Original Article DOI: 10.1111/j.1476-5829.2010.00223.x Calcitriol (1,25-dihydroxycholecalciferol) enhances mast cell tumour chemotherapy and receptor tyrosine kinase inhibitor activity in vitro and has single-agent activity against spontaneously occurring canine mast cell tumours * E. K. Malone 1 , K. M. Rassnick 1 , J. J. Wakshlag 1 , D. S. Russell 2† , R. Al-Sarraf 3 , D. M. Ruslander 4 , C. S. Johnson 5 and D. L. Trump 6 1 Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY USA 2 Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY, USA 3 Animal Emergency and Referral Associates, Fairfield, NJ, USA 4 Veterinary Specialty Hospital of the Carolinas, Cary, NC, USA 5 Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Buffalo, NY, USA 6 Department of Medicine, Roswell Park Cancer Institute, Buffalo, NY, USA Abstract Calcitriol potentiates the effect of multiple chemotherapy agents in a variety of tumour models. In this study, we examine whether calcitriol increases chemotherapy or tyrosine kinase inhibitor in vitro cytotoxicity in canine mastocytoma C2 cells. We also evaluate the in vivo effect of DN101, a highly concentrated oral formulation of calcitriol designed specifically for cancer therapy, as a single-agent therapy in dogs with mast cell tumours (MCTs). Calcitriol exhibits synergistic, antiproliferative activity when used in combination with CCNU, vinblastine, imatinib or toceranib in vitro. The concentrations required for 50% growth inhibition were generally two- to six-fold lower when the drugs were used in combination than when used individually. High-dose oral calcitriol induced remission in 4 of 10 dogs (one complete remission, three partial remissions), although the majority experienced toxicity, necessitating discontinuation of the trial. Further evaluation of calcitriol in combination therapy for dogs with MCTs is warranted. Keywords chemotherapy, in vitro model, oncology, small animal, tyrosine kinase Introduction Calcitriol (1,25-dihydroxyvitamin D 3 ) is the natu- ral ligand for the vitamin D receptor (VDR) and is a key regulator of bone metabolism and calcium ∗ Presented in part at the 29th Annual Conference of the Veterinary Cancer Society, Austin, TX, October 16 – 19, 2009. † Present address: Department of Veterinary Biosciences, The Ohio State University, Columbus, OH, USA homeostasis. Numerous investigations have shown the biological actions of calcitriol also include antineoplastic activity through potent antiprolifer- ative effects, prodifferentiating activity, activation of apoptotic pathways and inhibition of angiogen- esis through interaction with the nuclear VDR, affecting target gene transcription. 1,2 In addition to anticancer activity as a single agent, studies indi- cate that calcitriol acts synergistically to potentiate Correspondence address: K. M. Rassnick Department of Clinical Sciences College of Veterinary Medicine Cornell University Box 31, Ithaca, NY 14853, USA e-mail: kmr32@cornell.edu © 2010 Blackwell Publishing Ltd 209