buccal epithelium, and is the dominant mechanism of action of chitosan and its derivatives, chelating agents (EDTA, EGTA), and bile salts. TJ opening increases the effective pore size of the paracellular channel from the basal 3–5 nm radius to considerably larger dimensions, enabling the transit of hydrophilic molecules that would otherwise be excluded [48].

Chitosan a cationic polysaccharide derived from crustacean chitin deacetylation exerts its TJ-modulating effect through its positively charged amino groups (pKa ~6.5) interacting with the negatively charged components of TJ protein complexes. Chitosan has been shown to redistribute claudin-4, occludin, and ZO-1 proteins from the paracellular junctions to intracellular compartments (a process analogous to TJ internalization), transiently widening paracellular permeability. This effect is concentration-dependent, pH-dependent (chitosan is maximally cationic below pH 6.5 and requires protonation for TJ interaction), and fully reversible within 30–120 minutes following chitosan removal [49].

N-trimethyl chitosan (TMC)a permanently cationic quaternary ammonium derivative retains TJ-opening activity independent of pH, overcoming the pH limitation of unmodified chitosan in the neutral buccal environment. Thiolated chitosan’s (thiomers) with pendant thiol groups provide additional TJ-modulating activity through disulfide bond formation with cysteine-rich TJ proteins, while simultaneously offering improved mucoadhesive performance through covalent thiol-mucin interactions [50].

Chelating agents such as disodium EDTA and EGTA disrupt TJ integrity through a distinct mechanism chelation of Ca2+ ions that are essential for the maintenance of TJ protein conformation and ZO protein scaffolding. Calcium depletion causes rapid TJ disassembly with a consequent increase in paracellular flux. While highly effective, EDTA-mediated TJ disruption is less target-selective than chitosan, potentially disrupting barrier integrity more broadly and raising safety concerns about co-absorption of unintended macromolecular substances [51].

6.3 Solubilization and Extraction of Membrane Lipids

At concentrations above their critical micellar concentration (CMC), surfactants and bile salts form mixed micelles with the lipid components of the intercellular domains, effectively extracting these lipids from the membrane and creating transient aqueous channels. This solubilization mechanism is concentration-dependent and partially irreversible at higher concentrationsa key safety concern that necessitates strict formulation concentration control. Cyclodextrins, particularly methylated-beta-cyclodextrin (RAMEB), solubilize membrane cholesterol and phospholipids through inclusion complex formation, disrupting lipid bilayer organization and increasing membrane fluidity [52].

6.4 Enzymatic Inhibition

Enzyme inhibitor permeation enhancers do not directly modify the membrane barrier but instead protect drug molecules (particularly peptides and proteins) from enzymatic degradation within the buccal environment, effectively increasing the concentration of intact drug available for absorption. Aprotinin (a Kunitz-type serine protease inhibitor), bacitracin (a cyclic peptide protease inhibitor), the Bowman-Birk inhibitor (a soybean-derived protease inhibitor), and synthetic inhibitors such as nafamostat and camostat suppress salivary and mucosal protease activity, substantially extending the half-life of peptide drugs in the buccal cavity [53,54].

DPP-IV inhibitors are particularly relevant for buccal GLP-1 analogue delivery DPP-IV is abundantly expressed on epithelial surfaces and rapidly degrades the N-terminal dipeptide of GLP-1, rendering it inactive. Co-formulation of GLP-1 analogues with DPP-IV inhibitors such as sitagliptin within buccal films represents a rational strategy for combining enzyme protection with controlled release [55].

6.5 Efflux Pump Inhibition

P-glycoprotein (P-gp) and other ABC transporters expressed on the apical surface of buccal epithelial cells actively export drug molecules that have entered the transcellular pathway back into the mucosal lumen, limiting net transepithelial flux. Agents that inhibit these efflux pumps including verapamil (P-gp inhibitor), GF120918 (elacridar, dual P-gp/BCRP inhibitor), quercetin, and piperine can substantially increase the net transepithelial absorption of efflux-susceptible drugs. Vitamin E TPGS (D-alpha-tocopheryl polyethylene glycol 1000 succinate) is a particularly well-characterised multifunctional excipient that simultaneously solubilizes poorly soluble drugs, enhances membrane permeability through surfactant action, and inhibits P-gp and BCRP efflux at low concentrations [56].

6.6 Mucoadhesion-Based Enhancement

Mucoadhesive permeation enhancers primarily biopolymers such as chitosan, sodium alginate, Carbopol (polyacrylic acid), hyaluronic acid, and gelati contribute to permeation enhancement indirectly by prolonging the residence time of the drug and associated enhancer at the mucosal surface. Extended mucosal contact time translates to a sustained high drug concentration gradient across the epithelium, maximising the thermodynamic driving force for passive diffusion and the duration of direct enhancer-membrane interaction. Mucoadhesion is mediated by physical entanglement between polymer chains and mucin glycoprotein strands, hydrogen bonding, and electrostatic interactions [57].

7. APPLICATIONS ACROSS DRUG CATEGORIES:

7.1 Peptide and Protein Drugs

The buccal delivery of therapeutic peptides and proteins represents the most challenging but clinically significant application of permeation enhancers, driven by the exponential growth of biologic therapies and the unmet need for non-injectable administration routes. Insulin is the most extensively studied protein for buccal delivery, representing both the scientific archetype and commercial aspiration of transmucosal biologics delivery. Insulin's pharmacokinetic challenges hydrophilicity, high molecular weight (5808 Da), susceptibility to salivary proteolysis, and extremely low passive permeability require a multi-pronged enhancement strategy incorporating enzyme inhibition, TJ modulation, and membrane perturbation [58].

Buccal insulin delivery systems have combined sodium deoxycholate (lipid fluidizer + TJ modulator) with protease inhibitors (aprotinin, bacitracin) to achieve enhancement ratios of 10–30-fold compared to unaided buccal delivery in animal models. Lectin-functionalized microspheres targeting buccal epithelial cell surface receptors have demonstrated receptor-mediated endocytosis-based insulin uptake. Despite the promise of commercial products such as Oralin (Generex) buccal insulin spray the first buccal insulin product to receive regulatory approval in Ecuador and India clinical adoption has been limited by pharmacokinetic variability and inconsistent glycaemic control [59].

Salmon calcitonin (sCT, 3432 Da), used in osteoporosis and Paget's disease, and desmopressin (a vasopressin analogue, 1069 Da) for diabetes insipidus and enuresis have been investigated for buccal delivery with bile salt and chitosan enhancers, demonstrating moderate but clinically relevant bioavailability’s of 5–15% in animal models. Cyclosporine (BCS Class IV, 1202 Da), an immunosuppressant with highly variable oral bioavailability (10–89%), has been formulated in buccal gels and patches with combination fatty acid/bile salt enhancers, achieving more consistent systemic exposure profiles [60,61].

7.2 Cardiovascular Agents

]Cardiovascular drug delivery via the buccal route is clinically established for certain agents and represents an expanding research frontier for others. Nitro-glycerine (glyceryl trinitrate), a highly lipophilic vasodilator (log P 1.62, MW 227 Da) with essentially zero oral bioavailability due to complete first-pass metabolism, is absorbed rapidly through the buccal and sublingual mucosa without requiring exogenous permeation enhancers its physicochemical properties allow direct transcellular permeation. Buccal nitro-glycerine formulations (Suscard Buccal) provide sustained release over several hours and are used for the prophylaxis of angina [62].

Propranolol (BCS Class II, log P 2.97) and metoprolol (BCS Class II, log P 1.88) have been studied in buccal formulations to bypass their extensive hepatic first-pass metabolism (bioavailability 25–35% and 40–50%, respectively, via oral route). Buccal patches incorporating chitosan and propylene glycol as combined mucoadhesive/permeation enhancers have demonstrated 2–4-fold improvements in propranolol buccal flux in ex vivo studies. Verapamil buccal patches with oleic acid and sodium lauryl sulphate as enhancers have shown substantially improved flux and bioavailability in animal models [63].

7.3 Central Nervous System Drugs

CNS drug delivery via the buccal route offers particular advantages for rescue therapy in neurological emergencies and for medications requiring avoidance of first-pass metabolism. Buprenorphine (log P 4.98, BCS Class II), a partial mu-opioid receptor agonist widely used in opioid dependence treatment and chronic pain management, is commercially available as buccal film (Belbuca) and sublingual tablet (Subutex), achieving therapeutic plasma levels within 30–60 minutes. The commercial buccal film formulation demonstrates bioavailability comparable to sublingual administration owing to the drug's inherent lipophilicity, without requiring exogenous permeation enhancers [64].

Fentanyl buccal tablet (Fentora, Cephalon) employs an effervescent system combining citric acid and sodium bicarbonate upon contact with saliva, the effervescent reaction transiently increases local pH from 7.0 to approximately 8.0, shifting the ionization equilibrium of fentanyl (pKa 8.99) toward the uncharged form, thereby increasing the fraction of highly lipophilic unionized drug available for rapid transcellular absorption. This pH-shift strategy constitutes an elegant physicochemical permeation enhancement approach, and clinical studies demonstrated onset of analgesic effect within 10–15 minutes, significantly faster than conventional oral opioids [65,66].

Midazolam, a water-soluble benzodiazepine with established first-line use in seizure rescue therapy, has been formulated in buccal solutions and gels with ethanol, cyclodextrin, and chitosan enhancers for paediatric and adult seizure management. Several pharmaceutical companies have pursued mucoadhesive buccal films of clonazepam, lorazepam, and alprazolam for anxiety and seizure applications [67].

7.4 Anti-Infective Agents

Acyclovir (BCS Class III, MW 225 Da, low permeability despite reasonable aqueous solubility) is the most extensively investigated anti-infective for buccal delivery. Its hydrophilic nature and low passive permeability necessitate permeation enhancement; studies have evaluated sodium lauryl sulphate, polysorbate 80, sodium glycocholate, and chitosan as enhancers for buccal mucoadhesive tablets and films. Chitosan-based systems incorporating sodium caprate (C10) have demonstrated up to 5–7-fold enhancement in acyclovir permeation across porcine buccal mucosa ex vivo, with significantly improved bioavailability in rabbit models [68].

Metronidazole and tinidazole, both BCS Class I drugs with adequate aqueous solubility but subject to first-pass hepatic metabolism, have been investigated in buccal films with propylene glycol and sodium deoxycholate as combined enhancers. Antifungal triazoles (fluconazole, itraconazole, voriconazole) with poor and variable oral bioavailability have been explored in buccal films using cyclodextrin-based solubilization and bile salt-based permeation enhancement to improve systemic exposure in oropharyngeal and systemic fungal infections [69].

7.5 Antidiabetic Drugs

Metformin hydrochloride, the universal first-line antidiabetic, is a BCS Class III drug characterized by high aqueous solubility but extremely poor membrane permeability (due to its highly hydrophilic cationic guanidinium nature). Its oral bioavailability is limited to approximately 50–60% even at optimal doses, and GI adverse effects (nausea, diarrhoea) are frequent. Buccal delivery of metformin has been investigated as a strategy to simultaneously improve bioavailability and reduce GI adverse effects. Mucoadhesive buccal tablets incorporating chitosan and HPMC with oleic acid as permeation enhancer have demonstrated moderate improvements in metformin buccal flux [70].

Glibenclamide (BCS Class II) mucoadhesive patches with sodium lauryl sulphate, propylene glycol, and oleic acid as combination permeation enhancers have achieved extended delivery over 6–8 hours with significantly improved bioavailability in preclinical models. The clinical opportunity for buccal antidiabetic drug delivery is significant, given the large patient population requiring chronic pharmacotherapy and the dose-size limitations inherent to the buccal route for high-dose drugs such as metformin [71].

7.6 Anticancer Agents

The buccal route for anti-cancer agents serves both local purposes (treatment of oral mucositis, oral leukoplakia, oral squamous cell carcinoma) and systemic delivery (chemotherapy for circulating and distant tumours). 5-Fluorouracil (5-FU, BCS Class III) has been formulated in buccal mucoadhesive films incorporating poloxamer, chitosan, and carbopol for the local management of oral leukoplakia and potentially for hepatic metastasis management. Controlled release over 6–8 hours with sustained mucosal drug concentrations has been demonstrated in preclinical studies [72].

Tamoxifen, an estrogen receptor antagonist with extensive hepatic first-pass metabolism converting it to the active metabolite endoxifen (with high inter-individual CYP2D6 variability), has been investigated for buccal delivery to bypass CYP2D6-mediated metabolic variability and improve dose predictability using oleic acid and bile salt enhancers. EGFR tyrosine kinase inhibitors (erlotinib, gefitinib) with poor and variable oral bioavailability have been explored in buccal nano emulsion systems. A unique challenge is balancing the need for high systemic exposures with mucosal safety, as the same cytotoxicity that targets tumour cells can damage buccal epithelial cells requiring careful therapeutic window analysis [73].

7.7 Natural Products and Herbal Bioactive

Curcumin, the polyphenolic bioactive principal of turmeric, exemplifies the dual BCS Class IV challenge negligible aqueous solubility (solubility ~11 ng/mL) combined with very low membrane permeability, resulting in oral bioavailability below 1% in humans. Multiple buccal delivery strategies have been investigated: HP-beta-cyclodextrin complexation (achieving 900-fold solubility enhancement), phospholipid complexes (phytosomes), self-nanoemulsifying systems (SNEDDS), and nanostructured lipid carriers incorporating medium-chain fatty acids as both solubilizers and permeation enhancers [74].

Quercetin, resveratrol, berberine, and other polyphenolic flavonoids with similar BCS Class IV profiles have been investigated in buccal nano emulsion gels and liposomal films with lipid-based enhancers. Piperine, the alkaloid from black pepper, is a particularly interesting compound as it not only suffers from poor bioavailability itself (BCS Class IV) but simultaneously functions as a bioavailability enhancer for co-administered drugs through CYP3A4 inhibition and P-gp modulation creating a bifunctional natural permeation enhancer opportunity [75].

8. FORMULATION STRATEGIES INCORPORATING PERMEATION ENHANCERS:

8.1 Mucoadhesive Films and Patches

Mucoadhesive buccal films and patches are the most extensively researched and commercially successful dosage forms for buccal drug delivery. These thin, flexible polymer matrices adhere to the buccal mucosa, maintain intimate contact with the absorptive surface, provide controlled drug release, and incorporate permeation enhancers in close proximity to the absorption site. Film-forming polymers commonly include HPMC (E5, E15, K4M), HPC, CMC, polyvinyl alcohol (PVA), polyethylene oxide (PEO), carbopol 934/940P, and sodium alginate, often in combinations optimized for flexibility, tensile strength, and mucoadhesive force [76].

Bilayer (bi-laminate) patch designs address the fundamental limitation of unidirectional drug delivery. The drug-containing layer faces the mucosa (releasing drug toward the absorptive surface), while a backing layer of hydrophobic polymer (ethyl cellulose, Eudragit RL100/RS100) prevents drug loss into the saliva. Permeation enhancers are incorporated primarily in the mucosal layer to maximize concentration at the epithelial interface. Commercial products such as Belbuca (buprenorphine buccal film) and Striant (testosterone buccal system) exemplify the successful application of this design approach [77].

8.2 Mucoadhesive Tablets

Buccal mucoadhesive tablets prepared by direct compression or wet granulation offer advantages of precise dose delivery, robust physical stability, and scalable manufacturing. Carbopol, HPMC, sodium CMC, and chitosan are the most widely used polymers; their hydrogel formation upon saliva contact creates a viscous layer promoting mucosal adhesion and controlling drug release over 6–12 hours. Drug and enhancer are co-embedded in the polymer matrix, with enhancer release kinetics governing the drug absorption window [78].

Effervescent buccal tablets incorporating citric acid/tartaric acid with sodium bicarbonate create transient CO2 evolution upon saliva contact, promoting tablet disintegration, mucosal wetting, pH modulation, and drug solubilization. This approach exemplified commercially by Fentorasimultaneously optimizes drug dissolution and creates a physicochemical permeation enhancement effect through pH-dependent ion suppression [79].

8.3 Gels and In Situ Gelling Systems

Buccal gels and semi-solid systems offer formulation flexibility and ease of application, particularly for paediatric and geriatric patients, and for drug substances that cannot be compressed into tablets. Bio adhesive hydrogels (carbopol, poloxamer 407, gelatin, hyaluronic acid) can be engineered for in situ gelation upon buccal application through pH-triggered (carbopol, low viscosity below pH 5, gelling at buccal pH 6.5–7.5), thermosensitive (poloxamer 407, liquid below 25°C, gelling at 35–37°C), or ion-triggered (gellan gum, pectin gelating via Ca2+ in saliva) mechanisms [80].

Thermosensitive poloxamer-based gels loaded with poorly soluble drugs and medium-chain fatty acid permeation enhancers have demonstrated sustained buccal drug delivery over 4–8 hours in animal models, with the sol-gel transition at body temperature providing excellent initial mucosa-coating characteristics followed by prolonged gelled contact. The gel matrix concentrates the permeation enhancer at the mucosal interface and simultaneously provides enzyme protection for peptide drugs [81].

8.4 Nanoparticulate Systems

Nanoparticulate drug delivery systems represent a paradigm shift in buccal delivery, enabling simultaneous drug solubilization, enzymatic protection, enhanced mucosal retention, and modified intracellular delivery. Polymeric nanoparticles (PLGA, chitosan, gelatin) can encapsulate both drug and permeation enhancer, providing spatially and temporally controlled co-delivery at the mucosal surface. The nanometric size (100–400 nm range optimal for buccal mucosa interaction) enhances contact efficiency with the epithelial surface and enables endocytic uptake mechanisms not accessible to larger particles [82].

Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) are particularly well-suited for incorporating lipophilic permeation enhancers alongside poorly soluble drugs within their lipid matrix. The lipid matrix serves dual functions solubilizing the drug and delivering the enhancer directly to the lipid-rich intercellular domains of the buccal epithelium. Chitosan-coated PLGA nanoparticles combine mucoadhesion (surface chitosan) with TJ modulation (chitosan) and sustained drug/enhancer release (PLGA matrix), offering a multifunctional platform for buccal delivery of macromolecular drugs [83].

Self-nanoemulsifying drug delivery systems (SNEDDS) spontaneously forming O/W nanoemulsions upon dilution with buccal fluid represent a practical approach for BCS Class II/IV drugs. The oil phase (often medium-chain fatty acids with inherent permeation enhancement activity), surfactant, and co-surfactant can be optimized to simultaneously maximize drug solubilization and enhancer delivery, with minimal formulation complexity suitable for manufacturing scale-up [84].

8.5 Vesicular and Colloidal Systems

Vesicular drug delivery systems liposomes, transfersomes, ethosomes, niosomes, and dendrimers offer unique advantages for buccal drug delivery through their membrane-biomimetic structures and cargo protection capabilities. Conventional liposomes, while limited by poor mucosal stability, are modified by deformability (transfersomes with edge activators such as sodium cholate), ethanol-enrichment (ethosomes, where ethanol simultaneously acts as permeation enhancer and vesicle component), or coating with mucoadhesive polymers (chitosan-coated liposomes) to improve buccal performance [85].

Transfersomes are particularly noteworthy highly elastic, stress-responsive vesicles capable of squeezing through epithelial junctions with a driving force provided by the transepithelial osmotic gradient. They have demonstrated substantially improved transmucosal delivery of insulin and calcitonin in animal models compared to conventional liposomes. Dendrimers (generation 4 PAMAM dendrimers surface-modified with fatty acid or amino acid chains) provide a multivalent platform for permeation-enhancing surface chemistry alongside drug encapsulation [86].

9. EVALUATION METHODS FOR BUCCAL PERMEATION ENHANCEMENT:

9.1 In Vitro Permeation Models

Franz diffusion cell studies employing excised porcine buccal mucosa constitute the standard preliminary screening tool for assessing buccal drug permeation and enhancer efficacy. Porcine buccal tissue is preferred due to its histological and biochemical similarity to human tissue comparable epithelial thickness, lipid composition, and enzymatic activity profile. The tissue is mounted between donor compartment (drug formulation, typically 0.5–1 cm2 exposure area) and receptor compartment (PBS pH 7.4 or simulated salivary fluid, 5–10 mL) maintained at 37°C with constant stirring. Cumulative permeation versus time profiles is generated from periodic receptor fluid sampling and drug quantification by HPLC [87].

Cell-based permeation models using the TR146 human buccal carcinoma cell line (forming stratified semi-differentiated epithelial monolayers with TEER values of 80–150 Ω·cm2) provide mechanistic information on transcellular and paracellular transport pathways. The parallel artificial membrane permeability assay (PAMPA) adapted for buccal conditions using phospholipid-impregnated synthetic membranes enables high-throughput permeability screening for large compound libraries during early drug discovery [88].

9.2 Ex Vivo and In Vivo Models

Ex vivo permeation studies using freshly excised buccal tissue (porcine, bovine, ovine, or human cadaveric) provide more physiologically relevant assessments than cell culture models, as they preserve the intact epithelial architecture including the intercellular lipid barrier. Tissue viability is confirmed by MTT reduction, LDH release, and histological examination before and after experimentation. Ex vivo models also enable safety sectioning of enhancer-treated tissue reveals epithelial lifting, intercellular space widening, and nuclear condensation as markers of mucosal damage [89].

In vivo models range from rodent (rat buccal pouch, hamster cheek pouch) to porcine and canine models for pharmacokinetic studies. The hamster cheek pouches an easily accessible, everted mucosal sacis the most widely used in vivo buccal model for proof-of-concept studies. Porcine models are preferred for studies intended to support regulatory submissions due to anatomical and physiological similarity to humans. Clinical pharmacokinetic studies comparing buccal formulations against intravenous (absolute bioavailability) and oral reference (relative bioavailability) formulations in healthy volunteers or patients represent the definitive translational evidence standard [90].

9.3 Biophysical Characterization Techniques

ATR-FTIR spectroscopy of enhancer-treated buccal tissue provides molecular-level evidence of lipid fluidization shifts in the CH2 symmetric stretching frequency from ~2848 to ~2854 cm-1 indicate lipid disordering. DSC of tissue lipid extracts reveals shifts in lipid phase transition temperatures, quantifying the degree of membrane perturbation. TEER (transepithelial electrical resistance) measurements using Ussing-type chambers or Millicell systems quantify changes in paracellular permeability in cell monolayer models with high sensitivity and real-time resolution [91].

Confocal laser scanning microscopy (CLSM) using fluorescently labelled drug surrogates or enhancers enables visualization of penetration depth and distribution pathways within epithelial layers. Transmission electron microscopy (TEM) visualizes ultrastructural changes in intercellular lipid lamellae and tight junction morphology following enhancer exposure, providing direct mechanistic evidence at the nanoscale. Fluorescence recovery after photobleaching (FRAP) and two-photon microscopy offer dynamic information on lipid mobility and drug diffusion dynamics within the epithelium [92].

9.4 Key Permeability Parameters

Table 2: Key Parameters and Evaluation Methods for Buccal Permeation Studies

10. SAFETY, TOXICOLOGY, AND REGULATORY PERSPECTIVES:

10.1 Local Mucosal Toxicity

The safety of permeation enhancers at the buccal mucosal surface is the most critical determinant of clinical viability. Since buccal formulations are intended for repeated administration in chronic conditions, any enhancer-induced mucosal damage must be strictly transient and fully reversible within the dosing interval. Irritation, erythema, and localized inflammation are the most common adverse effects at effective enhancer concentrations. At excessive concentrations or after prolonged exposure, more severe effects epithelial ulceration, desquamation, and submucosal oedema can occur with ionic surfactants (SDS), unconjugated bile salts, and strong chelating agents (EDTA) [93].

Histological assessment of enhancer-treated mucosal tissue examining epithelial lifting, basement membrane disruption, inflammatory infiltrate, and vascular changes provides the most definitive local toxicity evidence. Ionic surfactants (SDS, cetylpyridinium chloride) exhibit the highest mucosal irritation potential and must be employed at the minimum effective concentration, typically below 0.5% w/w. Non-ionic surfactants (polysorbate 80, Brij-35) demonstrate superior mucosal safety profiles at equivalent permeation-enhancing concentrations [94].

10.2 In Vitro Cytotoxicity Evaluation

The TR146 buccal cell line is the standard model for first-line cytotoxicity screening of permeation enhancers. The MTT reduction assay quantifies mitochondrial metabolic activity as a surrogate for cell viability; LDH release indicates membrane lysis; and neutral red uptake assesses lysosomal integrity. IC50 values are determined for each enhancer, and the working formulation concentration is typically set at one-tenth of the IC50 value to provide a 10-fold safety margin. The safety index (SI = IC50 / effective enhancing concentration) provides a practical parameter for early-stage enhancer selection [95].

Structure-activity relationships of enhancer toxicity have revealed important principles: fatty acid cytotoxicity increases with decreasing chain length (medium-chain > long-chain in terms of toxicity at equal concentrations); ionic surfactants are generally 5–10 times more toxic than non-ionic surfactants; unconjugated bile salts are more toxic than conjugated forms; and terpenes demonstrate concentration-dependent transitions from safe enhancement to cytotoxicity over relatively narrow concentration ranges. These SAR insights guide rational enhancer concentration optimization [96].

10.3 Reversibility Assessment

Reversibility complete restoration of mucosal barrier integrity following enhancer removal is a non-negotiable requirement for clinical-stage permeation enhancers. TEER recovery after enhancer washout is the standard quantitative measure in cell models; a return to baseline TEER values within 1–4 hours following washing is generally considered acceptable for a dosing interval of 8–24 hours. Chitosan-induced TJ opening has been demonstrated to be fully reversible within 30–120 minutes in TR146 cell models [97].

In ex vivo tissue models, post-enhancer histological integrity and permeability recovery of a marker molecule (mannitol, fluorescein) after washing provides additional reversibility evidence. The distinction between concentration-dependent reversible enhancement and irreversible membrane damage is criticalthe therapeutic concentration range must remain well below the irreversibility threshold, and this must be verified for each drug-enhancer-formulation combination [98].

10.4 Regulatory Framework

The regulatory acceptability of permeation enhancers in buccal drug products is governed by their classification as inactive pharmaceutical ingredients, GRAS status, and precedent in approved products. The FDA Inactive Ingredient Database specifies maximum acceptable amounts for buccal administration. GRAS-classified enhancers (polysorbate 80, propylene glycol, ethanol, lecithin, medium-chain fatty acids) have established safety precedent and are generally accepted at concentrations within specified limits. Chitosan has regulatory approval in numerous markets as a pharmaceutical excipient [99].

Novel permeation enhancers without established safety data require complete non-clinical packages per ICH guidelines before regulatory acceptance, including: mucosal irritation studies (ISO 10993-23 for cytotoxicity; dermal sensitization and irritation assays adapted for mucosal application), systemic toxicity (28-day repeated-dose toxicology), genotoxicity (Ames test, micronucleus assay), and local tolerance in an appropriate animal model. ICH Q8(R2) provides the framework for risk-based justification of enhancer selection and concentration. EMA guidelines on non-oral drug products (including mucosal routes) provide the European regulatory pathway [100].

11. RECENT ADVANCES AND EMERGING STRATEGIES:

11.1 Combination Enhancer Strategies

The recognition that different enhancer classes act on distinct mucosal barrier components has stimulated interest in combination strategies exploiting mechanistic synergism to achieve superior enhancement at lower individual enhancer concentrations than required for either agent alone. Synergistic combinations such as oleic acid (lipid fluidizer) with sodium deoxycholate (lipid extractor + TJ modulator), or chitosan (TJ opener + mucoadhesive) with EDTA (calcium chelator + TJ disruptor)have demonstrated enhancement ratios significantly exceeding additive predictions. The combination approach also improves the safety profile by allowing sub-toxic individual concentrations [101].

Ternary combination approaches incorporating an enzyme inhibitor alongside a physical permeation enhancer and a mucoadhesive polymer represent the emerging gold standard for peptide drug delivery. For instance, combinations of (i) chitosan (TJ modulator, mucoadhesive), (ii) bacitracin (protease inhibitor), and (iii) EDTA (TJ disruptor, metalloprotease inhibitor) have been shown to achieve synergistic enhancement for insulin buccal delivery across multiple animal model studies [102].

11.2 Nano-Enhancer Systems

Nanostructured lipid carriers (NLCs) incorporating permeation enhancers as structural lipid components represent a convergence of nanotechnology and chemical enhancement. In these systems, medium-chain fatty acids (capric, lauric, oleic acid) serve as both the liquid lipid component of the NLC matrix and the source of permeation enhancement upon NLC-mucosa interaction. The high surface area of nanoparticles and their close apposition to the epithelium amplify enhancer-membrane interactions substantially. NLCs of poorly soluble drugs have demonstrated enhancement ratios of 8–20-fold relative to unenriched aqueous dispersion in ex vivo buccal models [103].

Exosome-inspired lipid nanoparticles surface-modified with fusogenic lipids (DOPE, DOTAP) facilitate direct membrane fusion rather than endocytic uptake, delivering cargo directly to the cytoplasm and bypassing endosomal degradation. This approach has shown promise for nucleic acid and protein delivery via mucosal surfaces. Hybrid nanoparticles combining an inorganic silica or gold core with a lipid shell incorporating permeation enhancers offer additional capabilities including controlled release via silica mesopore architecture and plasmonic photothermal drug release triggering [104].

11.3 Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs)short (5–30 amino acid) cationic or amphipathic sequences capable of directly translocating across biological membranes represent a biomolecular permeation enhancement strategy with unique attributes for macromolecular drug delivery. The HIV-1 TAT peptide (residues 47–57: GRKKRRQRRRPQ), penetrating from the Antennapedia homeodomain, transporting, and Pep-1 have been studied as buccal enhancers for proteins, siRNA, and nanoparticles [105].

TAT peptide-functionalized nanoparticles have demonstrated significantly enhanced uptake across TR146 buccal cell monolayers and ex vivo porcine buccal tissue compared to unmodified nanoparticles, via combined micropinocytosis, caveolae-mediated endocytosis, and direct membrane translocation mechanisms. D-amino acid incorporation or N-methylation of CPP sequences substantially improves their proteolytic stability in salivary environments, addressing a key practical limitation for clinical translation [106].

11.4 Physical Enhancement Synergistic Approaches

Physical permeation enhancement techniques can be combined with chemical enhancers for synergistic or additive effects. Iontophoresis applies a small electrical current (typically 0.1–0.5 mA/cm2) to drive ionized drug molecules across the mucosa by electrophoresis and electroosmosis; when combined with TJ-modulating chemical enhancers that simultaneously widen the paracellular pathway, substantially augmented transmucosal flux of charged macromolecules is achievable. Studies combining iontophoresis with chitosan have demonstrated enhancement ratios of 50–100-fold for model macromolecules in ex vivo buccal tissue [107].

Microneedle arrays designed for buccal application whether solid polymer (PLGA, maltose), hollow (stainless steel), or dissolving formulations create transient microchannels (50–250 µm depth) in the buccal epithelium that rapidly flood with drug solution containing chemical enhancers, dramatically improving absorption without the mechanical discomfort of conventional injection. Buccal microneedle patches in clinical development have demonstrated substantially improved insulin delivery in preclinical models [108].

11.5 Mucus-Penetrating Nanoparticles

Dense PEG surface coatings on nanoparticles create mucus-penetrating particles (MPPs) that exploit steric shielding to resist adhesive interactions with mucin fibres, enabling rapid diffusion through the mucus layer toward the epithelial surface. This approach contrasts fundamentally with traditional mucoadhesive strategies rather than adhering to mucus (which is subsequently cleared by mucociliary action), MPPs penetrate through it to deliver drug directly to the epithelial surface where absorption occurs. MPPs loaded with poorly absorbed drugs and chemical permeation enhancers have demonstrated markedly improved buccal drug absorption compared to both free drug solutions and conventional mucoadhesive nanoparticles in preclinical models [109].

11.6 Artificial Intelligence and Machine Learning in Formulation Optimization

The application of AI and ML to buccal formulation development represents a transformative capability that can dramatically reduce the empirical trial-and-error burden. ML models trained on structured datasets of drug physicochemical properties, enhancer concentrations, polymer types, and observed permeation outcomes can predict optimal formulation compositions with accuracy exceeding traditional statistical design of experiments approaches. Deep learning models trained on molecular structure datasets can predict permeation enhancement activity of candidate excipients prior to synthesis, enabling de novo enhancer design [110].

Molecular dynamics simulations of drug-enhancer-membrane interactions at the atomistic level provide mechanistic insights unachievable by experimental means alone, guiding rational enhancer design and predicting the effects of molecular modifications on enhancement potency and selectivity. High-throughput buccal permeation screening platforms combining Franz cell automation with PAMPA assays and computational prediction can evaluate thousands of formulation combinations per month, compressing formulation development timelines from years to months [111].

11.7 Three-Dimensional Printed Buccal Dosage Forms

3D printing technologies fused deposition modelling (FDM), selective laser sintering (SLS), stereolithography (SLA), and inkjet/binder jetting have emerged as powerful tools for buccal dosage form fabrication with precise geometric, compositional, and drug release control impossible to achieve by conventional manufacturing. 3D-printed buccal patches with gradient permeation enhancer concentrations highest at the mucosal interface, lowest at the oral-facing surface maximize enhancement efficiency while minimizing systemic enhancer exposure. Personalized buccal dosage forms with patient-specific drug loading, release kinetics, and geometry can be produced on-demand using digital pharmaceutical manufacturing workflows [112].

Multi-material 3D printing enables simultaneous printing of drug-loaded, enhancer-containing, and impermeable backing layers within a single dosage form, achieving complex drug-enhancer spatial architectures that direct drug release unidirectionally and precisely. Inkjet printing of drug and enhancer solutions onto mucoadhesive polymer substrates allows sub-milligram dose precision critical for potent peptide and CNS drugs requiring tight dose control [113].

12. COMMERCIALLY MARKETED BUCCAL DRUG PRODUCTS:

Table 3: Commercially Approved and Marketed Buccal Drug Delivery Systems and Their Permeation Enhancement Strategies