Comparative and Veterinary Pharmacology

then released for approximately 135 days. The history of the development of the
device components, together with the influence of dissolved gas and the radius of
the exit port in achieving the required PK profile has been described in detail
(Cardinal 1997 ). A second example is Paratect 1 bolus (Pfizer Animal Health).
This is based on the depot principle. This oral i.r. device for cattle continuously
releases morantel tartrate over a period of 90 days for the control of gastrointestinal
roundworms (Jones 1983 ). It consists of a stainless steel tube, which has porous
polyethylene discs at each end and which contain cellulose acetate. A reservoir
containing morantel and polyethylene glycol (PEG) is created in the main body of
the cylinder (Fig.1b). Devices designed for pulsatile release have also been
successfully developed. These systems are generally based on sequential, timed
release of tablets as the outer cylinder is eroded by ruminal fluids. Several studies
suggest that pulsed release of oxfendazole may be superior to SR oral formulations
in prevention of parasitism in calves, perhaps in part due to decreased risk of
resistance (reviewed in Cardinal 1997 ).
The anatomical and physiological variability between the gastrointestinal tracts
of companion animals and humans, do however, mean that information on IR and
CR oral formulations is limited (Sutton 2004 ). Intelligent formulation is the
simplest example of a delivery “device”. In this regard, significant advances have
been made in respect of standardising testing methods for once-a-day palatable

Semi-permeable shell

Tablet

Partition

High density

Flow Flow

Disc

Band

a

b

PEG/morantel

Tube

Ivermectin Hole

Fig. 1Intraruminal devices. (a) Osmotically-driven mini-pump containing an osmotic tablet, a
partition layer, the formulation and high density metal (e.g. IVOMEC-SR 1 bolus, Merck Corp.).
(b) Reservoir-based PEG/drug system containing a stainless steel chamber capped at each end with
polymer-impregnated polyethylene discs (e.g. Paratect 1 bolus morantel system, Pfizer Animal
Health)

Drug Delivery Systems in Domestic Animal Species 85