Docetaxel‐loaded solid lipid nanoparticles: a novel drug delivery system

Abstract

Over the past few years, taxanes have emerged as a new class of anticancer drugs. Docetaxel (DTX) the prototype of this class has been approved for the treatment of broad range of cancers. However, to date the commercial preparation of DTX (Taxotere^®) is accompanying adverse side effects, intolerance, and poor solubility, which can be overcome by encapsulating them using solid lipid nanoparticles (SLNs). SLNs represent versatile delivery system of drugs with newer forms such as polymer–solid lipid hybrid, surface modified and long circulating nanoparticles bringing forth improved prospects for cancer chemotherapy. In this review, the authors have discussed the current uses of various SLNs formulations of DTX with key emphasis on controlled and site‐specific drug delivery along with enhanced antitumour activity elucidated via in vitro and in vivo studies. Furthermore, the review article highlights few approaches that can be used in combination with existing DTX‐loaded SLNs to supplement DTX drug delivery.

1 Introduction

Docetaxel (DTX) is a semi‐synthetic antineoplastic agent from the taxoid family which was discovered in 1980s by Pierre Potier from the needles of the western yew tree, Taxus baccata [1]. DTX is a structural analogue of paclitaxel with slight difference in phenylpropionate and functional group on carbon 10 [2]. DTX is an effective anti‐mitotic chemotherapeutic agent for the therapy of a broad spectrum of human cancers [3, 4, 5, 6, 7]. The antitumor activity of DTX is attributed to its high affinity for the binding site of β ‐tubulin protein that initiates tubulin assembly into microtubules. As a result, depolymerisation of microtubules is inhibited at the G2/M phase ultimately resulting in cell cycle arrest and apoptosis [8]. DTX also exhibits anti‐angiogenic effect [9], and is considered more potent anti‐microtubule agent than doxorubicin, paclitaxel, and fluorouracil [10]. Despite these advantages, there are few limitations for DTX including poor water solubility, high systematic toxicity and non‐specific distribution [11, 12, 13]. To address these problems, researchers have focused to develop nanocarriers to deliver DTX. These strategies provide a promising way to enhance water solubility and target the delivery of anticancer drugs to the tumour site, ultimately reducing adverse effects. Several nanocarriers of DTX have been developed based on polymers, dendrimers, inorganic nanoparticles, solid lipid nanoparticles (SLNs) and liposomes [14, 15, 16, 17, 18, 19, 20, 21, 22]. Herein, we discuss recent formulations of SLNs for DTX delivery, and implications of such studies in future formulations of SLNs.

2 DTX marketed formulation: current pitfalls

Taxotere^® is the first commercial product of DTX marketed by Aventis and it was initially approved in 1996 for metastatic breast cancer treatment. Taxotere^® is now approved for therapy of different cancers including prostate cancer, non‐small‐cell lung cancer, head and neck cancer, ovarian cancer, and advanced gastric cancer. Taxotere^® contains 40 mg/ml DTX along with 1040 mg/ml polysorbate 80 (Tween 80) and need prior dilution with ethanol, which is used to dissolve DTX before intravenous administration [23, 24, 25]. After patent expiration of Taxotere^®, various generics products entered the market such as Docefrez and DTX injection. These products contain considerable quantity of alcohol, which may possibly cause alcohol intoxication during and after treatment. Therefore, the FDA approved a new non‐alcoholic formulation (injection) of DTX, free from the risk of alcohol intoxication [26]. Unfortunately, the use of Tween 80 and ethanol solvents in the available formulation leads to hypersensitivity reactions and decreased accumulation in tumour tissues [24, 27]. Thus, non‐selectivity for target tissues, cytotoxicity and side effects of Taxotere^® has raised concern over its use [2, 24, 28]. In addition, during storage the precipitation risk of infusion is the major reason for short shelf life of premix solutions. This risk is potentiated by various reasons such as temperature, inadequate mixing and excessive agitation of the premix solution [29]. To overcome the challenges such as uncontrolled drug release, non‐specific distribution of drug with high toxicity and instability of Taxotere^® during storage, much attention has been diverted to the development of a novel delivery vehicle for DTX. A poly[ethylene glycol] (PEG)ylated DTX derivative (NKTR‐105) is in developmental stage by Nektar Therapeutics and has entered dose‐escalation phase I clinical study in solid tumours patients [30]. Recently, a DTX‐encapsulated polymeric nanoparticles (NP) (DTX‐PNP; Samyang Pharmaceuticals, Seoul, Korea) is undergoing a phase I trial for the treatment of solid malignancies [31]. Another prostate‐specific membrane antigen (PSMA)‐targeted PNP of DTX, named BIND‐014 (BIND Therapeutics, Cambridge, MA, USA) has entered phase I trials in 2011 for the treatment of solid tumour. During phase I clinical study, BIND‐014 presents promising outcomes in patients with metastatic tumours [32]. Currently, liposomal formulation of DTX (LE‐DT) developed by NeoPharm, Inc., (Lake Forrest, IL, US) is undergoing a phase II clinical trial [2]. Moreover, due to several demerits such as (i) rapid leakage of hydrophilic drugs, (ii) low encapsulation efficiency (EE), and (iii) poor storage stability, the use of liposomes is limited [33]. SLNs drug carriers in comparison with other nanocarriers are emerging as promising agents for drug delivery.

3 SLNs: a new era in drug delivery

SLNs also known as solid lipid nanospheres or lipospheres are a comparatively novel class of colloidal carriers. SLNs were introduced in 1991, as an alternate and improved drug delivery vehicle to conventional colloidal vehicles such as emulsions, polymeric nanoparticles, and liposomes [34]. In comparison to other colloidal carriers, SLNs possess various advantages such as (i) prevent degradation of encapsulated drug, (ii) increased physical stability, and (iii) maintain controlled release of drug; additionally, they can be easily prepared in absence of organic solvents and sterilised on a huge scale [35, 36]. SLNs enhance therapeutic effectiveness and reduce toxicity of anticancer agents. This reduced toxicity is attributed to the fact that the lipid core of SLNs is composed of physiological biodegradable lipids which declines the risk of chronic and acute toxicity [37]. The sizes of SLNs are distributed in the submicron ranging from 50 to 1000 nm [38]. Submicron‐sized designed particulate matters have an enhanced tendency to concentrate preferentially in tumour tissue due to the enhanced permeability and retention (EPR) effect. EPR effect can emerge advantageous in developing better nanoparticulate system such as SLN to increase antitumour activity of DTX [39, 40].

SLNs contain a solid core of a high melting point lipid encapsulated by layer of safer surfactant [41]. The lipids used in SLNs include: (i) partial glycerides: glyceryl palmitostearate, glyceryl monostearate, and glyceryl behenate, (ii) saturated monoacid triglycerides: trilaurin (TL), trimyristin (TM), tristearin (TS), and tripalmitin (TP), (iii) fatty acids: behenic acid, palmitic acid, stearic acid, and decanoic acid, (iv) waxes: cetyl palmitate, and (v) steroids: cholesterol. The lipids used in SLNs remain solid at room and body temperatures [42]. Various surfactants and their combination have been used to provide stability to the lipid NPs in dispersions by covering their surface. Most commonly used category is non‐ionic surfactants such as Pluronic, Poloxamer 188, Poloxamer 407, Spans and Tween. Phospholipids and phosphatidylcholines (PCs) are commonly employed in SLNs preparation as amphoteric surfactants [43]. Depending on the location of the encapsulated drug molecule, SLNs have three different models of morphologies (Fig. 1). (i) Homogenous matrix model: in this case a solid solution of active ingredient and lipid can be obtained when the particles are prepared by the cold homogenisation process without using drug‐solubilising surfactant. There are pronounced interactions between active ingredient and lipid [44], (ii) drug‐enriched shell model: this model is attained when NPs are prepared by hot homogenisation process. Formation of inner solid lipid core occurs at recrystallisation temperature of lipid. On decreasing the temperature of the obtained dispersion, the active molecules steadily concentrate in the remaining melt lipid of outer shell [45]. (iii) Drug‐enriched core model: core model is attained when the concentration of active drug is high in the melted lipid and leads to supersaturation of the drug. On cooling the nanoemulsion, the active ingredient precipitates prior to lipid recrystallisation. Additional cooling of the dispersion finally leads to the recrystallisation of lipid encapsulating the drug as a layer [46].

Fig. 1.

Structure of various SLN models

(A) Solid solution (homogenous matrix) model, (B) Drug‐enriched shell model, (C) Drug‐enriched core model

The stability of SLNs at different pH is important for both the production and the application of such carrier system. It has been observed that the Para‐acyl‐calix‐arene‐based SLNs are stable at the pH range from 2 to 8 [47]. As observed previously, some SLNs showed aggregation in the gastric environment at low pH. SLNs formulation needs to be stable to acquire high bioavailability. Eike Zimmermann et al. showed that it is possible to develop gastrointestinal tract (GIT) stable SLNs by an optimising stabiliser composition for each lipid. For instance, Cutina CP‐based SLNs exhibited better GIT stability with the sugar ester S1670 stabiliser; however, the stabilisation with the emulsifier mixture Span 85/Tween 80 was less effective. Moreover, Pluronic F68 stabilised SLN, consisting of the lipid compritol ATO 888 but not the lipid Imwitor 900‐SLN sufficiently [48]. Addition of acid to the optimised stearic acid‐based SLNs increased the polydispersity index (PDI) value, whereas addition of base lowered the PDI values in comparison with the control formulation. Moreover, addition of acid decreased the magnitude of the zeta potential of the formulation, which relates with the higher PDI values. The lower zeta potential of about −60 mV produces a physically stable formulation [49].

SLNs are appropriate for parenteral application because they are well tolerated and they have longer stability after sterilisation and/or lyophilisation [50, 51]. Advantages and disadvantages of SLNs in comparison with liposomes and polymeric nanoparticle formulations are enlisted in Table 1. In general, intravenous administration of SLNs decreases the expected side effects of encapsulated drug with improved bioavailability. These remarkable benefits of SLNs make them very attractive drug delivery systems even for pharmaceutical industries. Cationic SLNs, namely TransoPlex^® was developed by PharmaSol drug delivery systems (Germany) for gene transfer [56]. AlphaRx (USA) is producing SLNs of gentamicin and vancomycin with trade names Zysolin™ and Vansolin™. Various pharmaceutical companies are involved in the development of SLNs for oral drug delivery. A cyclosporine SLNs formulation was developed by Pharmatec (Italy) for oral administration [57]. SLNs powders or granulates can be compressed into tablets, incorporated into pellets or filled into capsules [58]. A recent study reported that the surface modification of SLN with PEG‐stearate leads to better stability and resistance to lipolytic enzymes [59]. Rifampicin loaded SLN (RifamsolinTM) is also under developing by AlphaRx and has entered preclinical phase. Rifampicin is generally used for the treatment of tuberculosis, which requires long treatment duration due to limited cellular penetration of antibiotics. AlphaRx aims to develop SLNs that can deliver rifampicin inside the cell, to enhance efficacy and patient compliance [60]. Another research group encapsulated hydrophobic drugs (estradiol hemihydrates, pilocarpine and hydrocortisone) into SLNs. They observed high drug loading and extended drug release in all the formulations [61]. The nebulisation of SLNs carrying anticancer drugs for treatment of lung cancer is a novel and upcoming research area. Biodistribution of inhaled radio‐labelled SLNs has been evaluated and the results showed significantly increased uptake of the radio‐labelled SLNs in the respiratory system after inhalation [62].

Table 1.

Advantages and disadvantages of SLNs over liposomes and polymeric nanoparticle formulations

Advantages of SLNs	Disadvantages of SLNs
SLNs are more biocompatible than polymeric nanocarriers due to avoidance of organic solvents and use of physiological lipids in their preparation which decreases the risk of systemic toxicity [52]	unpredictable particle growth and gelation tendency [46]
solid matrix of SLN prevents drug leakage as commonly observed in liposomes [53]	unexpected drug expulsion after lipid transition during storage [46]
SLNs have possibility of encapsulating both hydrophilic and lipophilic drugs and enhanced the EE of drug (compared with other nanoparticle formulations, e.g. polymeric nanoparticles) [54]	unexpected crystallisation of drugs [46]
SLNs have longer physical stability than liposomes. SLNs formulation remain stable for even the years [53]	high‐pressure during production induced degradation of drug [46]
the core of SLNs is composed of a solid lipid instead of an aqueous solution which provides better protection to drug cargo against chemical degradation than liposomes [55] and enhancing the bioavailability of incorporated bioactive compounds [52]	the limited capacity to load hydrophilic drugs owing to partitioning effects during the fabrication process [46]
avoid clearance by the RES and thus bypass spleen and liver filtration [46]
temporally and spatially controlled release of the encapsulated drug can be achieved [53]
easy to scale‐up and less expensive than polymeric carriers. SLNs can be commercially sterilised and lyophilised [46]

4 SLN for DTX delivery: advances and challenges

In terms of pharmacokinetics, drug stability, anticancer activity, and biodistribution, SLN formulations of cytotoxic drugs have offered better anticancer response than the corresponding free drug solutions. Several studies about DTX‐loaded SLN formulations have been published with notable outcomes which are explained in the succeeding sections. Table 2 enlists the comparison of different SLNs formulations for DTX delivery.

Table 2.

Comparison of different SLNs formulation for DTX delivery

Solid matrix	Surfactant	Preparation technique	Targeted ligand	Focus of studies	Advantages	Reference
TM	DOPE, ePC, lactobionic acid	homogenisation method at elevated temperature (65°C)	galactose moiety targeting for hepatoma	synthesis, characterisation, in vivo studies on nude mice bearing hepatoma and in vitro studies on hepatoma cell line BEL7402	sustained release, specific targeting, and enhanced antitumour activity	[63]
glycerol tristearate	soybean lecithin, Poloxamer 188	emulsification and solvent evaporation	NA	synthesis, characterisation, PK/biodistribution studies in rats and antitumour activity studies in mice bearing ovarian tumour	elongated t 1/2 and MRT, enhanced antitumour efficacy	[64]
TM	soybean lecithin	high‐pressure homogenisation method	NA	synthesis, characterisation and systemic toxicity studies in rabbits, guinea pigs, mice, and beagle dogs	less acute and long‐term toxicity, reduced side effects associated with Tween 80	[65]
glyceryl monostearate	soy lecithin, Tween 80	emulsification and solvent evaporation	folic acid targeting for brain	synthesis, characterisation, in vitro studies in brain endothelial cell lines, and PK and Kin studies in rats	folate conjugation resulted in increased permeation to the brain	[66]
monostearin	soy lecithin Tween 80	emulsification and solvent evaporation	conjugate of stearylamine and betreliesoxybutyric acid (SA–HBA) targeting for brain	synthesis, characterisation, in vitro in brain endothelial cell lines and PK and Kin studies in rats	SA–HBA conjugate resulted in increased brain uptake of DTX	[67]
compritol, precirol	Poloxamer 188, H‐SPC	probe sonication and microemulsion	NA	synthesis, characterisation, in vitro studies in A‐375 and C‐26 cell lines and in vivo studies in C‐26‐implanted mice	higher cellular uptake and improved tumour regression compared with TXT	[7]
TM	soybean lecithin	high‐pressure homogenisation	NA	synthesis, characterisation, in vitro studies in MCF‐7 cell lines and in vivo studies in nude mice bearing MCF‐7 cells	enhanced maximum tolerated dose (MTD) and reduce toxicity especially myelosuppression	[68]
high melting point triglycerides (TM, TL, TS, and TP)	Poloxamer 188, DOPE, and ePC	modified emulsion/solvent evaporation method	NA	synthesis, characterisation, in vitro studies in murine cancer cells and in vivo studies in mouse bearing murine cancer cells	ensure formulation stability and improved antitumour activity with decreased systemic toxicity	[69]
glycerol tristearate	Tween 80	solvent‐diffusion method in an aqueous system	NA	synthesis, characterisation, in vitro drug release studies and in vivo PK and toxicity studies in rats	enhanced intestinal absorption, lymphatic uptake, and oral bioavailability	[70]
compritol, glyceryl monostearate	Poloxamer 188	hot melt‐emulsification method and optimised by face centred‐central composite design	folic acid for tumour targeting	synthesis, characterisation, in vitro studies MDA‐MB‐231 and MCF‐7 cell lines and in vivo studies in rats	folic acid incorporation resulted in increased cellular uptake and cytotoxicity	[71]

4.1 Enhanced stability and controlled release of DTX by SLNs encapsulation

SLNs have enhanced stability and make upgradability to production scale easier in comparison with liposomes. These characteristics of SLNs may be beneficial for various modes of targeting. SLNs develop the origin of drug delivery system, which has storage capability of at least 1 year [55]. Various drug‐loaded SLNs are prepared from different solid lipid and emulsifier by using different methods as shown in Fig. 2. In the pharmaceutical industry, SLNs have been used for enhancing the bioavailability and controlled release of encapsulated active substance by altering the dissolution rate in injectable formulations [72]. DTX‐loaded SLNs composed of precirol, compritol, and hydrogenated soy PCs (H‐SPC), with the size of 180 nm, were prepared and characterised by Mosallaei and colleagues. During stability test the SLNs showed a better stability for about 2 months; however, these SLNs presented a considerable size reduction in the first week of stability test. This can be justified by the observation that Poloxamer has achieved sufficient time to be fixed in the SLNs, and more surfaces layer of SLNs were stabilised and particle sizes were reduced during the first seven days of storage [7]. Similar phenomenon was observed by Heiati et al. [73] during storage of SLNs. Sustained release of DTX from DTX‐loaded SLNs was attributed to four main reasons: (i) encapsulation of amorphous DTX within core of SLNs, (ii) decrease crystallisation rate of precirol–compritol at the stated ratio, (iii) entrapment of a hydrophobic drug inside a hydrophobic basis, and (iv) longest diffusion pathway induced by spherical shape of particles [7]. Precirol can generate a lipid matrix structure with imperfections as well as with controlled release. This feature of formulation can be enhanced when precirol is used in combination with compritol [74]. Naguib and colleagues focused on the use of four different triglycerides such as TL, TM, TS, and TP as the core constituent of the DTX‐loaded SLNs to inhibit droplet coalescence and to achieve stable formulation. During stability tests of these SLNs for short period, no considerable change was observed in DTX content and particle size of any of the four types of SLNs. The TL‐based and TS‐based SLNs exhibited comparatively faster drug release. A considerable change was observed in zeta potential of TL‐based and TP‐based SLNs, which is used as indicator of nanoparticle instability, whereas trimyristin‐based DTX‐loaded SLNs were comparatively more stable and were observed to release the drug at slowest rate. Thus, TM‐based SLNs were preferred for further formulation [69]. SLNs composed of galactosylated‐dioleoylphosphatidyl ethanolamine (DOPE) and PC (ePC) were developed and characterised by Xu et al. for specific targeted delivery for DTX to hepatoma cells. In vitro drug release profile revealed that these SLNs released about 18.2% of DTX at the end of first day with low burst effect, which suggested that DTX remain encapsulated in the SLNs and the nanoparticles were taken up into the hepatoma cells instead of free drug. Following the first day, DTX release profiles revealed a sustained release of 83.4% of DTX released at the end of 30 days [63]. Pawar and his colleagues developed folic acid functionalised SLNs for co‐delivery of CRM and DTX (F‐DC‐SLNs) to increase therapeutic efficacy of DTX in metastatic breast cancer cells. In vitro study of the drug release revealed that F‐DC‐SLNs showed a sustained release of 67.94% of DTX and 89.27% of CRM in 48 h, whereas 80% of drug was released from Taxotere^® at the end of 20 h (in a medium to SLN formulation except 50% v/v ethanol is replaced by 3% w/v Tween 80) [71]. Venishetty et al. [75] developed DTX and ketoconazole‐loaded SLNs (DK‐SLNs) and modified the surface of these SLNs with folate conjugation for brain targeting. Drug release profile of DK‐SLNs revealed that release of ketoconazole was faster as compared with DTX. The folate‐grafted DK‐SLNs (FDK‐SLNs) exhibited controlled release of both DKs as compared with DK‐SLNs. In comparison to FDK‐SLNs, the faster release of DTX from DTX‐loaded SLNs was attributed to the higher surface area and smaller size of DTX‐loaded SLNs [75].

Fig. 2.

Schematic representation of various drug‐loaded SLNs nanoparticle strategies

4.2 Improved pharmacokinetics and biodistribution of DTX‐loaded SLN

In ideal situation, the antitumour drug can trigger its anticancer activity only when it reaches the tumour site, for this reason, the biodistribution and pharmacokinetics of the drug must be considered from clinical perspective. Venishetty et al. created a novel formulation by conjugating betreliesoxybutyric acid (HBA) with DTX‐loaded SLNs. These SLNs showed about five times increased area under the curve (AUC) in addition to five times higher half‐life of drug as compared with Taxotere^® [67]. In accordance with the previous study, DTX‐encapsulated SLN demonstrated increased plasma concentrations and AUC as compared with Taxotere^®. The co‐delivery of DK in SLNs further increased plasma AUC as compared with DTX‐encapsulated SLN. F‐SLNs showed six times higher concentrations of DTX in the brain in comparison with available formulations of DTX [66]. Moreover, non‐PEGylated DTX‐loaded SLNs were developed and pharmacokinetic behaviour was elucidated in murine ovarian cancer model. The half‐life of these SLNs was considerably prolonged due to the redistribution of DTX from reticulo‐endothelial system (RES) to systemic circulation. These results propose that RES not only eliminates foreign particles but may also be a reservoir of drug [64]. Furthermore, PEGylated DTX‐loaded SLNs when intravenously injected in mice exhibited five times higher concentration of DTX in the plasma than in those mice that were intravenously injected with the DTX in T80/ethanol formulation [69].

In comparison to Taxotere^®, PEGylated DTX‐loaded SLNs significantly increased the amount of DTX in tumour tissues with decreased level of DTX in other major organs such as heart, kidneys, liver, lungs, and spleen. However, the effect of PEGylation on RES clearance was not elucidated in this paper. Interestingly, mononuclear phagocyte system mostly named as RES clearance is considered as main obstacle to the successful delivery of drug by polymeric nanoparticles [76, 77]. To overcome RES clearance, drug delivery particles are coated with stable, biocompatible, and hydrophilic polymers such as PEG, poloxamers, or poloxamines [77]. These types of polymer‐coated drug delivery systems are named as ‘stealth’, because of their ability to delay or avoid RES clearance and as a result it prolongs drug circulation time [78, 79]. This phenomenon was supported by a pharmacokinetic study conducted on F‐DC‐SLNs. F‐DC‐SLNs showed increased AUC in comparison with DTX‐loaded SLNs and Taxotere^® due to slow release of DTX from the SLNs matrix and evasion of SLN by RES uptake because of the presence of PEG on its surface [71]. It should be noted that there are not sufficient studies which compared pharmacokinetics of stealth versus non‐stealth DTX‐loaded SLN. Therefore, it is difficult to conclude without any further studies, whether RES clearance as key factor in polymeric drug delivery system holds true for drug‐loaded SLN.

4.3 Antitumour activity with reduced systemic toxicity by SLN: one step further

Most of the cytotoxic anticancer drugs have very poor ability to bind specifically to tumour site and this is the most vital challenge to achieve better anticancer treatment. This directs to many adverse effects such as severe anaemia, gastrointestinal disturbances, fatigue, and alopecia which are commonly induced by the majority anticancer agents [80, 81]. The toxicity profile of anticancer drugs is generally induced by their ability to bind non‐specifically to healthy rapid proliferating tissues such as in hair follicles, bone marrow, gonadal tissue, and gastrointestinal mucosa [82]. In previous studies, the cytotoxicity of DTX was evaluated when loaded in SLN and compared with Taxotere^®. In one study, the soybean lecithin and glycerol tristearate‐based DSN showed enhanced antitumour activity comparable with Taxotere^®. The enhanced antitumour activity of DTX‐loaded SLNs could be due to optimal particle sizes of SLNs and extended mean residence time (MRT) of DTX. Furthermore, these SLNs showed reduced toxicity with higher MTD about 280 mg/kg, whereas the MTD of Taxotere^® was 100 mg/kg. The lesser toxicity of SLNs was attributed to the improved biodistribution and the decreased use of toxic constituents in preparation of SLNs such as polysorbate 80 [64].

Gao et al. performed comparative toxicity studies between TM‐based DTX‐loaded SLNs and Taxotere^®. Acute toxicity studies revealed that MTD of these SLNs was nearly four times higher as compared with Taxotere^®. As per long‐term toxicity study at the equal dose of these SLNs and Taxotere^®, SLNs caused more minor haemotoxicity, hepatotoxicity, and cardiac toxicity, slighter reduction in body weight and myelosuppression than Taxotere^®. The decrease in acute and prolong toxicity was attributed to the biomimetic structure of the SLNs, which prevent the direct contact of DTX with the tissue. Additionally, the DTX was released at slowest rate from the SLNs, which might avoid the injury of healthy organs caused by high level of DTX. [65]. Furthermore, Yuan et al. developed DTX‐encapsulated SLNs with same composition by using similar method as established earlier by Gao et al. to reduce myelosuppression and anaphylaxis caused by Tween 80, while still maintaining its antitumour activity. Myelosuppression is the major side effect of Taxotere^® in clinical application. These newly designed SLNs have increased MTD of DTX in comparison with Taxotere^® [68]. The SLNs have significant different IC50 values in comparison with Taxotere^®. The IC50 values of FDK‐SLNs, DK‐SLNs, and Taxotere^® were observed to be 0.05 ± 0.01 ng/ml, 0.16 ± 0.02 ng/ml, and 36.1 ± 0.7 ng/ml. respectively. In comparison to Taxotere^®, DTX‐loaded SLNs had 18‐times reduced IC50 value. The reason for the reduction in IC50 values as compared with Taxotere^® could be the enhanced cellular uptake of DTX using SLNs [75]. The galactosylated SLNs developed for hepatoma‐targeted delivery, with EE 92.5 ± 3.7%, were observed to have superior cytotoxicity and antitumour efficacy over Taxotere^®. Galactosylation of the SLNs improved the uptake of DTX into the hepatoma cells. Both the increased cellular uptake of SLNs and enhanced accumulation of DTX in tumour could be the reasons for enhanced antitumour activity [63].

4.4 Delivering on a promise: SLNs carry DTX to tumours

To enhance cytotoxicity of cancer cell selectively, one recent approach that has gained attention is the targeted therapy. This strategy designs cancer cell specific moiety on drug delivery system that allows molecular targeting of cancer cells avoiding lethal damage to healthy cells. Folic acid functionalised SLNs have been reported to exhibit active tumour targeting. Folate receptors (FRs) are over expressed in wide range of cancers; folic acid binds FR with high affinity and results in internalisation of receptor via receptor‐mediated endocytosis [83]. In this context, folic acid grafted SLNs showed significantly enhanced DTX uptake and cytotoxicity than non‐targeted SLNs in MCF‐7 cells [71]. Moreover, surface‐modified SLNs have been considered to increase DTX uptake by brain cells via active targeting. As previously reported, p‐glycoprotein (p‐gp) in the blood brain barrier restricts penetration of DTX in the brain [84]. FDK‐SLNs demonstrated 44 times higher DTX concentration in the brain than that of Taxotere^®. Ketoconazole inhibits p‐gp efflux of DTX; therefore, targeted SLNs formulation taken up by receptor‐mediated endocytosis using folic acid (FA) transporter persisted in the brain for a longer time [66]. In addition, brain endothelial cells utilise monocarboxylic transporters to transport HBA across blood brain barrier [85]. This concept was exploited by Venishetty et al. in an independent study to further enhance transport of DTX‐loaded SLN across blood brain barrier. As a result, surface‐modified DTX‐loaded SLN with HBA showed 57‐folds greater brain permeation coefficient of DTX than Taxotere^® [67]. DTX incorporated in galactosylated SLNs enhanced cellular uptake of DTX into hepatocellular carcinoma cells. Asialoglycoprotein receptors are highly expressed on human hepatoma cell lines and undergo ligand receptor interaction with galactosylated DTX‐loaded SLNs. Thus, these SLNs are endocytosed by both hepatoma carcinoma and healthy cells. However, histological findings showed no harmful effect on normal liver and liver with fibrosis [63]. Taken together, these findings demonstrate advantage of targeted‐SLN formulations and it would be worth considering this when developing new DTX‐loaded SLN formulations.

5 New approaches and future prospects

SLNs offer a new prototype by encapsulating antitumour drugs into nanocarriers which could be modified for drug targeting besides providing sustained and site‐specific targeted delivery. Hence, in the past few decades SLN has attracted interest of scientists. Here, we have discussed few aspects that can provide a platform to modify existing DTX‐loaded SLNs formulations to achieve desired advantages. The advanced strategies adapted in preparation of DTX‐loaded SLNs are schematically represented in Fig. 3.

Fig. 3.

Schematic representation of future strategies adapted in preparation of DTX‐loaded SLNs

5.1 Antibody‐based targeted delivery of DTX‐loaded SLNs

One of the most significant goals of drug delivery is to get the highest accumulation of chemotherapeutic agents at the tumour site, thereby reducing possible toxicity to healthy organs. Spatial targeting leads to increased local drug concentration and offers strategies for more effective therapy [86]. Among the various nanoparticulate vehicles, SLNs hold potential for getting the goal of targeted drug delivery to specific tumour site and hence attracted interest of key researchers [87]. To get better therapeutic window, several approaches have been investigating to target antitumour drug such as DTX to tumour site specifically with the aim to attain higher tumour accumulation and reducing toxicity to other normal tissues. Targeted delivery of chemotherapy can be achieved by antibodies with inbuilt ability to target cancer specific antigens. Monoclonal antibody (mAb) is usually used as target moiety for delivery of nanoparticles to specific tumour sites [88, 89, 90]. The conjugation of a drug to single mAb alters antibody–receptors interaction thus is unable to attain the therapeutic level within the targeted cells. Also, the coupling of mAb to a polymer chain results in relatively large size particles, which are mostly cleared by the mononuclear phagocytic system [91]. A single solution for all these limitations is the encapsulation of drug inside the lipid core and guide the drug carrying nanoparticle via a targeting antibody to the specific site for effective treatment [92].

PSMA, epidermal growth factor receptor (EGFR or HER‐1), transferrin receptor (TfR), and human EGFR 2 (HER2) are the potential targets for mAb‐mediated delivery system. DTX is the first‐line remedy for hormone‐refractory prostate cancer. The targeted delivery of DTX with anti‐EGFR therapies may decrease toxicity and enhance efficacy and make it practical to decrease the doses of DTX [93]. The efficient HER2‐targeted therapy is considered as a most important achievement in treatment of breast cancer. Trastuzumab was the first approved anti‐HER2 drug for treatment of HER2 overexpressed breast cancer [94]. The combination of DTX to trastuzumab plus pertuzumab was demonstrated to be effective in the phase III trial for the first‐line therapy of HER2 overexpressed metastatic breast cancer [95]. BIND‐014, presently in clinical trials, is used for treatment of PSMA overexpressed prostate cancer by PSM targeting [96]. In another study, anti‐PSMA antibody grafted liposomes are under investigation for targeted therapy [97]. The in vivo results revealed that TfR‐targeted liposomes could be a potential carrier for brain targeting of the DXT with better treatment efficacy as compared with the non‐targeted formulations [98]. In this context, DXT‐loaded SLNs can be surface modified with these earlier mentioned approaches to develop SLNs with targeted delivery plus controlled release for cancer treatment.

5.2 Co‐delivery of DTX and chemosensitising small interfering RNAs (siRNA) by SLN

Chemotherapy resistance is the main obstacle in treatment of cancers; hence, it is of huge clinical significance to overcome the cancer cell sensitivity to anticancer drugs. In the cancer treatment, siRNAs are used to improve the chemosensitivity of cancer cells by silencing the expression of cancer‐related genes and oncogenes. siRNA is a novel technique that can be used as a better therapeutic option for treatments of cancers than the conventional combination of radiotherapy and chemotherapy [99]. However, poor intracellular uptake, short serum half‐life, and unwanted non‐specific immune stimulations are the main obstacles toward the effective application of siRNA as therapeutic agents [100]. SLNs can be a promising carrier system for co‐delivery of therapeutic nucleic acids and anticancer drugs [101]. The co‐delivery of siRNA and anticancer drugs strategy may considerably increase the anticancer activity compared with either agent alone. Moreover, the co‐delivery of these two agents by a lipid nanocarrier can reduce side effects and toxicity associated with chemotherapy [102]. Cationic SLNs can proficiently attach to negatively charged siRNA directly through ionic interaction and mediate siRNA delivery to cancer cells [103]. Zheng et al. [104] reported that the co‐delivery of siRNA‐Bcl‐2 and DTX markedly induce down‐regulation of the Bcl‐2 anti‐apoptotic gene and improved antitumour efficacy with a relatively small dose of DTX, showing synergistic tumour suppression effect as compared with either the DTX or siRNA treatments alone. Meng and Tao [105] demonstrated that silencing stathmin‐1 gene could enhance the sensitivity of gastric cancer to DTX, increasing the fraction of cells at the sub‐G1 stage and promote apoptosis. The combined action of DTX and stathmin‐1 siRNA can considerably increase the efficacy of DTX for the treatment of gastric cancer. Future study of gene expression will certainly open a new avenue for the development of cationic SLNs as effective nanocarrier for co‐delivery of siRNA and anticancer drugs due to their significant potential of penetrating cells and ability to attain temporally and spatially controlled release for silencing cancer‐related genes.

5.3 Image‐guided therapy

Functionalised magnetic nanoparticles (MNPs) have come in prospective as a cutting edge theranostic tool which provides dual purpose in one single nanocarrier; that is, drug delivery and monitoring disease progression and therapeutic efficacy followed by magnetic resonance imaging (MRI). This pioneering idea was first proposed by Freeman et al. that uses external magnetic field generated by permanent magnets to guide drug‐loaded MNPs or antibodies to tissues altered by disease. The non‐invasive technique results in accumulating drug at the site of tumour [106] along with low systemic concentrations [107]. Moreover, MRI has high spatial resolution and efficient soft tissue contrast in comparison with other imaging techniques. In addition, MNPs are taken up by cells via endocytosis and within lysosomal region they are degraded into ions [108]. Beside these, MNPs lose their magnetism when external magnetic field is withdrawn [109]. Iron oxide‐based MNPs such as magnetite Fe₃ O₄ and maghemite γ ‐Fe₂ O₃ are most widely used nanoparticles as contrast agent in MRI due to their stability and biocompatibility [109, 110, 111]. There are numerous theranostic systems being studied including micelles‐liposomes or polymer‐based materials [112, 113, 114]. Nevertheless, SLN loaded with iron oxide nanoparticles have been inadequately explored. Recently, this phenomenon was exploited by Oumzil et al., where nucleoside lipids were used to form stable SLNs loaded with drug and iron oxide. The new SLNs turned out of having superior characteristics in comparison with clinically available contrast agent such as Feridex, suggesting the SLNs as future of image‐guided therapy [115].

6 Authors conclusion

In the last decade, the development of nanoformulations for DTX delivery has received tremendous interest due to the advantages of improved pharmacokinetic properties, enhanced efficacies, and ability to reduce side effects. On the basis of previous preclinical data, it was demonstrated that SLNs are superior to Taxotere^® interms of the delivery of DTX. The present review highlights the features of SLN formulations for effective delivery of DTX, offering an overview of strategies for prolonged circulation time, increased accumulation into tumour cells, and recent advances for targeted delivery. In the near future, it is expected that patients will benefit from both the therapeutic effectiveness and the low cost of SLNs.