Elevation of radiolabelled thymidine uptake in RIF-1 fibrosarcoma and HT29 colon adenocarcinoma cells after treatment with thymidylate synthase inhibitors

Thymidine Metabolism as a Confounding Factor for 3'-Deoxy-3'-18F-Fluorothymidine Uptake After Therapy in a Colorectal Cancer Model

Noninvasive monitoring of tumor therapy response helps in developing personalized treatment strategies. Here, we performed sequential PET and diffusion-weighted MRI to evaluate changes induced by a FOLFOX-like combination chemotherapy in colorectal cancer xenografts, to identify the cellular and molecular determinants of these imaging biomarkers. Methods: Tumor-bearing CD1 nude mice, engrafted with FOLFOX-sensitive Colo205 colorectal cancer xenografts, were treated with FOLFOX (5-fluorouracil, leucovorin, and oxaliplatin) weekly. On days 1, 2, 6, 9, and 13 of therapy, tumors were assessed by in vivo imaging and ex vivo analyses. In addition, HCT116 xenografts, which did not respond to the FOLFOX treatment, were imaged on day 1 of therapy. Results: In Colo205 xenografts, FOLFOX induced a profound increase in uptake of the proliferation PET tracer 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) accompanied by increases in markers for proliferation (Ki-67, thymidine kinase 1) and for activated DNA damage response (γH2AX), whereas the effect on cell death was minimal. Because tracer uptake was unaltered in the HCT116 model, these changes appear to be specific for tumor response. Conclusion: We demonstrated that 18F-FLT PET can noninvasively monitor cancer treatment-induced molecular alterations, including thymidine metabolism and DNA damage response. The cellular or imaging changes may not, however, be directly related to therapy response as assessed by volumetric measurements.

Gemcitabine Mechanism of Action Confounds Early Assessment of Treatment Response by 3'-Deoxy-3'-[18F]Fluorothymidine in Preclinical Models of Lung Cancer

3'-Deoxy-3'-[18F]fluorothymidine positron emission tomography ([18F]FLT-PET) and diffusion-weighted MRI (DW-MRI) are promising approaches to monitor tumor therapy response. Here, we employed these two imaging modalities to evaluate the response of lung carcinoma xenografts in mice after gemcitabine therapy. Caliper measurements revealed that H1975 xenografts responded to gemcitabine treatment, whereas A549 growth was not affected. In both tumor models, uptake of [18F]FLT was significantly reduced 6 hours after drug administration. On the basis of the gemcitabine concentration and [18F]FLT excretion measured, this was presumably related to a direct competition of gemcitabine with the radiotracer for cellular uptake. On day 1 after therapy, [18F]FLT uptake was increased in both models, which was correlated with thymidine kinase 1 (TK1) expression. Two and 3 days after drug administration, [18F]FLT uptake as well as TK1 and Ki67 expression were unchanged. A reduction in [18F]FLT in the responsive H1975 xenografts could only be noted on day 5 of therapy. Changes in ADCmean in A549 xenografts 1 or 2 days after gemcitabine did not seem to be of therapy-related biological relevance as they were not related to cell death (assessed by caspase-3 IHC and cellular density) or tumor therapy response. Taken together, in these models, early changes of [18F]FLT uptake in tumors reflected mechanisms, such as competing gemcitabine uptake or gemcitabine-induced thymidylate synthase inhibition, and only reflected growth-inhibitory effects at a later time point. Hence, the time point for [18F]FLT-PET imaging of tumor response to gemcitabine is of crucial importance. Cancer Res; 76(24); 7096-105. ©2016 AACR.

3'-Deoxy-3'-18F-fluorothymidine PET for the early prediction of response to leucovorin, 5-fluorouracil, and oxaliplatin therapy in patients with metastatic colorectal cancer

The aim of this study was to evaluate 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET for early prediction of the standard anatomic response and survival outcomes in patients with metastatic colorectal cancer (mCRC) receiving leucovorin, 5-fluorouracil (5-FU), and oxaliplatin (FOLFOX).

METHODS

The main eligibility criteria included histologically confirmed mCRC, ≥ 1 extrahepatic measurable lesions, and no prior chemotherapy in a metastatic setting. Chemotherapy consisted of leucovorin on day 1, followed by the continuous infusion of 5-FU on days 1 and 2, and oxaliplatin on day 3. In the second and subsequent cycles of chemotherapy, oxaliplatin was administered simultaneously with leucovorin on day 1. (18)F-FLT PET scans were obtained 3 times during the first cycle of chemotherapy: before chemotherapy, 24 h after infusion of 5-FU (day 2), and 48 h after completion of chemotherapy (day 5). The maximum standardized uptake value (SUVMAX) of (18)F-FLT was measured. Treatment responses were assessed by CT after 3 cycles of FOLFOX.

RESULTS

Eighteen patients were included in the study. The response rate after 3 cycles of FOLFOX was 27.8% (5/18). The SUVMAX was increased in responders (P = 0.043) and nonresponders (P < 0.001) on day 2 and was decreased, compared with baseline values, on day 5 in responders only (P = 0.043). Receiver-operating-characteristic curve analysis indicated that the use of a threshold of an SUVMAX increase on day 2 of ≤ 45.8% resulted in a sensitivity of 100%, specificity of 69.2%, and relative risk of 2.250 (P = 0.029) for the diagnosis of responders. Use of a threshold of an SUVMAX decrease on day 5 of ≥ 10.6% resulted in a sensitivity of 100%, specificity of 76.9%, and relative risk of 2.667 (P = 0.007). Patients with low (18)F-FLT flare tended to have longer survivals than patients with high flare (2-y overall survival rate, 77.8% vs. 44.4%; P = 0.051).

CONCLUSION

The (18)F-FLT flare observed during 5-FU infusion was associated with poor treatment response in patients with mCRC. The degree of (18)F-FLT flare might be used to predict the outcome of patients who receive infusional 5-FU-based chemotherapy.

Wheat germ agglutinin-functionalised crosslinked polyelectrolyte microparticles for local colon delivery of 5-FU: in vitro efficacy and in vivo gastrointestinal distribution

We have previously reported the development and characterisation of wheat germ agglutinin (WGA)-functionalised chitosan-Ca-alginate (CTS-Ca-ALG) microparticles (MPs) loaded with acid-resistant particles of 5-fluorouracil (5-FU). In the present work, our goal was to evaluate the potential of these carriers for efficient treatment of colon cancer by studying in vitro permeability and cell association of 5-FU and [methyl-³H]thymidine uptake in Caco-2 cells, as well as in vivo gastrointestinal distribution. The amount of 5-FU permeated through Caco-2 cells was 15.1, 7.7 and 6.5% for 5-FU solution, CTS-Ca-ALG MPs and WGA conjugates. The concentration of 5-FU associated with Caco-2 cells was significantly greater when delivered from MPs. By incorporation of 5-FU into MPs and further decoration with WGA, an increased [methyl-³H]thymidine uptake was observed few hours after continuous drug treatment followed by significantly reduced uptake after 6 h. Gastrointestinal distribution was in favour of increased localisation and concentration of the particles in colon region.

The effects of 5-fluoruracil treatment on 3'-fluoro-3'-deoxythymidine (FLT) transport and metabolism in proliferating and non-proliferating cultures of human tumor cells

3'-Fluoro-3'-deoxythymidine (FLT) positron emission tomography (PET) has been proposed for imaging thymidylate synthase (TS) inhibition. Agents that target TS and shut down de novo synthesis of thymidine monophosphate increase the uptake and retention of FLT in vitro and in vivo because of a compensating increase in the salvage pathway. Increases in both thymidine kinase-1 (TK1) and the equilibrative nucleoside transporter hENT1 have been reported to underlie this effect. We examined whether the effects of one TS inhibitor, 5-fluorouracil (5FU), on FLT uptake require proliferating cells and whether the effects are limited to increasing TK1 activity.

METHODS

The effects of 5FU on FLT transport and metabolism, TK1 activity, and cell cycle progression were evaluated in the human tumor cell line, A549, maintained as either a proliferating or non-proliferating culture.

RESULTS

There were dose-dependent increases in FLT uptake that peaked after a 10 μM 5FU exposure and then declined to baseline levels or below at higher doses in both proliferating and non-proliferating cultures. The dose-dependence for FLT uptake was mirrored by changes in TK1 activity. S phase fraction did not correlate with FLT uptake in proliferating cultures. Chemical inhibition of hENT1 reduced overall levels of FLT uptake but did not affect the low dose increase in FLT uptake.

CONCLUSIONS

5FU only affects FLT uptake in proliferating A549 cells and increases in FLT uptake are directly related to increased TK1 activity. Our studies did not support a role for hENT1 in the increased uptake of FLT after exposure to 5FU. Our studies with A549 cells support the suggestion that FLT-PET could provide a measure of TS inhibition in vivo.

Induction of thymidine kinase 1 after 5-fluorouracil as a mechanism for 3'-deoxy-3'-[18F]fluorothymidine flare

Imaging the pharmacodynamics of anti-cancer drugs may allow early assessment of anti-cancer effects. Increases in 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) uptake early after thymidylate synthase inhibition (TS) inhibition, the so-called flare response, is considered to be largely due to an increase in binding sites for type-1 equilibrative nucleoside transporter. We investigated the induction of thymidine kinase 1 (TK1) after 5-fluorouracil (5-FU) treatment as one of mechanisms for [(18)F]FLT flare. Exposure of nine cancer cell lines to 5-FU for 24h induced a 2.5- to 3.5-fold increase in [(18)F]FLT uptake, significantly higher than the 1.5-fold increase observed 2h after treatment. The increase of [(18)F]FLT uptake 24h after 5-FU exposure accompanied TK1 induction in most cell lines. In representative cell lines (A431 and HT29), 5-FU time-dependently increased [(18)F]FLT uptake, kinase activity and the levels of protein and mRNA for TK1, sequential cyclin E and A induction, and G(1)-S phase transition. Cycloheximide treatment and knockdown of TK1 completely inhibited 5-FU-induced [(18)F]FLT flare. On the other hand, HCT8 cells showed a biphasic [(18)F]FLT flare with lacked TK1 induction in response to the dosage of 5-FU. Cycloheximide did not inhibit 5-FU-induced [(18)F]FLT flare in this cells. In vivo dynamic [(18)F]FLT-PET and ex vivo analysis in HT29 tumor-bearing mice showed significantly increased [(18)F]FLT flux and TK1 activity of tumor tissue 24h after 5-FU administration (P<0.05). Conclusively, 5-FU induced TK1 and TK1-mediated high [(18)F]FLT flare in most of cell lines. [(18)F]FLT-PET may be used to assess pharmacodynamics of TS inhibitor by a mechanism involving TK1 induction.

[18F]FDG and [18F]FLT uptake in human breast cancer cells in relation to the effects of chemotherapy: an in vitro study

Increased 2′-deoxy-2′-[¹⁸F]fluoro-D-glucose (FDG) uptake is the most commonly used marker for positron emission tomography in oncology. However, a proliferation tracer such as 3′-deoxy-3′-[¹⁸F]fluorothymidine (FLT) might be more specific for cancer. 3′-deoxy-3′-[¹⁸F]fluorothymidine uptake is dependent on thymidine kinase 1 (TK) activity, but the effects of chemotherapeutic agents are unknown. The aim of this study was to characterise FDG and FLT uptake mechanisms in vitro before and after exposure to chemotherapeutic agents. The effects of 5-fluorouracil (5-FU), doxorubicin and paclitaxel on FDG and FLT uptake were measured in MDA MB231 human breast cancer cells in relation to cell cycle distribution, expression and enzyme activity of TK-1. At IC₅₀ concentrations, 5-FU resulted in accumulation in the G1 phase, but doxorubicin and paclitaxel induced a G2/M accumulation. Compared with untreated cells, 5-FU and doxorubicin increased TK-1 levels by >300. At 72 h, 5-FU decreased FDG uptake by 50% and FLT uptake by 54%, whereas doxorubicin increased FDG and FLT uptake by 71 and 173%, respectively. Paclitaxel increased FDG uptake with >100% after 48 h, whereas FLT uptake hardly changed. In conclusion, various chemotherapeutic agents, commonly used in the treatment of breast cancer, have different effects on the time course of uptake of both FDG and FLT in vitro. This might have implications for interpretation of clinical findings.

Imaging pharmacodynamics of the alpha-folate receptor-targeted thymidylate synthase inhibitor BGC 945

The assessment of tissue-specific pharmacodynamics is desirable in the development of tumor-targeted therapies. Plasma deoxyuridine (dUrd) levels, a measure of systemic thymidylate synthase (TS) inhibition, has limited application for studying the pharmacodynamics of novel TS inhibitors targeted to the high affinity alpha-folate receptor (FR). Here, we have evaluated the utility of [(18)F]fluorothymidine positron emission tomography ([(18)F]FLT-PET) for imaging the tissue pharmacodynamics of BGC 945, an FR-targeted antifolate TS inhibitor; the nontargeted antifolate BGC 9331 was used for comparison. TS inhibition by both drugs induced a concentration-dependent increase in [(3)H]thymidine uptake in FR-positive human epidermoid KB cells. Membrane-associated equilibrative nucleoside transporter type 1 levels increased from 55,720 +/- 6,101 to 118,700 +/- 5,193 and 130,800 +/- 10,800 per cell at 100 mug/mL of BGC 9331 and BGC 945, respectively, suggesting this as a potential mechanism of increased nucleoside uptake. In keeping with these in vitro findings, tumor [(18)F]FLT accumulation in KB xenografts increased by >/=2-fold after drug treatment with maximal levels at 1 to 4 hours and 4 to 24 hours after BGC 9331 and BGC 945 treatment, respectively. Of interest to FR targeting, BGC 9331, but not BGC 945, induced accumulation of [(18)F]FLT uptake in intestine, a proliferative and TS-responsive tissue. For both drugs, quantitative changes in tumor [(18)F]FLT uptake were associated with increased tumor dUrd levels. In conclusion, we have validated the utility of [(18)F]FLT-PET to image TS inhibition induced by antifolates and shown the tumor-specific activity of BGC 945. This imaging biomarker readout will be useful in the early clinical development of BGC 945.

Redistribution of nucleoside transporters to the cell membrane provides a novel approach for imaging thymidylate synthase inhibition by positron emission tomography

Thymidylate synthase (EC 2.1.1.45) is a key enzyme for the de novo synthesis of DNA and as such a target for anticancer drug development. There is a need to develop noninvasive methods for assessing thymidylate synthase inhibition in tumors. The aim of this study was to assess the potential of 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) positron emission tomography (PET) for early measurement of thymidylate synthase inhibition and to elucidate the cellular mechanisms involved. Radiation-induced fibrosarcoma-1 tumor-bearing mice were injected with a single i.p. dose of the thymidylate synthase inhibitor 5-fluorouracil (5-FU; 165 mg/kg) and imaged by [(18)F]FLT-PET at 1 to 2 hours after treatment. Deoxyuridine, thymidine kinase 1 (cytoplasmic thymidine kinase; EC2.7.1.21), and ATP levels in excised tumors were measured. Cellular assays for membrane transport were also done. There was a 1.8-fold increase in the 60-minute [(18)F]FLT tumor/heart radioactivity ratio in drug-treated mice compared with vehicle controls (P = 0.0016). Plasma and tumor deoxyuridine levels increased significantly but thymidine kinase and ATP levels were unchanged. Whole-cell assays implicated a (low level) functional role for the type-1 equilibrative nucleoside transporter (ENT). There was an increase in type-1 ENT-binding sites per cell from 49,110 in untreated cells to 73,142 (P = 0.03) in cells treated with 10 microg/mL 5-FU for 2 hours, without a change in transporter affinity (P = 0.41). We conclude that [(18)F]FLT-PET can be used to measure thymidylate synthase inhibition as early as 1 to 2 hours after treatment with 5-FU by a mechanism involving redistribution of nucleoside transporters to the plasma membrane.