Gentamicin Sulfate

(gentamicin)

Brand

Gentocin

Availability

Oral solution: 5 mg/mL
Injectable: 40 mg/mL, 100 mg/mL. *Note: injectable form diluted in normal, or hypertonic, saline can also be used when nebulizing required.

Pharmacology

Gentamicin is an aminoglycoside antibiotic that is considered bactericidal, and concentration dependent. It acts on susceptible bacteria by binding, irreversibly, to the 30S ribosomal subunit and inhibiting protein synthesis. Gentamicin acts primarily against Gram-negative aerobes, and some Gram-positive aerobic organisms.

It is widely distributed in extracellular fluid and does not penetrate ocular tissue, nor readily crosses the blood-brain barrier. The drug tends to accumulate in the kidneys, and the inner ear, explaining the reason for nephro and ototoxicity.

Aminoglycosides are able to cross the placental barrier and may cause nerve or nephrotoxic effects to the fetus. It is not recommended that these drugs be used in pregnant rats.

Excretion of gentamicin is almost exclusively eliminated by way of glomerular filtration, unchanged in urine; as well as, a very small amount excreted in bile.

Gentamicin, as with other aminoglycosides, should be used with caution in existing renal conditions or neuromuscular disease. The drug is not intended for long term use.

Indications

For use on Gram-negative infections such as pseudomonas, proteus and serratia, and some Gram-positive infections such as staphylococcus. Useful in skin/soft tissue, bone, abdominal and urinary tract infections and severe respiratory infections.

Drug Interactions or Contraindications

Use caution when giving with other aminoglycosides, cephalosporins, and loop diuretics such as furosemide, which may increase nephro or ototoxicity.
Per the manufacturer it is not a recommendation to mix gentamicin in with other drug compounds.

Adverse Reactions

EENT: ototoxicity

CNS: lethargy,neuromuscular blockade, facial edema, tremors

Resp: bronchoconstriction and/or bronchospasm can occur when nebulizing, but are reportedly rare

GI: Affects GI flora

GU: nephrotoxicity

Skin: itching, swelling

Other: loss of coordination

*Note: all aminoglycosides have the potential for causing ototoxicity, nephrotoxicity and neuromuscular blockade due to high serum and tissue concentrations, as well as prolonged use. This is especially prevalent when given by parenteral administration (e.g., IM, IV, SQ) due to systemic absorption. Studies have shown no evidence of toxicity when nebulized in therapeutic doses even with prolonged use. The lack of toxicity seen with nebulized aminoglycosides may be due to minimal or no systemic absorption, having lower unsustain serum concentrations due to a lower dosage used, and being directly delivered to the area of infection. A noted concern of prolonged or repetitive nebulizing of antimicrobials, such as the aminoglycosides, is the potential for the overgrowth of resistant organisms.¹^, ²^, ³

Dosage Recommendations

1 mg/lb to 2 mg/lb, SQ, IM, BID ⁴, or if using a
40 mg/mL solution then give .025mL/lb to .05mL/lb; BID for 7 to 14 days ⁴

or

2 mg/kg to 4 mg/kg, SQ, IM, q8hr to q24h ¹^, ²⁶^, ²⁷^, ²⁸

or

2 mg/kg to 5 mg/kg, SQ, IM, q24hr ⁴¹^, ⁴⁴

or

5 mg/kg to 8 mg/kg, SQ, IM, divided q8hr to q24hr ²⁹

or

4 mg/kg to 20 mg/kg, IM, q12hr to q24hr ⁴²^, ⁴⁵

or

4 mg/kg to 24 mg/kg, SQ, IM, q12hr ⁴¹^, ⁴⁴

or

20 mg/kg, SQ, q24hr ²⁶^, ³⁴^, ⁴¹^, ⁴⁴

Gentamicin in Nebulizing Treatment Regimen

The following is a dosage recommendation by Dr. Michael Hutchinson, DVM; Animal General, Cranberry Township, PA.. ¹⁸, for the use of Gentocin along with albuterol by nebulization in a treatment regimen for chronic and repeated respiratory infection, and serious to advanced lung infection not responding to enrofloxacin (Baytril) and doxycycline alone:

Nebulize 15 minutes, 2-3 times a day, for 14 days with the following mixture:
8 mL sterile saline
0.5mL Gentocin injectable 100 mg/mL
0.5mL Albuterol 0.083% Inhalation

*Note: excess mixture for nebulization can be refrigerated for up to 3 days.

For complete treatment regimen recommended by Dr. Hutchinson see articles Mycoplasma or Pneumonia in the Health section of the Rat Guide. Also see individual monographs for more information on the above drugs in the Medication section of the Rat Guide.

See Fig. 3: of First Aid Supplies in the Health section of the Rat Guide and in The Medical Corner of the RMCA site for information on nebulizer use and set up.

See Nursing Care section in articles Mycoplasma or Pneumonia for information on type of nebulizer and where to purchase.

Considerations

Be sure pet rat has access to plenty of fluids, or fluid containing foods (e.g., fruits that are juicy), while receiving the drug in order to minimize chemical irritation in renal tubules.
Give yogurt, Lactobacillus capsules/granules, or Bene-Bac to help maintain normal gut flora.
In treating suspected polymicrobial infections, where a broader coverage may be needed, synergistic or combination drugs may be used. The following drugs may be seen used simultaneously with gentamicin: ampicillin, amoxicillin, penicillin, fluoroquinolones (e.g., enrofloxacin or orbifloxicin), cephalosporins, trimethoprim-sulfa, and metronidazole. ¹
Please note that it is imperative to discuss the changing or adding of any medications during your rat’s treatment with your veterinarian to prevent future resistance of microbes to the drugs prescribed.
Injectable and oral can be stored at room temperature.

References

Campbell, P., & Saiman, L. (1999). Use of aerosolized antibiotics in patients with cystic fibrosis. Chest, 116(3), 775-88. Retrieved October 6, 2010, from http://chestjournal.chestpubs.org/content/116/3/775.long
Cohn, L. (2009). Inhalant therapy: Finding its place in small-animal practice. Veterinary Medicine, 104(7), 336-341. Retrieved October 6, 2010, from http://veterinarymedicine.dvm360.com/vetmed/article/articleDetail.jsp?id=608394
Prober, C., Walson, P., Jones, J., & Committee on Infectious Diseases and Committee on Drugs. (2000). Technical report: precautions regarding the use of aerosolized antibiotics. Pediatrics, 106(6), e89. Retrieved October 6, 2010, from http://pediatrics.aappublications.org/cgi/content/full/106/6/e89