Penicillin is a bactericidal antibiotic of the beta-lactam class which acts by inhibition of bacterial cell wall synthesis. Neomycin belongs to the aminoglycoside class of antibiotics and acts by inhibition of protein synthesis by interfering with ribosomal function.Penicillin and neomycin have a broad spectrum of activity against both gram-positive and gram-negative bacteria and exhibit synergistic activity when used in combination.

Neomycin is not absorbed from the gastro-intestinal tract, and the absorption of procaine benzylpenicillin is poor.

After intramuscular administration both compounds are well resorbed. The distribution volume of both compounds is relatively low, but in the case of procaine benzylpenicillin the tissue penetration in inflamed tissues is increased. The main elimination pathway for both compounds is via the kidneys, mainly in the unchanged form.

Pharmacokinetics of procaine benzylpenicillin and neomycin after intramuscular administration are similar in equine, bovine, ovine and porcine species. Maximum blood levels of procaine benzylpenicillin (1 -2 I.U./ml) and neomycin (10-15 µg/ml) were achieved within hours and elimination is rapid. No accumulation occurs with a dosing interval of 24 hours.

In the dog, maximum benzylpenicillin levels of approximately 3 I.U/ml occur one to two hours after administration. Maximum neomycin levels occur slightly earlier and are higher and more variable than with large animals. In the cat the time at which maximum levels occurred were comparable to those observed in the dog. Maximum levels of benzylpenicillin were consistently higher in the cat, whereas neomycin concentrations obtained were comparable between the cat and dog.