Recreating Human Skin In Vitro: Should the Microbiota Be Taken into Account?

Skin plays crucial roles in the human body: besides protecting the organism from external threats, it acts as a thermal regulator, is responsible for the sense of touch, hosts microbial communities (the skin microbiota) involved in preventing the invasion of foreign pathogens, contains immunocompetent cells that maintain a healthy immunogenic/tolerogenic balance, and is a suitable route for drug administration. In the skin, four defense levels can be identified: besides the physical, chemical, and immune barriers that are inherent to the tissue, the skin microbiota (i.e., the numerous microorganisms living on the skin surface) provides an additional barrier. Studying the skin barrier function or the effects of drugs or cosmetic agents on human skin is a difficult task since snapshot evidence can only be obtained using bioptic samples where dynamic processes cannot properly be followed. To overcome these limitations, many different in vitro models of human skin have been developed that are characterized by diverse levels of complexity in terms of chemical, structural, and cellular composition. The aim of this review is to summarize and discuss the advantages and disadvantages of the different human skin models so far available and to underline how the insertion of a proper microbiota would positively impact an in vitro human skin model in an attempt to better mimic conditions in vivo.

An Innovative Fluid Dynamic System to Model Inflammation in Human Skin Explants.

Skin is a major administration route for drugs, and all transdermal formulations must be tested for their capability to overcome the cutaneous barrier. Therefore, developing highly reliable skin models is crucial for preclinical studies. The current in vitro models are unable to replicate the living skin in all its complexity; thus, to date, excised human skin is considered the gold standard for in vitro permeation studies. However, skin explants have a limited life span. In an attempt to overcome this problem, we used an innovative bioreactor that allowed us to achieve good structural and functional preservation in vitro of explanted human skin for up to 72 h. This device was then used to set up an in vitro inflammatory model by applying two distinct agents mimicking either exogenous or endogenous stimuli: i.e., dithranol, inducing the contact dermatitis phenotype, and the substance P, mimicking neurogenic inflammation. Our in vitro system proved to reproduce inflammatory events observed in vivo, such as vasodilation, increased number of macrophages and mast cells, and increased cytokine secretion. This bioreactor-based system may therefore be suitably and reliably used to simulate in vitro human skin inflammation and may be foreseen as a promising tool to test the efficacy of drugs and cosmetics.

An ex vivo experimental system to track fluorescent nanoparticles inside skeletal muscle.

The development of novel nanoconstructs for biomedical applications requires the assessment of their biodistribution, metabolism and clearance in single cells, organs and entire organisms in a living environment. To reduce the number of in vivo experiments performed and to refine the methods used, in accordance with the 3Rs principle, this work proposes an ex vivo experimental system to monitor, using fluorescence microscopy, the distribution of nanoparticles in explanted murine skeletal muscle maintained in a bioreactor that can preserve the structural and functional features of the organ for long periods of time. Fluorescently-labelled liposomes and poly(lactide-co-glycolide) (PLGA)-based nanoparticles were injected into the intact soleus muscle (in the distal region close to the tendon) immediately after explants, and their distribution was analysed at increasing incubation times in cross cryosections from the proximal region of the belly. Both nanocarriers were clearly recognized in the muscle and were found to enter and migrate inside the myofibres, whereas their migration in the connective tissue seemed to be limited. In addition, some fluorescent signals were observed inside the macrophages, demonstrating the physiological clearance of the nanocarriers that did not enter the myofibres. Our ex vivo system therefore provides more information than previous in vitro experiments on cultured muscle cells, highlighting the need for the appropriate functionalization of nanocarriers if myofibre targeting is to be improved.

Repurposing pentamidine using hyaluronic acid-based nanocarriers for skeletal muscle treatment in myotonic dystrophy.

In a context of drug repurposing, pentamidine (PTM), an FDA-approved antiparasitic drug, has been proposed to reverse the splicing defects associated in myotonic dystrophy type 1 (DM1). However, clinical use of PTM is hinder by substantial toxicity, leading to find alternative delivery strategies. In this work we proposed hyaluronic acid-based nanoparticles as a novel encapsulation strategy to efficiently deliver PTM to skeletal muscles cells. In vitro studies on C2C12 myoblasts and myotubes showed an efficient nanoparticles' internalization with minimal toxicity. More interestingly, our findings evidenced for the first time the endosomal escape of hyaluronic acid-based nanocarriers. Ex vivo studies showed an efficient nanoparticles' internalization within skeletal muscle fibers. Finally, the therapeutic efficacy of PTM-loaded nanosystems to reduce the number of nuclear foci has been demonstrated in a novel DM1 in vitro model. So far, current data demonstrated the potency of hyaluronic acid-based nanosystems as efficient nanocarrier for delivering PTM into skeletal muscle and mitigate DM1 pathology.

Ex Vivo Evaluation of Ethosomes and Transethosomes Applied on Human Skin: A Comparative Study.

In this study, the transdermal fate of vesicular nanosystems was investigated. Particularly, ethosomes based on phosphatidylcholine 0.9% w/w and transethosomes based on phosphatidylcholine 0.9 or 2.7% w/w plus polysorbate 80 0.3% w/w as an edge activator were prepared and characterized. The vesicle mean size, morphology and deformability were influenced by both phosphatidylcholine and polysorbate 80. Indeed, the mean diameters of ethosome were around 200 nm, while transethosome's mean diameters were 146 or 350 nm in the case of phosphatidylcholine 0.9 or 2.7%, w/w, respectively. The highest deformability was achieved by transethosomes based on phosphatidylcholine 0.9%, w/w. The three types of vesicular nanosystems were applied on explanted human skin maintained in a bioreactor. Transmission electron microscopy demonstrated that all vesicles were able to enter the skin, keeping their structural integrity. Notably, the vesicle penetration capability was influenced by their physical-chemical features. Indeed, ethosomes reached keratinocytes and even the dermis, phosphatidylcholine 0.9% transethosomes were found in keratinocytes and phosphatidylcholine 2.7% transethosomes were found only in corneocytes of the outer layer. These findings open interesting perspectives for a differentiated application of these vesicles for transdermal drug delivery as a function of the cutaneous pathology to be addressed.

A spectrofluorometric analysis to evaluate transcutaneous biodistribution of fluorescent nanoparticulate gel formulations.

The investigation of the absorption of drug delivery systems, designed for the transport of therapeutic molecules inside the body, could be relatively simplified by the fluorophore association and tracking by means of bio-imaging techniques (i.e., optical in vivo imaging or confocal and multiphoton microscopy). However, when a fluorescence signal comes out from the skin, its specific detection can be problematic. Skin high autofluorescence can hinder the observation of administered exogenous fluorophores conjugated to drug delivery systems, making it more challenging to detect their biodistribution. In the present study, we have developed a method based on the spectrofluorometric analysis of skin samples to discriminate the fluorescent signal coming from administered fluorescent molecules from the background. Moreover, we gave a semi-quantitative evaluation of the signal intensity. Thus, we distinguished two gel formulations loading the fluorophore rhodamine B (called GEL RHO and GEL SLN-RHO). The two formulations of gels, one of which containing solid lipid nanoparticles (GEL RHO-SLN), were administered on skin explants incubated in a bioreactor, and the penetration was evaluated at different time points (2 and 6 hours). Cryostatic sections of skin samples were observed with confocal laser scanning microscopy, and a spectrofluorometric analysis was performed. Significantly higher signal intensity in the samples administered with SLN-RHO GEL, with a preferential accumulation in the hair bulbs, was found. Reaching also the deeper layers of the hair shaft after 6 hours, the solid lipid nanoparticles thickened with polymer represent a suitable drug delivery system for transcutaneous administration.

Integrated Microscopy and Metabolomics to Test an Innovative Fluid Dynamic System for Skin Explants In Vitro.

The in vitro models are receiving growing attention in studies on skin permeation, penetration, and irritancy, especially for the preclinical development of new transcutaneous drugs. However, synthetic membranes or cell cultures are unable to effectively mimic the permeability and absorption features of the cutaneous barrier. The use of explanted skin samples maintained in a fluid dynamic environment would make it possible for an in vitro experimentation closer to in vivo physiological conditions. To this aim, in the present study, we have modified a bioreactor designed for cell culture to host explanted skin samples. The preservation of the skin was evaluated by combining light, transmission, and scanning electron microscopy, for the histo/cytological characterization, with nuclear magnetic resonance spectroscopy, for the identification in the culture medium of metabolites indicative of the functional state of the explants. Our morphological and metabolomics results demonstrated that fluid dynamic conditions ameliorate significantly the structural and functional preservation of skin explants in comparison with conventional culture conditions. Our in vitro system is, therefore, reliable to test novel therapeutic agents intended for transdermal administration in skin samples from biopsies or surgical materials, providing predictive information suitable for focused in vivo research and reducing animal experimentation.

Improving the Cellular Uptake of Biomimetic Magnetic Nanoparticles.

Magnetococcus marinus magnetosome-associated protein MamC, expressed as recombinant, has been proven to mediate the formation of novel biomimetic magnetic nanoparticles (BMNPs) that are successful drug nanocarriers for targeted chemotherapy and hyperthermia agents. These BMNPs present several advantages over inorganic magnetic nanoparticles, such as larger sizes that allow the former to have larger magnetic moment per particle, and an isoelectric point at acidic pH values, which allows both the stable functionalization of BMNPs at physiological pH value and the molecule release at acidic (tumor) environments, simply based on electrostatic interactions. However, difficulties for BMNPs cell internalization still hold back the efficiency of these nanoparticles as drug nanocarriers and hyperthermia agents. In the present study we explore the enhanced BMNPs internalization following upon their encapsulation by poly (lactic-co-glycolic) acid (PLGA), a Food and Drug Administration (FDA) approved molecule. Internalization is further optimized by the functionalization of the nanoformulation with the cell-penetrating TAT peptide (TATp). Our results evidence that cells treated with the nanoformulation [TAT-PLGA(BMNPs)] show up to 80% more iron internalized (after 72 h) compared to that of cells treated with BMNPs (40%), without any significant decrease in cell viability. This nanoformulation showing optimal internalization is further characterized. In particular, the present manuscript demonstrates that neither its magnetic properties nor its performance as a hyperthermia agent are significantly altered due to the encapsulation. In vitro experiments demonstrate that, following upon the application of an alternating magnetic field on U87MG cells treated with BMNPs and TAT-PLGA(BMNPs), the cytotoxic effect of BMNPs was not affected by the TAT-PLGA enveloping. Based on that, difficulties shown in previous studies related to poor cell uptake of BMNPs can be overcome by the novel nanoassembly described here.

Ultrastructural features of the double capsulated connective tissue around silicone prostheses.

The implantation of breast prostheses for both aesthetic and reconstructive purposes has been growing exponentially in the last 20 years. Safety and prosthesis lifespan are majorly debated issues in relation to the correlated long-term complications. Mainly the periprosthetic capsule that develops around the implant is often the cause of complications and particularly for macrotextured silicone breast implants. Some reports have tried to elucidate the mechanism by which macrotextured silicone implants undergo damage and cause double capsule formation. In this study, we investigated the morphological characteristics of double capsule of macrotextured implants surgically removed from patients. With the use of microscopy techniques, this work analyzed the newly formed tissue observed in the interaction between synthetic and biological surfaces.

Imaging techniques in nanomedical research.

About twenty years ago, nanotechnology began to be applied to biomedical issues giving rise to the research field called nanomedicine. Thus, the study of the interactions between nanomaterials and the biological environment became of primary importance in order to design safe and effective nanoconstructs suitable for diagnostic and/or therapeutic purposes. Consequently, imaging techniques have increasingly been used in the production, characterisation and preclinical/clinical application of nanomedical tools. This work aims at making an overview of the microscopy and imaging techniques in vivo and in vitro in their application to nanomedical investigation, and to stress their contribution to this developing research field.

Colloidal polymer-coated Zn-doped iron oxide nanoparticles with high relaxivity and specific absorption rate for efficient magnetic resonance imaging and magnetic hyperthermia.

Colloidally stable nanoparticles-based magnetic agents endowed with very high relaxivity and specific absorption rate are extremely desirable for efficient magnetic resonance imaging and magnetic hyperthermia, respectively. Here, we report a water dispersible magnetic agent consisting of zinc-doped superparamagnetic iron oxide nanoparticles (i.e., Zn-SPIONs) of 15 nm size with high saturation magnetization coated with an amphiphilic polymer for effective magnetic resonance imaging and magnetic hyperthermia of glioblastoma cells. These biocompatible polymer-coated Zn-SPIONs had 24 nm hydrodynamic diameter and exhibited high colloidal stability in various aqueous media, very high transverse relaxivity of 471 mM-1 s-1, and specific absorption rate up to 743.8 W g-1, which perform better than most iron oxide nanoparticles reported in the literature, including commercially available agents. Therefore, using these polymer-coated Zn-SPIONs even at low concentrations, T2-weighted magnetic resonance imaging and moderate magnetic hyperthermia of glioblastoma cells under clinically relevant magnetic field were successfully implemented. In addition, the results of this in vitro study suggest the superior potential of Zn-SPIONs as a theranostic nanosystem for brain cancer treatment, simultaneously acting as a contrast agent for magnetic resonance imaging and a heat mediator for localized magnetic hyperthermia.

Ozone at low concentrations does not affect motility and proliferation of cancer cells in vitro.

Exposure to low ozone concentrations is used in medicine as an adjuvant/complementary treatment for a variety of diseases. The therapeutic potential of low ozone concentrations relies on their capability to increase the nuclear translocation of the Nuclear factor erythroid 2-related factor 2 (Nrf2), thus inducing the transcription of Antioxidant Response Elements (ARE)-driven genes and, through a cascade of events, a general cytoprotective response. However, based on the controversial role of Nrf2 in cancer initiation, progression and resistance to therapies, possible negative effects of ozone therapy may be hypothesised in oncological patients. With the aim to elucidate the possible changes in morphology, migration capability and proliferation of cancer cells following mild ozone exposure, we performed wound healing experiments in vitro on HeLa cells treated with low ozone concentrations currently used in the clinical practice. By combining a multimodal microscopy approach (light and fluorescence microscopy, scanning electron microscopy, atomic force microscopy) with morphometric analyses, we demonstrated that, under our experimental conditions, exposure to low ozone concentrations does not alter cytomorphology, motility and proliferation features, thus supporting the notion that ozone therapy should not positively affect tumour cell growth and metastasis.

A Correlative Imaging Study of in vivo and ex vivo Biodistribution of Solid Lipid Nanoparticles.

PURPOSE: Solid lipid nanoparticles are largely used in biomedical research and are characterized by high stability and biocompatibility and are also able to improve the stability of various loaded molecules. In vitro studies demonstrated that these nanoparticles are low cytotoxic, while in vivo studies proved their efficiency as nanocarriers for molecules characterized by a low bioavailability. However, to our knowledge, no data on the systemic biodistribution and organ accumulation of solid lipid nanoparticles in itself are presently available. METHODS: In this view, we investigated the solid lipid nanoparticles biodistribution by a multimodal imaging approach correlating in vivo and ex vivo analyses. We loaded solid lipid nanoparticles with two different fluorophores (cardiogreen and rhodamine) to observe them with an optical imager in the whole organism and in the excised organs, and with fluorescence microscopy in tissue sections. Light and transmission electron microscopy analyses were also performed to evaluate possible structural modification or damage due to nanoparticle administration. RESULTS: Solid lipid nanoparticles loaded with the two fluorochromes showed good optic characteristics and stable polydispersity. After in vivo administration, they were clearly detectable in the organism. Four  hours after the injection, the fluorescent signal occurred in anatomical districts corresponding to the liver and this was confirmed by the ex vivo acquisitions of excised organs. Brightfield, fluorescence and transmission electron microscopy confirmed solid lipid nanoparticles accumulation in hepatocytes without structural damage. CONCLUSION: Our results support the systemic biocompatibility of solid lipid nanoparticles and demonstrate their detailed biodistribution from the whole organism to organs until the cells.

Negative Pressure From an Internal Spiral Tissue Expander Generates New Subcutaneous Adipose Tissue in an In Vivo Animal Model.

BACKGROUND: Tissue expanders are widely utilized in plastic surgery. Traditional expanders usually are "inflatable balloons," which are planned to grow additional skin and/or to create space to be filled, for example, with an implant. In very recent years, reports suggest that negative pressure created by an external device (ie, Brava) induces both skin expansion and adipogenesis. OBJECTIVES: The authors evaluated and assessed the adipogenetic potential of a novel internal tissue expander in an in vivo animal model. METHODS: New Zealand female rabbits were enrolled in the study. A prototype spiral inner tissue expander was employed. It consisted of a-dynamic conic expander (DCE) with a valve at the end: when empty, it is flat (Archimedean spiral), whereas when filled with a fluid, it takes a conic shape. Inside the conic spiral, a negative pressure is therefore created. DCE is implanted flat under the latissimus dorsi muscle in experimental animals (rabbit) and then filled to reach the conical shape. Animals were investigated with magnetic resonance imaging, histology, and transmission electronic microscopy at 3, 6, and 12 months. RESULTS: Magnetic resonance imaging revealed a marked increase in newly formed adipose tissue, reaching its highest amount at 12 months after the DCE implantation. Histology confirmed the existence of new adipocytes, whereas transmission electronic microscopy ultrastructure confirmed that most of these new cells were mature adipocytes. CONCLUSIONS: Tensile stress, associated with negative-pressure expanders, generated newly white subcutaneous adipose tissue.

Imatinib-loaded gold nanoparticles inhibit proliferation of fibroblasts and macrophages from systemic sclerosis patients and ameliorate experimental bleomycin-induced lung fibrosis.

Interstitial lung involvement in Systemic Sclerosis (SSc-ILD) is a complication with high morbidity and mortality. Specifically, engineered gold nanoparticles (GNPs) are proposed as targeted delivery system increasing efficacy of drugs with antifibrotic effect, such as tyrosine kinases. We aimed to test in vitro and in vivo the activity of targeted Imatinib (Im)-loaded GNP on SSc-ILD patients derived cells and in experimental model of lung fibrosis. GNPs functionalized with anti-CD44 and loaded with Im (GNP-HCIm) were synthesized. Lung fibroblasts (LFs) and alveolar macrophages from bronchoalveolar lavage fluids of SSc-ILD patients were cultured in presence of nanoparticles. GNP-HCIm significantly inhibited proliferation and viability inducing apoptosis of LFs and effectively reduced IL-8 release, viability and M2 polarization in alveolar macrophages. Anti-fibrotic effect of tracheal instilled GNP-HCIm was evaluated on bleomycin lung fibrosis mouse model comparing effect with common route of Im administration. GNP-HCIm were able to reduce significantly lung fibrotic changes and collagen deposition. Finally, electron microscopy revealed the presence of GNPs inside alveolar macrophages. These data support the use of GNPs locally administered in the development of new therapeutic approaches to SSc-ILD.

Evaluating the effects of fluorine on biological properties and metabolic stability of some antitubulin 3-substituted 7-phenyl-pyrroloquinolinones.

A small number of fluorinated 7-phenyl-pyrroloquinolinone (7-PPyQ) derivatives was synthesized in an attempt to improve the metabolic stability of 3N-ethyl-7-PPyQ and 3N-benzoyl-7-PPyQ. The possible impacts of the fluorine-hydrogen isosterism on both biological activity and metabolic stability were evaluated. Introduction of a fluorine atom in the 2 or 3 position of the 7-phenyl ring yielded the 7-PPyQ derivatives 12, 13 and 15, which showed potent cytotoxicity (low micromolar and sub-nanomolar GI50s) both in human leukemic and solid tumor cell lines. None of them induced significant cell death in quiescent and proliferating human lymphocytes. Moreover, 12, 13 and 15 exhibited remarkable cytotoxic activity in the multidrug-resistant cell line CEMVbl100, suggesting that they are not substrates for P-glycoprotein. All compounds inhibited tubulin assembly and the binding of [3H]colchicine to tubulin, with the best activity occurring with compound 15. Mechanistic studies carried out on compound 12 indicated that it caused (a) a strong G2/M arrest; (b) apoptosis in a time- and concentration-dependent manner; (c) a significant production of ROS (in good agreement with the observed mitochondrial depolarization); (d) caspase-3 and poly (ADP-ribose) polymerase activation; and (e) a decrease in the expression of anti-apoptotic proteins. In vivo experiments in a murine syngeneic tumor model demonstrated that compounds 12 and 15 significantly reduced tumor mass at doses four times lower than that required for the reference compound combretastatin A-4 phosphate. Neither monofluorination of the 7-phenyl ring of 3N-ethyl-7-PPyQ nor replacement of the benzoyl function of 3N-benzoyl-7-PPyQ with a 2-fluorobenzoyl moiety led to any improvement in the metabolic stability.

Mild ozonisation activates antioxidant cell response by the Keap1/Nrf2 dependent pathway.

Treatment with low-dose ozone is successfully exploited as an adjuvant therapy in the treatment of several disorders. Although the list of medical applications of ozone therapy is increasing, molecular mechanisms underlying its beneficial effects are still partially known. Clinical and experimental evidence suggests that the therapeutic effects of ozone treatment may rely on its capability to mount a beneficial antioxidant response through activation of the nuclear factor erythroid-derived-like 2 (Nrf2) pathway. However, a conclusive mechanistic demonstration is still lacking. Here, we bridge this gap of knowledge by providing evidence that treatment with a low concentration of ozone in cultured cells promotes nuclear translocation of Nrf2 at the chromatin sites of active transcription and increases the expression of antioxidant response element (ARE)-driven genes. Importantly, we show that ozone-induced ARE activation can be reverted by the ectopic expression of the Nrf2 specific inhibitor Kelch-like ECH associated protein (Keap1), thus proving the role of the Nrf2 pathway in the antioxidant response induced by mild ozonisation.

Innovative approach to safely induce controlled lipolysis by superparamagnetic iron oxide nanoparticles-mediated hyperthermic treatment.

During last years, evidence has been provided on the involvement of overweight and obesity in the pathogenesis and aggravation of several life-threatening diseases. Here, we demonstrate that, under appropriate administration conditions, polyhedral iron oxide nanoparticles are efficiently and safely taken up by 3T3 cell line-derived adipocytes (3T3 adipocytes) in vitro. Since these nanoparticles proved to effectively produce heat when subjected to alternating magnetic field, 3T3 adipocytes were submitted to superparamagnetic iron oxide nanoparticles-mediated hyperthermia treatment (SMHT), with the aim of modulating their lipid content. Notably, the treatment resulted in a significant delipidation persisting for at least 24h, and in the absence of cell death, damage or dedifferentiation. Interestingly, transcript expression of adipose triglyceride lipase (ATGL), a key gene involved in canonical lipolysis, was not modulated upon SMHT, suggesting the involvement of a novel/alternative mechanism in the effective lipolysis observed. By applying the same experimental conditions successfully used for 3T3 adipocytes, SMHT was able to induce delipidation also in primary cultures of human adipose-derived adult stem cells. The success of this pioneering approach in vitro opens promising perspectives for the application of SMHT in vivo as an innovative safe and physiologically mild strategy against obesity, potentially useful in association with balanced diet and healthy lifestyle.

Nanoformulations for dimethyl fumarate: Physicochemical characterization and in vitro/in vivo behavior.

Dimethyl fumarate has been demonstrated useful in relapsing remitting multiple sclerosis treatment (Tecfidera®). Nevertheless, since Tecfidera® capsules induce flushing, gastro-intestinal events and other more serious drawbacks, in this investigation a nanoparticle based system to be administered by an alternative way is proposed. In particular this study describes the preparation and characterization of dimethyl fumarate-containing solid lipid nanoparticles (SLN). Namely SLN based on tristearin, tristearin SLN treated with polysorbate 80 and cationic SLN constituted of tristearin in mixture with dimethyldioctadecylammonium chloride were investigated. The effect of the presence of dimethyl fumarate, functionalization by polysorbate 80 and dimethyldioctadecylammonium chloride was studied on morphology and dimensional distribution of SLN, by photon correlation spectroscopy and cryogenic transmission electron microscopy. Dimethyl fumarate release from SLN, studied by Franz cell, evidenced a Fickian dissolutive type kinetic in the case of SLN treated by polysorbate 80. Moreover fluorescent SLN were produced and characterized in order to investigate their in vitro permeability and in vivo biodistribution in mice. An in vitro study of fluorescent SLN permeability performed through a model of mouse brain microvascular endothelial cells, indicated that cationic SLN displayed higher permeability values with respect to neutral SLN and SLN treated by polysorbate 80. Biodistribution of polysorbate 80 treated SLN was studied by fluorescent imaging after intraperitoneal or intranasal administration in mice. The in vivo images indicate that polysorbate 80 treated SLN were able to reach the brain, even if they prevalently accumulated in liver and spleen, especially by intraperitoneal route.

Thiol-Activated Anticancer Agents: The State of the Art.

The thiol or sulfhydryl group, as part of low molecular weight non-peptide biomolecules, as well as part of the cysteine residues in peptides and proteins, is known to play extremely important roles in several aspects of cellular function. Glutathione (γ-Glu-Cys-Gly; GSH) is the most abundant thiol-containing peptide in mammals, being present intracellularly in the low millimolar concentration range, but only in the low micromolar concentration range in the majority of extracellular fluids. Notably, intracellular levels of GSH have been found to be significantly upregulated in a number of human cancers, a phenomenon thought to contribute, in concert with overexpression of some GSHassociated enzymes, to the development of tumor cell chemo- and radioresistance. On the other hand, various natural and synthetic chemical entities of different sizes show significant cytotoxic activity only upon interaction with a thiol, and can therefore exploit the GSH-rich intracellular environment of tumors. This review article attempts to summarize the current structural and pharmacological knowledge in the field of thiol-activated anticancer agents, with a focus on the mechanism(s) of their activation. Even though a great part of the available thiol-activated anticancer compounds is still in the preclinical phase of testing, some of them are undergoing trials in cancer patients.