A real or apparent decrease in glomerular filtration rate in patients using olaparib?

PURPOSE: Olaparib is a poly (ADP-ribose) polymerase (PARP) inhibitor indicated for ovarian and metastatic breast cancer. Increased serum creatinine levels have been observed in patients taking olaparib, but the underlying mechanism is unknown. This study aimed to investigate if patients receiving olaparib have increased creatinine levels during olaparib treatment and whether this actually relates to a declined glomerular filtration rate (GFR). METHODS: We retrospectively identified patients using olaparib at the Netherlands Cancer Institute - Antoni van Leeuwenhoek (NKI-AVL) from 2012 until 2020. Patients with at least one plasma or serum sample available at baseline/off treatment and during olaparib treatment were included. Cystatin C levels were measured, creatinine levels were available and renal function was determined by calculating the estimated glomerular filtration rate (eGFR) using the Creatinine Equation (CKD-EPI 2009) and the Cystatin C Equation (CKD-EPI 2012). RESULTS: In total, 66 patients were included. Olaparib treatment was associated with a 14% increase in median creatinine from 72 (inter quartile range (IQR): 22) µmol/L before/off treatment to 82 (IQR: 20) µmol/L during treatment (p < 0.001) and a 13% decrease in median creatinine-derived eGFR from 86 (IQR: 26) mL/min/1.73 m2 before/off treatment to 75 (IQR: 29) mL/min/1.73 m2 during treatment (p < 0.001). Olaparib treatment had no significant effect on median cystatin C levels (p = 0.520) and the median cystatin C-derived eGFR (p = 0.918). CONCLUSIONS: This study demonstrates that olaparib likely causes inhibition of renal transporters leading to a reversible and dose-dependent increase in creatinine and does not affect GFR, since the median cystatin C-derived eGFR was comparable before/off treatment and during treatment of olaparib. Using the creatinine-derived eGFR can give an underestimation of GFR in patients taking olaparib. Therefore, an alternative renal marker such as cystatin C should be used to accurately calculate eGFR in patients taking olaparib.

Study protocols of three parallel phase 1 trials combining radical radiotherapy with the PARP inhibitor olaparib.

BACKGROUND: Poly (ADP-ribose) Polymerase (PARP) inhibitors are promising novel radiosensitisers. Pre-clinical models have demonstrated potent and tumour-specific radiosensitisation by PARP inhibitors. Olaparib is a PARP inhibitor with a favourable safety profile in comparison to clinically used radiosensitisers including cisplatin when used as single agent. However, data on safety, tolerability and efficacy of olaparib in combination with radiotherapy are limited. METHODS: Olaparib is dose escalated in combination with radical (chemo-)radiotherapy regimens for non-small cell lung cancer (NSCLC), breast cancer and head and neck squamous cell carcinoma (HNSCC) in three parallel single institution phase 1 trials. All trials investigate a combination treatment of olaparib and radiotherapy, the NSCLC trial also investigates a triple combination of olaparib, radiotherapy and concurrent low dose cisplatin. The primary objective is to identify the maximum tolerated dose of olaparib in these combination treatments, defined as the dose closest to but not exceeding a 15% probability of dose limiting toxicity. Each trial has a separate dose limiting toxicity definition, taking into account incidence, duration and severity of expected toxicities without olaparib. Dose escalation is performed using a time-to-event continual reassessment method (TITE-CRM). TITE-CRM enables the incorporation of late onset toxicity until one year after treatment in the dose limiting toxicity definition while maintaining an acceptable trial duration. Olaparib treatment starts two days before radiotherapy and continues during weekends until two days after radiotherapy. Olaparib will also be given two weeks and one week before radiotherapy in the breast cancer trial and HNSCC trial respectively to allow for translational research. Toxicity is scored using common terminology criteria for adverse events (CTCAE) version 4.03. Blood samples, and tumour biopsies in the breast cancer trial, are collected for pharmacokinetic and pharmacodynamic analyses. DISCUSSION: We designed three parallel phase 1 trials to assess the safety and tolerability of the PARP inhibitor olaparib in combination with radical (chemo-)radiotherapy treatment regimens. PARP inhibitors have the potential to improve outcomes in patients treated with radical (chemo-)radiotherapy, by achieving higher locoregional control rates and/or less treatment associated toxicity. TRIAL REGISTRATION: ClinicalTrials.gov Identifiers: NCT01562210 (registered March 23, 2012), NCT02227082 (retrospectively registered August 27, 2014), NCT02229656 (registered September 1, 2014).

Ten-year results of the PORTEC-2 trial for high-intermediate risk endometrial carcinoma: improving patient selection for adjuvant therapy.

BACKGROUND: PORTEC-2 was a randomised trial for women with high-intermediate risk (HIR) endometrial cancer, comparing pelvic external beam radiotherapy (EBRT) with vaginal brachytherapy (VBT). We evaluated long-term outcomes combined with the results of pathology review and molecular analysis. METHODS: 427 women with HIR endometrial cancer were randomised between 2002-2006 to VBT or EBRT. Primary endpoint was vaginal recurrence (VR). Pathology review was done in 97.4%, combined with molecular analysis. RESULTS: Median follow-up was 116 months; 10-year VR was 3.4% versus 2.4% for VBT vs. EBRT (p = 0.55). Ten-year pelvic recurrence (PR) was more frequent in the VBT group (6.3% vs. 0.9%, p = 0.004), mostly combined with distant metastases (DM). Ten-year isolated PR was 2.5% vs. 0.5%, p = 0.10, and DM 10.4 vs. 8.9% (p = 0.45). Overall survival for VBT vs. EBRT was 69.5% vs. 67.6% at 10 years (p = 0.72). L1CAM and p53-mutant expression and substantial lymph-vascular space invasion were risk factors for PR and DM. EBRT reduced PR in cases with these risk factors. CONCLUSION: Long-term results of the PORTEC-2 trial confirm VBT as standard adjuvant treatment for HIR endometrial cancer. Molecular risk assessment has the potential to guide adjuvant therapy. EBRT provided better pelvic control in patients with unfavourable risk factors.

Evaluation of clinical and endoscopic toxicity after external beam radiotherapy and endorectal brachytherapy in elderly patients with rectal cancer treated in the HERBERT study.

INTRODUCTION: The HERBERT study evaluated a high-dose-rate endorectal brachytherapy boost (HDREBT) after EBRT in medically inoperable/elderly patients with rectal cancer. The response-rates are promising but not without risk of toxicity. The current analysis provides a comprehensive overview of patient reported, physician reported and endoscopically observed toxicity. MATERIAL AND METHODS: A brachytherapy dose finding study was performed in 38 inoperable/elderly patients with T2-T4N0-1 rectal cancer. Patients received EBRT (13 × 3 Gy) followed by three weekly HDREBT applications (5-8 Gy). Toxicity was assessed via three methods: patient and physician (CTCAEv3) reported rectal symptoms and endoscopically. Wilcoxon's signed rank test, paired t-test and Spearman's correlation were used. RESULTS: Patient reported bowel symptoms showed a marked increase at the end of EBRT and two weeks after HDREBT. Acute grade 2 and 3 proctitis occurred in 68.4% and 13.2% respectively while late grade 2 and ≥3 proctitis occurred in 48% and 40%. Endoscopic evaluation mainly showed erythema and telangiectasia. In three patients frank haemorrhage or ulceration occurred. Most severe toxicity was observed 12-18 months after treatment. CONCLUSION: For elderly patients with rectal cancer, definitive radiotherapy can provide good tumour response but has a substantial risk of toxicity. The potential benefit and risks of a HDREBT boost above EBRT alone must be further evaluated.

Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism.

Thymidylate synthase (TS) is a key enzyme in the de novo synthesis of 2'-deoxythymidine-5'-monophosphate (dTMP) from 2'-deoxyuridine-5'-monophosphate (dUMP), for which 5,10-methylene-tetrahydrofolate (CH(2)-THF) is the methyl donor. TS is an important target for chemotherapy; it is inhibited by folate and nucleotide analogs, such as by 5-fluoro-dUMP (FdUMP), the active metabolite of 5-fluorouracil (5FU). FdUMP forms a relatively stable ternary complex with TS and CH(2)THF, which is further stabilized by leucovorin (LV). 5FU treatment can induce TS expression, which might bypass dTMP depletion. An improved efficacy of 5FU might be achieved by increasing and prolonging TS inhibition, a prevention of dissociation of the ternary complex, and prevention of TS induction. In a panel of 17 colon cancer cells, including several variants with acquired resistance to 5FU, sensitivity was related to TS levels, but exclusion of the resistant variants abolished this relation. For antifolates, polyglutamylation was more important than the intrinsic TS level. Cells with low p53 levels were more sensitive to 5FU and the antifolate raltitrexed (RTX) than cells with high, mutated p53. Free TS protein down-regulates its own translation, but its transcription is regulated by E2F, a cell cycle checkpoint regulator. Together, this results in low TS levels in stationary phase cells. Although cells with a low TS might theoretically be more sensitive to 5FU, the low proliferation rate prevents induction of DNA damage and 5FU toxicity. TS levels were not related to polymorphisms of the TS promoter. Treatment with 5FU or RTX rapidly induced TS levels two- to five-fold. In animal models, 5FU treatment resulted in TS inhibition followed by a two- to three-fold TS induction. Both LV and a high dose of 5FU not only enhanced TS inhibition, but also prevented TS induction and increased the antitumor effect. In patients, TS levels as determined by enzyme activity assays, immunohistochemistry and mRNA expression, were related to a response to 5FU. 5FU treatment initially decreased TS levels, but this was followed by an induction, as seen with an increased ratio of TS protein over TS-mRNA. The clear retrospective relation between TS levels and response now forms the basis for a prospective study, in which TS levels are measured before treatment in order to determine the treatment protocol.

Downstream molecular determinants of response to 5-fluorouracil and antifolate thymidylate synthase inhibitors.

Thymidylate synthase (TS) is an essential enzyme for the de novo synthesis of thymidylate and subsequently DNA synthesis. TS has been used as a target for cancer chemotherapy in the development of fluoropyrimidines such as 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine and of novel folate-based TS inhibitors such as ZD1694 (Tomudex, Raltitrexed), ZD9331, LY231514 (ALIMTA, Pemetrexed), AG337 (Thymitaq, Nolatrexed) and AG331. Although TS has been considered as a target for chemotherapy, the precise mechanism by which TS inhibition leads to cell death is still not completely resolved. TS inhibition results in depletion of dTTP, an essential precursor for DNA, and an increase in dUTP. This results in the so-called thymine-less death due to misincorporation of dUTP into DNA; its excision, catalysed by uracil-DNA glycosylase, results in DNA damage. Both this imbalance in dTTP/dUTP and DNA damage can result in induction of downstream events, leading to apoptosis. On the other hand a specific interaction exists between oncogenes and TS, by binding of TS protein to the p53 and c-myc RNA, while wt p53 can also inhibit TS promotor activity. TS inhibition by either 5-FU or antifolates can also result in a depression of TS protein mediated inhibition of TS mRNA translation leading to induction of more TS protein synthesis, and p53 protein may further deregulate this process. These complex indirect and direct interactions between oncogenes and TS may have as yet unclear clinical implications, since most data are based on in vitro or in vivo studies and some results are contradictive. In some preliminary clinical studies evidence was postulated for a combined prognostic role for TS and p53. This knowledge should be used to design clinical studies with the aim to deliver effective treatment to potentially sensitive patients both in the adjuvant setting and in advanced stage disease.

Thymidylate synthase expression in patients with colorectal carcinoma using a polyclonal thymidylate synthase antibody in comparison to the TS 106 monoclonal antibody.

Colorectal cancer is one of the most common human cancers, for which 5-fluorouracil (5FU) is usually part of the treatment. Thymidylate synthase (TS), the target enzyme for 5FU, can be predictive for the outcome of 5FU-based therapy. TS levels in tumor samples can be determined with radiochemical enzyme assays, RT-PCR, and immunohistochemical staining. We validated TS immunohistochemistry with a polyclonal rabbit anti-human TS antibody using the avidin-biotin method. This antibody can be used on paraffin-embedded, formalin-fixed material using an antigen retrieval method with citrate buffer and microwave treatment. The antibody shows a granular cytosolic staining pattern. The reproducibility in cross-sections from colorectal tumors from 50 patients was 90% and the interobserver variability was acceptable with a kappa of 0.45. On Western blotting it detects purified TS at 36 kD, while in 5FU-treated cells the ternary complex between FdUMP, TS, and 5, 10-methylene-tetrahydrofolate is clearly visible at 38 kD, with no other interfering bands. In a separate set of tumors, immunostaining was compared with enzyme levels; Western blots correlated with enzyme levels. Because both this polyclonal antibody and the monoclonal antibody TS-106 are being used for large-scale studies, we also determined whether they could be used interchangeably. No differences were observed. This polyclonal antibody is specific and gives reproducible results. A study on a larger scale is ongoing to determine the role of TS as a predictive parameter in patients with colorectal cancer treated either with postoperative adjuvant 5FU/levamisole or with surgery only.

Molecular downstream events and induction of thymidylate synthase in mutant and wild-type p53 colon cancer cell lines after treatment with 5-fluorouracil and the thymidylate synthase inhibitor raltitrexed.

Inhibition of the key enzyme in DNA synthesis, thymidylate synthase (TS), by 5-fluorouracil (5-FU) and the novel antifolate raltitrexed (Tomudex; ZD1694), induces dTTP depletion, resulting in DNA strand breaks, which can initiate pathways leading to an apoptotic mode of cell death. We studied 5-FU- and ZD1694-induced TS inhibition in relation to the expression of p53, p21, Bcl-2 and Bax in six colon carcinoma cell lines, two with a wild-type (wt) p53 (Lovo, LS174T) and four with a mutant (mt) p53 (WiDr, WiDr/F, HT29 and SW948) phenotype. In untreated cells, a reciprocal correlation between p53 and Bcl-2 was found: in cells with a low wt p53, Bcl-2 expression was present; whilst in cells with mt p53, Bcl-2 expression was not detectable. Exposure to 5-FU (50 and 100 microM) and ZD1694 (50 and 100 nM) for 24 and 48 h induced p53 and p21 expression in wt p53 cells, but not in mt p53 cells. TS was induced approximately 2-10-fold in all cell lines. TS induction was highest after ZD1694 exposure in the mt p53 cells HT29 and WiDr/F (6-10-fold). After 5-FU treatment, TS was present both as the free enzyme and in the ternary complex; however, predominantly as the ternary complex between TS, FdUMP and 5,10-methylenetetrahydrofolate. In wt p53 cells, both drugs increased Bax expression up to 5-fold, whereas in mt p53 cells, only a very slight induction was found. In wt p53 cells, Bcl-2 expression hardly changed after drug treatment. These results indicate a p53-independent induction of TS but a regulatory role of wt p53 in the synthesis of Bax in the colon carcinoma cell lines after TS inhibition.

Prognostic role of thymidylate synthase, thymidine phosphorylase/platelet-derived endothelial cell growth factor, and proliferation markers in colorectal cancer.

5-Fluorouracil (5FU)-based therapy is given to patients with advanced colorectal cancer and as adjuvant treatment. Thymidylate synthase (TS) is the target for 5FU, and may have a prognostic role for the outcome of 5FU-based therapy together with proliferation markers such as p53 and Ki67. Thymidine phosphorylase (TP, also known as platelet-derived endothelial cell growth factor) may be of importance both in the 5FU drug activation pathway and in tumor angiogenesis, similar to vascular endothelial growth factor (VEGF). TS and TP levels were determined biochemically in fresh-frozen tumor specimens of 32 untreated patients with colorectal cancer, whereas in paraffin-embedded tissue samples, immunohistochemistry was performed for TS, TP, and additional prognostic markers such as p53, Ki67, and VEGF as well as microvessel density. All factors were correlated with patient characteristics such as age, gender, Dukes' stage, angio-invasion, and differentiation grade. TS and TP as measured by various assays were correlated with overall and disease-free survival in this patient group. TP enzyme activity and protein expression correlated with each other. A significant correlation was found between TP enzyme activity and 5-fluoro-2'-deoxyuridine-5'-monophosphate binding activity. VEGF expression correlated significantly with TP immunostaining and Ki67 index. Survival analysis revealed a significant relation of TS levels to the overall survival in this small patient group and a significant correlation between TP activity and disease-free survival. TS and TP both were of prognostic significance in these patients with colorectal cancer. The interesting relationship of TS and TP with angiogenesis and proliferation needs further investigation.

Thymidylate synthase: a target for combination therapy and determinant of chemotherapeutic response in colorectal cancer.

In this review, we describe the role of thymidylate synthase (TS) as a determinant of chemotherapeutic response. TS is an essential enzyme for the de novo synthesis of thymidylate and subsequent DNA synthesis. TS has been used as a target for cancer chemotherapy in the development of fluoropyrimidines such as 5-fluorouracil (5FU) and 5-fluorodeoxyuridine and novel folate-based TS inhibitors such as ZD1694 (Tomudex, Raltitrexed), LY231514, AG337 (Thymitaq) and GW1843U89. All these antifolates have different transport mechanisms and various polyglutamylation and accumulation patterns which are all important for drug sensitivity. An update of clinical studies with TS inhibitors (antifolate-based TS inhibitors and 5FU combination studies) will be given and new perspectives for clinical development will be discussed. The prognostic or predictive value of TS (in vitro, in vivo and in patients) will be reviewed, focusing on colorectal cancer.

Thymidylate synthase level as the main predictive parameter for sensitivity to 5-fluorouracil, but not for folate-based thymidylate synthase inhibitors, in 13 nonselected colon cancer cell lines.

Thymidylate synthase (TS), a critical enzyme in the de novo synthesis of thymidylate, is an important target for fluoropyrimidines and folate-based TS inhibitors. In a panel of 13 nonselected human colon cancer cell lines, we evaluated the role of TS levels in sensitivity to 5-fluorouracil (5FU) and four folate-based TS inhibitors that have been introduced recently into the clinic: ZD1694 (Tomudex, Raltitrexed, TDX), GW1843U89 (GW), LY231514 (LY), and AG337 (Thymitaq, AG). Because the latter compounds have different transport and polyglutamylation characteristics, we also related these parameters with drug sensitivity, measured by the sulforhodamine B assay after 72 h of drug exposure. For 5FU, the IC50s varied from 0.8 to 43.0 microM. Leucovorin (LV) potentiated the activity of 5FU in only 4 of 13 cell lines. Sensitivity to folate-based TS inhibitors was variable; IC50s were in the range of: 5.3-59.0 nM TDX; 11.0-1570 nM LY; and 0.5-8.9 nM GW. Eleven of 13 cell lines had an IC50 for AG between 1.3 and 5.3 microM. Two cell lines were resistant to AG, Colo201 and SW1116, with IC50s of 27 and 29 microM, respectively. TS catalytic activity (conversion of dUMP to dTMP) varied from 62 to 777 pmol/h/10(6) cells. The number of FdUMP binding sites varied from 32 to 231 fmol/10(6) cells. Regression analysis showed a significant relation between TS catalytic activity and IC50s for 5FU and 5FU/LV. Kis for FdUMP showed a significant Spearman rank correlation with the IC50s of AG and GW. The role of antifolate transport, accumulation, and polyglutamylation was determined with [3H]methotrexate (MTX) as a reference compound. [3H]MTX influx via the reduced folate carrier varied from 18.6 to 150 fmol/10(6) cells/min. Folylpolyglutamate synthetase (FPGS) activity showed a range from 47 to 429 pmol/10(6) cells/h. A total of 24 h of [3H]MTX accumulation showed a 20-fold variation, from 1.2 to 21.8 pmol/10(6) cells. FPGS levels showed a Spearman rank positive correlation with cytotoxicity to TDX. In conclusion, in a heterogeneous nonselected human colon cancer cell line panel, the best predictor for sensitivity to 5FU and 5FU/LV was TS activity. Multiple sensitivity determinants were of importance for antifolate TS inhibitors, including FPGS activity and TS enzyme kinetics.

Cross-resistance to antifolates in multidrug resistant cell lines with P-glycoprotein or multidrug resistance protein expression.

Resistance to some (lipophilic) antifolates has been associated with P-glycoprotein (P-gp)-mediated multidrug resistance (MDR). A possible relationship with non-P-gp MDR has not been established. We studied resistance to antifolates in SW-1573 human lung carcinoma cells, a P-gp overexpressing variant SW-1573/2R160 and a multidrug resistance protein (MRP) overexpressing variant SW-1573/2R120. In this study, thymidylate synthase (TS) inhibitors with different properties concerning the efficiency of membrane transport and the efficiency of polyglutamylation were tested for cross-resistance in SW-1573/2R120 and SW-1573/2R160 cells. Growth inhibition patterns in this cell line panel were measured by the Sulforhodamine B (SRB) assay. Resistance factors for TS inhibitors were: 2.4 and 0.4 for 5-fluorouracil (5FU), 18.8 and 8.8 for ZD1694, 17 and 0.7 for AG337, and 40 and 8.3 for BW1843U89 in SW-1573/2R160 and SW-1573/2R120, respectively. This study showed changes in the TS enzyme kinetics during the induction of doxorubicin resistance in both SW-1573 variants, resulting in 2-fold lower Km values for 2'-deoxyuridine-5'-monophosphate (dUMP) in both resistant variants compared to the parental cell line. TS activity, TS protein induction and TS mRNA expression all had 2-fold increased in the SW-1573/2R120 compared to the SW-1573/2R160. 3H-MTX influx was 2-fold lower in SW-1573/2R160 cells compared to SW-1573/2R120 and SW-1573 cells. In the SW-1573/2R160 cell line, an aberrant intracellular trafficking towards the target TS was observed, compared to SW-1573/2R120 and SW-1573 cells as measured by the TS in situ assay. The rate of TS inhibition by the TS inhibitors used in this study was similar in all cell lines. In conclusion, collateral sensitivity to 5FU and the lipophilic AG337 and cross-resistance to other antifolates were observed in non-P-gp MDR SW-1573/2R120 cells, as well as resistance to all antifolates in P-gp SW-1573/2R160 cells. The mechanism of resistance in SW-1573/2R160 cells possibly involves reduced influx and changes in intracellular trafficking routes. For the SW-1573/2R120 cell line, several changes related to the TS enzyme possibly play a role in the observed cross-resistance and collateral sensitivity pattern.

Current chemotherapeutic possibilities in the treatment of colorectal cancer.

To date, the best treatment modality for colorectal cancer is a surgical excision of the primary tumour. Adjuvant therapy can be added to the surgical treatment and can consist of adjuvant chemo-, immuno- or radiotherapy. In the U.S.A., adjuvant chemotherapy with 5-fluorouracil (5FU) and levamisole is advocated as standard treatment for patients with localised poor risk (Dukes stage C) colon cancer. Not every clinician is convinced of the usefulness of adjuvant chemotherapy. Therefore, confirmatory clinical trials are still ongoing to compare no adjuvant treatment with 5FU/levamisole adjuvant treatment. Treatment with 5FU/leucovorin has been shown to be effective as adjuvant therapy. In rectal cancer, radiotherapy can be added to the primary surgical treatment. It is still unproven whether radiotherapy should be given pre-, peri, or postoperatively, and whether chemotherapy should be added to this multimodality regimen. If chemotherapy is applied as a radio-sensitiser, a continuous infusion is preferable to daily bolus injection. Much effort has been put into the improvement of the response rate of 10-15% 5FU, used as a single agent in the treatment of advanced colorectal cancer. Biochemical modulation of 5FU with leucovorin and interferon, different schedules of 5FU administration and hepatic arterial therapy have all been attempted. Higher response rates have been reported with these treatment modalities, unfortunately without improvement of survival, except for the intra-arterial approach. Recently, two new drugs have shown efficacy in the treatment of advanced colorectal cancer. A phase II trial with Tomudex (ZD1694), a new antifolate thymidiylate synthase inhibitor, produced a response rate of 25% in patients with advanced colorectal cancer.(ABSTRACT TRUNCATED AT 250 WORDS)

Thymidylate synthase and drug resistance.

Thymidylate synthase is an important target for both fluorinated pyrimidines and for new folate analogues. Resistance to 5-fluorouracil (5FU) can be related to insufficient inhibition of thymidylate synthase. The 5FU-nucleotide FdUMP induces inhibition of thymidylate synthase which is enhanced and retained for longer in the presence of increased folate pools, for which leucovorin is a precursor. In a murine model system, 5FU treatment caused a 4-fold induction of thymidylate synthase levels which may have contributed to resistance. Addition of leucovorin to this treatment prevented this induction and increased the antitumour effect 2-3-fold. In the clinical setting, 5FU administration to patients resulted in approximately 50% inhibition of TS after 48 h. The combination with leucovorin resulted in a more pronounced inhibition after 48 h (approximately 70%). A significant relationship was observed with outcome of treatment; when thymidylate synthase levels were high and inhibition was low, no response was observed. A separate study showed that low thymidylate synthase levels appeared to be an independent prognostic factor for adjuvant therapy.