Comparative Medicine Copyright 2001 by the American Association for Laboratory Animal Science Vol 51, No 5 October 2001 430 Cyclosporine A-Induced Mammary Hyperplasia and Hyperprolactinemia in New Zealand White Rabbits Ruta Petraitiene, MD, 1 Vidmantas Petraitis, MD, 1 John Bacher, DVM, 2 Saroj R. Das, PhD, 3 Albert F. Parlow, PhD, 4 and Thomas J. Walsh, MD 1,* Received: 2/16/01. Revision requested: 3/20/01. Accepted: 8/3/01. 1 Immunocompromised Host Section, Pediatric Oncology Branch, National Can- cer Institute, National Institutes of Health, Building 10, Room 13N240, Center Drive, Bethesda, Maryland 20892, 2 Surgery Service, Veterinary Resources Pro- gram, Office of Research Services, National Institutes of Health, Bethesda, Mary- land, 3 Ani Lytics Incorporated, Gaithersburg, Maryland, 4 National Hormone & Peptide Program, Harbor-UCLA, Medical Center, Research & Education Insti- tute, Torrance, California, 90502. *Corresponding author. In 1983, cyclosporine A (CsA) was approved for clinical prac- tice as an effective immunosuppressive cyclic peptide. This com- pound has been extensively used for prevention of organ rejection in transplant recipients and for treatment of autoim- mune diseases (1). It also has been used as an effective immu- nosuppressive agent in rabbits for transplantation immunology (2-6), toxicology (7-13), treatment of experimentally induced uveitis or keratitis (14-17), and for study of the pathogenesis and treatment of various opportunistic infections (18-24). During recent treatment of rabbits with CsA and methyl- prednisolone or only CsA, we found that virtually all rabbits de- veloped multiple abdominal masses of uncertain etiology. Fur- ther study of these masses revealed striking hyperplasia of mammary tissues. To further investigate this phenomenon, we prospectively studied a cohort of rabbits treated with CsA and methylprednisolone or only CsA. Materials and Methods Animals. One hundred three nulliparous female New Zealand White rabbits (Oryctolagus cuniculus; Hazleton Research Prod- ucts, Inc., Denver, Pa.; aged 5 to 6 months), which ranged in body weight from 2.4 to 3.5 kg, were studied. The experiments were ap- proved by the Animal Care and Use Committee of the National Cancer Institute. All rabbits were housed and monitored under hu- mane care and use in facilities approved by the Association for As- sessment and Accreditation of Laboratory Animal Care (AAALAC International) and according to National Institutes of Health (NIH) guidelines for animal care and in fulfillment of criteria of AAALAC International and the National Research Council Com- mittee Guidelines (25). Rabbits were housed individually in stain- less steel cages (26  26  17 in.) with slotted floors. The animals were fed a commercial rabbit diet (Lab Diet 5326 Hi-Fiber Rabbit, PMI Nutrition International, Inc., Brentwood, Mo.), and water was available ad libitum. Rabbits were maintained in a controlled environment at a temperature of 18 to 21 C (65 to 70F), relative humidity of 40 to 65%, and photoperiod of 12/12-h light/dark. Ventilation in the animal room was 100% fresh air with approxi- mately 12 complete air changes/h. Vascular access was established in each rabbit by surgical placement of a silastic tunneled cen- tral venous catheter (26), which permitted non-traumatic venous access for repeated blood sampling and parenteral administration of various agents. Immunosuppression. To simulate the conditions of immu- nosuppression in our experiments, therapy with CsA (Sandim- mune ampules for injection, Sandoz Pharma Ltd., Basle, Switzer- land for Sandoz Pharmaceuticals Corp., East Hanover, N.J.) was administered intravenously on a daily basis. The CsA was dis- solved at a concentration of 2.5 mg/ml in normal saline, stored protected from light at 4C, and used within 24 h. The rabbits Purpose: To investigate the potential activity of cyclosporin A (CsA) to induce mammary hyperplasia in New Zealand White (NZW) rabbits. Methods: Female NZW rabbits were used throughout experiments. To simulate the conditions of immunosuppres- sion, CsA (10 mg/kg of body weight/d) was administered intravenously on a daily basis for 14 days and methylpredniso- lone (5 mg/kg/d) was administered on the first two days. The CsA (10 mg/kg/d) also was administered without methyl- prednisolone for 14 days to another cohort of rabbits. Mammary tissue of each rabbit was palpated and serially mea- sured during this treatment period. The CsA was discontinued, and rabbits were monitored for 14 more days during the washout period. Sequential plasma concentrations of prolactin, 17 β-estradiol, and progesterone in each blood sample were determined by use of radioimmunoassay. Results: All NZW rabbits treated with CsA and methylprednisolone for immunosuppression consistently devel- oped striking mammary tissue hyperplasia. At the end of treatment with CsA and methylprednisolone, mammary glands had extensive changes consistent with actively lactating glands. Similar but less extensive hyperplasia de- veloped in response to CsA alone. Plasma concentration of prolactin increased during treatment and decreased during the washout period. Plasma concentration of 17 β-estradiol increased during treatment and continued to increase during the washout period. Plasma progesterone concentration decreased at the end of treatment. On discon- tinuation of CsA, mammary hyperplasia regressed. Conclusions: Cyclosporine A, with or without methylprednisolone, induces mammary hyperplasia and hyperprolac- tinemia in NZW rabbits. This rabbit model may be a reliable in vivo system by which to study immunosuppressant- induced structural and functional changes of mammary glands similar to those observed in humans. Pages 430-435