J Toxicol Pathol 2007; 20: 125–132 Review Use of Miniature Pig for Biomedical Research, with Reference to Toxicologic Studies Tetsuo Nunoya1, Kazumoto Shibuya1, Toshiki Saitoh1, Hajime Yazawa1, Keigo Nakamura1, Yasuko Baba1, and Takuya Hirai1 1Nippon Institute for Biological Science, 9–2221–1 Shinmachi, Ome, Tokyo 198–0024, Japan Abstract: The pigs have been a well-recognized experimental animal in biomedical research for many years. Minipigs particularly have gained in massive importance in biomedical research over the last few years. Pigs are increasingly being used as an alternative non-rodent animal species to the dog or monkey in toxicology because of the morphological and physiological similarities between porcine and human organs, especially the skin, cardiovascular system, gastrointestinal tract, and the urinary system. Accumulating data indicate that the minipig can be used for all routes of administration and is preferable to the dog or monkey in many cases. The advantages of the minipig compared to the domestic pig are its smaller size, even at full maturity, slower growth during studies, ease of handling, and controlled genotype as well as microbiologically obvious characteristics. Minipigs also have an advantage over traditional non- rodent animals because of increasing ethical concerns about the use of them in experiments. Reservoir of information from studies using minipigs is the keystone for the future diffusion of them as a good alternative to the non-rodent animals traditionally used in toxicology. (J Toxicol Pathol 2007; 20: 125–132) Key words: miniature pig, animal model, biomedical research, toxicology Introduction and regulatory studies are liable to be avoided as far as possible from the perspective of animal protection and their A crucial prerequisite for the development of safe rising costs. Along with the increased use of pigs in preclinical protocols in biomedical research is to ensure the research, technical advancements in surgery, anesthesia, suitability of animal models that would allow human-related husbandry and handling of the animal as well as improving validation of the valuable research information gathered the humane care have accelerated making it easier to use pigs from experimentation with them. Among animal models, in research institutions. Compared with the domestic pig, the pig has played a significant role in the scientific the advantages of the minipig are its ease of handling community as an alternative non-rodent species to the dog or because of its inherently smaller size, even at full maturity, monkey. The miniature pigs (minipigs) in particular have which is particularly suitable for long-term studies, and its gained increasing importance in the biomedical research well characterized and controlled genotype as well as areas of radiology, traumatology, dermatology, pathology, phenotype. Accordingly minipigs such as Yucatan, Hanford embryology, gastroenterology, cardiology, nephrology, and Göttingen have been widely used for chronic studies, in surgery, pediatrics, and odontology over the last few years. which the significantly rapid growth of domestic breeds More recently, minipigs have been used for such fields of would be an issue. Minipigs enable collection of larger investigation as the generation of organs with human volumes of multiple samples of body fluids and biopsies transgene expression for xenotransplantation1, and compared to rodents, thus making it possible to conduct neurotransplantation using porcine neural stem cells grafted studies that approximate to those performed in humans. into inbred miniature pigs as an alternative model to non- Minipigs are mild and good in nature, and the commercially human primates xenografted with human cells2. Behind the available ones tend to be more tractable than the domestic research trend is the fact that the anatomy of pig’s organ breeds raised in an agricultural setting, allowing successful systems, as well as physiological and pathophysiological performance of experiments under unstressed conditions. responses, has been found similar to those of the human3 and Minipigs are bred and raised under strictly controlled that monkeys and dogs widely used for drug development conditions including a defined environment, restricted access to food, and microbiological monitoring. Received: 1 May 2007, Accepted: 14 May 2007 General Considerations Mailing address: Tetsuo Nunoya, Nippon Institute for Biological Science, 9–2221–1 Shinmachi, Ome, Tokyo 198–0024, Japan The pig (Sus scrofa domestica) has been widely used in TEL: 81-428-33-1059 FAX: 81-428-31-6166 E-mail: [email protected] biomedical research for decades because of its 126 Miniature Pig in Toxicology morphological and physiological similarities to the human, Types of Minipigs e.g. with respect to the cardiovascular, integumentary, digestive, and urinary systems. Most of the pigs used as There are several breeds of minipigs available in the experimental animals have been small domestic breeds, but US, Europe and Japan. The well known breeds are the the domestic pig was found unsuitable for specific purposes Yucatan16, Hanford14, and Sinclair minipigs17 in the US, such as chronic studies because of the disadvantage of its Göttingen in Europe3, and Ohmini18, CLAWN19 and NIBS20 rapid growth during the studies. The development of minipigs in Japan. minipigs has resulted in strains of more manageable size The Yucatan minipig is a naturally occurring breed of with genetically and microbiologically obvious miniature swine and the only miniature breed in North characteristics. The main difference between the farm pig America derived from one primary gene pool without the and the minipig is the size at birth and sexual maturity. The introduction of genes from other breeds21. The Yucatan former typically increases from an average weight of 1 kg at minipig was developed at Colorado State University from birth to 100 kg at 4 months of age and the latter usually foundation animals imported from the Yucatan peninsula in grows from a birth weight of 0.5 kg to 12–45 kg at 4 months 196022. The Yucatan minipig is slate-gray, essentially of age4,5. Minipigs reach puberty at an earlier age than larger hairless, and has a docile temperament. The tractable nature domestic pigs. Gilts reach puberty at 3.5–6 months of age of this pig minimizes risks for animal handlers as well as and boars have been known to successfully serve and sire 4– researchers. The absence of a hair coat facilitates a number 8 litters at 3–6 months of age with breed variation6. of laboratory procedures and reduced the heavy odor found The cardiovascular system of the pig is anatomically in most breeds of swine16. With regular human contact, close to that of human, excepting the presence of the left these pigs become amenable to handling and tolerate azygous vein which enters the coronary sinus7. The growth restraint well. Their mean birth weight is 0.74 kg, and their of the heart and great vessels in young pigs is comparable to mean weaning weight at 60 days is 6.47 kg. The mean that of growing children, and the heart of a 40–50 kg weight of mature non-obese boars is 83 kg while that of miniature pig is approximately equivalent to that of an adult mature sows is 70 kg16. The weight at sexual maturity has human. Moreover, the heart size to body weight ratio in pigs been lowered through selective breeding to, currently, commonly used in the laboratory (25–30 kg) is identical to between 30 and 50 kg, with an ultimate goal of 20–25 kg22. that of humans8. Though the hemodynamics of the pig have The Hanford miniature swine was developed at the been shown to be similar in cardiac function to humans, Hanford Laboratory (Battelle Northwest Research variations among breeds and age of the animal need to be Laboratories) in Richard, Washington in 1958. The Hanford taken into consideration9. Experimental atherosclerosis has minipig was produced by crossing Falouse gilts with a been induced by feeding minipigs an atherosclerotic diet and Pittman-Moore boar which had been bred at the University the induced lesions appeared to be similar to those observed of Minnesota and derived from swamp hogs from the in humans10,11. southern US. The Hanford minipig has a white haircoat and The skin of the pig is relatively hairless and tightly skin with less subcutaneous fat than other breeds and weighs attached to the subcutaneous tissues like that of humans. 75 kg at maturity24, making them excellent for dermal With the exception of the absence of apocrine sweat glands, studies. The heart size of the 40–50 kg Hanford minipig is the similarities of pig skin to that of humans have long been similar in size to that of humans, with few collaterals, recognized in overall morphology including epidermal making the pig excellent cardiovascular model4. thickness, cellular composition and cutaneous blood supply The Sinclair miniature swine was developed at the characteristics12,13. Because of these anatomical as well as University of Missouri Comparative Medicine Research physiological similarities, more than any other animal Farm, i.e. the Sinclair Farm23. The Sinclair minipig has a species, pigs have been used as a standard model for surgical significant incidence of cutaneous melanoma with a and wound healing studies of the skin and as a model of histopathology similar to that of humans17 and has been dermal and transdermal toxicology4. extensively studied as an appropriate animal model for the The digestive and urinary systems of pigs function like