High-dose 5-Fluorouracil with uridine-diphosphoglucose rescue increases thymidylate synthase inhibition but not 5-Fluorouracil incorporation into RNA in murine tumors

Nucleo(s)tide metabolism as basis for drug development; the Anne Simmonds award lecture

Aberrant metabolism of purines and pyrimidines led to development of drugs for treatment of various diseases, such as inflammatory, neurological, cardiovascular, viral infections and cancer. Purine and Pyrimidine Symposia are characterized by close interactions, leading to extensive cross-fertilization on methodology and translating not only from bench-to-bedside, but also between various disciplines such as medicinal chemistry, pharmacology, oncology, virology, rheumatology, biochemistry, pediatrics, cardiology, surgery and immunology. This background was fundamental in our studies on how to optimize application of existing drugs (5-fluorouracil [5FU], thiopurines, antifolates such as methotrexate) but also to support development of novel drugs such as gemcitabine, novel antifolates, S-1, TAS-102 and fluorocyclopentenylcytosine. Knowledge of their metabolism helped to design rational combinations such as of gemcitabine with cisplatin, one of the most widely used drug combinations for various cancers. The combination of 5FU with uridine, led to the development of triacetyluridine registered for emergency treatment of patients with lethal 5FU toxicity. Mechanisms of action were studied by careful analysis of their metabolism, using classical enzyme assays with radioactive precursors and HPLC analysis. Drug metabolism moved from manually operated HPLC systems with UV-detection for peak identification and paper rolls for quantification, to computer-operated HPLC with automatic multi-wavelength and fluorometric peak detection and more recently to ultrasensitive, highly specific mass-spectrometry-based systems. Some aspects, however, never changed; careful analysis of the results and being prepared for the unexpected. The latter actually led to the most interesting results. Investigation of (nucleoside/nucleotide) metabolism remains an exciting field of research.

Prompt treatment with uridine triacetate improves survival and reduces toxicity due to fluorouracil and capecitabine overdose or dihydropyrimidine dehydrogenase deficiency

Uridine triacetate has been shown to be an effective antidote against mortality and toxicity caused by either overdoses or exaggerated susceptibility to the widely used anticancer agents 5-fluorouracil (5-FU) and capecitabine. However, a direct assessment of efficacy based on when emergency treatment was initiated was not clinically feasible. In this study we used mouse models of 5-FU overdose and of dihydropyrimidine dehydrogenase (DPD) deficiency to compare the efficacy of uridine triacetate in reducing toxicity and mortality when treatment was initiated at time points from 4 to 144 h after administration of 5-FU. We found that uridine triacetate was effective both in the 5-FU overdose and DPD deficiency models. Starting treatment within 24 h was most effective at reducing toxicity and mortality in both models, while treatment starting more than 96 to 120 h after 5-FU was far less effective. Uridine triacetate also reduced mortality in the DPD deficiency model when mice were treated with the 5-FU prodrug capecitabine. The results of this study are supportive of clinical observations and practice, indicating that efficacy declined progressively with later and later treatment initiation. Prompt treatment with uridine triacetate, within 24 h, conferred the greatest protection against 5-FU overexposure.

Identification of thymidylate synthase as a potential therapeutic target for lung cancer

Background:

Thymidylate synthase (TS), a key enzyme in the de novo synthesis of thymidine, is an important chemotherapeutic target for malignant tumours including lung cancer. Although inhibition of TS has an antiproliferative effect in cancer cells, the precise mechanism of this effect has remained unclear.

Methods:

We examined the effects of TS inhibition with an RNA interference-based approach. The effect of TS depletion on the growth of lung cancer cells was examined using colorimetric assay and flow cytometry.

Results:

Measurement of the enzymatic activity of TS in 30 human lung cancer cell lines revealed that such activity differs among tumour histotypes. Almost complete elimination of TS activity by RNA interference resulted in inhibition of cell proliferation in all tested cell lines, suggestive of a pivotal role for TS in cell proliferation independent of the original level of enzyme activity. The antiproliferative effect of TS depletion was accompanied by arrest of cells in S phase of the cell cycle and the induction of caspase-dependent apoptosis as well as by changes in the expression levels of cyclin E and c-Myc. Moreover, TS depletion induced downregulation of the antiapoptotic protein X-linked inhibitor of apoptosis (XIAP), and it seemed to activate the mitochondrial pathway of apoptosis.

Conclusion:

Our data provide insight into the biological relevance of TS as well as a basis for clinical development of TS-targeted therapy for lung cancer.

Selective protection by uridine of growth inhibition by 5-fluorouracil (5FU) mediated by 5FU incorporation into RNA, but not the thymidylate synthase mediated growth inhibition by 5FU-leucovorin

Fluorouracil (5FU) acts by RNA-incorporation and inhibition of thymidylate synthase; the first action is counteracted by uridine, and the second is enhanced by leucovorin (LV). Growth inhibition of C26-10 colon cancer cells by 5FU was enhanced by LV and rescued by uridine, but 5FU-LV was only partially rescued by uridine. In WiDr cells, 5FU sensitivity was not enhanced by LV, while both 5FU and 5FU-LV were rescued by uridine. Intermediate trends were found in SW948 and HT29 cells. Uridine rescue in mice allowed 1.5-fold increase in 5FU dose, leading to 2-fold increase in the antitumor effect and thymidylate synthase inhibition in resistant Colon-26 tumors. In the sensitive Colon-26-10 tumor, uridine rescue decreased 5FU-RNA incorporation > 10-fold, without affecting the antitumor activity. The use of LV and uridine can differentiate between two mechanisms of action of 5FU.

5-Fluorouracil dose escalation enabled with PN401 (triacetyluridine): toxicity reduction and increased antitumor activity in mice

PURPOSE

PN401, an oral prodrug of uridine yields more bioavailable uridine than oral administration of uridine itself. PN401 may therefore be useful for permitting dose escalation of 5-fluorouracil (5-FU) with consequent improvements in antitumor efficacy.

EXPERIMENTAL DESIGN

Female BALB/c mice (Colon 26 adenocarcinoma) were treated with 5-FU with PN401 to define the MTD, and pharmacokinetic analyses were done. A comparison of 5-FU/PN401 was made to 5-FU/eniluracil (EU) and 5-FU/LV. The best timing of the first dose of PN401 relative to 5-FU was evaluated by administering groups of mice PN401 beginning 2, 24, or 48 h after 5-FU dose.

RESULTS

The MTD of 5-FU was 100 mg/kg/week whereas the MTD of 5-FU + PN401 was 200 mg/kg/week. A complete response (CR) of 80% and partial response (PR) of 20% was observed with 5-FU (200 mg/kg) + PN401, CR of 40% and PR of 60% with 5-FU (175 mg/kg) + PN401, PR of 10% with 5-FU (150 mg/kg) + PN401 while no response with 5-FU (100 mg/kg) + PN401. Analysis of 5-FU pharmacokinetics displayed nonlinearity as a function of administered dose in mice. In the comparison study, the best response was achieved with PN401 when compared to EU and LV. Mice that did not receive PN401 died by day 12, while other groups were alive at day 31. The proportion of mice surviving was highest in the group which received PN401 at 2 h followed by 24 and 48 h.

CONCLUSIONS

There is a threshold 5-FU dose after which the efficacy is dramatically improved-in mice bearing Colon 26 adenocarcinoma, that threshold is a dose of >150 mg/kg/week, and the increased efficacy correlates with about a fourfold increase in the AUC of 5-FU. PN401 used to rescue mice from the lethal toxicity of 5-FU entails that PN401 can be used as an antidote even when used up to 48 h after a 5-FU overdose.

Pharmacokinetics of bolus 5-fluorouracil: relationship between dose, plasma concentrations, area-under-the-curve and toxicity

The pharmacokinetics of 5-fluorouracil (5FU) have been related to toxicity and antitumor activity, in particular for continuous infusion schedules, but to a lesser extent for frequently used bolus injections. The use of intensive sampling schedules limits the application of pharmacokinetics to optimize individual dosing or to define the ideal combination with other drugs. We therefore reanalyzed a pharmacokinetic study in order to develop a limited sampling schedule. Patients received escalating doses of 5FU at 500, 600 and 720 mg/m2 as a bolus until toxicity developed. Blood samples were analyzed until 24 h after administration. The area under the concentration time curve from 0-90 min (AUC(0-90)) was strongly correlated with dose and also with toxicity (p = 0.0009). The 5FU concentrations at 30 and 60 min were correlated to the AUC(30-240) and to that of the AUC(0-90) (r2 = 0.970). The use of limited sampling (30, 60, 90 min) in a patient given 353 mg/m2 5FU with severe toxicity at initial dosing at 500 mg/m2 revealed that the AUC(0-90) at 353 mg/m2 was higher than the normal AUC(0-90) for 500 mg/m2. This patient appeared to have an 8-fold lower activity of the 5FU degradation enzyme dihydropyrimidine dehydrogenase. Limited sampling will allow us to define potential aberrant kinetics of pharmacokinetic interaction of 5FU with other drugs being developed for treatment of colorectal cancer.

Prospective clinical trials using a pharmacogenetic/pharmacogenomic approach

The antitumor activity of most anticancer agents is limited by a number of different factors, such as their cellular targets and activating enzymes, while constitutive genetic polymorphisms may limit drug bioavailability and influence either antitumor efficacy or toxic side effects. An example of a drug for which clear predictive parameters have been identified is 5-fluorouracil (5FU): its antitumor activity is limited by either a high activity of the target enzyme thymidylate synthase (TS) and/or a high activity of its degrading enzyme, dihydropyrimidine dehydrogenase (DPD). Retrospective studies showed a clear correlation between a high expression of TS and a poor response, which was stronger when DPD was included in the evaluation (high DPD, poor response). Therefore we initiated a clinical prospective study in which we treated previously untreated patients with advanced colorectal cancer with tailored chemotherapy: at a low TS-mRNA and low DPD-mRNA patients were stratified to receive a standard weekly 5FU-leucovorin regimen. At a high TS and/or DPD, patients were stratified to receive a combination of oxaliplatin and irinotecan. Up to now this proof-of-principle study demonstrated that selection of patients is possible and can clearly improve the clinical outcome. The next step is to develop algorithms to select patients for combination chemotherapy with 5FU-leucovorin and new compounds, such as oxaliplatin or irinotecan, or novel targeted agents such as bevacizumab or cetuximab. For these combination schedules the optimal combination of predictive factors has to be explored.

A non-radioactive sensitive assay to measure 5-fluorouracil incorporation into DNA of solid tumors

A non-radioactive method to determine 5-Fluorouracil (5FU) incorporation into DNA has been developed. Isolated DNA was enzymatically degraded to bases and the resulting 5FU was measured with standard gas-chromatography coupled to mass spectrometry (GC-MS) and compared with that of radioactive 5FU in a cell line. Incorporation into DNA of the murine Colon 26-B tumor treated with maximal tolerated doses of 5FU and fluorodeoxyuridine (FUdR) was maximal after 2 hour and was 15.4 and 71.0 fmol/microg DNA, respectively. After a plateau for about 3 days a decrease was observed to +/- 2 fmol/microg DNA after 10 days. The assay is very sensitive and reproducible and can be used in a clinical setting.

5-Fluorouracil incorporation into RNA and DNA in relation to thymidylate synthase inhibition of human colorectal cancers

BACKGROUND

The mechanism of action of 5-fluorouracil (5-FU) has been associated with inhibition of thymidylate synthase (TS) and incorporation of 5-FU into RNA and DNA, but limited data are available in human tumor tissue for the latter. We therefore measured incorporation in human tumor biopsy specimens after administration of a test dose of 5-FU alone or with leucovorin.

PATIENTS AND METHODS

Patients received 5-FU (500 mg/m(2)) with or without high-dose leucovorin, low-dose leucovorin or l-leucovorin, and biopsy specimens were taken after approximately 2, 24 or 48 h. Tissues were pulverized and extracted for nucleic acids. 5-FU incorporation was measured using gas chromatography/mass spectrometry after complete degradation to bases of isolated RNA and DNA.

RESULTS

Maximal incorporation into RNA (1.0 pmol/micrograms RNA) and DNA (127 fmol/micrograms DNA) of 59 and 46 biopsy specimens, respectively, was found at 24 h after 5-FU administration. Incorporation into RNA but not DNA was significantly correlated with intratumoral 5-FU levels. However, DNA incorporation was significantly correlated with the RNA incorporation. Primary tumor tissue, liver metastasis and normal mucosa did not show significant differences, while leucovorin had no effect. Neither for RNA (30 patients) nor DNA (24 patients) incorporation was a significant correlation with response to 5-FU therapy found. However, in the same group of patients, response was significantly correlated to TS inhibition (mean TS in responding and non-responding groups 45 and 231 pmol/h/mg protein, respectively; P=0.001).

CONCLUSIONS

5-FU is incorporated at detectable levels into RNA and DNA of human tumor tissue, but no relation between the efficacy of 5-FU treatment and incorporation was found, in contrast to TS.

Scope and epidemiology of cancer therapy-induced oral and gastrointestinal mucositis

OBJECTIVE

To discuss the scope and epidemiology of cancer therapy-induced mucositis.

DATA SOURCE

Peer-reviewed articles and book chapters.

CONCLUSION

Mucositis is a frequent and costly complication of cancer treatment. The risk of cancer therapy-induced mucositis varies depending on a number of patient- and treatment-related factors.

IMPLICATIONS FOR NURSING PRACTICE

An awareness of the risk factors associated with mucositis will allow nurses to identify cancer patients at greatest risk and incorporate supportive care measures into their management plans.