P750 LOGBOOK 5 STUDENT ID : 6469969
- Age related nuclear cataract is the anterior segment disease and it is the clouding of ocular lens characterized by reduced antioxidant levels in the lens core. Antioxidant cystine is a amino acid having molecular weight 240.3 g/mol and solubility of 50 mg/ml in 1 M Hcl is used to prevent cataract formation.
To treat cataract, cystine is applied topically in the form of eye drops. The factors that reduce cystine bioavailability are,
Tear film :- Topically applied drugs will first encounter tear film and it is considered as first protective structure.
Nasolacrimal drainage system :- After the application cystine eye drops tear fluid turnover doubles it is called as washout effect. Due to pH and foreign body sensation reflex tearing will occur.
Cornea :- It is the main mechanical barrier. Due to its sandwich like structure drugs with molecular weight less than 5 K Da and partition coefficient of 10 to 100 can pass through it. Cornea is composed of three layers, the outermost layer is epithelium which is lipophilic in nature, middle layer is stroma which is hydrophilic in nature and innermost layer is endothelium which is lipophilic in nature.
FORMULATION PARAMETERS : – The formulation parameters to be considered in formulating a topical cystine eye drops are,
- Physico – chemical drug properties :-
- Partition coefficient – log p of the formulation should range from 10 to 100.
- Molecular weight – molecular weight of the formulation should be less than 5 K Da.
- Charge – charge of the drug should be positive.
- Buffer capacity and pH :- eye drops should be formulated with a pH range of 7.0 to 7.7.
- If the pH is 7, 99% of the drug remains in unionized form and favors the permeation through lipophilic epithelium.
- If the pH is more than 7, most of the drug ionizes and easily diffuses through the hydrophilic stroma.
- Viscosity : – viscosity of the eye drop preparation must be around 15 m Pa.
- Instillation volume :- instillation volume must be less than 30 µl because cul – de – sac can hold up to 30 µl.
- Osmotic pressure :- osmotic pressure of the eye drop must range between 310 to 350 m osm/kg
- Antioxidants :- cystine itself acts as a antioxidant, no other preservatives are required.
FORMULATION APPROACH : – Colloidal ocular delivery systems like micro emulsions are used to deliver the antioxidant cystine in the form of topical eye drops.
Micro emulsions : –
- Micro-emulsions acts as vehicles for the delivery of antioxidant cystine in the form of topical eye drops.
- In micro emulsions phase transition will occur between the bi-continuous micro-emulsion, oil in water emulsion, water in oil emulsion, and lamellar crystals.
- Components of micro emulsions are,
- Oils like mineral oil, vegetable oils, di and triglycerides, fatty acid ester.
- Surfactant – non ionic, amphoteric and less commonly anionic and cationic surfactants are used.
- Co-surfactant – short and medium chain alcohols are used as co-surfactants.
- Advantages of phase transitions of micro-emulsions
- W/O micro :-emulsion : these micro-emulsions are responsible for protection of water soluble drugs and sustained release of water soluble drugs.
- O/W micro-emulsions :- these micro-emulsions are responsible for increasing solubility of lipophilic drugs.
- Bi-continuous micro-emulsions :- these micro-emulsions are having good wetting and spreading properties on the ocular surface hence these are used in ocular drug delivery systems.
- Micro-emulsions are having low viscosity hence it is easy to instill.
- These micro-emulsions are thermodynamically stable.
- These are easy to prepare, no mixing is required.
- In these micro-emulsions we can detect the phase separation easily, drug precipitation and microbial contamination.
- Micro-emulsions are used as vehicles due to solubilization of hydrophilic and lipophilic drugs.
- By using the micro-emulsions as vehicles we can increase the bio-availability of drugs.
- Glaucoma is anterior segment disease characterized by raised intra ocular pressure, which results in the loss of regional ganglia cells and degeneration of optic nerve. Glaucoma is the main ocular disease responsible for blindness. Antisense oligonucleotides (As ODN) helps in curing glaucoma disease.
Antisense oligonucleotides (As ODN) : –
- These are single stranded DNA fragments of 10 to 30 nucleotides, complementary to the target mRNA.
- Mechanism of action – Generally, DNA transcripts mRNA in the nucleus and this mRNA enters into the cytoplasm and ribosomes translate the mRNA. Finally results in the formation of proteins. Antisense oligonucleotides (As ODN) having complementary base sequence to that of mRNA binds with mRNA and prevents the formation of Cx proteins from it.
Delivery route : – I would like to choose intra-vitreous delivery route because, periocular injections of antisense oligonucleotides is responsible for poor stability and it is difficult for antisense oligonucleotides to penetrate through the cells.
Intra-vitreous injection : – It is the injection of antisense oligonucleotides into the vitreous with the help of a needle. In the treatment of many of the ocular diseases intra-vitreous administration of drugs is used.
Advantages of intra-vitreous injection :
- This delivery route is responsible for achieving high concentration of drug in the vitreous.
- There will be no side effects, because it is not a systemic administration.
Disadvantages of intra-vitreous injection :
- From the vitreous, many drugs are rapidly cleared due to blood – retinal barrier, therefore repeated dose administration is required.
- Frequent injections leads to endopthalmitis, lens damage, detachment of retina.
FORMULATION PARAMETERS :-
Chemical modification : – chemical modification of antisense oligonucleotides leads to increased or decreased solubility, stability etc..
- Phospho-diester group :-
- By replacing oxygen from the phospho-diester group with sulphur, stability and solubility increases and RNase H cleavage will occur.
- By replacing oxygen with methyl group there is increased stability but decrease in solubility, due to lack of charge cellular uptake will not occur, no RNase H activity.
- Sugar moiety modification :-
- By adding a alkyl group at the 2nd position of ribose, there is increase in hydrophilicity and binding affinity but mismatches will occur and no RNase H cleavage.
- Other modification :-
- Replacement of phospho-diester group with polyamide results in high affinity to mRNA but aqueous solubility decreases, cellular uptake decreases and no RNase H cleavage.
FORMULATION APPROACH :-
To prevent antisense oligonucleotides (As ODN) from enzymatic degradation and to improve the cellular uptake a variety of formulation approaches have been put forward. They are, liposomes, nanoparticles, peptides, dendrimers, and physical methods. Among these i would like to choose multi functional dendrimer carriers.
Dendrimer carriers :-
Dendrimers are branched molecules and spherical in nature. Dendrimers are divided into low molecular weight and high molecular weight species. The properties of dendrimers depends on the functional groups at their molecular level. Dendrimers are cationic in nature and possess positive charge on it.
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Antisense oligonucleotides shows its therapeutic effect at cytoplasmic level and these antisense oligonucleotides are anionic in nature and possess negative charge. During the formulation, dendrimer undergoes complexation and condensation with antisense oligonucleotides. The functional groups present at the molecular level are.
- Cell penetrating peptide :-
TAT peptide is derived from human immunodeficiency virus and it acts as a cell penetrating peptide. TAT helps in cellular uptake of dendrimer antisense oligonucleotide complex.
- Fusogenic peptide :-
Dendrimer- antisense oligonucleotide complex binds to the cell membrane than enters in to the cell through endocytosis. These fusogenic peptide helps in endisomal escape. Influenza virus hemagglutinin subunit-2 is a fusogenic peptide.
- Lipoamino acid :-
This functional group helps in improving permeability and stability. Example, C14
Disadvantage of dendrimer carrier : – cytotoxicity increases due to the presence of cell penetrating peptides and fusogenic peptides
- Dr Ilva Rupenthal lecture notes given on 4th April 2014.
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