Biochemical factors affecting the tightness of 5-fluorodeoxyuridylate binding to human thymidylate synthetase

Treatment of patients with carcinomas in advanced stages with 5-fluorouracil, folinic acid and pyridoxine in tandem

The effect of high-dose pyridoxine (PN) on activity of 5-fluorouracil (FUra) and folinic acid (FA)-containing regimens was studied in 50 patients including 14 with digestive tract, and 36 with breast carcinomas (BC) in advanced stages with poor prognostic characteristics. Patients with colorectal, and pancreas adenocarcinoma received oxaliplatin, irinotecan, FUra, FA (Folfirinox), and patients with squamous cell carcinoma of the esophagus had paclitaxel, carboplatin, FUra, FA (TCbF). Patients with BC received AVCF (doxorubicin, vinorelbine, cyclophosphamide, FUra, FA) followed by TCbF or TCbF only, and patients who overexpressed HER2 received TCbF plus trastuzumab and pertuzumab. PN (1000-3000 mg/day iv) preceded each administration of FUra and FA. 47 patients (94%) responded, including 16 (32%) with CR. Median tumor reduction was 93%. Median event-free survival (EFS) was 37.7 months. The 25 patients with tumor shrinkage ≥ 91% had EFS of 52% from 42 months onwards. Unexpected toxicity did not occur. PN enhances potency of chemotherapy regimens comprising FUra and FA.

Pharmacologic modulation of 5-fluorouracil by folinic acid and pyridoxine for treatment of patients with advanced breast carcinoma

High concentration pyridoxal 5’-phosphate, the cofactor of vitamin B6, potentiates cytotoxicity in cancer cells exposed to 5-fluorouracil (FUra) and folinic acid (FA). We studied the effect of high-dose pyridoxine on antitumor activity of regimens comprising FUra and FA in 27 advanced breast carcinoma patients. Of 18 previously untreated patients, 12 had tumors that did not overexpress HER2 (Group I), and 6 that overexpressed HER2 (Group II). Nine patients (Group III) had prior chemotherapy. Group I received AVCF (doxorubicin, vinorelbine, cyclophosphamide, FUra, FA) or FAC (doxorubicin, cyclophosphamide, FUra, FA) followed by TCbF (paclitaxel carboplatin, FUra, FA). Groups II, and III received TCbF. Pyridoxine iv (1000–3000 mg/day) preceded each FA and FUra. Group II also received trastuzumab and pertuzumab. 26 patients responded. Three patients in Group I had CRs and 9 had PRs with 62–98% reduction rates; 4 patients in Group II had CRs and 2 had PRs with 98% reduction. Of 7 measurable patients in Group III, 2 attained CRs, and 5 had PRs with 81–94% reduction rates. Median time to response was 3.4 months. Unexpected toxicity did not occur. This pilot study suggests that high-dose vitamin B6 enhances antitumor potency of regimens comprising FUra and FA.

Pharmacologic modulation of 5-fluorouracil by folinic acid and high-dose pyridoxine for treatment of patients with digestive tract carcinomas

Supplementation of cancer cells exposed to 5-fluorouracil (FUra) and folinic acid (FA) with high concentration pyridoxal 5′-phosphate, the cofactor of vitamin B6, potentiates the cytotoxicity of FUra in a synergistic interaction mode. We report a pilot study in 13 patients with previously untreated advanced carcinoma of the digestive tract to assess the impact of high-dose pyridoxine (PN) on the antitumor activity of regimens comprising FUra and FA. Five patients had colorectal adenocarcinoma (CRC); 5 had pancreas adenocarcinoma (PC); and 3 had squamous cell carcinoma of the esophagus (EC). Patients with CRC and with PC received oxaliplatin, irinotecan, FUra and FA, and patients with EC had paclitaxel, carboplatin, FUra and FA. PN iv from 1000 to 3000 mg/day preceded each administration of FA and FUra. Eleven patients responded. Two patients with CRC attained CRs and 3 had PRs with reduction rates ≥ 78%. Two patients with PC attained CRs, and 2 had PRs with reduction rates ≥ 79%. Responders experienced disappearance of most metastases. Of 3 patients with EC, 2 attained CRs. Median time to attain a response was 3 months. Unexpected toxicity did not occur. Results suggest that high-dose vitamin B6 enhances antitumor potency of regimens comprising FUra and FA.

Relationship between folate concentration and expression of folate-associated genes in tissue and plasma after intraoperative administration of leucovorin in patients with colorectal cancer

Purpose

The aim of study was to investigate the relationship between folate concentration and expression of folate-associated genes in tumour, mucosa and plasma of patients with colorectal cancer, after intraoperative administration of bolus leucovorin (LV).

Methods

Eighty patients were randomized into four groups to receive 0, 60, 200, or 500 mg/m² LV, respectively. Tissue and plasma folate concentrations were assessed by LC–MS/MS. Gene expression of ABCC3/MRP3, FPGS, GGH, MTHFD1L, SLC46A1/PCFT, and SLC19A1/RFC-1 was determined using quantitative PCR.

Results

The folate concentration in tumour increased with increasing dosage of LV. Half of the patients treated with 60 mg/m² did not reach a level above the levels of untreated patients. A significant correlation between folate concentration in tumour and mucosa was found in untreated patients, and in the group treated with 60 mg/m² LV. The 5-MTHF/LV ratio correlated negatively with folate concentration in mucosa, whereas a positive correlation was found in tumour of patients who received 200 or 500 mg/m² LV. A positive correlation was found between folate concentration and expression of all genes, except MTHFD1L, in patients who received LV. There was a negative correlation between 5-MTHF concentration in plasma of untreated patients and expression of GGH and SLC46A1/PCFT in tumour.

Conclusions

The results indicate the possibility of using the individual plasma 5-MTHF/LV ratio after LV injection as a surrogate marker for tissue folate concentration. Expression of several folate-associated genes is associated with folate concentration in tissue and plasma and may become useful when predicting response to LV treatment.

Electronic supplementary material

The online version of this article (10.1007/s00280-018-3690-9) contains supplementary material, which is available to authorized users.

Enhancement of 5-Fluorouracil Cytotoxicity by Pyridoxal 5'-Phosphate and Folinic Acid in Tandem

The current study originates from the assumption that, in tumors, levels of naturally occurring pyridoxal 5'-phosphate (PLP) are too small to allow conversion of tetra hydro pteroylglutamate (H₄PteGlu) into methylene tetra hydro pteroylglutamate (CH₂-H₄PteGlu) in amounts required to improve inhibition of thymidylate synthase by 5-fluorouracil (FUra) through ternary complex stabilization. The hypothesis relates to the low affinity for cofactor of the PLP-dependent serine hydroxymethyl transferase (SHMT), the enzyme that catalyzes formation of CH₂-H₄PteGlu by transfer of the Cβ of serine to H₄PteGlu. Intracellular concentrations of PLP are smaller than the dissociation constant of SHMT for cofactor, which suggests that enzyme activity should be sensitive to PLP level changes. Three cancer cell lines were supplemented with PLP to investigate the influence of this cofactor on FUra cytotoxicity. Cells were exposed to FUra, FUra and folinic acid (FA), FUra and PLP, and FUra combined with both FA and PLP. The median-effect principle for concentration-effect analysis and combination indices were used to determine interactions on cytotoxicity. FUra cytotoxicity in vitro was enhanced by FA and PLP in tandem. Synergistic cytotoxic interaction of FUra with FA and PLP was demonstrated in HT29 and L1210 cells. Summation was found in HCT116 cells. Parenteral pyridoxamine was administered in mice to explore erythrocyte production of PLP in vivo. Cofactor attained levels in the range of the K_D for binding to SHMT, and it was rapidly cleared from cells. Pharmacokinetics of pyridoxamine suggests that modulation of FUra by vitamin B6 could be achieved in vivo.

Capecitabine in Combination with Docetaxel in First Line in HER2-Negative Metastatic Breast Cancer: an Observational Study

Due to the limited experience with capecitabine plus docetaxel (XT) combination in the first-line treatment of metastatic breast cancer in Hungary, the main objective of the study was to analyze the effectiveness and tolerability of XT therapy. A prospective, open-label, non-randomized, single-arm, multicenter, observational study was designed. All female patients were eligible whose metastatic breast cancer could be treated with the XT protocol according to the summary of product characteristics of the drugs. The median progression free survival was 9.9 ± 3.0 months. Time to treatment failure was 4.6 ± 5.1 months on average. The overall response rate was 28.9 %, the clinical benefit rate was 73.3 %. The treatment was discontinued in 35.6 % of patients due to disease progression and in 20.0 % due to adverse events (AE). 33 patients with a total of 73 AEs have been reported, and 13 of them had serious adverse events (SAE). The efficacy and the safety profile of XT chemotherapy proven in the study are consistent with the results demonstrated in randomized trials. First-line XT chemotherapy effectively improves the PFS in metastatic breast cancer.

Epigenetics and chemoresistance in colorectal cancer: an opportunity for treatment tailoring and novel therapeutic strategies

Colorectal cancer is the second leading cause of cancer-related deaths in the world. Despite many therapeutic opportunities, prognosis remains dismal for patients with metastatic disease, and a significant portion of early-stage patients develop recurrence after chemotherapy. Epigenetic gene regulation is a major mechanism of cancer initiation and progression, through the inactivation of several tumor suppressor genes. Emerging evidence indicates that epigenetics may also play a key role in the development of chemoresistance. In the present review, we summarize epigenetic mechanisms triggering resistance to three commonly used agents in colorectal cancer: 5-fluorouracil, irinotecan and oxaliplatin. Those epigenetic biomarkers may help stratify colorectal cancer patients and develop a tailored therapeutic approach. In addition, epigenetic modifications are reversible through specific drugs: histone-deacetylase and DNA-methyl-transferase inhibitors. Preclinical studies suggest that these drugs may reverse chemoresistance in colorectal tumors. In conclusion, an epigenetic approach to colorectal cancer chemoresistance may pave the way to personalized treatment and to innovative therapeutic strategies.

CoFactor: Folate Requirement for Optimization of 5-Fluouracil Activity in Anticancer Chemotherapy

Intracellular reduced folate exists as a "pool" of more than 6 interconvertable forms. One of these forms, 5,10 methylenetetrahydrofolic acid (CH(2)THF), is the key one-carbon donor and reduced folate substrate for thymidylate synthase (TS). This pathway has been an important target for chemotherapy as it provides one of the necessary nucleotide substrates for DNA synthesis. The fluoropyrimidine 5-fluorouracil (5-FU) exerts its main cytotoxic activity through TS inhibition. Leucovorin (5-formyltetrahydrofolate; LV) has been used to increase the intracellular reduced folate pools and enhance TS inhibition. However, it must be metabolized within the cell through multiple intracellular enzymatic steps to form CH2THF. CoFactor (USAN fotrexorin calcium, (dl)-5,10,-methylenepteroyl-monoglutamate calcium salt) is a reduced folate that potentiates 5-FU cytotoxicity. According to early clinical trials, when 5-FU is modulated by CoFactor instead of LV, there is greater anti-tumor activity and less toxicity. This review presents the emerging role of CoFactor in colorectal and nongastrointestinal malignancies.

Capecitabine for locally advanced and metastatic colorectal cancer: A review

Capecitabine (Xeloda^®) is an oral fluoropyrimidine which is produced as a pro-drug of fluorouracil, and shows improved tolerability and intratumor drug concentrations following its tumor-specific conversion to the active drug. We have searched the Pubmed and Cochrane databases from 1980 to 2009 with the purpose of reviewing all available information on Capecitabine, focusing on its clinical effectiveness against colorectal cancer. Special attention has been paid to trials that compared Capecitabine with standard folinic acid (leucovorin, LV)-modulated intravenous 5-fluorouracil (5-FU) bolus regimens in patients with metastatic colorectal cancer. Moreover the efficacy of Capecitabine on metastatic colorectal cancer, either alone or in various combinations with other active drugs such as Irinotecan and Oxaliplatin was also assessed. Finally, neoadjuvant therapy consisting of Capecitabine plus radiation therapy, for locally advanced rectal cancer was analysed. This combination of chemotherapy and radiotherapy has a special role in tumor down staging and in sphincter preservation for lower rectal tumors. Comparative trials have shown that Capecitabine is at least equivalent to the standard LV-5-FU combination in relation to progression-free and overall survival whilst showing a better tolerability profile with a much lower incidence of stomatitis. It is now known that Capecitabine can be combined with other active drugs such as Irinotecan and Oxaliplatin. The combination of Oxaliplatin with Capecitabine represents a new standard of care for metastatic colorectal cancer. Combinating the Capecitabine-Oxaliplatin regimen with promising new biological drugs such as Bevacizumab seems to give a realistic prospect of further improvement in time to progression of metastatic disease. Moreover, preoperative chemo-radiation using oral capecitabine is better tolerated than bolus 5-FU and is more effective in the promotion of both down-staging and sphincter preservation in patients with locally advanced rectal cancer. Finally, the outcomes of recently published trials suggest that capecitabine seems to be more cost effective than other standard treatments for the management of patients with colorectal cancer.

Evaluating the drug-target relationship between thymidylate synthase expression and tumor response to 5-fluorouracil. Is it time to move forward?

Thymidylate synthase is a target of 5-fluoruracil, a pyrimidine analog used to treat gastrointestinal and other cancers. The 5-fluorouracil metabolite, fluoro-deoxyuridine monophosphate, forms a ternary complex with thymidylate synthase and 5,10-methylene tetrahydrofolate. The purpose of this study was to evaluate the time-honored connection between thymidylate synthase and 5-fluorouracil. From our literature search spanning reports from 1995 to 2007 published in journals having an impact factor greater than 2, we stratified the tumors within each article, according to low versus high thymidylate synthase expression. These groups were subdivided into responders, stable disease or disease progression. The relationship between thymidylate synthase expression and 5-fluorouracil response was analyzed for the overall group, as well as for subsets. Overall, the literature supported an approximately 2-fold inverse relationship between thymidylate synthase expression and response to 5-fluoruracil. We found no change in the trend for a relationship between thymidylate synthase and 5-fluorouracil when the literature was stratified by date of publication, impact factor of the journal in which the report was published, or substrate (mRNA versus protein) for measuring thymidylate synthase expression. Of note, there is no significant change in the trend when comparing 5-fluorouracil treatment alone or in combination with leucovorin. We found a decline of this trend when certain chemotherapeutics were used in combination with 5-fluorouracil. In sum, the connection between thymidylate synthase expression and patient response to 5-fluorouracil does not satisfy expectations for an effective drug-target relationship; and thus, studies of the thymidylate synthase tandem repeat status might only be clinically valuable in regards to patient toxicity. Thus, we question the reliability of thymidylate synthase expression as a clinical predictor of 5-fluorouracil response. Future research could perhaps be directed towards alternate targets and metabolites of 5-fluorouracil, in an effort to find a clinically relevant biomarker panel for response and to optimize fluoropyrimidine-based therapy.

Phase III study of cyclophosphamide, doxorubicin, and fluorouracil (CAF) plus leucovorin versus CAF for metastatic breast cancer: Cancer and Leukemia Group B 9140

PURPOSE

To determine whether biochemical modulation with LV (leucovorin) enhances the efficacy of CAF (cyclophosphamide, doxorubicin, and fluorouracil) against metastatic breast cancer.

PATIENTS AND METHODS

Women with histologically confirmed stage IV breast cancer, Cancer and Leukemia Group B (CALGB) performance status 0 to 2, and no prior chemotherapy for metastatic disease were randomly assigned to receive CAF (cyclophosphamide 500 mg/m2 day 1, doxorubicin 40 mg/m2 day 1, and fluorouracil [FU] 200 mg/m2 intravenous bolus days 1 to 5) with or without LV (LV 200 mg/m2 over 30 minutes days 1 to 5 given 1 hour before FU).

RESULTS

Two hundred forty-two patients were randomly assigned to treatment; 124 patients had visceral crisis and 40 patients had a CALGB performance status score of 2. The median follow-up was 6 years. The two study arms were similar with regard to serious adverse events; four patients died from treatment-related causes, two patients on each study arm. Predictive variables for time to treatment failure and survival were visceral disease and performance status. The overall response rate was 29% for CAF versus 28% for CAF plus LV. The median time to treatment failure (9 months) and median survival (1.7 years) did not differ by treatment arm.

CONCLUSION

Modulation of CAF with LV improved neither response rates nor survival among women with metastatic breast cancer, compared with CAF alone. Multivariate analyses confirmed the prognostic importance of performance status and visceral crisis. However, the overall and complete response rates, response durations, time to treatment failure, and survival were the same in the two treatment arms.

A phase II study of combined oral uracil and ftorafur with leucovorin for patients with squamous cell carcinoma of the head and neck

BACKGROUND

The objective of this Phase II study was to define the response rate, safety profile, and toxicity of oral uracil and ftorafur (UFT) with leucovorin (UFT/LV) as a palliative treatment for patients with squamous cell carcinoma of the head and neck (SCCHN).

METHODS

Patients with metastatic or recurrent SCCHN with an Eastern Cooperative Oncology Group performance status < 2 and adequate organ function were enrolled in an institutional review board-approved trial. Prior induction or adjuvant chemotherapy was permitted provided 6 months had elapsed since the last chemotherapy. Patients were treated with UFT 300 mg/m(2) per day and leucovorin 90 mg per day administered in three doses daily for 28 days followed by a 7-day break for a 35-day cycle. Planned intrapatient dose modifications were based on individual toxicity. Patients were removed from the study for progression of disease or unacceptable toxicity.

RESULTS

One hundred six cycles of UFT/LV had been administered to 42 patients as of January 1, 2000. The most common toxicities, in descending order of incidence, were anemia, pain, fatigue, diarrhea, nausea, mucositis, and anorexia. Clinically significant toxicities attributable to UFT/ LV were primarily gastrointestinal. On an intent-to-treat basis, three patients (7%) achieved a complete response, and six patients (14%) achieved a partial response. The overall response rate was 21% (95% confidence interval, 10--37%).

CONCLUSIONS

UFT/LV therapy is feasible in this patient population and is generally well tolerated. Response rates are similar to the rates expected with continuous-infusion 5-fluorouracil. UFT/LV should be studied further both alone and in combination therapy for patients with SCCHN.

Cytotoxic synergism of methioninase in combination with 5-fluorouracil and folinic acid

Potentiation of the cytotoxic activity of 5-fluorouracil (FUra) by folinic acid (5-HCO-H4folate) is due to elevation of the methylene tetrahydrofolate (CH2-H4folate) level, which increases the stability of the ternary complex of thymidylate synthase (TS), fluorodeoxyuridine monophosphate, and CH2-H4folate that inactivates the TS. Methionine deprivation results in the production of tetrahydrofolate (H4folate) and, subsequently, CH2-H4folate from methyl tetrahydrofolate, as a consequence of the induction of methionine synthesis. We hypothesized that the efficacy of FUra could be augmented by the combination of high-concentration 5-HCO-H4folate and recombinant methioninase (rMETase), a methionine-cleaving enzyme. Studies in vitro were performed with the cell line CCRF-CEM. Cytotoxic synergism of FUra + rMETase and FUra + 5-HCO-H4folate + rMETase was demonstrated with the combination index throughout a broad concentration range of FUra and rMETase. A subcytotoxic concentration of rMETase reduced the IC50 of FUra by a factor of 3.6, and by a factor of 7.5, in the absence and in the presence of 5-HCO-H4folate, respectively. 5-HCO-H4folate increased the intracellular concentrations of CH2-H4folate and H4folate from their baseline levels. Concentrations of folates were not changed by exposure to rMETase. Levels of free TS in cells treated with FUra + 5-HCO-H4folate and with FUra + rMETase were lower than those in cells exposed to FUra alone. The decrease of TS was still more pronounced in cells treated with FUra + 5-HCO-H4folate + rMETase. The synergism described in this study will be a basis for further exploration of combinations of fluoropyrimidines, folates, and rMETase.

Effects of 5-fluorouracil and leucovorin in the treatment of pancreatic-biliary tract adenocarcinomas

Adenocarcinomas of the pancreas and biliary tract are highly malignant neoplasms, which are found in the advanced stage. Chemotherapy commonly plays a palliative role in the treatment of pancreatic and biliary tract cancers. 5-Fluorouracil (5-FU) is the most widely studied single agent; the response rate of 5-FU is only 20%. Recently, some reports presented interesting results, in which 5-FU, modulated with levofolinic acid (leucovorin), was active in patients with colorectal cancer. In relation, we performed a phase II study of 5-FU, modulated with leucovorin, in patients affected by advanced pancreatic or biliary tract cancer. Fifty-one patients with nonresectable carcinomas of the pancreas or biliary tract admitted to Korea University Hospital between May 1995 and December 1998 were included in this study. Chemotherapy consisted of leucovorin 25 mg/m2/day by 2-hour intravenous infusion, followed by 5-FU 375 mg/m2/day by bolus intravenous infusion, from day I to day 5. The treatment was repeated every 3 to 4 weeks. A total of 51 eligible patients with advanced adenocarcinoma of the pancreas or biliary tract were enrolled. Of 23 enrolled patients with pancreatic adenocarcinoma, one patient showed complete remission with a survival duration of 13 months (response duration was 9 months). Three patients had partial responses (PRs) with survival times of 6, 12, and 15 months, respectively. The overall response rate was 17.4% (95% confidence interval [CI], 7.2%-36.2%). The median time of overall survival was 6 months (range: 1-15 months). Of 28 enrolled patients with biliary tract cancer, complete responses were observed in 2 patients (7.1%) with survival time of 14 and 16 months, respectively. Seven patients had PRs with a median survival of 8 months. The overall response rate was 32.1% (95% CI, 20.3%-57.5%). The median time of overall survival was 6 months (range: 1-16 months). The most prominent toxicity was mucositis. Hematologic toxicity was less severe. 5-Fluorouracil in modulation with intravenous leucovorin is well tolerated by patients with stage IV pancreatic adenocarcinoma or biliary tract cancer. Although the response rate for patients with pancreatic adenocarcinoma is not better than that achieved using 5-FU monochemotherapy, the 32.1% overall response rate achieved in patients with biliary tract cancer suggests that 5-FU modulation with leucovorin is active in biliary tract cancer.

Tegafur/uracil + calcium folinate in colorectal cancer: double modulation of fluorouracil

The oral chemotherapeutic agent tegafur/uracil (UFT) is the first of a new class of anticancer drugs called dihydropyrimidine dehydrogenase inhibitory fluoropyrimidines. Tegafur/uracil combines uracil with the fluorouracil prodrug tegafur in a 4:1 molar ratio. Uracil competitively inhibits the degradation of fluorouracil, which results in the concentration of fluorouracil remaining at sustained levels in both plasma and tumour. Tegafur/uracil has been commercially available in Japan since 1983 and examined extensively in various tumours. Trials conducted in the US have focused on the combination of tegafur/uracil plus calcium folinate (calcium leucovorin) [ORZEL]. Several phase I and II trials have evaluated the maximum tolerated dose, pharmacokinetics, efficacy, and safety of this combination in the treatment of colorectal cancer. Results have shown that tegafur/uracil at 300 mg/m2/day in divided doses given every 8 hours for 28 days provides prolonged exposure to fluorouracil. Furthermore, tegafur/uracil + calcium folinate is well tolerated, with dose-limiting toxicity manifesting as diarrhoea. Compared with intravenous fluorouracil plus folinic acid (leucovorin) regimens, tegafur/uracil + calcium folinate has similar efficacy with less toxicity and is more convenient because it is an oral regimen. Early studies have also shown potential cost savings because of fewer complications.

Chemical stability and human plasma pharmacokinetics of reduced folates

The in vitro stability and plasma pharmacokinetics of 5,10-methylenetetrahydrofolic acid (CH2FH4), tetrahydrofolic acid (FH4), 5-methyltetrahydrofolic acid (CH3FH4), and 5-formyltetrahydrofolic acid (5-CHOFH4) were studied in view of their potential usefulness in cancer chemotherapy. Analysis of reduced folates was done on a high-performance liquid chromatography (HPLC) system. The high sensitivity of FH4 and CH2FH4 to oxidation can be circumvented by use of high concentrations of the folates, addition of ascorbate, and by thorough exclusion of atmospheric O2. Intravenous injection of 200 mg FH4 or CH2FH4 resulted in average peak concentrations of 69.2 +/- 3.2 nmol/ml and 46.3 +/- 2.6 nmol/ml, respectively. The plasma concentration curves support the concept that these highly oxygen-sensitive reduced folates can be reliably administered as pharmaceuticals to cancer patients through the use of a suitable air-occlusive system for their preparation and administration.

The additivity of substrate fragments in enzyme-ligand binding

BACKGROUND

Enzymes have evolved to recognise their target substrates with exquisite selectivity and specificity. Whether fragments of the substrate--perhaps never available to the evolving enzyme--are bound in the same manner as the parent substrate addresses the fundamental basis of specificity. An understanding of the relative contributions of individual portions of ligand molecules to the enzyme-binding interaction may offer considerable insight into the principles of substrate recognition.

RESULTS

We report 12 crystal structures of Escherichia coli thymidylate synthase in complexes with available fragments of the substrate (dUMP), both with and without the presence of a cofactor analogue. The structures display considerable fidelity of binding mode and interactions. These complexes reveal several interesting features: the cofactor analogue enhances the localisation of substrate and substrate fragments near the reactive thiol; the ribose moiety reduces local disorder through additional specific enzyme-ligand interactions; the pyrimidine has multiple roles, ranging from stereospecificity to mechanistic competence; and the glycosidic linkage has an important role in the formation of a covalent attachment between substrate and enzyme.

CONCLUSIONS

The requirements of ligand-protein binding can be understood in terms of the binding of separate fragments of the ligand. Fragments which are subsystems of the natural substrate for the enzyme confer specific contributions to the binding affinity, orientation or electrostatics of the enzymatic mechanism. This ligand-binding analysis provides a complementary method to the more prevalent approaches utilising site-directed mutagenesis. In addition, these observations suggest a modular approach for rational drug design utilising chemical fragments.

Mechanisms of acquired resistance to modulation of 5-fluorouracil by leucovorin in HCT-8 human ileocecal carcinoma cells

Repeated (10x) exposure of HCT-8 human ileocecal carcinoma cells to 5-fluorouracil (5-FU) for 2 or 72 hr, both incubations in the continuous presence of 20 microM leucovorin (LV), yielded two stable modulation-resistant sublines, FL2h and FL72h. Although LV potentiated growth inhibition by 5-FU 2-fold in parental HCT-8 cells, it did not potentiate the effect of 5-FU in the FL2h or FL72h sublines. LV modulation of 5-fluorodeoxyuridine (5-FdUrd) was also reduced (FL72h) or eliminated (FL2h). In the FL2h and FL72h sublines, the level of thymidylate synthase (TS) protein and TS activity in cell extracts, TS activity in situ, the rate of cellular uptake and metabolism of LV, and the level of 5-FU incorporation into total cellular RNA were similar to those in parental HCT-8 cells. However, LV significantly (P < 0.01) potentiated the inhibition of TS activity in situ in HCT-8 cells at 24 hr after a 2-hr treatment with either 5-FU or 5-FdUrd, but had no such activity in the FL2h and FL72h sublines (P > 0.1). Resistance to modulation of 5-FU by LV was associated with the inability of LV to increase the formation of intracellular TS-FdUMP-methylenetetrahydrofolate ternary complexes, and these complexes dissociated more rapidly (T1/2 > 1.5- to 3-fold faster) in the presence of different concentrations of 5,10-methylenetetrahydropteroylpentaglutamate. Thus, decreased stability of ternary complexes appears to be the mechanism of acquired resistance to the LV modulation of fluoropyrimidine cytotoxicity, possibly due to mutation(s) of TS in these two modulation-resistant HCT-8 sublines.

Biochemical mechanisms of interferon modulation of 5-fluorouracil activity in colon cancer cells

The antiproliferative effect of 5-fluorouracil (5-FU) in colon cancer can be enhanced by interferons (IFN-alpha and IFN-gamma). The mechanisms by which IFNs modulate 5-FU activity are not completely elucidated. IFN-alpha may elevate the levels of the active 5-FU metabolite 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) in the cell, possibly leading to increased inhibition of the target enzyme thymidylate synthase (TS), which might enhance DNA damage. It has been shown that IFN-gamma can prevent 5-FU induced overexpression of TS. We studied IFN modulation in three colon cancer cell lines (SW948, WiDr, human; C26-10, murine) and the sublines WiDr/F and C26-10/F, which were adapted to low folate levels. A 1.5-fold increase in 5-FU sensitivity was observed in C26-10 and C26-10/F (by murine IFN-alpha, beta); in SW948, WiDr and WiDr/F (by human IFN-gamma) and in SW948 and WiDr/ F (by human IFN-alpha). In none of the cell lines did human IFN-alpha, IFN-gamma or murine IFN-alpha, beta increase FdUMP levels after exposure to 5-FU. TS activity, indirectly measured by incorporation of [6-3H]-deoxyuridine into DNA, was inhibited by 5-FU, but the IFNs did not enhance inhibition. DNA damage was measured as a drug-induced decrease of double-stranded (dss) DNA compared to control cells. After 5-FU exposure, dss DNA decreased to 60-75% in WiDr, WiDr/F and SW948 cells. Human IFN-alpha alone caused minimal DNA damage (95% dss DNA), but increased 5-FU-induced effects to 35-50% dss DNA. IFN-gamma did not cause DNA damage and did not enhance 5-FU-mediated DNA damage. Expression of TS protein, analysed by ELISA, was increased after 5-FU exposure of SW948 cells, but this increase was not affected by addition of either IFN-alpha or IFN-gamma. It is concluded that one of the mechanisms involved in modulation of 5-FU activity is the effect of IFN-alpha on 5-FU-mediated DNA damage, but for IFN-gamma no mechanism of action was found.

The cellular interaction of 5-fluorouracil and cisplatin in a human colon carcinoma cell line

The combination of 5-fluorouracil (5-FU) and cisplatin (CDDP) has been shown to have synergistic cytotoxicity in human tumours, but the biochemical mechanism for this interaction remains unclear. Therefore, the aim of this study was to investigate the interaction of 5-FU and CDDP in a human colon carcinoma cell line, NCI H548. A 24 h exposure to 5-FU resulted in a 5-FU IC50 value of 24.2 +/- 4.5 microM, Dm 22.6 microM; exposure to CDDP for 2 h resulted in a IC50 value of 20.8 +/- 8.0 microM, Dm 21.9 microM. When cells were exposed simultaneously to 5-FU for 24 h and CDDP for the initial 2 h, or when cells were treated with CDDP for 2 h followed by various concentrations of 5-FU for 24 h, no greater than additive cytotoxicity was observed. In contrast, when cells were treated with 5-FU for 24 h prior to CDDP for 2 h, a greater than additive interaction was noted (5-FU IC50 1.2 +/- 0.6 microM, Dm 1.3 microM, CI 0.45). Thymidine 10 microM partially reversed the growth inhibitory effects of the 5-FU/ CDDP combination. Using both immunological and biochemical assays, no notable differences in the total amount of TS enzyme or the fraction of bound TS enzyme could be detected in the absence or presence of CDDP. No notable differences could be detected in intracellular reduced folate pools, FdUMP or FUTP pools, or 5-FU incorporation into RNA or DNA with the addition of CDDP to 5-FU. DNA fragmentation studies revealed that the combination of 5-FU followed by CDDP demonstrated a greater degree of single-stranded DNA fragments in parental (P = 0.024) and newly synthesised DNA (P = 0.025) compared with the administration of CDDP prior to 5-FU or either drug alone. The increase in smaller DNA fragment size was reversed with the addition of thymidine (10 microM). These findings suggest that the interaction of 5-FU and CDDP is associated with a greater degree of fragmentation of both nascent and parental DNA.

Treatment of advanced adenocarcinomas of the exocrine pancreas and the gallbladder with 5-fluorouracil, high dose levofolinic acid and oral hydroxyurea on a weekly schedule. Results of a multicenter study of the Southern Italy Oncology Group (G.O.I.M.)

BACKGROUND

To date there is no established chemotherapeutic treatment for patients with unresectable locally advanced and/or metastatic carcinomas of the exocrine pancreas or the gallbladder. A multicenter Phase II trial has been performed by the Southern Italy Oncology Group with the aim of evaluating the clinical effectiveness and tolerability of weekly 5-fluorouracil (5-FU) in modulation with intravenous (i.v.) high dose levofolinic acid and oral hydroxyurea.

METHODS

A total of 70 patients fulfilling the standard eligibility for a Phase II study were enrolled in the trial. Forty patients had advanced pancreatic adenocarcinoma and 30 had advanced gallbladder carcinoma. The treatment schedule was: levofolinic acid, 100 mg/m2, in 500 mL of normal saline over 2-hour infusion followed by 5-FU, 600 mg/m2 i.v. bolus, and oral hydroxyurea, 1000 mg/m2, for 1 day every week for 6 consecutive weeks followed by 15 days of rest.

RESULTS

Among the 40 patients with pancreatic adenocarcinoma, 5 (12.5%; 95% confidence level [CL], 8.5-16.5%) showed a partial response with a median duration of 5.6+ months, and 13 had stable disease. Twenty-two patients progressed. Median survival was 5.8 months. Among patients with advanced gallbladder carcinoma, 9 of 30 had a partial response (30%; 95% CL, 26-34%) with a median duration of 6.5 months, and 8 (27%) had stabilization of disease. Thirteen patients showed progressive disease. Median overall survival was 8 months. Toxicity was mild, with Grade 1 to 2 leukopenia and gastrointestinal toxicity the most frequent side effects. No chemotherapy-related deaths were observed.

CONCLUSIONS

5-FU in modulation with i.v. levofolinic acid and oral hydroxyurea on a weekly schedule is well tolerated by the vast majority of patients with locally advanced and/or metastatic carcinoma of the pancreas or the gallbladder. Although response rate and overall survival for patients with pancreatic adenocarcinoma are far from acceptable, the 30% overall response rate achieved in patients with advanced gallbladder carcinoma suggests that 5-FU in modulation with levofolinic acid and hydroxyurea is active in this neoplasm. The combination of modulated 5-FU with other antineoplastic drugs seems worthy of clinical testing in further controlled trials.

Tomudex (ZD1694): from concept to care, a programme in rational drug discovery

Folate-based anticancer drugs with specificity for thymidylate synthase (TS) have come of age. Ideas nurtured in the early 1970s led to the first-generation of antifolates with TS and dihydrofolate reductase (DHFR) inhibitory activities. Compounds with increased selectivity for TS followed with the highly specific inhibitor, CB3717 being synthesised in 1979 at the Institute of Cancer Research (ICR). CB3717 had significant clinical activity but its development had to be abandoned because its low aqueous solubility led to occasional nephrotoxicity. Collaborative laboratory studies between the ICR and ICI Pharmaceuticals (later to become Zeneca Pharmaceuticals) led to the discovery of ZD1694 (Tomudex), the first antifolate to be licensed for the treatment of cancer (UK 1995) in nearly 40 years and the first new drug for colorectal cancer in about 35 years. Tomudex belongs to a class of compounds that use the reduced-folate carrier (RFC) for uptake into cells and which are excellent substrates for folylpolyglutamate synthetase (FPGS). This paper reviews the underlying philosophies, and the milestones reached during the development of Tomudex.

Biochemical and molecular studies of human methenyltetrahydrofolate synthetase

5-FormylH4folate is administered clinically under the name Leucovorin in association with the antineoplastic agent 5-fluorouracil (5-FU) to enhance the cytotoxic effects of 5-FU. The combination has been shown to be superior to 5-FU alone in the treatment of patients with metastatic colorectal carcinoma. Methenyltetrahydrofolate synthetase (MTHFS) catalyzes the transformation of 5-formyl-tetrahydrofolate to methenylH4folate, which is the obligatory initial metabolic step prior to the intracellular conversion of 5-formylH4folate to other reduced folates and the increase in intracellular folate pools required for 5-FU potentiation. In the following paper, we will summarize results of biochemical and molecular studies of human MTHFS.

Role of leucovorin dosing and administration schedule

Leucovorin (LV) is commonly administered in association with 5-fluorouracil (5-FU) to enhance its cytotoxic effects. In this paper, the cellular and clinical pharmacology of 5-FU potentiation by LV are reviewed, and the dosing and administration schedules are discussed in relation to reported clinical trials. In vitro experimental data suggest that prolonged cellular exposures to relatively low LV concentrations simultaneously with prolonged 5-FU administration are the optimal conditions to enhance 5-FU efficacy. Clinical studies of 5-FU/LV in metastatic colorectal carcinoma have established that 5-day bolus 5-FU with low-dose bolus LV injections yield therapeutic benefits equivalent to those obtained with intravenous bolus schedules using higher doses of LV. It remains to be determined, however, if bolus administration schedules are the optimal clinical treatment regimens. Infusional 5-FU/LV regimens appear to be a strategy worthy of further clinical investigation.

Thymidylate synthase gene amplification in human colon cancer cell lines resistant to 5-fluorouracil

A series of 5-fluorouracil (5-FU)-resistant human colon H630 cancer cell lines were established by continuous exposure of cells to 5-FU. The concentration of 5-FU required to inhibit cell proliferation by 50% (IC50) in the parent colon line (H630) was 5.5 microM. The 5-FU IC50 values for the resistant H630-R1, H630-R10, and H630-R cell lines were 11-, 29-, and 27-fold higher than that for the parent H630 cell line. Using both the radioenzymatic 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) binding and catalytic assays for measurement of thymidylate synthase (TS) enzyme activity, there was significantly increased TS activity in resistant H630-R1 (13- and 23-fold), H630-R10 (37- and 40-fold), and H630-R (24- and 34-fold) lines, for binding and catalytic assays, respectively, compared with the parent H630 line. The level of TS protein, as determined by western immunoblot analysis, was increased markedly in resistant H630-R1 (23-fold), H630-R10 (33-fold), and H630-R (26-fold) cells. Northern analysis revealed elevations in TS mRNA levels in H630-R1 (18-fold), H630-R10 (39-fold), and H630-R (36-fold) cells relative to parent H630 cells. Although no major rearrangements of the TS gene were noted by Southern analysis, there was significant amplification of the TS gene in 5-FU-resistant cells, which was confirmed by DNA slot blot analysis. These studies demonstrate that continuous exposure of human colon cancer cells to 5-FU leads to TS gene amplification and overexpression of TS protein with resultant development of fluoropyrimidine resistance.

5,10-Dideazatetrahydrofolic acid reduces toxicity and deoxyadenosine triphosphate pool, expansion in cultured L1210 cells treated with inhibitors of thymidylate synthase

5,10-Dideazatetrahydrofolic acid (DDATHF) reduces de nova purine biosynthesis by inhibiting glycinamide ribonucleotide transformylase. ICI D1964 and CB3717 are folate-based inhibitors of thymidylate synthase (TS). Fluorodeoxyuridine (FdUrd) following metabolism to FdUMP also inhibits TS. In cultured L1210 cells DDATHF reduced the toxicity of ICI D1694, CB3717 and FdUrd in a concentration-dependent manner. This protection correlated with a prevention of the increase in intracellular dATP pools seen in cells exposed to the TS inhibitors alone. The possibility that DDATHF protection might be due to competition for cell entry is not likely since CB3717 and FdURD but not ICI D1694 enter the cell by independent transport processes. Exogenous hypoxanthine (HX) had no effect on the toxicity of TS inhibitors. However, HX increased the protective effect of DDATHF from ICI D1694 toxicity, had no effect on the CB3717-DDATHF interaction, and reduced the protective effect of DDATHF on FdUrd toxicity. HX prevented the fall in dATP levels caused by DDATHF addition to cells treated with TS inhibitors. HX had different effects on dTTP levels in cells treated with DDATHF and quinazoline TS inhibitors compared to FdUrd. Together these results support the hypothesis that imbalance in dTTP and dATP pools is an important determinant of cytotoxicity in antifolate-treated cells. In addition, these findings suggests that intracellular reduced folates interconvert to catalyse reactions in metabolically perturbed cells.

A phase I-II study of cyclophosphamide, epidoxorubicin, levofolinic acid/5-fluorouracil and recombinant human granulocyte colony stimulating factor in metastatic breast carcinoma

Thirty patients with measurable metastatic breast carcinoma were treated with a combination of cyclophosphamide 600 mg/m2 on day 1, levofolinic acid 100 mg/m2 plus 5-fluorouracil 375 mg/m2 on days 1-3, and epidoxorubicin (EDXR) in three refracted doses on days 1-3 with G-CSF rescue for 10 days. In the phase I part of the study, groups of 3 patients received EDXR 20, 25, 30, 35, and 40 mg/m2/day until the dose limiting toxicity (DLT) was reached. At the dose of 40 mg/m2/day prolonged grade 4 leukopenia, severe proctitis, and grade 3 diarrhea represented the DLT. All subsequent patients were treated at the maximal tolerated dose of EDXR (35 mg/m2/day). In the group of 18 patients treated at 35 mg/m2/day the overall response rate was 78%, with 22% CR and 56% PR. Four patients did not respond. Objective responses were seen at all tumor sites including bone and viscera, which usually are rather chemotherapy insensitive. Toxicity was generally acceptable. Although the response rate was quite high, the duration of objective tumor regression and patients' survival were not impressive. In conclusion, we do not recommend routine use of such an aggressive regimen for palliation of advanced breast cancer. Results of the present and similar studies may, however, be useful for planning of neoadjuvant or adjuvant trials with curative intent.

Phase I study of 5-day continuous infusion fluorodeoxyuridine and high-dose folinic acid with oral hydroxyurea

Fluorodeoxyuridine (FUdR), the deoxynucleoside metabolite of 5-fluorouracil (5-FU), can be converted in a single step to fluorodeoxyuridine monophosphate (FdUMP), which binds covalently to thymidylate synthase (TS). Ribonucleotide reductase, an obligatory enzyme in the synthesis of deoxynucleotides, can be inhibited by hydroxyurea. Recognizing the well-established synergism between 5-FU and folinic acid (leucovorin), we hypothesized that the simultaneous administration of FUdR, leucovorin, and hydroxyurea might afford more effective inhibition of TS. Thirty-six patients with neoplastic disease considered refractory to standard therapy were entered into this phase I protocol. Treatment was administered on days 1 through 5 of a 28-day cycle and consisted of folinic acid (500 mg m-2 day-1) and FUdR at escalating doses of 0.1, 0.15, or 0.2 mg kg-1 day-1 both administered by continuous i.v. infusion, and hydroxyurea given p.o. once per day at doses ranging from 0 to 250o mg in 500-mg increments. The hydroxyurea and FUdR levels were escalated in a sequential fashion. The majority of patients had refractory breast or lung cancer. Dose-limiting toxicities were mucositis and diarrhea at the maximally tolerated dose of 0.15 mg/kg FUdR and 2000 mg hydroxyurea per day in conjunction with high-dose folinic acid. Hematological toxicity was minor. Of the 18 patients in whom response could be evaluated, none had evidence of objective disease regression. Mucositis and diarrhea are the dose-limiting toxicities when continuous infusions of FUdR and high-dose folinic acid are combined with oral hydroxyurea, effects that are consistent with the observed toxicities for FUdR when administered alone or in combination with leucovorin.

Quantitation of thymidylate synthase in human tumors using an ultrasensitive enzyme-linked immunoassay

Thymidylate synthase (TS; EC 2.1.1.45) is an important therapeutic target for fluoropyrimidine cytotoxic drugs that are widely used for the treatment of solid tumors. Using the monoclonal antibody TS 106, we have developed an ultrasensitive enzyme-linked immunoassay (ELISA) for the detection and quantitation of TS. Using a chemiluminescent ELISA technique, TS was detectable in serially diluted lysates from NCI H630 and HCT 116 human colon carcinoma cell lines. The ELISA assay was reliably able to detect activity down to a level of 30 attamol of TS protein above background (P2 = 0.016). The usable range of detection was from 0.03 to 500 fmol of enzyme. There was a close correlation between the optical density signal and the total TS enzyme between both cell lines (r2 = 0.96). The ELISA was used to measure TS in cytosolic extracts from human tumor samples, and it was able to quantitate TS levels using as little as 1-mg tumor biopsy samples. The mean total TS measured by ELISA in seven tumor samples from patients with breast cancer and sarcomas was 131 fmol/mg cytosolic protein (range 60-240) compared with a mean TS of 85 fmol/mg cytosolic protein (range 35-163) using the fluorodeoxyuridine monophosphate binding assay. While the TS levels were uniformly higher when measured by ELISA, there was close proportional agreement between both assays (r2 = 0.84). Thus, the chemiluminescent TS ELISA would appear to be an extremely sensitive and specific assay that may be used to quantitate TS in tumor tissue specimens.

Interaction of pyridoxal phosphate with thymidylate synthase: spectral and equilibrium dialysis studies

1. Changes in the spectrum of pyridoxal phosphate (PLP) were produced by adding an equimolar amount of native thymidylate synthase, but not by adding denatured enzyme or enzyme modified by sulfhydryl-blocking reagents. 2. The dissociation constant of the thymidylate synthase-PLP complex determined by equilibrium dialysis was 9 +/- 1.6 microM, the maximum number of PLP molecules bound per molecule of native thymidylate synthase was 2.5 +/- 0.4, and the Hill coefficient was 0.97. 3. No evidence of PLP binding was found with denatured thymidylate synthase, and only slight binding was observed when enzyme SH groups were blocked or when the active site was blocked with 5-fluorodeoxyuridylate (FdUMP) and methylene tetrahydrofolate. 4. The presence of dUMP, dTMP, or FdUMP interfered with the binding of PLP to thymidylate synthase, and the presence of equimolar amounts of PLP interfered with the binding of dUMP.

Dependence of the thymidylate synthase inhibition rate on the interval after the last administration of tegafur in sigmoid colon cancer patients

The thymidylate synthase (TS) inhibition rate was measured after tegafur (FT) administration (1.5 g/day, at least 10 days) in 7 sigmoid colon cancer patients. The TS inhibition rate decreased as the interval between the time of the last administration and the time of the tumor resection increased longer. This study provides basic data for considering methods of drug administration and assessment of modification, for example, by leucovorin.

Ternary complex formation and reduced folate in surgical specimens of human adenocarcinoma tissues

BACKGROUND AND METHODS

Various factors, including thymidylate synthase, thymidine kinase, 5-fluorouracil phosphorylation and degradation pathways, folate concentrations, and the stability of ternary complex, which influence thymidylate synthase inhibition rate of fluoropyrimidines, were studied in 87 human adenocarcinoma tissues.

RESULTS

The activity of the 5-fluorouracil degradation pathway was not significantly lower than the activity of the 5-fluorouracil phosphorylation pathway. The activity of the catabolism pathway of 5-fluorouracil should be considered in human adenocarcinoma tissue during chemotherapy. On the other hand, the means plus or minus standard deviations (means +/- SD) of the concentration of 5,10-methylenetetrahydrofolate and tetrahydrofolate were 0.69 +/- 0.54 and 1.25 +/- 0.69 nM, respectively, for the adenocarcinoma tissues without previous chemotherapy.

CONCLUSIONS

Because the half-life of tritium-labeled ternary complex and folate concentration in cytosol were correlated well, the differences in folate concentration among tumors must influence the dynamic equilibrium of ternary complex formation. Moreover, these results show that the ratio of 5,10-methylenetetrahydrofolate concentration to thymidylate synthase concentration influences the thymidylate synthase inhibition rate in tumor, and that the new synthesis of 5,10-methylenetetrahydrofolate and tetrahydrofolate from other endogenous reduced folates is also important in tumors with high thymidylate synthase concentrations.

Pharmacokinetics and in vitro studies of l-leucovorin. Comparison with the d and d,l-leucovorin

BACKGROUND

The active isomer of leucovorin (LV), LV-6S was compared with the racemic form, LV-6R,S and the inactive form, LV-6R.

MATERIALS AND METHODS

Pharmacokinetic studies of LV-6S and 6R,S were performed on normal volunteers and patients. Growth of Pediococcus Cerevisiae (PC), a LV-dependent strain, was measured with the 3 forms of LV. CCRF-CEM, a leukemic human cell line, was used to compare the effect of LV-6S, 6R,S and 6R on the rescue of methotrexate (MTX) and on the cytotoxicity of 5-fluorouracil (5-FU).

RESULTS

LV-6S exhibits pharmacokinetic patterns similar to those obtained with LV-6R,S whatever the route used, oral or intravenous. The growth of PC was similar with the active isomer and the racemic form while the unnatural isomer, LV-6R did not promote any growth. Cells exposed to MTX (10(-7) to 10(-5) M) were rescued from MTX cytotoxicity with LV-6S and LV-6R,S at concentrations 100 and 200 fold higher, whereas LV-6R did not reverse toxic effects of MTX. An enhancement of the cytotoxicity induced by 5-FU (10(-4) M) was obtained after preexposure of cells to LV-6S or LV-6R,S while LV-6R did not exhibit any synergistic effect.

CONCLUSION

LV-6S has similar effects to LV-6R,S in vitro and in vivo but at half doses; its clinical use prevents the possible interference of the inactive isomer, especially in patients receiving high doses of LV.

5-Fluorouracil combined with the [6S]-stereoisomer of folinic acid in high doses for treatment of patients with advanced colorectal carcinoma. A phase I-II study of two consecutive regimens

BACKGROUND

Potentiation of the antitumor activity of 5-fluorouracil (5-FU) by folinic acid has been demonstrated in patients with colorectal adenocarcinoma. Modulation is due to the interaction of thymidylate synthase (TS), fluorodeoxyuridylate (FdUMP), and methylene tetrahydrofolate (5,10-CH2-FH4), which leads to the formation of a stable ternary complex with concomitant enzyme inactivation. Folinic acid consists of a mixture of equal parts of two stereoisomers differing in chirality at the C6 carbon of the pteridine ring. Only the levorotatory [6S]-folinic acid is transformed into active folate cofactors. However, the [6R]-stereoisomer is not inert; it was shown to interfere with the [6S] form at the cellular level. The possibility of a deleterious effect of the unnatural stereoisomer on the modulation of 5-FU led us to carry out 2 consecutive phase I-II studies of 5-FU combined with the [6S]-stereoisomer of folinic acid given in high doses for treatment of patients with advanced colorectal carcinoma.

PATIENTS AND METHODS

Treatment comprised 5-FU by i.v. infusion for 2 hours (the initial dose was 350 mg/m2/d; it was incremented by 25 mg/m2/d until a maximal dose of 550 mg/m2/d) and [6S]-folinic acid (100 mg/m2/d by rapid i.v. injection in Regimen 1, and 100 mg/m2 by rapid i.v. injection followed by a 2-hour infusion of 250 mg/m2 in Regimen 2) for 5 days, every 21 days. Twenty-five pts and 27 pts were assessed in Regimen 1 and in Regimen 2, respectively. They had had no prior chemotherapy. The median follow-up time was 9 months and 15.5 months for patients treated with Regimen 1 and Regimen 2, respectively. For pts treated with Regimen 1, the response rate was 52% (CR, 12%; PR, 40%). The median time to disease progression was 9.2 months. The probability of survival at 12 months was 73%. For pts treated with Regimen 2, the response rate was 37% (CR, 7%; PR, 30%). The median time to disease progression was 8.9 months. The probability of survival at 12 months was 67%. Improvement in quality of life was achieved in most patients who had symptoms due to the tumor before the start of treatment. The dose-limiting toxic effects (WHO grades > or = 3) were diarrhea, dermatitis, and mucositis. One single episode of grade 4 diarrhea occurred. After injection according to the schema in Regimen 1, [6S]-folinic acid was rapidly cleared from plasma (mean t 1/2 alpha and t 1/2 beta of 7.2 and 126 minutes, respectively). The mean concentration of the [6S]-stereoisomer two hours after injection was 5.8 mmol/L. After a rapid i.v. injection of 100 mg/m2 followed by a 2-hour infusion of 250 mg/m2, the mean concentration of [6S]-folinic acid two hours after the injection was 57.5 mmol/L. Pharmacokinetic data suggests saturation of the metabolic conversion of [6S]-folinic acid when large doses are administered.

CONCLUSION

The [6S]-form of folinic acid potentiates the antitumor effect of 5-FU given concomitantly. However, increase of the daily dose of the folate did not result in a therapeutic improvement. The present results justify a more complete exploration of the pure active stereoisomer as a modulator of the fluoropyrimidines.

The history of the development and clinical use of CB 3717 and ICI D1694

The antifolate thymidylate synthase inhibitors represent an exciting area in new drug development and show that with an understanding of the structural basis for toxicity, new drugs can be synthesised which have a more manageable spectrum of side effects whilst retaining activity. ICI D1694 does not show the nephrotoxicity which affected the development of CB 3717. Myelosuppression and gut toxicity are seen and are more typical of the toxicities one associates with this class of agent. Changes in hepatic enzymes have been seen with both drugs, and are also seen with other anti-folates including MTX, but these changes settle with repeat dosing and with cessation of treatment. We await the results of the planned phase II trials of ICI D1694 with great interest.

Genetic variation in thymidylate synthase confers resistance to 5-fluorodeoxyuridine

The human colorectal tumor cell line HCT 116 was resident to the cytotoxic effects of 5-fluorodeoxyuridine (FdUrd). The response to FdUrd was increased only slightly by the presence of 10 microM folinic acid (CF). HCT 116 formed FdUMP and CH2H4PteGlu polyglutamates after exposure to FdUrd and CF. The sensitivity to FdUrd correlated well with the extent of TS inhibition. The role of TS in the resistance of the cells to FdUrd was examined. HCT 116 expresses two TS enzymes, which differ in pI. The more basic TS has been detected in only HCT 116 cells. The other TS is identical in pI to the enzymes detected in other human cells. The variant TS differs from the common by His replacement of Tyr at residue 33. The variant TS exhibited a 3-fold lower affinity for FdUMP than the common TS. The enzymes co-expressed in HCT 116 exhibited an FdUMP binding constant similar to that of the variant TS. TS-deficient cells were transfected with cDNAs encoding the two TS polypeptides. Transfectants expressing the variant TS were more resistant to FdUrd cytotoxicity than cells expressing the common TS. Thus, the structural variation in TS reduced enzyme affinity for FdUMP and conferred resistance to FdUrd.

Weekly 5-fluorouracil and folinic acid plus escalating doses of cisplatin with glutathione protection in patients with advanced head and neck cancer

Twenty-two patients with advanced head and neck carcinoma were treated with 5FU 400 mg-2 m-1 week and folinic acid 500 mg m-2 week-1 plus CDDP in escalating doses from 20 to 40 mg m-2 week-1 without forced diuresis. Reduced glutathione at the dose of 1.5 g m-2 was employed to protect patients from CDDP-related nephrotoxicity. The aims of the study were: a) to evaluate the therapeutic efficacy of this schedule, and b) to evaluate reduced glutathione as uroprotector. Out of 20 evaluable patients 14 (70%) had a major objective response. A CR with a mean duration of 9.0+ months was achieved in 15% of the patients, a PR of 5.8+ months in 55% of the patients, while 3 patients had stable disease and 4 progressed. It was possible to escalate CDDP up to 35 mg m-2 week-1, but at the dose of CDDP 40 mg m-2 week-1 the occurrence of grade 2 renal toxicity provoked a severe reduction of dose-intensity. Overall, this treatment has been very well tolerated by most patients with few cases of grade 3 gastrointestinal or hematological toxicity. In conclusion, the schedule seems effective and may be safely given to patients with advanced head and neck cancer on outpatient basis. Reduced glutathione seems to be able to reduce, at least partially, CDDP-related nephrotoxicity permitting the delivery of higher CDDP doses.