Schedule-dependent interaction between raltitrexed and 5-fluorouracil in human colon cancer cell lines in vitro

Structural Bases for the Synergistic Inhibition of Human Thymidylate Synthase and Ovarian Cancer Cell Growth by Drug Combinations

Simple Summary

Drug combinations may help overcome drug resistance, a relevant cause of failure of ovarian cancer therapy. However, designing successful combinations requires a lengthy preclinical validation process. We have analyzed combinations of 5-fluorouracil and raltitrexed, two anticancer drugs that target thymidylate synthase, a key enzyme for the nucleotide synthesis. We have observed administration sequence specific and synergistic combined effects of the two drugs against cisplatin sensitive and resistant ovarian cancer cells. However, the focus of this work was to show that a high stability of the complex of the enzyme with the two drugs, as highlighted by X-ray crystallography, and synergistic inhibition of the enzyme represent indicators, if not prerequisites, for this drug combination to be synergistically active against sensitive and resistant ovarian cancer cells. We thus propose that structural and mechanistic information acquired during the preclinical research can help predict a successful therapeutic application of a drug combination.

Abstract

Combining drugs represent an approach to efficiently prevent and overcome drug resistance and to reduce toxicity; yet it is a highly challenging task, particularly if combinations of inhibitors of the same enzyme target are considered. To show that crystallographic and inhibition kinetic information can provide indicators of cancer cell growth inhibition by combinations of two anti-human thymidylate synthase (hTS) drugs, we obtained the X-ray crystal structure of the hTS:raltitrexed:5-fluorodeoxyuridine monophosphate (FdUMP) complex. Its analysis showed a ternary complex with both molecules strongly bound inside the enzyme catalytic cavity. The synergistic inhibition of hTS and its mechanistic rationale were consistent with the structural analysis. When administered in combination to A2780 and A2780/CP ovarian cancer cells, the two drugs inhibited ovarian cancer cell growth additively/synergistically. Together, these results support the idea that X-ray crystallography can provide structural indicators for designing combinations of hTS (or any other target)-directed drugs to accelerate preclinical research for therapeutic application.

Synergistic antitumour effect of raltitrexed and 5-fluorouracil plus folinic acid combination in human cancer cells

5-Fluorouracil, usually in combination with folinic acid, is widely used in the treatment of both colorectal and head and neck squamous cell cancer patients. Since 5-fluorouracil plus folinic acid and the antifolate thymidylate synthase inhibitor; raltitrexed have distinct mechanisms of action and toxicity profiles, we have evaluated the potential synergistic antitumor interaction between these two agents combined with a sequential schedule of administration in KB (wt-p53) and Cal27 (mut-p53) head and neck squamous cell carcinomas, and LoVo (wt-p53) and HT29 (mut-p53) colorectal cell lines. The combination between a 24-h exposure to raltitrexed followed by a 4-h exposure to 5-fluorouracil plus folinic acid was globally synergistic, as assessed by the median effect principle and combination index. A specific contribution of folinic acid to the cytotoxic effect of the raltitrexed/5-fluorouracil combination was clearly demonstrated by the evaluation of the potentiation factor. In all cell lines, a 1.5- up to 17-fold reduction in the IC50 of both raltitrexed and 5-fluorouracil plus folinic acid was observed in the combination setting compared with the concentrations of the each drug used alone. Moreover, we demonstrated that raltitrexed/5-fluorouracil plus folinic acid induced a distinct S-phase block of the cell cycle, as well as a potentiation of the apoptotic cell death, compared with 5-fluorouracil plus folinic acid or raltitrexed/5-fluorouracil combination. This preclinical work represents, at least to our knowledge, the first demonstration of a synergistic interaction between raltitrexed and 5-fluorouracil modulated by folinic acid, and could represent a rationale for further clinical investigation of raltitrexed/5-fluorouracil plus folinic acid combination.

Raltitrexed plus levofolinic acid and bolus/continuous infusion 5-fluorouracil on a biweekly schedule for elderly patients with advanced colorectal carcinomas

BACKGROUND

The aim of the study was to evaluate the safety and efficacy of the raltitrexed/5-fluorouracil/levofolinic acid combination regimen as first-line chemotherapy for elderly patients with advanced/metastatic colorectal cancer.

PATIENTS AND METHODS

Previously untreated patients with metastatic colorectal cancer received raltitrexed 2 mg/m(2) i.v. plus levofolinic acid and 5-fluorouracil according to the De Gramont' schedule given every 2 weeks as first-line chemotherapy. Patients were re-evaluated after six cycles and chemotherapy was continued up to tolerance or disease progression.

RESULTS

Seventy patients aged >/=65 years were accrued from 11 centers between September 2001 and July 2002. According to the intention-to-treat analysis, the overall response rate was 35% (95% CI 29.5% to 40.5%) including one complete response (1%) and 24 partial responses (34%). Twenty patients (31%) showed a stabilization of disease for a tumor growth control rate of 64% (95% CI 57% to 71%). The median overall survival was 12.5 months and the median time to disease progression was 6.5 months. No toxic deaths or allergic reaction were recorded. Grade 4 toxicities were non-existent. The main hematological toxicity was grade 3 neutropenia, which occurred in 9% of patients, and grade 3 anemia in only one case, while no case of graded 3 thrombocytopenia was observed. Grade 3 non-hematological toxicities were asthenia (11%), transient increase of transaminases (10%) and diarrhea (4%).

CONCLUSIONS

The results of this study suggest that the raltitrexed/5-fluorouracil/levolofinic acid combination is an effective and well tolerated regimen for the treatment of elderly patients with advanced colorectal cancer. Its ease of administration and patient's tolerance warrant further investigation over 5-fluorouracil/folinic acid regimens.

Effects of antifolates on the binding of 5-fluoro-2'-deoxyuridine monophosphate to thymidylate synthase

Folate based inhibitors of thymidylate synthase (TS) might facilitate binding of 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) to TS similar to the natural reduced folate 5,10-methylenetetrahydrofolate (CH(2)-H(4)-folate). We studied the lipophilic, non-polyglutamatable antifolates Nolatrexed (NTX) and AG331 and antifolates, that can have a polyglutamate side chain like the natural folate CH(2)-H(4)-folate; GW1843U89, Raltitrexed (RTX) and Multi-targetted antifolate (MTA) and pentaglutamates (RTX-Glu(5) and MTA-Glu(5)). The capacity of these compounds to facilitate the binding of [(3)H]FdUMP to Lactobacillus casei TS and an ammoniumsulphate precipitate of human TS was investigated. Only NTX, RTX-Glu(5) and MTA-Glu(5) facilitated FdUMP binding to L. casei TS and their dissociation constant K(d) (0.2-0.7 microM) was low compared to CH(2)-H(4)-folate (2.0 microM). The small lipophilic molecule NTX was favorable to the larger AG331. Polyglutamylation, as indicated by the difference in effect of RTX vs. RTX-Glu(5) and MTA vs. MTA-Glu(5), seems to be important for a classical antifolate to facilitate binding of FdUMP to bacterial TS. Effects of antifolates on FdUMP binding to human TS were different. At a low concentration (0.05 microM) NTX, RTX-Glu(5) and MTA-Glu(5) facilitated 3-5 times higher binding of [(3)H]FdUMP to TS than CH(2)-H(4)-folate. At higher concentrations (0.3-5 microM) of NTX, RTX-Glu(5) and MTA-Glu(5) the FdUMP binding decreased. The complex remained stable in the absence of (anti)folate for at least 24hr. The K(d) values of the antifolates for human TS varied from 19 to 387 nM, while the K(d) of CH(2)-H(4)-folate for human TS was 351 nM. The Hill coefficients, which indicated the type of cooperativity of the antifolates in the binding of FdUMP to TS were positive (0.58-0.99) at low concentrations (<0.3 microM) and negative (-0.35 to -0.81) at concentrations >0.3 microM except for GW1843U89, which only showed negative cooperativity (-1.70). It was shown with [(14)C]NTX that when the binding of FdUMP decreased at high NTX concentrations, the binding of NTX to TS still increased. This also held for the natural substrate dUMP. The negative cooperativity of the antifolates was clearly concentration dependent. The difference between human and L. casei TS in the FdUMP binding assays with antifolates can possibly be explained by interaction of the two subunits of human TS, which was absent in L. casei TS. The binding of antifolates to one of the two subunits induced a conformational change of the other subunit. This change no longer allowed the binding of FdUMP or dUMP at the active site. In conclusion this study showed that antifolates enhanced the binding of FdUMP to TS, especially at low antifolate concentrations, that are also clinically achievable, e.g. in human plasma.

Schedule-dependent synergism and antagonism between raltitrexed ("Tomudex") and methotrexate in human colon cancer cell lines in vitro

The folate-dependent enzymes are attractive targets for cancer chemotherapy. Methotrexate (MTX), which inhibits dihydrofolate reductase, has been widely used for the treatment of solid tumors and hematological cancers. Raltitrexed ("Tomudex") ), which inhibits thymidylate synthase, is a novel anticancer agent active against colorectal cancer and some other solid tumors. We studied the optimal schedule of raltitrexed and MTX in combination against four human colon cancer cell lines Colo201, Colo320, LoVo, and WiDr. These cells were simultaneously exposed to raltitrexed and MTX for 24 h, or sequentially exposed to raltitrexed for 24 h followed by MTX for 24 h, or vice versa. Cell growth inhibition after 5 days was determined by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The effects of drug combinations at the concentrations of drug that produced 80% and 50% cell growth inhibition (IC(80) and IC(50)) were analyzed by the isobologram method (Steel and Peckham, 1979). Cytotoxic interactions between raltitrexed and MTX were schedule-dependent. The simultaneous exposure to raltitrexed and MTX showed additive effects in Colo201, LoVo and WiDr cells and antagonistic effects in Colo320 cells. The sequential exposure to raltitrexed followed by MTX produced additive effects in all four cell lines. The sequential exposure to MTX followed by raltitrexed produced synergistic effects in Colo201, LoVo and WiDr cells and additive effects in Colo320 cells. These findings suggest that the sequential administration of MTX followed by raltitrexed produces more than the expected cytotoxicity and may be the optimal schedule at the cellular level. Further in vivo and clinical studies will be necessary to determine the toxicity and to test the antitumor effects of sequential administration of MTX followed by raltitrexed proposed on the basis of the in vitro synergism.