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Protein-protein interactions play a crucial role in the development of drug delivery devices for the increasingly important biologicals, including antibodies, growth factors and cytokines. The understanding thereof might offer opportunities for tailoring carriers or drug proteins specifically for this purpose and thereby allow controlled delivery to a chosen target. The possible applications range from trigger-dependent release to sustained drug delivery and possibly permanently present stimuli, depending on the anticipated mechanism.
Silk fibroin (SF) is a biomaterial that is suitable as a carrier for protein drug delivery devices. It combines processability under mild conditions, good biocompatibility and stabilizing effects on incorporated proteins.
As SF is naturally produced by spiders and silkworms, the understanding of this process and its major factors might offer a blueprint for formulation scientists, interested in working with this biopolymer. The natural process of silk spinning covers a fascinating versatility of aggregate states, ranging from colloidal solutions through hydrogels to solid systems. The transition among these states is controlled by a carefully orchestrated process in vivo. Major players within the natural process include the control of spatial pH throughout passage of the silk dope, the composition and type of ions, and fluid flow mechanics within the duct, respectively. The function of these input parameters on the spinning process is reviewed before detailing their impact on the design and manufacture of silk based drug delivery systems (DDS). Examples are reported including the control of hydrogel formation during storage or significant parameters controlling precipitation in the presence of appropriate salts, respectively. The review details the use of silk fibroin to develop liquid, semiliquid or solid DDS with a focus on the control of SF crystallization, particle formation, and drug-SF interaction for tailored drug load.
Although we were able to show many examples for SF drug delivery applications and there are many publications about the loading of biologics to SF systems, the mechanism of interaction between both in solution was not yet extensively explored. This is why we made this the subject of our work, as it might allow for direct influence on pharmaceutical parameters, like aggregation and drug load.
In order to understand the underlying mechanism for the interaction between SF and positively charged model proteins, we used isothermal titration calorimetry for thermodynamic characterization. This was supported by hydrophobicity analysis and by colloidal characterization methods including static light scattering, nanoparticle tracking analysis and zeta potential measurements. We studied the effects of three Hofmeister salts – NaCl (neutral), NaSCN (chaotropic) and Na2SO4 (cosmotropic) – and the pH on the interaction of SF with the model proteins in dependence of the ratio from one to another. The salts impacted the SF structure by stabilizing (cosmotropic) or destabilizing (chaotropic) the SF micelles, resulting in completely abolished (cosmotropic) or strongly enhanced (chaotropic) interaction. These effects were responsible for different levels of loading and coacervation when varying type of salt and its concentration. Additionally, NaCl and NaSCN were able to prolong the stability of aqueous SF solution during storage at 25°C in a preliminary study.
Another approach to influence protein-protein interactions was followed by covalent modification. Interleukin-4 (IL-4) is a cytokine driving macrophages to M2 macrophages, which are known to provide anti-inflammatory effects. The possibility to regulate the polarization of macrophages to this state might be attractive for a variety of diseases, like atherosclerosis, in which macrophages are involved. As these cases demand a long-term treatment, this polarization was supposed to be maintained over time and we were planning to achieve this by keeping IL-4 permanently present in an immobilized way. In order to immobilize it, we genetically introduced an alkyne-carrying, artificial amino acid in the IL-4 sequence. This allowed access to a site-specific click reaction (Cu(I)-catalyzed Huisgen azide-alkyne cycloaddition) with an azide partner. This study was able to set the basis for the project by successful expression and purification of the IL-4 analogue and by proving the availability for the click reaction and maintained bioactivity. The other side of this project was the isolation of human monocytes and the polarization and characterization of human macrophages. The challenge here was that the majority of related research was based on murine macrophages which was not applicable to human cells and the successful work was so far limited to establishing the necessary methods.
In conclusion, we were able to show two different methods that allow the influence of protein-protein interactions and thereby the possible tailoring of drug loading. Although the results were very promising for both systems, their applicability in the development of drug delivery devices needs to be shown by further studies.
Cell culture models are helpful tools to study inflammatory diseases, like rheumatoid arthritis (RA), osteoarthritis (OA), arteriosclerosis or asthma, which are linked to increased matrix metalloproteinase (MMP) activity. Such cell culture models often focus on the secretion of cytokines and growth factors or the direct effects of disease on tissue destruction. Even though the crucial role of MMPs in inflammatory diseases is known, the results of MMP studies are contradictious and the use of MMPs as biomarkers is inconsistent. MMPs play an important role in disease pathology, as they are involved in elastin degradation in the walls of alveoli in chronic obstructive pulmonary disease (COPD), tumor angiogenesis and metastasis and in cartilage and bone degradation in arthropathies. In RA and OA MMPs are secreted by osteocytes, synoviocytes, and by infiltrating immune cells in response to the increased concentration of inflammatory mediators, like growth factors and cytokines. MMPs are zinc and calcium-dependent proteinases and play an important role in physiological and pathological extracellular matrix (ECM) turn over. Their substrate specificity gives them the ability to degrade all major ECM components, like aggrecan, elastin, gelatin, fibronectin and all types of collagen even the triple helix of collagen monomers. The ECM consists of two large three-dimensional cross-linked macromolecule classes: one are fibrous proteins, like collagen and elastin fibers that are responsible for ECM’s structure, tensile strength, resiliency, reversible extensibility, and deformability and the second class is comprised of proteoglycans composed of glycosaminoglycan (GAG) chains covalently attached to protein cores that are multifunctionally involved in signaling pathways and cell interactions. ECM is present within all tissues and organs and changes in ECM structure contribute to pathogenesis, e.g. wounded and fibrotic tissue, COPD or tumours.
This thesis primarily focuses on the development of a diagnostic peptide system, that enables to gain information on MMP activity from ECM by deploying the isobaric mass encoding strategy. The core element of the developed system is an isotopically labelled peptide sequence (mass tag), that is released in response to elevated levels of MMPs and allows multiplexed detection in tandem mass spectrometry (LC-MS/MS). The mass reporters possess a modular structure with different functionalities. C-terminal either a transglutaminase (TG) recognition sequence or a high molecular weight polyethylene glycol (PEG) moiety was attached to immobilize the mass reporters covalently or physically at the injection site. The following matrix metalloproteinase substrate sequence (MSS) is incorporated in two different versions with different sensitivity to MMPs. The MSS were applied in pairs for relative quantification consisting of the cleavable version synthesized with natural L-amino acids and the non-cleavable D-amino acid variant. The mass tag was synthesized with isotopically labelled amino acids and is separated from the MSS by a UV light-sensitive molecule. N-terminal the mass tag is followed by a tobacco etch virus protease (TEV) sensitive sequence, that is responsible to separate the mass tag from the affinity tag, which was either the Strep-tag II sequence or biotin and were added for purification purposes.
Chapter 1 presents a step-by-step protocol on how to design a mass tag family allowing for multiplexed analysis by LC-MS/MS. The multiplexing is achieved by developing an isobar mass tag family with four family members, which are chromatographically indistinguishable, but due to the mass encoding principles they fragment in distinct y-type ions with a mass difference of 1 or 2 Da each in MS2. Furthermore, it is explained how to covalently attach the mass reporter peptides onto ECM by the activated calcium-catalyzed blood coagulation transglutaminase factor XIII (FXIIIa). The lysine of mass reporter’s TG sequence (D-domain of insulin-like growth factor-I (IGF-I)) and a glutamine in fibronectin are covalently crosslinked by FXIIIa and build an isopeptide bond. Elevated levels of MMP release the mass reporters from ECM by recognizing the inter-positioned MSS.
The designed mass reporters were able to monitor enzyme activity in an in vitro setting with cell-derived ECM, which was shown in Chapter 2. The modular structured mass reporters were investigated in a proof of concept study. First, the different modules were characterized in terms of their MMP responsiveness and their sensitivity to TEV protease and UV light. Then the FXIIIa-mediated coupling reaction was detailed and the successful coupling on ECM was visualized by an immunosorbent assay or confocal laser scanning microscopy. Finally, the immobilized mass reporters on ECM were incubated with MMP-9 to investigate their multiplexing ability of MMP activity. The cleaved mass reporter fragments were purified in three steps and mass tags were analyzed as mix of all four in LC-MS/MS.
Chapter 3 describes the change from an immobilizing system as seen in chapter 1 and 2 to a soluble enzyme activity monitoring system that was applied in an osteoarthritic mouse model. Instead of the immobilizing TG sequence the C-terminal MMS was extended with two amino acids where one holds an azide moiety to perform a strain-promoted azide-alkyne cycloaddition to a high molecular weight dibenzocyclooctyne-polyethylene glycol (DBCO-PEG), which was chosen to retain the mass reporters at the injection site. Furthermore, the N-terminal affinity tag was extended with a 2.5 kDa PEG chain to increase the half-life of the mass reporter peptides after MMP release. The systems biocompatibility was proved but its enzyme monitoring ability in an in vivo setting could not be analyzed as samples degraded during shipping resulting from the Chinese customs blocking transport to Germany.
In summary the diagnostic peptide system was developed in two variants. The immobilized version one from chapter 1 and 2 was designed to be covalently attached to ECM by the transglutaminase-mediated cross-linking reaction. In an in vitro setting the functionality of the mass reporter system for the detection of MMP activity was successfully verified. The second variant comprises of a soluble mass reporter system that was tested in an OA mouse model and showed biocompatibility. With these two designed systems this thesis provides a flexible platform based on multiplexed analysis with mass-encoded peptides to characterize cell culture models regarding their MMP activity, to deploy cell-derived ECM as endogenous depot scaffold and to develop a mass tag family that enables simultaneous detection of at least four mass tags.
Each year millions of plastic and reconstructive procedures are performed to regenerate soft tissue defects after, for example, traumata, deep burns or tumor resections. Tissue engineered adipose tissue grafts are a promising alternative to autologous fat transfer or synthetic implants to meet this demand for adipose tissue. Strategies of tissue engineering, especially the use of cell carriers, provide an environment for better cell survival, an easier positioning and supplemented with the appropriate conditions a faster vascularization in vivo. To successfully engineer an adipose tissue substitute for clinical use, it is crucial to know the actual intended application. In some areas, like the upper and lower extremities, only a thin subcutaneous fat layer is needed and in others, large volumes of vascularized fat grafts are more desirable. The use and interplay of stem cells and selected scaffolds were investigated and provide now a basis for the generation of fitted and suitable substitutes in two different application areas.
Complex injuries of the upper and lower extremities, in many cases, lead to excessive scarring. Due to severe damage to the subcutaneous fat layer, a common sequela is adhesion formation to mobile structures like tendons, nerves, and blood vessels resulting in restricted motion and disabling pain [Moor 1996, McHugh 1997]. In order to generate a subcutaneous fat layer to cushion scarred tissue after substantial burns or injuries, different collagen matrices were tested for clinical handling and the ability to support adipogenesis. When testing five different collagen matrices, PermacolTM and StratticeTM showed promising characteristics; additionally both possess the clinical approval. Under culture conditions, only PermacolTM, a cross-linked collagen matrix, exhibited an excellent long-term stability. Ranking nearly on the same level was StratticeTM, a non-cross-linked dermal scaffold; it only exhibited a slight shrinkage. All other scaffolds tested were severely compromised in stability under culture conditions. Engineering a subcutaneous fat layer, a construct would be desirable with a thin layer of emerging fat for cushioning on one side, and a non-seeded other side for cell migration and host integration. With PermacolTM and StratticeTM, it was possible to produce constructs with ASC (adipose derived stem cells) seeded on one side, which could be adipogenically differentiated. Additionally, the thickness of the cell layer could be varied. Thereby, it becomes possible to adjust the thickness of the construct to the surrounding tissue. In order to reduce the pre-implantation time ex vivo and the costs, the culture time was varied by testing different induction protocols. An adipogenic induction period of only four days was demonstrated to be sufficient to obtain a substantial adipogenic differentiation of the applied ASC. Thus, seeded with ASC, PermacolTM and StratticeTM are suitable scaffolds to engineer subcutaneous fat layers for reconstruction of the upper and lower extremities, as they support adipogenesis and are appropriately thin, and therefore would not compromise the cosmesis.
For the engineering of large-volume adipose tissue, adequate vascularization still represents a major challenge. With the objective to engineer vascularized fat pads, it is important to consider the slow kinetics of revascularization in vivo. Therefore, a decellularized porcine jejunum with pre-existing vascular structures and pedicles to connect to the host vasculature or the circulation of a bioreactor system was used. In a first step, the ability of a small decellularized jejunal section was tested for cell adhesion and for supporting adipogenic differentiation of hASC mono-cultures. Cell adhesion and adipogenic maturation of ASC seeded on the jejunal material was verified through histological and molecular analysis. After the successful mono-culture, the goal was to establish a MVEC (microvascular endothelial cells) and ASC co-culture; suitable culture conditions had to be found, which support the viability of both cell types and do not interfere with the adipogenic differentiation. After the elimination of EGF (epidermal growth factor) from the co-culture medium, substantial adipogenic maturation was observed. In the next step, a large jejunal segment (length 8 cm), with its pre-existing vascular structures and arterial/venous pedicles, was connected to the supply system of a custom-made bioreactor. After successful reseeding the vascular structure with endothelial cells, the lumen was seeded with ASC which were then adipogenically induced. Histological and molecular examinations confirmed adipogenic maturation and the existence of seeded vessels within the engineered construct. Noteworthily, a co-localization of adipogenically differentiating ASC and endothelial cells in vascular networks could be observed. So, for the first time a vascularized fat construct was developed in vitro, based on the use of a decellularized porcine jejunum. As this engineered construct can be connected to a supply system or even to a patient vasculature, it is versatile in use, for example, as transplant in plastic and reconstruction surgery, as model in basic research or as an in vitro drug testing system.
To summarize, in this work a promising substitute for subcutaneous fat layer reconstruction, in the upper and lower extremities, was developed, and the first, as far as reported, in vitro generated adipose tissue construct with integrated vascular networks was successfully engineered.
Agrochemicals like systemic active ingredients (AI) need to penetrate the outermost barrier of the plant, known as the plant cuticle, to reach its right target site. Therefore, adjuvants are added to provide precise and efficient biodelivery by i.a. modifying the cuticular barrier and increasing the AI diffusion. This modification process is depicted as plasticization of the cuticular wax which mainly consists of very long-chain aliphatic (VLCA) and cyclic compounds. Plasticization of cuticular waxes is pictured as an increase of amorphous domains and/or a decrease of crystalline fractions, but comprehensive, experimental proof is lacking to date. Hence, the objective of this thesis was to i) elucidate the permeation barrier of the plant cuticle to AIs in terms of the different wax fractions and ii) holistically investigate the modification of this barrier using selected oil and surface active adjuvants, an aliphatic leaf wax and an artificial model wax. Therefore, the oil adjuvant methyl oleate (MeO) and other oil derivatives like methyl linolenate (MeLin), methyl stearate (MeSt) and oleic acid (OA) were selected. Three monodisperse, non-ionic alcohol ethoxylates with increasing ethylene oxide monomer (EO) number (C10E2, C10E5, C10E8) were chosen as representatives of the group of surface active agents (surfactants). Both adjuvant classes are commonly used as formulation aids for agrochemicals which are known for its penetration enhancing effect. The aliphatic leaf wax of Schefflera elegantissima was selected, as well as a model wax comprising the four most abundant cuticular wax compounds of this species. Permeation, transpiration and penetration studies were conducted using enzymatically isolated cuticles of Prunus laurocerasus and Garcinia xanthochymus.
Cuticular permeability to the three organic solutes theobromine, caffeine and azoxystrobin differing in lipophilicity was measured using a steady-state two-chamber system separated by the isolated leaf cuticles of the evergreen species P. laurocerasus and G. xanthochymus. Treating the isolated cuticles with methanol selectively removed the cyclic fraction, and membrane permeability to the organic compounds was not altered. In contrast, fully dewaxing the membranes using chloroform resulted in a statistically significant increase in permeance for all compounds and species, except caffeine with cuticles of G. xanthochymus due to a matrix-specific influence on the semi-hydrophilic compound. Crystalline regions may reduce the accessibility to the lipophilic pathway across the waxes and also block hydrophilic domains in the cuticle.
Knowing that the aliphatic wax fraction builds the cuticular diffusion barrier, the influence of the adjuvants on the phase behaviour of an aliphatic cuticular wax as well as the influence on the cuticular penetration of AIs were investigated. Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) were selected to investigate the phase behaviour and thus possible plasticization of pure Schefflera elegantissima leaf wax, its artificial model wax comprising the four most abundant compounds (n-nonacosane, n-hentriacontane, 1-triacontanol and 1-dotriacontanol) and wax adjuvant mixtures. DSC thermograms showed a shift of the melting ranges to lower temperatures and decreased absolute values of the total enthalpy of transition (EOT) for all adjuvant leaf wax blends at 50 % (w/w) adjuvant proportion. The highest decrease was found for C10E2 followed by MeO > OA and C10E8 > MeLin > MeSt. The aliphatic crystallinity determined by FTIR yielded declined values for the leaf and the artificial wax with 50 % MeO. All other adjuvant leaf wax blends did not show a significant decrease of crystallinity. As it is assumed that the cuticular wax is formed by crystalline domains which consist of aliphatic hydrocarbon chains and an amorphous fraction comprising aliphatic chain ends and functional groups, the plasticizers are depicted as wax disruptors influencing amorphization and/or crystallization. The adjuvants can increase crystalline domains using the aliphatic tail whereas their more hydrophilic head is embedded in the amorphous wax fraction. DSC and FTIR showed similar trends using the leaf wax and the model wax in combination with the adjuvants.
In general, cuticular transpiration increased after adding the pure adjuvants to the surface of isolated cuticles or leaf envelopes. As waxes build the cuticular permeation barrier not only to AIs but also to water, the adjuvant wax interaction might affect the cuticular barrier properties leading to increased transpiration. Direct evidence for increased AI penetration with the adjuvants was given using isolated cuticles of P. laurocerasus in combination with the non-steady-state setup simulation of foliar penetration (SOFP) and caffeine at relative humidity levels (RH) of 30, 50 and 80 %. The increase in caffeine penetration was much more pronounced using C10E5 and C10E8 than MeO but always independent of RH. Only C10E2 exhibited an increased penetration enhancing effect positively related to RH. The role of the molecular structure of adjuvants in terms of humectant and plasticizer properties are discussed.
Hence, the current work shows for the first time that the cuticular permeation barrier is associated with the VLCAs rather than the cyclic fraction and that adjuvants structurally influence this barrier resulting in penetration enhancing effects. Additionally, this work demonstrates that an artificial model wax is feasible to mimic the wax adjuvant interaction in conformity with a leaf wax, making it feasible for in-vitro experiments on a larger scale (e.g. screenings). This provides valuable knowledge about the cuticular barrier modification to enhance AI penetration which is a crucial factor concerning the optimization of AI formulations in agrochemistry.
The bile system in vertebrates is an evolutionary conserved endogenous solubilization system for hydrophobic fats and poorly water-soluble vitamins. Bile pours out from the gallbladder through the common bile duct into the duodenum triggered by cholecystokinin. Cholecystokinin is released from enteroendocrine cells after food intake. The small intestine is also the absorption site of many orally administered drugs. Most emerging drug candidates belong to the class of poorly water-soluble drugs (PWSDs). Like hydrophobic vitamins, these PWSDs might as well be solubilized by bile. Therefore, this natural system is of high interest for drug formulation strategies. Simulated intestinal fluids containing bile salts (e.g., taurocholate TC) and phospholipids (e.g., lecithin L) have been widely applied over the last decade to approximate the behavior of PWSDs in the intestine. Solubilization by bile can enhance the oral absorption of PWSDs being at least in part responsible for the positive “food effect”. The dissolution rate of PWSDs can be also enhanced by the presence of bile. Furthermore, some PWSDs profit from supersaturation stabilization by bile salts. Some excipients solubilizing PWSDs seemed to be promising candidates for drug formulation when investigated in vitro without bile. When tested in vivo, these excipients reduced the bioavailability of drugs. However, these observations have been hardly examined on a molecular level and general links between bile interaction in vitro and bioavailability are still missing.
This thesis investigated the interplay of bile, PWSDs, and excipients on a molecular level, providing formulation scientists a blueprint for rational formulation design taking bile/PWSD/excipient/ interaction into account. The first chapter focus on an in silico 1H nuclear magnetic resonance (NMR) spectroscopy-based algorithm for bile/drug interaction prediction. Chapter II to IV report the impact of excipients on bioavailability of PWSDs interacting with bile. At last, we summarized helpful in vitro methods for drug formulation excipient choice harnessing biopharmaceutic solubilization in chapter V.
Chapter I applies 1H NMR studies with bile and drugs on a large scale for quantitative structure-property relationship analysis. 141 drugs were tested in simulated intestinal media by 1H NMR. Drug aryl-proton signal shifts were correlated to in silico calculated molecular 2D descriptors. The probability of a drug interacting with bile was dependent on its polarizability and lipophilicity, whereas interaction with lipids in simulated intestinal media components was dependent on molecular symmetry, lipophilicity, hydrogen bond acceptor capability, and aromaticity. The probability of a drug to interact with bile was predictive for a positive food effect. This algorithm might help in the future to identify a bile and lipid interacting drug a priori.
Chapter II investigates the impact of excipients on bile and free drug fraction. Three different interaction patterns for excipients were observed. The first pattern defined excipients that interacted with bile and irreversibly bound bile. Therefore, the free drug fraction of bile interacting drugs increased. The second pattern categorized excipients that formed new colloidal entities with bile which had a high affinity to bile interacting drugs. These colloids trapped the drug and decreased the free drug fraction. The last excipient pattern described excipients that formed supramolecular structures in coexistence with bile and had no impact on the free drug fraction. These effects were only observed for drugs interacting with bile (Perphenazine and Imatinib). Metoprolol’s free drug fraction, a compound not interacting with bile, was unaffected by bile or bile/excipient interaction. We hypothesized that bile/excipient interactions may reduce the bioavailability of bile interacting drugs.
Chapter III addresses the hypothesis from chapter II. A pharmacokinetic study in rats revealed that the absorption of Perphenazine was reduced by bile interacting excipients due to bile/excipient interaction. The simultaneous administration of excipient patterns I and II did not further reduce or enhance Perphenazine absorption. Conversely, the absorption of Metoprolol was not impacted by excipients. This reinforced the hypothesis, that drugs interacting with bile should not be formulated with excipients also interacting with bile.
Chapter IV further elaborates which in vitro methods using simulated intestinal fluids are predictive for a drug’s pharmacokinetic profile. The PWSD Naporafenib was analyzed in vitro with simulated intestinal fluids and in presence of excipients regarding solubility, supersaturation, and free drug fraction. Naporafenib showed a strong interaction with TC/L from simulated bile. Assays with TC/L, but not without identified one excipient as possibly bioavailability reducing, one as supersaturation destabilizing, and the last as bile not interacting and supersaturation stabilizing excipient. A pharmacokinetic study in beagle dogs outlined and confirmed the in vitro predictions.
The Appendix summarizes in vivo predictive methods as presented in chapter I to IV and rationalizes experimental design paving the way towards a biopharmaceutic excipient screening. The first presented preliminary decision tree is transformed into a step-by-step instruction. The presented decision matrix might serve as a blueprint for processes in early phase drug formulation development.
In summary, this thesis describes how a drug can be defined as bile interacting or non-interacting and gives a guide as well how to rate the impact of excipients on bile. We showed in two in vivo studies that bile/excipient interaction reduced the bioavailability of bile interacting drugs, while bile non-interacting drugs were not affected. We pointed out that the bile solubilization system must be incorporated during drug formulation design. Simulated gastrointestinal fluids offer a well-established platform studying the fate of drugs and excipients in vivo. Therefore, rational implementation of biopharmaceutic drug and excipient screening steers towards efficacy of oral PWSD formulation design.
This thesis aimed at searching for new effective agents against Multidrug-Resistant Enterobacteriaceae. This is necessitated by the urgent need for new and innovative antibacterial agents addressing the critical priority pathogens prescribed by the World Health Organization (WHO). Among the available means for antibiotics discovery and development, nature has long remained a proven, innovative, and highly reliable gateway to successful antibacterial agents. Nevertheless, numerous challenges surrounding this valuable source of antibiotics among other drugs are limiting the complete realization of its potential. These include the availability of good quality data on the highly potential natural sources, limitations in methods to prepare and screen crude extracts, bottlenecks in reproducing biological potentials observed in natural sources, as well as hurdles in isolation, purification, and characterization of natural compounds with diverse structural complexities.
Through an extensive review of the literature, it was possible to prepare libraries of plant species and phytochemicals with reported high potentials against Escherichia coli and Klebsiella pneumnoniae. The libraries were profiled to highlight the existing patterns and relationships between the reported antibacterial activities and studied plants’ families and parts, the type of the extracting solvent, as well as phytochemicals’ classes, drug-likeness and selected parameters for enhanced accumulation within the Gram-negative bacteria. In addition, motivations, objectives, the role of traditional practices and other crucial experimental aspects in the screening of plant extracts for antibacterial activities were identified and discussed.
Based on the implemented strict inclusion criteria, the created libraries grant speedy access to well-evaluated plant species and phytochemicals with potential antibacterial activities. This way, further studies in yet unexplored directions can be pursued from the indicated or related species and compounds. Moreover, the availability of compound libraries focusing on related bacterial species serves a great role in the ongoing efforts to develop the rules of antibiotics penetrability and accumulation, particularly among Gram-negative bacteria. Here, in addition to hunting for potential scaffolds from such libraries, detailed evaluations of large pool compounds with related antibacterial potential can grant a better understanding of structural features crucial for their penetration and accumulation. Based on the scarcity of compounds with broad structural diversity and activity against Gram-negative bacteria, the creation and updating of such libraries remain a laborious but important undertaking.
A Pressurized Microwave Assisted Extraction (PMAE) method over a short duration and low-temperature conditions was developed and compared to the conventional cold maceration over a prolonged duration. This method aimed at addressing the key challenges associated with conventional extraction methods which require long extraction durations, and use more energy and solvents, in addition to larger quantities of plant materials. Furthermore, the method was intended to replace the common use of high temperatures in most of the current MAE applications. Interestingly, the yields of 16 of 18 plant samples under PMAE over 30 minutes were found to be within 91–139% of those obtained from the 24h extraction by maceration. Additionally, different levels of selectivity were observed upon an analytical comparison of the extracts obtained from the two methods. Although each method indicated selective extraction of higher quantities or additional types of certain phytochemicals, a slightly larger number of additional compounds were observed under maceration. The use of this method allows efficient extraction of a large number of samples while sparing heat-sensitive compounds and minimizing chances for cross-reactions between phytochemicals.
Moreover, findings from another investigation highlighted the low likelihood of reproducing antibacterial activities previously reported among various plant species, identified the key drivers of poor reproducibility, and proposed possible measures to mitigate the challenge. The majority of extracts showed no activities up to the highest tested concentration of 1024 µg/mL. In the case of identical plant species, some activities were observed only in 15% of the extracts, in which the Minimum Inhibitory Concentrations (MICs) were 4 – 16-fold higher than those in previous reports. Evaluation of related plant species indicated better outcomes, whereby about 18% of the extracts showed activities in a range of 128–512 μg/mL, some of the activities being superior to those previously reported in related species.
Furthermore, solubilizing plant crude extracts during the preparation of test solutions for Antibacterial Susceptibility Testing (AST) assays was outlined as a key challenge. In trying to address this challenge, some studies have used bacteria-toxic solvents or generally unacceptable concentrations of common solubilizing agents. Both approaches are liable to give false positive results. In line with this challenge, this study has underscored the suitability of acetone in the solubilization of crude plant extracts. Using acetone, better solubility profiles of crude plant extracts were observed compared to dimethyl sulfoxide (DMSO) at up to 10 %v/v. Based on lacking toxicity against many bacteria species at up to 25 %v/v, its use in the solubilization of poorly water-soluble extracts, particularly those from less polar solvents is advocated.
In a subsequent study, four galloylglucoses were isolated from the leaves of Paeonia officinalis L., whereby the isolation of three of them from this source was reported for the first time. The isolation and characterization of these compounds were driven by the crucial need to continually fill the pre-clinical antibiotics pipeline using all available means. Application of the bioautography-guided isolation and a matrix of extractive, chromatographic, spectroscopic, and spectrometric techniques enabled the isolation of the compounds at high purity levels and the ascertainment of their chemical structures.
Further, the compounds exhibited the Minimum Inhibitory Concentrations (MIC) in a range of 2–256 µg/mL against Multidrug-Resistant (MDR) strains of E. coli and K. pneumonia exhibiting diverse MDR phenotypes. In that, the antibacterial activities of three of the isolated compounds were reported for the first time. The observed in vitro activities of the compounds resonated with their in vivo potentials as determined using the Galleria mellonella larvae model. Additionally, the susceptibility of the MDR bacteria to the galloylglucoses was noted to vary depending on the nature of the resistance enzymes expressed by the MDR bacteria. In that, the bacteria expressing enzymes with higher content of aromatic amino acids and zero or positive net charges were generally more susceptible. Following these findings, a plausible hypothesis for the observed patterns was put forward.
The generally challenging pharmacokinetic properties of galloylglucoses limit their further development into therapeutic agents. However, the compounds can replace or reduce the use of antibiotics in livestock keeping as well as in the treatment of septic wounds and topical or oral cavity infections, among other potential uses.
Using nature-inspired approaches, a series of glucovanillin derivatives were prepared following feasible synthetic pathways which in most cases ensured good yields and high purity levels. Some of the prepared compounds showed MIC values in a range of 128 – 512 μg/mL against susceptible and MDR strains of Klebsiella pneumoniae, Methicillin-Resistant Staphylococcus aureus (MRSA) and Vancomycin-Resistant Enterococcus faecium (VRE). These findings emphasize the previously reported essence of small molecular size, the presence of protonatable amino groups and halogen atoms, as well as an amphiphilic character, as crucial features for potential antibacterial agents.
Due to the experienced limited success in the search for new antibacterial agents using purely synthetic means, pursuing semi-synthetic approaches as employed in this study are highly encouraged. This way, it is possible to explore broader chemical spaces around natural scaffolds while addressing their inherent limitations such as solubility, toxicity, and poor pharmacokinetic profiles.
Serum half-life elongation as well as the immobilization of small proteins like cytokines is still one of the key challenges for biologics. This accounts also for cytokines, which often have a molecular weight between 5 and 40 kDa and are therefore prone to elimination by renal filtration and sinusoidal lining cells. To solve this problem biologics are often conjugated to poly(ethylene glycol) (PEG), which is the gold standard for the so called PEGylation. PEG is a synthetic, non-biodegradable polymer for increasing the hydrodynamic radius of the conjugated protein to modulate their pharmacokinetic performance and prolong their therapeutic outcome. Though the benefits of PEGylation are significant, they also come with a prize, which is a loss in bioactivity due to steric hindrance and most often the usage of heterogeneous bioconjugation chemistries. While PEG is a safe excipient in most cases, an increasing number of PEG related side-effects, such as immunological responses like hypersensitivity and accelerated blood clearance upon repetitive exposure occur, which highlights the need for PEG alternative polymers, that can replace PEG in such cases.
Another promising method to significantly prolong the residence time of biologics is to immobilize them at a desired location. To achieve this, the transglutaminase (TG) Factor XIIIa (FXIIIa), which is an important human enzyme during blood coagulation can be used. FXIIIa can recognize specific peptide sequences that contain a lysine as substrates and link them covalently to another peptide sequence, that contains a glutamine, forming an isopeptide bond. This mechanism can be used to link modified proteins, which have a N- or C-terminal incorporated signal peptide by mutation, to the extracellular matrix (ECM) of tissues.
Additionally, both above-described methods can be combined. By artificially introducing a TG recognition sequence, it is possible to attach an azide group containing peptide site-specifically to the TG, recognition sequence. This allows the creation of a site-selective reactive site at the proteins N- or C-terminus, which can then be targeted by cyclooctyne functionalized polymers, just like amber codon functionalized proteins.
This thesis has focused on the two cytokines human Interferon-α2a (IFN-α2a) and human, as well as murine Interleukin-4 (IL-4) as model proteins to investigate the above-described challenges. IFN-α2a has been chosen as a model protein because it is an approved drug since 1986 in systemic applications against some viral infections, as well as several types of cancer. Furthermore, IFN-α2 is also approved in three PEGylated forms, which have different molecular weights and use different conjugation techniques for polymer attachment. This turns it into an ideal candidate to compare new polymers against the gold standard PEG. Interleukin-4 (IL-4) has been chosen as the second model protein due to its similar size and biopotency. This allows to compare found trends from IFN-α2a with another bioconjugate platform and distinguish between IFN-α2a specific, or general trends. Furthermore, IL-4 is a promising candidate for clinical applications as it is a potent anti-inflammatory protein, which polarizes macrophages from the pro-inflammatory M1 state into the anti-inflammatory M2 state.
Poor or variable oral bioavailability is of major concern regarding safety and efficacy for the treatment of patients with poorly water-soluble drugs (PWSDs). The problem statement of this work involves a pharmaceutical development perspective, the physicochemical basis of the absorption process and physiological / biopharmaceutical aspects. A methodology was developed aiming at closing the gap between drug liberation and dissolution on the one hand and the appearance of drug in the blood on the other. Considering what is out of control from a formulation development perspective, a clear differentiation between bioavailability and bioaccessibility was necessary. Focusing on the absorption process, bioaccessibility of a model compound, a poorly soluble but well permeable weak base, was characterized by means of flux across artificial biomimetic membranes. Such setups can be considered to reasonably mimic relevant oral absorption resistances in vitro in terms of diffusion through an unstirred water layer (UWL) and a lipidic barrier. Mechanistic understanding of the driving force for permeation was gained by differentiating drug species and subsequently linking them to the observed transfer rates using a bioaccessibility concept. The three key species that need to be differentiated are molecularly dissolved drug, drug associated in solution with other components (liquid reservoir) and undissolved drug (solid reservoir). An innovative approach to differentiate molecularly dissolved drug from the liquid reservoir using ultracentrifugation in combination with dynamic light scattering as control is presented. A guidance for rational formulation development of PWSDs is elaborated based on the employed model compound. It is structured into five guiding questions to help drug formulation scientists in selecting drug form, excipients and eventually the formulation principle. Overall, the relevance but also limitations of characterizing bioaccessibility were outlined with respect to practical application e.g. in early drug formulation development.
Most medicines are taken orally. To enter the systemic circulation, they dissolve in the intestinal fluid, cross the epithelial barrier, and pass through the liver. Intestinal absorption is driven by the unique features of the gastrointestinal tract, including the bile colloids formed in the lumen and the mucus layer covering the intestinal epithelium. Neglecting this multifaceted environment can lead to poor drug development decisions, especially for poorly water-soluble drugs that interact with bile and mucus. However, there is a lack of a rationale nexus of molecular interactions between oral medicines and gastrointestinal components with drug bioavailability. Against this background, this thesis aims to develop biopharmaceutical strategies to optimize the presentation of oral therapeutics to the intestinal epithelial barrier.
In Chapter 1, the dynamics of bile colloids upon solubilization of the poorly-water soluble drug Perphenazine was studied. Perphenazine impacted molecular arrangement, structure, binding thermodynamics, and induced a morphological transition from vesicles to worm-like micelles. Despite these dynamics, the bile colloids ensured stable relative amounts of free drug substance. The chapter was published in Langmuir.
Chapter 2 examined the impact of pharmaceutical polymeric excipients on bile-mediated drug solubilization. Perphenazine and Imatinib were introduced as model compounds interacting with bile, whereas Metoprolol did not. Some polymers altered the arrangement and geometry of bile colloids, thereby affecting the molecularly soluble amount of those drugs interacting with bile. These insights into the bile-drug-excipient interplay provide a blueprint to optimizing formulations leveraging bile solubilization. The chapter was published in Journal of Controlled Release.
Chapter 3 deals with the impact of bile on porcine intestinal mucus. Mucus exposed to bile solution changed transiently, it stiffened, and the overall diffusion rate increased. The bile-induced changes eased the transport of the bile-interacting drug substance Fluphenazine, whereas Metoprolol was unaffected. This dichotomous pattern was linked to bioavailability in rats and generalized based on two previously published data sets. The outcomes point to a bile-mucus interaction relevant to drug delivery. The chapter is submitted.
The Appendix provides a guide for biopharmaceutical characterization of drug substances by nuclear magnetic resonance spectroscopy aiming at establishing a predictive algorithm.
In summary, this thesis deciphers bile-driven mechanisms shaping intestinal drug absorption. Based on these molecular insights, pharmaceuticals can be developed along a biopharmaceutical optimization, ultimately leading to better oral drugs of tomorrow.
Successful formulation development of novel, particularly organic APIs of low molecular weight as candidates for ground-breaking pharmaceutical products is a major challenge for the pharmaceutical industry because of the poor aqueous solubility of most of these compounds.
The hit identification strategies of drug development in use today apply high throughput screening techniques for the investigation of thousands of substances. This approach led to a systematical increase in molecular weight and lipophilicity and a decrease of water solubility of lead compounds reaching market access.
The high lipophilicity causes an excellent permeability of the compounds which favours the absorption process from the small intestine, but it causes a decrease of water-solubility. It becomes evident that an adequate aqueous solubility is necessary for absorption of the API from the gastrointestinal fluids into the systemic circulation and hence for efficacy of the pharmaceutical product. Only an dissolved API is getting absorbed and becomes efficacious. The precipitated proportion is resigned directly. Therefore, the development of an individual formulation aligning the physicochemical characteristics is necessary for every API to produce supersaturated solutions in the small intestine and to reach an adequate bioavailability after absorption into the systemic circulation.
In this thesis a specific formulation development was investigated for two exemplary poorly water-soluble APIs to replace the empirical approach often used today. The basic tyrosine-kinase inhibitor imatinib and six different acetylated amino acids were transferred into ILs. As compared to the free base and the mesylate salt, which is marketed by Novartis AG as Gleevec®, the dissolution rate as well as the supersaturation time was increased significantly. By changing the mesylate anion with its potential genotoxic risks, the total toxicity of the drug product could be decreased. The amorphous ILs proved adequate stability under forcing conditions and there was no recrystallization of the free base observed. The amorphous character of the ILs caused an increased amount of water vapour sorption which can be compensated by special packaging materials. Taken together, the presentation of imatinib as an IL is intended for oral administration as a tablet and can cause a reduction of dose because of the increased solubility. Therefore, the occurrence of side effects can be reduced as compared to Gleevec®. If there is actually an increased bioavailability to observe, has to be proved by the execution of animal trials.
The novel NOX inhibitor VAS3947 is intended for the treatment of endothelial dysfunctions causing diseases like heart failure and stroke. The compounds poor aqueous solubility hindered further clinical development so far and make the drug candidate to remain in a very early stage of the drug development process. Therefore, different formulation concepts were evaluated in this study:
An amorphous solid dispersion prepared from VAS3947 and Eudragit® L100 by means of spray drying was able to increase the dissolution rate and solubility of the compound significantly, but with the accomplished kinetic solubility being in the low µM range it is not possible to reach therapeutic plasma concentrations.
In contrast, the incorporation into cyclodextrins resulted in an 760-fold increased solubility. Different cyclodextrins were evaluated. Especially the lipophilic derivatives of the β-cyclodextrin showed to be the most adequate excipients. The incorporation of the API into the cyclodextrin cavity was proved by means of NMR spectroscopy. Additionally, a formulation of VAS3947 and hydroxypropyl-β-cyclodextrin was prepared. This formulation is intended for the intravenous application during animal trials, which have to be conducted to get to know the pharmacokinetics of VAS3947. This formulation reached a concentration of 1 mg/mL spending striking protection of VAS3947 against degradation.
Presentation of VAS3947 as a microemulsion system led also to increase the aqueous solubility of the compound, but not in the same extent as the cyclodextrin formulation. Beside the formulation development a physicochemical characterization was performed to get to know important parameters such as log P and pKa values of VAS3947. An HPLC method was developed and validated to analyse the extent of solubility improvement.
A major issue of the compound VAS3947 and all related triazolopyrimidine derivatives, developed by Vasopharm GmbH, is the insufficient chemical stability because of presence of a hemiaminal moiety in the chemical structure. Stability investigations and an extensive biopharmaceutical characterization confirm the hindering of further clinical development by insufficient drug stability and high cytotoxicity. Poor aqueous solubility is an additional disadvantage which can be handled by a concerted formulation development.