Zeolitic Imidazole Framework (ZIF) Nanospheres ... - ACS Publications

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Molecular Pharmaceutics

Zeolitic imidazole framework (ZIF) nanospheres for easy encapsulation and controlled release of an anticancer drug doxorubicin under different external stimuli: A way towards smart drug delivery system Chandan Adhikari, Anupam Das and Anjan Chakraborty*

Department of Chemistry Indian Institute of Technology Indore IET, M-Block, Email: [email protected] Fax: + 91-731-2431482

ACS Paragon Plus Environment

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Abstract: The conventional drug delivery systems made from organic or inorganic based materials suffer from some problems associated with uncontrolled drug release, biocompatibility, cytotoxicity etc. To overcome these problems zeolitic imidazole framework (ZIF) like hybrid materials can be one of the solutions. Here, we report a very easy and successful encapsulation of an anticancer drug DOX inside two ZIFs namely ZIF-7 and ZIF-8 which are little explored as drug delivery systems and studied the controlled release of the drug from these two ZIFs under external stimuli like change in pH and upon contact with biomimetic systems. Experimental results demonstrate that ZIF-7 remains intact upon change in pH from physiological condition to acidic condition while ZIF-8 successfully releases drug under acidic condition. Interestingly, both the ZIFs are excellent for drug release when come in contact with micelles or liposomes. In case of ZIF-8, the drug delivery can be controlled for 3 hours whereas its analogue ZIF-7 delivers the drug for a time span of 10 hours. We explained the reluctance of ZIF-7 towards drug release in terms of rigidity. So; this study highlights that by using different ZIFs and liposomes the drug release rate can easily be modulated which implies a lot of possibility for ZIFs as a good drug delivery system. The study over all shows a novel strategy for easy drug encapsulation and its release in a controlled manner which will help future development of the drug delivery system.

Key words: Doxorubicin, ZIFs, drug delivery, liposomes, controlled release


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Introduction: To bring a drug into the market from its synthesis, it takes almost 15 years with cost of more than $500 million.1 But most of the drugs suffer from high side effects, poor adsorption, poor solubility, high drug dosing, minimum efficiency, uncontrolled, nonspecific delivery with high cytotoxicity, which limit their uses.2 When it comes for the chemotheraptics drugs, the concern should be more because most of the anticancer drugs are highly toxic to the healthy cells and damage the healthy cells along with the cancer cells which give lots of side effects to the patients. These problems can be overcome and toxicity can be reduced if the drugs are delivered through a vehicle that delivers the drugs precisely on demand. To achieve the precise and targeted delivery of the drugs, we need a drug delivery system (DDS) that delivers the drugs to our target cells without affecting the healthy cells with a controlled manner.3,4 There are different types of drug delivery systems prepared both from organic molecules and inorganic materials in the past few decades. Organic based drug delivery system comprised of polymer, dendrimer, amino acid, protein, peptide, nucleic acid, and polysaccharide has attracted scientists over several years due to their biocompatibility and biodegradability.5,6 Due to the different interesting properties including controllable size and shape, ability to conjugate with different types of molecules, biocompatibility and easy functionalization, recently inorganic materials become great interest for biomedical application.7,8 Although both organic based DDS and inorganic based DDS have been used for a long time, both the systems possess some problems associated with the drug loading and controlled release and thus let us for the search of a new kind of delivery system which will be free from these difficulties.9 Metal organic framework (MOF), which has advantages of both organic as well as inorganic DDS can be one of the solution for this purpose.15


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MOFs are composed of metals which are connected by organic linkers.10 Organic linkers can be imidazolates, amines, pyridyl, phenolates, phosphonates, polycarboxilates, whereas the metals can be nontoxic Fe, Zn, Ca, Mg etc.11, 47-51. Broad range of porosity, (0.4 to 6 nm), high surface area, tunable shape, size and easy functionalization make MOFs very useful in different field of science including catalysis, gas storage, separation, adsorption, gas sensing, biomedical imaging, solar harvesting systems etc.12-14 One of the recent and promising application of MOFs is their application in biomedical sciences. Due to high drug loading capacity and ability to protect the drug from leakage, MOFs show a great potential for its application in drug delivery.16 Other than these, the microenvironment of MOFs which are either hydrophilic or hydrophobic gives them ability to encapsulate both lipophilic as well as lipophobic drugs.17 Different types of forces play an important role for drug loading and its release. These forces can be Van Der Waals force, hydrogen bonds, coordination bond, pi-pi interactions, electrostatic interaction etc. Out of these all, electrostatic interaction plays the major role in drug loading and its release.

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Although

several studies have already been done to show the great potentiality of MOFs based DDS; still further research is required to optimize this DDS. The previously reported MOFs DDS have problems associated with the size, stability, drug loading, biocompatibility, drug release etc. 19-21 hence; more studies are required to optimize this MOFs DDS. In this paper, we would like to report that zeolitic imidazole frameworks (ZIFs) are brilliant systems for the controlled release of drug molecules. ZIFs are one kind of metal organic frameworks composed of zinc as metal ion and imidazole or its derivatives as organic linkers.22 Due to some interesting properties (high porosity, high surface area, exceptional thermal and chemical stability), from the very beginning ZIFs are very attracting materials to both chemist as well as biologist.23 Although significant amount of works have been reported on ZIFs for


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different applications (gas storage, gas separation, catalysis, sensing etc.) ,24-26 only few reports are there, where ZIFs can be used as drug delivery systems.27-28 Here, we encapsulated doxorubicin (DOX) in two ZIFs namely ZIF-7 and ZIF-8 and studied release of the drug in a controlled manner by using different triggering agents. We have chosen ZIF-8 because this ZIF has already been explored for encapsulation and release study of an anionic drug molecule and its biocompatibility has also been studied.37 So; it is desirable to explore the possibility of encapsulation of a cationic drug in ZIFs and its release in presence of external stimuli. The reason behind the selection of ZIF-7 was that ZIF-7 is biocompatible and an analog of ZIF-8. Therefore; we wanted to know whether it is able to deliver Dox or not and we successfully have shown the encapsulation and release of Dox from it. DOX, which is also known as adriyamycin is one kind of anthracyclin antibiotic extensively used for the treatment of solid tumors, breast, prostate, uterus, ovary, stomach, and liver tumors etc.29 DOX acts through DNA intercalation mechanism and it inhibits topoisomerase II.30Although DOX has been used for a long time but due to its severe side effects on healthy tissues of brain, liver, heart and kidney limits its dosage and frequency by which it is administered to treat cancer cells.31 To overcome these problems we need to develop an efficient and targeted drug delivery system. In this paper, we demonstrated controlled release of DOX from ZIFs by using different stimuli as triggering agents. There are various external stimuli like pH, light, temperature, magnetic field, redox potential, electric field, ultrasound etc.32, 33 Among all the external stimuli pH gradients have been used for a long time to design stimuli responsive DDS.34 The reason behind use of pH is that the disease cells are more acidic than normal cells and the pH of the cancer cells vary from 4 to 6. 35, 36

Along with pH, we are also keen to understand how the drug delivery system responds upon contact with cell membrane like environment.38 For this purpose, we used SDS micelles and


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negatively charged DMPC-DMPG liposomes (9:1) to show the controlled release of the drug. To the best of our knowledge, this is the first report where biomimetic systems have been used to know the behavior of ZIF-DDS. The novelty of the present manuscript lies in its very simple and easy encapsulation of Dox with high loading inside ZIFs and the controlled release over a period of 3-10 hours under external stimuli. This is the first report where the effect of biomimetic membrane (liposomes) on the drug loaded ZIFs has been investigated thoroughly and its consequences has been explained. We also compared the release of Dox from two ZIFs and we explained the differences in drug release from this two ZIFs based on their stability. Although more research is needed in this field, we hope that this study will help further for the development of smart drug delivery system.

Experimental Section Materials:

DOX,

2-methyl

imidazole,

benzimidazole,

zinc

nitrate

hexahydrate,

dimethylformamide, methanol, sodium dodecyl sulfate (SDS), 1, 2-dimyristoyl-sn-glycero-3phosphocholine (DMPC), 1,2-Dimyristoyl-sn-glycero-3-phospho-rac-(1-glycerol)sodium salt (DMPG), Sodium dihydrogen phosphate, disodium hydrogen phosphate were purchased from Sigma-Aldrich. All the reagents were used without further purification. Preparation of drug solution: Drug solution was prepared by dissolving required amount of DOX in desired amount of methanol. Synthesis of ZIF-8: We prepared ZIF-7 and ZIF-8 using a previously reported procedure with some modifications.39,40 Briefly, for ZIF-8, 600 mg of zinc nitrate hexahydrate dissolved in 10


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ml of methanol in a scintillation vial and in another vial 1.3 gm. of 2-methyl imidazole was dissolved in 10 ml of methanol. Now 2-methyl imidazole solution was poured into zinc nitrate hexahydrate solution with stirring and it is stirred for 30 minutes. The product was collected by centrifuged and washed 5 times with methanol to completely get rid of the unreacted reactants. The product was dried in an oven at 100 °C for overnight to remove the solvents and kept at room temperature for further use. Synthesis of ZIF-7: To prepare ZIF-7, we took 3 gm. of zinc nitrate hexahydrate and 10 gm. of benzimidazole were taken in a 100 ml round bottom flask and 100 ml DMF was added to this mixture was stirred for 48 hours. The product was separated by centrifugation and washed with DMF for 5 times to completely get rid of the unreacted reactants. The product was dried at 100 °C for overnight to remove the organic solvents and kept at room temperature for further use. Procedure for drug encapsulation: DOX cannot be encapsulated inside ZIFs by simply insitu synthesis method. This is because of the fact that DOX remains mostly as positively charged at our experimental conditions and so are ZIFs. Thus to incorporate DOX inside ZIFs, a more stringent condition is required in post synthesis period of ZIFs. Here, we report a very easy and simple procedure for encapsulation of drugs inside ZIFs. Briefly, 2 ml of 0.5 mM drug solution was added to the 100 mg of ZIFs and was stirred for 48 hours. After that the drug loaded ZIFs were separated by centrifugation and washed several times by methanol and dried before doing further experiments. The method has its own benefit of drug encapsulation over the conventional methods. Most of the cases conventional methods follow drug loading during synthesis but it has one disadvantage that the framework and drug should have opposite charge for the successful drug encapsulation.37


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Preparation of liposomes: Liposomes were made by a previously reported protocol.41 Briefly, aqueous buffer solution of pH 7.4 was taken in a round bottom flask and the temperature was kept above the phase transition temperature of the lipids. Then required amount of lipids were dissolved in ethanol (less than 1% V/V) and the ethanolic lipid solutions were rapidly injected into the buffer solution above the phase transition temperature. Instrumentation: We characterized ZIFs and DOX loaded ZIFs by powder X-ray diffraction, field emission scanning electron microscopy (FE-SEM) and infrared (IR) spectroscopy. Powder X-ray diffraction (PXRD) was done in an Automated Multipurpose X-ray Diffractometer from Rigaku SmartLab, X-ray generator: A 3 kW sealed tube x-ray generator (Max. voltage 60kV, Max. current 50mA, with Cu target). FE-SEM study was conducted in ZEISS Supra55 field emission scanning electron microscopy. Steady state absorption spectra were recorded on a Varian UV-vis spectrometer (Model: Cary 100). Thermogravimetric analysis (TGA) was conducted in a TGA instrument from Mettler Toledo, Switzerland, Model: TGA/DSC1. The drug released was monitored by steady state fluorescence spectroscopy using a Fluoromax-4p spectrofluorimeter from Horiba Jobin Yvon (Model: FM-100). The samples were excited at 480 nm. For the time resolved studies, we used a picosecond time correlated single photon counting (TCSPC) system from IBH (Model: Fluorocube-01-NL). The experimental setup for TCSPC has been described elsewhere.46 The samples were excited at 480 nm using a picosecond diode laser and the decays were collected at 600 nm. The repetition rate was 5 MHz. The signals were collected at magic angle (54.70°) polarization using a photomultiplier tube (TBX-07C) as detector. The FWHM of the decay function was around 140 ps. IR spectra were taken in a Fourier Transform Infrared Spectrometer (FT-IR), Tensor 27, BRUKER.


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Results and discussion We first characterized our nano-ZIFs by scanning electron microscopy and the images (Figure 1) exactly match with the earlier literature reports.39, 40

Figure 1: SEM images of (a) ZIF-7 (b) ZIF-8 nanospheres

UV and IR results: After characterization, we move to our main objective which is to use these ZIFs as Drug delivery system. The first criterion of a good DDS is their ability to encapsulate the drug molecules inside it.37 Encapsulation can be done during synthesis or after synthesis. As we already mentioned in the earlier section that ZIFs framework are positively charged, so; it is easy to encapsulate a negatively charged drug than a positively charged drug as reported earlier.37 But here, we successfully incorporated DOX, a well celebrated drug, which most likely remains as positively charged in the experimental condition. To the best of our knowledge, this is the first report where DOX is incorporated inside ZIF-7. Figure 2(a) and 2(b) represent the absorption spectra for DOX in aqueous solution and after incorporation in ZIF-8 and ZIF-7 respectively. It is revealed from Figure (2) that after encapsulation of drug molecules inside ZIFs absorptions


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becomes halved. We estimated the relative percentage of drug encapsulation inside ZIFs and it was found that 40% and 52% drug was incorporated inside ZIF-7 and ZIF-8 respectively. The phenomenon can be explained based on the bigger cavity size of ZIF-7 than the ZIF-8. 0.9 1.2

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0.8

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Figure 2: Absorption spectra for (a) DOX and ZIF-8 encapsulated DOX (b) DOX and ZIF-7 encapsulated DOX

We further confirmed encapsulation of DOX inside the ZIFs by IR spectra. The representative IR spectra of ZIF-8 and drug encapsulated ZIF-8 are shown in Figure 3 (a, b). The peak at 1142 cm1, 1304 cm-1, 1381 cm-1, 1624 cm-1, 1720 cm-1 , cm-1, 2360 cm-1, and 3437 cm-1 corresponds to N-H, of NH3+, C-O, C-O of ether group, C=C, C=O, O-H (carboxylic acid), and O-H of DOX respectively. The peak at 2933 cm-1 arises due to N-H group of imidazole of ZIF-8 framework.

TGA and PXRD characterization of ZIF and DOX encapsulated ZIFs: TGA was performed to gain the information about thermal stability of the nano-ZIFs before and after drug encapsulation. Figure 3c shows that nano-ZIFs i.e. ZIF-8 and ZIF-7 are thermally stable up to


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460 °C and 540 °C respectively. The more thermal stability of ZIF-7 indicates that it is more rigid than ZIF-8. Drug encapsulated nano-ZIFs; ZIF-7-DOX and ZIF-8-DOX are thermally stable up to 640 °C and 500 °C respectively. Another observation from the TGA curve is that drug encapsulated ZIFs are thermally more stable than blank ZIFs. As the cavity of the ZIFs frameworks are filled by drug molecules, so they become more rigid and become more thermally stable. A sharp weight loss step was observed for all the cases beyond 600 °C and indicates almost 70 % weight loss for all the nano-ZIFs. This weight loss indicates thermal decomposition of the nano-ZIFs framework in that temperature range. Figure 3(d) shows the PXRD patterns of ZIF-7, ZIF-8, and DOX encapsulated ZIF-8 and DOX encapsulated ZIF-7. PXRD pattern of pure ZIF-7 and ZIF-8 match with the earlier literature reports.39, 40 The sharp peak of PXRD pattern indicates narrow size distribution of the synthesized ZIFs.


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(a)

(b)

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ZIF-8@DOX

Transmittance [%]

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O-H C=C

N-H

C-O

O-H C=O

N-H Salt

C-O

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80

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ZIF-8 ZIF-8-DOX ZIF-7 ZIF-7-DOX

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ZIF-8-DOX

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0

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Figure 3: IR spectra of (a) ZIF-8 and DOX encapsulated ZIF-8 (b) expanded spectra of ZIF-8@DOX.(c) TGA curve for blank nano-ZIFs and DOX encapsulated nano-ZIFs. (d) PXRD patterns of ZIF-7, ZIF-8, ZIF-8@DOX and ZIF-7@DOX.


12

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Therefore, it is clear from IR, UV, TGA and PXRD data that drug is incorporated inside the ZIFs framework. It is noteworthy that although ZIF-8 has been used as a DDS for camptothecin and other drug molecules as reported earlier,

43-45

there are no such reports where drug delivery has

been shown from ZIFs framework in presence of biomimetic systems like liposomes and micelle. Another important point which should be mentioned that this is the first time ZIF-7 is used as drug delivery system for anticancer drug like DOX. In the due course of manuscript we will demonstrate that release of the drug molecules can be controlled in both the cases under suitable external stimuli. Drug release under pH stimuli from ZIF-8: We start our discussion with the release of drug from ZIF-8 using pH stimuli. We monitored the drug release at pH 7.4, 6, 5 and 4 respectively. This range of pH will be able to unravel how the drug release is affected while going from physiological pH to little acidic pH. The cancer cells are little bit acidic (pH varies from 4 to 6); thus an ideal DDS should release drug molecule at this pH range. It is reported

37

that the ZIF-8

is normally stable at neutral pH while the framework gets dissociated in acidic pH. The reason behind this dissociation is detachment of the coordination between metal ion and ligands in this range of pH. This gives an opportunity to release the drug at this acidic pH. It is revealed from Figure 4 that there is a continuous increase in the fluorescence intensity at 590 nm as the time increases. This gradual increase in intensity implies the release of drug molecules from ZIF-8 framework. As expected the release was very less at pH 7.4 as ZIF-8 is stable at this pH and there is not significant increment in the florescence intensity. This observation is highly important with respect to its use because as the pH of the healthy cells is around 7.4, so; the drug will remain intact inside ZIF framework and the healthy cells will be little affected as the drug release is very less in this pH range.


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Figure 4: The emission spectra of DOX in phosphate buffer at different pH (a) pH 7.4 (b) pH 6 (c) pH 5 at different time varying from 0-2.5 hours. (d) Comparison of rate of drug release at different pH. In the inset it is shown that there is no drug release from ZIF-7 upon change in pH.


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Scheme 1: Encapsulation of DOX inside ZIFs framework and (1) its release at acidic pH due to dissociation of coordination between metal ion and ligands (2) release of drug by liposome/micelle due to the binding of DOX with micelle/liposomes

As ZIF-8 is dissociated at acidic pH;

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it is expected that the drug release should be more

pronounced in this range of pH as compared to that in physiological pH. It is evident from the


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Figure 4(b) that the fluorescence intensity increases almost four times from its initial value within a time span of 2.5 hours which means drug has been released faster than the neutral pH. Next we decrease the pH to 5 and did the same experiment. This time the release of the drug is even faster and the Figure 4(c) reveals that there is almost six times increment of the intensity than its initial value. However, further decrease in pH did not score any change in rate of release. This result show that the release of the drug is controlled by the pH of the medium and easy drug release takes place at acidic pH while the drug can be intact within the ZIF framework at neutral pH. Thus ZIFs may be used as a potential drug delivery system and the drug release can easily be controlled by varying pH. Drug release using biomimetic membrane from ZIF-8: Although drug release takes place at lower pH and the release rate can be controlled by varying pH but as mentioned earlier, we would like to see the possibility of drug release from ZIFs upon contact with biomimetic membrane. Keeping this in mind, we used SDS micelle, DMPC and DMPC-DMPG (9:1) liposome which represent cell membrane like environment. The CMC of SDS is 8 mM, so; we took three solutions of SDS having concentration of below CMC (3 mM), at CMC (8 mM) and above CMC (30 mM). It is revealed from the Figure 5(a) and 5(b) that there is almost negligible increment in the florescence intensity at below CMC and at CMC of SDS solution. This implies that there is almost no drug release for the first two concentration of SDS. The little increment is ascribed to the electrostatic interaction between SDS monomer and positively charged DOX. However, the drug release significantly increased when the SDS concentration was kept above CMC and this time intensity increased by 1.5 times within a time span of 1.5 hours. It is clear from the Figure 5(d) that much more amount of drugs is released in case of 30 mM of SDS whereas the release of drug is very slow at CMC. There are two reasons behind the choice of this


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surfactant. The first is that SDS micelle creates little acidic environment 42 which helps to release the drug. The second and most important reason is that, SDS above their critical micelle concentration (CMC) can bind with DOX which helps to drug come out from the ZIF frameworks.

(a)

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Figure 5: The normalized emission spectra of DOX in SDS at different concentrations (a) 3 mM (b) 8 mM (c) 30 mM at different time varying from 0-1.5 hours and (d) comparison of the rate of drug release at 8 mM and 30 mM


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Next we studied liposomes mediated drug release and for this purpose first we chose a zwitterionic liposome composed of DMPC lipid but in this case there is almost no drug release (only 1.1 times increment in the fluorescence intensity, Figure 6(a). Then we used a mixed liposome composed of DMPC and DMPG lipids (9:1). Figure 6(b) shows that there is almost 1.5 times increment in the florescence intensity in presence of liposome implying that the drug has been released from ZIF frameworks. From Figure 6(c) it is clear that the more number of drug molecules are released in case of DMPC/DMPG liposome while the rate is very slow in case of DMPC liposome. The probable reason could be that DOX remains as positively charged, therefore; the negatively charged liposome is able to bind with it due to electrostatic interactions which results in release of drugs from ZIF frame works. But in case of zwitterionic liposome, this kind of interaction is not possible; hence there is no significant drug release. In order to establish this fact, we studied interaction of DOX with bare liposomes and SDS micelles. The results are shown in supporting information. It is clear that DOX binds with SDS micelles and liposomes through electrostatic interaction. We estimated lifetime of DOX in SDS micelles and liposomes (Figure S1, Table S1 and S2, in supporting information). It is revealed that DOX exhibits a single exponential decay at 600 nm in aqueous solution with a time components around 1.0 ns. In micelles and in liposomes the lifetime becomes bi-exponential consisting of a longer component around 3.1 ns. Therefore, it is clear that micelles and negatively charged liposomes are able to bind with DOX through electrostatic interaction and this is the driving force for the release of drug from ZIF-8.


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Figure 6: The normalized emission spectra of DOX in liposomes (a) DMPC liposomes (b) DMPG/DMPC liposomes and (c) comparison of the rate of drug release at DMPC liposomes and DMPG/DMPC liposomes.


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Drug release under pH stimuli and biomimetic membrane from ZIF-7 and comparison with ZIF-8: After successful drug encapsulation and release from ZIF-8, we moved to investigate release of DOX from ZIF-7. In this case, we did not observe any drug release upon change in pH from 7.4 to 4.0. This fact implies that unlike ZIF-8, ZIF-7 does not dissociate at lower pH and thus prevent the drug to come out from the framework. Next we used micelle for the drug release as we know from our previous result that ZIF-8 is susceptible to drug release in presence of SDS micelle. Figure 7(a) reveals that there is almost 1.5 times increment in the fluorescence intensity within time span of 3 hours which clearly signify that a significant amount of drug is released by SDS. Although in presence of DMPC/DMPG (9:1) liposomes, we observe a drug release as revealed by Figure 7(b); however, the rate of release is much slower compared to ZIF-8. The phenomenon further supports the conjecture that ZIF-8 is more susceptible towards drug release as compared to ZIF-7 under external stimuli. The obvious reason behind the slower drug release from ZIF-7 is the more stable framework of ZIF-7 than that of ZIF-8. Due to the less stability of ZIF-8 framework it is easier for SDS and liposome to release the drug from its framework whereas the more stable ZIF-7 took more time to release the drug.


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1.8

(a)

SDS 30 mM (0-150 min)



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DMPG/DMPC (1:9 w/w) (0-600 min)

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Ratio of intensity (a.u.)

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(c)

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ZIF-7@dox by liposome ZIF-8@dox by liposome

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Figure 7: The emission spectra of DOX in (a) SDS (b) DMPG/DMPC liposomes and (c) comparison of the rate of drug release by liposome in two different ZIFs (ZIF-7 and ZIF-8)


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Summary: In summary, we successfully encapsulate a widely used anticancer drug DOX in ZIF-7 and ZIF-8 through a very simple procedure with high encapsulation efficiency. We find that the ZIF-8 is excellent for release of DOX in a controlled manner under pH change whereas ZIF-7 is not sensitive towards change in pH. We explain this phenomenon by the fact that ZIF-7 is more stable at acidic pH than ZIF-8 framework. Interestingly, both the ZIFs seem to be excellent in releasing drug when come in contact with lipid membranes and micelles. This study will help further in the development of drug delivery system based on nanomaterials.


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Author information Dr. Anjan Chakraborty Department of Chemistry Indian Institute of Technology Indore IET, M-Block, Email: [email protected] Fax: +91(0)731-2366382 Tel: +91(0)731-2438706

Acknowledgement: The authors would like to thank SIC, IIT Indore for providing the facility and infrastructure. We are grateful to the Powder X-ray diffraction (P-XRD), Facility equipped at Sophisticated Instrument Centre (SIC), IIT Indore. CA and AD thank IIT Indore for providing fellowship. References: 1. Bolten, B. M.; DeGregorio, T. Trends in development cycles. Nat. Rev. Drug Discovery 2002, 1, 335-336. 2. Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Polymeric systems for controlled drug release. Chem. Rev. 1999, 99, 3181-3198. 3. Duncan, R. The dawning era of polymer therapeutics. Nat. Rev. Drug Discovery, 2003, 2, 347-360. 4. Balmert, S. C.; Little, S. R. Biomimetic Delivery with Microand Nanoparticles. Adv. Mater. 2012, 24, 3757–3778.


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185x215mm (300 x 300 DPI)


Zeolitic Imidazole Framework (ZIF) Nanospheres ... - ACS Publications

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