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Limited efficiency of drug delivery to specific intracellular organelles using subcellularly-‘targeted’ drug delivery systems Amit Ranjan Maity, and David Stepensky Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.5b00697 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Molecular Pharmaceutics

Limited efficiency of drug delivery to specific intracellular organelles using subcellularly-‘targeted’ drug delivery systems

Amit Ranjan Maity and David Stepensky*

Department of Clinical Biochemistry and Pharmacology, The Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel

*

Correspondence:

David Stepensky, Department of Clinical Biochemistry and Pharmacology, The Faculty of Health Sciences, Ben-Gurion University of the Negev, P.O.Box 653, Beer Sheva 84105, Israel. Tel.: +972-8-6477381; Fax: +972-8-6479303; E-mail address: [email protected] (David Stepensky).

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Graphical abstract

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ABSTRACT: Many drugs have been designed to act on intracellular targets and to affect intracellular processes inside target cells. In order to exert the desired effects, these drugs should permeate target cells and reach specific intracellular organelles. Subcellular drug targeting approach has been proposed for enhancement of accumulation of these drugs in the target organelles and for improvement of their efficiency. This approach is based on drug encapsulation in drug delivery systems (DDSs) and/or their decoration with specific targeting moieties that are intended to enhance the drug/DDS accumulation in the intracellular organelle of interest. During the recent years, there has been a constant increase in interest in DDSs targeted to specific intracellular organelles, and many different approaches have been proposed for attaining efficient drug delivery to the specific organelles of interest. However, it appears that in many studies insufficient efforts have been devoted to quantitative analysis of the major formulation parameters, of the DDSs disposition (efficiency of DDSs endocytosis and endosomal escape, intracellular trafficking, and efficiency of DDS delivery to the target organelle) and of the resulting pharmacological effects. Thus, in many cases, claims regarding efficient delivery of drug/DDS to the specific organelle and efficient subcellular targeting appear to be exaggerated. Based on the available experimental data, it appears that drugs/DDSs decoration with specific targeting residues can affect their intracellular fate and result in preferential drug accumulation within an organelle of interest. However, it is not clear whether these approaches will be efficient in in vivo settings and can be translated into pre-clinical and clinical applications. Studies that quantitatively assess the mechanisms, barriers and efficiencies of subcellular drug delivery and of the associated toxic effects are required to determine the therapeutic potential of subcellular DDSs targeting.

KEYWORDS: subcellular drug targeting; drug delivery systems; decoration with targeting residues; barriers for subcellular targeting; limited targeting efficiency; exaggerated claims.

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INTRODUCTION Cells maintain complex structure that is based on numerous bioactive molecules and their interactions compartmentalized by intracellular membranes into individual organelles. This complex environment provides numerous opportunities for pharmacological modulation using exogenous active compounds. Indeed, many drugs were developed to affect specific intracellular processes and targets (see selected examples in Table 1). In order to exert desired effects, these drugs should permeate inside the target cells and reach specific organelles within these cells. Subcellularly-acting drugs are characterized by inherent pharmacokinetic limitation. On one hand, in order to reach their site of action, these drugs should possess high permeability through different biological barriers (capillary walls, cell membranes, etc.). On the other hand, high permeability leads to extensive drug distribution in the body, limits its accumulation at the site of desired action (specific organelles of the target cells), and can negatively affect its safety profile (due to undesired pharmacological effects at other cells and organelles). Intracellularly-acting compounds that are characterized by inefficient permeability through biological barriers are unable to reach the target organelles and possess limited pharmacological activity that precludes their pharmacological application. In order to overcome this pharmacokinetic limitation of the intracellularly-acting drugs, subcellular drug targeting approach can be used (i.e., drug targeting on a subcellular level to a specific intracellular compartment/organelle). This approach is based on decoration of drug (or of a drug delivery system, DDS, that encapsulates the drug) with specific targeting moieties that are intended to enhance the drug/DDS accumulation in the intracellular organelle of interest. During the recent years, there was a constant increase in interest in drugs/DDSs targeted to specific intracellular organelles, and many different approaches have been proposed for attaining efficient intracellular drug delivery to the organelle of interest. In

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this review, we summarize the approaches for subcellular drug/DDS targeting, their efficiency, and the factors that limit the development and application of subcellularly-targeted drugs/DDSs.

APPROACHES FOR SUBCELLULAR DRUG TARGETING Drug delivery systems used for drug delivery to specific intracellular organelles can be classified according to their structure and composition into liposomes, particles, micelles, dendrimers and conjugates (see Table 2). Detailed description of the composition and properties of different DDSs intended for delivery into specific organelles appears in several recent reviews 1-4. Different types of DDSs are characterized by specific properties that can be important for subcellular drug delivery and targeting: a) size and composition, b) ability to encapsulate different drugs (e.g., hydrophilic and lipophilic small molecular weight drugs, nucleic acids, therapeutic proteins, etc.), c) drug loading efficiency and release kinetics, d) stability in different locations in the body (body fluids and cells) and ability to control it. Specifically, efficiency of subcellular drug targeting using a specific DDS is expected to be highly dependent on its surface properties and drug release kinetics. Indeed, DDS surface is the major site of interaction between the DDS and biological systems, and the surface properties of a specific DDS have important effect on its disposition (distribution and elimination) pathways, such as endocytosis (in in vitro and in vivo settings, e.g., by the endothelial, phagocytic and target cells), interaction with soluble factors (activation of complement, formation of ‘corona’ made of endogenous proteins), etc. 5, 6. Certain components can be incorporated into the DDSs in order to alter their endocytosis, intracellular disposition and subcellular targeting (see Table 2). For instance, PEG residues can reduce DDSs clearance and interaction with soluble factors 7-9, penetration enhancers and surface-active agents can increase DDSs permeability via the biological barriers

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, etc.


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Moreover, DDSs can be decorated with specific targeting residues (see Table 3) with intention to increase drug accumulation in a specific intracellular organelle, i.e., to attain subcellular drug/DDS targeting. Majority of these residues originate from endogenous ‘tag’ peptides that mediate intracellular trafficking of endogenous proteins and their delivery to the target organelle. Indeed, many such peptides were identified by cell biologists, and in some cases correlation between ‘tag’ sequence and subcellular targeting efficiency of a protein bearing this ‘tag’ was investigated in a detailed fashion

12-14

. Alternatively, drugs/DDSs can

be decorated with non-peptidic chemical compounds that possess affinity to a specific intracellular organelle, such as triphenylphosphonium (TPP) cations that preferentially accumulate in the mitochondrial matrix due to their ionization properties

15, 16

. Numerous

approaches have been developed for decoration of drugs/DDSs with targeting residues (see Table 4) that can be attached directly to the drug (to generate drug-targeting residue conjugate) or to the DDSs surface (via covalent or non-covalent interactions), or can be incorporated into the DDSs during their preparation. It should be noted that during decoration of DDS with targeting residues, interaction between the reactants can proceed via multiple processes (e.g., covalent conjugation and non-covalent adsorption of targeting residues to the DDS surface 17, formation of unintended by-products, etc.). Thus, the surface properties of the decorated DDSs can be different from the expected ones. DDS decoration with targeting residues can be even more complex in cases when sequential multi-stage DDS decoration approaches are applied (e.g., conjugation of linker followed by a targeting residue) 17, 18.

SUMMARY OF STUDIES THAT INVESTIGATED SUBCELLULAR DRUG/DDS TARGETING We performed an extensive search of scientific research publications that reported drug/DDSs delivery and/or targeting to specific intracellular organelle/s in the PubMed and Web of

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Science databases. We were able to identify 77 such publications, with the earliest one published in 1998 19. For detailed analysis of these publications, the readers are referred to our recently published manuscript

20

. In this review, we will present only some of the major

trends in the field of subcellular drug/DDS targeting that emerge from analysis of the content of the 77 identified publications. We analyzed the data on the target organelles, DDS type and properties, decoration approach, experimental systems and efficiency of subcellular DDS targeting that have been reported in the individual identified publications. The nucleus and the mitochondrion have been the most common targets for drug/DDS delivery (in 50% and 33% of the identified publications, respectively), followed by the endoplasmic reticulum (11%), Golgi apparatus (4%), and other organelles (cytoskeleton, lysosomes, etc.). Nanoparticle-based formulations (NPs; including nanotubes, nano-dots, nanorods, and other non-liposomal formulations) have been used more frequently for the purpose of subcellular targeting than liposomes, conjugates and QDs (58%, 16%, 13% and 13% of the identified publications, respectively). The majority of the identified publications reported results of in vitro studies that involved incubation of the studied DDSs with specific cell lines, and analysis of subcellular DDSs localization. Only in 7 studies (9%) the studied DDSs have been investigated in ex vivo or/and in vivo settings. None of the studies performed detailed toxicity assessment of the studied DDSs. DDS decoration with specific targeting residues have been applied in 65% of cases, and peptidic targeting residues have been used more frequently for this purpose, as compared to the charged and lipophilic compounds (48%, 12% and 5%, respectively). The most commonly used types of peptidic residues have been nuclear localization signals (NLS), mitochondria targeting peptides, and endoplasmic reticulum (ER) signal peptides. In 24% of the publications, targeting residues have been incorporated into the formulation (usually for liposomes) during DDSs preparation. In other cases, targeting residues have been attached to

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pre-formed formulations using specific chemical reactions, such as carbodiimide reaction, coupling between maleimide and thiol groups, and Click chemistry (in 26%, 9% and 6% of the identified publications, respectively). Our analysis revealed that many of the identified publications did not report the major formulation parameters that affect the subcellular drug/DDS targeting efficiency. Specifically, 18% of the identified publications did not report the dimensions (diameter) of the investigated DDSs; 90% of the identified publications did not contain quantitative data on the density of targeting residues on the DDS surface; and 77% of the identified publications did not report quantitative data that allowed calculation of the DDS subcellular targeting efficiency (see below). Despite the lack of these data, many of the identified studies claimed successful subcellular drug/DDS targeting using the applied formulations and expressed expectations for successful clinical translation of the obtained results.

EFFICIENCY OF SUBCELLULAR DRUG/DDS TARGETING Optimistic claims regarding the efficiency and the clinical translation of the DDSs intended for subcellular targeting (see above) apparently originate from the expectation that decoration of DDSs with subcellular targeting residues should lead to their targeting to the specific intracellular organelle. Thus, the commonly applied research approach is based on preparation of DDSs decorated with specific targeting residues (and control formulation/s, without these targeting residues), basic characterization of these formulations (e.g., DDSs size and ζpotential), and in vitro experiments to determine their presence in the target organelles following incubation with cultured cells. Some studies apply qualitative analytical techniques (e.g., electron microscopy) to identify presence of the studied DDSs in the specific intracellular organelles

21, 22

. In other

cases, fluorescence-based imaging techniques are applied to analyze the colocalization of the

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DDS with the specific organelle that is labeled with fluorescent marker (such as LysoTracker 23-25

or MitoTracker) or using immunofluorescence technique

. More rarely, the

ultracentrifugation technique is used to separate the contents of the analyzed cells followed by analysis of DDSs accumulation in the individual intracellular organelles. Unfortunately, these approaches can provide incorrect results and are prone to artefacts. For instance, fluorescencebased techniques can lead to biased or erroneous conclusions due to leaking of fluorescent probe from DDS, nonspecific binding of the organelle staining markers, photobleaching of the fluorophores, autofluorescence of the studied samples, and other reasons

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. Thus, careful

validation of the applied analytical techniques is required for reliable analysis of the presence and of the content of the studied DDSs in the specific intracellular organelles. Unfortunately, none of the identified publications (including the publications that reported quantitative data of drug/DDS content in the target organelle, see above) contained description of the validation characteristics of the applied detection and quantification methods. Detection of drug/DDS in a specific intracellular organelle using the above-described methods does not necessarily indicate that the drug has reached its site of action in the target organelle and can elicit the desired pharmacological effect

27

. For instance, drugs acting in

mitochondria should interact with specific targets in the outer mitochondrial membrane (apoptosis- or necrosis-inducing drugs), intermembrane space or the inner mitochondrial membrane (drugs that affect the proteins or coenzymes of the electron transport chain), or the mitochondrial matrix (siRNA against mitochondrial DNA)

16, 28, 29

. Decoration of drug/DDS

with a specific mitochondrial targeting sequence can lead to its preferential mitochondrial accumulation that can be detected by fluorescence-based, biochemical (subcellular fractionation) or other analytical techniques. However, these techniques cannot differentiate between the drug that is present in vicinity to the mitochondria or inside this organelle, in pharmacologically-active or inactive (e.g., encapsulated inside the DDS) form. Therefore,

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reports on ‘efficient targeting’ of drug/DDS to the organelle of interest need to be verified by analysis of drugs’ pharmacological effect on the specific target in this organelle. Additional problem with analysis of subcellular drug/DDS targeting efficiency is derived from several possible definitions of this term (summarized in Table 5). It can be seen that analysis of drug/DDS targeting that is based on the relative concentration requires quantitative assay of the drug/DDS content in different intracellular organelles. For analysis of subcellular drug/DDS targeting efficiency using other approaches (the concentration above

the threshold value and the fraction of dose), quantitative analysis of the drug/DDS content in a single target organelle is sufficient. Qualitative experimental results are insufficient for reaching conclusions regarding the efficiency of subcellular drug/DDS targeting. It should be noted that the above-described targeting efficiency definitions refer to the drug content in the target cells, and presence of non-target cells in the analyzed sample (that can accumulate the drug) can reduce efficiency of drug/DDS targeting to the intracellular site of action in the target cells. Moreover, subcellular drug/DDS targeting efficiency and exposure of the target organelle to the drug can change with time due to the processes of drug/DDS distribution and elimination in the analyzed sample. Thus, selection of sampling time points (e.g., 1, 4 or 24 h after the start of drug/DDS incubation with specific cells) can have a major impact on the measured subcellular targeting efficiency of the studied DDS, and can complicate the interpretation of experimental findings and their comparison with the results of other studies.

UNSOLVED PROBLEMS IN THE FIELD OF SUBCELLULAR DRUG TARGETING Intracellular disposition pathways and their efficiency Efficient subcellular drug/DDS targeting usually implies success at all the following stages: DDS endocytosis by the target cells, endosomal escape, targeting to the specific organelle of

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interest (or in vicinity of this organelle, see above), release of the encapsulated drug (for particle- and vesicle-based DDSs), and drug-target interaction in the organelle of interest. Each one of these processes can pose a barrier for drug accumulation in the target organelle 30. For instance, many types of cells possess low endocytic behavior that precludes the subsequent targeting stages. Moreover, DDSs which enter cells via endocytosis tend to trap inside endosomes and undergo degradation during their maturation into late endosomes and lysosomes. This endosomal trapping is a major obstacle for efficient subcellular targeting, as it prevents drug/DDS permeation to the cytosol (where the targeting residues are expected to be active) and its accumulation in the target organelle 31. DDS formulation properties can affect subcellular drug targeting efficiency at multiple stages. For instance, dense PEGylation can reduce DDS endocytosis

8

and diminish drug

release from the DDS leading to reduced drug interaction with its intracellular target. Due to the above-raised issues (insufficient DDS characterization, unreliable analysis of subcellular targeting efficiency), and other problems, no clear conclusions can be made regarding the relationships between the DDS formulation properties (formulation type, size, charge, type and density of surface residues, their stability) and the efficiency of the DDS uptake and intracellular trafficking mechanisms. The best investigated process appears to be the DDS endocytosis by some types of cells where some important discoveries regarding the effects of DDS size/shape/charge have been made recently 5, 32, 33. On the other hand, efficiency of DDS endosomal escape, spontaneous or mediated by specific endosome-destabilizing formulation components, is much less characterized. Moreover, it is not clear particles of which size can be handled by the intracellular trafficking mechanisms, which types of surface targeting residues are best suited for this, and to which extent these properties differ between cells of different origin.

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Extensive research is needed to clarify these issues, and to identify the formulation properties that lead to efficient subcellular targeting. This research should involve detailed investigation of drug/DDS uptake and intracellular disposition in the target and non-target cells, in different experimental settings. Otherwise, the expectations regarding targeted drug/DDS delivery can turn out to be exaggerated. For instance, based on results of in vitro studies, it is generally expected that DDS decoration with cell penetrating peptides (CPPs) will enhance its permeability to the target organelle and will enhance pharmacological activity of the encapsulated drugs

10, 11, 34

. It appears, though, that majority of the known CPPs are

non-specific, increase the DDS permeability via different (target and non-target) membranes, make non-target side effects much more likely and reduce the ratio of therapeutic vs. side effects 35. Development of tissue- and cell-specific CPPs can help to overcome this problem and to pave the way for pre-clinical and clinical application of CPP-decorated DDSs.

Translation into clinical applications Subcellular drug/DDS targeting is usually studied nowadays in in vitro (and sometimes in ex

vivo) experimental settings, and the currently available DDSs apparently offer limited subcellular targeting efficiency (by either definition of this term, see above). Even if efficient subcellular DDSs targeting to organelles of interest would be attained in vitro/ex vivo in the near future, these DDSs would most probably possess limited targeting efficiency in in vivo settings. The reason for this is multiple pathways of DDSs degradation/elimination in the blood and biological fluids that can preclude DDSs accumulation in the target cells and subsequent targeting to specific intracellular organelles

5, 6

. Specifically, DDSs can undergo

degradation, aggregation, uptake by mononuclear phagocyte system (MPS) cells, dendritic cells, or endothelial cells, interaction with antibodies that bind immunogenic components of DDSs, etc.

36-38

. All these pathways can interfere with DDSs ability to reach the cells in the

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target organs, which is a prerequisite for efficient subcellular targeting. It should be noted that some pathways, such as enhanced permeability and retention effect (EPR, which efficiency varies in different types of cancer) 39, 40, or endocytosis by macrophages and dendritic cells 41, 423

can enhance DDSs accumulation in the target cells (e.g., for anti-cancer DDSs and

vaccines, respectively). However, such endogenous ‘cell-targeting’ pathways exist only for selected types of target cells, and for numerous other types of cells efficient subcellular drug/DDSs targeting in vivo requires overcoming numerous obstacles and barriers, which apparently is not feasible based on the existing pharmaceutical technologies.

Safety aspects Induction of toxic effects by DDSs can pose an additional obstacle for their pre-clinical and clinical applications. Different types of DDSs can induce profound toxic effects at the intended site of action (intracellular organelle or specific cells) or at other locations in the body. The same formulation parameters that govern DDSs disposition and targeting (formulation type, size, charge, type and density of surface residues, etc.; see above) also determine their toxicity profile

43-45

. Changes in these parameters (e.g., alterations in decoration efficiency, change in

the sequence of targeting residues) can affect both targeting efficiency and toxicity of DDSs. For instance, experimental approaches that promote ‘endosomal escape’ of drug/DDS

19, 34, 46

inevitably destabilize endo/lysosomes and can lead to cell toxicity. Detailed, quantitative, mechanism-based analyses of toxic effects induced by the DDSs (using the available in vitro,

ex vivo and in vivo tests)47, 48 and of their targeting efficiency are needed for development of effective and safe subcellularly-targeted DDSs.

SUMMARY: MECHANISMS AND EFFICIENCY OF DRUG/DDS DELIVERY TO SPECIFIC INTRACELLULAR ORGANELLES

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Subcellularly-targeted drug delivery is a promising approach for overcoming the pharmacokinetic limitations of intracellularly-acting drugs, and for enhancing and controlling their pharmacological activities. There is an increasing interest in specialized drug delivery systems for subcellular drug targeting, and numerous approaches have been proposed for attaining efficient drug/DDS delivery to the organelle of interest. There is general expectation that decoration of DDSs with specific targeting residues (such as cell penetration peptides, nuclear or mitochondrial targeting residues) enhances accumulation of drug/DDS in the target organelle, and efficient subcellular drug/DDS targeting has been claimed in many scientific publications. However, it appears that in many of these studies insufficient efforts have been devoted to quantitative analysis of the major formulation parameters, of the DDSs disposition (efficiency of DDSs endocytosis and endosomal escape, intracellular trafficking, and efficiency of DDS delivery to the target organelle), and of the resulting pharmacological effects. Thus, in many cases, claims regarding efficient delivery of drug/DDS to the specific organelle and efficient subcellular targeting appear to be exaggerated. Based on the available experimental data, that originates mostly from in vitro studies in cultured cells, it appears that DDS decoration with specific targeting residues can affect their intracellular fate and result in preferential drug accumulation within an organelle of interest. However, it is not clear whether these approaches will be efficient in in vivo settings and can be translated into preclinical and clinical applications. Studies that quantitatively assess the mechanisms, barriers and efficiencies of subcellular drug delivery and of the associated toxic effects are required to determine the therapeutic potential of subcellular DDSs targeting. Specifically, there is a need to identify more selective and more efficient intracellular targeting residues, and to clarify which formulation properties of DDSs (i.e., size, charge, type and density of surface residues) will lead to their efficient targeting to specific intracellular organelles. This knowledge will pave the way for development of DDSs with controllable intracellular disposition that will

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efficiently deliver intracellularly-acting drugs to their sites of action and will enhance their efficiency in pre-clinical and clinical settings.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: (+972) 86477381. Notes The authors declare no competing ﬁnancial interest. Research of Dr. Amit Ranjan Maity is supported by the Planning and Budgeting Committee of the Israeli Council for Higher Education for Outstanding Post-doctoral Fellows from China and India.

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Table 1. Examples of intracellularly-acting drugs with sites of action in specific organelles and intracellular locations. Organelle Nucleus Mitochondrion

Diseases genetic diseases cancer cancer neurodegenerative diseases neuromuscular disorders

Endoplasmic reticulum and Golgi complex

cancer infectious diseases protein folding diseases

Lysosome Peroxisome Cytosol

lysosomal storage diseases peroxisomal disorders cancer infectious diseases

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Drugs DNA, shRNA drugs anti-cancer agents pro-apoptotic peptides antioxidants mitochondrial DNA mitochondrial proteins antigentic peptides antibiotics chaperons glycosylation enzymes lysosomal enzymes peroxisomal enzymes anti-cancer agents siRNA, miRNA drugs antibiotics proteins



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Table 2. Drug delivery systems and model formulations applied for investigation of subcellular targeting, and types of their components that can affect the targeting efficiency. Formulation type

Components that can affect drug/DDS endocytosis, intracellular disposition and targeting • liposomes • intracellular targeting residues (e.g., nuclear localization sequence) • particles (nanocapsules, solid lipid particles, nanospheres, • penetration enhancers (e.g., fusogenic lipids or cellnanotubes, nano-rods, nanopenetrating peptides) dots, quantum dots, etc.) • surface-active agents (e.g., chitosan, surfactants) • micelles • PEG residues (covalent or sheddable) • dendrimers • pH-sensitive and/or endosome disrupting components (e.g., dioleoylphosphatidylethanolamine, DOPE) • conjugates • enzyme-dependent components (e.g., endosomal cathepsin-sensitive peptides)

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Table 3. Examples of targeting residues that can be applied for subcellular targeting of drugs and drug delivery systems. Type

Peptide

Nonpeptide

Target organelle Examples

ER

• ER insertion sequence (signal peptide) • KKXX or KXKXX retrieval signal • RXR ER retention/retrieval signal • KDEL ER retention/retrieval signal • C-terminal hydrophobic sequence in tail-anchored proteins

Nucleus

Nuclear Localization Signal (NLS): • SSDDEPPKKKRKV • R11KC

Mitochondrion

Mitochondrial Targeting Signal (MTS): • MSVLTPLLLRGLTGSARRLPVPRAKIHWLC

Peroxisome

Peroxisomal Targeting Signal (PTS): • SKLKANL

Mitochondrion

• triphenylphosphonium cations

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Table 4. Methods of decoration of drug delivery systems with targeting residues.

Method Targeting group incorporated during DDS preparation

Covalent conjugation

Interaction type

Active groups on DDS and targeting residues

non-covalent binding

lipophilic chains with active group

addition of stearyl octaarginine during liposome preparation

carbodiimide

-NH2, -COOH

amide bond formation

sulfhydryl

-SH

maleimide-thiol interaction

-N3, -C≡C-, nitrone, bioorthogonal reactions hydrazine, ketone/aldehyde van der Waals interaction

Non-covalent binding

Example

Click chemistry, hydrazone formation reaction

electrostatic opposite charged groups interaction, gold-thiol interaction

hydrophobic interaction hydrophobic chains

interaction between lipophilic groups

avidin-biotin interaction protein, ligand

streptavidin-biotin interaction

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Table 5. Indexes that can be applied for quantitative analysis of subcellular drug/DDS targeting efficiency. Analyzed parameter

Index

Relative concentration

concentration of the drug/DDS in the target organelle vs. other intracellular organelles

Concentration above threshold value

concentration of the drug/DDS in the target organelle in comparison to a specific threshold value (e.g., the lower boundary of therapeutic window)

Fraction of dose

% of drug/DDS dose that reached the target organelle

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graphical abstract 92x38mm (150 x 150 DPI)


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âTargetedâ Drug Delivery System - ACS Publications - American

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