3′-Deoxy-3′-[ 18 F]fluorothymidine ([ 18 F]FLT) has been proposed as a new marker for imaging tumor proliferation by positron emission tomography (PET). The uptake of [ 18 F]FLT is regulated by cytosolic S-phase-specific thymidine kinase 1 (TK1). In this article, we have investigated the use of [ 18 F]FLT to monitor the response of tumors to antiproliferative treatment in vivo . C3H/Hej mice bearing the radiation-induced fibrosarcoma 1 tumor were treated with 5-fluorouracil (5-FU; 165 mg/kg i.p.). Changes in tumor volume and biodistribution of [ 18 F]FLT and 2-[ 18 F]fluoro-2-deoxy-d-glucose ([ 18 F]FDG) were measured in three groups of mice ( n = 8–12/group): ( a ) untreated controls; ( b ) 24 h after 5-FU; and ( c ) 48 h after 5-FU. In addition, dynamic [ 18 F]FLT-PET imaging was performed on a small animal scanner for 60 min. The metabolism of [ 18 F]FLT in tumor, plasma, liver, and urine was determined chromatographically. Proliferation was determined by staining histological sections for proliferating cell nuclear antigen (PCNA). Tumor levels of TK1 protein and cofactor (ATP) were determined by Western blotting and bioluminescence, respectively. Tumor [ 18 F]FLT uptake decreased after 5-FU treatment (47.8 ± 7.0 and 27.1 ± 3.7% for groups b and c , respectively, compared with group a ; P 18 F]FLT uptake was significantly more pronounced than that of [ 18 F]FDG. The PET image data confirmed lower tumor [ 18 F]FLT retention in group c compared with group a , despite a trend toward higher radiotracer delivery for group c . Other than phosphorylation in tumors, [ 18 F]FLT was found to be metabolically stable in vivo . The decrease in tumor [ 18 F]FLT uptake correlated with the PCNA-labeling index ( r = 0.71, P = 0.031) and tumor volume changes after 5-FU treatment ( r = 0.58, P = 0.001). In this model system, the decrease in [ 18 F]FLT uptake could be explained by changes in catalytic activity but not translation of TK1 protein. Compared with group a , TK1 levels were lower in group b (78.2 ± 5.2%) but higher in group c (141.3 ± 9.1%, P a to b to c ( P 18 F]FLT and PET. In our model system, the radiotracer uptake was correlated with PCNA-labeling index. The decrease in [ 18 F]FLT uptake after 5-FU was more pronounced than that of [ 18 F]FDG. [ 18 F]FLT is, therefore, a promising marker for monitoring antiproliferative drug activity in oncology that warrants additional testing.

https://cancerres.aacrjournals.org/content/canres/63/13/3791.full.pdf

3'-deoxy-3'-[18F]fluorothymidine as a new marker for monitoring tumor response to antiproliferative therapy in vivo with positron emission tomography.

There is an unmet need to develop imaging methods for the early and objective assessment of breast tumors to therapy. 3′-Deoxy-3′-[ 18 F]fluorothymidine ([ 18 F]FLT)–positron emission tomography represents a new approach to imaging thymidine kinase activity, and hence, cellular proliferation. We compared graphical, spectral, and semiquantitative analytic methodologies for quantifying [ 18 F]FLT kinetics in tumor and normal tissue of patients with locally advanced and metastatic breast cancer. The resultant kinetic parameters were correlated with the Ki-67 labeling index from tumor biopsies. [ 18 F]FLT accumulation was detected in primary tumor, nodal disease, and lung metastasis. In large tumors, there was substantial heterogeneity in regional radiotracer uptake, reflecting heterogeneity in cellular proliferation; radiotracer uptake in primary tumors also differed from that of metastases. [ 18 F]FLT was metabolized in patients to a single metabolite [ 18 F]FLT-glucuronide. Unmetabolized [ 18 F]FLT accounted for 71.54 ± 1.50% of plasma radioactivity by 90 minutes. The rate constant for the metabolite-corrected net irreversible uptake of [ 18 F]FLT (K i ) ranged from 0.6 to 10.4 × 10 −4 and from 0 to 0.6 × 10 −4 mL plasma cleared/s/mL tissue in tumor (29 regions, 15 patients) and normal tissues, respectively. Tumor K i and fractional retention of radiotracer determined by spectral analysis correlated with Ki-67 labeling index ( r = 0.92, P r = 0.92, P 18 F]FLT-positron emission tomography is a promising clinical tool for imaging cellular proliferation in breast cancer, and is most predictive when analyzed by graphical and spectral methods.

http://cancerres.aacrjournals.org/content/canres/65/21/10104.full.pdf

Quantification of cellular proliferation in tumor and normal tissues of patients with breast cancer by [18F]fluorothymidine-positron emission tomography imaging: evaluation of analytical methods.

Substrate specificity and phosphorylation of antiviral and anticancer nucleoside analogues by human deoxyribonucleoside kinases and ribonucleoside kinases.

Oncolytic viruses (OVs) are viral strains that can infect and kill malignant cells while spare their normal counterparts. OVs can access cells through binding to receptors on their surface or through fusion with the plasma membrane and establish a lytic cycle in tumors, while leaving normal tissue essentially unharmed. Multiple viruses have been investigated in humans for the past century. IMLYGIC™ (T-VEC/Talimogene Laherparepvec), a genetically engineered Herpes Simplex Virus, is the first OV approved for use in the United States and the European Union for patients with locally advanced or non-resectable melanoma. Although OVs have a favorable toxicity profile and are impressively active anticancer agents in vitro and in vivo the majority of OVs have limited clinical efficacy as a single agent. While a virus-induced antitumor immune response can enhance oncolysis, when OVs are used systemically, the antiviral immune response can prevent the virus reaching the tumor tissue and having a therapeutic effect. Intratumoral administration can provide direct access to tumor tissue and be beneficial in reducing side effects. Immune checkpoint stimulation in tumor tissue has been noted after OV therapy and can be a natural response to viral-induced oncolysis. Also for immune checkpoint inhibition to be effective in treating cancer, an immune response to tumor neoantigens and an inflamed tumor microenvironment are required, both of which treatment with an OV may provide. Therefore, direct and indirect mechanisms of tumor killing provide rationale for clinical trials investigating the combination of OVs other forms of cancer therapy, including immune checkpoint inhibition.

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Review: Oncolytic virotherapy, updates and future directions

Objective: Early detection is a prerequisite to the effective reduction of morbidity and mortality from breast cancer. The present study intended to employ a high-throughput membran

Combination of Multiple mRNA Markers (PTTG1, Survivin, UbcH10 and TK1) in the Diagnosis of Taiwanese Patients with Breast Cancer by Membrane Array

Thymidine kinase 1 expression defines an activated G1 state of the cell cycle as revealed with site-specific antibodies and ArrayScan assays

Characterization of a peptide antibody against a C-terminal part of human and mouse cytosolic thymidine kinase, which is a marker for cell proliferation

Thymidine kinase 1 (TK1), an enzyme involved in the synthesis of precursors for DNA, and thus proliferation dependent, has been suggested as a good tumour marker. We have recently developed poly/monoclonal antibodies against TK1, which proved useful for diagnostics in both serum and immunohistochemistry of cancer patients. The anti-TK1 monoclonal antibodies (mAbs) 1D11 and 1E3 were characterized by Western blot, immunoprecipitation and flow cytometry. TK1 mAbs and Ki-67 mAb were then used for immunohistochemistry staining of tumour sections from 54 patients with ductal infiltrated breast carcinoma. Results showed the relative number of patients with positively stained tumours for TK1 (mAb 1D11) and for Ki-67 (mAb MIB-1) were 47 and 41%, respectively, significantly related (p=0.007). Combination of TK1 mAbs 1D11 and 1E3 increased this number to 56%, due to detection of a significantly higher number of patients with grade 2 tumours. Patients with stage II and grade 2 tumours showed significantly higher TK1 staining when compared to stage I and grade 1. Ki-67 staining was significantly higher in stage III and grade 3. The tumours only stained for TK1 represented higher stages and grades, while tumours staining only for Ki-67 were of lower stages and grades. Combining TK1 and Ki-67 increased the number of patients with positively stained tumours to 69%. In conclusion, TK1 is a reliable marker for identification of patients with grade 2 tumours. The highest number of patients with positively stained tumours were obtained when both TK1 and Ki-67 markers were used.

Cytosolic thymidine kinase is a specific histopathologic tumour marker for breast carcinomas.

The activity of the proliferation related enzyme thymidine kinase 1 (TK1) was reported to be 3-fold higher in extracts from normal kidney tissue as compare to renal carcinoma extracts [3]. To verify these unexpected results, determinations of the protein levels of TK1 in normal kidney and in samples from different types of renal cell carcinoma (RCC) were done with immunohistochemistry and Western blot analysis. Two anti-TK1 peptide antibodies reacting with different TK1 epitops were used. TK1 levels were high in tubule cells as compared to glomerulus cells and connective tissue cells, while an intermediary TK1 was observed in renal cell carcinoma (RCC) cells. Western blot analysis demonstrated high levels of TK1 in extract from normal kidney, and lower levels of TK1 in the RCC extracts. The specificity of TK1 staining was demonstrated in competition experiments with excess TK1 antigen. The high TK1 levels in normal kidney tubule cells suggest that they are in a form of activated G1-state. The relatively low TK1 level in RCC, representing TK1 expression in S-phase cells, is in accordance with the low overall proliferation rate of these tumors. These results suggest that cell cycle regulation of TK1 in normal tubule cells differ from that in other type of normal and malignant renal cells.

Naining Wang

Papers

Thymidine kinase 1 expression defines an activated G1 state of the cell cycle as revealed with site-specific antibodies and ArrayScan assays

Characterization of a peptide antibody against a C-terminal part of human and mouse cytosolic thymidine kinase, which is a marker for cell proliferation

Cytosolic thymidine kinase is a specific histopathologic tumour marker for breast carcinomas.

The Proliferation Marker Thymidine Kinase 1 Level is High in Normal Kidney Tubule Cells Compared to other Normal and Malignant Renal Cells