Injections and Implants


Liquid injection formulations can readily be administered to animals through conventional syringes and needles. Small animals use the same gauge as humans do (e.g., 27 to 32 gauge). In contrast, large animals require larger needle gauges to ensure they penetrate the hair and skin and do not break during use (e.g., 14 to 21 gauge). Fortunately large animals are less needle shy than humans. Larger bore needles do offer the formulator some additional formulation benefits as more viscous preparations can, theoretically, be administered through a larger bore needle. Volumes administered are much greater than those administered in humans (up to 10 mL subcutaneously in cattle), which again, affords the formulator some scope in the dosage form design. A major challenge when formulating a product for injection is the mass of active that needs to be incorporated into the dosage form. An in-depth knowledge of solubility science is required for this area of veterinary formulation science.

The physical characteristics of the dosage form require tailoring around the injectability profile of the product. Ease of administration is a key marketing and use characteristic and should be borne in mind during the development of such a dosage form. To understand syringing acceptability for products in development, Foster has categorized liquid formulations into five model formulations representing a wide range of viscosity and Newtonian and non-Newtonian systems. These model formulations were then tested

Figure 13 Administration to the middle third of the ear of a cow.

by veterinarians using 14 to 21 gauge needles and 3 to 35 cc syringes. The veterinarians determined whether the product could be withdrawn from a vial and injected through the syringe/needle combination. New products in development then need only to be classified as being similar to one of the five model formulations, and immediate understanding of how acceptable veterinarians will find the syringing of the product will be known (54).

As mentioned previously in this chapter, the availability of the animal in veterinary dosage form development is an opportunity that the formulation scientist can capitalize upon. They can be used to advantage in the development of injectable formulations since injectability depends not only on the internal diameter of the needle, the viscosity of the preparation, and the syringe characteristics, but also on the environment of the injection site, something that is difficult to model on the bench.

Administration sites are a great consideration when developing an injectable formulation for large animals. Intramuscular injections are less popular than the subcutaneous route as meat damage can occur, which can devalue the animal at slaughter.

Often the ear has been chosen as the site for administration since it represents an inedible portion of the carcass (except for swine in certain regions of the world) and is discarded on slaughter, accessible to the administrator, well perfused with blood, and administration is relatively pain free and, therefore, provides minimal distress to the animal. Pfizer's one-time administered cephalosporin for cattle (Excenel®) is administered into the middle third of the ear (Fig. 13). This minimizes tissue residue and was considered unique and granted a U.S. patent (55).


Implants are a favored subcutaneous delivery system for veterinary formulation scientists as it affords long release of drug, and in contrast to its human counterparts, animals better tolerate the rather invasive administration procedures that are associated with implants. An example of a commercially available subcutaneous implanter is shown in Figure 12.

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