Evaluation of Diphenhydramine as an Antihistamine in Dogs Anesthetized for Surgical Excision of Mast Cell Tumours by Andrea Sanchez A Thesis presented to The University of Guelph In partial fulfillment of requirements for the degree of Doctor of Veterinary Science Guelph, Ontario, Canada © Andrea Sanchez, August, 2015 ABSTRACT EVALUATION OF DIPHENHYDRAMINE AS AN ANTIHISTAMINE IN DOGS ANESTHETIZED FOR SURGICAL EXCISION OF MAST CELL TUMOURS Andrea Sanchez Advisor: University of Guelph, 2015 Alexander Valverde This thesis determined in phase 1 the pharmacokinetics and cardio-respiratory effects of diphenhydramine (DHP) in conscious research dogs administered 1 mg/kg, IV, or 2 mg/kg, IM. Phase 2 consisted of a blinded clinical trial to investigate the effectiveness of DPH in anesthetized dogs to prevent the negative cardiovascular effects associated with potential histamine release during surgical excision of mast cell tumours (MCT). Dogs were anesthetized with a balanced anesthetic and analgesic technique and then allocated to receive DPH (1 mg/kg, IV) or saline. Plasma DPH and histamine concentrations were measured and correlated with cardio-respiratory parameters. Phase 1 results provided descriptive pharmacokinetics of DPH administered IV or IM in healthy dogs. Cardio-respiratory parameters remained within normal limits during the experiment and no behavioural changes were associated with DPH administration. The IV protocol was chosen for the clinical phase 2 under anesthesia, due to a shorter time to maximal concentrations (Tmax) of 6.0 ± 0.3 min versus 45 ± 5.1 min for the IM group. During the clinical phase 2, plasma concentrations of DPH remained above concentrations considered therapeutic (25 ng/mL) in humans until the end of surgery. Despite the lack of statistical differences in histamine concentrations throughout anesthesia between groups, higher histamine concentrations were measured during maximal manipulation of the tumour. Mean arterial and diastolic blood pressures were significantly lower in DPH than in the saline group during surgical excision of the tumour. These results contradict the assumption that hypotension is more likely in dogs undergoing MCT excision that have not received DPH. Values for cardio-respiratory parameters in both groups were considered within acceptable limits for anesthetized dogs. In dogs, DPH can be administered by either IV or IM routes at 1 mg/kg and 2 mg/kg, respectively, to yield plasma concentrations that exceed therapeutic concentrations in humans without noticeable adverse behavioural or cardio-respiratory side effects. The administration of DPH prior to surgical removal of MCT in dogs did not have clear clinical anesthetic differences related to cardio-respiratory responses compared to dogs receiving a saline placebo. ACKNOWLEDGEMENTS I would like to thank for their support and contribution to this thesis to all the members of my committee: Dr. Conny Mosley, Dr. Ameet Singh, Dr. Melissa Sinclair, Dr. Brad Hanna, and Dr. Tony Mutsaers. I would like to specially thank my supervisor; Dr. Alex Valverde, this thesis has become a reality because of his patience and dedication. Hopefullly someday I can repay him for all the energy and effort that he put in helping me reach my goals over these years. Thanks for helping me find my way. Thanks to my family and friends both sides of the Atlantic for being always there even in the distance. I would also like to acknowledge all the anesthesia technicians and residents that not only helped me during the clinical part of this thesis but also filled my path these years with happiness, friendship and fun. Finally I would like to dedicate this work to the most important men in my life. To my husband, Dr. Luis Gaitero, for surviving your second residency by my side, for keeping me calm and steady when I needed it the most. To my dad, the best person I ever met, because all the work and effort that I put in everything in life is hoping that he is proud of me, wherever he is. iv TABLE OF CONTENTS ACKNOWLEDGEMENTS................................................................................................iv TABLE OF CONTENTS....................................................................................................v DECLARATION OF WORK PERFORMED....................................................................ix LIST OF TABLES...............................................................................................................x LIST OF FIGURES...........................................................................................................xii LIST OF ABBREVIATIONS............................................................................................xv CHAPTER I GENERAL LITERATURE REVIEW 1.1. Introduction...................................................................................................................1 1.2. Objectives and hypothesis............................................................................................2 1.3. Histamine…..................................................................................................................3 1.4. Histamine receptors......................................................................................................6 1.5. Biological functions of histamine receptors.................................................................7 1.5.a. H1 receptor.....................................................................................................8 1.5.b. H2 receptor.....................................................................................................9 1.5.c. H3 receptor...................................................................................................10 1.5.d. H4 receptor...................................................................................................12 1.6. Plasma histamine concentration determinations…………………………………….13 1.7. Cardiovascular effects of histamine release……….……………….………………..15 1.8 H1-Antihistamines……………………………………………..……………………..19 1.8.a. Side effects of first generation H1-antihistamines…………………………22 v 1.9. Diphenhydramine……………………………...…………………………………….24 1.9.a. Pharmacokinetics of DPH…………….……………………...……………24 Systemic availability….…....…………………………………….26 Volume of distribution…………………………………………...27 Elimination half-life……………………………………………...29 Clearance…………………………………………………………32 1.9.b. Clinical uses in veterinary medicine............................................................34 1.10. Mast cell tumours......................................................................................................38 1.10.a. Mast cells...................................................................................................39 1.10.b. Canine mast cell tumours…………………....……………………….…..41 Epidemiology…………………………………………….………41 Pathogenesis……………………………………..……….………41 Pathophysiology……………………………………………….…43 1.11. Anesthetic considerations and effects of histamine in dogs…….…….……...........44 1.12. Cardiovascular monitoring under anesthesia…….…….…………………………..50 1.12.a. Arterial blood pressure monitoring and the significance of hypotension..50 1.12.b. Cardiac output monitoring…………………………………………….…53 1.13. References….………………………….…………………………………………...59 CHAPTER II THE PHARMACOKINETICS OF DIPHENHYDRAMINE AFTER ADMINISTRATION OF A SINGLE INTRAVENOUS OR INTRAMUSCULAR DOSE IN HEALTHY DOGS vi 2.1. Summary.................................................................................................................... 83 2.2. Introduction.................................................................................................................85 2.3. Materials and methods................................................................................................87 2.3.a. Animals.............................................................................................................87 2.3.b. Experimental design.........................................................................................87 2.3.c. Study protocol...................................................................................................87 2.3.d. Plasma Diphenhydramine analysis...................................................................89 Method validation..........................................................................90 2.3.e. Pharmacokinetic Analysis................................................................................91 2.3.f. Data analysis.....................................................................................................94 2.4. Results.........................................................................................................................95 2.5. Discussion...................................................................................................................96 2.6. References.................................................................................................................105 CHAPTER III EVALUATION OF THE ANTIHISTAMINIC EFFECTS OF DIPHENHYDRAMINE IN DOGS UNDERGOING EXCISION OF MAST CELL TUMOURS 3.1. Summary.................................................................................................................. 117 3.2. Introduction...............................................................................................................120 3.3. Materials and methods..............................................................................................123 3.3.a. Animals...........................................................................................................123