Arginine-based cationic surfactants: Biodegradable auxiliary agents for the formation of hydrophobic ion pairs with hydrophilic macromolecular drugs

Interplay of Drug-Polymer Interactions and Release Performance for HPMCAS-Based Amorphous Solid Dispersions

The interplay between drug and polymer chemistry and its impact on drug release from an amorphous solid dispersion (ASD) is a relatively underexplored area. Herein, the release rates of several drugs of diverse chemistry from hydroxypropyl methylcellulose acetate succinate (HPMCAS)-based ASDs were explored using surface area normalized dissolution. The tendency of the drug to form an insoluble complex with HPMCAS was determined through coprecipitation experiments. The role of pH and the extent of drug ionization were probed to evaluate the role of electrostatic interactions in complex formation. Relationships between the extent of complexation and the drug release rate from an ASD were observed, whereby the drugs could be divided into two groups. Drugs with a low extent of insoluble complex formation with HPMCAS tended to be neutral or anionic and showed reasonable release at pH 6.8 even at higher drug loadings. Cationic drugs formed insoluble complexes with HPMCAS and showed poor release when formulated as an ASD. Thus, and somewhat counterintuitively, a weakly basic drug showed a reduced release rate from an ASD at a bulk solution pH where it was ionized, relative to when unionized. The opposite trend was observed in the absence of polymer for the neat amorphous drug. In conclusion, electrostatic interactions between HPMCAS and lipophilic cationic drugs led to insoluble complex formation, which in turn resulted in ASDs with poor release performance.

Counterion optimization for hydrophobic ion pairing (HIP): Unraveling the key factors

In the present study, various surfactants were combined with insulin (INS), bovine serum albumin (BSA) and horseradish peroxidase (HRP) via hydrophobic ion pairing to increase lipophilicity and facilitate incorporation into self-emulsifying drug delivery systems (SEDDS). Lipophilicity of model proteins was successfully increased, achieving log Dn-butanol/water values up to 3.5 (INS), 3.2 (BSA) and 1.2 (HRP). Hereby, key factors responsible for complex formation were identified. In particular, surfactants with branched alkyl chains or chain lengths greater than C12 showed favorable properties for hydrophobic ion pairs (HIP). Furthermore, flexibility of the carbon chain resulted in higher lipophilicity and suitability of polar head groups of surfactants for HIP decreased in the rank order sulfonate > sulfosuccinate > phosphate = sulfate > carbonate > phosphonic acids = sulfobetaines. Stability studies of formed HIP complexes were performed in various gastrointestinal fluids and their solubility was determined in commonly used SEDDS excipients. Formed complexes were stable in simulated gastrointestinal fluids and could be incorporated into SEDDS formulations (C1: 10% caprylocaproyl polyoxyl-8 glycerides, 20% PEG-40 hydrogenated castor oil, 20% medium-chain triglycerides, 50% n-butanol; C2: 10% caprylocaproyl polyoxyl-8 glycerides, 20% PEG-40 hydrogenated castor oil, 20% medium-chain triglycerides, 40% n-butanol, 10% 1,2-butanediol), resulting in suitable payloads of up to 11.9 mg/ml for INS, 1.0 mg/ml for BSA and 1.6 mg/ml for HRP.

Thiolated Cellulose: A Dual-Acting Mucoadhesive and Permeation-Enhancing Polymer

This study aims to design an anionic, thiolated cellulose derivative and to evaluate its mucoadhesive and permeation-enhancing properties utilizing enoxaparin as a model drug. 2-Mercaptosuccinic acid-modified cellulose (cellulose-mercaptosuccinate) was synthesized by the reaction of cellulose with S-acetylmercaptosuccinic anhydride. The chemical structure of the target compound was confirmed by FTIR and 1H NMR spectroscopy. The thiol content was determined by Ellman's test. The conjugate exhibited 215.5 ± 25 μmol/g of thiol groups and 84 ± 16 μmol/g of disulfide bonds. Because of thiolation, mucoadhesion on porcine intestinal mucosa was 9.6-fold enhanced. The apparent permeability (Papp) of the model dye Lucifer yellow was up to 2.2-fold improved by 0.5% cellulose-mercaptosuccinate on a Caco-2 cell monolayer. Enoxaparin permeation through rat intestinal mucosa increased 2.4-fold in the presence of 0.5% cellulose-mercaptosuccinate compared with the drug in buffer only. In vivo studies in rats showed an oral bioavailability of 8.98% using cellulose-mercaptosuccinate, which was 12.5-fold higher than that of the aqueous solution of the drug. Results of this study show that the modification of cellulose with 2-mercaptosuccinic acid provides mucoadhesive and permeation-enhancing properties, making this thiolated polymer an attractive excipient for oral drug delivery.

Polyphosphate coated nanoparticles: Enzyme-activated charge-reversal gene delivery systems

AIM

The current study aimed to develop enzyme-activated charge-reversal lipid nanoparticles (LNPs) as novel gene delivery systems.

METHODS

Palmitic acid was covalently bound to protamine being utilised as transfection promoter to anchor it on the surfaces of LNPs. Green fluorescent protein (GFP) encoding plasmid DNA (pDNA) was ion paired with various cationic counter ions to achieve high encapsulation in LNPs. Protamine-decorated LNPs were prepared by solvent injection method followed by coating with sodium tripolyphosphate (TPP) to generate a bio-inert anionic outer surface. Resulting LNPs were characterised regarding size, polydispersity, zeta potential and encapsulation efficiency. Enzyme-triggered charge-reversal of LNPs was investigated using isolated alkaline phosphatase (ALP) monitoring changes in zeta potential as well as monophosphate release. Furthermore, monophosphate release, cell viability and transfection efficiency were evaluated on a human alveolar epithelial (A549) cell line.

RESULTS

Protamine-decorated and TPP-coated (Prot-pDNA/DcChol-TPP) LNPs displayed a mean size of 298.8 ± 17.4 nm and a zeta potential of -13.70 ± 0.61 mV. High pDNA encapsulation was achieved with hydrophobic ion pairs of pDNA with 3ß-[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol hydrochloride (DcChol). Zeta potential of Prot-pDNA/DcChol-TPP LNPs reversed to positive values with a total Δ26.8 mV shift upon incubation with ALP. Conformably, a notable amount of monophosphate was released upon incubation of Prot-pDNA/DcChol-TPP LNPs with isolated as well as cell-associated ALP. A549 cells well tolerated LNPs displaying more than 95 % viability. Compared with naked pDNA, unmodified LNPs and control LNPs, Prot-pDNA/DcChol-TPP LNPs showed a significantly increased transfection efficiency.

CONCLUSION

Prot-pDNA/DcChol-TPP LNPs can be regarded as promising gene delivery systems.

Rivastigmine-DHA ion-pair complex improved loading in hybrid nanoparticles for better amyloid inhibition and nose-to-brain targeting in Alzheimer's

Rivastigmine hydrogen tartrate (RIV-HT) is given orally for Alzheimer's disease. However, oral therapy shows low brain bioavailability, short half-life and gastrointestinal-mediated adverse effects. RIV-HT intranasal delivery can avoid these side effects, but its low brain bioavailability remains challenging. These issues could be solved with hybrid lipid nanoparticles with enough drug loading to enhance RIV-HT brain bioavailability while avoiding oral route side effects. The RIV-HT and docosahexaenoic acid (DHA) ion-pair complex (RIV:DHA) was prepared to improve drug loading into lipid-polymer hybrid (LPH) nanoparticles. Two types of LPH, i.e., cationic (RIV:DHA LPH(+ve)) and anionic LPH (RIV:DHA LPH(-ve)) were developed. The effect of LPH surface charge on in-vitro amyloid inhibition, in-vivo brain concentrations and nose-to-brain drug targeting efficiency were investigated. LPH nanoparticles showed concentration dependant amyloid inhibition. RIV:DHA LPH(+ve) demonstrated relatively enhanced Aβ1-42 peptide inhibition. The thermoresponsive gel embedded with LPH nanoparticles improved nasal drug retention. LPH nanoparticles gel significantly improved pharmacokinetic parameters compared to RIV-HT gel. RIV:DHA LPH(+ve) gel showed better brain concentrations than RIV:DHA LPH(-ve) gel. The histological examination of nasal mucosa treated with LPH nanoparticles gel showed that the delivery system was safe. In conclusion, the LPH nanoparticle gel was safe and efficient in improving the nose-to-brain targeting of RIV, which can potentially be utilized in managing Alzheimer's.

Biodegradable Cationic and Ionizable Cationic Lipids: A Roadmap for Safer Pharmaceutical Excipients

Cationic and ionizable cationic lipids are broadly applied as auxiliary agents, but their use is associated with adverse effects. If these excipients are rapidly degraded to endogenously occurring metabolites such as amino acids and fatty acids, their toxic potential can be minimized. So far, synthesized and evaluated biodegradable cationic and ionizable cationic lipids already showed promising results in terms of functionality and safety. Within this review, an overview about the different types of such biodegradable lipids, the available building blocks, their synthesis and cleavage by endogenous enzymes is provided. Moreover, the relationship between the structure of the lipids and their toxicity is described. Their application in drug delivery systems is critically discussed and placed in context with the lead compounds used in mRNA vaccines. Moreover, their use as preservatives is reviewed, guidance for their design is provided, and an outlook on future developments is given.

Biodegradable arginine based steroid-surfactants: Cationic green agents for hydrophobic ion-pairing

The aim of this study was to evaluate the safety and efficacy for hydrophobic ion-pairing of surfactants based on arginine (Arg). The prepared Arg-cholesteryl ester (ACE) and Arg-diosgenyl ester (ADE) were characterized regarding solubility, pKa, critical micellar concentration (CMC), biodegradability as well as membrane- and aquatic toxicity using DOTAP as reference. The ability for hydrophobic ion-pairing was evaluated and the lipophilicity of formed complexes was determined. NMR, FT-IR and MS confirmed successful synthesis of Arg-surfactants. The slightly soluble single-charged Arg-surfactants (pH < pKa₃ (ACE = 10.42 ± 0.52; ADE = 10.38 ± 0.27)) showed CMCs of 27.17 µM for ACE and 35.67 µM for ADE. CMCs of the sparingly soluble double-charged species (pH < pKa₂ (ACE = 5.30 ± 0.20; ADE = 5.55 ± 0.06)) were determined at concentrations of ≥ 250 µM for ACE and ≥ 850 µM for ADE. The enzymatic- and environmental biodegradability was proven by an entire cleavage of Arg-surfactants within 24 h, whereas DOTAP remained stable. Arg-surfactants exhibited lower membrane- (> 2-fold) and aquatic toxicity (> 15-fold) than DOTAP. The complexes formed with Arg-surfactants and insulin showed higher lipophilicity than the DOTAP-complex. According to these results, Arg-surfactants might be a promising safe tool for the delivery of peptide drugs.

Entirely S-protected thiolated hydroxyethylcellulose: Design of a dual cross-linking approach for hydrogels

AIM

The aim of this study was the synthesis and evaluation of entirely S-protected thiolated hydroxyethylcellulose (HEC) with low and high viscosity, as well as thiolated poly-L-lysine (poly-L-Lys) used as dual-acting ionic as well as thiol-disulfide exchange mediated cross-linking hydrogel.

METHODS

Bis(mercaptosuccinic acid) was covalently attached to low and high viscous HECs via Fisher esterification, obtaining S-protected polymers. Poly-L-Lys-cysteine was synthesized via amidation of poly-L-Lys-HBr with cysteine (Cys). Thiolated polymers were examined in terms of cytotoxicity and rheological behavior of hydrogels containing these thiomers was evaluated with a cone-plate rheometer.

RESULTS

Thiomers showed less cytotoxicity compared to the corresponding unmodified polymers. Rheological studies showed that cross-linking occurred between the two polymers via thiol-disulfide exchange reactions facilitated by the complementary charges. Employing poly-L-Lys-Cys in a concentration of either 0.5 or 5% (m/v) resulted in a 34.5-fold or 17.3-fold as well as a 53.6-fold or 29.6-fold improvement in dynamic viscosity within 5 min at 37 °C on S-protected thiolated low and high viscous HEC, compared to the corresponding unmodified HECs, respectively.

CONCLUSION

By the combination of anionic S-protected thiolated polymers with a cationic thiolated polymer, dual-acting hydrogels exhibiting a time dependent increase in viscosity can be designed.

Modelling of n-Hexadecane bioremediation from soil by slurry bioreactors using artificial neural network method

Diesel oil is known to be one of the major petroleum products that can pollute water and soil. Soil pollution caused by petroleum hydrocarbons has substantially impacted the environment, especially in the Middle East. In this study, modeling and optimization of hexadecane removal from soil was performed using two pure cultures of Acinetobacter and Acromobacter and consortium culture of both bacterial species using artificial neural network (ANN) method. Then the best ANN structure was proposed based on mean square error (MSE) as well as correlation coefficient (R) for pure cultures of Acinetobacter and Acromobacter as well as their consortium. The results showed that the correlations between the actual data and the data predicted by ANN (R2) in Acromobacter, Acinetobacter and consortium of both cultures were 0.50, 0.47 and 0.63, respectively. Despite the low correlation between the experimental data and the data predicted by the ANN, the correlation coefficient and the precision of ANN for the consortium was higher. As a result, ANN had desirable precision to predict hexadecan removal by the cobsertium culture of Ochromobater and Acintobacter.

Ethyl lauroyl arginate-based hydrophobic ion pair complex in lipid nanocapsules: A novel oral delivery approach of rosmarinic acid for enhanced permeability and bioavailability

Limited oral bioavailability due to high hydrophilicity restricts the beneficial use of Rosmaranic acid (RM) that is characterized by many biological and pharmacological effects. The present work was addressed to extract RM from Rosmarinus officinalis L. leaves and then increase its lipophilicity and permeability through the application of hydrophobic ion pair (HIP) approach using ethyl lauroyl arginate (ELA) as a novel counter-ion. Different RM:ELA ratios were screened to optimize HIP formation process. The encapsulation of the optimized HIP into lipid nanocapsules (LNCs) was then achieved to facilitate oral administration. The results of % transmittance, % complexation efficiency (87.32 ± 0.19%) and partition coefficient revealed the successful formation of the HIP complex occurred at RM:ELA molar ratio of 1:2. The formed HIP was successfully loaded into spherical small sized (39.32 ± 0.18 nm) LNCs. The ex vivo permeability studies across porcine intestine showed that the cumulative RM amount permeated/area after 6 h from HIP and LNCs were 3.79 ± 0.57 and 5.71 ± 0.32 µg/cm², respectively. Pharmacokinetic study results showed that the maximum RM concentrations in plasma (C_max) can be arranged in a descending manner as follows; 61.33 ± 8.89 < 42.13 ± 11.22 < 20.96 ± 3.12 ng/ml attained after 4.80, 8.00 and 10.40 h in case of LNC, HIP and solution, respectively. Moreover, the HIP and LNC formulae showed higher total drug amounts in plasma reaching 1.46 and 1.88-fold relative to RM solution, respectively. In conclusion, the HIP complex and HIP loaded LNCs prosper in enhancing the permeability and absorption of the low permeable drugs.

Synthesis and evaluation of sulfosuccinate-based surfactants as counterions for hydrophobic ion pairing

Hydrophobic ion pairing is a promising strategy to raise the lipophilic character of therapeutic peptides and proteins. In past studies, docusate, an all-purpose surfactant with a dialkyl sulfosuccinate structure, showed highest potential as hydrophobic counterion. Being originally not purposed for hydrophobic ion pairing, it is likely still far away from the perfect counterion. Thus, within this study, docusate analogues with various linear and branched alkyl residues were synthesized to derive systematic insights into which hydrophobic tail is most advantageous for hydrophobic ion pairing, as well as to identify lead counterions that form complexes with superior hydrophobicity. The successful synthesis of the target compounds was confirmed by FT-IR, ¹H-NMR, and ¹³C-NMR. In a screening with the model protein hemoglobin, monostearyl sulfosuccinate, dioleyl sulfosuccinate, and bis(isotridecyl) sulfosuccinate were identified as lead counterions. Their potential was further evaluated with the peptides and proteins vancomycin, insulin, and horseradish peroxidase. Dioleyl sulfosuccinate and bis(isotridecyl) sulfosuccinate significantly increased the hydrophobicity of the tested peptides and proteins determined as logP or lipophilicity determined as solubility in 1-octanol, respectively, in comparison to the gold standard docusate. Dioleyl sulfosuccinate provided an up to 8.3-fold higher partition coefficient and up to 26.5-fold higher solubility in 1-octanol than docusate, whereas bis(isotridecyl) sulfosuccinate resulted in an up to 6.7-fold improvement in the partition coefficient and up to 44.0-fold higher solubility in 1-octanol. The conjugation of highly lipophilic alkyl tails to the polar sulfosuccinate head group allows the design of promising counterions for hydrophobic ion pairing. STATEMENT OF SIGNIFICANCE: Hydrophobic ion pairing enables efficient incorporation of hydrophilic molecules into lipid-based formulations by forming complexes with hydrophobic counterions. Docusate, a sulfosuccinate with two branched alkyl tails, has shown highest potential as anionic hydrophobic counterion. As it was originally not purposed for hydrophobic ion pairing, its structure is likely still far away from the perfect counterion. To improve its properties, analogues of docusate with various alkyl tails were synthesized in the present study. The investigation of different alkyl residues allowed to derive systematic insights into which tail structures are most favorable for hydrophobic ion pairing. Moreover, the lead counterions dioleyl sulfosuccinate and bis(isotridecyl) sulfosuccinate bearing highly lipophilic alkyl tails provided a significant improvement in the hydrophobicity of the resulting complexes.

Oral delivery of therapeutic peptides and proteins: Technology landscape of lipid-based nanocarriers

The oral administration of therapeutic peptides and proteins is favoured from a patient and commercial point of view. In order to reach the systemic circulation after oral administration, these drugs have to overcome numerous barriers including the enzymatic, sulfhydryl, mucus and epithelial barrier. The development of oral formulations for therapeutic peptides and proteins is therefore necessary. Among the most promising formulation approaches are lipid-based nanocarriers such as oil-in-water nanoemulsions, self-emulsifying drug delivery systems (SEDDS), solid lipid nanoparticles (SLN), nanostructured lipid carriers (NLC), liposomes and micelles. As the lipophilic character of therapeutic peptides and proteins can be tremendously increased such as by the formation of hydrophobic ion pairs (HIP) with hydrophobic counter ions, they can be incorporated in the lipophilic phase of these carriers. Since gastrointestinal (GI) peptidases as well as sulfhydryl compounds such as glutathione and dietary proteins are too hydrophilic to enter the lipophilic phase of these carriers, the incorporated therapeutic peptide or protein is protected towards enzymatic degradation as well as unintended thiol/disulfide exchange reactions. Stability of lipid-based nanocarriers towards lipases can be provided by the use to excipients that are not or just poorly degraded by these enzymes. Nanocarriers with a size <200 nm and a mucoinert surface such as PEG or zwitterionic surfaces exhibit high mucus permeating properties. Having reached the underlying absorption membrane, lipid-based nanocarriers enable paracellular and lymphatic drug uptake, induce endocytosis and transcytosis or simply fuse with the cell membrane releasing their payload into the systemic circulation. Numerous in vivo studies provide evidence for the potential of these delivery systems. Within this review we provide an overview about the different barriers for oral peptide and protein delivery, highlight the progress made on lipid-based nanocarriers in order to overcome them and discuss strengths and weaknesses of these delivery systems in comparison to other technologies.

Synergism and properties of binary mixtures based on an arginine dodecyl ester surfactant

The properties of binary mixtures of new cationic amino acid surfactant arginine dihydrochloride dodecyl ester (ADDE) with alkyl poly glycosides (APGs) were studied systematically by evaluating surface tension, conductivity, dynamic...

Self-Assembling Drug Formulations with Tunable Permeability and Biodegradability

This review focuses on key topics in the field of drug delivery related to the design of nanocarriers answering the biomedicine criteria, including biocompatibility, biodegradability, low toxicity, and the ability to overcome biological barriers. For these reasons, much attention is paid to the amphiphile-based carriers composed of natural building blocks, lipids, and their structural analogues and synthetic surfactants that are capable of self-assembly with the formation of a variety of supramolecular aggregates. The latter are dynamic structures that can be used as nanocontainers for hydrophobic drugs to increase their solubility and bioavailability. In this section, biodegradable cationic surfactants bearing cleavable fragments are discussed, with ester- and carbamate-containing analogs, as well as amino acid derivatives received special attention. Drug delivery through the biological barriers is a challenging task, which is highlighted by the example of transdermal method of drug administration. In this paper, nonionic surfactants are primarily discussed, including their application for the fabrication of nanocarriers, their surfactant-skin interactions, the mechanisms of modulating their permeability, and the factors controlling drug encapsulation, release, and targeted delivery. Different types of nanocarriers are covered, including niosomes, transfersomes, invasomes and chitosomes, with their morphological specificity, beneficial characteristics and limitations discussed.

Nanostructured Lipid Carriers (NLCs) for Oral Peptide Drug Delivery: About the Impact of Surface Decoration

This study was aimed to evaluate the impact of surfactants used for nanostructured lipid carriers (NLCs) to provide enzymatic protection for incorporated peptides. Insulin as a model peptide was ion paired with sodium dodecyl sulfate to improve its lipophilicity. Three NLC formulations containing polyethylene glycol ester (PEG-ester), polyethylene glycol ether (PEG-ether), and polyglycerol ester (PG-ester) surfactants were prepared by solvent diffusion method. NLCs were characterized regarding particle size, polydispersity index, and zeta potential. Biocompatibility of NLCs was assessed on Caco-2 cells via resazurin assay. In vitro lipolysis study was performed using a standard lipid digestion method. Proteolytic studies were performed in simulated gastric fluid containing pepsin and simulated intestinal fluid containing pancreatin. Lipophilicity of insulin in terms of log Poctanol/water was improved from -1.8 to 2.1. NLCs were in the size range of 64-217 nm with a polydispersity index of 0.2-0.5 and exhibited a negative surface charge. PG-ester NLCs were non-cytotoxic up to a concentration of 0.5%, PEG-ester NLCs up to a concentration of 0.25% and PEG-ether NLC up to a concentration of 0.125% (w/v). The lipolysis study showed the release of >90%, 70%, and 10% of free fatty acids from PEG-ester, PG-ester, and PEG-ether NLCs, respectively. Proteolysis results revealed the highest protective effect of PEG-ether NLCs followed by PG-ester and PEG-ester NLCs for incorporated insulin complex. Findings suggest that NLCs bearing substructures less susceptible to degrading enzymes on their surface can provide higher protection for incorporated peptides toward gastrointestinal proteases.

Highly-loaded protein nanocarriers prepared by Flash NanoPrecipitation with hydrophobic ion pairing

The efficient encapsulation of therapeutic proteins into delivery vehicles, particularly without loss of function, remains a significant research hurdle. Typical liposomal formulations achieve drug loadings on the order of 3-5% and encapsulation efficiencies around 50%. We demonstrate the encapsulation of model proteins with isoelectric points above and below pH 7 into nanocarriers (NCs) with protein loadings as high as 46% and encapsulation efficiencies above 95%. This is done by combining the continuous nanofabrication process Flash NanoPrecipitation (FNP) with the technique of hydrophobic ion pairing by forming and encapsulating an ionic complex within a nanocarrier stabilized by a block copolymer surface layer. We complex and encapsulate lysozyme with two anionic hydrophobic counterions, sodium oleate and sodium dodecyl sulfate, using either a pre-formed complex or in situ pairing. The strategy successfully forms NCs ~150 nm in diameter and achieves encapsulation efficiencies over 95%. Protein release rate from the NCs in physiological conditions and the bioactivity of released lysozyme are measured, and both are found to vary with the complexing counterion and the protein/counterion ratio used during formulation. Protein release on the time scale of weeks is observed, and up to 100% bioactivity is measured from released lysozyme. 16 quaternary ammonium cationic counterions are tested to encapsulate ovalbumin in 32 formulations. Of these, 19 successfully form ~150 nm NCs with loadings up to 29% and encapsulation efficiencies up to 88%. We divide the formulations into four regimes and identify chemical factors responsible for the success or failure of a given counterion to formulate NCs with the desirable size, loading, and encapsulation efficiency. A successful ovalbumin NC formulation was then tested in vivo in a mouse nasal vaccine model and found to induce a higher titer of OVA-specific IgG than unencapsulated ovalbumin. Taken together, these findings suggest that Flash NanoPrecipitation with hydrophobic ion pairing is an attractive platform for encapsulating high molecular weight proteins into NCs. In particular, the ability to tune protein release rate by varying the counterion or protein/counterion ratio used during formulation is a useful feature.

Oral peptide delivery: challenges and the way ahead

Peptides and proteins have emerged as potential therapeutic agents and, in the search for the best treatment regimen, the oral route has been extensively evaluated because of its non-invasive and safe nature. The physicochemical properties of peptides and proteins along with the hurdles in the gastrointestinal tract (GIT), such as degrading enzymes and permeation barriers, are challenges to their delivery. To address these challenges, several conventional and novel approaches, such as nanocarriers, site-specific and stimuli specific delivery, are being used. In this review, we discuss the challenges to the oral delivery of peptides and the approaches used to tackle these challenges.

Mucolytic self-emulsifying drug delivery systems (SEDDS) containing a hydrophobic ion-pair of proteinase

AIM

The aim of this study was to form hydrophobic ion-pairs of proteinase with cationic surfactants and to incorporate them into self-emulsifying drug delivery systems (SEDDS) to improve their mucus permeating properties.

METHODS

Proteinase was ion-paired with benzalkonium chloride (BAK), hexadecylpyridinium chloride (HDP), alkyltrimethylammonium bromide (ATA) and hexadecyltrimethylammonium bromide (HDT) at pH 8.5-9.0, and subsequently incorporated into SEDDS consisting of Cremophor EL, propylene glycol, and Capmul 808-G (40/20/40). Mucus permeation of SEDDS containing proteinase complexes was evaluated via rotating tube technique and cell-free Transwell® insert system. Additionally, enzymatic activity of proteinase complexes as well as their potential cytotoxicity was evaluated.

RESULTS

Among all tested hydrophobic ion-pairs, proteinase/BAK showed highest potential. Mucus diffusion of SEDDS containing proteinase/BAK complex yielded in 2.3-fold and 2.5-fold higher mucus permeability with respect to blank SEDDS at Transwell® insert system and rotating tube technique, respectively. Furthermore, proteinase/BAK complex maintained the highest enzymatic activity of 50.5 ± 5.6% compared to free proteinase. At a SEDDS concentration as low as 0.006% cell viability was just 80%. The addition of proteinase complexes to SEDDS increased cytotoxicity on Caco-2 cells in a concentration-dependent manner.

CONCLUSION

SEDDS loaded with proteinase/BAK complexes are promising nanocarriers because of enhanced mucus permeating properties.

Self-Assembly of Amphiphilic Compounds as a Versatile Tool for Construction of Nanoscale Drug Carriers

This review focuses on synthetic and natural amphiphilic systems prepared from straight-chain and macrocyclic compounds capable of self-assembly with the formation of nanoscale aggregates of different morphology and their application as drug carriers. Since numerous biological species (lipid membrane, bacterial cell wall, mucous membrane, corneal epithelium, biopolymers, e.g., proteins, nucleic acids) bear negatively charged fragments, much attention is paid to cationic carriers providing high affinity for encapsulated drugs to targeted cells. First part of the review is devoted to self-assembling and functional properties of surfactant systems, with special attention focusing on cationic amphiphiles, including those bearing natural or cleavable fragments. Further, lipid formulations, especially liposomes, are discussed in terms of their fabrication and application for intracellular drug delivery. This section highlights several features of these carriers, including noncovalent modification of lipid formulations by cationic surfactants, pH-responsive properties, endosomal escape, etc. Third part of the review deals with nanocarriers based on macrocyclic compounds, with such important characteristics as mucoadhesive properties emphasized. In this section, different combinations of cyclodextrin platform conjugated with polymers is considered as drug delivery systems with synergetic effect that improves solubility, targeting and biocompatibility of formulations.

Lipophilic Arginine Esters: The Gateway to Preservatives without Side Effects

This study hypothesized that long carbon chain cationic arginine (Arg) esters can be considered as toxicologically harmless preservatives. Arg-esters with C₁₈ and C₂₄ carbon chains, namely, arginine-oleate (Arg-OL) and arginine-decyltetradecanoate (Arg-DT), were synthesized. Structures were confirmed by FT-IR, ¹H NMR, and mass spectroscopy. Both Arg-esters were tested regarding hydrophobicity in terms of log P_{octanol/water}, critical micelle concentration (CMC), biodegradability, cytotoxicity, hemolysis, and antimicrobial activity against Escherichiacoli (E. coli), Staphylococcusaureus (S. aureus), Bacillussubtilis (B. subtilis), and Enterococcusfaecalis (E. faecalis). Log P_{octanol/water} of arginine was raised from −1.9 to 0.3 and 0.6 due to the attachment of C₁₈ and C₂₄ carbon chains, respectively. The critical micelle concentration of Arg-OL and Arg-DT was 0.52 and 0.013 mM, respectively. Both Arg-esters were biodegradable by porcine pancreatic lipase. In comparison to the well-established antimicrobials, benzalkonium chloride (BAC) and cetrimide, Arg-esters showed significantly less cytotoxic and hemolytic activity. Both esters exhibited pronounced antimicrobial properties against Gram-positive and Gram-negative bacteria comparable to that of BAC and cetrimide. The minimum inhibitory concentration (MIC) of Arg-esters was <50 μg mL^–1 against all tested microbes. Overall, results showed a high potential of Arg-esters with long carbon chains as toxicologically harmless novel preservatives.

Lysine-Based Biodegradable Surfactants: Increasing the Lipophilicity of Insulin by Hydrophobic Ion Paring

AIM

The aim of this study was to evaluate biodegradable cationic surfactants based on lysine.

METHODS

Lysine was esterified with cholesterol, oleyl alcohol and 1-decanol resulting in cholesteryl lysinate (CL), oleyl lysinate (OL) and decyl lysinate (DL). Esters were investigated regarding their log D_{n-octanol/water}, critical micelle concentration (CMC) and biodegradability. Hemolytic potential of CL, OL, DL and the already established hexadecyl lysinate (HL) was determined and complexes with insulin (INS) were formed by hydrophobic ion pairing (HIP). Lipophilic characteristics of ion-pairs were examined by analyzing their log P_{n-butanol/water}.

RESULTS

Successful synthesis of CL, OL and DL was confirmed by IR, NMR and MS. Log D analysis revealed amphiphilic properties for the esters and a CMC of 0.01 mM, 2.0 mM and 6.0 mM was found for CL, OL and DL, respectively. Biodegradability was proven, as over 99% of OL and DL were degraded by isolated enzymes within 30 min and after 3 h 97% of CL was cleaved by membrane bound enzymes. OL as well as DL displayed no hemolytic effect and for CL cytotoxicity was significantly reduced in comparison to HL. INS/CL complex exhibited highest lipophilicity.

CONCLUSION

Cholesterol-amino acid based surfactants seem to be promising agents for HIP.

Less Reactive Thiol Ligands: Key towards Highly Mucoadhesive Drug Delivery Systems

As less reactive s-protected thiomers can likely interpenetrate the mucus gel layer to a higher extent before getting immobilized via disulfide bond formation with mucins, it was the aim of this study to develop a novel type of s-protected thiomer based on the less reactive substructure cysteine-N-acetyl cysteine (Cys-NAC) in order to obtain improved mucoadhesive properties. For this purpose, two types of s-protected thiomers, polyacrylic acid-cysteine-mercaptonicotinic acid (PAA-Cys-MNA) and polyacrylic acid-cysteine-N-acetyl cysteine (PAA-Cys-NAC), were synthesized and characterized by Fourier-transform infrared spectroscopy (FT-IR) and the quantification of attached disulfide ligands. The viscosity of both products was measured in the presence of NAC and mucus. Both thiomers were also evaluated regarding swelling behavior, tensile studies and retention time on the porcine intestinal mucosa. The FT-IR spectra confirmed the successful attachment of Cys-MNA and Cys-NAC ligands to PAA. The number of attached sulfhydryl groups was in the range of 660–683 µmol/g. The viscosity of both s-protected thiomers increased due to the addition of increasing amounts of NAC. The viscosity of the mucus increased in the presence of 1% PAA-Cys-MNA and PAA-Cys-NAC 5.6- and 10.9-fold, respectively, in comparison to only 1% PAA. Both s-protected thiomers showed higher water uptake than unmodified PAA. The maximum detachment force (MDF) and the total work of adhesion (TWA) increased in the case of PAA-Cys-MNA up to 1.4- and 1.6-fold and up to 2.4- and 2.8-fold in the case of PAA-Cys-NAC. The retention of PAA, PAA-Cys-MNA, and PAA-Cys-NAC on porcine intestinal mucosa was 25%, 49%, and 76% within 3 h, respectively. The results of this study provide evidence that less reactive s-protected thiomers exhibit higher mucoadhesive properties than highly reactive s-protected thiomers.

S-protected thiolated hyaluronic acid: In-situ crosslinking hydrogels for 3D cell culture scaffold

The purpose of this study was to synthesize S-protected thiolated hyaluronic acid (HA) and to evaluate its potential for 3D cell culture scaffold. S-protected thiolated HA was synthesized by the covalent attachment of N-acetyl-S-((3-((2,5-dioxopyrrolidin-1-yl)oxy)-3-oxopropyl)thio)cysteine hydrazide ligand to the HA. Hydrogels were characterized for texture, swelling behavior and rheological properties. Furthermore, the potential of S-protected thiolated HA hydrogels as a scaffold for tissue engineering was evaluated by cell proliferation studies with Caco-2 and NIH 3T3 cells. It showed enhanced cohesion upon addition of N-acetyl cysteine (NAC). Dynamic viscosity of S-protected thiolated HA hydrogel was increased up to 19.5-fold by addition of NAC and 10.1-fold after mixing with mucus. Furthermore, Caco-2 and NIH 3T3 cells encapsulated into hydrogels proliferated in-vitro. As this novel S-protected thiolated HA is stable towards oxidation and forms highly cohesive gels when getting into contact with endogenous thiols due to disulfide-crosslinking, it is a promising tool for 3D cell culture scaffold.

Cationic Surfactants: Self-Assembly, Structure-Activity Correlation and Their Biological Applications

The development of biotechnological protocols based on cationic surfactants is a modern trend focusing on the fabrication of antimicrobial and bioimaging agents, supramolecular catalysts, stabilizers of nanoparticles, and especially drug and gene nanocarriers. The main emphasis given to the design of novel ecologically friendly and biocompatible cationic surfactants makes it possible to avoid the drawbacks of nanoformulations preventing their entry to clinical trials. To solve the problem of toxicity various ways are proposed, including the use of mixed composition with nontoxic nonionic surfactants and/or hydrotropic agents, design of amphiphilic compounds bearing natural or cleavable fragments. Essential advantages of cationic surfactants are the structural diversity of their head groups allowing of chemical modification and introduction of desirable moiety to answer the green chemistry criteria. The latter can be exemplified by the design of novel families of ecological friendly cleavable surfactants, with improved biodegradability, amphiphiles with natural fragments, and geminis with low aggregation threshold. Importantly, the development of amphiphilic nanocarriers for drug delivery allows understanding the correlation between the chemical structure of surfactants, their aggregation behavior, and their functional activity. This review focuses on several aspects related to the synthesis of innovative cationic surfactants and their broad biological applications including antimicrobial activity, solubilization of hydrophobic drugs, complexation with DNA, and catalytic effect toward important biochemical reaction.

Hydrophobic ion pairing: encapsulating small molecules, peptides, and proteins into nanocarriers

Hydrophobic ion pairing has emerged as a method to modulate the solubility of charged hydrophilic molecules ranging in class from small molecules to large enzymes. Charged hydrophilic molecules are ionically paired with oppositely-charged molecules that include hydrophobic moieties; the resulting uncharged complex is water-insoluble and will precipitate in aqueous media. Here we review one of the most prominent applications of hydrophobic ion pairing: efficient encapsulation of charged hydrophilic molecules into nano-scale delivery vehicles - nanoparticles or nanocarriers. Hydrophobic complexes are formed and then encapsulated using techniques developed for poorly-water-soluble therapeutics. With this approach, researchers have reported encapsulation efficiencies up to 100% and drug loadings up to 30%. This review covers the fundamentals of hydrophobic ion pairing, including nomenclature, drug eligibility for the technique, commonly-used counterions, and drug release of encapsulated ion paired complexes. We then focus on nanoformulation techniques used in concert with hydrophobic ion pairing and note strengths and weaknesses specific to each. The penultimate section bridges hydrophobic ion pairing with the related fields of polyelectrolyte coacervation and polyelectrolyte-surfactant complexation. We then discuss the state of the art and anticipated future challenges. The review ends with comprehensive tables of reported hydrophobic ion pairing and encapsulation from the literature.