Novel HPMA copolymer-bound constructs for combined tumor and mitochondrial targeting

Hydrophilic biomaterials: From crosslinked and self-assembled hydrogels to polymer-drug conjugates and drug-free macromolecular therapeutics

This "Magnum Opus" accentuates my lifelong belief that the future of science is in the interdisciplinary approach to hypotheses formulation and problem solving. Inspired by the invention of hydrogels and soft contact lenses by my mentors, my six decades of research have continuously proceeded from the synthesis of biocompatible hydrogels to the development of polymer-drug conjugates, then generation of drug-free macromolecular therapeutics (DFMT) and finally to multi-antigen T cell hybridizers (MATCH). This interdisciplinary journey was inspiring; the lifetime feeling that one is a beginner in some aspects of the research is a driving force that keeps the enthusiasm high. Also, I wanted to illustrate that systematic research in one wide area can be a life-time effort without the need to jump to areas that are temporarily en-vogue. In addition to generating general scientific knowledge, hydrogels from my laboratory have been transferred to the clinic, polymer-drug conjugates to clinical trials, and drug-free macromolecular systems have an excellent potential for personalizing patient therapies. There is a limit to life but no limit to imagination. I anticipate that systematic basic research will contribute to the expansion of our knowledge and create a foundation for the design of new paradigms based on the comprehension of mechanisms of physiological processes. The emerging novel platform technologies in biomaterial-based devices and implants as well as in personalized nanomedicines will ultimately impact clinical practice.

Enhanced apoptosis and mitochondrial cell death by paclitaxel-loaded TPP-TPGS1000-functionalized nanoemulsion

Background: The present research was designed to develop a nanoemulsion (NE) of triphenylphosphine-D-α-tocopheryl-polyethylene glycol succinate (TPP-TPGS₁₀₀₀) and paclitaxel (PTX) to effectively deliver PTX to improve breast cancer therapy. Materials & methods: A quality-by-design approach was applied for optimization and in vitro and in vivo characterization were performed. Results: The TPP-TPGS₁₀₀₀-PTX-NE enhanced cellular uptake, mitochondrial membrane depolarization and G₂M cell cycle arrest compared with free-PTX treatment. In addition, pharmacokinetics, biodistribution and in vivo live imaging studies in tumor-bearing mice showed that TPP-TPGS₁₀₀₀-PTX-NE had superior performance compared with free-PTX treatment. Histological and survival investigations ascertained the nontoxicity of the nanoformulation, suggesting new opportunities and potential to treat breast cancer. Conclusion: TPP-TPGS₁₀₀₀-PTX-NE improved the efficacy of breast cancer treatment by enhancing its effectiveness and decreasing drug toxicity.

Photochemical Targeting of Mitochondria to Overcome Chemoresistance in Ovarian Cancer †

Ovarian cancer is the most lethal gynecologic malignancy with a stubborn mortality rate of ~65%. The persistent failure of multiline chemotherapy, and significant tumor heterogeneity, has made it challenging to improve outcomes. A target of increasing interest is the mitochondrion because of its essential role in critical cellular functions, and the significance of metabolic adaptation in chemoresistance. This review describes mitochondrial processes, including metabolic reprogramming, mitochondrial transfer and mitochondrial dynamics in ovarian cancer progression and chemoresistance. The effect of malignant ascites, or excess peritoneal fluid, on mitochondrial function is discussed. The role of photodynamic therapy (PDT) in overcoming mitochondria-mediated resistance is presented. PDT, a photochemistry-based modality, involves the light-based activation of a photosensitizer leading to the production of short-lived reactive molecular species and spatiotemporally confined photodamage to nearby organelles and biological targets. The consequential effects range from subcytotoxic priming of target cells for increased sensitivity to subsequent treatments, such as chemotherapy, to direct cell killing. This review discusses how PDT-based approaches can address key limitations of current treatments. Specifically, an overview of the mechanisms by which PDT alters mitochondrial function, and a summary of preclinical advancements and clinical PDT experience in ovarian cancer are provided.

Inactivation of PDH can Reduce Anaplastic Thyroid Cancer Cells' Sensitivity to Artemisinin

BACKGROUND

Anaplastic Thyroid Cancer (ATC) is a rare subtype of thyroid tumors with a high mortality rate. Targeted therapies against ATC are ineffective and mostly transient. Artemisinin has shown excellent anti-tumor activity in several cancers, but its effects on ATC are still unknown.

OBJECTIVE

To evaluate the effects of artemisinin on ATC cells and assess the mechanism underlying drug resistance.

METHODS

The viability and proliferation rates of the artemisinin-treated CAL-62 and BHT-101 cells were analyzed by MTT and EdU incorporation assays. The protein expression levels were determined by Tandem Mass Tag (TMT) labeling quantitative proteomics and western blotting.

RESULTS

Artemisinin treatment significantly decreased the expression levels of COX2 and COX7A2 and increased that of COX14, YEM1l1, ALAS1, and OAT after 48h. In addition, FTL was upregulated in the CAL-62 cells and downregulated in BHT-101 cells. The CAL-62 cells showed transient and reversible resistance to artemisinin, which was correlated to time-dependent changes in HIF1α, PDK1, and PDHA levels.

CONCLUSION

Artemisinin targets the mitochondrial respiratory chain proteins in ATC cells. CAL-62 cells show transient resistance to artemisinin via PDH downregulation, indicating that PDH activation may enhance the cytotoxic effects of artemisinin on ATC cells.

Preparation and characterization of chitosan derivatives modified with quaternary ammonium salt and quaternary phosphate salt and its effect on tropical fruit preservation

In this paper, HACC modified with (5-Carboxypentyl) (triphenyl) phosphonium bromide (HA-CS-NP) was synthesized. Then, a multifunctional food packaging composite film with good thermal stability and antibacterial functions was fabricated by HA-CS-NP and poly (vinyl alcohol) (PVA). The tensile strength and elongation at break of HA-CS-NP/PVA composite film at the weight ratio of 3/7 were 20.32 ± 1.02 MPa and 65.73 ± 3.29%, respectively. And, the inhibition rates of HA-CS-NP (0.5%) on Mango C. lagenarium and Papaya C. gloeosporioides on day 6 were up to 80.92 ± 4.12%. Compared with CK group, the weight loss of experimental groups were 23.96 ± 2.46 g/206 ± 7.25 g (mangoes) and 59.45 ± 3.06 g/496 ± 6.37 g (papaya), reduced by 35.76 ± 1.15%. Moreover, the final hardness value of the fruits coated with composite films was 4.94 ± 0.23 kg/cm³ and increased by 20.79 ± 1.04%, and the rot index was reduced by 71.43 ± 3.24%. The multifunctional HA-CS-NP/PVA coating has broad prospects in the application of food packaging.

Synthesis of Triphenylphosphonium Phospholipid Conjugates for the Preparation of Mitochondriotropic Liposomes

Surface modification of liposomes with a ligand is facilitated by the conjugation of the ligand to a hydrophobic molecule that serves to anchor the ligand to the liposomal bilayer. We describe here a simple protocol to conjugate a triphenylphosphonium group to several commercially available functionalized phospholipids. The resulting triphenylphosphonium-conjugated lipids can be used to prepare liposomes that preferentially associate with mitochondria when exposed to live mammalian cells in culture.

Emerging insights into mitochondria-specific targeting and drug delivering strategies: Recent milestones and therapeutic implications

Mitochondria are a major intracellular organelle for drug targeting due to its functional roles in cellular metabolism and cell signaling for proliferation and cell death. Mitochondria-targeted treatment strategy could be promising to improve the therapeutic efficacy of cancer while minimizing the adverse side effects. Over the last decades, several studies have explored and focused on mitochondrial functions, which has led to the emergence of mitochondria-specific therapies. Molecules in the mitochondria are considered to be prime targets, and a wide range of molecular strategies have been designed for targeting mitochondria compared with that of the cytosol. In this review, we focused on the molecular mechanisms of mitochondria-specific ligand targeting and selective drug action strategies for targeting mitochondria, including those premised on mitochondrial targeting of signal peptides (MTS), cell-penetrating peptides (CPPs), and use of lipophilic cations. Furthermore, most research has concentrated on specific conjugation of ligands to therapeutic molecules to enhance their effectiveness. There are several variations for the ideal design and development for mitochondrial-targeted drugs, such as selecting a suitable ligand and linker targets. However, some challenges related to drug solubility and selectivity could be resolved using the nanocarrier system. Nanoparticles yield excellent advantages for targeting and transmitting therapeutic drugs, and they offer elegant platforms for mitochondria-specific drug delivery. We explain many of the advanced and proven strategies for multifunctional mitochondria-specific targets, which should contribute to achieving better anticancer therapies in a promising future.

Polymer nanomedicines

Polymer nanomedicines (macromolecular therapeutics, polymer-drug conjugates, drug-free macromolecular therapeutics) are a group of biologically active compounds that are characterized by their large molecular weight. This review focuses on bioconjugates of water-soluble macromolecules with low molecular weight drugs and selected proteins. After analyzing the design principles, different structures of polymer carriers are discussed followed by the examination of the efficacy of the conjugates in animal models and challenges for their translation into the clinic. Two innovative directions in macromolecular therapeutics that depend on receptor crosslinking are highlighted: a) Combination chemotherapy of backbone degradable polymer-drug conjugates with immune checkpoint blockade by multivalent polymer peptide antagonists; and b) Drug-free macromolecular therapeutics, a new paradigm in drug delivery.

Mitochondrial Targeted Doxorubicin-Triphenylphosphonium Delivered by Hyaluronic Acid Modified and pH Responsive Nanocarriers to Breast Tumor: in Vitro and in Vivo Studies

Multidrug resistance (MDR) is the major obstacle for chemotherapy. In a previous study, we have successfully synthesized a novel doxorubicin (DOX) derivative modified by triphenylphosphonium (TPP) to realize mitochondrial delivery of DOX and showed the potential of this compound to overcome DOX resistance in MDA-MB-435/DOX cells. (1) To introduce specificity for DOX-TPP to cancer cells, here we report on the conjugation of DOX-TPP to hyaluronic acid (HA) by hydrazone bond with adipic acid dihydrazide (ADH) as the acid-responsive linker, producing HA- hydra-DOX-TPP nanoparticles. Hyaluronic acid (HA) is a natural water-soluble linear glycosaminoglycan, which was hypothesized to increase the accumulation of nanoparticles containing DOX-TPP in the mitochondria of tumor cells upon systemic administration, overcoming DOX resistance, in vivo. Our results showed HA- hydra-DOX-TPP to self-assemble to core/shell nanoparticles of good dispersibility and effective release of DOX-TPP from the HA- hydra-DOX-TPP conjugate in cancer cells, which was followed by enhanced DOX mitochondria accumulation. The HA- hydra-DOX-TPP nanoparticles also showed improved anticancer effects, better tumor cell apoptosis, and better safety profile compared to free DOX in MCF-7/ADR bearing mice.

Unique Triphenylphosphonium Derivatives for Enhanced Mitochondrial Uptake and Photodynamic Therapy

In this study, unique methyl-functionalized derivatives (T*PP⁺) of the drug carrier triphenylphosphonium (TPP⁺) that exhibit significant enhancement of the accumulation of both the cation and its conjugated cargo in cell mitochondria are designed. We show that the presence of methyl group(s) at key positions within the phenyl ring results in an increase in the hydrophobicity and solvent accessible surface area of T*PP⁺. In particular, when the para position of the phenyl ring in T*PP⁺ is functionalized with a methyl group, the cation is most exposed to the surrounding environment, leading to a large decrease in water entropy and an increase in the level of van der Waals interaction with and partition into a nonpolar solvent. Therefore, stronger binding between the hydrophobic T*PP⁺ and mitochondrial membrane occurs. This is exemplified in a (hexachloro-fluorescein)-TPP⁺ conjugate system, where an ∼12 times increase in the rate of mitochondrial uptake and a 2 times increase in photodynamic therapy (PDT) efficacy against HeLa and FU97 cancer cells are achieved when TPP⁺ is replaced with T*PP⁺. Importantly, nearly all the FU97 cells treated with the (hexachloro-fluorescein)-T*PP⁺ conjugate are killed as compared to only half the population of cells in the case of the (hexachloro-fluorescein)-TPP⁺ conjugate at a similar PDT light dosage. This study thus forms a platform for the healthcare community to explore alternative TPP⁺ derivatives that can act as optimal drug transporters for enhanced mitochondrially targeted therapies.

A pH-responsive sequential-disassembly nanohybrid for mitochondrial targeting

Cationic materials have been reported as promising tools for targeting to mitochondria which are the "power houses" and "metabolic garbage keepers" of cells. However, their positive nature also restricts their in vivo application due to the quick clearance. Herein, we fabricated a nanohybrid consisting of the pH-responsive N-(2-hydroxypropyl)methacrylamide (HPMA) co-polymer (R-P) shells and positive mesoporous silica nanoparticle cores via electrostatic interaction. The anticancer drug, docetaxel (DTX), was encapsulated in the positive MSN cores (MSN-DTX). Once concealed by the anionic R-P shield, the assembled nanohybrid R-P@MSN-DTX will achieve prolonged blood circulation thereby leading to an enhanced EPR effect. At mildly acidic tumor environmental pH, first-stage charge reversion took place due to the hydrolysis of the amide bond on HPMA co-polymers. The de-attachment of the HPMA co-polymer occurred because of the positive charge repulsion and partial exposure of the positively charged MSN core promoted the cell internalization. The second-stage pH-responsiveness in the endo/lysosomes with a more acidic environment accelerates the disassembly of the nanohybrid and the leakage of the core facilitated the endo/lysosome escape and mitochondrial targeting with the help of intracellular compartmental acidity. Gathering up the characteristics of neutralized charge and stepwise pH-responsiveness, the R-P@MSN-DTX acquired a good tumor inhibition rate of 72.6% on nude mice. Our report provided a reference for systemic mitochondrial targeting achieved by the union of "assembly-disassembly".

Delivery of drugs and macromolecules to the mitochondria for cancer therapy

Mitochondria are organelles that have pivotal functions in producing the energy necessary for life and executing the cell death pathway. Targeting drugs and macromolecules to the mitochondria may provide an effective means of inducing cell death for cancer therapy, and has been actively pursued in the last decade. This review will provide a brief overview of mitochondrial structure and function, how it relates to cancer, and importantly, will discuss different strategies of mitochondrial delivery including delivery using small molecules, peptides, genes encoding proteins and MTSs, and targeting polymers/nanoparticles with payloads to the mitochondria. The advantages and disadvantages for each strategy will be discussed. Specific examples using the latest strategies for mitochondrial targeting will be evaluated, as well as potential opportunities for specific mitochondrial compartment localization, which may lead to improvements in mitochondrial therapeutics. Future perspectives in mitochondrial targeting of drugs and macromolecules will be discussed. Currently this is an under-explored area that is prime for new discoveries in cancer therapeutics.

Effect of the Conjugation Density of Triphenylphosphonium Cation on the Mitochondrial Targeting of Poly(amidoamine) Dendrimers

Many clinically relevant diseases with known poor therapeutic outcomes, including cancer and neurodegenerative disorders, have been directly linked to mitochondrial dysfunction. The ability to efficiently target therapeutics to intracellular organelles such as mitochondria may represent new opportunities for the effective treatment of such ailments. The present study reports the synthesis, cellular uptake, cytotoxicity, and mitochondrial colocalization of conjugates of triphenylphosphonium cation (TPP) to amine-terminated, generation 4, poly(amidoamine) (PAMAM) dendrimer (G4NH2) nanocarriers. The mitochondrial-targeting moiety TPP was either directly conjugated to G4NH2 (G4NH2-TPP) or to the dendrimer through a flexible polyethylene glycol (PEG) linker (G4NH2-PEGTPP). Conjugation was done at various TPP densities to assess their biological activity and potential for mitochondrial-targeted drug delivery. Tests in an in vitro model of the human alveolar carcinoma (A549 cells) showed that even at a low TPP density (∼5 TPP) both the cellular internalization and mitochondrial targeting increase significantly, as determined by fluorescence activated cell sorting (FACS) and confocal microscopy (CM), respectively. At a density of ∼10 TPP per G4NH2, further increase in cellular internalization and mitochondrial targeting was achieved. However, at this higher density, the nanocarriers also showed pronounced cytotoxicity. It was observed that the toxicity of the conjugates is decreased upon the addition of a PEG linker between the dendrimer and TPP (G4NH2-PEGTPP), while the mitochondrial targeting ability of the nanocarriers is not affected as the PEG density increases. The proposed strategies indicate that TPP-conjugated G4NH2 dendrimers represent a potentially viable strategy for the targeting of therapeutic molecules to mitochondria, which may help improve therapeutic outcomes of diseases related to mitochondrial dysfunction.

Synthesis of triphenylphosphonium phospholipid conjugates for the preparation of mitochondriotropic liposomes

Surface modification of liposomes with a ligand is facilitated by the conjugation of the ligand to a hydrophobic molecule that serves to anchor the ligand to the liposomal bilayer. We describe here a simple protocol to conjugate a triphenylphosphonium group to several commercially available functionalized phospholipids. The resulting triphenylphosphonium conjugated lipids can be used to prepare liposomes that preferentially associate with mitochondria when exposed to live mammalian cells in culture.

Macromolecular therapeutics

This review covers water-soluble polymer-drug conjugates and macromolecules that possess biological activity without attached low molecular weight drugs. The main design principles of traditional and backbone degradable polymer-drug conjugates as well as the development of a new paradigm in nanomedicines - (low molecular weight) drug-free macromolecular therapeutics are discussed. To address the biological features of cancer, macromolecular therapeutics directed to stem/progenitor cells and the tumor microenvironment are deliberated. Finally, the future perspectives of the field are briefly debated.

Nanopreparations for organelle-specific delivery in cancer

To efficiently deliver therapeutics into cancer cells, a number of strategies have been recently investigated. The toxicity associated with the administration of chemotherapeutic drugs due to their random interactions throughout the body necessitates the development of drug-encapsulating nanopreparations that significantly mask, or reduce, the toxic side effects of the drugs. In addition to reduced side effects associated with drug encapsulation, nanocarriers preferentially accumulate in tumors as a result of its abnormally leaky vasculature via the Enhanced Permeability and Retention (EPR) effect. However, simple passive nanocarrier delivery to the tumor site is unlikely to be enough to elicit a maximum therapeutic response as the drug-loaded carriers must reach the intracellular target sites. Therefore, efficient translocation of the nanocarrier through the cell membrane is necessary for cytosolic delivery of the cargo. However, crossing the cell membrane barrier and reaching cytosol might still not be enough for achieving maximum therapeutic benefit, which necessitates the delivery of drugs directly to intracellular targets, such as bringing pro-apoptotic drugs to mitochondria, nucleic acid therapeutics to nuclei, and lysosomal enzymes to defective lysosomes. In this review, we discuss the strategies developed for tumor targeting, cytosolic delivery via cell membrane translocation, and finally organelle-specific targeting, which may be applied for developing highly efficacious, truly multifunctional, cancer-targeted nanopreparations.

Clinical response to chemotherapy in oesophageal adenocarcinoma patients is linked to defects in mitochondria

Chemotherapeutic drugs kill cancer cells, but it is unclear why this happens in responding patients but not in non-responders. Proteomic profiles of patients with oesophageal adenocarcinoma may be helpful in predicting response and selecting more effective treatment strategies. In this study, pretherapeutic oesophageal adenocarcinoma biopsies were analysed for proteomic changes associated with response to chemotherapy by MALDI imaging mass spectrometry. Resulting candidate proteins were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and investigated for functional relevance in vitro. Clinical impact was validated in pretherapeutic biopsies from an independent patient cohort. Studies on the incidence of these defects in other solid tumours were included. We discovered that clinical response to cisplatin correlated with pre-existing defects in the mitochondrial respiratory chain complexes of cancer cells, caused by loss of specific cytochrome c oxidase (COX) subunits. Knockdown of a COX protein altered chemosensitivity in vitro, increasing the propensity of cancer cells to undergo cell death following cisplatin treatment. In an independent validation, patients with reduced COX protein expression prior to treatment exhibited favourable clinical outcomes to chemotherapy, whereas tumours with unchanged COX expression were chemoresistant. In conclusion, previously undiscovered pre-existing defects in mitochondrial respiratory complexes cause cancer cells to become chemosensitive: mitochondrial defects lower the cells' threshold for undergoing cell death in response to cisplatin. By contrast, cancer cells with intact mitochondrial respiratory complexes are chemoresistant and have a high threshold for cisplatin-induced cell death. This connection between mitochondrial respiration and chemosensitivity is relevant to anticancer therapeutics that target the mitochondrial electron transport chain.

Polymer-drug conjugates: origins, progress to date and future directions

This overview focuses on bioconjugates of water-soluble polymers with low molecular weight drugs and proteins. After a short discussion of the origins of the field, the state-of-the-art is reviewed. Then research directions needed for the acceleration of the translation of nanomedicines into the clinic are outlined. Two most important directions, synthesis of backbone degradable polymer carriers and drug-free macromolecular therapeutics, a new paradigm in drug delivery, are discussed in detail. Finally, the future perspectives of the field are briefly discussed.

Liposomes loaded with paclitaxel and modified with novel triphenylphosphonium-PEG-PE conjugate possess low toxicity, target mitochondria and demonstrate enhanced antitumor effects in vitro and in vivo

Previously, stearyl triphenylphosphonium (STPP)-modified liposomes (STPP-L) were reported to target mitochondria. To overcome a non-specific cytotoxicity of STPP-L, we synthesized a novel polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugate with the TPP group attached to the distal end of the PEG block (TPP-PEG-PE). This conjugate was incorporated into the liposomal lipid bilayer, and the modified liposomes were studied for their toxicity, mitochondrial targeting, and efficacy in delivering paclitaxel (PTX) to cancer cells in vitro and in vivo. These TPP-PEG-PE-modified liposomes (TPP-PEG-L), surface grafted with as high as 8 mol% of the conjugate, were less cytotoxic compared to STPP-L or PEGylated STPP-L. At the same time, TPP-PEG-L demonstrated efficient mitochondrial targeting in cancer cells as shown by confocal microscopy in co-localization experiments with stained mitochondria. PTX-loaded TPP-PEG-L demonstrated enhanced PTX-induced cytotoxicity and anti-tumor efficacy in cell culture and mouse experiments compared to PTX-loaded unmodified plain liposomes (PL). Thus, TPP-PEG-PE can serve as a targeting ligand to prepare non-toxic liposomes as mitochondria-targeted drug delivery systems (DDS).

Surface conjugation of triphenylphosphonium to target poly(amidoamine) dendrimers to mitochondria

Dendrimers have emerged as promising carriers for the delivery of a wide variety of pay-loads including therapeutic drugs, imaging agents and nucleic acid materials into biological systems. The current work aimed to develop a novel mitochondria-targeted generation 5 poly(amidoamine) (PAMAM) dendrimer (G(5)-D). To achieve this goal, a known mitochondriotropic ligand triphenylphosphonium (TPP) was conjugated on the surface of the dendrimer. A fraction of the cationic surface charge of G(5)-D was neutralized by partial acetylation of the primary amine groups. Next, the mitochondria-targeted dendrimer was synthesized via the acid-amine-coupling conjugation reaction between the acid group of (3-carboxypropyl)triphenyl-phosphonium bromide and the primary amines of the acetylated dendrimer (G(5)-D-Ac). These dendrimers were fluorescently labeled with fluorescein isothiocyanate (FITC) to quantify cell association by flow cytometry and for visualization under confocal laser scanning microscopy to assess the mitochondrial targeting in vitro. The newly developed TPP-anchored dendrimer (G(5)-D-Ac-TPP) was efficiently taken up by the cells and demonstrated good mitochondrial targeting. In vitro cytotoxicity experiments carried out on normal mouse fibroblast cells (NIH-3T3) had greater cell viability in the presence of the G(5)-D-Ac-TPP compared to the parent unmodified G(5)-D. This mitochondria-targeted dendrimer-based nanocarrier could be useful for imaging as well as for selective delivery of bio-actives to the mitochondria for the treatment of diseases associated with mitochondrial dysfunction.

Liposomes loaded with paclitaxel and modified with novel triphenylphosphonium-PEG-PE conjugate possess low toxicity, target mitochondria and demonstrate enhanced antitumor effects in vitro and in vivo

Previously, stearyl triphenylphosphonium (STPP)-modified liposomes (STPP-L) were reported to target mitochondria. To overcome a non-specific cytotoxicity of STPP-L, we synthesized a novel polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugate with the TPP group attached to the distal end of the PEG block (TPP-PEG-PE). This conjugate was incorporated into the liposomal lipid bilayer, and the modified liposomes were studied for their toxicity, mitochondrial targeting, and efficacy in delivering paclitaxel (PTX) to cancer cells in vitro and in vivo. These TPP-PEG-PE-modified liposomes (TPP-PEG-L), surface grafted with as high as 8 mol% of the conjugate, were less cytotoxic compared to STPP-L or PEGylated STPP-L. At the same time, TPP-PEG-L demonstrated efficient mitochondrial targeting in cancer cells as shown by confocal microscopy in co-localization experiments with stained mitochondria. PTX-loaded TPP-PEG-L demonstrated enhanced PTX-induced cytotoxicity and anti-tumor efficacy in cell culture and mouse experiments compared to PTX-loaded unmodified plain liposomes (PL). Thus, TPP-PEG-PE can serve as a targeting ligand to prepare non-toxic liposomes as mitochondria-targeted drug delivery systems (DDS).

Anticancer drugs targeting the mitochondrial electron transport chain

SIGNIFICANCE

Mitochondria are emerging as highly intriguing organelles showing promise but that are yet to be fully exploited as targets for anticancer drugs.

RECENT ADVANCES

A group of compounds that induce mitochondrial destabilization, thereby affecting the physiology of cancer cells, has been defined and termed 'mitocans.' Based on their mode of action of targeting in and around mitochondria, we have placed these agents into several groups including hexokinase inhibitors, compounds targeting Bcl-2 family proteins, thiol redox inhibitors, VDAC/ANT targeting drugs, electron transport chain-targeting drugs, lipophilic cations targeting the inner membrane, agents affecting the tricarboxylic acid cycle, drugs targeting mtDNA, and agents targeting other presently unknown sites.

CRITICAL ISSUES

Mitocans have a potential to prove highly efficient in suppressing various malignant diseases in a selective manner. They include compounds that are currently in clinical trial and offer substantial promise to become clinically applied drugs. Here we update and redefine the individual classes of mitocans, providing examples of the various members of these groups with a particular focus on agents targeting the electron transport chain, and indicate their potential application in clinical practice.

FUTURE DIRECTIONS

Even though reactive oxygen species induction is important for the anticancer activity of many mitocans, the precise sequence of events preceding and following this pivotal event are not yet fully clarified, and warrant further investigation. This is imperative for effective deployment of these compounds in the clinic.

A novel mitotropic oligolysine nanocarrier: Targeted delivery of covalently bound D-Luciferin to cell mitochondria

New and emerging therapeutic approaches focus on the targeted delivery of therapeutic agents to cell mitochondria with high specificity. Herein we present a novel mitotropic nanocarrier based on an oligolysine scaffold by addition of two triphenylphosphonium cations per oligomer. Although the parent oligolysine failed to enter healthy cells, the triphenylphosphonium modified carrier, with or without D-Luciferin, attached as cargo molecule, demonstrated striking mitochondrial specificity. Furthermore, the oligolysine bound d-Luciferin exhibited chemiluminescence, of lower intensity than free d-Luciferin, yet of remarkably longer steady-state temporal profile.

Synthesis and characterization of poly(ε-caprolactone)-block-poly[N-(2-hydroxypropyl)methacrylamide] micelles for drug delivery

Amphiphilic block copolymers based on HPMA and ε-CL were synthesized by ring-opening polymerization of ε-CL followed by RAFT polymerization of HPMA. A copolymer composed of 34 kDa PHPMA and 8.5 kDa PCL associated into micelles with CMC of 5.4 µg · mL(-1) . A novel retinoid, 3-Cl-AHPC-OMe, was incorporated into micelles with 25 wt.-% loading by dialysis method. The effective diameter of drug loading micelles was 117 nm. Incubation of micelles in PBS at 37 °C indicated 86 wt.-% of the drug was released after 96 h. Cytotoxicity studies performed with C4-2 prostate cancer cells showed the IC(50) dose was 1.96 µM after 72 h of incubation, whereas the micelles without drug showed no cytotoxicity.

Approaches for targeting mitochondria in cancer therapy

The recognition of the role that mitochondria play in human health and disease is evidenced by the emergence in recent decades of a whole new field of "Mitochondrial Medicine". Molecules located on or inside mitochondria are considered prime pharmacological targets and a wide range of efforts are underway to exploit these targets to develop targeted therapies for various diseases including cancer. However the concept of targeting, while seemingly simple in theory, has multiple subtly different practical approaches. The focus of this article is to highlight these differences in the context of a discussion on the current status of various mitochondria-targeted approaches to cancer therapy.

HPMA copolymers: origins, early developments, present, and future

The overview covers the discovery of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers, initial studies on their synthesis, evaluation of biological properties, and explorations of their potential as carriers of biologically active compounds in general and anticancer drugs in particular. The focus is on the research in the authors' laboratory - the development of macromolecular therapeutics for the treatment of cancer and musculoskeletal diseases. In addition, the evaluation of HPMA (co)polymers as building blocks of modified and new biomaterials is presented: the utilization of semitelechelic poly(HPMA) and HPMA copolymers for the modification of biomaterial and protein surfaces and the design of hybrid block and graft HPMA copolymers that self-assemble into smart hydrogels. Finally, suggestions for the design of second-generation macromolecular therapeutics are portrayed.

Intracellular trafficking and subcellular distribution of a large array of HPMA copolymers

The basic physicochemical properties that determine the distribution and fate of synthetic macromolecules in living cells were characterized using fluorescently labeled HPMA (N-(2-hydroxypropyl)methacrylamide) copolymers. Twelve different classes of water-soluble copolymers were created by incorporating eight different functionalized comonomers. These comonomers possessed functional groups with positive or negative charges or contained short hydrophobic peptides. The copolymers were fractionated to create parallel "ladders" consisting of 10 fractions of narrow polydispersity with molecular weights ranging from 10 to 200 kDa. The intracellular distributions were characterized for copolymer solutions microinjected into the cytoplasm of cultured ovarian carcinoma cells. Even the highest molecular weight HPMA copolymers were shown to quickly and evenly diffuse throughout the cytoplasm and remain excluded from membrane-bound organelles, regardless of composition. The exceptions were the strongly cationic copolymers, which demonstrated a pronounced localization to microtubules. For all copolymers, nuclear entry was consistent with passive transport through the nuclear pore complex (NPC). Nuclear uptake was shown to be largely dictated by the molecular weight of the copolymers, however, detailed kinetic analyses showed that nuclear import rates were moderately, but significantly, affected by differences in comonomer composition. HPMA copolymers containing amide-terminated phenylalanine-glycine (FG) sequences, analogous to those found in the NPC channel protein, demonstrated a potential to regulate import to the nuclear compartment. Kinetic analyses showed that 15 kDa copolymers containing GGFG, but not those containing GGLFG, peptide pendant groups altered the size-exclusion characteristics of NPC-mediated nuclear import.