Comparison of the pharmacokinetics of penicillin G and ampicillin in the horse.

The research discusses the comparison of two antibiotics, penicillin G and ampicillin, in relation to their binding capacity with horse serum proteins and their elimination rates in horses. The binding capacity refers to the percentage of each antibiotic bound to serum proteins and the elimination rates refer to the time taken for the concentrations of the antibiotics to reduce by half, post administration.

Protein Binding Capacity of Penicillin G and Ampicillin

• This research was conducted by performing equilibrium dialysis or ultrafiltration technique. These methods are used to measure the affinity and binding capacity of certain substances to serum proteins.
• Horse serum proteins showed a higher level of affinity and binding capacity for penicillin G as compared to ampicillin.
• Therapeutic concentration ranges indicated that serum protein binding accounted for 6.8-8% of the total ampicillin concentration. This implies that only a small fraction of the administered ampicillin binds to serum proteins, leaving a larger fraction free to exert its therapeutic effect.
• Penicillin G displayed a significantly higher protein binding fraction of 52-54% of its total serum concentration, meaning that more than half of the administered penicillin G is bound to, and thus inactive in serum proteins.

Elimination Rates of Penicillin G and Ampicillin

• The rate at which the body eliminates a drug, also known as the biological half-life, was measured by tracking serum concentrations of both penicillin G and ampicillin following single intravenous injections.
• Ampicillin exhibited a longer half-life of 93 minutes, indicating that it persists in the horse’s body for a longer period than penicillin G, therefore producing a more prolonged therapeutic effect.
• Penicillin G on the other hand, was eliminated more quickly with a half-life of 53 minutes, implying a shorter duration of action compared to ampicillin.
• These measurements were taken in healthy adult horses with no recorded impairment in circulation or kidney and liver function, which would otherwise affect the pharmacokinetics of these drugs.