Introduction: Erythrocytes, the most abundant cells in the human body, have potential carrier capabilities for the delivery of drugs. Erythrocytes are biocompatible, biodegradable, possess long circulation half lives and can be loaded with a variety of biologically active compounds using various chemical and physical methods. Erythrocytes, also known as red blood cells, have been extensively studied for their potential carrier capabilities for the delivery of drugs and drug-loaded microspheres. Such drug-loaded carrier erythrocytes are prepared simply by collecting blood samples from the organism of interest, separating erythrocytes from plasma, entrapping drug in the erythrocytes, and resealing the resultant cellular carriers. Hence, these carriers are called resealed erythrocytes. The overall process is based on the response of these cells under osmotic conditions. Upon reinjection, the drug-loaded erythrocytes serve as slow circulating depots and target the drugs to a reticuloendothelial system (RES).
Morphology and physiology of erythrocytes
Erythrocytes are the most abundant cells in the human body (5.4 million cells/mm 3 blood in a healthy male and 4.8 million cells/mm 3 blood in a healthy female).
Its crucial role in oxygen delivery to various parts of the body.
These are biconcave discs with an average diameter of 7.8 µm, a thickness of 2.5 µm in periphery, 1 µm in the center and a volume of 85–91 µm 3 .
The flexible, biconcave shape enables erythrocytes to squeeze through narrow capillaries, which may be only 3 µm wide.
Erythrocytes are a highly specialized O 2 carrier system in the body. Because a nucleus is absent, all the intracellular space is available for O 2 transport. Also, mitochondria are absent and energy is generated anaerobically in erythrocytes.
Erythrocytes live only about 120 days.
Worn-out erythrocytes are removed from circulation and destroyed in the spleen and liver (RES), and the breakdown products are recycled. The process of erythrocyte formation within the body is known as erythropoiesis. In a mature human being, erythrocytes are produced in red bone marrow under the regulation of a hemopoietic hormone called erythropoietin .
Source and isolation of erythrocytes:
Erythrocytes from mammalians like mice, cattle, pigs, dogs, sheep, goats, monkeys, chicken, rats and rabbits.
Fresh whole blood is the blood that is collected heparinized tubes by venipuncture withdrawn from cardiac/splenic puncture in a syringe containing a drop of anti coagulant, immediately chilled to 4ºC and stored for less than 2 days.
The erythrocytes are then seperated from serum coats by centrifugation at 2500rpm for 5 min at 4 ±1ºC and washed 3 times with phosphate buffer solution (pH= 7.4).
The washed cells are suspended in buffer solutions at various hematocrit values as desired and are often stored in acid-citrate-dextrose buffer at 4ºC for as long as 48 h before use.
Properties of resealed erythrocyte of novel drug delivery carriers:
The drug should be released at target site in a controlled manner.
It should be appropriate size, shape and should permit the passage through capillaries and Minimum leakage of drug should take place.
It should be biocompatible and should have minimum toxic effect.
It should possess the ability to carry a broad spectrum of drug.
It should possess specific physicochemical properties by which desired target size could be recognized.
The degradation product of the carriers system, after release of the drug at the selected site should be biocompatible. It should be physico-chemically compatible with drug.
The carrier system should have an appreciable stability during storage.
Their biocompatibility, particularly when autologous cells are used, hence no possibility of triggered immune response.
Their biodegradability with no generation of toxic products.
The considerably uniform size and shape of the carrier.
Relatively inert intracellular environment .
Prevention of degradation of the loaded drug from inactivation by endogenous chemicals.
The wide variety of chemicals that can be entrapped.
The modification of pharmacokinetic and pharmacodynamic parameters of drug attainment of steady-state plasma concentration decreases.
They are non-immunogenic in action and can be targeted to disease tissue/organ.
They prolong the systemic activity of drug while residing for a longer time in the body.
They protect the premature degradation, inactivation and excretion of proteins and enzymes and act as a carrier for number of drugs.
They facilitate incorporation of proteins and nucleic acid in eukaryotic cells by cell infusion with RBC.
They have a limited potential as carrier to non-phagocyte target tissue.
Possibility of clumping of cells and dose dumping may be there.
Characteristic of Resealed Erythrocytes:
When erythrocytes are suspended in a hypotonic medium they swell to about one and a half times their normal size, and the membrane rupture in the formation of pores with diameters of 200 to 500 Å. The pores allow equilibration of the intracellular and extracellular solution.
If the ionic strength of the medium then is adjusted to isotonic and the cells are incubated at 37ºC, the pores will close and cause the erythrocyte to “Resealed”. Using this technique upto 40% of drug can be entraped in the resealed erythrocytes from the extracellular solution. So, this systems are used for targeted delivery via intravenous injection .
Normal aging erythrocytes, slightly damaged erythrocytes, slightly damaged erythrocytes and those coated lightly with antibodies are sequestered in the spleen after intravenous vein fusion, but heavily damaged or modified erythrocytes are removed from the circulation by the Liver.
Red cells placed in hypotonic drug solutions Lysis of cells and entry of drug Resealed red cells loaded with drug
Membrane perturbation Electro encapsulation Hypo-osmotic lysis Lipid fusion method Dialysis method Preswell method Dilution method METHODS OF DRUG LOADING IN RESEALED ERYTHROCYTES Osmotic-lysis Endocytosis method Normal transport mechanism
HYPO OSMOTIC LYSIS METHOD
In this method, erythrocyte suspension + Drug solution -> dialysis tube and both ends are tied with thread.
This tube is placed in bottle containing 100ml of swelling solution at stored at 4ºC for lysis.
After, the dialysis tube is transferred to 100ml resealing solution (isotonic PBS, pH 7.4 ) at room temp. (25º-30ºC) for resealing.
These resealed cells are removed and washed with PBS at 4ºC and finally suspended in PBS solution.
The limitation of dilution method can be overcome by carrying out lysis and resealing in the same dialysis tube.
When these RBC are placed in the hypotonic (0.4% NaOH) they get ruptures permitting escape of cellular content and equilibrium is achieved within 1min, which results in swelling upto 1.6 time its initial volume.
At 0ºC, opening permits to attain equilibrium for inter- and extra cellular fluids through the pore size of 200-500Å.
Increasing the ionic strength at 37ºC, results in resealing of cell membrane and restoring the osmotic property.
This method is simple and faster, but have very low encapsulation efficiency (1-8%) and lose of cytoplasmic constituents during osmotic lysis.
Low mol. wt. drugs like ß-glucosidase and ß-galactosidase can be encapsulated.
In this technique, haemolysis in isotonic solution can be achieved by either chemical or physical means or both.
Erythrocytes are incubated in drug solution with high transerythrocytic membrane permeability like PEG or NH 4 Cl or Urea which offers osmotic diffusion until it reaches equilibrium.
Then the solution was diluted with an isotonic-buffered drug solution.
These cell are separated, washed and resealed at 37ºC.
In this method, 1vol. of packed erythrocytes + 9 vol. of buffer (containing 2.5mM ATP, 2.5mM MgCl 2 and 1mM CaCl 2 ) and incubated for 2min at room temperature.
The pores created in this method are resealed by using 154mM of NaCl and incubate at 37 ºC for 2min.
The entrapment of drug was obtained by endocytosis.
In this method, the vesicle membrane separates the endocytosed substance from the cytoplasm containing drug, which are sensitive to inactivation of cytoplasmic enzyme and protect from erythrocyte membrance.
Lipid vesicles containing a drug can be directly fused to human erythrocytes, which leads to an exchange with a lipid-entrapped drug.
Entrapment efficiency is very low ( ~1%).
Eg: Inositol monophosphate to improve the O 2 carrying capacity of RBC.
This method is based on increase in membrane permeability of erythrocytes when exposed to certain chemicals.
Eg: Amphotericin-B, Daunomycin using Halothane.
NORMAL TRANSPORT MECHANISM:
The biological active components are entrapped in erythrocytes without dispruting the erythrocyte membrane by incubating in drug solution for varying period of time.
30-35% of drug can be entrapped.
- The most common storage media is Hank’s balanced salt solution and acid-citrate-dextrose at 4 ºC and can retain their physiological and carrier characteristics for at least 2weeks.
Addition calcium-chelating agent (or) purine nucleosides improve circulation survival time of cell upon reinjection.
DMSO, dimethyl-3,3-di-thio-bispropionamide, gluteraldehyde, toulene-2,4-diisocynate are used to stabilize the membrane of RSE and enhance stability upon storage.
Disadv.:- At high conc. of membrane stabilizers, reduces circulation survival time.
Hank’s balanced salt solution (HBSS) 5.55 mM D-Glucose 05. 136.89 mM Sodium Chloride 04. 0.34 mM Sodium Phosphate, Dibasic 03. 5.37 mM Potassium Chloride 02. 0.44 mM Potassium Phosphate 01. Quantity Chemicals S.No
CHARACTERIZATION OF RESEALED ERYTHROCYTES
Drug content estimation,
In-vitro drug release and hemoglobin content,
Percent cell recovery,
Erythrocyte sedimentation rate (ESR),
Determination of entrapped magnetite,
Drug content estimation:
- 0.5ml of loaded cells + 2.0ml of Acetonitrile and centrifuge at 2500rpm.
- The supernatant liquid is collected and analyzed for drug content.
(b) Percent cell recovery:
It is determined by counting no. of intact cells per cubic mm of packed erythrocytes using laser scattering.
(c) In-vitro drug release and hemoglobin content
- Initially collect cell suspension (5% hemotocrit in PBS) and stored in 4 ºC in amber colored bottle.
- Periodically the clear supernatant are withdrawn using hypodermic needle equipped with 0.45µ filter and deprotinised using methanol and were estimated for drug content.
(d) Osmotic fragility:
Normal and loaded erythrocytes of drug are incubated separately in stepwise decreasing % of NaCl solution (0.9 and 0.1%) at 37 ±2ºC for 10min and centrifugation at 2000 rpm for 10min. The supernatant examined for drug and hemoglobin content.
(e) Osmotic shock:
In this study, 1ml of erythrocyte suspension were diluted with distilled water of 5ml and centrifuged at 3000rpm for 15min. The supernatent liquid was estimated for hemoglobin spectrophotometrically.
= A 540 of sample - A 540 of background A 540 of 100% hemoglobin % Hemoglobin release
(f) Turbulence shock:
- It is the measure of simulating destruction of loaded cells during injection.
- In this method, the normal and drug loaded cells are passed through a 23 gauge hypodermic needle at a flow rate of 10ml/min which is comparable to that of blood. The samples are with drawn and centrifuged for 2000rpm for 10min and estimated for hemoglobin content.
- Drug loaded erythrocytes found to be less resistant to flow and causes distruction.
(g) Determination of entrapped magnitude:
- Initially, Hcl is added to a fixed amount of magnitude bearing erythrocytes and content are heated to 60 ºC for 2hrs.
- to this added 20%w/v Tricholoroacetic acid is added and supernatent liquid is obtained after centrifugation.
- This was analyzed for magnitude concentration by using AAS.
(h) Erythrocyte sedimentation rate (ESR):
- Sedimentation depends upon the size and no. of cells and relative concentration to the plasma proteins, fibrogen and α - & ß- globulines.
- The test is performed by determination of rate of sedimentation in the standard tube.
- Normal blood ESR is 0-15mm/hr. Higher rate indicates active but obscure disease processes.
Applications of resealed erythrocytes:
Drug targeting: these can act as drug carriers and targeting tool. Used to target organs of mononuclear phagocytic system/ reticuloendothelial system because the changes in the membrane are recognized by macrophages.
Eg: Urease capsules in treatment of kidney failure for maintenance of serum urea levels.
Targeting RES organs: damaged erythrocytes are rapidly cleared from circulation of phagocytic Kupffur cells in liver and spleen.
- Targeting organs like liver and spleen, by modification of Resealed erythrocyte membrane with various antibiotics, gluteraldehyde, carbohydrate, sulphydryl, etc.,
Treatment of hepatic tumors: Antineoplastic drugs like Methotrexate, Bleomycin, Asparginase and Adrimycin delivered by RES.
Treatment of parasitic diseases: Antimalarial, antileishmanial and antiamoebic drugs can be delivered.
Eg: Pentamidine loaded immunoglobin-G coated erythrocytes against macrophage-cantained leishmania.
Removal of RES iron overload: Desferrioxamine, an iron-chelating drug in erythrocyte ghosts with a view to promote excretion of iron in patients with excess body stores.
- Most of the body store of iron is present as intracellular ferritin and hemosiderin deposits.