Naincy Jain*
Antimicrobial resistance represents one of the defining biomedical challenges of our era. According to a landmark 2024 Lancet systematic analysis, bacterial AMR was associated with approximately 4.95 million deaths in 2019, with 1.27 million deaths directly attributable to resistant infections and figures that are projected to escalate dramatically without concerted global intervention [1]. The World Health Organization (WHO) has classified AMR as a global health emergency, identifying pathogens such as methicillin-resistant Staphylococcus aureus (MRSA), carbapenem-resistant Klebsiella pneumoniae, and extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli as priority threats [2][3].
A central, yet frequently underappreciated, contributor to therapeutic failure is the physicochemical inadequacy of existing antimicrobial agents rather than resistance alone. Approximately 40% of drugs in active development and up to 60% of synthesized small-molecule candidates suffer from poor aqueous solubility, a formidable barrier to effective drug delivery [4][5]. Drugs classified under the Biopharmaceutics Classification System (BCS) as Class II (low solubility, high permeability) or Class IV (low solubility, low permeability) present persistent challenges: inadequate absorption, unpredictable bioavailability, sub-therapeutic plasma concentrations, and dose-dependent toxicity arising from attempts to compensate through dose escalation [6][7].
The pharmacological arsenal against resistant microorganisms is further constrained by the physiological impermeability of some infection niches, notably bacterial biofilms, intracellular reservoirs within macrophages, and mucosal barriers which attenuate drug concentrations at the site of action. Biofilms, for instance, are known to resist antibiotic concentrations 100–1000 times higher than those effective against planktonic bacteria, rendering most conventional therapies inadequate [8][9].
Nanotechnology-based drug delivery systems have garnered considerable scientific and commercial attention as transformative solutions to these intertwined challenges [10]. Among the diversity of nano formulations, liposomes, polymeric nanoparticles, solid lipid nanoparticles, dendrimers, nanoemulsions[11]. Nanosuspensions occupy a unique niche by virtue of their simplicity, they consist essentially of pure drug nanocrystals without a carrier matrix, thus offering exceptionally high drug loading, minimal excipient burden, and relatively straightforward scale-up [12].
The present review comprehensively examines the scientific landscape of nanosuspensions as platforms for antimicrobial drug delivery. It critically analyzes preparation methodologies, characterization tools, mechanistic insights, therapeutic applications across diverse infectious disease categories, and addresses the evolving safety considerations.
- Concept and Physicochemical Principles of Nanosuspensions
Nanosuspensions are biphasic colloidal systems comprising solid drug particles reduced to the nanometer range (typically 10–1000 nm), dispersed in an aqueous or non-aqueous dispersion medium and stabilized by appropriate surfactants or polymers [13]. Distinct from liposomes or polymer-matrix nanoparticles in which the drug is encapsulated within or adsorbed onto a carrier material, nanosuspensions consist predominantly of 100% active pharmaceutical ingredient (API), with excipients limited to stabilizers and dispersion media. This carrier-free architecture confers exceptionally high drug loading, practical scalability, and compatibility with a broad range of API chemistries [14].
2.1 Ostwald–Freundlich and Noyes–Whitney Principles
The pharmaceutical rationale for nanosuspension technology is based on two physicochemical laws. The Ostwald–Freundlich equation relates particle curvature to enhanced saturation solubility: as particle radius decreases below 1 μm, solubility increases nonlinearly, yielding a substantially larger concentration gradient driving dissolution [15]. The Noyes–Whitney equation quantifies this effect: the rate of dissolution is directly proportional to surface area (A) and the solubility gradient (Cs − C), and inversely proportional to diffusion layer thickness [16]. Nanosizing simultaneously maximizes surface area and steepens the gradient, producing dissolution rates of magnitude faster than bulk drug particles [17]. These phenomena collectively translate into measurably improved oral bioavailability, particularly for BCS Class II drugs such as itraconazole, clofazimine, and rifampicin [18].
2.2 Stabilization Mechanisms
Because nanoscale particles carry high surface free energy, they are thermodynamically predisposed to aggregation and Ostwald ripening, a process in which smaller particles dissolve and re-deposit onto larger ones, progressively shifting the size distribution toward coarser particles [19]. Stabilizers counteract this tendency via two primary mechanisms: steric stabilization, in which polymer chains (e.g., HPMC, PVP, poloxamers) adsorb onto particle surfaces to create a physical barrier against coalescence; and electrostatic stabilization, in which ionic surfactants (e.g., SDS, TPGS) impart surface charges that generate repulsive forces[20][21]. Optimal formulations typically exploit a combination of both mechanisms. Zeta potential measurements are indispensable in this context: values ≥ ±30 mV are conventionally regarded as indicative of adequate electrostatic repulsion and long-term physical stabilit Abhi 2
Critically, stabilizer selection also influences vivo behavior. Chitosan-based stabilizers, for instance, impart mucoadhesive properties relevant to oral and pulmonary delivery, while PEGylated surfactants extend circulatory half-life by reducing opsonization [23][24]. The antimicrobial nanosuspension studies increasingly recognize stabilizer role as a pharmacological variable, because the surface chemistry of nanoparticles directly governs their interaction with bacterial membranes and host cell uptake pathways [25].
- METHODS OF PREPERATION
Nanosuspension preparation methods are broadly categorized into three strategies: top-down (size reduction), bottom-up (particle growth from solution), and combination or hybrid approaches that leverage the advantages of both. The choice of method is dictated by drug physicochemical properties, desired particle size and distribution, scalability requirements, and regulatory considerations [26].
3.1. Top-Down Methods
Top-down technologies are the dominant industrial approach, benefiting from established scale-up precedents and applicability to diverse drug classes. They operate by applying mechanical energy to reduce coarse drug crystals to the nanoscale [27].
3.1.1 Wet Media Milling (Pearl Milling): Wet media milling (WMM) is the most widely employed top-down technique, In this process, drug particles dispersed in an aqueous medium are subjected to high shear generated by rotating milling beads (typically zirconium oxide, yttria-stabilized zirconia, or polystyrene)[28]. Critical process parameters include bead size, filling volume, milling speed, and milling time, all of which interact in complex ways captured by microhydrodynamic modeling to predict breakage kinetics and final particle size distribution [29]. A key advantage is the ability to achieve particles below 200 nm; a limitation is potential contamination from bead erosion and the risk of drug degradation from heat generation during extended milling. Studies in the context of itraconazole,a BCS class II antifungal drug confirmed spherical particles of 294 nm with preserved crystallinity following pearl milling with Poloxamer 407 and zirconium oxide beads [30].
3.1.2 High-Pressure Homogenization (HPH): HPH is the second dominant top-down platform, applying shear forces, cavitation, and collision to a coarse drug suspension passed through a narrow gap at pressures of 500–350 MPa [31]. Two commercial configurations are prevalent: the piston-gap homogenizer (Micron LAB 40) and the microfluidizer (IDD-P® technology). HPH is scalable, GMP-compatible, and suitable for thermolabile drugs when conducted under controlled temperatures [32]. In aqueous media (Dissocubes®) or non-aqueous media (Nanopure®), HPH yields nanosuspensions of reproducible particle size, though typically requiring 10–25 homogenization cycles to achieve the target size distribution [33]. For antimicrobial applications, amphotericin B nanosuspensions prepared by HPH yielded 528 nm particles with demonstrated in vivo efficacy against visceral leishmaniasis, a benchmark study in the antiparasitic nanosuspension literature [34].
3.2 Bottom-Up Methods
Bottom-up approaches induce drug nanoparticle formation through controlled precipitation from supersaturated solutions, exploiting nucleation and crystal growth kinetics to modulate particle size distribution [35].
Liquid Antisolvent Precipitation (LAS): Drug dissolved in a water-miscible organic solvent is injected into an aqueous antisolvent under controlled mixing, inducing rapid nucleation. Parameters governing particle size include supersaturation ratio, solvent addition rate, temperature, and stabilizer concentration. Bottom-up methods excel in cost-effectiveness and scalability, but face challenges with residual organic solvents, Ostwald ripening during scale-up, and difficulty achieving the very small particle sizes attainable by WMM [17][36].
Supercritical Fluid Technology (SCF): SCF methods, including rapid expansion of supercritical solutions (RESS) and supercritical anti-solvent (SAS) produce highly pure nanoparticles without organic solvent residues, making them particularly attractive for parenteral antimicrobial formulations. Carbon dioxide (CO₂) at supercritical conditions (31.1°C, 73.8 bar) serves as solvent or antisolvent, enabling precise particle size control. While yield and scalability challenges persist, recent advances in continuous SCF processing have renewed interest for pharmaceutical manufacturing [37][38].
3.3 Combination (Hybrid) Methods
Combination approaches, exemplified by H42 technology (HPH of lyophilized precipitation product), NanomorpH® (precipitation + HPH), and Nanoedge® (precipitation-followed-by-WMM) utilises the particle size advantages of top-down processing while leveraging the cost and time efficiency of bottom-up pre-processing [39]. These strategies have gained traction in recent years, with combination methods featuring in many recent nanosuspension publications. For complex antimicrobials such as rifampicin and clofazimine, combination processing reduces overall milling time while preserving drug stability[40][41][42].
- CHARACTERIZATION TECHNIQUES
Comprehensive characterization is indispensable to both scientific reproducibility and regulatory compliance with antimicrobial nanosuspensions. Characterization encompasses physical, chemical, and biological dimensions, each providing distinct and complementary information about formulation performance.
Table 1. Characterization Techniques for Antimicrobial Nanosuspensions
|
Technique |
Parameter Measured |
Significance in Antimicrobial Applications |
|
Dynamic Light Scattering (DLS) |
Particle size, PDI, hydrodynamic diameter |
Predicts dissolution rate, mucosal penetration, and phagocytic uptake by macrophages [43]. |
|
Zeta Potential (ZP) |
Surface charge (mV) |
Values ≥ ±30 mV ensure electrostatic stabilization and anti-aggregation; charge influences bacterial membrane interactions [44]. |
|
Scanning Electron Microscopy (SEM) |
Surface morphology, shape |
Confirms nanometer-scale dimensions and absence of aggregates; critical for inhalation/IV safety [45] |
|
Powder X-Ray Diffraction (PXRD) |
Crystallinity, polymorphism |
Amorphous vs. crystalline state affects solubility; polymorphic changes alter antimicrobial efficacy [46] |
|
Differential Scanning Calorimetry (DSC) |
Thermal events, glass transition, melting |
Detects excipient-drug interactions; confirms lyophilized powder integrity for long-term storage [47] |
|
In Vitro Drug Release / Dissolution |
Drug release profile, dissolution rate |
Compared against bulk drug and marketed formulation; predicts in vivo bioavailability enhancement [48] |
|
Saturation Solubility |
Intrinsic drug solubility (mg/mL) |
Demonstrates Ostwald–Freundlich enhancement; key BCS II/IV validation metric [49] |
|
Fourier-Transform Infrared (FTIR) |
Functional group interactions |
Confirms drug-stabilizer compatibility; rules out chemical degradation during milling [50] |
|
Transmission Electron Microscopy (TEM) |
Internal morphology, core-shell structures |
Essential for hybrid nanoplatforms; validates particle architecture for targeted antimicrobial delivery [51] |
|
Nanoparticle Tracking Analysis (NTA) |
Particle size & concentration |
Gold-standard for heterogeneous suspensions; provides absolute particle count per mL [52] |
Physical characterization using Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA) provides complementary insights into nanosuspension systems. DLS yields intensity-weighted hydrodynamic diameter and polydispersity index (PDI), where values <0.25 generally indicate acceptable monodispersity, while NTA enables visualization of particle size distributions at the individual particle level and provides absolute particle concentration, a parameter increasingly recognized for its biological and pharmacological relevance. Zeta potential, typically measured by electrophoretic light scattering, serves as an indicator of colloidal stability; values below ±20 mV or multimodal distributions may indicate insufficient electrostatic stabilization and a higher risk of aggregation [43][44][52].
Powder X-ray diffraction (PXRD) is particularly critical in nanosuspensions as crystallinity strongly influences dissolution behavior [53]. Differential scanning calorimetry (DSC) complements PXRD by identifying thermal transitions such as melting points and glass transition temperatures, which can indicate polymorphic transformations or drug–excipient incompatibilities introduced during processing [47]. Fourier-transform infrared (FTIR) spectroscopy further confirms the chemical integrity of the active pharmaceutical ingredient (API) and the absence of covalent interactions with stabilizers, an important requirement for regulatory compliance [54].
In vitro drug release studies, commonly performed using dialysis membrane or Franz diffusion cell systems in biorelevant media, provide insight into dissolution enhancement achieved through nanosizing [48][55]. These results must be interpreted relative to bulk drug and marketed formulations; for example, itraconazole nanosuspensions have demonstrated significantly improved dissolution profiles compared to conventional capsule formulations in acidic media, supporting their clinical relevance [30].
- MECHANISMS OF ENHANCED ANTIMICROBIAL ACTIVITY
The antimicrobial superiority of nanosuspensions over conventional drug formulations is multifactorial, including biopharmaceutical, pharmacokinetic, and pharmacodynamic dimensions. Understanding these mechanisms is critical for rational formulation design and for anticipating the clinical conditions under which nanosuspension advantage will be most pronounced.
5.1 Increased Surface Area and Dissolution Velocity
Nanosizing exponentially increases drug surface area per unit mass, driving faster dissolution and a higher concentration gradient across biological membranes. For a drug particle reduced from 10 μm to 100 nm, a 100-fold size reduction leads to surface area increase by approximately 10,000-fold. This translates, via the Noyes–Whitney equation, into dramatically accelerated dissolution velocities, shifting the dissolution rate-limited absorption of BCS II drugs toward permeability-limited behavior[56][57].This mechanism is especially relevant for antifungals (itraconazole, voriconazole), antimycobacterials (rifampicin, clofazimine), and antiparasitic agents (atovaquone, amphotericin B) where solubility is the primary bioavailability barrier[58][59][60].
5.2 Improved Mucosal and Intracellular Penetration
Nanoparticles in the 100–500 nm range exhibit significantly enhanced capacity to penetrate biological barriers that ordinarily exclude microparticulate drugs. For pulmonary infections, aerosolized nanosuspensions achieve deep lung deposition within the alveolar space, a critical advantage for pathogens such as Mycobacterium tuberculosis and Pseudomonas aeruginosa that colonize the lower airways [61].
For intracellular pathogens including Leishmania spp., Salmonella typhi, and Mycobacterium avium, nanosuspension-derived particles are internalized by alveolar macrophages and dendritic cells via phagocytosis, a mechanism that has been deliberately exploited as a passive targeting strategy [62].
5.3 Enhanced Biofilm Penetration
Bacterial biofilms present a significant physical and metabolic barrier to antimicrobials. The extracellular polymeric substance (EPS) matrix comprising polysaccharides, proteins, and extracellular DNA, creates heterogeneous network that restricts diffusion and reduces effective antibiotic penetration [63]. Nanoparticles sized below 300 nm have been shown to penetrate biofilm matrices more effectively than larger particles or free drug molecules, due to their ability to exploit channels and voids within the EPS [64]. Furthermore, surface-engineered Nanosuspensions with cationic charge (e.g., chitosan-coated) exploit electrostatic attraction to the negatively charged biofilm matrix, concentrating the antimicrobial payload at the target site [65].
5.4 Reactive Oxygen Species Generation and Direct Membrane Disruption
Metal-incorporated antimicrobial nanosuspensions most prominently silver (AgNPs), zinc oxide (ZnO), and copper oxide (CuO) nanoparticles exert direct antibacterial effects through ROS generation, membrane disruption, and interference with intracellular metabolic pathways [66][67]. Unlike conventional antibiotics, which often act on a single molecular target, metal nanoparticles exert multi-target effects that reduce the likelihood of resistance development. When engineered at the nanoscale, their high surface area enhances reactivity and antimicrobial efficacy compared with bulk materials or larger particles, often resulting in lower minimum inhibitory concentrations (MICs) [68].
- THERAPEUTIC APPLICATIONS
6.1 Antibacterial Therapy
Post covid, the antibacterial application domain has been the most extensively explored. Nanosuspensions of conventional antibiotics rifampicin, linezolid, vancomycin, and ciprofloxacin have demonstrated significant MIC reductions against resistant strains compared to their conventional counterparts.
Table : Antibacterial Nanosuspensions
|
Antibiotic |
Observed Effect on MIC |
Impact on Resistant Strains |
|
Rifampicin |
Significant reduction |
When compared to the commercial product, the nanosuspension increased the rifampicin concentration 2-fold. [69] |
|
Linezolid |
Significant reduction |
The antimicrobial efficacy of developed linezolid nanoemulsions can be approximately 3 times higher than pure linezolid, particularly against Mycobacterium smegmatis [70] |
|
Vancomycin |
Significant reduction |
Increased potency against resistant strains, drastically increase its activity against Gram-negative bacteria (up to 100 times) and disrupt mature biofilms, broadening its conventional Gram-positive-only spectrum [71] |
|
Ciprofloxacin |
Significant reduction |
Better penetration and effectiveness, improved solubility, dissolution rate, and bioavailability of the poorly soluble antibiotic ciprofloxacin. Often prepared via nanoprecipitation or high-pressure homogenization, these systems typically improve antibacterial efficacy against gram-negative and gram-positive bacteria by increasing drug penetration [72]. |
Critically, however, clinical translation of inhaled antibiotic nanosuspensions remains hampered by particle size stability of post-nebulization, a challenge requiring further engineering innovation [73].
6.2 Antifungal Therapy
Antifungal therapeutics is historically richest for nanosuspension applications, given the poor aqueous solubility of major antifungal agents: itraconazole (Class II, solubility ~1 μg/mL), voriconazole, amphotericin B (AmB), and posaconazole.
|
Drug |
Key Findings (In Vitro) |
Key Findings (In Vivo / Clinical Relevance) |
|
Itraconazole [74] |
Mean particle size ~294 nm; preserved crystallinity; significantly enhanced dissolution and drug release in 0.1N HCl vs marketed capsules |
Improved bioavailability; enhanced antifungal efficacy (IV nanosuspension vs solution in rats) |
|
Amphotericin B [75] |
Reduced hemolytic activity maintained antifungal efficacy |
Improved safety profile (reduced nephrotoxicity); enhanced antifungal activity. |
6.3 Antiviral Therapy
Nanosuspension applications in antiviral therapy have gained momentum since 2019, catalyzed by the COVID-19 pandemic's acceleration of nanomedicine innovation. Poorly water-soluble antivirals including lopinavir, ritonavir, efavirenz, and remdesivir present formulation challenges amenable to nanosuspension technology.
|
Drug |
Formulation Challenge |
Nanosuspension Strategy |
Benefits |
|
Lopinavir |
Poor aqueous solubility; variable oral bioavailability |
Particle size reduction via nanosuspension |
Lopinavir nanosuspensions are submicron, surface-stabilized drug particles (typically 100–500 nm) designed to overcome the drug's poor solubility, low bioavailability and extensive hepatic metabolism [76]. |
|
Ritonavir |
Low solubility; acts as CYP3A4 inhibitor but has formulation limitations |
Nanosuspension to improve dispersion and stability |
Ritonavir nanosuspension is a drug delivery system designed to enhance the low aqueous solubility and oral bioavailability of the HIV protease inhibitor ritonavir (a BCS Class II drug) [77]. |
|
Efavirenz |
BCS Class II (low solubility, high permeability); erratic absorption |
Nanosuspension formulation |
colloidal dispersions of drug nanoparticles (typically 200–600 nm) designed to overcome the poor aqueous solubility (BCS Class II) and low bioavailability of the HIV medication Efavirenz [78] |
- LIMITATIONS AND CHALLENGES
Despite the advantages, several significant challenges stop the clinical translation of antimicrobial nanosuspensions. Physical stability during long-term storage remains the most clinically critical; Ostwald ripening, aggregation, and polymorphic transformation can alter particle size distribution, drug release profiles, and ultimately therapeutic efficacy during shelf life [79]. While lyophilization and spray drying provide partial solutions, the reconstituted product may not fully recover the original particle size distribution, a phenomenon especially problematic for IV formulations where particle size directly influences pharmacokinetics and safety [80].
Manufacturing challenges present additional hurdles. High-energy milling can cause thermal degradation of heat-labile drugs, contamination from bead erosion (introducing heavy metal impurities), and induction of undesirable polymorphic or amorphous transitions [81]. Batch-to-batch variability—attributable to the inherent complexity of milling and HPH processes demands rigorous in-process controls and sophisticated PAT (Process Analytical Technology) integration to satisfy GMP requirements. Scale-up from laboratory to commercial manufacturing is non-trivial, with micro hydrodynamic models providing only approximate guidance [82].
From a nanotoxicological perspective, the long-term safety of nanosuspension excipients and the potential for nanoparticle accumulation in non-target tissues (liver, spleen, kidney) through the mononuclear phagocyte system (MPS) remain incompletely characterized [83]. The ROS-generating capacity of metal-containing nanosuspensions highly desirable for antimicrobial activity carries dual-use risk by inducing oxidative damage in host tissues at elevated concentrations [84]. Regulatory requirements for comprehensive toxicological profiling including genotoxicity, immunotoxicity, and reproductive toxicity studies; add substantially to development timelines and costs [85].
CONCLUSION
Nanosuspension technology represents a strategically significant advancement in antimicrobial drug delivery, addressing both physicochemical limitations of drugs and biological barriers associated with resistant infections. Nanosuspensions reduce drug particle size into nanometer scales; nanosuspensions increase drug dissolution rate, saturation solubility, and bioavailability, thus increasing therapeutic activity in terms of poorly soluble antimicrobials. The high penetrating ability of biofilm, intracellular infectious areas, and direct antimicrobial action of certain nanosuspension formulations make them a highly efficient means for combating drug-resistant pathogens.The versatility in terms of preparation procedures, such as top-down, bottom-up, and hybrid approaches, and advanced characterizations contribute to their broad applicability and reproducibility. Numerous antibacterial, antifungal, and antiviral trials reveal increased efficacy, lower MIC, and better pharmacokinetics results in favor of nanosuspensions.
Nevertheless, the transfer of nanosuspensions from scientific investigations to practical implementation is limited by physical stability issues, complexity of scaling up, and unclear toxicology profiles. Solving these problems will allow further progress in this direction. In conclusion, nanosuspensions hold substantial promise as next-generation antimicrobial delivery systems, offering a viable pathway to extend the clinical utility of existing drugs and combat the escalating threat of antimicrobial resistance.
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