Dextran-polylactide micelles loaded with doxorubicin and DiR for image-guided chemo-photothermal tumor therapy

Highlights

• Dextran-polylactide copolymers with various degrees of polylactide substitution are synthesized.

• The selected dextran-polylactide is assembled with doxorubicin and DiR to construct desirable micelles.

• The optimized micelles have good stability and high photothermal conversion efficiency.

• Micelles can accumulate inside tumors sustainably and persistently emit strong fluorescence signals.

• Micelles show the ability to highly inhibit tumor growth with adequate in vivo safety.

Abstract

Image-guided chemo-photothermal therapy based on near-infrared (NIR) theranostic agents has found promising applications in treating tumors. In this multimodal treatment, it is of critical importance to image real-time distribution of photothermal agents in vivo and to monitor therapeutic outcomes for implementing personalized treatment. In this study, an optimally synthesized dextran-polylactide (DEX-PLA) copolymer was assembled with doxorubicin (DOX) and DiR, a kind of NIR dye, to construct desirable micelles ((DiR + DOX)/DEX-PLA) for performing image-guided chemo-photothermal therapy. These (DiR + DOX)/DEX-PLA micelles had good physical and photothermal stability in aqueous media and showed high photothermal efficiency in vivo. Based on the H22-tumor-bearing mouse model, (DiR + DOX)/DEX-PLA micelles were found to accumulate inside tumors sustainably and to emit strong fluorescence signals for more than three days. The (DiR + DOX)@DEX-PLA micelles together with NIR laser irradiation were able to highly inhibit tumor growth or even eradicate tumors with one injection and two dose-designated 5-minute laser irradiations at the tumor site during 14 days of treatment. Furthermore, they showed almost no impairment to the body of the treated mice. These (DiR + DOX)@DEX-PLA micelles have confirmative translational potential in clinical tumor therapy on account of their persistent image-guided capacity, high antitumor efficacy and good in vivo safety.

Introduction

Chemotherapy is one of the important approaches used for cancer treatments to date. Nevertheless, chemotherapy based on the drug administration in free formulation generally causes severe side effects while often showing limited therapeutic efficacy against cancers because of unspecific drug delivery [1], [2]. In addition to these, chemotherapy performed with repeated drug administration could induce the migration and aggregation of cancer stem cells [3], multidrug resistance [4], [5], and tumor metastasis [6], [7]. Taking account of the drawbacks of regular chemotherapy, many efforts have already been undertaken to develop alternative therapeutic modalities such as photothermal therapy, photodynamic therapy and immunotherapy [8]. Nowadays, different tumor therapeutic modalities are often used in designed combinatorial modes to achieve the improved therapeutic efficiency while minimizing the side effects of chemical therapeutics [9]. Among different combination modes, the image-guided chemo-photothermal therapy based on near-infrared (NIR) theranostic agents has attracted a lot of fundamental research interest and clinical attention in cancer treatments due to its several potential advantages such as the localized hyperthermia, adjustability of radiation dose, and synergistically enhanced efficacy [10], [11], [12].

In chemo-photothermal therapy, varied kinds of inorganic nanoparticles, including silica-coated gold nanorods, gold nanoshells on silica nanorattles, gold nanorods-capped magnetic core/silica shell nanoellipsoids, DNA-coated gold nanorods, graphene and transition metal sulfide, have been employed as photothermal conversion agents (PTCAs), and in some cases, also as imaging agents [10], [13]. In general, inorganic photothermal therapeutics or imaging agents could be prepared into nanoparticles with high stereo-tacticity and very small size, and many of them have the nature of repetitive heat generation and persistent image signal emission under the appropriate photostimulation [8], [9], [11], [12]. Although many pre-clinical studies signify the applicability of inorganic photothermal or imaging agents so far, there are various difficulties to translate them into clinical usage since they are often correlated to complex chemistry, tedious preparation and potential long-term toxicity. Hence, in the field of image-guided chemo-photothermal cancer treatment, it is still highly desirable to develop other types of photothermal and imaging agents with the characteristics of simple composition, easy and repeatable preparation, safety and high efficiency, and meanwhile, with high potential for their clinical translation.

Besides eligible PTCAs and imaging agents, carriers with sufficient biocompatibility and satisfactory safety are also very important for building nanomedicines that are intended for use in image-guided chemo-photothermal therapy. Typical polymer nanocarriers that are commonly used in intravenous theranostic administration include conjugates, micelles, complexes, and vesicles. Among them, micelles that are self-assembled from biocompatible amphiphilic polymers have been very often used in the field of nanomedicines owing to their specific core-shell-like structure, ease of preparation without involving additional chemicals, and the feature of a single carrier material. The hydrophobic core-like portion of micelles can serve as a reservoir for loading poorly water-soluble agents whereas their antifouling hydrophilic shell-like surface would be conducive to the colloidal stability and to the formation of intrinsic stealth effect [14]. In general, polymer materials utilized for micelle construction need to be biodegradable because non-degradable polymers could accumulate inside the human tissues or organs, and result in certain toxicity [14], [15].

To date, various kinds of water-soluble polysaccharides have been modified to achieve amphiphilic copolymers for building micelles due to their meritorious biocompatibility and biodegradability [16], [17], [18]. Dextran (DEX) is a biocompatible polysaccharide specifically suitable for the micelle construction due to its several advantages: (1) high in vivo safety, being evidenced by its common use as a plasma volume expander and antithrombotic agent for years [19]; (2) a broad range of derivatization produced via chemical modifiability through its hydroxyl groups, imparting DEX with many possibilities to construct diverse micelles with designed structures and intended functions; and (3) excellent water solubility, allowing DEX chains to carry a large number of hydrophobic branches [20]. Among available DEX-based amphiphilic derivatives, the copolymers synthesized by grafting polylactide (PLA) onto DEX backbone are particularly useful for building micelles because the length of PLA side chains and the degree of PLA substitution for DEX-PLA copolymers can be effectively controlled, and the fabricated DEX-PLA micelles could have finely tunable sizes and well-constructed structure [21], [22], [23]. Besides, DEX-PLA micelles also have high safety since only two biocompatible and biodegradable components, DEX and PLA, are involved [16], [20].

In addition to the aforementioned inorganic PTCAs, several kinds of organic molecules with NIR responsive characteristics have also been employed for preparing nanomedicines. These nanomedicines are often utilized in tracking cells, guiding therapy and assessing the post-treatment outcomes [12], [24]. Despite many successful applications, the NIR molecules that have a suitable NIR window, adequate safety, sufficient fluorescence quantum yield, and high photothermal efficiency are very few [25]. 1,1-dioctadecyl-3,3,3,3-tetramethyl indotricarbocyanine iodide (DiR) is a kind of cyanine NIR fluorescent dye with good biocompatibility when applied at a safe dosage in vivo [26]. DiR has the photothermal conversion capacity in the NIR region with a high quantum yield at its maximal emission wavelength [25], [26]. Furthermore, DiR is capable of imaging the targeting tissues with minimal auto-fluorescence interference [27], allowing it to effectively function in sensitive tumor detection and precise implementation of localized hyperthermia generation via NIR laser irradiation [25], [28]. However, DiR molecules are prone to self-aggregate in an aqueous environment and have a short lifetime in vivo, which considerably limits their use [26], [27], [29].

Up to now, many anticancer nanomedicines with specialized compositions, elaborately constructed structures and multi-functionalities have been developed, and abundant results basing on animal models have demonstrated their applicability [1], [2], [8]. Although some of them have already advanced to the pre-clinical stage or even to a certain phase of clinical trials [14], [30], [31], to date, very few of them have been approved by the regulatory agencies for clinical use [8], [9], [30], [31], [32], [33]. Specific to the image-guided chemo-photothermal tumor therapy, the development of theranostic nanomedicines with satisfactory efficacy, adequate safety and high potential in the actualization of clinic translation remains great challenge for industry and regulatory agencies [34], [35].

To this end, an effort was made in this study to assemble DEX-PLA with DiR and doxorubicin (DOX) to achieve certain desired (DiR + DOX)/DEX-PLA micelles for implementing image-guided chemo-photothermal tumor therapy. The results obtained from in vivo experiments revealed that the optimally built micelles were able to accumulate in the tumor with persistent emission of strong fluorescence signals and to highly inhibit tumor growth with only one injection and two designated 5-minute NIR laser irradiations during 14 days of treatment. They also showed adequate in vivo safety. Since DEX, PLA, DiR and DOX are all clinically used substances [16], [20], [26], [27], these (DiR + DOX)@DEX-PLA micelles could find an auspicious avenue for the clinical translational in tumor therapy.