Dex-Tobrin

Uses

This sterile Eye Suspension is used for steroid-responsive inflammatory ocular conditions for which a corticosteroid is used and where bacterial infection or a risk of bacterial ocular infection exists.

Dex-Tobrin is also used to associated treatment for these conditions: Acne Rosacea, Acute Gouty Arthritis, Acute Otitis Externa, Acute Otitis Media, Adrenal cortical hypofunctions, Adrenocortical Hyperfunction, Alopecia Areata (AA), Ankylosing Spondylitis (AS), Anterior Segment Inflammation, Aspiration Pneumonitis, Asthma, Atopic Dermatitis (AD), Berylliosis, Bullous dermatitis herpetiformis, Bursitis, Chorioretinitis, Choroiditis, Congenital Adrenal Hyperplasia (CAH), Congenital Hypoplastic Anemia, Conjunctivitis, Conjunctivitis allergic, Corneal Inflammation, Cushing's Syndrome, Dermatitis, Dermatitis exfoliative generalised, Dermatitis, Contact, Diabetic Macular Edema (DME), Discoid Lupus Erythematosus (DLE), Drug hypersensitivity reaction, Edema of the cerebrum, Epicondylitis, Episcleritis, Erythroblastopenia, Eye Infections, Eye allergy, Eye swelling, Glaucoma, Hypercalcemia, Idiopathic Thrombocytopenic Purpura, Infection, Inflammation, Inflammation of the External Auditory Canal, Intraocular Inflammation, Iridocyclitis, Iritis, Keloid Scars, Leukemia, Acute, Lichen Planus (LP), Lichen simplex chronicus, Loeffler's syndrome, Macular Edema, Malignant Lymphomas, Middle ear inflammation, Mucosal Inflammation of the eye, Multiple Myeloma (MM), Muscle Inflammation caused by Cataract Surgery of the eye, Mycosis Fungoides (MF), Necrobiosis lipoidica diabeticorum, Noninfectious Posterior Uveitis, Ocular Infections, Irritations and Inflammations, Ocular Inflammation, Ocular Inflammation and Pain, Ocular Irritation, Ophthalmia, Sympathetic, Optic Neuritis, Otitis Externa, Pemphigus, Perennial Allergic Rhinitis (PAR), Phlyctenular keratoconjunctivitis, Post-traumatic Osteoarthritis, Postoperative Infections of the eyes caused by susceptible bacteria, Regional Enteritis, Rheumatoid Arthritis, Rheumatoid Arthritis, Juvenile, Sarcoidosis, Scleritis, Seasonal Allergic Conjunctivitis, Seasonal Allergic Rhinitis, Secondary thrombocytopenia, Serum Sickness, Severe Seborrheic Dermatitis, Stevens-Johnson Syndrome, Synovitis, Systemic Lupus Erythematosus (SLE), Trichinosis, Tuberculosis (TB), Tuberculosis Meningitis, Ulcerative Colitis, Uveitis, Vernal Keratoconjunctivitis, Acquired immune hemolytic anemia, Acute nonspecific tenosynovitis, Acute rheumatic carditis, Corticosteroid-responsive dermatoses, Ear infection-not otherwise specified caused by susceptible bacteria, Granuloma annulare lesions, Non-suppurative Thyroiditis, Ocular bacterial infections, Severe Psoriasis, Steroid-responsive inflammation of the eye, Varicella-zoster virus acute retinal necrosis, Watery itchy eyesBacterial Peritonitis, Bone Infection, Cystic fibrosis, Pseudomonas aeruginosa infection, Eye Infections, Inflammation of the External Auditory Canal, Intra-Abdominal Infections, Lower respiratory tract infection bacterial, Meningitis, Bacterial, Ocular Inflammation, Septicemia gram-negative, Skin and Subcutaneous Tissue Bacterial Infections, Corticosteroid-responsive Disorder of the Ophthalmic, Ear infection-not otherwise specified caused by susceptible bacteria, Ocular bacterial infections, Recurrent Complicated Urinary Tract Infection, Steroid-responsive inflammation

How Dex-Tobrin works

The short term effects of corticosteroids are decreased vasodilation and permeability of capillaries, as well as decreased leukocyte migration to sites of inflammation. Corticosteroids binding to the glucocorticoid receptor mediates changes in gene expression that lead to multiple downstream effects over hours to days.

Glucocorticoids inhibit neutrophil apoptosis and demargination; they inhibit phospholipase A2, which decreases the formation of arachidonic acid derivatives; they inhibit NF-Kappa B and other inflammatory transcription factors; they promote anti-inflammatory genes like interleukin-10.

Lower doses of corticosteroids provide an anti-inflammatory effect, while higher doses are immunosuppressive. High doses of glucocorticoids for an extended period bind to the mineralocorticoid receptor, raising sodium levels and decreasing potassium levels.

Tobramycin is a 4,6-disubstituted 2-deoxystreptamine (DOS) ring-containing aminoglycoside antibiotic with activity against various Gram-negative and some Gram-positive bacteria. The mechanism of action of tobramycin has not been unambiguously elucidated, and some insights into its mechanism rely on results using similar aminoglycosides. In general, like other aminoglycosides, tobramycin is bactericidal and exhibits both immediate and delayed killing, which are attributed to different mechanisms, as outlined below.

Aminoglycosides are polycationic at physiological pH, such that they readily bind to bacterial membranes ("ionic binding"); this includes binding to lipopolysaccharide and phospholipids within the outer membrane of Gram-negative bacteria and to teichoic acid and phospholipids within the cell membrane of Gram-positive bacteria. This binding displaces divalent cations and increases membrane permeability, which allows aminoglycoside entry. Additional aminoglycoside entry ("energy-dependent phase I") into the cytoplasm requires the proton-motive force, allowing access of the aminoglycoside to its primary intracellular target of the bacterial 30S ribosome. Mistranslated proteins produced as a result of aminoglycoside binding to the ribosome (see below) integrate into and disrupt the cell membrane, which allows more of the aminoglycoside into the cell ("energy-dependent phase II"). Hence, tobramycin and other aminoglycosides have both immediate bactericidal effects through membrane disruption and delayed bactericidal effects through impaired protein synthesis; observed experimental data and mathematical modelling support this two-mechanism model.

Inhibition of protein synthesis was the first recognized effect of aminoglycoside antibiotics. Structural and cell biological studies suggest that aminoglycosides bind to the 16S rRNA in helix 44 (h44), near the A site of the 30S ribosomal subunit, altering interactions between h44 and h45. This binding also displaces two important residues, A1492 and A1493, from h44, mimicking normal conformational changes that occur with successful codon-anticodon pairing in the A site. Overall, aminoglycoside binding has several negative effects, including inhibiting translation initiation and elongation and ribosome recycling. Recent evidence suggests that the latter effect is due to a cryptic second binding site situated in h69 of the 23S rRNA of the 50S ribosomal subunit. Also, by stabilizing a conformation that mimics correct codon-anticodon pairing, aminoglycosides promote error-prone translation; mistranslated proteins can incorporate into the cell membrane, inducing the damage discussed above.

Although direct mutation of the 16S rRNA is a rare resistance mechanism, due to the gene being present in numerous copies, posttranscriptional 16S rRNA modification by 16S rRNA methyltransferases (16S-RMTases) at the N7 position of G1405 or the N1 position of A1408 are common resistance mechanisms in aminoglycoside-resistant bacteria. These mutants also further support the proposed mechanism of action of aminoglycosides. Direct modification of the aminoglycoside itself through acetylation, adenylation, and phosphorylation by aminoglycoside-modifying enzymes (AMEs) are also commonly encountered resistance mutations. Finally, due to the requirement for active transport of aminoglycosides across bacterial membranes, they are not active against obligately anaerobic bacteria.

Dosage

Dex-Tobrin dosage

Insert 1 drop into the conjunctival sac 3-5 times per day. During the initial 24 to 48 hours, the dosage may be increased to 1 drop every 2 hours.

Side Effects

The most frequent side effects to topical ocular Tobramycin are localized ocular toxicity and hypersensitivity, including lid itching and swelling and conjuntival erythema. The reactions due to the steroid component are elevation of intraocular pressure (IOP) with possible development of glaucoma and infrequent optic nerve damage, posterior subcapsule cataract formation.

Toxicity

The oral LD₅₀ in female mice was 6.5g/kg and 794mg/kg via the intravenous route.

Overdoses are not expected with otic formulations. Chronic high doses of glucocorticoids can lead to the development of cataract, glaucoma, hypertension, water retention, hyperlipidemia, peptic ulcer, pancreatitis, myopathy, osteoporosis, mood changes, psychosis, dermal atrophy, allergy, acne, hypertrichosis, immune suppression, decreased resistance to infection, moon face, hyperglycemia, hypocalcemia, hypophosphatemia, metabolic acidosis, growth suppression, and secondary adrenal insufficiency. Overdose may be treated by adjusting the dose or stopping the corticosteroid as well as initiating symptomatic and supportive treatment.

Toxicity information regarding tobramycin is not readily available. Patients experiencing an overdose are at an increased risk of severe adverse effects such as nephrotoxicity, ototoxicity, neuromuscular blockade, and respiratory failure/paralysis. Symptomatic and supportive measures are recommended; hemodialysis may help clear excess tobramycin. Accidental ingestion of tobramycin is unlikely to result in an overdose, as aminoglycosides are poorly absorbed in the gastrointestinal tract.

Poor gastrointestinal absorption is reflected in animal studies. When administered by the intraperitoneal or subcutaneous route, the LD50 for mice and rats ranges from 367-1030 mg/kg while the oral LD50 values are more than 7500 mg/kg.

Precaution

Shake the bottle well before use. To prevent contamination do not touch the tip of the bottle to affected eye, eyelid or any surface of of the affected eye. Keep the bottle tightly closed after use.

Interaction

No specific interaction studies were performed with this combination eye drops. In case of concomitant therapy with other topical ophthalmic medicines, an interval of 10 minutes should be allowed between successive applications.

Volume of Distribution

A 1.5mg oral dose of dexamethasone has a volume of distribution of 51.0L, while a 3mg intramuscular dose has a volume of distribution of 96.0L.

Inhalation tobramycin had an apparent volume of distribution in the central compartment of 85.1 L for a typical cystic fibrosis patient.

Elimination Route

Absorption via the intramuscular route is slower than via the intravenous route. A 3mg intramuscular dose reaches a C_max of 34.6±6.0ng/mL with a T_max of 2.0±1.2h and an AUC of 113±38ng*h/mL. A 1.5mg oral dose reaches a C_max of 13.9±6.8ng/mL with a T_max of 2.0±0.5h and an AUC of 331±50ng*h/mL. Oral dexamethasone is approximately 70-78% bioavailable in healthy subjects.

Tobramycin administered by inhalation in cystic fibrosis patients showed greater variability in sputum as compared to serum. After a single 112 mg dose, the serum C_max was 1.02 ± 0.53 μg/mL, which was reached in one hour (T_max), while the sputum C_max was 1048 ± 1080 μg/g. Comparatively, for a 300 mg dose, the serum C_max was 1.04 ± 0.58 μg/mL, which was also reached within one hour, while the sputum C_max was 737 ± 1028 μg/g. The systemic exposure (AUC_0-12) was also similar between the two doses, at 4.6 ± 2.0 μg∙h/mL for the 112 mg dose and 4.8 ± 2.5 μg∙h/mL for the 300 mg dose. When tobramycin was administered over a four-week cycle at 112 mg twice daily, the C_max measured one hour after dosing ranged from 1.48 ± 0.69 μg/mL to 1.99 ± 0.59 μg/mL.

Half Life

The mean terminal half life of a 20mg oral tablet is 4 hours. A 1.5mg oral dose of dexamethasone has a half life of 6.6±4.3h, while a 3mg intramuscular dose has a half life of 4.2±1.2h.

Tobramycin has an apparent serum terminal half-life of ~3 hours following a single 112 mg inhaled dose in cystic fibrosis patients.

Clearance

A 20mg oral tablet has a clearance of 15.7L/h. A 1.5mg oral dose of dexamethasone has a clearance of 15.6±4.9L/h while a 3.0mg intramuscular dose has a clearance of 9.9±1.4L/h.

Inhaled tobramycin has an apparent serum clearance of 14.5 L/h in cystic fibrosis patients aged 6-58 years.

Elimination Route

Corticosteroids are generally eliminated predominantly in the urine. However, dexamethasone is 15

Tobramycin is primarily excreted unchanged in the urine.

Pregnancy & Breastfeeding use

Safety for use during pregnancy and lactation in humans has not been established

Contraindication

Epithelial herpes simplex keratitis (dendritic keratitis), vaccinia, varicella and other viral disease of the cornea and conjunctiva. Mycobacterial infections of the eye caused by, but not limited to, acid-fast bacilli such as Mycobacterium tuberculosis, Mycobacterium leprae, or Mycobacterium avium. Fungal diseases of ocular structures. Untreated purulent infection of the eye. Hypersensitivity to any component of the medication.

Special Warning

Use in paediatric: Safety and effectiveness in paediatric patients below 2 years have not been established.

Acute Overdose

Overdose of this eye drops may be flushed from the eye(s) with lukewarm tap water.

Storage Condition

Store at room temperature. Close the bottle immediately after use. Do not use for longer than one month after opening the bottle.

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