Proliferation indices have been histochemically evaluated in tumor tissue and can predict biologic behavior more precisely than tumor grade in some cases. Thymidine kinase (TK) is a soluble biomarker present in S-phase of a salvage pathway for DNA synthesis, and can be measured in serum. TK activity correlates with stage, prognosis, and relapse in dogs and humans with lymphoma. The purpose of this study was to compare TK concentrations among dogs with various tumors and to healthy dogs.

Serum samples previously stored at -80°C at the ACC-CSU were assayed: transitional cell carcinoma (TCC, n=18), osteosarcoma (n=15), lymphoma (n=14, with 9 B-cell, 3 T-cell, and 2 null cell), and hemangiosarcoma (n=18). Sera from age-matched clinically normal dogs comprised a control population (n=33). An ELISA using AZT as a TK1 substrate was used. Comparisons among groups were made using 1- and 2-tailed student T-tests as appropriate.

TK activity in normal canine serum ranged from 0-5.6 U/L (mean, 2.8 U/L). An upper limit of normal (mean+2SD) was established at 6.16 U/L. TK activity was significantly higher than control (p<0.0001) in dogs with hemangiosarcoma and lymphoma (mean+/-SD = 34.2+/-36.5 and 59.0+/-43.4 respectively). There was a significant difference between B- and T- cell lymphoma (77 vs. 1.2 U/L, p=0.0035). In 22%, 13%, 71% and 80% of dogs with TCC, osteosarcoma, lymphoma, and hemangiosarcoma, respectively, TK activity was above the normal range.

TK activity may be a useful for the early detection of canine lymphoma and hemangiosarcoma, and may help differentiate between lymphoma immunophenotypes in dogs.

Originally presented at the Veterinary Cancer Society Annual Conference, Fort Lauderdale, FL, November 2007.