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The thymidine kinases are enzymes that convert deoxythymidine to deoxythymidine monophosphate and have a function in DNA synthesis. Rapidly proliferating cells will have higher levels of thymidine kinase. Serum thymidine kinase activity (sTK) is a useful tumour marker in humans and dogs, with utility as a prognostic indicator in lymphoma. In the current study serum samples were collected from 49 clinically healthy cats, 33 with lymphoma, 55 with inflammatory disease and 34 with non-haematopoietic neoplasia (NHPN). sTK was measured using a radioenzyme assay and a reference interval (1.96 × SD) was established from the clinically healthy cats (<5.5 U/l). Mean sTK activity for healthy cats was 2.2 U/l (range 0.8-8.4, ± SD 1.7). Mean sTK activity for cats with lymphoma was 17.5 U/l (range 1.0-100.0 SD ± 27.4). Mean sTK activity for cats with NHPN was 4.2 U/l (range 1.0-45.0, SD ± 8.6). Mean sTK activity for the inflammatory group was 3.4 U/l (range 1.0-19.6, SD 3.9). Cats with lymphoma had significantly higher sTK activity than healthy cats or cats with inflammatory disease (P <0.0001) and cats with NHPN (P <0.0002). sTK activity is a potentially useful biomarker for feline lymphoma and further study is required to assess its utility as a prognostic indicator.

Thymidine kinase 1 (TK), which is involved in the synthesis of DNA precursors, is only expressed in S-G2 cells. Serum TK levels correlate to the proliferative activity of tumor disease. Determinations of TK levels have so far relied on radio enzyme assay (REA) and experimental ELISA methods, which have limited the clinical use of this biomarker, although recent studies in dogs with malignant lymphoma (ML) demonstrate its wide potential. A non-radiometric method based on a competitive immunoassay with specific anti-3'-azido-deoxythymidine monophosphate (AZTMP) antibodies has been further developed into the fully automated Liaison TK assay (DiaSorin). Sera from healthy dogs (n=30), and dogs with leukemia (LEUK) (n=35), ML (n=84), non-hematological tumors (n=50), and inflammatory disease (n=14) were tested using both methods. Lymphoma and LEUK samples were available before and during chemotherapy. The coefficients of variation for the Liaison TK assay in this study were 6.3 and 3.4% (low/high TK, respectively), and the correlation between TK REA (X) and the Liaison TK assay (Y) was y=0.9203x+1.3854 (R2=0.9501). The TK1 levels measured during chemotherapy gave very clear differences between dogs in complete remission and dogs out of remission. A Tukey-Kramer analysis showed that all LEUKs and MLs out of remission differed significantly from the other groups. The Liaison TK assay showed high precision, high sensitivity and a good correlation to the TK REA. The Liaison TK assay provides valuable clinical information in the treatment and management of canine LEUK and ML, with a potential to be further validated in human trials.

The aim of this study was to evaluate an enzyme-linked immunosorbent assay (ELISA), for determination of serum thymidine kinase 1 (sTK1) activity in dogs with malignant lymphoma (ML) and compare it with a thymidine kinase (TK) radioenzymatic assay (TK-REA). The TK-REA has recently been shown to be useful in determining the clinical stage and prognosis in canine ML. In addition, serum lactate dehydrogenase (LDH) was measured. Forty-five dogs were included in the study. Sixty serum samples from these dogs, stored in a tumour serum sample bank (stored at -20 degrees C), were analysed. Apart from 37 dogs with ML, four normal dogs as well as two dogs with mammary carcinomas, one dog with bladder carcinoma, and one dog with malignant fibrous histiocytoma were included. Staging of ML was based on the modified World Health Organization (WHO) staging system for canine ML. The diagnosis of all tumours was verified by histopathology. The TK activity (units per litre [U/L]) ranged from 1.0 to 607.9 in the TK-REA analysis and from 1.1 to 510 in the TK-ELISA (normal reference value <7U/L). The range for LDH was between 12 and 1194 U/L (normal reference value <228 U/L). There was a significant correlation between the TK-REA and the TK-ELISA. The correlation coefficient (CC) was 0.97 and the standard error of the estimate (SEE) was 3.7 U/L. There was no correlation between LDH and either the TK-REA or the TK-ELISA (CC=0.53 for both assays; SEE=26.7 and 12.7 U/L, respectively). Most of the variation in LDH was still within the normal reference range. The mean LDH in dogs with high-stage (stage IV+V) disease was 201.9 U/L. The corresponding values for the TK-REA and TK-ELISA were 109 and 109.9 U/L, respectively. The significant relation between the TK-REA and the TK-ELISA was confirmed by Bland-Altman analysis. The TK-ELISA assay, because of its relative simplicity, will permit measurement of TK in cases of ML in dogs to become a routine procedure.