Resealed erythrocytes - Fundamentals and applications, formulation, preparation methods and evaluation.

1. RESEALED
ERYTHROCYTES as
CARRIERS
1
PREPARED BY:
K. ARSHAD AHMED KHAN
M.Pharm, (Ph.D)
Dept. of Pharmaceutics
RIPER.

2. INTRODUCTION
• A large number of drug carriers are available today but
an ideal carrier is still lacking.
• Among the various carriers used for targeting of drugs
to various body tissues, the erythrocytes meet several
criteria desirable in clinical applications, among the
most important being biocompatibility, nom-
immunogenicity of carrier and its degradation products.
• Leukocytes, platelets and erythrocytes have been
proposed as cellular carrier systems.
• Among these, the erythrocytes have been the most
investigated and have found to possess great potential
in drug delivery.
Rationale: Damaged erythrocytes are removed from
circulation by phagocytic reticulo-endothelial cells. 2

3.  Blood is biphasic liquid.
 Plasma has water (90-92%) and
protein (7%).
 RBC / erythrocytes are flexible,
biconcave, nucleated structure
with an average diameter of 7-8
μm, a thickness of 2.5 μm in
periphery, 1 μm in the center.
 The shape enables erythrocytes to
squeeze through narrow
capillaries, which may be only 3
μm wide.
 Erythrocytes are a highly
specialized O2 and CO2 carrier
system in the body.
Blood & RBC
3

4. • RBC contains proteins (40-60%) and
lipids (10-12%).
• The lipid fraction contains phosphatides
(lecithins, cephalin), cholesterol,
cholesterol esters, neutral fats,
cerebrocides, sialic acid glycoproteins.
• Blood constitutes 7% of body weight in
males and 6.5% in females.
• Normal erythrocyte count is 5.4 million
cells/mm3 blood in a healthy male and
4.8 million cells/mm3 blood in a healthy
female).
• Roughly blood volume is 5 lit in 70 kg
man.
4

5.  RBC newly released from bone marrow are called
reticulocytes.
 Erythrocytes contain a red pigment called hemoglobin.
 The pore size of erythrocyte membrane is 200-500Ao.
 Red cells can be used as carriers for drugs, enzymes,
DNA and other macro molecules.
Hematocrit value:
1. This is the volume ratio of packed cells to total blood
volume.
2. This is an index of both the number and size of the
RBC.
3. This is determined by centrifugation of blood sample.
5

6. Introduction to resealed erythrocytes
• Attempts for encapsulating substances in RBC started in
1970. Ihler & Zimmerman suggested resealed
erythrocytes are useful carriers.
• The term “Carrier RBC ” was introduced in 1979.
• The RBC membrane encloses cytoplasm and hemoglobin,
when erythrocytes are lysed in a controlled manner by
different techniques and again resealed, the cells loses
some of the properties of normal erythrocytes and
referred as RESEALED ERYTHROCYTES.
• During resealing, exchange of intra cellular and extra
cellular solute occurs, some of the hemoglobin is lost and
other cellular contents are retained.
• Resealed erythrocytes that contain no or little
hemoglobin are called ghosts. Pink ghosts are superior to
white ghosts as slow release carriers. 6

7. Properties of resealed erythrocyte as novel drug carriers
1. The drug should be released at target site in a controlled
manner.
2. It should be appropriate size, shape and should permit the
passage through capillaries and minimum leakage of drug
should take place.
3. It should be biocompatible and should have minimum toxic
effect.
4. It should possess the ability to carry a broad spectrum of drug.
5. It should possess specific physicochemical properties by which
desired target site could be recognized.
6. The degradation product of the carriers system, after release
of the drug at the selected site should be biocompatible.
7. It should be physico-chemically compatible with drug.
8. The carrier system should have an appreciable stability during
storage.
7

8. Advantages:
1. They are the natural product of the body which are
biodegradable in nature.
2. Isolation of erythrocyte is easy and larger amount of drug
can be encapsulated in a small volume of cells.
3. The entrapment of drugs does not require any chemical
modification of the substance to be entrapped.
4. They are non-immunogenic , non-toxic in action and can
be targeted to disease tissue/ organ.
5. They prolong the systemic activity of drug while residing
for a longer time in the body
6. They protect the premature degradation, inactivation and
excretion of proteins and enzymes
7. They can target the drugs within reticuloendothelial
system.
8

9. 8. They facilitate incorporation of proteins and nucleic acids
in eukaryotic cells by cell infusion with RBC.
9. A longer life span in circulation as compared to other
synthetic carrier.
10. Isolation of RBC is easy, collection and storage techniques
are well established.
Disadvantages:
1. They have a limited potential as carrier to non-
phagocytic target tissue & compounds extensively
metabolized in liver.
2. Possibility of clumping of cells and dose dumping.
3. Fragility of RBC membrane, their permeability to various
drugs and leakiness.
4. These are not suitable for highly polar and non-diffusable
drugs (heparin, gentamycin). 9

10. Isolation of RBC
1. Blood is collected into heparinized tubes by vein
puncture.
2. Blood is withdrawn from cardiac/splenic puncture( in
small animals) and through veins (in large animals) in a
syringe containing a drop of anti coagulant.
3. The whole blood is centrifuged at 2500 rpm for 5 min at
4±10 C in a refrigerated centrifuge.
4. The serum and buffy coats are carefully removed and
packed cells washed three times with phosphate buffer
saline (pH=7.4).
5. The washed erythrocytes are diluted with PBS and
stored at 4oC until used.
10

11. • Erythrocytes from mammalians like mice, cattle, pigs,
dogs, sheep, goats, monkeys, chicken, rats and rabbits.
• EDTA or heparin can be used as anticoagulants agents.
11

12. Drug Entrapment methods
• The following methods have been employed for drug
entrapment in erythrocytes:
1. Hypo- osmotic lysis method
a. Dilution method
b. Dialysis method
c. Pre-swell method
d. Isotonic osmosis lysis method
2. Electrical breakdown method
3. Endocytosis method
4. Membrane perturbation method
5. Normal transport method
6. Lipid fusion method
7. Osmotic pulse method/ Incubation method
12

13. 1. Hypo- osmotic lysis method:
In this process, the intracellular and extracellular solutes are
exchanged by osmotic lysis and resealing.
The drug present will be encapsulated with in the erythrocytes
membrane .
a. Dilution method
13

14. 1. Erythrocytes have little capacity to resist increase in volume.
2. When these RBC are placed in the hypotonic (0.4% NaCl),
erythrocyte volume increases by 50-75% they get ruptures
permitting escape of cellular content and equilibrium is achieved
within 1min, which results in swelling up to 1.6 time its initial
volume.
3. At 0oC, opening permits to attain equilibrium for inter- and extra
cellular fluids through the pore size of 200-500A0.
4. Increasing the ionic strength at 37oC, results in resealing of cell
membrane and restoring the osmotic property.
5. This method is simple and faster, but have very low encapsulation
efficiency (1-8%) and lose of cytoplasmic constituents during
osmotic lysis.
6. Low mol. wt. drugs like s-glucosidase and s-galactosidase can be
encapsulated.
7. These cells loose most of cell constituents and are removed by RE
cells, thus viability in-vivo is reduced.
14

15. b. Dialysis method:
1. In this method, erythrocyte suspension + Drug
solution, loaded in dialysis tube with 25% air bubble
and both ends are tied with thread.
2. This tube is placed in bottle containing 100ml of
swelling solution, stored at 4oC for lysis.
3. After, the dialysis tube is transferred to 100ml resealing
solution (isotonic PBS, pH 7.4) at room temp. (25-30oC)
for resealing.
4. These resealed cells are removed and washed with PBS
at 4oC and finally suspended in PBS solution.
5. High entrapment efficiency (30-40%) is attained by
carrying out lysis and resealing in the same dialysis
tube.
15

16. Large molecular weight substances are entrapped by
using low hematocrit erythrocyte suspension.
Disadvantages:
1. Time consuming method.
2. Drug concentration in individual ghosts may vary.
16

17. C. Pre-swell dilution technique:
17

18. This technique is based upon initial controlled swelling of RBC with
out lysis in hypotonic buffered solution.
1. Erythrocyte suspension (2ml, 50% hematocrit), centrifuged at 1000
rpm for 10 min at 4oC to obtain packed RBC.
2. Remove supernatant. Collect packed RBC + 4 ml of 0.65% NaCl
(hypotonic pre-swelling solu.) and centrifuge 600 rpm for 5 min to
collect swollen RBC cells.
3. To swollen RBC, add small volumes of drug solution until they
reach the point of lysis.
4. Point of lysis is detected by appearance of thin layer of white
ghosts on centrifugation.
5. After 10 min hypertonic saline solu. Is added and suspension is
incubated at 37oC for 10 min to restore isotonicity and resealing.
6. Cells are washed thrice with washing buffer to remove hemoglobin
and unentraped drug. Cells are finally suspended in PBS buffer.
This techniques results in good retention of cytoplasmic constituents,
high drug entrapment (72%) and good in-vivo survival.
Eg: Thyroxin, Ibuprofen, etc., 18

19. d. Isotonic osmotic lysis technique:
In this technique, haemolysis in isotonic solution can
be achieved by either chemical or physical means or
both.
1. Propylene glycol increases transient permeability
and drug diffusion through erythrocyte wall
without disturbing isotonicity.
2. The lysed erythrocytes are resealed under isotonic
condition by dilution with a glycol free medium.
Eg: DMSO, monosaccharides, sucrose.
19

20. 20

21. 2. ELECTRIC BREAKDOWN TECHNIQUE
Principle: Electrical break down of cell membrane is observed
when the membrane is polarized very rapidly (nano-microseconds)
using voltage of 1 volt.
• This is due to electromechanical compression of membrane
which leads to formation of pores.
• The break down at lipid region / lipid-protein junction in
membrane.
• At this potential difference, increase in membrane conductance is
observed.
• The potential difference across the membrane can be built up
either directly (inter/ intra cellular electrodes) or indirectly by
applying electric field pulse to cell suspension.
• Mechanical rupture of membrane occurs when polarized with 50-
350 mv for the period exceeding 10 µsec resulting in 10 pores/cm
with pore size 3 nm.
21

22. Method:
1. Firstly, erythrocytes are suspended in an isotonic buffered
solution in electric discharge chamber connected to the
capacitor and external circuit.
2. The charge is discharged at definite voltage within
definite time interval through cell suspension to produce
a square-wave potential.
3. The optimum intensity of an electric field is between 1-10
kW/cm and optimum discharge time is 20-160μs.
4. The compound which is to be entrapped was added to
the medium.
5. After certain time the cell suspension was transferred to a
pre-cooled tubes and kept at 4oC.
6. Resealing of electrically perforated erythrocytes
membrane is then done by incubation at 37oC in an
osmotically balanced medium. 22

23. • Major advantage is uniform distribution of drug loading
is achieved and possible to entrap up to 35% of drug.
• Disadvantage is need sophisticated and special
instruments and time consuming.
• This is inferior to dialysis and pre-swell methods.
Eg: Sucrose, Urease, Methotrexate, Isoniazid, Glucophorin.
23

24. 3. ENDOCYTOSIS METHOD:
• Intra cellular vesicles with small molecules, drugs, enzymes,
viruses (100 nm) can be induced in erythrocytes.
• The vesicle membrane separates the endocytosed substance
from the cytoplasm, thus drug which are sensitive to inactivation
of cytoplasmic enzyme are protected.
1. In this method, 1vol. of packed erythrocytes + 9 vol. of buffer
(containing 2.5mM ATP, 2.5mM MgCl2 and 1mM CaCl2) and
incubated for 2min at room temperature.
2. The pores created in this method are resealed by using 154mM of
NaCl and incubate at 37oC for 2min.
3. The entrapment of drug was obtained by endocytosis.
• Endocytosis can be induced by exposure to certain membrane
activated drugs as given below.
Eg: 8-amino quinolines, Vinblastin, Chlorpromazine, Phenothiazines,
hydrocortisone, propanolol, tetracine, Vit-A, etc.,
24

25. 25

26. 4. MEMBRANE PERTURBATION:
• Antibiotics such as amphotericin-B damage micro-
organisms by increasing the permeability of their
membrane to metabolites and ions.
• This property could be exploited for loading of drug
into erythrocytes.
• Amphotericin-B was used to load erythrocytes with
anti-leukaemic drug daunomycin.
• Amphotericin-B interacts with the cholesterol of the
plasma membrane of eukaryotic cells causing change
in permeability of the membrane.
26

27. 27

28. 5. 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.
• After infusion the drug would, exit from the cell
following kinetics compared to those observed for entry.
• Drug of 30-35% can be entrapped.
28

29. 6. Lipid fusion method:
• Lipid vesicles containing drug can be directly fused with
human erythrocytes leading to exchange of lipid
entrapped drug.
• This technique was used for loading inositol
hexaphosphate into resealed erythrocytes.
• Cell fusion takes place by cell swelling, followed by cell
adhesion.
• This method gives very low encapsulation efficiency (1%).
• Fusion can be induced by carboxylic acids, their esters,
retinol, tocopherol.
29

30. 7. Osmotic pulse method/ Incubation method:
This method has 4 steps.
Step-1: DMSO incubation:
The RBC suspension can be incubated with different
concentration of DMSO with variable hematocrit.
Step-2: Isotonic dilution with the material to be
encapsulated:
The drug can be added with constant mixing.
Step-3: Post dilution incubation with cellular swelling:
The RBC swell and drug is encapsulated through pores.
Step-4: Return to original shape:
Drug loaded resealed erythrocytes are obtained.
30

31. 31

32. CHARACTERIZATION OF RESEALED ERYTHROCYTES
1. Drug content estimation,
2. In-vitro drug release and hemoglobin content,
3. Percent cell recovery,
4. Morphology,
5. Osmotic fragility,
6. Osmotic shock,
7. Turbulence shock,
8. Determination of entrapped magnetite,
9. Erythrocyte sedimentation rate (ESR),
10. Zeta sedimentation ratio
11. Hematological parameters
12. Biochemical parameters
13. Lipid content
14. Membrane fluidity
15. Rheological parameters.
32

33. 1. Drug content:
packed loaded erythrocytes (0.5ml) are first deproteinized
with acetonitrile (2ml) and subjected to centrifugation at
2500 rpm for 10 min.
The clear supernatant is analyzed for the drug content
using specified estimation methodology for entrapped
drug.
2. In-vitro drug release and hemoglobin content:
- Initially collect cell suspension (5% hemotocrit in PBS) and
stored in 4oC in amber colored bottle.
- Periodically the clear supernatant are withdrawn using
hypodermic needle equipped with 0.45μ filter and
deprotenised using methanol and were estimated
for drug content.
33

34. • Hemoglobin Conc. is measured spectrophotometrically
after conversion of hemoglobin, oxyhemoglobin,
cyanamethamoglobin to hematin.
• This conversion is done by adding strong base (NaOH) to
pH 10 / alkaline cyanide potassium ferricyanide reagent.
• Results are expressed in “g hemoglobin/ 100 ml of blood”
34

35. 3. Percent cell recovery:
It is determined by counting no. of intact cells per cubic
mm of packed erythrocytes before and after drug loading.
The counting is performed in a haemocytometer.
4. Morphology:
• Phase contrast & electron microscopy for mormal
and drug loaded RBC.
• For electron microscopy sample is washed with
cacodylate buffer and stained negatively with 0.01%
uranyl acetate.
• SEM/ TEM – monitoring and evaluation of all stages
of carrier cell preparation.
35

36. 5. Osmotic fragility:
 This indicates resistance of cell to haemolysis in
decreasing conc. of hypotonic saline.
 This is reliable parameter for in-vitro shelf life, in-vivo
survival and effect of encapsulated substances.
 Normal and loaded erythrocytes of drug are incubated
separately in stepwise decreasing % of NaCl solution
(0.9 and 0.1%) at 37oC for 10min and centrifugation at
2000 rpm for 10min.
 The supernatant examined for drug and hemoglobin
content.
 Increased osmotic fragility indicates acquired hemolytic
anemia, increased resistance is observed in
thalassemia, sickle cell anemia, hypochromic anemia.
36

37. 6. Osmotic shock:
• Drug loaded erythrocytes are suddenly exposed to an
environment, which is far from isotonic and the ability
of resealed erythrocytes to withstand the stress,
maintain their integrity as well as appearance is
evaluated.
• In this study, 1ml of erythrocyte suspension were
diluted with distilled water of 5ml and centrifuged at
3000rpm for 15min.
• The supernatant liquid was estimated for hemoglobin
spectrophotometrically.
37

38. 7. 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
turbulence indicating destruction on shaking.
38

39. 8. Determination of entrapped magnetite:
• Magnetically responsive drug loaded erythrocytes are
developed by loading iron compound (magnetite) in
RBC.
• Initially, Hcl is added to a fixed amount of magnetite
bearing erythrocytes and contents are heated to 60oC
for 2hrs.
• To this added 20%w/v Tricholoroacetic acid is added
and supernatent liquid is obtained after centrifugation.
• This was analyzed for magnetite concentration by using
AAS (Atomic Absorption Spectroscopy).
39

40. 9. Erythrocyte sedimentation rate (ESR):
• Sedimentation depends upon the size and no. of cells
and relative concentration to the plasma proteins.
• ESR increases with rise in O2, cholesterol, fibrinogen, α-
globulin. ESR decreases with rise in Co2, albumin,
nucleoprotein, lecithin.
• The test is performed in a standard tube placed
vertically.
• Height of supernatant plasma in mm separated out at
the top of the vertical column of blood after 1 hr is
noted.
• Normal blood ESR is 0-15mm/hr. Higher rate indicates
active but obscure disease progress.
40

41. 10. Zeta sediment ratio:
This measures the closeness with which RBC approach each
other after dispersion.
Velocity of RBC aggregation can be measured in a rheoscope.
11. Hematological parameters:
This predicts effect of loaded drug on integrity of RBC.
Parameters include protein content, hemoglobin content,
mean corpuscular volume (MCV), mean corpuscular
hemoglobin (MCH), mean corpuscular hemoglobin
concentration (MCHC), red cell distribution width (DW).
12. Biochemical parameters:
This is helpful in detection of presence of disease.
Parameters include cell debris, content measurement of thoil,
glutathione, oxidized glutathione, creatinine, initial hemolysis
and K+ leakage rate, etc.,
41

42. 13. Lipid content:
Lipid content and the spatial arrangement of lipids on the
external surface of resealed erythrocytes affects their in-
vivo half life.
The longer half-life of loaded RBC indicated it maintains
cell surface similar to normal RBC which prevents its
removal by RE cells.
14. Membrane fluidity:
This is measured by a hydrophobic fluorescent probe and
the values are related to lipid composition.
15. Rheological parameters:
Viscosity and relative density are measured by brook field
viscometer and relative density bottle respectively.
42

43. 43

44. 44

45. Applications of resealed erythrocytes
These have been proposed for a variety of applications in
human and veterinary medicines.
1. Applications In-Vitro
2. Applications In-Vivo
a) Targeting bioactive agents to RE system,
b)Erythrocytes as circulating carriers,
c) Erythrocytes as circulating bioreactors,
d)Erythrocytes in enzyme delivery,
e) Prevention of thromboembolism,
f) Treatment of poisoning,
g) Prodrug,
h)Drug targeting other than RES.
45

46. 1.Applications In-Vitro
1. Carrier RBC is used in in-vitro tests.
2. For in-vitro phagocytosis, cells have been used to
facilitate the uptake of enzymes by phagolysosomes.
3. The enzyme content with in carrier RBC can be
visualized with cytochemical technique.
4. Biochemical defects like glucose-6-phosphate
dehydrogenase deficiency can be treated with
resealed erythrocytes.
5. RBC are most frequently used in microinjections.
6. Ribosome inactivation proteins and nucleic acids are
injected in to RBC by fusion process.
7. Anti-body injected RBC are useful to confirm the site of
action of diphtheria toxin.
46

47. 1. Applications In-Vivo
a) Targeting bioactive agents to RE system
 Resealed erythrocytes with modified characteristics are
removed form circulation by phagocytic cells located in
liver and spleen.
 This means bioactive agents can be targeted to these
organs using erythrocytes.
 The drug entrapped erythrocytes are used for RES
Targeting in treatment of following diseases.
1)Treatment of lysosomal storage disease:
2)Treatment of Gaucher’s disease:
3)Treatment of liver tumors.
4)Treatment of parasitic diseases
5)Removal of RES iron overload.
6)Targeting mycotoxins to RES. 47

48. 1. Treatment of lysosomal storage disease:
Resealed erythrocytes have been proposed to deliver
lysosomal enzymes to lysosomes of the erythrophagocytic
cells, thus resulting in replacement of the missing enzyme.
Ex: β- glucoronidase, β-galactoronidase and β-glucosiade.
2. Treatment of Gaucher’s disease:
Gaucher’s disease is due to accumulation of glucocerebroside
from catabolised erythrocytes and leukocytes in spleen,
liver and bone marrow macrophages.
This disease was treated by encapsulating
glucocerebrosidase in erythrocyte.
3. Treatment of liver tumors:
Anticancer agents like bleomycin, adriamycin,
Lasparaginase, doxorobucin and methotrexate are
encapsulated in erythrocyte to treat hepatic carcinomas.
48

49. 3. Treatment of parasitic diseases.
The ability of resealed erythrocytes to selectively accumulate
within RES organs make them useful tool during the delivery
of anti-parasitic agents.
Eg: Pentamidine loaded immunoglobin-G coated
erythrocytes against macrophage-containing leishmania.
Liver targeting– Glutaraldehyde treated erythrocytes
(Primaquine phosphate {anti-malarial} + Metronidazole
{anti-amoebic})
4. Removal of RES iron overload:
Most of the body store of iron is present as intracellular
ferritin and hemosiderin deposits.
Desferrioxamine, an iron-chelating drug in erythrocyte
ghosts with a view to promote excretion of iron in patients
with excess body stores. 49

50. 5. Targeting of mycotoxins to RES:
Erythrocytes can target large doses of T-2 toxin
(tricothecene mycotoxin) to macrophages of liver and
spleen for treatment of Schistosomiasis and prolong the
retention time of it.
b. Erythrocytes as circulating carriers:
Various bioactive agents are encapsulated in erythrocytes
for their slow release in circulation for treatment of
parasitic diseases in cattle.
Ex: anti cancers, vitamins, steroides, antibiotics, Homodium
bromide, Imidicocrab dipropionate, tetracycline.
c. Erythrocytes as citculating bioreactors:
To diminish the level of circulating metabolites(arginine, uric
acid) enzyme (arginase, urease) loaded erythrocytes are
used. 50

51. d. Erythrocytes in enzyme delivery:
Enzyme loaded erythrocytes are used to replace missing
enzyme in metabolic disorders/ to degrade toxic
compounds in blood.
Diseases ---Gauchers disease, kidney failure, hyperuricemia.
Enzymes --- L-Aspariginase (Leukemia), araginase, alcohol
oxidase, acetaldehyde dehydrogenase, hexokinase, etc.,
Adv:-This eliminate or minimize the problems related to
immunologic responses and toxicity.
e. Prevention of thromboembolism:
Encapsulated heparin is liberated from circulating
erythrocytes at the site of thrombus formation thus
reducing the risk of further thrombus growth.
Ex: Aspirin + ferromagnetic colloid loaded  Thrombosis.
51

52. f. Treatment of poisoning:
Rhondanase + Sodium thosulphate loaded erythrocytes are
used for treatment of cyanide poisoning by converting
cyanide to less toxic thiocyanate.
g. Prodrug:
2,3-dideoxycytidine (ddCyd) is potent antiviral for killing HIV.
2,3-dideoxycytidine-5-phosphate (ddCMP) is prodrug
loaded in erythrocytes for treatment of HIV.
Adv: Toxicity is reduced, slow delivery with long plasma
Conc.
h. Drug targeting other than RES:
Urease loaded erythrocytes are used in treatment of kidney
failure.
52

53. REFERENCES:
1. Controlled and novel drug delivery; N.K.JAIN. 2017, CBS
publishers.
2. Introduction to novel drug delivery system; N.K.JAIN.
2017, Vallabh prakashan.
3. Textbook of industrial pharmacy, drug delivery system &
cosmetic and herbal drug technology. SHOBHA RANI. RH.
2014, universities press.
4. Targeted and controlled drug delivery novel carrier
system; SP. VYAS & RK.KHAR. 2010, CBS publishers.

54. Thank you……
54