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Journal of veterinary pharmacology and therapeutics2026; doi: 10.1111/jvp.70087

Comparative Pharmacokinetics of Gentamicin in Healthy Young-Adult and Geriatric Horses.

Abstract: Aging may modify the pharmacokinetic disposition and excretion of gentamicin, although drug dose adjustments in aged horses are uncommon in clinical practice. Since high-dose, once daily dosing of gentamicin is considered therapeutically most effective, a comparative single-dose study was conducted to evaluate the differences in pharmacokinetics between healthy young-adult (5-10 years) and geriatric (≥ 25 years) horses receiving 6.6 mg/kg gentamicin intravenously. Blood samples were collected at designated time-points following drug administration and frozen at -80°C until assayed by a validated immunoassay. Gentamicin plasma concentrations versus time plots were analyzed by noncompartmental analysis using commercial software (WinNonlin-v8.4). Baseline physical examination and hematological parameters did not differ between age groups, except for a lower mean bodyweight in the geriatric group (477 ± 4 kg vs. 402 ± 6 kg). None of the pharmacokinetic parameters were statistically different between age groups. The oldest geriatric horse (41 years) had a longer half-life and lower clearance of 5.3 h and 22.79 L/h, respectively, compared to a range of 1.39-2.56 h and 26.39-40.59 L/h for the remainder of the geriatric group (25-29 years). Further studies may be indicated in horses > 30 years old to determine if dose reduction is necessary in this population.
© 2026 John Wiley & Sons Ltd.
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Publication Date: 2026-06-13 PubMed ID: 42287236DOI: 10.1111/jvp.70087Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study compared how the antibiotic gentamicin is processed in the bodies of healthy young-adult horses versus geriatric horses after a single intravenous dose.
  • The goal was to determine if aging affects the drug’s pharmacokinetics, which could inform whether dose adjustments are needed for older horses.

Background

  • Gentamicin is an antibiotic commonly used in horses, often administered at high doses once daily for therapeutic effectiveness.
  • Pharmacokinetics (PK) refers to the absorption, distribution, metabolism, and excretion of a drug in the body.
  • Aging can potentially alter PK parameters, such as the drug’s half-life and clearance, which could impact dosing requirements.
  • Despite this, dose adjustments for gentamicin in aged horses are not commonly done in clinical practice.

Study Design

  • A comparative, single-dose pharmacokinetic study was performed.
  • Two groups of healthy horses were selected:
    • Young-adults aged 5 to 10 years.
    • Geriatric horses aged 25 years or older.
  • Each horse received a dose of 6.6 mg/kg gentamicin intravenously.
  • Blood samples were taken at specific times after drug administration to measure plasma gentamicin concentrations.

Methodology

  • Samples were frozen at -80°C to preserve drug levels until analysis.
  • Gentamicin concentrations were measured using a validated immunoassay technique.
  • Pharmacokinetic parameters were calculated using noncompartmental analysis with commercial software (WinNonlin v8.4), which does not assume a specific compartmental model but uses the plasma concentration-time data directly.
  • Physical exams and hematological tests were performed and compared between the age groups to rule out underlying health differences.

Results

  • Key baseline characteristics were generally similar between young-adult and geriatric horses, except that geriatric horses had a lower average body weight (402 ± 6 kg vs. 477 ± 4 kg in young-adults).
  • No statistically significant differences were observed in any pharmacokinetic parameters (such as clearance, half-life, volume of distribution) between the young-adult and geriatric groups overall.
  • An exception was noted in the oldest horse (41 years old) within the geriatric group, which exhibited:
    • A longer half-life of gentamicin (5.3 hours), indicating the drug remained in the body longer.
    • A lower clearance rate (22.79 L/h), showing reduced efficiency in eliminating the drug from the bloodstream.
  • The rest of the geriatric horses aged 25-29 years had half-lives ranging from 1.39 to 2.56 hours and clearance rates of 26.39-40.59 L/h, more in line with younger horses.

Interpretation and Implications

  • Most aged horses up to around 29 years old process gentamicin similarly to younger horses, implying that routine dose adjustments based purely on age may not be necessary in this age range.
  • The markedly altered pharmacokinetics in the oldest horse suggests that very advanced age (30+ years) might affect gentamicin metabolism and excretion.
  • This finding highlights the need for further studies focusing specifically on horses older than 30 years to determine if gentamicin dosing guidelines should be modified to avoid drug accumulation and potential toxicity.
  • Clinicians should be aware that while standard dosing may be appropriate for most aged horses, exceptional cases may require individualized treatment plans.

Conclusion

  • The study provided evidence that gentamicin pharmacokinetics do not significantly differ between healthy young-adult and most geriatric horses (up to about 29 years old).
  • Additional research in very old horses (>30 years) is suggested to ascertain if dosage adjustments are warranted in this population to ensure therapeutic efficacy and safety.

Cite This Article

APA
Ceresia ML, Bedenice D, Gestrich A, McKinney-Aguirre CA, Paradis MR, Zaghloul I. (2026). Comparative Pharmacokinetics of Gentamicin in Healthy Young-Adult and Geriatric Horses. J Vet Pharmacol Ther. https://doi.org/10.1111/jvp.70087

Publication

Journal of veterinary pharmacology and therapeutics
ISSN: 1365-2885
NlmUniqueID: 7910920
Country: England
Language: English

Researcher Affiliations

Ceresia, Michelle L
  • Department of Large Animal Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, Massachusetts, USA.
  • School of Pharmacy, MCPHS University, Boston, Massachusetts, USA.
Bedenice, Daniela
  • Department of Large Animal Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, Massachusetts, USA.
Gestrich, Andrew
  • Department of Large Animal Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, Massachusetts, USA.
McKinney-Aguirre, Caroline A
  • Department of Large Animal Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, Massachusetts, USA.
Paradis, Mary Rose
  • Department of Large Animal Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University, North Grafton, Massachusetts, USA.
Zaghloul, Iman
  • School of Pharmacy, MCPHS University, Boston, Massachusetts, USA.

Grant Funding

  • Dorothy Russell Havemeyer Foundation

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