Background: Thymidine kinase 1 (TK1) is a pyrimidine salvage pathway enzyme that is up-regulated in malignant tissues and elevated in the serum of cancer patients. While TK1 has been well established as a tumor biomarker, little has been done to explore its potential as a tumor target. Recently, we reported the membrane expression of TK1 on malignant cells, but not on normal cells. This study explores the possible use of monoclonal antibodies for the targeting of membrane associated TK1 in lung, breast, colon and prostate cancer cells. Methods: We generated and evaluated a panel of monoclonal antibodies against six different epitopes exposed in the tetrameric form of TK1. Antibodies were developed with hybridoma technology and validated with Western blot, siRNA TK1 knockdown, enzyme-linked immunosorbent assay (ELISA) and flow cytometry. The therapeutic potential of the antibodies was evaluated in vitro in antibody-dependent cell-mediated-cytotoxicity (ADCC) experiments. Results: Binding of the antibodies to TK1 was confirmed by Western blot in purified recombinant protein, cancer serum, and cell lysate. After a TK1 knockdown was performed, a reduction of TK1 expression was observed with five antibodies. Using indirect ELISA, we identified 3B2E11, 9C10, 7H2, 3B4, 8G2 among the most sensitive antibodies (LOD = 10.73-66.9 pg/ml). Surface expression of TK1 on the membrane of various cancer cell lines was analyzed with flow cytometry. Antibodies 8G2, 3B4, 7HD and 5F7G11 detected TK1 on the membrane of various cancer cell lines, including lung, prostate, colon and breast. No significant binding was detected on normal lymphocytes. Increased cytolysis of lung (~ 70%. p = 0.0001), breast (~ 70%, p = 0.0461) and colon (~ 50% p = 0.0216) cancer cells by effector cells was observed when anti-TK1 antibodies were added during ADCC experiments. Conclusions: The antibodies developed showed potential to be used to detect and target TK1 on the membrane of various tumor cells. The targeting of TK1 in malignant cells using monoclonal antibodies may be a feasible approach for the elimination of high TK1 expressing tumor cells.

Thymidine Kinase 1 (TK1) is primarily known as a cancer biomarker with good prognostic capabilities for both hematological and solid malignancies. However, recent studies targeting TK1 at protein and mRNA levels have shown that TK1 may be useful as a therapeutic target. In order to examine the use of TK1 as a therapeutic target, it is necessary to develop therapeutics specific for it. Single domain antibodies (sdAbs), represent an exciting approach for the development of immunotherapeutics due to their cost-effective production and higher tumor penetration than conventional antibodies. In this study, we isolated sdAb fragments specific to human TK1 from a human sdAb library. A total of 400 sdAbs were screened through 5 rounds of selection by monoclonal phage ELISA. The most sensitive sdAb fragments were selected as candidates for preclinical testing. The sdAb fragments showed specificity for human TK1 in phage ELISA, Western blot analysis and had an estimated limit of detection of 3.9 ng/ml for the antibody fragments 4-H-TK1_A1 and 4-H-TK1_D1. The antibody fragments were successfully expressed and used for detection of membrane associated TK1 (mTK1) through flow cytometry on cancer cells [lung (~95%), colon (~87%), breast (~53%)] and healthy human mononuclear cells (MNC). The most sensitive antibody fragments, 4-H-TK1_A1 and 4-H-TK1_D1 were fused to an engineered IgG1 Fc fragment. When added to cancer cells expressing mTK1 co-cultured with human MNCs, the anti-TK1-sdAb-IgG1_A1 and D1 were able to elicit a significant antibody-dependent cell-mediated cytotoxicity (ADCC) response against lung cancer cells compared to isotype controls (P<0.0267 and P<0.0265, respectively). To our knowledge this is the first time that the isolation and evaluation of human anti-TK1 single domain antibodies using phage display technology has been reported. The antibody fragments isolated here may represent a valuable resource for the detection and the targeting of TK1 on tumor cells.

Single domain antibodies or nanobodies (Nbs) have positively proved effective for the targeting of multiple tumor targets. This study describes the isolation of human Nbs against the tumor proliferation biomarker thymidine kinase 1 (TK1). The anti-TK1 Nbs were used to target membrane associated TK1 in the non-small cell lung carcinoma cell lines NCI-H460 and A549 cells. TK1 is a pyrimidine salvage enzyme that is upregulated in malignant tissues. Recently, we have reported the presence of a TK1 form that is associated to the cell membrane in breast, lung and colon tumors as well as leukemia. Our findings suggest that targeting of TK1 in proliferating cancer cells may be an alternative approach for the treatment of cancer. Using phage display technology, we isolated a total of 237 nanobodies from the Daniel Christ DAb library. After 2-4 rounds of selection the affinity of the nanobodies to TK1 was tested with monoclonal phage ELISA using human recombinant TK1. Ten of the most sensitive anti-TK1 Nbs were then selected and evaluated with dose response curves as possible candidates for preclinical testing. The dose response curves showed R squares values ranging from 0.9738-0.9980 fitting a sigmoidal curve. Thus, indicating a strong antibody dose response relationship specific for TK1. In addition, the specificity of the TK1 antibodies was tested by screening against unrelated proteins and cell lysate from an siRNA TK1 knockdown using monoclonal phage ELISA. Nanobodies showed negligible binding when screened against TK1 non related proteins. Validation of the anti-TK1 Nbs with TK1 siRNA knockdown showed a significant reduction on the Nbs binding compared to wild type. The sensitivity of the anti-TK1 Nbs to detect TK1 was within the nanomolar range (between 100-20 ng/ml) and had thermostable standing temperatures between 50-75 °C. The anti-TK1 Nbs were used to detect membrane associated TK1 in the non-small cell lung carcinoma cell lines NCI-H460 and A549. The phage nanobodies showed binding capacity to TK1 on cancer cells with 30- 60% of positive cells for NCI-H460 and 25 to 40% for A549 cells. Human Nbs represent a valuable resource for the detection and the targeting of TK1 in proliferating tumor cells. Further work is required to enable the exploration of the therapeutic uses of anti-TK1 Nb-based therapies and their incorporation to cell adoptive therapies such as chimeric antigen receptor (CAR) therapies. Citation Format: Edwin J. Velazquez, Jordan D. Cress, Kathryn R. Smith, David M. Bellini, Jonathan R. Skidmore, Zachary D. Ewell, Richard A. Robison, Kim L. O'Neill. Exploring the potential of human nanobodies against thymidine kinase 1 for the targeting of lung cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 519.

This study explores the potential therapeutic use of monoclonal antibodies against the tumor proliferation biomarker TK1 for the immunotargeting of lung and breast cancer cells. Recent clinical trials have led to the approval of monoclonal antibodies for the treatment of several solid tumors including lung and breast tumors. However, a common limitation that antibody-based therapies face is that a significant portion of the current tumor targets are expressed on normal tissues, thus creating off-target tumor effects. TK1 is a tumor proliferation biomarker that is up-regulated in malignant tissues. Recently, we have reported the expression of TK1 on the cell membrane of several cancer cell lines, mononuclear cells (MNC) from patients with leukemia and cells from breast and colon tumors. No membrane expression of TK1 was found on normal cells. To further explore the potential of TK1 as an immunotherapeutic target, we evaluated the capacity of a novel panel of anti TK1 antibodies to target TK1 on the cell membrane of lung and breast cancer cells and to elicit an antibody-dependent cell-mediated-cytotoxicity (ADCC) response in vitro. Antibodies previously developed in our lab were validated to specifically target six different regions of the TK1 molecule. Four different antibodies targeting three different regions in the TK1 molecule were selected for ADCC experiments. Nuclear restricted GFP versions of the NCI-H460 and MDA-MB-231 cell lines were utilized in conjunction with the real-time cell imaging system ImageXpress® Pico to measure the ADCC response. Mono nuclear cells (MNCs) from healthy donors were co-cultured with target cells and 10ug/ml of each anti-TK1 antibody were added to the media. An antibody with the same isotype than the TK1 antibodies, but that doesn't bind to any human surface antigen was utilized as a control. All ADCC responses elicited by the anti-TK1 antibodies were observed between 24-72hrs. The highest ADCC responses were elicited by antibodies 4G10 and 1B12 in both breast and lung cancer cell lines while a minimal response was observed with antibodies 7D1 and 3B2E11. A 50-60% increment in the ADCC response against NCI-H460 cells was observed with antibody 4G10 in comparison with the isotype control (p= 0.001). A 40% increase in the ADCC response against MDA-MB-231 cells was observed using antibody 1B12 (p = 0.003) compared to the its isotype control. This preliminary data suggests that anti TK1 monoclonal antibodies can be used for the immunotargeting of tumor cells expressing membrane associated TK1. The further exploration of TK1 as a tumor target could lead to the development of new TK1-based immunotherapies. Citation Format: Edwin J. Velazquez, Jordan D. Cress, Kathryn R. Smith, Taylor D. Brindley, Gajendra Shrestha, Richard A. Robison, Kim L. O'Neill. Monoclonal antibodies against thyimidine kinase 1 for the immunotargeting of lung and breast cancer cells, a preclinical evaluation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 543.