INTRODUCTION:

Enzymes are the bio-molecules present in the body performing a variety of functions. Enzymes consist of the non-protein part (co-factor), which is responsible for binding to a substrate and catalyze the reaction. An enzyme without co-factor (apo-enzyme) is a protein, whereas an enzyme with all necessary components (holoenzyme). Enzymes hasten the rate of a biochemical reaction and consists of active sites on their surface to which substrate binds. The main functions of enzymes viz., bio-catalysts, synthesizing the molecules, etc.

Some of the enzymes are specific in particular organs (E.g., Renin in kidneys, rennin, pepsin and chymotrypsin from Gut, Ptyalin in mouth oral cavity and others having significant functions are carbonic anhydrase, hexokinase, glycogen phosphorylase and protease in a certain type of specialised cells.

Proteases regulate most of the biological and pathological processes by proteolysis². Proteolysis, a type of irreversible regulatory mechanism, is accountable for discerning cleavage of certain substrates³. The levels of proteases can act as biomarkers in the finding and prediction of tumors⁴. Proteases are involved in controlling various pathways viz., DNA replication and transcription, cell proliferation and differentiation, cell mobilization, inflammation, immunity, necrosis, apoptosis, hemostasis and blood coagulation⁵. Therefore, abnormalities in proteolytic actions result in cancer, cardiovascular and neurodegenerative disorders.

Another class of enzymes called matrix metalloproteinases (MMPs) are mainly involved in the cancer initiation and act as biomarkers and therapeutic targets. Recent emerging trends deal with the ideal expansion of MMP receptive drug delivery thereby targeting the tumour cells⁶. It was established that the diseases befalling by the dysfunctioning of enzymes laid the foundation for drug development to act in such sites or conditions of the physiological status.

Now a day, nanoscience is an emerging technology including EADDS finds its significance in drug delivery to various sites to the enzymes present in that region. The designing of the systems as small-sized entities like nanoparticles (NP) have advantages that they can easily diffuse into the membrane through the intracellular pores of very small in size.

It was a known fact that certain type of drug molecules shows rather an enhanced action when their size gets reduced to the nanometer level. Hence the NP are described as the good carriers of drug delivery owing to their small size, shape and surface characteristics. It also offers the prolonged release of entrapped drug by the aid of an external or internal impact⁷.

Another type of well-known strategy is the prodrug mechanism, which is described as the inactive form of drug molecules. Upon the action of certain enzymes, chemical or environment stimuli, prodrugs transform to release the active drug in-vivo. With this approach, the features of the drug-like solubility, stability, permeability, and distribution can also be enhanced⁸. It includes the chemical conjugation of the drug with a suitable moiety, can be described as the prodrug⁹. Because of these significant aspects, the prodrug approach has been in practice in developing various novel formulations. Approximately 5-7% of currently approved drugs as a prodrug. The designing of prodrug should be such that the promoiety should get removed after the respective enzymatic action. It provides an idea that the prodrug gets transformed into its active form only when the enzyme of interest, having the promoiety as substrate, cleaves it. So, the drug will be released at a specific site where the enzyme is in overexpressed condition. There are so many difficulties arising that even though the anti-neoplastic drugs are much potent, they may also act on the normal cells and thus inhibiting the normal physiological process like cell division leading to serious adverse effects. Due to the drug resistance developed by the patients, it leads to the let-down of treatment. Such a kind of problem can be overcome by the aid of several approaches of which one of them is EADDS¹⁰.

Now the current article gives a clear idea of how the drug can be delivered to a particular type of organ or site based on the enzyme present at that respective site. The drug is supplied to the particular site, showing action on that site thus avoiding the unsolicited side effects on other types of tissues or organs. Before studying the EADDS, it is necessary to have a brief idea on rate-controlled drug delivery systems (RCDDS), it refers to the delivery of the drug to a particular site of which the rate can be determined by the various factors. The RCDDS can be classified as illustrated in chart 1.

Chart 1: Classification of rate-regulated drug delivery systems

MATERIAL AND METHODS:

Enzyme activated drug delivery system (EADDS):

In this article, the authors highlighted EADDS, which is a bio-chemical activated type of rate-controlled system. This type of drug delivery is utilized for the treatment of various ailments including cancer. However, the traditional system of cancer treatment called chemotherapy was in practice, it shows rapid toxicity to other types of cells because of its non-specificity. In ESDDS, the discharge of drugs from the system is activated by the enzymatic process¹¹. Here the drug is initially dissolved in a suitable medium to form the drug reservoir and can be enclosed physically as microspheres or by chemical means which includes the using of biopolymers like albumins or polypeptides to which the drug is bounded to their polymer chains. The chief procedure involved in the hydrolysis of biopolymers by the distinguishing type of enzyme present in the target site. E.g., Albumin microspheres of 5-Fluorouracil can be made targeted to the cancer cells based on the enzyme called protease present in the cancer cells, that act specifically on the proteins like albumin. In this system, the degradation of albumin microspheres occurs by the action of the protease enzyme.

Past successful attempts on EADDS:

The scientific report by Dzamukova et.al. 2015,¹²revealed the successful delivery of brilliant green (an anti-neoplastic and anti-septic agent) delivered into the human cells by using the halloysite nanotube (HNT) carriers of 50nm size. They have used dextrin end stoppers, which are physically adsorbed for regulating the discharge of brilliant green. The interpenetration of the carrier particles can be achieved and their uptake by the cells is reliant on upon the cellular growth rate and creation. Glycosyl hydrolase is the enzyme present in the cells that act on the system and causes the dextrin tube end stoppers to undergo decay to release the loaded-brilliant green to act upon the human lung carcinoma cells rather than on the hepatoma cells thus the hepatic damage can be evaded. In this study, they have gone through the assortment of two kinds of cells i.e., Adenocarcinomic human alveolar basal epithelial cells and human hepatoma cells (Hep3b) that are explored for the cellular uptake as a purpose as they display diverse rates of proliferation. These cells were cultured in a manner of increasing concentrations of halloysite formulation of DX-HNTs (25-100μg/1, 00,000 cells, BG-free) for incubation up to 24 h. The cells were analysed microscopically by using the enhanced dark-field (EDF) microscopy and was found characteristic uptake nature of A549 cells and Hep3b, such that A549 cells appeared as obviously evident aggregates, while Hep3b cells have random delivery. The results obtained from TEM images revealed that A549 is arbitrarily dispersed in lysosomes, defining the enzymatic breakdown of dextrin tube-end stoppers with an improved discharge. The results were drawn from various other investigations and prophesied that the dextrin stoppers substantially reduced the toxicity of the formulation towards Hep3b and an improved release at the A549 cells. Their study has proven that the discharge of brilliant green by using the dextrin-coated halloysite nanotubes (HNT) is one of the most appropriate methods for handling cancer.

Bernardos et al., 2010,¹³ designed a system with Doxorubicin, entailing of hydrocarbon molecules which are covalently attached to the silences. Doxorubicin release from mesoporous silica NP that are capped with saccharides, occurs by the enzyme called glycoside hydrolase. This system upon coming into the cancer cells comprising the enzyme glycoside hydrolase undergoes putrefaction to discharge the drug into the cells thus producing the effect.

Yildiz et al., 2018,¹⁴came with another work using Doxorubicin prepared by polymeric NP for the treatment of cancer. Here, the system gets activated by the enzyme called protease that acts on proteins. They have employed poly (lactic-co-glycolic acid)-b-poly-l-lysine and poly (lactic acid)-b-poly (ethylene glycol), among which the first polymer with Poly-l-lysine covalently adapted with near-IR 750 molecules. Doxorubicin delivery primed by the nanoprecipitation of copolymer blends were called as Theraneustic nanomedicines with a mean size of 60-80nm and was shown a controlled release of drug for 30 days. The discharge of NP to breast cancer cells was studied by fluorescence microscopy and was concluded that they were seemly as controlled release systems and contrast agents in the areas of imaging of cancer cells.

Sun et al., 2019,¹⁵ worked on the improvement of the anti-tumour competence of Gemcitabine as a prodrug by FAPα-mediated activation. Gemcitabine agonizes with low uptake by tumor cells and low competence. So, the 4-amino group of Gemcitabine was modified to form Z-GP- Gemcitabine that advances the specificity and cleavage via FAPα-enzyme activation in tumor environment. In contrast, the prodrug form was found to have an improved uptake of tumor cells and enhanced inhibition effect on both growths of the 4T1 tumor cells and pulmonary metastasis in mice bearing orthotopic type of 4T1 breast tumors along with a reduction of tumor-associated fibroblast (TAF) was observed during animal testing. Therefore, it was established as an anticipated tactic for the treatment of cancer treatment by the route of intravenous administration.

Phillips and Pombeiro 2016,¹⁶ described the transition metal-based prodrugs for anticancer drug delivery by taking Cisplatin as a model. The adverse effects of Cisplatin can be overcome by converting them into prodrug that gets activated and released by the differences in oxygen concentration or pH, by the action of overexpressed enzymes, by differences in metabolic rates, etc., through which the cancer cells can be easily differentiated from normal ones. By this technique, the pharmacological activity of the Cisplatin will be enhanced and becomes more inert.

Qing et al., 2018,¹⁷ provided the novel idea of MMP (Matrix Metallo Proteinases) responsive smart drug delivery and tumor targeting systems. These are the extracellular enzymes that become overexpressed in the neoplastic conditions. However, under normal physiologic conditions, MMPs get regulated by the tissue inhibitors of metalloproteinases and they will be in limited quantity. Three types of MMPs are described in which MMP-2, MMP-7, and MMP-9 are found in cancer conditions. This concept can be utilized in developing the MMP inhibitors for targeting as nanocarriers of size <200nm. The nanocarriers can be covalently linked to drug and coated with PEG (PEGylation). The mechanism is such that this system will reduce the apprehending of the Mono Nuclear Phagocytic (MNP) system, thereby promotes circulation and improved permeability and retention effect.

Paul et al., 2007,¹⁸ described the role of polymer hydrogel particles in the case of bio-responsive polymer hydro gel for prolonged discharge. As per the view, the major advantage is that the active ingredient was protected from premature degradation using polymeric carrier. In this study, they described the carriers as “smart” materials because of their nature of altering their physical possessions in rejoinder to the applied stimuli, involving enzyme activity. The main components he used in emerging the system are enzyme cleavable linkers and hydrogels which are chemically cross-linked and are sensitive to a disease state enzyme. Upon the enzymatic action, these gels undergo a macroscopic change and get disintegrate to release the drug molecules that are entangled in it. For this work, he used PEGA-800 (copolymers of polyethylene glycol and acrylamide, 800). These PEGA particles were then modified using diglycine or dialanine as ECL (Enzyme Cleavable Linkers). They were found to respond for three types of enzymes thermolysin, chymotrypsin, and elastase. Later, they were evaluated for size characteristics, swelling nature of the hydrogel in response to the enzymatic action, and release physiognomies upon hydrolysis of the hydrogel by the enzymes. The enzymatic response of the PEGA particles was determined by three complementary methods.

· Analysis of the hydrogel accessibility to a fluorescently labeled dextran marker by two-photon microscopy

· Analysis of cleaved peptide fragments by HPLC

· Optical microscopy for particle diameters

Rationale:

The basis in the development of EADDS is the activation of the delivery system by the physiological enzymes present at respective sites to which the drug is envisioned to be delivered. Usually, in certain disease conditions like cancer, intracellular enzymes become overexpressed to uphold the rapid cell proliferation and obtaining more nutrients for cellular growth. Such an altered cellular enzymatic expression can be made as the target for drug delivery by which the system gets activated at the respective site or organ. Abnormal functioning of enzymes leads to the severe diseases that laid the basis for drug development that act in such sites or conditions of the physiological status¹⁹.

Delivery carriers:

These are the substances used to deliver the drug, in which the drug molecule gets entrapped/linked to its polymer chains. Nanotubes of carbon molecules are the effectual carriers of the drug because they get easily internalized by mammalian cells and acts as the room for drugs until they reach the target cell. They are less preferable owing to its toxic effects²⁰.

Strategies:

Polymer type nanoparticles:

One of the methods for preparing the NP is by using the polymers of concerned physicochemical properties. The extensively used polymers as nanocarriers are polyethylene glycol (PEG), dextran, hydroxypropyl methyl acrylamide (HPMA)^21-23. Various types of polymers can be used for preparing the NP that offers a targeted drug delivery especially in the case of cancer conditions. They not only meant for site targeting but also provides an enhanced and retention type of effect. During the time of designing, there is a need to incorporate a molecule that can be identified and acted by the respective enzyme to satisfy enzyme-based drug release²⁴. Moreover, the polymer should have its properties, non-toxic, no influence on drug action and a chemical modification can be done²⁵. This technique of combining healing and diagnostic drugs within a single nanoscale “theranostic” offers significant potential for personalized nanomedicine to cancer patients²⁶. Theranostic type of NP employed in various conditions including cancer treatment, diabetes²⁷, neural disorders²⁸, cardiovascular²⁹, inflammatory or autoimmune disease³⁰ and pulmonary³¹ afflictions.

Over 10 years, several types of NP have been established for imaging and treating cancer³², based on organic and inorganic materials including quantum dots, carbon nanotubes, gold, silica, iron oxide, and polymers. NP approach for enzymatic drug delivery was shown in figure 1.

Fig 1: Nanoparticulate approach for enzymatic drug delivery

Liposomes:

Liposomes are the combination of phospholipids in water, by which many types of drugs can be delivered and offers the most efficient targeting of drugs to the particular site³³. The drugs need to contain both lipophilicity and hydrophilicity for effective absorption as well as dissolution in the biological membrane. It was conversant that the cell membranes of most of the organs covering the lipid layers and followed by the hydrophilic layers due to which the permeability of drugs get limited and accounting a lot of difficulties to permeate into the membrane. This can be easily overcome by drug delivery through liposomes in which both hydrophilic and lipophilic drugs can be incorporated and delivered³⁴. Due to their bio-compatibility and feasibility, liposomes are used in drug delivery³⁵.

Mesoporous Silica Nanoparticles:

They have a variety of applications in the delivery of drugs due to their large surface area, pore volume and high stability³⁶. An organo-silane precursor was used for their preparation and the drug is incorporated through the pores. Along with these, the enzyme responsive materials should also be present which include lipase responsive, hydrolase responsive, and protease responsive NP etc.³⁷

Another type of nanocarriers is clay nanotubes where the drug gets encumbered into the lumen of tubule and also provides surface modification³⁸. An example of clay nanotubes is halloysite which is an aluminosilicate clay tubule having its external diameter of 50–60nm. At a neutral pH, the surface of silicon dioxide has a negative charge and the aluminium oxide inner lumen has positive charge³⁹. It has good bio-compatibility. Another example of the polymer used for surface modification is PEG and thereby increase the aqueous solubility. Another type of polymer which is in the clinical study at present is HPMA (n- hydroxypropylmethacrylamide)⁴⁰

Microspheres:

Here the drug is initially dissolved in a suitable medium to form the drug reservoir and can be enclosed physically as microspheres or by chemical means which includes the using of biopolymers like albumins or polypeptides to which the drug is bounded to their polymer chains. The main process involved in the hydrolysis of biopolymers by the characteristic type of enzyme present in the target site. Here, is an example that albumin microspheres of 5-Fluorouracil can be made targeted drug delivery to the cancer cells based on the enzyme called protease present in the cancer cells, that act specifically on the proteins like albumin. In this system, the degradation of albumin microspheres occurs by the action of the protease enzyme⁴¹. Representation of the mechanism of EADDS is represented in figure 2.

Fig. 2. Representation of mechanism of Enzyme activated drug delivery system

Prodrug:

Fig. 3. Pro-drug approach for Enzyme activated DDS

Co-drug:

Conjugating the active moiety with those drugs that will inhibit the functioning of certain efflux transporters will improve the drug absorption into the target site or cell with overexpressed enzyme functioning. One of such examples is conjugating the anti-neoplastic drug-like Doxorubicin with the drugs quinidine, verapamil that inhibit the efflux transporters like p-gp or MRP will promote the cellular uptake of doxorubicin by temporary inactivation of efflux transporter.

Preparation:

Initially, the drug was loaded into the lumen of carrier nanotube with the help of vacuum suction technique⁴³. These drug-loaded carrier nanotubes were then coated with the polymer-like dextrin for achieving the enzymatic response and surface functionalization as well as clogging at the tube end. The drug may be loaded by mixing with the solvents like ethanol and this liquid state drug was further conditioned to take a dry powder form. The drugs can be loaded into carrier nanotube as concentrated solutions or by melting. The loaded nanotubes are dried and were kept for a long period, the discharge of drug occurs in around 10 h when exposed to water. Examples of the drugs that achieved prominent results through the EADDS are dexamethasone, gentamycin, tetracycline, ciprofloxacin, and brilliant green. The coating with polymeric material ceases the tube ends and controls the rate of drug release for prolonged period⁴⁴.

CONCLUSION:

A study on these works revealed that enzyme activated drug delivery offers the targeted release in the controlled manner. When compared to the uncoated tubes, those coated with the stoppers will delay the release of drugs and thus allows for enhanced delivery of the drug. This technique is very easy when compared to the covalent linkage of nanotubes. Hence, this type of system can have its importance in the treatment of various diseases especially cancer to which targeted drug delivery is required to prevent the unwanted side effects associated with traditional chemotherapy. It provides a clear idea that the drug gets released only when the enzyme of interest, having the respective substrate, cleaves it. So, the drug will be released at a specific site where the enzyme is in overexpressed condition.

ACKNOWLEDGMENT:

The authors are grateful to the college management for providing facilities for this work.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

REFERENCES:

1. Mura S, Nicolas J, Couvreur P. Stimuli-responsive nanocarriers for drug delivery. Nature Materials. 2013 Nov; 12(11): 991-1003.

2. Choi KY, Swierczewska M, Lee S, Chen X. Protease-activated drug development. Theranostics. 2012; 2(2):156.

3. Lopez-Otín C, Hunter T. The regulatory crosstalk between kinases and proteases in cancer. Nature Reviews Cancer. 2010 Apr; 10(4): 278.

4. Orlowski RZ, Kuhn DJ. Proteasome inhibitors in cancer therapy: lessons from the first decade. Clinical Cancer Research. 2008 Mar 15; 14(6):1649-57.

5. Lopez-Otín C, Bond JS. Proteases: multifunctional enzymes in life and disease. Journal of Biological Chemistry. 2008 Nov 7; 283(45): 30433-7.

6. Yao Q, Kou L, Tu Y, Zhu L. MMP-responsive ‘smart’ drug delivery and tumor targeting. Trends in Pharmacological Sciences. 2018 Aug 1; 39(8): 766-81.

7. Yildiz T, Gu R, Zauscher S, Betancourt T. Doxorubicin-loaded protease-activated near-infrared fluorescent polymeric nanoparticles for imaging and therapy of cancer. International Journal of Nanomedicine. 2018; 13: 6961.

8. Mahato R, Tai W, Cheng K. Prodrugs for improving tumor targetability and efficiency. Advanced Drug Delivery Reviews. 2011 Jul 18; 63(8): 659-70.

9. Sun J, Yang D, Cui SH, Zhang HT, Fu Y, Wang JC, Zhang Q. Enhanced anti-tumor efficiency of gemcitabine prodrug by FAPα-mediated activation. International Journal of Pharmaceutics. 2019 Mar 25; 559: 48-57.

10. Chien Y. Novel drug delivery systems. CRC Press; 1991 Oct 31.

11. Thornton PD, Mart RJ, Ulijn RV. Enzyme‐responsive polymer hydrogel particles for controlled release. Advanced Materials. 2007 May 7; 19(9): 1252-6.

12. Dzamukova MR, Naumenko EA, Lvov YM, Fakhrullin RF. Enzyme-activated intracellular drug delivery with tubule clay nanoformulation. Scientific Reports. 2015 May 15; 5: 10560.

13. Bernardos A, Mondragon L, Aznar E, Marcos MD, Martinez-Manez R, Sancenon F, Soto J, Barat JM, Perez-Paya E, Guillem C, Amoros P. Enzyme-responsive intracellular controlled release using nanometric silica mesoporous supports capped with “saccharides”. Acs Nano. 2010 Oct 19; 4(11): 6353-68.

14. Yildiz T, Gu R, Zauscher S, Betancourt T. Doxorubicin-loaded protease-activated near-infrared fluorescent polymeric nanoparticles for imaging and therapy of cancer. International Journal of Nanomedicine. 2018; 13: 6961.

15. Sun J, Yang D, Cui SH, Zhang HT, Fu Y, Wang JC, Zhang Q. Enhanced anti-tumor efficiency of gemcitabine prodrug by FAPα-mediated activation. International Journal of Pharmaceutics. 2019 Mar 25; 559: 48-57.

16. Phillips AM, Pombeiro AJ. Recent advances in organocatalytic enantioselective transfer hydrogenation. Organic & Biomolecular Chemistry. 2017; 15(11): 2307-40.

17. Yao Q, Kou L, Tu Y, Zhu L. MMP-responsive ‘smart’drug delivery and tumor targeting. Trends in Pharmacological Sciences. 2018 Aug 1; 39(8): 766-81.

18. Thornton PD, Mart RJ, Ulijn RV. Enzyme‐responsive polymer hydrogel particles for controlled release. Advanced Materials. 2007 May 7; 19(9): 1252-6.

19. Orlowski RZ, Kuhn DJ. Proteasome inhibitors in cancer therapy: lessons from the first decade. Clinical Cancer Research. 2008 Mar 15; 14(6):1649-57.

20. Kam NW, O'Connell M, Wisdom JA, Dai H. Carbon nanotubes as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction. Proceedings of the National Academy of Sciences. 2005 Aug 16; 102(33): 11600-5.

21. Patel K, Angelos S, Dichtel WR, Coskun A, Yang YW, Zink JI, Stoddart JF. Enzyme-responsive snap-top covered silica nanocontainers. Journal of the American Chemical Society. 2008 Feb 27; 130(8): 2382-3.

22. Ahad HA, Kumar CS, Kishore Kumar SC. Designing and evaluation of Diclofenac sodium sustained release matrix tablets using Hibiscus Rosa-Sinensis leaves mucilage. Int J of Pharm Sci Rev and Res. 2010; 1(2): 29-31.

23. Taurin S, Greish K. Enhanced vascular permeability in solid tumors: a promise for anticancer nanomedicine. In. Tight Junctions in Cancer Metastasis, Springer, Dordrecht. 2013: 81–118.

24. Colilla M, González B, Vallet-Regí M. Mesoporous silica nanoparticles for the design of smart delivery nanodevices. Biomaterials Science. 2013; 1(2): 114-34.

25. Wan Q, Xie L, Gao L, Wang Z, Nan X, Lei H, Long X, Chen ZY, He CY, Liu G, Liu X. Self-assembled magnetic theranostic nanoparticles for highly sensitive MRI of minicircle DNA delivery. Nanoscale. 2013; 5(2): 744-52.

26. Lammers T, Kiessling F, Hennink WE, Storm G. Nanotheranostics and image-guided drug delivery: current concepts and future directions. Molecular Pharmaceutics. 2010 Oct 6; 7(6): 1899-912.

27. Bolognesi ML, Gandini A, Prati F, Uliassi E. From companion diagnostics to theranostics: a new avenue for alzheimer’s disease? Miniperspective. Journal of Medicinal Chemistry. 2016 May 6; 59(17): 7759-70.

28. Aulić S, Bolognesi ML, Legname G. Small-molecule theranostic probes: a promising future in neurodegenerative diseases. International Journal of Cell Biology. 2013; 2013.

29. McCarthy JR. Multifunctional agents for concurrent imaging and therapy in cardiovascular disease. Advanced Drug Delivery Reviews. 2010 Aug 30; 62(11): 1023-30.

30. Singh AV, Khare M, Gade WN, Zamboni P. Theranostic implications of nanotechnology in multiple sclerosis: a future perspective. Autoimmune Diseases. 2012: 6; 1–12.

31. Howell M, Wang C, Mahmoud A, Hellermann G, Mohapatra SS, Mohapatra S. Dual-function theranostic nanoparticles for drug delivery and medical imaging contrast: perspectives and challenges for use in lung diseases. Drug Delivery and Translational Research. 2013 Aug 1; 3(4): 352-63.

32. Xie J, Lee S, Chen X. Nanoparticle-based theranostic agents. Advanced drug delivery reviews. 2010 Aug 30; 62(11): 1064-79.

33. Joo KI, Xiao L, Liu S, Liu Y, Lee CL, Conti PS, Wong MK, Li Z, Wang P. Crosslinked multilamellar liposomes for controlled delivery of anticancer drugs. Biomaterials. 2013 Apr 1;34(12): 3098-109.

34. Gabizon AA. Liposomal drug carrier systems in cancer chemotherapy: current status and future prospects. Journal of Drug Targeting. 2002 Jan 1; 10(7): 535-8.

35. May JP, Undzys E, Roy A, Li SD. Synthesis of a gemcitabine prodrug for remote loading into liposomes and improved therapeutic effect. Bioconjugate Chemistry. 2015 Dec 31; 27(1): 226-37.

36. Fu C, Liu T, Li L, Liu H, Chen D, Tang F. The absorption, distribution, excretion and toxicity of mesoporous silica nanoparticles in mice following different exposure routes. Biomaterials. 2013 Mar 1; 34(10): 2565-75.

37. Singh RK, Kim TH, Kim JJ, Lee EJ, Knowles JC, Kim HW. Mesoporous silica tubular nanocarriers for the delivery of therapeutic molecules. RSC Advances. 2013; 3(23): 8692-704.

38. Lvov Y, Aerov A, Fakhrullin R. Clay nanotube encapsulation for functional biocomposites. Advances in Colloid and Interface Science. 2014 May 1; 207: 189-98.

39. Lee Y, Jung GE, Cho SJ, Geckeler KE, Fuchs H. Cellular interactions of doxorubicin-loaded DNA-modified halloysite nanotubes. Nanoscale. 2013; 5(18): 8577-85.

40. Shutava TG, Fakhrullin RF, Lvov YM. Spherical and tubule nanocarriers for sustained drug release. Current Opinion in Pharmacology. 2014 Oct 1; 18: 141-8.

41. Crielaard BJ, Lammers T, Schiffelers RM, Storm G. Drug targeting systems for inflammatory disease: one for all, all for one. Journal of Controlled Release. 2012 Jul 20; 161(2): 225-34.

42. Rautio J, Kumpulainen H, Heimbach T, et al. Prodrugs: Design and clinical applications. Nat Rev Drug Discov, 2008: 7; 255-70.

43. Wei W, Minullina R, Abdullayev E, Fakhrullin R, Mills D, Lvov Y. Enhanced efficiency of antiseptics with sustained release from clay nanotubes. Rsc Advances. 2014; 4(1): 488-94.

44. Lvov Y, Abdullayev E. Functional polymer–clay nanotube composites with sustained release of chemical agents. Progress in Polymer Science. 2013 Oct 1; 38(10-11):1690-719.

Received on 05.12.2019 Modified on 08.02.2020

Research J. Pharm. and Tech. 2021; 14(1):516-522.

DOI: 10.5958/0974-360X.2021.00094.9