Vaccines remain one of the most powerful tools in modern medicine, having revolutionized public health by preventing millions of deaths and controlling the spread of infectious diseases worldwide. However, conventional needle-based vaccines face several limitations, including pain and discomfort, the need for cold-chain infrastructure, reliance on trained healthcare personnel, risk of cross-contamination, and limited accessibility in low-resource settings. These challenges have spurred the development of non-invasive vaccination approaches that promise safer, more accessible, and patient-friendly immunization. Non-invasive immunizations not only eliminate the need for needles but may also enhance compliance and enable mucosal immune responses. To harness the full potential of these innovative delivery routes, a comprehensive understanding of their formulation strategies and mechanism of action is essential. This review aims to comprehensively discuss recent advancements in oral, intranasal, microneedle, buccal, sublingual, and vaginal vaccinations and highlight their underlying immunological mechanisms, formulation strategies in preclinical studies, examples of marketed products, and ongoing clinical trials.

Background/Objectives: Cervical dysplasia, a precursor to cervical cancer, represents a significant global health challenge, particularly in regions like Central America, Africa, and Southeast Asia. Current management approaches rely on surgical or ablative interventions, which can lead to complications, for example, preterm birth and cervical insufficiency. Therefore, developing non-invasive, localized therapeutic alternatives is of great clinical interest. Curcumin is a natural compound that suppresses the progression of cervical cancer, but it has poor oral bioavailability and high clearance. Methods: We incorporated curcumin into solid lipid nanoparticles, which were then loaded into rapidly dissolving films. These films show the sustained release profile of curcumin at the localized vaginal site, demonstrating release kinetics consistent with the Korsmeyer–Peppas model. Results: The curcumin solid lipid nanoparticles yielded a size of 341 nm and a polydispersity index of 0.373, and the zeta potential was −23.4 mV. The encapsulation efficiency of curcumin solid lipid nanoparticles was 77.27% using a validated HPLC method. FTIR analysis supported successful incorporation of curcumin into the lipid matrix. A Box–Behnken Design of Experiments optimized the key film formulation parameters and yielded a film with a tensile strength of 2.8 mPa, disintegration time of 3 min, folding endurance of 263, film thickness of 0.426 mm and a pH of 4.0. Conclusions: In vitro assays in human cervical carcinoma cells demonstrated enhanced mortality and autophagosome formation by the curcumin solid lipid nanoparticles when compared to free curcumin. Surface expression of MHC I, MHCII, CD40 and CD80 in peripheral dendritic cells was significantly higher in the curcumin solid lipid nanoparticles than in free curcumin. Results show that solid lipid nanoparticles loaded with curcumin effectively stimulate and activate dendritic cells, supporting immune cell activation outside the tumor microenvironment. The proposed pain-free self-administration strategies will lead to increased patient compliance.

Background: Adjuvants enhance the immune response to antigens incorporated in vaccine formulations. Given that the majority of infectious agents enter the body through mucosal surfaces, efficacious vaccines must generate protective immunity at these sites, which serve as the first line of defense. There is a need for mucosal adjuvants; hence, we explored the potential of repurposing existing drugs with established safety profiles in humans. Polymyxins are a class of clinically used antibiotics. They are cationic peptides and mast cell activators, which are a novel class of vaccination adjuvants. The goal was to assess the adjuvant properties of Polymyxin B microparticles in combination with vaccine candidates previously developed in our laboratory, such as microparticulate gonorrhea, influenza, measles, Zika, and canine coronavirus vaccines, and to compare their performance with FDA-licensed adjuvants, such as MF59 and Alum. Methods: Polymyxin microparticles were formulated using a double emulsion method, and the toxicity profile and autophagosome generation of Polymyxin B microparticles were assessed. The immunogenic potential of Polymyxin B in these vaccines was studied using multiple parameters such as nitric oxide release using Griess assay and immune profiling using flow cytometry for markers such as MHC I, MHC II, CD40, and CD80. Results: Polymyxin B microparticles were found to be non-cytotoxic to dendritic cells up to 500 μg/mL. Polymyxin B promoted autophagosome formation and nitric oxide release, and showed the upregulation of MHC I, MHC II, CD80, and CD40 pathways. Conclusions: The adjuvant activity of Polymyxin B across various vaccine platforms is significantly comparable to FDA-approved adjuvants, which is a critical requirement for generating T cell responses such as helper T cell and cytotoxic CD8+ T cell responses.

Background/Objectives: Particulate vaccine formulations require delivery systems and immunostimulatory components that support antigen-presenting cell interaction while maintaining formulation stability and cytocompatibility. This study examines Guanine-α-D-Fructose (GDF), a guanine–fructose small-molecule immunostimulatory candidate, incorporated into poly(lactic acid-co-glycolic acid) (PLGA) microparticles and benchmarked in vitro against established adjuvant formulations containing Alum or AddaVax. Methods: GDF-loaded microparticles were prepared and characterized for particle size, morphology, surface charge, entrapment efficiency, and release behavior. Their in vitro immunostimulatory activity was evaluated using murine dendritic cells. To explore formulation compatibility, GDF MPs were tested in combination with multiple particulate antigen formulations, including measles, gonorrhea, SARS-CoV-2, influenza A (H3N2), and Zika. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was utilized to measure cell viability, while nitric oxide generation was measured with the Griess assay. Dendritic cell activation-associated surface marker expression was analyzed by flow cytometry using MHC I, MHC II, CD80, and CD40. Results: The particles showed consistent spherical morphology, sustained GDF release, and favorable cytocompatibility at concentrations up to 250 µg/mL, with reduced viability at higher concentrations. GDF MPs increased nitric oxide production and activation-associated marker expression, with responses benchmarked against Alum and AddaVax and comparable in several assay conditions. Increased autophagosome-associated fluorescence was also observed, suggesting modulation of autophagy-related cellular activity; however, direct antigen-processing or antigen-presentation assays are still required. Conclusions: Overall, these findings support GDF-loaded PLGA microparticles as an in vitro immunostimulatory particulate formulation suitable for further mechanistic and in vivo evaluation in vaccine-related systems.

Background: Immunomodulatory compounds can modify or regulate the immune responses. Given that vaccine-induced immune responses can vary in magnitude and durability depending on antigen properties and adjuvant selection. Immunomodulators that enhance antigen-specific immune responses with low toxicity may complement existing adjuvant systems. Recent studies indicate that adenosine receptor–mediated signaling can modulate dendritic cell (DC) function through mechanisms distinct from classical pathogen-associated molecular pattern (PAMP)-driven Toll-like receptor pathways. Methods: In this context, the present study comparatively evaluates poly-(lactic-co-glycolic acid) (PLGA) microparticle–encapsulated β-L-adenosine (BLA MPs) alongside established FDA-approved adjuvants to assess their immunomodulatory potential under limited-antigen conditions. FDA-approved PLGA was used to encapsulate BLA in combination with multiple viral antigens, including H1N1 influenza, Zika virus, and canine coronavirus, to enable sustained delivery, antigen protection, and efficient uptake by antigen-presenting cells. Results: Physicochemical characterization demonstrated uniform particle size distribution, a low polydispersity index, and a stable negative surface charge. Release studies showed more than 50% payload release within 12 h, with release kinetics best described by the Korsmeyer–Peppas model. Cytotoxicity evaluation using DC2.4 cells confirmed that BLA MPs were non-cytotoxic at concentrations up to 250 μg/mL. Comparative in vitro immunological assessments revealed that BLA MPs induced dendritic cell activation, including upregulation of antigen-presenting and co-stimulatory molecules, at levels largely comparable to those observed with Alum- and MF59-based formulations across multiple antigen groups. Nitric oxide production remained within comparable ranges, indicating balanced immunostimulatory activity without excessive inflammatory signaling. In select conditions, co-formulation of BLA MPs with MF59 further enhanced DC activation, supporting its role as a complementary immunomodulatory component. Conclusion: These findings align with previously reported adenosine-dependent pathways involved in DC maturation and antigen presentation. Overall, this comparative study demonstrates that PLGA-encapsulated β-L-adenosine functions as an effective immunomodulatory agent, with performance comparable to that of established FDA-approved adjuvants across diverse vaccine antigens. Further in vivo studies are warranted to evaluate dose dependency, cytokine profiles, and antibody responses to define its role within combinatorial vaccine adjuvant strategies.

Background/Objectives: Gonorrhea remains one of the most prevalent sexually transmitted infections globally, with an estimated 82–87 million new infections annually and no licensed vaccine available. Antigenic variability and the lack of durable protective immunity following natural infection have been major obstacles to vaccine development. We previously developed an adjuvanted, formalin-inactivated whole-cell gonococcal microparticle (GC-MP) vaccine delivered by dissolving microneedles and demonstrated that it induced functional antibodies and enhanced bacterial clearance in murine models. Here, we evaluated this GC-MP microneedle platform in Rhesus macaques to provide proof-of-concept data in a relevant large-animal model and to compare different adjuvants. Methods: Rhesus macaques were allocated into five groups and vaccinated, and serum antigen-specific IgG and IgA responses were quantified by ELISA, with functional activity assessed by serum bactericidal assay. In parallel, the potency of delta-inulin-, aluminum-phosphate-, or aluminum-hydroxide-adjuvanted N. gonorrhoeae-loaded microparticles to activate dendritic cells was assessed in vitro via changes in surface expression of MHC I, MHC II, CD40, and CD80. Results: The serum from vaccinated primates demonstrated elevated gonococcal-specific IgG and IgA responses compared with naïve controls and measurable bactericidal activity against live, homologous N. gonorrhoeae strains. All adjuvanted formulations increased dendritic cell expression of MHC I, MHC II, CD40, and CD80. Conclusions: These findings indicate that an adjuvanted whole-cell inactivated gonorrhea vaccine delivered transdermally via microneedles elicits clearly elevated, relative to controls, functional antibody responses, supporting its advancement toward clinical evaluation.