Stimuli-sensitive Systems-an emerging delivery system for drugs

Abstract

Objectives: Development of controlled and sustained drug delivery system (DDS) remains a great thrust of human beings for the successful delivery of drugs due to various drawbacks of existing systems. In order to overcome these drawbacks, various stimuli-sensitive DDSs were developed in the recent years. Key findings: Stimuli are a state of responsiveness to sensory stimulation or excitability. Stimuli sensitive systems are those systems which deal with the changes in the physiology of body with respective to the environment changes. These systems may be very beneficial for the controlled and sustained delivery of drug in the body if proper work would be carried out on these types of systems. Controlled drug delivery became the standard criteria in modern pharmaceutical product design and an intensive research is still going on in achieving much better drug product with features like effectiveness, reliability, and safety. Many changes like photo and light, temperature, pH, ion, glucose, and redox affect the release of drug from the delivery system. These stimuli-sensitive systems are used for various purposes in various forms like in parenteral, ocular, peroral, rectal, vaginal, nasal, dermal and transdermal drug delivery. Summary: Various literature surveys revealed that stimuli-sensitive DDSs can be explored as a potential tool for the delivery of a variety of macromolecules that are not effectively delivered by conventional techniques.

Keywords::

• glucose
• ocular
• redox
• stimuli
• transdermal

Introduction

Drug delivery is a method or a process of administering a pharmaceutical compound to achieve a better therapeutic effect in humans or animals (Zhang et al. 2011, Traitel et al. 2000). Drug delivery technologies are patent-protected formulation technologies that modify drug release profile, absorption, distribution, and elimination for the benefit of improving product efficacy and safety, as well as patient convenience and compliance (Carreira et al. 2010). Drug targeting mainly include two components: (1) selective delivery of drugs to the target sites and (2) drug release from drug carriers at the target sites.

One of the best ways to achieve the selective drug targeting and release is stimuli-sensitive drug release. Basically, stimuli- sensitive is a state of responsiveness to sensory stimulation or excitability. Stimuli sensitivity includes the changes in the pH levels of certain enzymes and externally applied physical signals (e.g., high temperature, light, magnetic field, and ultrasound). Among these internal and external stimuli, high temperature is one of the best signals in terms of easy and safe medical applications. The merits of high temperature are due to the substantial strides that medical researchers have found in applications of the hyperthermia therapy to solid tumors (Nakayama et al. 2006). The use of stimuli-responsive nanocarriers also offers an interesting opportunity for drug and gene delivery where the delivery system becomes an active participant, rather than passive vehicle in the optimization of therapy. Several families of molecular assemblies are employed as stimuli-responsive nanocarriers for either passive or active targeting. Liposomes, polymeric nanoparticles, block copolymer micelles, and dendrimers are colloidal molecular assemblies. The composition of each class of these molecular assemblies can be manipulated to obtain nanocarrier of desired stimuli- responsive property. The benefit of stimuli-responsive nanocarriers is especially important when the stimuli are unique to disease pathology, allowing the nanocarrier to respond specifically to the pathological “triggers”. Selective examples of biological stimuli that can be exploited for targeted drug and gene delivery include pH, temperature, and redox microenvironment (Ganta et al. 2008).

Recent studies related to stimuli-sensitive delivery system

Too many research works have been carried out in the last decades on stimuli-sensitive drug delivery systems (DDSs). The main objective of all these works was to establish a standard and controlled DDS which overcomes all the limitations related to all other delivery system. List of recent studies related to stimuli-sensitive DDS are shown in Table I.

Table I. Various stimuli sensitive drug delivery systems.

Aim	Objective	Pharmaceutical uses	References
A work on environmentally responsive peptides as anticancer drug carriers had been carried out.	This study involved the development of environmentally responsive delivery systems by use of tumor pathology to trigger release of therapeutic agents at the target site.	Potential application includes the use of such technology for treatment of tumors with the help of signals.	Aluri et al. (2009)
A work on novel environmentally responsive ophthalmic drug delivery system had been carried out.	The objective of present study was to develop a temperature-triggered and ion-activated environmentally responsive system using ofloxacin as drug candidate.	In present study, temperature-triggered and ion-activated environmentally responsive systems were prepared for ocular drug delivery by using poloxamer-407 and sodium alginate.	Deshmukh and Fursule (2010)
Another work on temperature-responsive drug delivery system in conjunction with thermotherapy has been carried out.	–	These thermosensitive polymers are useful for the preparation of temperature-sensitive dendrimers.	Kojima (2010)
A work on the highly temperature-sensitive liposomes based on a thermosensitive block copolymer for tumor-specific chemotherapy has been done.	Incorporation of poly [2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE)], which exhibits a lower critical solution temperature (LCST) around 40°C, provides temperature-sensitive properties for stable liposomes.	Liposome release can be controlled by adjusting a LCST of thermosensitive polymers at desired temperatures.	Strehl et al. (2013)
A work on redox-responsive polyplexes represent a promising class of nonviral gene delivery vectors.	Polyplexes were investigated as promising delivery vectors for a variety of nucleic acid therapeutics.	Polyplexes capable of responding to environmental changes by altering their properties and their behavior seemed to promise a significant improvement in delivery efficacy.	Manickam et al. (2010)
A preparation of thermo-responsive polymer membranes has been carried out.	The purpose of this study was to develop thermo-responsive membranes by using liquid crystals (LC) for controlled drug delivery system.	Thermo-responsive system found to be useful tool for drug delivery system which responds to the change of body temperature and/or the external thermal stimuli.	Sortino (2008)

Various stimuli-sensitive DDSs

The various stimuli-sensitive DDSs are described below as:

• 1) pH-sensitive DDS.

• 2) Temperature-sensitive DDS.

• 3) Redox-sensitive DDS.

• 4) Photo-sensitive DDS.

• 5) Thermo-sensitive DDS.

• 6) Hydrogel-sensitive DDS.

• 7) Gluocose-sensitive DDS.

• 8) Ion-sensitive DDS.

General introduction of each system and their therapeutic applications

pH-sensitive (responsive) DDS

pH of the human body varies and can be utilized for various pH-responsive DDS. While considering oral DDS, there is a pH difference between the stomach and intestine. Cancer and inflammatory tissues are slightly acidic with pH 6.5–7.2, whereas intracellular cytosol, endosome, and lysosome have pH values of 7.4, 5.0–6.5, and 4.5–5.0, respectively (Kojima 2010). Figure 1 represents the different levels of pH in various parts of the body. The pH is an important signal, which can be addressed through pH-responsive materials. Ionizable polymers with a pKa value between 3 and 10 are good candidates for pH-responsive systems. Weak acids and bases like carboxylic acids, phosphoric acid, and amines, respectively exhibit a change in the ionization state upon variation of the pH. It leads to a change in the conformation for the soluble polymers. Swelling behavior of the hydrogels also changes when these ionizable groups are linked to the polymer structure. This behavior leads to induce controlled release of model compounds like caffeine, indomethacin, or cationic proteins like lysozome.

Figure 1. Different levels of pH in the various parts of body.

The use of stimuli-responsive nanocarriers offers an interesting approach for drug and gene delivery. The extracellular and intracellular pH profile of biological system is greatly affected by diseases. Various cellular and tissue compartments have different pH value in normal condition, which get changed during disease conditions. pH of various cellular and tissue compartments in normal condition are shown in Table II. The pH profile of pathological tissues, such as upon acquisition of inflammation, infection, and cancer, is significantly different from that of the normal tissue. The pH at systemic sites of infections, primary tumor, and metastasized tumor are lower than the pH of the normal tissue. Cellular components such as the cytoplasm, endosomes, lysosomes, endoplasmic reticulum, golgi bodies, mitochondria, and nuclei are known to maintain their own pH values. The pH, surface charge, and density of low-density lipoprotein receptors are the factors that show notable differences among the normal and tumor tissues. All these properties are known to influence the drug's physicochemical properties and are exploited for enhanced delivery to the target site. Since tumor proliferate very rapidly, the vasculature of tumor is often insufficient to supply enough nutritional and oxygen needs for the expanding population of tumor cells (Ganta et al. 2008, Schmaljohann 2006).

Table II. pH in various cellular and tissue compartments under normal conditions.

Cellular/Tissue compartment	pH
Blood	7.35–7.45
Duodenum	7.0–8.5
Colon	7.0–7.5
Stomach	2.0–3.0
Early endosome	6.0–6.5
Late endosome	5.0–6.0
Golgi	6.4
Lysosome	4.5–5.0
Tumors, extracellular	7.2–6.5

Therefore pH sensitive preparations have been designed in order to overcome such diseases. The pH-sensitive liposomes have been designed to deliver highly hydrophilic molecules or macromolecules into the cytoplasm. Bae and co-workers found that intracellular delivery of pH-sensitive polymeric micelles release anticancer drug doxycycline in response to acidic pH at endosomes (pH 5.0–6.0) and lysosomes (pH 4.0–5.0), which may be helpful for the treatment of tumor.

pH-sensitive shielding of nonviral vectors that exploit the endosomal acidification process after endocytosis for deshielding of the delivery system were studied by Meyer et al. (Meyer and Wagner 2006). In vivo systemic target delivery remains a major challenge in the field of biomolecular therapeutics. This hurdle has been partly overcome by inclusion of ligand-appended delivery but these targeted and pegylated carriers also do not reach the target site properly. Direct interactions of positively charged particles with the inner endosome membrane may lead to membrane disruption through perturbation or fusion. Additional endosomolytic moieties (either the carrier itself or linked domains) may be included to enhance gene transfer. Therefore, it is obvious that dynamic nonviral gene delivery systems have to be developed that undergo bioresponsive changes after reaching the target cell.

Environmentally responsive peptides as delivery systems have been developed, which make the use of tumor pathology to trigger release of therapeutic agents at the target site (Aluri et al. 2009). The tumor microenvironment has been widely studied, generating a host of biomarkers suitable for targeted delivery. A wide range of potential biomarkers have been available; however, it is likely that only few of biomarkers can be engineered into suitable triggers for targeted drug delivery. The list of potential biomarkers provided by the tumor microenvironment can be broadly classified into physical or molecular triggers. Physical triggers are activated by the nature of the tumor microenvironment. On the other hand molecular triggers include the target molecules that are upregulated in the tumor vasculature or within the tumor cells. These targets include vascular endothelial growth factor, integrins, matrix metalloproteases, and tumor necrosis factors. These microenvironmental biomarkers are being actively explored for the ability to produce environmentally responsive drug release in the tumor. One approach is to develop environmentally sensitive polymers, including peptides that respond to tumor microenvironment with targeted delivery of drug. Such approaches are intended to either increase the accumulation of drug carrier in the tumor or increase the release of active drugs from carriers that have already trafficked to the tumor. As development of cancer-targeted nanocarriers expands, peptides provide a unique source of functional units that may target disease. Various colloidal carriers and their therapeutic applications are shown in Table III.

Table III. Various colloidal carrier systems along with their therapeutic applications.

Carrier Type	Polymer/lipid	Drug/Gene	Therapeutic use	References
Liposome	Liposomes attached with a saccharide (lecithin) vector PEGylated liposome Copolymer of N-isopropylacryl amide and acryloylpyrrolidine (sterically stabilized liposome) Histidine-modified galactosylated cholesterol derivative—cationic liposome Anionic liposomes containing phosphatidylethanolamine (PE)	Sarcolysine Doxorubicin Hexadecyl phosphocholine Plasmid DNA Antisense oligonucleotids	Increased level of vectored liposome binding to malignant cells by 50–80% compared to liposome without vector Treatment of breast cancer due to higher efficacy and favorable safety profile Improved antitumor activity in breast carcinoma in nude mice Increased gene transfection to hepatocytes Effective in the treatment of viral infections cancer or inflammatory diseases	Vodovozova et al. (1998) Ganta et al. (2008) Arndt et al. (1997) Shigeta et al. (2007)
Niosomes	Amphiphilic co-polymer of poly (methoxy-polyethylene glycol cyanoacrylate-c0-n-hexadecyl cyano acrylate) (PEG-PH DCA) Polyethylene Poly (phthaloyl-L-lysine)	Hydroxy Camptothecin (HCPT) Doxorubicin Cisplatin	Antitumor activity, good tumor inhibition rate Helpful for the treatment of hepatic neoplasms Release the drug in a controlled fashion and also improve stability of vesicles both in vitro and in vivo.	Li and Wallace (2008) Li and Wallace (2008)
Dendrimers	Polyamidoamine dendrimer Drug linked to polyamidoamine dendrimer via pH-sensitive linker	Chlorambucil Doxorubicin	Drug was released in a pH-dependant fashion Improved cytotoxicity on Ca9-22 cells	Hui et al. (2005) Lai et al. (2007)
Nanoparticles/ Nanocapsules	Poly (alkylcyanoacrylate) nanoparticles with anticancer agents Poly (methylmethacrylate) nanoparticles with proteins and therapeutic agents Poly (alkylcyanoacrylate) Polyester nanoparticles with anti-parasitic/ antiviral agents Albumin, Chitosan, Dextran	Oligonucleotides Vaccine adjuvant Antiviral drugs Hydrophilic and protein affinity	Targeting, reduced toxicity, enhanced uptake of antitumor agents Enhanced bioavailability, protection from gastrointestinal enzymes Target reticuloendothelial systems for intracellular infections Targeted, improved and controlled delivery of drug	Vyas and Khar (2002) Vyas and Khar (2002) Vyas and Khar (2002) Vyas and Khar (2002)
Polymeric	Poly (N-isopropylacrylamide- cobutylmethacrylate-co-acrylic acid) Poly (N-isopropylacrylamide)	Insulin Doxorubicin	This system has a potential as a polypeptide drug carrier Showed improved anticancer activity in murine tumor models	Yong-Hee et al. (1994) Dufresne et al. (2004)

Targeting the drug to the appropriate site of action is usually one of the greatest challenges in drug delivery to the eye because of its anatomical and physiological nature. A novel, environmentally responsive, ophthalmic DDS was developed by Deshmukh et al. (Deshmukh and Fursule 2010). They developed a temperature-triggered and ion-activated environmentally responsive system for delivery of ofloxacin. It is a fluoroquinolone derivative used to treat external infections of eye such as acute and subacute bacterial conjunctivitis, keratitis, keratoconjuctivitis, and corneal ulcers, which can prevent frequent drug administration and enhance patient compliance. Sodium alginate was used as ion-activated gelling agent in combination with Poloxamer 407 as temperature-triggered gelling agent for formulation of ofloxacin eye drops (0.3%w/v), which undergo gelation in simulated tear fluid and provide sustained release of drug. Ion-activated environmentally responsive ophthalmic system (0.3% (w/v) using alginate as a gelling agent in combination with HPMC E50Lv as a viscosity-enhancing agent was successfully formulated. The gel formed in vitro produced sustained drug release over an 8 hrs period. They successfully developed temperature-triggered and ion-activated environmentally responsive system for the ocular drug delivery using poloxamer-407 and sodium alginate.

Advantages

1. Decreased side effects.

2. Decreased dose to be administered.

3. Drug directly available at target site.

4. Protection of mucosa from irritating drugs.

5. Protection of mucosa from irritating drugs.

6. Drug targeting to specific site like colon.

7. Improved patient compliance.

Temperature-sensitive DDS

One of the most widely used system for delivery of drug is temperature-sensitive DDS, which can be applied in conjunction with thermotherapy for the delivery of many drugs (Kojima 2010). A lot of work has already been carried out on different types of temperature-sensitive DDSs. Various temperature-sensitive DDSs and their applications are shown in Table IV. A number of temperature-sensitive polymers are available, poly (N-isopropylacrylamide) (PNIPAM) is one of them, which takes advantage of the design of thermosensitive DDS. These polymers have a phase transition temperature at which the solubility and other properties of systems drastically change. The “cloud point” or lower critical solution temperature (LCST) of PNIPAM is 32°C. Polymers containing an ethylene glycol unit, such as poly (2-(2’-methoxyehoxy) ethyl methacrylates), are also temperature-sensitive polymers. There is a cloud point which is heavily influenced by the balance between hydrophobicity and hydrophilicity of the polymer. These thermosensitive polymers are useful for the preparation of temperature-sensitive dendrimers.

Table IV. Various temperature sensitive drug delivery systems.

Polymer	Therapeutic use	Drug/Gene	References
N-isopropylacrylamide and N, N-dimethylaminoethyl methacrylate (PNIPAm-co-PDMAEMA)	Improved gene delivery and effective gene transfection	pDNA	Hinrichs and Suhuurmans (1999)
Linear PNI-Pam-co-DMAEMA polymer	Improved efficacy of transfection		Alexander (2006)
PEI-PNIPAm polymer with 46-kDa PNIPAm grafts	More effective transfection efficacy		Alexander (2006)
Poly[2-(2-ethoxy) ethoxy ethylvinyl ether (EOEOVE)	Provide temperature-sensitive properties to stable liposomes (tumor-specific chemotherapy)	Doxorubicin	Kono et al. (2010)
Multiblock copolymers synthesized from pluronic and di-(ethylene glycol) divinyl ether	Used as delivery vehicle for controlled and sustained release of plasmid DNA	Plasmid DNA	Namgung et al. (2009)
Polyethylenimine (BPEI)/pDNA complex	Improved transfection efficacy	Plasmid DNA	Namgung et al. (2009)
Co-polymerization of PVP and acrylic acid	Provide sustained release of drug, also provide temperature-dependent release of FITC-dextran	Fluorescein isothiocyanate-dextran (FITC-dextran)	Sahoo et al. (1998)
Pluronic-g-PAA copolymers	In situ gelling vehicle for ophthalmic drug delivery, prolong the precorneal resident time, and improve ocular drug bioavailability	(Model Drug)	Ma et al. (2008)

In the latter, temperature-modulated drug permeation have been tried through liquid crystal-embedded cellulose membranes (Atyabi et al. 2007). It was found that the stimuli-sensitive membranes may act as “on–off switches” or “permeability valves”, producing patterns of pulsatile release, where the period and rate of mass transfer can be controlled by external or environmental triggers. A study was conducted to determine the temperature-modulated drug permeation using cellulose nitrate and cellulose acetate monolayer membranes containing thermotropic liquid crystals (LC) as thermoresponsive barriers for drug permeation. Methimazole and paracetamol as hydrophilic and hydrophobic drug models were used for the study, respectively. It was found that upon changing the temperature of the system around the Tn–i, both cellulose membranes without LC showed no temperature sensitivity to drug permeation, whereas the results for LC-entrapped membranes exhibited a distinct jump in permeability when temperature was raised to above the Tn–1 of the liquid crystal for both drug models. They found that thermoresponsive drug permeation through the LC membranes was reversible, reproducible, and followed zero-order kinetics.

Kono studied the role of highly temperature-sensitive liposomes based on a thermosensitive block copolymer for tumor-specific chemotherapy (Strehl et al. 2013). They found that incorporation of poly [2-(2-ethoxy) ethoxyethyl vinyl ether (EOEOVE)], which exhibits a LCST around 40°C, provides temperature-sensitive properties to stable liposomes. The highly temperature-sensitive properties of the copolymer-incorporated liposomes might contribute to establishment of tumor selectivity and effective chemotherapy. Poly(N-isopropylacrylamide) is the best-studied thermosensitive polymer with a LCST at which its property changes between hydrophilic and hydrophobic. Advantage of this approach was that the temperature region where the liposomal contents release was triggered could be controlled by adjusting LCST of thermosensitive polymers at desired temperatures.

To prolong the precorneal resident time and improve ocular bioavailability of the drug, Pluronic-g-poly(acrylic acid) copolymers were developed by Ma et al. as a temperature-responsive in situ gelling vehicle for an ophthalmic DDS (Ma et al. 2008). Their rheogram and in vitro drug release study showed that the drug release rate decreased as acrylic acid/Pluronic molar ratio and copolymer solution concentration increased. The in vivo experimental results, along with the rheological properties and in vitro drug release study, demonstrated that in situ gels containing Pluronic-g-PAA copolymer may significantly prolong the drug resident time and thus improve bioavailability. Their result showed that the Pluronic-g-PAA copolymer could be a promising tool for in situ gelling system for ophthalmic drug delivery. Due to its unique thermoreversible gelation properties, Pluronic F127 became one of the most extensively investigated temperature responsive materials.

Advantages

• The precorneal resident time of drug could be raised by use of this delivery system.

• They play a promising role as DDSs for tumor-specific chemotherapy.

• Liposomes with high stability and high temperature sensitivity have been prepared.

• Ability of controlled and sustained release of pDNA in in vivo gene delivery in muscle cells also found to be improved by the use of this delivery system.

• High cytosolic delivery could be achieved by the use of this kind of delivery system.

Redox-sensitive DDS

One of the successfully used systems is redox-sensitive DDS, which showed that the properties of linkers or cross-linkers are critically important for the sustained and controlled delivery of drug to the target site (Ganta et al. 2008). The redox-sensitive property of the DDSs can be used to formulate prodrug, where role of linkers in the delivery of drug is highly important. Carrier-linked prodrug design involves the synthesis of inactive drug derivatives that are converted to an active form at the site of action by cleavage of a “linker” molecule. Prodrugs are designed to overcome physicochemical (e.g., solubility and chemical instability) or biopharmaceutical problems (e.g., bioavailability and toxicity) associated with the drug. Recently, the large difference between the extra- and intra-cellular redox environments has been exploited for drug delivery using redox-sensitive linkers. Macromolecular prodrugs have been developed which contain disulfide-based linkers that remains stable in the extra-cellular space while undergo some changes upon cellular uptake. One of the major drawbacks of using redox-sensitive linkers is that only a limited understanding is currently available about their basic mechanism of action.

Redox-responsive polyplexes were prepared for nonviral gene delivery (Manickam et al. 2010). Polyplexes are investigated as promising delivery vectors for a variety of nucleic acid therapeutics. Polyplexes capable of responding to environmental changes or stimuli by altering their properties and their behavior showed a much significant improvement in delivery efficacy.

Redox-sensitive DDS have many potential applications for the gene delivery. Gene-delivery systems containing disulfide linkages were prepared and those were taken by endocytosis may undergo disulfide cleavage in the lysosomal compartments. The glutathione pathway which controls the intracellular redox potential was significantly involved in this stimuli mechanism. The use of these reducible polymers for gene delivery result in the efficient gene transfection in vivo while limiting the chances of toxicity. With greater understanding of physiological differences between normal and disease tissues and advances in material design, there is an opportunity to develop nanocarrier systems for target-specific drug and gene delivery that will respond to the local stimuli (Balasubramaniam et al. 2003).

Role of the redox state of the photosynthetic and respiratory electron transport chains on the regulation of psbA expression in Synechocystis PCC 6803 were studied by Alfonso et al. (Traitel et al. 2000). They showed that the changes in the redox state of the inter system electron transport chain induce modifications of psbA transcript production and psbA mRNA stability. It has been recently demonstrated that occupancy of the Q0 site in the Cyt b6/f by a plastoquinol molecule is the signal for the activation of a light-dependent kinase of light-harvesting complex II involved in the balance of excitation energy between the two photosystems. The redox state of one of the electron carriers between the PQ pool and the PSI seems to be involved. This regulation was specific since several housekeeping genes, such as rpnB and trpA, do not respond to dark/light transitions or to changes in the redox state of the photosynthetic electron transport chain.

Advantages

• These systems are helpful to deliver nucleic acid therapeutics to tumors with altered redox state.

• A significant improvement in delivery efficacy has been achieved by use of redox-sensitive DDS.

• Redox-sensitive DDS is most suitable for sustained delivery of dopamine to brain.

• These systems are capable to slow the delivery of testosterone itself in the brain.

• Redox-sensitive chemical delivery system is helpful for delivery of steroids.

• Transfection activity of redox-responsive complexes is more than that of nonreducible complexes.

Role of nanotechnology in the drug delivery with respective to stimuli-sensitive DDS

It was found that nanotechnology has shown tremendous promise in target-specific delivery of drugs and genes in the body (Ganta et al. 2008). The use of stimuli-responsive nanocarriers offers an interesting opportunity for drug and gene delivery, where the delivery system becomes an active participant, rather than passive vehicle in the optimization of therapy. Several families of molecular assemblies are employed as stimuli-responsive nanocarriers for either passive or active targeting. Liposomes, polymeric nanoparticles, block copolymer micelles, and dendrimers are colloidal molecular assemblies. Improvements in target-to-nontarget concentration ratios, increased drug residence at the target site, and improved cellular uptake and intracellular stability are some of the major reasons for greater use of nanoparticulate delivery systems. Nanoparticulate carriers can be made from a variety of organic and inorganic materials including nondegradable and biodegradable polymers, lipids (liposomes, nanoemulsions, and solid-lipid nanoparticles), self-assembling amphiphilic molecules, dendrimers, metal, and inorganic semiconductor nanocrystals (Couvrer and Vouthier 2006). The selection of material for development of nanoparticulate carriers is mainly dictated by the desired diagnostic or therapeutic goal, type of payload, material safety profile, and the route of administration. The benefit of stimuli-responsive nanocarriers are especially important when the stimuli are unique to disease pathology, allowing the nanocarriers to respond specifically to the pathological “triggers”. Selected examples of biological stimuli that could be exploited for targeted drug and gene delivery include pH, temperature, and redox microenvironment. The extracellular and intracellular pH profile of biological system is greatly affected by diseases. For instance, in solid tumors, the extracellular pH tends to be significantly more acidic (˜6.5) than the pH of the blood (7.4) at 37°C. By selecting the right material composition, it is possible to engineer nanocarriers that can exploit these pH differences and allow the delivery of encapsulated payload to the specific site in select extracellular or intracellular part. Temperature is another variable that can be exploited in specifically releasing the nanocarrier-delivered drugs or genes to a select target site (Ganta et al. 2008). For instance, using temperature-sensitive nanocarriers one could envision a delivery system that will only release the payload at temperatures above 37°C. Such a system would keep the toxic drug encapsulated in the systemic circulation or upon contact with nontargeted tissue. However, on application of hyperthermic stimuli to the disease area, the drug would be readily available in a localized region. Another possible strategy is physical targeting of drugs and genes by external stimuli (magnetic field, ultrasound, light, and heat). An interesting example is targeted delivery of iron oxide nanoparticles using magnetic field. Ultrasound has been used to achieve the targeted delivery to the tumor by local sonication after the injection of micellar encapsulated drugs. In addition to tumor uptake, this technique also allows the uniform distribution of micelles and drug throughout the tumor tissue (Kojima 2010). Maeda and his colleagues first described the environmental polymorphism registry (EPR) effect in murine solid tumor models and this phenomenon has been further used by others. When polymer–drug conjugates are administered, 10–100 fold higher concentrations could be achieved in the tumor due to EPR effect as compared to administration of free drug. Using pH-sensitive poly (beta-amino ester) (PbAE) nanoparticles, they found significant enhancement in drug delivery and accumulation in the tumor mass as compared to drug administration in PCL nanoparticles, a non-pH-sensitive polymer and in aqueous solution (Shenoy et al. 2005). Recently, the role of combination paclitaxel and the apoptotic second messenger, C6-ceramide was studied. They were administered concurrently in PEO-modified PCL nanoparticles to overcome multidrug resistance cancer. When PEG-modified thiolated gelatin nanoparticles were encapsulated with plasmid DNA encoding for soluble vascular endothelial growth factor receptor 1 (sVEGFR-1 or sFlt-1), highly efficient transgene expression was observed in human breast cancer cells and in vivo in an orthotopic tumor model. Further, direct targeting to the disease site could be achieved by coupling with specific ligand on the surface of the nanoparticles. In addition, PEG modification of nanocarriers enhances the circulation time of the delivery system in the body (Wagnor 2007). Using solid tumor as an example again, there are several strategies that can be adopted for surface modification of nanocarrier systems for effective targeted delivery to the tumor cells or to endothelial cells of the tumor blood vessels. When the surface of nanocarriers was modified with folic acid, targeted drug delivery was achieved due to the over-expressed folate receptor on the tumor cells.

Figure 2 illustrates the intracellular delivery of folate-anchored nanocarrier through endocytosis process and releasing its contents in response to internal stimuli. The phage display method has been used to identify specific peptide sequences that can be used for targeting tumors and other disease areas in the body (Sergeeva et al. 2006). One of the FDA-approved targeted therapeutics is Adalimumab^® antibody; a human anti-TNF IgG1 used against rheumatoid arthritis is generated by phage display technique. The pH profile of pathological tissues, such as upon acquisition of inflammation, infection, and cancer, is significantly different from that of the normal tissue. The pH at systemic sites of infections, primary tumors, and metastasized tumors are lower than the pH of normal tissue. For example, pH of the region drops from 7.4 under normal conditions to 6.5 after 60 hrs following onset of inflammatory reaction. This behavior can be utilized for the preparation of stimuli-responsive drug or gene-delivery systems, which could exploit the biochemical properties at the diseased site for targeted delivery. The cellular components display transmembrane pH gradient in normal as well as pathological conditions, which can also be used for intracellular delivery of macromolecules. The apparent membrane partitioning of a weak acid increases significantly as pH decreases its pKa value and decreased membrane partitioning is observed for a weak base due to the neutralization of charges. These pH-responsive compounds can be incorporated into nanocarriers or conjugated with nanocarriers as such to macromolecules to achieve efficient intracellular delivery and subcellular localization of macromolecules. The selective targeting of drugs or macromolecules in tumor tissue is of high interest in cancer therapy. The pH, surface charge, and low-density lipoprotein receptors are the factors that show notable differences among the normal and tumor tissues. The insufficient oxygen in tumor leads to hypoxia and causes production of lactic acid, and hydrolysis of ATP in an energy-deficient environment contributes to an acidic microenvironment, which has been found in many tumors. In addition, tumor cells seem to be more sensitive to heat-induced damage than normal cells. The liposomes and nanoparticles provide a method for intracellular delivery and localization of the iron oxide particles.

Figure 2. The active drug targeting with surface-modified micelles.

Advantages

• These systems lead to targeted drug delivery especially in case of cancer.

• Nanocarriers are able to avoid reticuloendothelial system (RES) uptake mediated by macrophages.

• Sustained and controlled drug delivery could be achieved by the use of nanocarriers.

• Transfection efficacy of these systems is very high.

• Sequential delivery could be achieved by the use of these systems.

Photo- and light-sensitive DDS

Photo irradiation is another stimulus that can induce cytotoxicity in affected cells (Kojima 2010). Photo-related therapy includes photodynamic therapy (PDT) and photothermal therapy (PTT). PDT is a new clinical treatment for superficial tumors and age-related muscular degeneration and was approved in the 1990s. This technique involves the systemic administration of a photosensitive drug and light irradiation to the affected tissue. This photosensitizer generates singlet oxygen after light irradiation and causes oxidative damage to cells. PDT affects only the irradiated areas because singlet oxygen is short-lived, making it a site-specific and noninvasive treatment. Kataoka et al. reported that dendritic polymer micelles composed of a photosensitizer-core dendrimer, and PEG-block polymers had an efficient PDT effect. PTT involves the systemic administration of gold nanoparticles and light irradiation of the affected tissues, similar to PDT with photosensitizers. Gold nanoparticles generate heat after light irradiation and these damages the cells. Some biocompatible gold nanoparticles have been studied for PTT. It has been reported that a PEGylated dendrimer encapsulating a gold nanoparticle generated photothermal energy, which could be used for PTT. Photo-degradable dendrimers are also useful for photosensitive DDS.

PDT is a fast-developing modality in treating a variety of malignant tumor. A new generation of photosensitizers becomes available and technological advancements in the targeted delivery of both the photodynamic drug and light to tumor lesion are warranted. PDT is based on administrating the tumor localizing photosensitizer which, upon photoactivation with visible light of appropriate wavelength, generates locally superoxide radicals or reactive singlet oxygen (type II reaction) leading to tumor cell destruction. Hypericin (Hyp), a powerful naturally occurring photosensitizer, can be extracted from plants of genus Hypericum. Hyp holds as a promising agent in PDT of cancer due to its minimal dark toxicity, low concentration necessary to trigger photocytotoxicity, high clearance rate from the host body and due to its large absorption in the visible spectrum. Several strategies using various types of delivery systems have been adopted in order to enhance specific uptake of photodynamic drugs by targeted tissues and improve PDT efficiency (Fadel 2007).

Light-sensitive intelligent DDSs of coumarin-modified mesoporous bioactive glass (MBG) were developed by Lin et al. (Lin et al. 2010). Light-sensitive inorganic substrate systems that may be helpful to achieve improved control of the loading and release of guest substances were highlighted recently (Tao et al. 2012, Sortino 2008, Johansson et al. 2008, Aznar et al. 2007). Light-sensitivity is an attractive phenomenon for developing advanced DDS capable of precise external modulation of the site. Recently, photo-responsive pore size control of mesoporous silica by azobenzene and coumarin modification has been studied (Mal et al. 2003). Coumarin groups showed better results than azobenzene for photo-responsive pore size control of biomaterials. This study reported the photo-controlled storage and release of guest molecules in coumarin-modified MBG, which can be studied through the photoresponsive reversible intermolecular dimerization of coumarin derivatives attached preferentially to the pore outlets, and the development of systems for releasing guest molecules from MBG. The photo-controlled release is reversible, and the entrapped guest molecules can be delivered in installments according to UV wavelength. Coumarin-modified MBG was used as a drug delivery carrier to investigate drug storage/release characteristics using phenanthrene as a model drug. Irradiation with UV light (> 310 nm) induced photo-dimerization of the coumarin-modified MBG, which led to the pores’ closing with cyclobutane dimers and trapping of the guest phenanthrene in the mesopores. However, irradiating the dimerized-coumarin-modified MBG with shorter wavelength UV light (̴250 nm) regenerates the coumarin monomer derivative by the photo-cleavage of cyclobutane dimers, such that trapped guest molecules are released from the mesopores. The system demonstrates great potential in light-sensitive intelligent DDSs and disease therapy fields.

Advantages

• The system showed greater potential in the delivery of light-sensitive drugs, thus also known as intelligent DDS.

• The photo-controlled release is reversible and the entrapped guest molecules can be delivered in installments according to UV wavelength.

• PDT involves the systemic administration of a photosensitive drug and light irradiation to the affected tissue.

• PTT involves the systemic administration of a gold nanoparticles and light irradiation to affected tissue.

• PDT is an improved technique for treatment for superficial tumors and age-related muscular degeneration.

Thermo-responsive DDS

Thermosensitive DDSs play a tremendous role in the delivery of drugs. In the latter, large number of work were carried out related to thermosensitive DDS.

Thermo-responsive polymer membranes were prepared by Nozawa et al. The purpose of their study was to develop thermo-responsive membranes by using LC. The membranes containing LC have following advantages:

• (a) These membranes are electrically neutral and therefore compatible with ionized drugs;

• (b) thermo-response efficacy is so sharp that the membrane can control the drug release in response to minute temperature change.

In order to develop such thermosensitive system, two kinds of LC-entrapped membranes, (a) polymer-alloyed membrane and (b) LC-adsorption membrane, were prepared. They found that the drug permeation rate was sharply on–off controlled in response to minute temperature change by using the LC-adsorbed membrane. This thermoresponsive system was applicable to DDS, which responds to the change of body temperature and/or the external thermal stimuli (Nozawa et al. 1991).

With the help of thermosensitive DDSs, many hurdles of conventional DDSs were overcame such as efficacy-related problem, targeting- and drug toxicities-related issues. These systems can be broadly classified into either closed-loop or open-loop delivery systems, depending on the nature of drug release from the system. In closed-loop delivery systems, the drug is released in response to biochemical changes in the local environment, that is, the presence or absence of specific molecules. In contrast, open-loop delivery systems involve the release of the drug in response to an external stimuli such as ultrasound, electric and magnetic fields, light, mechanical forces, and temperature. These open-loop delivery systems are commonly referred to as “pulsatile” or “externally regulated” systems and they are by far the most frequently explored systems for controlled drug release (Bikram and West 2008).

Advantages

• The thermoresponsive system is applicable to DDS which responds to the change of body temperature and/or the external thermal stimuli.

• Thermoresponsive hydrogels utilize temperature change as the trigger for determining the gelling behavior without any additional external factors.

• Temperature-sensitive systems found to be the very promising stimuli-sensitive systems that have practical applications for sequential delivery of drug.

• The thermo- and pH-responsive micelles have great potential in the cellular delivery of drug.

Hydrogels-based DDS

Stimuli-sensitive hydrogels gained a considerable attention as an intelligent system in the recent years for their biochemical and biomedical fields, since they can sense environmental changes and induce structural changes by themselves.

Hydrogels DDSs are one of the upcoming classes of polymer-based controlled release DDSs. Besides exhibiting swelling-controlled drug release, hydrogels also show stimuli-responsive changes in their structural network and hence the drug release. Hydrogels have many applications like controlled drug release, stimuli-responsive delivery, especially pH-responsive drug release. Polymers from natural, synthetic, or semi-synthetic sources can be used for synthesizing hydrogels (Gupta et al. 2002).

Biomolecule-sensitive hydrogels also play important role in drug delivery. Hydrogels that exhibit both liquid-like and solid-like behavior have a variety of functional properties, such as swelling, mechanical, permeation, surface, and optical properties. Such properties have provided many potential applications of hydrogels in fields such as medicine, agriculture, biotechnology, etc. In addition, hydrogels have the unique property of undergoing abrupt volume changes from their collapsed and swollen states in response to environmental changes (Miyata et al. 2002).

Hydrogels are well known as wound-dressing material. They have the ability to absorb and retain the wound exudates along with the foreign bodies such as bacteria within its network structure. Hydrogels are able to protect the wound from external impurities, which leads to rapid wound healing and maintains the microclimate for biosynthetic reactions which is necessary for normal cellular activities (Lopez and Bodmeier 1997). Hydrogel sheets are applied over the wound surface with polymeric film and are attached at the wound surface by the use of adhesives or bandages (Ueno et al. 2001). In case of third-degree burn wounds, dextran-based hydrogels are useful as they promote neovascularization and skin regeneration (Sun et al. 2011).

Recombinant silk-elastin like polymer (SELP) hydrogels for the delivery of pRL-CMV for the treatment of human breast cancers were prepared (Mageed et al. 2004). Results of the study showed an increase in the transfection efficiency when SELP hydrogels were used. Inclusion of basic fibroblasts growth factor, insulin growth factor II (IGF-II) and collagen within the microcapsules showed proliferation and differentiation of encapsulated C2C12 myoblasts, thus reduction in the tumor to some extent (Li et al. 2006).

Hydrogels are also useful for transdermal drug delivery as they can be delivered for a long duration at a constant rate and can be easily interrupted on demand by simply removing the devices. They have higher water content, therefore, swollen hydrogels can provide a better environment for the skin in comparison to conventional ointments and patches (Chen et al. 2011).

Advantages

• These systems give a controlled drug release.

• They exhibit swelling-controlled drug release.

• These systems have application in wound healing.

• These systems have applications in tissue engineering.

• These systems are very useful for ocular drug delivery.

Glucose-sensitive DDS

Glucose-sensitive hydrogels also found to be very useful tool for the development of self-regulated insulin delivery systems and enable us to construct an artificial pancreas that can administer the necessary delivery amount of insulin in response to the blood glucose concentration (Miyata et al. 2002).

Three types of glucose-sensitive hydrogels have been used.

• a) Glucose oxidase-loaded hydrogels.

• b) Lectin-loaded hydrogels.

• c) Hydrogels with phenylboronic acid moieties

Glucose oxidase-loaded hydrogels

Combining glucose oxidase with pH-sensitive hydrogels to sense glucose and regulate insulin release is the method that many researchers used to develop glucose-sensitive insulin delivery systems. Within the pH-sensitive hydrogels containing glucose oxidase, glucose is converted to gluconic acid by glucose oxidase, thus lowering the pH in the hydrogels. Insulin can be released by the pH-sensitive swelling of the hydrogels. The poly (MAAc-g-EG) hydrogels containing glucose oxidase showed a glucose-sensitivity resulting from the combination of the catalytic reaction of glucose oxidase, and the pH-sensitivity leads to complex formation between carboxyl and etheric groups. Consequently, glucose-sensitive insulin release can be achieved by using pH-sensitive hydrogels containing insulin and glucose oxidase.

Lectin-loaded hydrogels

The unique carbohydrate-binding properties of lectins are very useful for the fabrication of glucose-sensitive systems. Therefore, some researchers have focused on the glucose-binding properties of Con A, a lectin possessing four binding sites. Development of glucose-sensitive insulin release systems using Con A found to be very helpful to synthesize a stable, biologically active, glycosylated insulin derivative which is able to form a complex with Con A and group (Seminoff et al. 1989, Kim et al. 1990). Further a new type of hydrogel capable of sol-gel phase-reversible transitions has been prepared which is based upon changes in the glucose concentration (Obaidat and Park 1996).

Hydrogels with phenylboronic acid moieties

It involves the preparation of glucose-sensitive hydrogels without biological components such as proteins, instead of complex formation between a phenylboronic acid group and glucose. The complex formation between phenylboronic acid and a polyol compound has many potential applications as a glucose-sensitive material. These results suggest that a glucose-sensitive insulin release system can be constructed by exploiting the complex formation properties of phenylboronic acid groups and glucose, without the use of biological components, such as proteins (Miyata et al. 2002).

Glucose-responsive microgels with a core-shell structure with a thermoresponsive core and a glucose-responsive shell, made of poly(N-isopropylacrylamide) (pNIPAM) and pNIPAM-coacrylamidophenylboronic acid (pNIPAM-co-APBA), respectively were prepared. The extent of core swelling was regulated by two processes: its own internal stimulus, i.e. temperature, and shell compression, which is proportional to glucose concentration even at physiological salinity. This work was used later on as a basic concept for the development of a temperature-responsive hydrogel where hydrophilic core combined with a glucose-responsive shell (Lapeyre et al. 2008).

Glucose-sensitive actuator for DDS including microvalve were prepared. A glucose-sensitive actuator for DDS consists of silicon boss, microchannel, and outlet of the microvalve. Two types of microchannels were fabricated using anisotropic etching of Deep-RIE and wet chemical etching using KOH solution and these were rectangular and trapezoidal microchannel. However, the process steps need to be optimized in order to obtain a smoother microchannel. Glucose-Micro valve have Bio MEMS application in DDS which allows better control of drug release through human bodies. Diabetic patients usually need long-term diet management along with daily insulin injection to control their glucose concentration. With this microvalve, the insulin can be released cordially to human body automatically based on the glucose concentration (Dzulkefli 2008).

Preparation of glucose-sensitive hydrogels varied accordingly to the types of different polymers, cross-linking agent and the presence of cationic pH-sensitive polymers containing immobilized insulin and glucose oxidase which can swell in response to blood glucose level releasing the entrapped insulin in a pulsatile fashion. Luo et al. studied the effect of environmental and pH on the mechanical characteristics of glucose-sensitive hydrogels and found that these changes greatly effect the insulin delivery from the glucose-sensitive hydrogels (Luo et al. 2009).

Advantages

• They found to be very useful for the development of self-regulated insulin delivery systems.

• A glucose-sensitive insulin release can be achieved using pH-sensitive hydrogels.

• Drug release can be modulated from the hydrogel-based delivery system according to the requirement like pulsatile release and sustained release.

Ion-activated stimuli

In this method, gelling of the solution is triggered by the ionic strength of the eye. Polymers may undergo phase transition in presence of various mono and divalent ions cationic ions (Ca²⁺, Mg²⁺, K⁺, and Na⁺). Some of the polysaccharides fall into the class of ion-sensitive ones. Various ion-sensitive DDS are given in Table V. For example, Gelrite is a polysaccharide, low acetyl gellan gum, and Alginate (Kelton), which forms a clear gel in the presence of mono or divalent cations. The concentration of sodium in human tears is 2.6 g/l is particularly suitable to cause gelation of the material when topically installed into the conjunctival sac.

Table V. Various ion-sensitive drug delivery systems.

Objective	Polymer	Drug	Therapeutic used	References
Formulation and evaluation of ion activated ocular gels of ketorolac Tromethamine	Gelrite	Ketorolac tromethamine	Improve the precorneal residence time and sustain the drug release	Vodithala et al. (2010)
Study of an alginate/HPMC-based in situ gelling ophthalmic delivery system for gatifloxacin	Alginate/HPmC	Gatifloxacin	Produced sustained drug release over an 8-h period.	Liu et al. (2006)
Sustained ophthalmic delivery of ofloxacin From an ion-activated in situ gelling system	Sod Alginate/HPC	Ofloxacin	Sustained the drug release over a period of 8 hours.	Abraham et al. (2009)
Ion-activated in situ gelling systems for sustained ophthalmic delivery of ciprofloxacin hydrochloride	Gelrite gellan gum + sod alginate	Ciprofloxacin HCL	Sustained the drug release over a period of 8 hours.	Balasubramaniam et al. (2003)
Enhancement of miotic potential of pilocarpine by tamarind gum based in situ gelling ocular dosage form	Tamarind	Pilocarpine	Enhancement of miotic potential of pilocarpine	Mishra and Gilhotra (2008)
Sustained ophthalmic delivery of Levofloxacin Hemihydrate from an Ion-activated in situ gelling system	Gelrite	Levofloxacin Hemihydrate	Sustained the drug release over a period of 8 hours.	Kavitha and Rajas (2011)

Ion-activated in situ gelling systems for sustained ophthalmic delivery of ciprofloxacin hydrochloride (HCl) were prepared later on and concluded that these formulated systems provide sustained release of the drug over an 8-h period in vitro (Balasubramaniam et al. 2003). Ion-activated, gelrite-based in situ ophthalmic gels of pefloxacin mesylate were also prepared and compared with conventional eye drops and concluded that these systems were capable for effective and controlled management of conjunctivitis for 12 h (Sultana et al. 2006). In situ gelling gelrite/alginate formulations as vehicles for ophthalmic drug delivery were prepared and found that the optimum concentration of gelrite solution for the in situ gel forming delivery systems was 0.3% (w/w) and that for alginate solution was 1.4% (w/w). The mixture of 0.2% Gelrite and 0.6% alginate solutions showed a significant enhancement in gel strength at physiological condition (Liu et al. 2010). Ion-activated ocular in situ gels of ketorolac tromethamine using gelrite as a polymer were prepared and found that these developed formulations showed sustained release of drug up to 6 hrs. The formulations were found to be nonirritating with no ocular damage (Vodithala et al. 2010).

Advantages

• This system was capable for effective and controlled management of conjunctivitis.

• Gelling of solution instilled is triggered by the ionic strength of the eye.

• Sustained ophthalmic delivery of Levofloxacin Hemihydrate from an ion-activated in situ gelling system is obtained.

Conclusion

Thus, it was found that the stimuli-sensitive DDS plays an important role in the delivery of drugs to various parts of the body. Various stimuli-sensitive DDS have been studied like pH-sensitive DDS, redox-sensitive DDS, temperature-sensitive DDS, thermosensitive DDS, etc. Different types of functional polymers and their properties have been studied for series of drugs. As a result, new and interesting controlled and sustained delivery strategies have become available. The fascinating properties of the stimuli-sensitive polymers seem promising in many future applications and offer possible use as the next generation materials in biological, biomedical, and pharmaceutical products. However, more detailed study in this field is still required. A detail study in the area may be very beneficial in the future in order to provide much better delivery systems and long-lasting action of drug with least or no side effects.

Acknowledgment

The authors are grateful to the ISF College of Pharmacy for providing all kinds of support and motivation to carry out this work.

Funding

This review received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Declaration of interest

The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.