INTRODUCTION:

Magnetic microspheres are the supramolecular particles that are small enough to circulate through the capillaries but are sufficiently susceptible to be enraptured in microvessels by applying magnetic fields of 0.4 T-0.8 T. Targeted drug deliveries can be done by two modes i.e. magnetic drug delivery and non magnetic drug delivery. Magnetic drug delivery by various novel carriers is an excellent method in which a drug is directly delivered to the diseased location/area. nonmagnetic drug delivery systems such as nanoparticles, microspheres and microparticles etc are successfully utilized for drug targeting but do not show a satisfactory site specificity and are rapidly cleared off by RES (reticuloendothelial system)¹ Magnetic polymer microspheres are usually composed of magnetic cores to ensure a strong magnetic response and polymeric shells to protect from particle aggregation. These microspheres exhibit features such as small and proper size, different shapes, and various functional groups on the surface.

They have therefore received much attention in recent years for wide potential applications such as immobilization of enzymes, protein separations, and various drug delivery processes. Thus magnetism plays an important role in living beings metabolism. For example, the hemoglobin is an iron complex present in blood and is magnetic in nature. Magnetite, Fe3O4, is a biocompatible structure and it has a cubic inverse spinal structure with oxygen forming a FCC closed packing and therefore it is one of the most commonly used biomaterials for biological and medical applications from cell separation and drug delivery to hyperthermia². Magnetically drug delivery is a excellent way, in which a drug is binded to a small biocompatible magnetically active component, entrapped in the biodegradable polymeric matrix and pharmacologically active stable formulation is formulated, which is injected into the stream of blood and a high-gradient magnetic field is used to pull them out of suspension in the target region. Controlled drug release and further biodegradation are important for developing successful formulations.

Mechanisms which involve Potential release are:

Desorption of surface-bound /adsorbed drugs

Diffusion through the carrier matrix

Carrier wall diffusion

Carrier matrix erosion

Combination of erosion /diffusion process^3-8.

Principle of Magnetic Targeting:

1. A drug or therapeutic radioisotope is encapsulated in a magnetic compound; injected into patient’s blood stream and powerful magnetic field in the target area is applied to stop it

2. Depending on the type of drug, it is then slowly released from magnetic, thus it reduces the loss of drug as freely circulating in body ^9-12.

Figure 1: Drug Targeting Via Magnetic and Non Magnetic Systems

Magnetic modulated systems Targeted systems are classified as follows: ^13-14

1. Magnetic microspheres

2. Magnetic liposomes

3. Magnetic nanoparticles

4. Magnetic resealed erythrocytes

5. Magnetic emulsions

Criteria for selection of drugs for formation of Magnetic microspheres^15-18

1. Magnetic microspheres are prepared when the drug is so dangerous that we cannot allow it to circulate freely into the blood stream.

2. When the agent is so expensive and we cannot afford to waste it.

3. Requires a selective regional effect to meet localized therapeutic objective.

4. Requires an alternative formulation essential to continue treatment in patients whose systemic therapy must be temporarily discontinued due to life threatening toxicity directed at selective organs.

Advantages of Magnetic Microspheres^19-21

1. Therapeutic responses in target organs can be achieved by only small fraction of the free drug dose.

2. Better drug utilization will improve the bioavailability and reduce the incidence or intensity of adverse effects.

3. Controlled drug release within target tissues for prolonged therapeutic effect.

4. Reduce the dosing frequency and thereby improve the patient compliance.

5. They could be injected into the body due to the spherical shape and smaller size.

6. Microsphere morphology allows a controllable variability in degradation and drug release.

Disadvantages of Magnetic Microspheres²²

1. It is an expensive technical approach and requires specialized manufacture and quality control system.

2. It needs specialized magnet for targeting, for monitoring, and trained personnel to perform procedures.

3. Magnets must have relatively constant gradients, in order to avoid focal over-dosing with toxic drugs.

4. A large fraction of the magnetite, which is entrapped in carriers, is deposited permanently in tissues.

5. Controlled release formulations generally contain a higher drug load and thus any loss of integrity of the release characteristics of the dosage form may lead to potential toxicity.

Magnetically Modulated Systems and Devices²³

Magnetically modulated polymeric controlled drug delivery systems that deliver the drugs at increased rate on demand have been developed extensively in recent years. These systems consist of polymeric matrix, in which drug powder is discrete. The polymeric matrix consists of ethylene vinyl acetate copolymer (EVAc) with some magnetic beads. The beads used are either (i) magnetic steel beads composed of iron (79%), chromium (17%), carbon (1%), manganese (1%), silicon (1%), molybdenum (0.75%), and phosphorus (0.04%) or (ii) small amount of samarium cobalt magnets. These systems are formulated by adding approximately 50% of drug-polymer mixture to a glass mold, which is cooled to −80°C using dry ice, after this the magnetic particles, are added followed by the remaining drug-polymer mixture. In case of in vivo experiment, the magnetic tablets are placed in glass vials. An oscillating external magnetic field, which is generated by a device that rotates the permanent magnets below the vials, controls the release rates.

Methods of Preparation of Magnetic Microspheres^24-27

Selection of Drugs:

In the selection of a drug for formulation of magnetic microspheres, following points are taken into consideration: (i) The drug is so dangerous or labile that we cannot allow it to circulate freely in the blood stream. (ii) The agent is so expensive, that we cannot afford to waste 99.9% of it. (iii) Requires a selective, regional effect to meet localized therapeutic objective. Requires an alternative formulation essential to continue treatment in patient whose systemic therapy must be temporarily discontinued due to life threatening toxicity directed at selective organs.

Continuous Solvent Evaporation:

In this method the drug and polymer (Carrier) are dissolved in appropriate volatile organic solvent and then magnetite (if magnetic microspheres) is added to this solution along with stirring in order to form a homogeneous suspension. This suspension is added to an immiscible auxiliary solution along with vigorous stirring. Now the volatile organic solvent is evaporated slowly at 22-30 °C to form microspheres. Microspheres are this centrifuged and freeze dried and stored at 4 °C ^18-20.

Phase Separation Emulsion Polymerization:

Homogenous aqueous suspension is prepared by adding albumin water-soluble drug and agent with magnetite in appropriate quantity of water (if magnetic microspheres). This aqueous suspension is then emulsified in the presence of suitable emulsifying agent to form spheres in emulsion. This aqueous proteineous sphere thus formed in the emulsion are stabilized either by heating at 100- 150 °C or by adding hydrophobic cross linking agents like formaldehyde, glutraldehyde or 2-3 butadiene, microspheres thus produced are centrifuged out and washed either in ether or some other appropriate organic solvent to remove excess of oil. Microspheres are freeze dried and stored at 4 °C ^18-20.

Multiple Emulsion Method:

Water dispersible magnetite with a PEG/PAA coating was added to the BSA-containing inner water phase. 0.2 mL of a 1 mg/mL BSA solution added to a 4 mL mixture of DCM and EA at a ratio of 3 to 1 containing 200 mg of PLGA (first w/o emulsion was prepared using a homogenizer (Polytron PT10-35; Kinematica, Luzern, Switzerland) in an ice bath at 26 000 r/min for 2.5 min). Fifteen mL of a 1% PVA solution poured directly into the primary emulsion and re-emulsified using the same homogenizer under the same conditions for another 2.5 min. W/o/w emulsion immediately poured into a beaker containing 85 mL of 1% PVA solution and stirred in a hood under an overhead Propeller for 2 h, allowing the solvent to evaporate. Solidified microspheres harvested by centrifugation at 2 500 r/min for 10 min and washed with distilled water three times

Cross Linking Method Reagents Used:

Acetate buffer–used as solvent for the chitosan polymer; Glutraldehyde-used as the cross-linker; Sodium hydroxide solution-used as medium. Synthesis of magnetic fluid: A 35% (w/v) ferrous sulfate solution, 54% (w/v) ferric chloride solution and 36% (w/v) sodium hydroxide solution were prepared using distilled water. Then the ferric salt and ferrous salt were mixed, stirred and heated. When the temperature reached 55 °C, the alkaline solution was added. The mixture was stirred for 30 min, and then 5 g of polyethylene glycol-10000 (PEG10000) was added. The temperature was raised to 80 °C and maintained for 30 min. The mixture was then neutralized while cooling, and the magnetic fluid was prepared. 1% (w/ w) chitosan was dissolved in acetate buffer at pH 4.5. The dissolved chitosan was added drop wise on the magnetic fluid. Formed chitosan magnetic microspheres were washed with deionized water and soaked in 1, 3, and 5 mol % glutraldehyde solution for 2 h, and then washed with deionized water.

Evaluation and Characterization of Magnetic Microspheres ^28-31

Particle Size and Shape: The most widely used procedures to visualize microspheres are conventional light microscopy (LM) and scanning electron microscopy (SEM). Both can be used to determine the shape and outer structure of microspheres. LM provides a control over coating parameters in case of double walled microspheres. The microspheres structures can be visualized before and after coating and the change can be measured microscopically. SEM provides higher resolution in contrast to the LM. SEM allows investigations of the microspheres surfaces and after particles are cross-sectioned, it can also be used for the investigation of double walled systems.

Electron Spectroscopy for Chemical Analysis:

The surface chemistry of the microspheres can be determined using the electron spectroscopy for chemical analysis (ESCA). ESCA provides a means for the determination of the atomic composition of the surface. The spectra obtained using ECSA can be used to determine the surface degradation of the biodegradable microspheres.

Thermal analysis:

Thermal analysis of microcapsule and its component can be done by using differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), differential thermometric analysis (DTA). Accurately the sample was weighed and heated on alumina pan at constant rate of 10oC/min under nitrogen flow of 40 ml/min.

Attenuated Total Reflectance Fourier Transform-Infrared Spectroscopy:

A TRFTIR is used to determine the degradation of the polymeric matrix of the carrier system. The surface of the microspheres is investigated measuring attenuated total reflectance (ATR). The IR beam passing through the ATR cell reflected many times through the sample to provide IR spectra mainly of surface material. The ATRFTIR provides information about the surface composition of the microspheres depending upon manufacturing procedures and conditions.

Swelling Index:

Swelling index was determined by measuring the extent of swelling of microspheres in the given buffer. To ensure the complete equilibrium, exactly weighed amount of microspheres were allowed to swell in given buffer. The excess surface adhered liquid drops were removed by blotting and the swollen microspheres were weighed by using microbalance. The hydrogel microspheres then dried in an oven at 60° for 5 h until there was no change in the dried mass of sample. The swelling index of the microsphere was calculated by using the formula:

Mass of swollen microspheres–mass of dry microspheres ×100

Swelling index= ---------------------------------------------

Mass of dried microspheres

Density:

Bulk density It is determined by pouring a sample of microspheres of known weight into a measuring cylinder without tapping and measuring its volume, then dividing the weight by the volume. Tapped density is determined by pouring a sample of microspheres of known weight into a measuring cylinder and thoroughly tapping it and measuring its volume, then dividing the weight by the volume.

Hausner Ratio:

Hausner ratio is the ratio of the tapped density to the bulk density of microspheres and can be used to predict of microspheres flow. Low Hausner ratio of <1.2 indicates free flowing microspheres.

Angle of Repose:

It is defined as the maximum angle to the horizontal that is attainable by a heap of microspheres. Among methods available for measuring the angle of repose are the fixed height cone and the fixed base cone. High angle of repose indicates poor flowing microspheres, while low angle indicates a free flowing microsphere.

Surface Charge Analysis:

Surface charge can be determined using the micro-electrophoresis. It is an apparatus used to measure the electrophoresis mobility of microspheres from which the iso-electric point can be determined. The mean velocity at different pH values ranging from 3-10 is calculated by measuring the time of particle movement over a distance of 1 mm. By using this data, the electrical mobility of the particle can be determined. The electrophoretic mobility can be related to surface contained charge and ion absorption nature of the microspheres.

Drug Release Profiles:

There is a need for experimental methods which allow the release characteristics and permeability of a drug through membrane to be determined. For this purpose, a number of in vitro and in vivo techniques have been reported. In vitro drug release studies have been employed as a quality control procedure in pharmaceutical production and product development. Standard USP or BP dissolution apparatus have been used to study in vitro release profiles using rotating elements, paddle and basket. Dissolution medium may be used for the study varied from 100-500 ml and speed of rotation from 50-100 rpm.

Determination of Microspheres Drug Content or Entrapment Efficiency:

Accurately weighed amount of microspheres are crushed using glass mortar and pestle and the powder microspheres is then suspended in a specific volume of suitable solvent. After 12 hours the solution was filtered and the filtrate is then analyzed for the drug content using UV-Visible spectrophotometer. Drug content is equal to entrapment efficiency is equal to ratio of actual drug content to theoretical drug content.

Applications of Magnetic Microspheres^32-33

a. Magnetic microsphere carriers have received considerable attention, because of their wide applications in the fields of biomedicine and bioengineering, biological and biomedical developments and trends such as enzyme immobilization, cell isolation, protein purification, and target drugs.

b. Magnetic vehicles are very attractive for delivery of therapeutic agents as they can be targeted to specific locations in the body through the application of a magnetic field gradient. The magnetic localization of a therapeutic agent results in the concentration of the therapy at the target site consequently reducing or eliminating the systemic drug side effects.

c. Magnetic microspheres are used in targeting drugs like mitoxantrone, paclitaxel and doxorubicin to tumor sites. Magnetic microsphere carriers labeled with radionuclide such as Rhenium-188 and Yttrium-90 have been also used in a preclinical study to treat liver and brain tumors.

d. Magnetic microspheres of cisplastin and paclitaxel were used in localized hyperthermia for treatment of cancer.

e. Magnetic microspheres can be used for stem cell extraction and bone marrow purging.

f. Magnetic polystyrene microspheres have been used as specific cell labeling.

g. Improvement in methods for isolating DNA, proteins, cells or cell organelles has been made and more recently, methods that rely on the use of solid phase have been proposed. Adsorbents such as silica that provide fast, efficient DNA purification are important for making this procedure amenable to automation. One of these kits involves isolation of DNA using silica coated magnetic particles.

CONCLUSION:

Magnetism seems to be a common function of opening a new vista of a multi-barrier of multi-step drug delivery. Their main advantage is the targeting of drug using an external magnet, which can be accomplished very easily. It does not exert side effects, neither on its way to the therapeutic target, not at the target site, nor during the clearance process. Thus magnetic microspheres have the potential for these objectives. It is a challenging area for future research in the drug targeting so more researches, long term toxicity study, and characterization will ensure the improvement of magnetic drug delivery system. The future holds lot of promises in magnetic microspheres and by further study this will be developed as novel and efficient approach for targeted drug delivery system.

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Received on 26.07.2021 Modified on 13.11.2021

Asian J. Res. Pharm. Sci. 2022; 12(1):57-61.

DOI: 10.52711/2231-5659.2022.00011