Photoresponsive Block Copolymer Prodrug Nanoparticles as Delivery Vehicle for Single and Dual Anticancer Drugs.

In recent decades, drug delivery systems (DDSs) based on polymer nanoparticles have been explored due to their potential to deliver drugs with poor water solubility. Some of the limitations of nanoparticle-based DDSs can be overcome by developing an appropriate polymer prodrug. In this work, poly(NIPA)-b-poly(HMNPPA)-b-poly(PEGMA-stat-BA) was synthesized using reversible addition fragmentation chain transfer polymerization and Chlorambucil (Cbl), an anticancer drug, was conjugated to the copolymer via 3-(3-(hydroxymethyl)-4-nitrophenoxy)propyl acrylate (HMNPPA) units to prepare the prodrug. A few biotin acrylate (BA) units were also incorporated to bring potential targeting capability to the prodrug in the copolymer. This polymer prodrug formed spherical micellar nanoparticles in physiological conditions, which were characterized by dynamic light scattering and transmission electron microscopy measurements. The very low critical aggregation concentration (cac) (0.011 mg/mL) of the prodrug, as measured from Nile Red fluorescence, makes it stable against dilution. The polymer prodrug was shown to release Cbl on photoirradiation by soft UV (λ ≥ 365 nm) and laser (λ = 405 nm) light. The prodrug micellar nanoparticles were capable of encapsulating a second drug (doxorubicin, DOX) in their hydrophobic core. On photoirradiation with UV and laser light of the DOX-loaded nanoparticles, both Cbl and DOX were released. Light-induced breaking of photolabile ester bond resulted in the release of Cbl and caused disruption of the nanoparticles facilitating release of DOX. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay confirmed the nontoxicity of the polymers and effectiveness of the dual drug-loaded micellar nanoparticles toward cancer cells. Confocal microscopy results showed a better cellular internalization capability of the DOX-loaded nanoparticles in cancer cells, possibly due to the presence of cancer cell targeting biotin molecules in the polymer. This new photoresponsive potentially biocompatible and cancer cell-targeted polymer prodrug may be useful for delivery of single and/or multiple hydrophobic drugs.

Introduction

In recent decades, tremendous efforts have been made to improve the available anticancer therapies, and chemotherapy has emerged as the most promising of these therapeutic applications. In this context, drug delivery vehicles based on polymer nanoparticles have been explored due to their ability to successfully deliver poorly aqueous soluble drugs like Doxorubicin (DOX).[1−3] In spite of a few promising results, most of these polymeric nanoparticle drug delivery systems (DDSs) have limitations, which has hampered their further translation to the clinic and eventually to the market. Polymer–drug conjugates could be an alternative delivery technology in the development pipeline. Conjugation of a drug with a polymer forms a so-called “polymer prodrug”. A polymer prodrug is a kind of therapeutic where the bioactive agent is not encapsulated but covalently linked to a biocompatible polymeric carrier.[4,5] The “prodrug” approach provides a powerful means of drug modification, for example, solubilization of hydrophobic drugs, elimination of initial burst release of drug micelles, and tuning of drug pharmacokinetics.[6]

There is an increasing interest among scientists to synthesize biocompatible amphiphilic photoresponsive polymer prodrug systems. The amphiphilic block copolymers generally have a tendency to self-assemble in aqueous medium to form stable nanoparticles like micelles[7,8] and vesicles.[9,10] These nanoparticles are known to be potent carriers of hydrophobic as well as hydrophilic molecules in aqueous solution. Attachment of stimuli-responsive functional groups to the block copolymers can form smart micellar nanoparticles that can be used as stimuli-regulated drug release systems.[11,12] For biomedical application, the smart nanoparticles should be biocompatible, nontoxic in nature, and preferably biodegradable. In this context, poly(ethylene glycol) (PEG)-based block copolymers have often been chosen for drug delivery systems due to their nontoxicity, high water solubility, excellent biocompatibility, and prolonged circulation time in blood stream.[13] Such properties make PEG-containing stimuli-responsive polymers excellent candidates for drug delivery systems (DDSs).[14,15] Similarly, poly(N-isopropylacrylamide) (PNIPA)-based polymeric systems have also been explored as DDSs.[16,17]

Recently, the combined administration of different drugs to enhance therapeutic effect has gained great attention.[18,19] Dual drug delivery systems (DDDS) are those that are capable of releasing two drugs from the same carrier.[20] There has been a continuous necessity to overcome the main drawback in combined therapy, which is to regulate each drug release in a controlled manner.[21] Another aspect that needs to be considered is how to deliver drug in a cell-specific manner. Recently, the passive targeting strategy has been widely used in anticancer application through enhanced permeation and retention (EPR) effect.[22−24] PEGylation results in longer circulation times for DDSs in cancer cells via EPR effect for the passive targeting strategy.[25] However, PEGylation significantly hampers endocytic uptake and endosomal escape by cancer cells. This so-called “PEG dilemma” causes significant loss of activity of drug carriers.[26] To resolve the problem of the PEG dilemma in the passive targeting strategy and to increase the cellular uptake, active targeting has been introduced that exploits specific ligands including protein antibodies and peptides that can recognize and bind to the specific proteins or antigens expressed on cancer cells.[27] Among these ligands, biotin, a low molecular weight vitamin, plays a significant role in all cell divisions, especially for the growth of cancer cells. Biotin binds to the biotin receptors, and it has been shown that biotin receptors are over expressed in cancer cells over normal cells.[28,29]

In this work, we have investigated photoresponsive biotin-functionalized polymer prodrug micellar nanoparticles as a dual drug delivery system as well as a single drug delivery system (SDDS). Accordingly, we have synthesized an amphiphilic block copolymer, poly(NIPA)-b-poly(HMNPPA)-b-poly(PEGMA-stat-BA), containing a few biotin acrylate (BA) repeat units. The anticancer drug Chlorambucil (Cbl) was conjugated to the copolymer via 3-(3-hydroxy methyl-4-nitrophenoxy)propyl acrylate (HMNPPA) units to prepare the prodrug. This potential cancer cell targeting polymer prodrug was investigated as a photoresponsive drug carrier. Doxorubicin was further loaded in the micellar nanoparticles formed by the prodrug in physiological conditions. It was observed that the prodrug and DOX-loaded prodrug can be employed to deliver a single drug and two drugs, respectively, by irradiating with soft UV light (≥365 nm) or laser light (405 nm).

Results and Discussion

Considering the importance of photoresponsive drug delivery systems, we synthesized a new photoresponsive block copolymer prodrug containing a photoreleasable anticancer drug, Cbl. Moreover, to enhance the therapeutic efficiency and make the prodrug more potent in cancer drug delivery, a targeting moiety (biotin) was conjugated with the block copolymer. Moreover, a second anticancer drug, DOX, was coloaded in the nanoparticles formed by the self-assembly of the prodrug to further improve the utility of the drug delivery system. Phototriggered drug delivery from this polymer-based system was investigated by the illumination of laser light (λ = 405 nm) and soft UV (λ ≥ 365 nm) in vitro.

Synthesis of the Monomers, Block Copolymers, and Polymer Prodrug

Synthesis of the Monomers

3-(3-(Hydroxymethyl)-4-nitrophenoxy)propyl acrylate (HMNPPA) was prepared from 3-bromopropyl acrylate as described in the Materials section (Scheme ). 4-Hydroxy-2-nitrobenzyl alcohol and K2CO3 were dissolved in dry dimethylformamide (DMF) and allowed to stir for 45 min under a N2 atmosphere at 70 °C. After that, 3-bromopropyl acrylate dissolved in dry DMF was added slowly to the solution. The reaction mixture was allowed to stir for 2 days at the same reaction conditions. The crude product was purified by column chromatography of silica gel and the desired product was found as a yellow solid with 58% yield. The monomer was characterized by proton NMR (Figure ) and 13C NMR (Figure S2, Supporting Information). As shown in Figure , the peak at 8.14–8.11 ppm is due to the deshielded nature of the Ar proton closer to the NO2 group and peaks at 7.34 and 7.04–7.02 ppm relate to the other two Ar protons in HMNPPA. The peaks at 6.36–6.31, 6.22–6.15, and 5.96–5.93 ppm are due to vinylic protons that confirm the presence of acrylate moieties in HMNPPA, peaks at 5.59–5.56 and 4.85–4.84 ppm are due to benzylic OH and benzylic CH2, respectively. The peaks at 4.29–4.26, 4.23–4.12, and 2.15–2.08 ppm are attributed to the propyl moiety in HMNPPA and confirm the successful synthesis of HMNPPA. Successful synthesis of HMNPPA was furthered confirmed by 13C NMR.

Scheme 1

Synthesis Scheme of Photocleavable Monomer 3-(3-(hydroxymethyl)-4-nitrophenoxy)propyl acrylate (HMNPPA)

Figure 1

1H NMR spectrum (in DMSO-d6) of 3-(3-(hydroxymethyl)-4-nitrophenoxy)propyl acrylate (HMNPPA).

As shown in Figure S2, the 13C NMR signal at 166.30 ppm is due to the carbonyl carbon of the ester group. The 13C NMR signals at 163.53, 140.58, 140.47, 128.31, 114.57, and 113.73 ppm are attributed to Ar carbons and signals at 131.30 and 128.12 ppm correspond to vinylic carbons in HMNPPA. The propyl moiety in HMNPPA was signified by the signals at 65.35, 63.00, and 28.56 ppm. The significant 13C NMR signal of benzylic carbon arises at 61.11 ppm.

Biotin acrylate was synthesized by a similar method to that for HMNPPA. d-Biotin and K2CO3 were dissolved in dry dimethylformamide (DMF) for 45 min at 70 °C under stirring. 3-Bromopropyl acrylate in dry DMF was slowly added to the reaction mixture and allowed to stir for 48 h at the same reaction condition. A pure product was found as an off white solid after recrystallization from ether. BA was characterized by proton NMR (Figure S3, Supporting Information) and 13C NMR (Figure S4, Supporting Information). As shown in Figure S3, Supporting Information, proton NMR signals at 6.41–6.30, 6.20–6.13, and 5.96–5.93 ppm signify the vinylic protons and NH protons of BA. The propyl moiety in BA was confirmed by the corresponding signals at 4.18–4.07 and 1.96–1.90 ppm. The proton NMR signals for the biotin moiety in BA were confirmed by the proton NMR signals at 6.41–6.30, 4.31–4.28, 4.18–4.07, 3.11–3.06, 2.83–2.79, 2.59–2.53, 2.31–2.27, 1.65–1.4, and 1.36–1.29 ppm. BA was furthered confirmed by 13C NMR (Figure S4, Supporting Information). The 13C NMR signal at 173.59 ppm corresponds to the carbonyl carbon of the ester linkage and the peak at 166.13 ppm is due to the carbonyl carbon of the acrylate moiety in BA. Vinylic carbons in BA showed 13C NMR signals at 130.99 and 128.26 ppm. The propyl moiety showed its significant 13C NMR signals at 61.14, 60.94, and 27.97 ppm. 13C NMR signals due to the biotin moiety are 163.65, 62.00, 60.18, 55.42, 40.53, 33.85, 28.36, 28.24, and 24.76 ppm.

Synthesis of the Block Copolymers

Poly(NIPA)-b-poly(HMNPPA) copolymer (P1) was synthesized by the reversible addition fragmentation chain transfer (RAFT) technique, as depicted in Scheme . At first, P0 was prepared using BCTPA as a chain transfer agent (CTA),[30] as confirmed by 1H NMR (Figure S5, Supporting Information). The molecular weight of P0 was determined by the absolute GPC method (Figure ) using dn/dc of the polymer as 0.12, determined at 30 °C in tetrahydrofuran (THF) using a differential refractometer. The molecular weight of P0 was found to be Mw = 4200 g/mol, Mn = 3900 g/mol, and polydispersity index (PDI) = 1.07, which corresponds to 34 NIPA units per polymer chain. P0 was further used as the CTA for the controlled polymerization of HMNPPA to form P1. Incorporation of HMNPPA in the copolymer was confirmed from proton NMR (Figure ), as is evident from the appearance of the peaks at 4.14 ppm due to two CH2 groups of the propyl chain in HMNPPA, 5.00 ppm due to benzylic CH2 of HMNPPA, and 8.09 ppm due to the most down shielded aryl proton near to the NO2 group. Molecular weight of P1 was determined as above using dn/dc as 0.108 and found to be Mw = 6350, Mn = 6000, and PDI = 1.07, which implies that the number of HMNPPA units per P1 chain was equal to seven. The third block was synthesized by radically polymerizing a mixture of poly(ethylene glycol) methyl ether acrylate (Mw = 480) and biotin acrylate (BA) in the presence of P1, which acted as chain transfer agent. The presence of poly(ethylene glycol) grafts is expected to increase the water solubility of the block copolymer in physiological temperature (at 37 °C) and the presence of biotin is expected to impart targeting ability to the polymer prodrug toward cancer cells. Incorporation of PEG moieties and BA moieties in P2 was confirmed by proton NMR (Figure ) and GPC (Figure ). The 1H NMR peaks of P2 at 2.75, 2.90, 3.18, 4.32, and 4.51 ppm are due to BA and those at 3.37, 3.64, and 4.15 ppm are due to PEGMA moieties. The molecular weight of P2 was determined by GPC as Mw = 11 150 g/mol, Mn 9650 g/mol and PDI 1.15 using dn/dc as 0.08.

Scheme 2

Synthesis Scheme of Poly(NIPA)-block-poly(HMNPPA) Copolymer (P1)

Figure 2

GPC traces of P0, P1, P2, and P3.

Figure 3

1H NMR spectrum (in CDCl3) of poly(NIPA)-b-poly(HMNPPA) copolymer (P1).

Figure 4

1H NMR spectrum (in CDCl3) of poly(NIPA)-b-poly(HMNPPA)-b-poly(PEGMA-stat-BA) copolymer (P2).

Synthesis of Polymer Prodrug (P3)

P2 was further utilized for DCC coupling with Cbl, an anticancer drug, to form prodrug P3. Coupling of Cbl with the HMNPPA moieties in the block copolymer was confirmed by proton NMR (Figure ). The peak corresponding to benzylic CH2 was shifted to 5.47 ppm due to formation of the ester bond. The molecular weight of P3 was determined to be Mw ∼ 13 650 g/mol, Mn ∼ 11 600, and PDI 1.17 using dn/dc as 0.09 (Figure ). The summary of the block copolymer characterization data is provided in Table .

Figure 5

1H NMR spectrum (in CDCl3) of poly(NIPA)-b-poly(HMNPPACbl)-b-poly(PEGMA-stat-BA) copolymer (P3).

Table 1

Molecular Weight Data of the Block Copolymers

symbol	description	Mn (g/mol)	Mw (g/mol)	PDI
P0	PNIPA34 macroCTA	3900	4200	1.07
P1	poly(NIPA34)-b-poly(HMNPPA7)	6000	6350	1.07
P2	poly(NIPA34)-b-poly(HMNPPA7)-b-poly(PEGMA8-stat-BA2)	9650	11 150	1.15
P3	poly(NIPA34)-b-poly(HMNPPA7Cbl)-b-poly(PEGMA8-stat-BA2)	11 600	13 650	1.17

Formation of Self-Assembled Nanoparticles from P3

We synthesized a new photoresponsive amphiphilic triblock copolymer prodrug, P3, which contains poly(NIPA34), poly(HMNPPA7Cbl) and poly(PEGMA8-stat-BA2). Amphiphilic block copolymers are well known for their self-assembling behavior in aqueous solution. To probe whether P3 undergoes self-assembly in aqueous solution, we monitored the fluorescence of Nile Red in aqueous solutions of increasing concentration of P3. Nile Red has been used for probing the self-assembly of polymers in aqueous solution by previous researchers as well.[31] As shown in Figure , initially, the fluorescence intensity of Nile Red (at 626 nm) was relatively low and nearly independent of P3 concentration till a certain concentration, above which, the fluorescence intensity increased drastically, indicating formation of hydrophobic pockets where Nile Red started getting encapsulated. This indicates the formation of self-assembled nanoparticles from P3 above a certain concentration. The concentration corresponding to the inflection point was considered as the critical aggregation concentration (cac) of the prodrug P3. The cac value was found to be 0.011 mg/mL in phosphate-buffered saline (PBS) buffer at physiological pH at 37 °C, which is significantly lower than the cac values generally observed for common amphiphilic block copolymers reported in the literature.[32,33] A very low cac for P3 is expected to provide excellent stability against dilution when injected into the blood.

Figure 6

Plot of fluorescence emission intensity of Nile Red at 626 nm vs prodrug concentration (mg/mL) in water at 37 °C and pH = 7.4.

Formation of self-assembled nanoparticles from P3 was furthered investigated by transmission electron microscopy (TEM) measurements as well as dynamic light scattering (DLS) studies with 1 mg/mL concentration of P3 in PBS buffer at physiological pH at 37 °C. TEM measurements revealed that the nanoparticles had spherical morphology with an average diameter of around 200 nm, confirming the formation of spherical micelles (Figure a). DLS was used to determine the size of the micelles of P3 and the result showed (Figure b) that the intensity-average hydrodynamic diameter (Dh) was 150 nm, which was a little lower than the size found in TEM measurements. The slightly higher size obtained in TEM could be due to flattening of micellar nanoparticles during sample preparation.

Figure 7

(a) TEM image and (b) DLS data of P3 prodrug nanoparticles in PBS buffer at pH = 7.4 at 37 °C.

Phototriggered Release of Chlorambucil (Cbl) from the Prodrug Nanoparticles

Stimuli-responsive micellar nanoparticles are one of the most potent carriers of hydrophobic drug molecules.[34,35] In this work, we synthesized polymer prodrug P3 containing Cbl by DCC coupling, which can work as a photoresponsive Cbl delivery system. Cbl is known to alkylate DNA, inducing DNA damage.[36,37] As shown in Figure , P2 exhibited absorption maxima at 313 nm and P3 exhibited a broad absorption peak in the range from 200 to 450 nm with absorption maxima at 300 and 259 nm at pH 7.4. The presence of a NO2 group in the phenyl ring is responsible for the UV absorption maximum at 313 nm in P2, and the conjugated Cbl drug with HMNPPA moieties in P3 is responsible for the absorption maxima at 300 and 259 nm. Hence, we used soft UV light (λ ≥ 365 nm) to investigate the photoresponsive release as P3 exhibits broad absorption in the range from 200 to 450 nm. To demonstrate the ability of P3 to release Cbl on photoirradiation, absorbance of P3 was monitored after UV irradiation of specific durations. The UV–vis absorbance measurements were carried out after waiting for 30 min post irradiation to let the released insoluble Cbl settle down. Figure a shows that the intensity at 259 and 300 nm decreased with more irradiation time, which indicated the release of Cbl molecules from the prodrug. Figure b shows the drug release profile of prodrug P3 on UV irradiation by high-performance liquid chromatography (HPLC) measurement. The released drug from P3 on UV irradiation for 1 h was found to be 50 and 56%, as determined by UV–vis spectroscopy and HPLC, respectively.

Figure 8

UV–vis absorption spectra of block copolymer P2 and the polymer prodrug P3 in aqueous solution at pH 7.4 and 25 °C. The spectrum for free Cbl presented here was recorded in 1% methanol in water.

Figure 9

(a) UV–vis spectra of P3 after UV irradiation of different durations. (b) In vitro drug release profile of prodrug P3 by HPLC study.

Coloading of Doxorubicin in the Prodrug Nanoparticles and Phototriggered Release of Two Drugs from Nanoparticles

To enhance the therapeutic efficiency of P3 in cancer treatment, DOX was coloaded as a second drug into the P3 nanoparticles. The drug loading capacity (DLC) and drug encapsulation efficiency (DEE) of DOX was found to be 19.6 and 61.0%, respectively, establishing the drug-encapsulating ability of the polymer prodrug. DOX-loaded micelles in PBS buffer at 37 °C were characterized by DLS and TEM measurements. The DLS results showed that after drug loading, the size of P3 nanoparticles increased from 150 to 440 nm (Figure a). This DLS result was also supported by TEM, which also showed an increase in average size to 480 nm from 200 nm on loading of DOX in P3 prodrug (Figure b). Prodrug nanoparticles got swollen to a significant extent due to encapsulation of DOX in the hydrophobic core as seen from the TEM images. The stability of P3 and P3–DOX nanoparticles was monitored by DLS and absorbance study, and recording the fluorescence of DOX in P3–DOX nanoparticles. No significant change in the size, absorbance, and DOX fluorescence was observed for the duration of studies, suggesting the stable nature of the prodrug P3 and DOX-loaded prodrug (Figures S6–S8, Supporting Information).

Figure 10

(a) DLS data of P3–DOX prodrug nanoparticles at 37 °C in PBS buffer (pH 7.4). TEM images of DOX-loaded P3 prodrug nanoparticles (b) before and (c) after irradiation for 1 h by UV light (λ ≥ 365 nm) at 37 °C in PBS buffer (pH 7.4).

DLS and TEM measurements were performed to monitor the fate of the nanoparticles on photoirradiation along with following the release of the two drugs, namely, Cbl and DOX. At first, the measurements were carried out on P3–DOX nanoparticles on which UV irradiation had been performed for 1 h. The hydrodynamic size for this UV-irradiated sample obtained from DLS was not stable. The size was increasing, although slowly, with time. Hence, we think that the aggregates that were formed post micellar disruption were not very well-defined and were probably formed by loose attachment of the polymer chains. The TEM image of the UV-irradiated DOX-loaded nanoparticles is presented in Figure c and shows that the loaded spherical structures of the micelle nanoparticles were disrupted and turned into irregular shaped aggregated structures. The sample for the TEM image presented in Figure c was prepared within 30 min of the UV irradiation before a significant extent of aggregation took effect. A similar effect on the size was also observed for 1 h laser irradiation on the DOX-loaded nanoparticles by DLS size measurement, which revealed that laser can also cause disruption of micellar nanoparticles. On photoirradiation, the hydrophobic HMNPPA moieties were converted to hydrophilic 3-(3-formyl-4-nitrosophenoxy)propyl acetate (FNPPA) moieties[38] (Figure S9, Supporting Information), which caused disruption of the P3 micelles. Detachment of the Cbl moieties was evident from the decrease of the peaks corresponding to Cbl in the UV–vis spectra recorded for UV-irradiated DOX-loaded nanoparticles. The disruption of micellar nanoparticles also triggered release of DOX from the nanoparticles, which is evident from the decrease in the fluorescence intensity of DOX in the nanoparticles on UV irradiation (Figure S10, Supporting Information).

To investigate the effect of laser irradiation on P3–DOX nanoparticles, the buffer solution of nanoparticles was also irradiated by laser light (λ = 405 nm) for different durations, and the release of the two drugs was followed spectroscopically. Generally, laser light is more preferred as a phototrigger as the lower energy of laser causes less damage to normal cells.[39]Figure a,b shows the UV–vis and fluorescence spectra recorded for laser-irradiated DOX-loaded nanoparticles. A gradual decrease in the absorbance and fluorescence intensity of DOX confirms release of DOX from the hydrophobic interior of the nanoparticles to the aqueous medium.

Figure 11

(a) UV absorbance and (b) fluorescence spectra of P3–DOX prodrug nanoparticles after irradiation with laser (λ = 405 nm) for various durations.

The cumulative drug release profiles were plotted against the duration of UV or laser irradiation. It can be noted from Figure that the DOX release percentages are quite close for both cases. 54.7% of loaded DOX was released by UV exposure on P3–DOX nanoparticles whereas 50.3% of loaded DOX was released by laser exposure. It could be concluded that DOX release did not significantly differ for the two different external light sources. Both UV or laser irradiation causes micellar disruption, which releases the encapsulated DOX. It can also be noted that 27.9% of Cbl was released upon 1 h of UV irradiation from P3–DOX nanoparticles. This value of Cbl release was lower than the release of Cbl from only P3 (without DOX loading). This may be because of the slow release of the detached Cbl molecules from the nanoparticles in which the core contained hydrophobic DOX molecules. This release percentage further decreased to 19.3% when laser light was used. Thus, Cbl release from P3 prodrug could be regulated by the external light source. All observations showed that P3 nanoparticles themselves can be used as a photoresponsive controlled drug delivery system for releasing a single drug (Cbl), whereas P3–DOX nanoparticles can be used in applications where slow release of multiple drugs is beneficial.

Figure 12

In vitro drug release profile from P3–DOX nanoparticles in PBS (pH 7.4) buffer upon different times of photoirradiation.

In Vitro Cytotoxic Effect

The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay is the conventional and suitable method for evaluating cell viability through mitochondrial reductase activity. The cytotoxic effects of P2, P3, and P3–DOX along with free DOX were evaluated against normal human dermal fibroblast cell lines as well as MDA MB-231 triple negative breast cancer cell lines. Figure a shows the nontoxic nature of P2, P3, and P3–DOX against the normal cells. The P3 treatment against breast cancer cells shows high cell viability even at very high doses of this prodrug, suggesting the nontoxic nature of the prodrug (Figure b) toward cancer cells. The cell viability in both normal and cancerous cell lines was significantly high for P3 even at higher doses of 200 μg/mL. In P3–DOX without UV radiation, Cbl was not released. Therefore, a low cytotoxic effect after treatment with these P3 prodrug molecules was observed on the MDA MB 231 breast cancer cell lines. However, P3–DOX shows slightly higher toxicity than P3, indicating the release of DOX from P3–DOX to some extent. On UV irradiation the cell viability of P3–DOX decreased significantly compared to that without UV irradiation (Figure b). The IC50 values of free DOX and P3–DOX (with UV treatment) were found to be 164 and 105 μg/mL, respectively. The higher cytotoxic effect of UV-irradiated P3–DOX is likely due to the effect of the dual drugs, namely, Cbl and DOX. These results confirmed the effectiveness of P3–DOX as a drug delivery system toward cancer cells.

Figure 13

Cell viability data for P2, P3, and P3–DOX along with free DOX as determined by MTT assay against (a) normal human dermal fibroblast cell lines and (b) MDA MB 231 breast cancer cell lines.

In Vitro Cell Uptake Study

In vitro internalization of DOX and P3–DOX was studied in a time dependent manner against MDA MB 231 breast cancer cell lines. Under confocal microscopy, the DOX internalization into the cell was indicated by red fluorescence of DOX. The nuclei of the cells were stained with DAPI, which emits blue fluorescence. The intensity of red fluorescence inside indicates the presence of DOX inside the cells after its uptake (Figure ). The study was carried out after 1, 2, and 4 h of treatment against breast cancer cell lines. After 4 h of incubation with free DOX, it showed moderate fluorescence according to its accumulation in the nucleus (Figure S11, Supporting Information). This might be due to passive diffusion of DOX to the cells. But, in the case of just P3–DOX, mild red fluorescence was observed in the cytoplasm, even after 1 h of treatment (Figure ). Gradually, the red fluorescence intensity increased and shifted toward the nuclei after further incubation. The smooth internalization of P3–DOX might be through an endocytic mechanism into the cells. This study clearly showed the better cellular internalization capability of P3–DOX than free DOX in cancer cells, possibly due to the presence of cancer cell targeting biotin molecules in P3. However, the lower toxicity of P3–DOX toward cancer cells was due to the slow release of DOX during the first 4 h from P3–DOX, which is expected to drastically improve on irradiation.

Figure 14

Confocal microscopic images of P3–DOX-treated MDA MB breast cancer cell lines over a time span of 1–4 h. The DOX in P3–DOX produces red color fluorescence. Nuclei were stained with DAPI showing blue coloration.

Conclusions

Photoresponsive amphiphilic poly(NIPA)-b-poly(HMNPPA)-b-poly(PEGMA-stat-BA) was successfully synthesized using RAFT polymerization. Chlorambucil was conjugated via a photoresponsive bond, and biotin acrylate was conjugated to the copolymer to bring potential targeting capability to the prodrug in the copolymer. This polymer prodrug formed self-assembled micellar nanoparticles in physiological conditions with an average size of about 200 nm and a very low critical aggregation concentration of 0.011 mg/mL. The low value of cac should provide stability against dilution if the polymer prodrug is injected in blood stream. On irradiation by UV or laser light, Cbl molecules detached from the polymer backbone, causing release of Cbl and disruption of the spherical nanoparticles. The nanoparticles were further utilized to encapsulate a second anticancer drug, DOX. The DOX-loaded prodrug nanoparticles showed controlled release of both DOX and Cbl on irradiation with UV and laser light. The nontoxic nature of the precursor polymer and the polymer prodrug, along with the toxic nature of the DOX-loaded prodrug nanoparticles toward cancer cells, especially on irradiation, established the potential of the prodrug as a drug delivery vehicle. This was further supported by the better cellular internalization capability of DOX-loaded nanoparticles in cancer cells, possibly due to the presence of cancer cell targeting biotin molecules in the polymer. Thus, this Cbl and biotin conjugated poly(NIPA)-b-poly(HMNPPA)-b-poly(PEGMA-stat-BA) prodrug may potentially be used for the delivery of single or multiple drugs preferentially to cancer cells on irradiation with either soft UV or laser light.

Experimental Section

Materials

3-Bromo-1-propanol, 5-hydroxy-2-nitro benzyl alcohol, N-isopropylacrylamide (NIPA), poly(ethylene glycol) methyl ether acrylate (PEGMA, Mw = 480), Cbl, and AIBN were purchased from Sigma-Aldrich and used as received. Doxorubicin hydrochloride was procured from Sigma-Aldrich and neutralized by a standard method. All other chemicals and solvents were purchased from local suppliers and purified by standard procedures. 3-(Benzylthiocarbonothioyl-thio)propanoic acid (BCTPA) was synthesized according to the literature procedure.[40] The procedure for synthesis of 3-bromopropyl acrylate is described in the Supporting Information. 3-(3-Hydroxy methyl-4-nitrophenoxy)propyl acrylate (HMNPPA) was synthesized by following a process described below.

Synthesis of 3-(3-(Hydroxymethyl)-4-nitrophenoxy)propyl Acrylate (HMNPPA) (Scheme )

4-Hydroxy-2-nitrobenzyl alcohol (0.7 g, 4.14 mmol) and K2CO3 (1.144 g, 8.28 mmol) were dissolved in 10 mL of dry DMF in a 50 mL two necked round bottomed flask under a N2 atmosphere at 70 °C. After 45 min, 3-bromopropyl acrylate dissolved in 5 mL of dry DMF was added slowly to the solution. Then, the reaction mixture was allowed to stir for 2 days at 70 °C under a nitrogen atmosphere. The reaction mixture was extracted with DCM and washed repeatedly with ice cold water. The crude product was purified by column chromatography of silica gel using DCM and 1% MeOH in DCM as mobile phase. A yellow solid was formed after evaporation of solvent. The yield obtained was 58%. 1H NMR (DMSO-d6, 400 MHz) (Figure ): δ (ppm) 2.15–2.08 (m, 2H), 4.23–4.12 (t, 2H), 4.29–4.26 (t, 2H), 4.85–4.84 (t, 2H), 5.59–5.56 (t, 1H), 5.96–5.93 (d, 1H), 6.22–6.15 (m, 1H), 6.36–6.31 (d, 1H), 7.04–7.02 (d, 1H), 7.34 (s, 1H), 8.14–8.11 (d, 1H). 13C NMR (100 Hz, CDCl3) (Figure S2, Supporting Information): δ (ppm) 28.56, 61.11, 63, 65.35, 113.73, 114.57, 128.12, 128.31, 131.30, 140.47, 140.58, 163.53, 166.30.

Synthesis of Poly(NIPA)-block-poly(HMNPPA) Copolymer (P1) (Scheme )

Among the different polymerization techniques employed for the synthesis of block copolymers, RAFT polymerization is perhaps the most suitable, due to its pertinence to various monomers in both aqueous and organic different solvents.[41−43] RAFT polymerization was utilized to synthesize the block copolymers in the present work. The process of synthesis of PNIPA macroCTA, P0, is provided in the Supporting Information. Synthesis of P1 was carried out by polymerization of HMNPPA using P0 as CTA and AIBN as radical initiator. P0 (0.3 g, 0.0467 mmol) was dissolved in 5 mL of 1,4 dioxane in a 25 mL round bottomed flask; AIBN (0.0023 g, 0.014 mmol) was added to it, followed by addition of HMNPPA (0.197 g, 0.7005 mmol). The mixture was degassed and allowed to stir overnight at 65 °C. Thereafter, the reaction was quenched by placing the flask inside a freezer, and the product was isolated by precipitation twice into cold ether and dried in vacuum. 1H NMR (CDCl3, 400 MHz) (Figure ): δ (ppm) 1.13 (br, CH(CH3)2 of NIPA), 1.62 (br, CHCH2 polymer back bone), 2.33 (br, CHCH2 polymer back bone), 3.99 (br, CH(CH3)2 of NIPA), 4.14 (br, CH2 of HMNPPA), 5.00 (br, benzylic CH2 of HMNPPA), 6.47 (br, NH of NIPA), 6.82 (br, Ar protons), 7.13 (br, Ar protons), 8.09 (br, Ar protons).

Synthesis of Biotin Acrylate (BA)

Biotin acrylate was also synthesized in a similar way to HMNPPA. d-Biotin (0.3 g, 1.23 mmol) and K2CO3 (0.34 g, 2.45 mmol) were put in a 50 mL two necked round bottomed flask, dissolved in 3 mL of dry DMF, and allowed to stir for 45 min at 70 °C under a nitrogen atmosphere. Then, 3-bromopropyl acrylate (0.237 g, 1.23 mmol) dissolved in 3 mL of dry DMF was slowly added to the reaction mixture and allowed to stir for 48 h in the same condition. The crude product was extracted with DCM and washed with ice cold water eight times. The combined DCM layer was dried over anhydrous MgSO4 and evaporated. A pure product (0.315 g, 74.9%) as an off white solid was found after recrystallization from ether. 1H NMR (DMSO-d6, 400 MHz) (Figure S3, Supporting Information): δ (ppm) 1.36–1.29 (m, 2H), 1.65–1.41 (m, 4H), 1.96–1.90 (m, 2H), 2.31–2.27 (t, 2H), 2.59–2.53 (dd, 1H), 2.83–2.79 (dd, 1H), 3.11–3.06 (m, 1H), 4.18–4.07 (m, 5H), 4.31–4.28 (t, 1H), 5.96–5.93 (d, 1H), 6.20–6.13 (m, 1H), 6.41–6.30 (t, 3H). 13C NMR (100 Hz, CDCl3) (Figure S4, Supporting Information): δ (ppm) 24.76, 27.97, 28.24, 28.36, 33.85, 40.53, 55.42, 60.18, 60.94, 61.14, 62.00, 128.26, 130.99, 163.65, 166.13, 173.59.

Synthesis of Poly(NIPA)-block-poly(HMNPPA)-block-poly(PEGMA-stat-BA) Copolymer (P2) (Scheme 3)

Freshly synthesized biotin acrylate (0.0776 g, 0.2177 mmol), poly(ethylene glycol) methyl ether acrylate (Mw = 480) (0.298 g, 0.622 mmol), and P1 (0.2 g, 0.0311 mmol) were dissolved in 5 mL of 1,4 dioxane in a 50 mL round bottomed flask. Then, AIBN (0.0015 g, 0.009 mmol) was added to it. The reaction mixture was degassed and allowed to stir for 12 h at 65 °C. After that, the reaction was quenched and P2 was isolated by precipitation twice into cold ether. 1H NMR (CDCl3, 400 MHz) (Figure ): δ (ppm) 1.13 (br, CH(CH3)2 of NIPA), 1.66 (br, CHCH2 polymer backbone), 2.34 (br, CHCH2 polymer backbone), 2.75 (br, CH2 of heterocycle in BA), 2.90 (br, CH2 of heterocycle in BA), 3.16 (br, CH of heterocycle in BA), 3.37 (br, terminal CH3 of PEGMA), 3.64 (br, CH2-CH2-O-PEG), 3.99 (br, CH(CH3)2 of NIPA), 4.15 (br, O-CH2-PEG, CH2 of HMNPPA), 4.32 (br, CH2 of BA), 4.5 (br, CH of heterocycle in BA), 5.00 (br, benzylic CH2 of HMNPPA), 6.42 (br, NH of NIPA), 6.84 (br, Ar protons), 7.13 (br, Ar protons), 8.11 (br, Ar protons).

Synthesis of Poly(NIPA)-b-poly(HMNPPACbl)-b-poly(PEGMA-stat-BA) Copolymer (P3) (Scheme 4)

P2 (0.15 g, 0.0155 mmol), Cbl (0.0331 g, 0.1088 mmol), and DMAP (0.0013 g, 0.0108 mmol) were added to a 50 mL two necked round bottomed flask and dissolved in 5 mL of dry DCM at 0 °C. After 30 min, DCC (0.0337 g, 0.1632 mmol) in 5 mL of dry DCM was slowly added for 5 min under a nitrogen atmosphere at 0 °C. The reaction mixture was allowed to stir initially at 0 °C for 1 h and then at room temperature for 12 h. The precipitate was filtered off and DCM was evaporated under vacuum. The product was furthered purified by dissolving the reaction mixture into water followed by centrifugation and freeze drying. 1H NMR (CDCl3, 400 MHz) (Figure ): δ (ppm) 1.15 (br, CH(CH3)2 of NIPA), 1.47 (br, CH2 of n butyl chain in BA), 1.66 (br, CHCH2 polymer backbone), 1.91 (br, NH of NIPA), 2.12 (br, CHCH2 polymer backbone), 2.35 (br, CHCH2 polymer backbone), 2.78 (br, CH2 of heterocycle in BA), 2.92 (br, CH2 of heterocycle in BA), 3.18 (br, CH of heterocycle in BA), (3.39 (br, terminal CH3 of PEGMA), 3.66 (br, CH2-CH2-O-), 4.01 (br, CH(CH3)2 of NIPA), 4.34 (br, CH2 of n propyl chain in BA), 4.53 (br, CH of heterocycle in BA), 5.47 (br, benzylic CH2 of ester linkage), 6.62 (br, Ar protons), 7.05 (br, Ar protons), 8.17 (br, Ar protons).

Methods

Polymer Characterization

1H NMR and 13C NMR were recorded using a Bruker DPX spectrometer operating at 400/600 and 100 MHz, respectively, at 25 °C in CDCl3/DMSO-d6 and were calibrated using TMS as the internal standard. The absolute molecular weight and polydispersity index (PDI) of the polymers were determined by a triple detector GPC (TDAmax, Malvern, U.K.) instrument using refractive index, differential pressure viscometry, and dual angle light scattering (λ = 670 nm, 90 and 7°) detectors, an Agilent 1200 model isocratic pump, and THF as eluent with a flow rate of 1 mL min–1. The light scattering detectors were calibrated using narrow disperse polystyrene standards using. dn/dc values of the polymers were determined by using a differential refractometer (WGE DrBures, Germany).

Size Determination

Dynamic light scattering (DLS) measurements were performed for the determination of hydrodynamic size and size distribution using Malvern Zetasizer Nano equipment with a temperature-controlled sample chamber by employing a 4.0 mW He–Ne laser operated at 632.8 nm. Analysis was performed at an angle of 173°. The raw data was processed by instrumental software to obtain the hydrodynamic diameter (Dh) and size distribution in terms of polydispersity index (PDI). Transmission electron microscopy (TEM) measurements were performed for structural analysis of polymeric aggregates using a JEOL model JEM 2100 transmission electron microscope at an operating voltage 80 kV.

Absorbance and Fluorescence Measurements

The UV–vis absorbance was recorded using a Shimadzu (model number, UV-2450) spectrophotometer. Steady-state fluorescence spectra were collected using a Hitachi (model no. F-7000) and Jobin Yvon-Spex Fluorolog-3 spectrofluorimeter using a quartz cuvette of 1 cm path length. HPLC measurements of the UV-irradiated P3 were carried out using acetonitrile as mobile phase at a flow rate of 1 mL/min (detection: UV 250 nm).

Determination of Critical Aggregation Concentration

Critical aggregation concentration of the block copolymer P3 was determined by using Nile Red as fluorescence probe.[44] A definite amount (5 μL) of stock solution of Nile Red in ethanol (1 mg/mL, 0.0031 mM) was added in different vials, which was followed by evaporation of the solvent. Varying amounts of polymer stock solution (0–0.07 mg/mL) in ethanol were added to each of these vials and the solvent was again evaporated. Thereafter, 1 mL of PBS buffer solution was added to each of these vials, sonicated for 5 min, and the mixtures were allowed to stir overnight at room temperature. The emission spectra of the resulting solutions were measured at an excitation wavelength of 550 nm while emission was recorded from 570 to 750 nm at 37 °C. Emission intensity at 626 nm was plotted against polymer concentration to determine the critical aggregation concentration.

Preparation of Drug-Loaded Micelle (P3–DOX)

Anticancer drug doxorubicin was loaded into the polymeric micelles by a simple dialysis method. To a solution of 5 mg of P3 in 1 mL of THF, 2 mg of DOX·HCl with 2 equiv of Et3N was added and allowed to stir for 1 h. Then, the solution was slowly added to PBS buffer (10 mL) and kept overnight in the dark under stirring. The solution was then dialyzed against buffer through a 12 000 Da molecular weight cut-off membrane for 12 h to eliminate free drug and THF. In the meantime, the dialysis medium was changed four times. The drug-loaded micelle solution was diluted to make the solution concentration of polymer ∼0.3 mg/mL. The drug loading capacity (DLC) and drug encapsulation efficiency (DEE) were calculated by the following equations.

Phototriggered Destabilization of P3 Micelles with and without DOX

The size and size distribution of P3 micelles with and without DOX were determined by DLS measurement. Transmission electron microscopy measurements were also performed for P3 micelles with and without DOX and the obtained images were compared with the TEM image of DOX-loaded P3 micelles (P3–DOX) after 1 h of photoirradiation. The used light sources for photoirradiation were UV light (≥365 nm, 125 mW cm–2 medium-pressure Hg lamp) and laser (=405 nm and 110 mW cm–2).

In Vitro Drug Release Study

In vitro drug release was studied in PBS buffer (pH 7.4) at 37 °C. Micellar solutions of P3 prodrug with and without DOX were prepared at a concentration of ∼0.3 mg/mL (10 mL) for the investigation of phototriggered drug release. After a certain time interval of UV light (λ ≥ 365 nm) or laser (λ = 405 nm) irradiation, the aliquot (1 mL) was taken out and analyzed by UV spectroscopy and fluorescence emission spectroscopy. For further confirmation of drug release from P3 nanoparticles, we performed HPLC. P3 (10 mg) was dissolved in acetonitrile (50 mL) and degassed with nitrogen. Then, half of the solution was irradiated under UV light (λ ≥ 365 nm). Aliquots (1 mL each) were taken out after certain time intervals and were analyzed by HPLC.

MTT Assay

The human triple negative breast cancer cell line MDA MB 231 and normal immortalized human dermal fibroblast cell line (hFB) were employed for the determination of cytotoxicity of P2, P3, and P3–DOX (with and without UV radiation) along with free DOX using MTT assay. Both cell lines were seeded in 96 well plates at a concentration of 1 × 106 cells per well with 100 μL of Dulbecco’s modified Eagle medium (DMEM) with 10% fetal bovine serum (FBS). The cells were then allowed to adhere to the surface of the wells in a 5% CO2 atmosphere and at 37 °C till the cells reached 70% confluency in 24 h. Then, the medium was replaced with the same volume of DMEM with different concentrations of P2, P3, and P3–DOX (in duplicate). After treatment with different formulations of the above-mentioned compounds, the cells were allowed to grow for another 24 h. To observe the UV responsive effect on DOX-loaded P3 on the cancerous cells, one of the two P3–DOX-treated MDA MB 231 cells were UV radiated at ≥365 nm for 15 min and placed back in the same cell culture environment. After elapse of a predefined time, the wells were treated with 100 μL of MTT solution and were allowed to incubate for 4 h at 5% CO2 and 37 °C. The formazon crystals formed after the MTT treatment were dissolved with dimethyl sulfoxide (DMSO). The used medium after MTT treatment was replaced by 100 μL of DMSO. The absorbance was measured by a BioRad iMARK microplate reader at 595 nm. The cell viability of the samples was appraised as (Asample/Acontrol) × 100.

Cell Uptake Study

For the cellular uptake study, MDA MB 231 cells were cultured in DMEM at 37 °C in a humidified atmosphere of 5% CO2, and the uptake was monitored under confocal microscopy (Olympus Fluoview FU1000). For microscopic study, cells were seeded at 0.5 × 105 cells per well and allowed to adhere to the surface of the well overnight. The cells were treated with free DOX and P3–DOX at time intervals of 1, 2, and 4 h. Cells were fixed with 4% paraformaldehyde for 30 min. Then, the fixed cells were stained with DAPI (4′,6-diamidino-2-phenylpndole, dihydrochloride) for 5 min to observe the nuclei of the cells. The cells were mounted and observed under a confocal microscope. DOX was similarly monitored under a confocal microscope.