Transepithelial transport of PAMAM dendrimers across isolated rat jejunal mucosae in ussing chambers

Anti-biofilm, drug delivery and cytotoxicity properties of dendrimers

Background and purpose

Treatments using antimicrobial agents have faced many difficulties as a result of biofilm formation by pathogenic microorganisms. The biofilm matrix formed by these microorganisms prevents antimicrobial agents from penetrating the interior where they can exact their activity effectively. Additionally, extracellular polymeric molecules associated with biofilm surfaces can absorb antimicrobial compounds, lowering their bioavailability. This problem has resulted in the quest for alternative treatment protocols, and the development of nanomaterials and devices through nanotechnology has recently been on the rise.

Research approach

The literature on dendrimers was searched for in databases such as Google Scholar, PubMed, and ScienceDirect.

Key results

As a nanomaterial, dendrimers have found useful applications as a drug delivery vehicle for antimicrobial agents against biofilm-mediated infections to circumvent these defense mechanisms. The distinctive properties of dendrimers, such as multi-valency, biocompatibility, high water solubility, non-immunogenicity, and biofilm matrix-/cell membrane fusogenicity (ability to merge with intracellular membrane or other proteins), significantly increase the efficacy of antimicrobial agents and reduce the likelihood of recurring infections.

Conclusion

This review outlines the current state of dendrimer carriers for biofilm treatments, provides examples of their real-world uses, and examines potential drawbacks.

Safety Challenges and Application Strategies for the Use of Dendrimers in Medicine

Dendrimers are used for a variety of applications in medicine but, due to their host–guest and entrapment characteristics, are particularly used for the delivery of genes and drugs. However, dendrimers are intrinsically toxic, thus creating a major limitation for their use in biological systems. To reduce such toxicity, biocompatible dendrimers have been designed and synthesized, and surface engineering has been used to create advantageous changes at the periphery of dendrimers. Although dendrimers have been reviewed previously in the literature, there has yet to be a systematic and comprehensive review of the harmful effects of dendrimers. In this review, we describe the routes of dendrimer exposure and their distribution in vivo. Then, we discuss the toxicity of dendrimers at the organ, cellular, and sub-cellular levels. In this review, we also describe how technology can be used to reduce dendrimer toxicity, by changing their size and surface functionalization, how dendrimers can be combined with other materials to generate a composite formulation, and how dendrimers can be used for the diagnosis of disease. Finally, we discuss future challenges, developments, and research directions in developing biocompatible and safe dendrimers for medical purposes.

The brain-gut axis: A prime therapeutic target in traumatic brain injury

Traumatic brain injury (TBI) is a significant cause of morbidity and mortality in trauma patients. The primary focus of treating TBI is to prevent additional injury to the damaged brain tissue, known as secondary brain injury. This treatment can include treating the body's inflammatory response. Despite promise in animal models, anti-inflammatory therapy has failed to improve outcomes in human patients, suggesting a more targeted and precise approach may be needed. There is a bidirectional axis between the intestine and the brain that contributes to this inflammation in acute and chronic injury. The mechanisms for this interaction are not completely understood, but there is evidence that neural, inflammatory, endocrine, and microbiome signals all participate in this process. Therapies that target the intestine as a source of inflammation have potential to lessen secondary brain injury and improve outcomes in TBI patients, but to develop these treatments we need to better understand the mechanisms behind this intestinal inflammatory response.

Tissue-based biosensor for monitoring the antioxidant effect of orally administered drugs in the intestine

For a better understanding of the effect of drugs and their interaction with cells and tissues, there is a need for in vitro and ex vivo model systems which enables studying these events. There are several in vitro methods available to evaluate the antioxidant activity; however, these methods do not factor in the complex in vivo physiology. Here we present an intestinal tissue modified oxygen electrode, used for the detection of the antioxidant effect of orally administered drugs in the presence of H₂O₂. Antioxidants are essential in the defense against oxidative stress, more specifically against reactive oxygen species such as H₂O_2. Due to the presence of native catalase in the intestine, with the tissue-based biosensor we were able to detect H₂O₂ in the range between 50 and 500 µM. The reproducibility of the sensor based on the calculated relative standard deviations was 15 ± 6%. We found that the O₂ production by catalase from H₂O₂ was reduced in the presence of a well-known antioxidant, quinol. This indirectly detected antioxidant activity was also observed in the case of orally administered drugs with a reported anti-inflammatory effect such as mesalazine and paracetamol, while no antioxidant activity was recorded with aspirin and metformin.

Amphiphilic Star Polypept(o)ides as Nanomeric Vectors in Mucosal Drug Delivery

Mucosal delivery across the gastrointestinal (GI) tract, airways, and buccal epithelia is an attractive mode of therapeutic administration, but the challenge is to overcome the mucus and epithelial barriers. Here, we present degradable star polypept(o)ides capable of permeating both barriers as a promising biomaterial platform for mucosal delivery. Star polypept(o)ides were obtained by the initiation of benzyl-l-glutamate N-carboxyanhydride (NCA) from an 8-arm poly(propyleneimine) (PPI) dendrimer, with subsequent chain extension with sarcosine NCA. The hydrophobic poly(benzyl-l-glutamate) (PBLG) block length was maintained at 20 monomers, while the length of the hydrophilic poly(sarcosine) (PSar) block ranged from 20-640 monomers to produce star polypept(o)ides with increasing hydrophilic: hydrophobic ratios. Transmission electron microscopy (TEM) images revealed elongated particles of ∼120 nm length, while dynamic light scattering (DLS) provided evidence of a decrease in the size of polymer aggregates in water with increasing poly(sarcosine) block length, with the smallest size obtained for the star PBLG₂₀-b-PSar₆₄₀. Fluorescein isothiocyanate (FITC)-conjugated PBLG₂₀-b-PSar₆₄₀ permeated artificial mucus and isolated rat mucus, as well as rat intestinal jejunal tissue mounted in Franz diffusion chambers. An apparent permeability coefficient (P_app) of 15.4 ± 3.1 ×10^-6 cm/s for FITC-PBLG₂₀-b-PSar₆₄₀ was calculated from the transepithelial flux obtained with the apical-side addition of 7.5 mg polypept(o)ide to jejunal tissue over 2 h. This P_app could not be accounted for by flux of unconjugated FITC. Resistance to trypsin demonstrated the stability of FITC-labeled polypept(o)ide over 2 h, but enzymatic degradation at the mucus-epithelial interface or during flux could not be ruled out as contributing to the P_app. The absence of any histological damage to the jejunal tissue during the 2 h exposure suggests that the flux was not associated with overt toxicity.

Pediatric oral formulation of dendrimer-N-acetyl-l-cysteine conjugates for the treatment of neuroinflammation

N-Acetyl-l-cysteine (NAC) commonly used as an antidote in acetaminophen poisoning has shown promise in the treatment of neurological disorders such as cerebral palsy (CP). However, NAC suffers from drawbacks such as poor oral bioavailability and suboptimal blood-brain-barrier (BBB) permeability limiting its clinical success. It was previously demonstrated that intravenous administration of dendrimer-NAC (D-NAC) conjugates have shown significant promise in the targeted treatment of neuroinflammation, in multiple preclinical models. Development of an oral formulation of D-NAC may open new administrative routes for this compound. Here, we report the gastrointestinal stability, in vitro transepithelial permeability, and in vivo oral absorption and pharmacokinetics in rats of a pediatric formulation of D-NAC containing Capmul MCM (glycerol monocaprylate) as a penetration enhancer. D-NAC was stable for 6 h in all five simulated gastrointestinal fluids with no signs of chemical degradation. The apparent permeability (P_app) of D-NAC increased 9-fold in the formulation containing Capmul. The area under the curve [AUC]_0-∞ of D-NAC with Capmul increased by 47% when compared to D-NAC alone. These results indicate that an oral pediatric formulation containing D-NAC and Capmul can be an effective option for the treatment of neuroinflammation.

1H NMR-Based Identification of Intestinally Absorbed Metabolites by Ussing Chamber Analysis of the Rat Cecum

The large intestine (cecum and colon) is a complex biochemical factory of vital importance to human health. It plays a major role in digestion and absorption by salvaging nutrients from polysaccharides via fermentation initiated by the bacteria that comprise the gut microbiome. We hypothesize that the intestinal epithelium absorbs a limited number of luminal metabolites with bioactive potential while actively excluding those with toxic effects. To explore this concept, we combined ¹H NMR detection with Ussing chamber measurements of absorptive transport by rat cecum. Numerous metabolites transported across the epithelium can be measured simultaneously by ¹H NMR, a universal detector of organic compounds, alleviating the need for fluorescent or radiolabeled compounds. Our results demonstrate the utility of this approach to delineate the repertoire of fecal solutes that are selectively absorbed by the cecum and to determine their transport rates.

Noncompetitive Active Transport Exploiting Intestinal Transferrin Receptors for Oral Delivery of Proteins by Tunable Nanoplatform

Here we present a "thinking-outside-the-box", tunable nanoplatform for oral delivery of proteins using insulin as a model protein. These nanosystems offer noncompetitive active transport exploiting transferrin receptors present in the intestine and permit tailored release in vivo. Such delivery approaches have the potential to individualize insulin therapy to a regimen that is compatible with the patient's glucose profile.

Molecular dynamics simulations in drug delivery research: Calcium chelation of G3.5 PAMAM dendrimers

Poly(amido amine) (PAMAM) dendrimers have been considered as possible delivery systems for anticancer drugs. One potential advantage of these carriers would be their use in oral formulations, which will require absorption in the intestinal lumen. This may require the opening of tight junctions which may be enabled by reducing the Ca2+ concentration in the intestinal lumen, which has been shown as an absorption mechanism for EDTA (ethylenediaminetetraacetic acid). Using molecular dynamics simulations, we show that the G3.5 PAMAM dendrimers are able to chelate Ca2+ at similar proportions to EDTA, providing support to the hypothesis that oral formulations of PAMAM dendrimers could use this high chelating efficiency as a potential mechanism for permeating the tight junctions of the intestines if other formulation barriers could be overcome.

Surface functionality affects the biodistribution and microglia-targeting of intra-amniotically delivered dendrimers

Cerebral Palsy (CP) is a chronic childhood disorder with limited therapeutic options. Maternal intrauterine inflammation/infection is a major risk factor in the pathogenesis of CP. In pre-clinical models, dendrimer-based therapies are viable in postnatal period, attenuating inflammation and improving motor function in vivo. However, treatment to the mother, in the prenatal period, may provide the possibility of preventing/resolving inflammation at early stages. Towards this goal, we used a maternal intrauterine inflammation-induced rabbit model of CP to study fetal-maternal transport and neuroinflammation targeting of intra-amniotically administrated dendrimers with neutral/anionic surface functionality. Our study suggested both hydroxyl-terminated 'neutral' (D-OH) and carboxyl-terminated 'anionic' (D-COOH) Polyamidoamine (PAMAM) dendrimers were absorbed by fetuses and demonstrated bi-directional transport between fetuses and mother. D-OH was more effective in crossing the fetal blood-brain barrier, and targeting activated microglia. The cell-specific targeting was associated with the extent of microglia activation. This study demonstrated intra-amniotically administered hydroxyl PAMAM dendrimers could be an effective drug delivery vehicle for targeting fetal inflammation and preventing subsequent neurologic injury associated with chorioamnionitis.

Advances in the transepithelial transport of nanoparticles

The intestinal epithelium represents a barrier to the delivery of nanoparticles (NPs). It prevents intact NPs from efficiently crossing the mucosa to access the circulation, thus limiting the successful application of NP-based oral drug delivery. Recent advances in nanotechnology have provided promising solutions to this challenge. This review describes the potential intestinal absorption pathways of NPs, including the transenterocytic pathway, paracellular pathway and M-cell-mediated pathway. NP properties that influence transcytosis are summarized; and the biodistribution of NPs after oral absorption is described and the future prospects of novel NPs are explored.

Poly(amido amine) dendrimers in oral delivery

Poly(amidoamine) (PAMAM) dendrimers have been extensively investigated for oral delivery applications due to their ability to translocate across the gastrointestinal epithelium. In this Review, we highlight recent advances in the evaluation of PAMAM dendrimers as oral drug delivery carriers. Specifically, toxicity, mechanisms of transepithelial transport, models of the intestinal epithelial barrier including isolated human intestinal tissue model, detection of dendrimers, and surface modification are discussed. We also highlight evaluation of various PAMAM dendrimer-drug conjugates for their ability to transport across gastrointestinal epithelium for improved oral bioavailability. In addition, current challenges and future trends for clinical translation of PAMAM dendrimers as carriers for oral delivery are discussed.

Ex Vivo Intestinal Sacs to Assess Mucosal Permeability in Models of Gastrointestinal Disease

The epithelial barrier is the first innate defense of the gastrointestinal tract and selectively regulates transport from the lumen to the underlying tissue compartments, restricting the transport of smaller molecules across the epithelium and almost completely prohibiting epithelial macromolecular transport. This selectivity is determined by the mucous gel layer, which limits the transport of lipophilic molecules and both the apical receptors and tight junctional protein complexes of the epithelium. In vitro cell culture models of the epithelium are convenient, but as a model, they lack the complexity of interactions between the microbiota, mucous-gel, epithelium and immune system. On the other hand, in vivo assessment of intestinal absorption or permeability may be performed, but these assays measure overall gastrointestinal absorption, with no indication of site specificity. Ex vivo permeability assays using "intestinal sacs" are a rapid and sensitive method of measuring either overall intestinal integrity or comparative transport of a specific molecule, with the added advantage of intestinal site specificity. Here we describe the preparation of intestinal sacs for permeability studies and the calculation of the apparent permeability (Papp) of a molecule across the intestinal barrier. This technique may be used as a method of assessing drug absorption, or to examine regional epithelial barrier dysfunction in animal models of gastrointestinal disease.

Transepithelial Transport of PAMAM Dendrimers Across Isolated Human Intestinal Tissue

Poly(amido amine) (PAMAM) dendrimers have shown transepithelial transport across intestinal epithelial barrier in rats and across Caco-2 cell monolayers. Caco-2 models innately lack mucous barriers, and rat isolated intestinal tissue has been shown to overestimate human permeability. This study is the first report of transport of PAMAM dendrimers across isolated human intestinal epithelium. It was observed that FITC labeled G4-NH2 and G3.5-COOH PAMAM dendrimers at 1 mM concentration do not have a statistically higher permeability compared to free FITC controls in isolated human jejunum and colonic tissues. Mannitol permeability was increased at 10 mM concentrations of G3.5-COOH and G4-NH2 dendrimers. Significant histological changes in human colonic and jejunal tissues were observed at G3.5-COOH and G4-NH2 concentrations of 10 mM implying that dose limiting toxicity may occur at similar concentrations in vivo. The permeability through human isolated intestinal tissue in this study was compared to previous rat and Caco-2 permeability data. This study implicates that PAMAM dendrimer oral drug delivery may be feasible, but it may be limited to highly potent drugs.

Regional Morphology and Transport of PAMAM Dendrimers Across Isolated Rat Intestinal Tissue

Intestinal permeability of PAMAM dendrimers has been observed, giving rationale for their use in oral drug delivery as potential carriers of associated molecules. This study assessed the apparent permeability coefficients (Papp) of dendrimers across isolated rat intestinal regional mucosae, along with estimation of the maximum non-toxic concentration. Caco-2 monolayers were also used to assess the comparative Papp values between isolated mucosae and cell culture models. Concentrations from 0.1 to 10 mM of anionic and cationic dendrimers were tested in mucosae to assess their Papp, membrane TEER, [(14)C]-mannitol Papp, and histology. 0.1 mM concentrations of dendrimers were assessed over 120 min in Caco-2 cell monolayers as concentrations above that were cytotoxic. Jejunal transport of dendrimers was higher than transport in colonic epithelium. Monolayer Papp values of dendrimers were comparable to those of jejunal mucosae. Mucosae exposed to dendrimer concentrations of 10 mM for 120 min caused significant reduction in TEER and changes in tissue morphology; however, G3.5 was the only analogue that caused significant TEER reduction and morphological changes at 1 mM concentrations. Transport in jejunal mucosae appears to be the greatest indicating that the small intestinal will be the most likely region to target for oral drug delivery using PAMAM dendrimers.