Gijsbert Veerman

2',2'-Difluoro-deoxycytidine (gemcitabine) incorporation into RNA and DNA of tumour cell lines

Gemcitabine (dFdC) is a new cytidine analogue which is active mainly by the incorporation of its triphosphate (dFdCTP) into DNA, leading to cell death. We determined incorporation of dFdC into nucleic acids of two solid tumour cell lines: the murine colon carcinoma cell line Colon 26-10, the human ovarian carcinoma cell line A2780, and the human leukemic cell line CCRF-CEM. dFdC was not only incorporated into DNA, but also into RNA. The extent of incorporation into DNA was highest in A2780 cells and lowest in CCRF-CEM cells (2-4-fold difference). The same pattern was observed for incorporation into RNA, but with a 10-20-fold difference. In A2780, incorporation into DNA was about twice that of the incorporation into RNA, in CEM cells 10-20-fold that of RNA. Incorporation into RNA was verified using two methods for separation of RNA and DNA, acid precipitation and CsCl-gradient centrifugation. Incorporation into DNA was time and concentration dependent, but incorporation into RNA seemed to be only concentration dependent. We also determined the effect of dFdC on DNA and RNA synthesis by measurement of thymidine and uridine incorporation, respectively, using similar conditions as for the incorporation studies. In all three cell lines DNA synthesis was inhibited almost completely, even at 0.1 microM dFdC and at 4-hr exposure. RNA synthesis inhibition did not exceed 50% in both solid tumour cell lines, even at 1 microM dFdC exposure for 24 hr. A clear concentration effect was only observed in the CCRF-CEM cell line and only after 24 hr exposure. At a 1 microM dFdC exposure for 24 hr, RNA synthesis was completely inhibited in these cells. Incorporation of dFdC into RNA and inhibition of RNA synthesis represent an unrecognized but possibly important mechanism of action of this drug.

Mechanisms of synergism between cisplatin and gemcitabine in ovarian and non-small-cell lung cancer cell lines

Summary2′,2′-Difluorodeoxycytidine (gemcitabine, dFdC) and cis-diammine-dichloroplatinum (cisplatin, CDDP) are active agents against ovarian cancer and non-small-cell lung cancer (NSCLC). CDDP acts by formation of platinum (Pt)–DNA adducts; dFdC by dFdCTP incorporation into DNA, subsequently leading to inhibition of exonuclease and DNA repair. Previously, synergism between both compounds was found in several human and murine cancer cell lines when cells were treated with these drugs in a constant ratio. In the present study we used different combinations of both drugs (one drug at its IC25 and the other in a concentration range) in the human ovarian cancer cell line A2780, its CDDP-resistant variant ADDP, its dFdC-resistant variant AG6000 and two NSCLC cell lines, H322 (human) and Lewis lung (LL) (murine). Cells were exposed for 4, 24 and 72 h with a total culture time of 96 h, and possible synergism was evaluated by median drug effect analysis by calculating a combination index (CI; CI < 1 indicates synergism). With CDDP at its IC25, the average CIs calculated at the IC50, IC75 IC90 and IC95 after 4, 24 and 72 h of exposure were < 1 for all cell lines, indicating synergism, except for the CI after 4 h exposure in the LL cell line which showed an additive effect. With dFdC at its IC25, the CIs for the combination with CDDP after 24 h were < 1 in all cell lines, except for the Cls after 4 h exposure in the LL and H322 cell lines which showed an additive effect. At 72 h exposure all Cls were < 1. CDDP did not significantly affect dFdCTP accumulation in all cell lines. CDDP increased dFdC incorporation into both DNA and RNA of the A2780 cell lines 33- and 79-fold (P < 0.01) respectively, and tended to increase the dFdC incorporation into RNA in all cell lines. In the AG6000 and LL cell lines, CDDP and dFdC induced > 25% more DNA strand breaks (DSB) than each drug alone; however, in the other cell lines no effect, or even a decrease in DSB, was observed. dFdC increased the cellular Pt accumulation after 24 h incubation only in the ADDP cell line. However, dFdC did enhance the Pt–DNA adduct formation in the A2780, AG6000, ADDP and LL cell lines (1.6-, 1.4-, 2.9- and 1.6-fold respectively). This increase in Pt–DNA adduct formation seems to be related to the incorporation of dFdC into DNA (r = 0.91). No increase in DNA platination was found in the H322 cell line. dFdC only increased Pt–DNA adduct retention in the A2780 and LL cell lines, but decreased the Pt–DNA adduct retention in the AG6000 cell line. In conclusion, the synergism between dFdC and CDDP appears to be mainly due to an increase in Pt–DNA adduct formation possibly related to changes in DNA due to dFdC incorporation into DNA.

In vivo Induction of Resistance to Gemcitabine Results in Increased Expression of Ribonucleotide Reductase Subunit M1 as the Major Determinant

Gemcitabine is a deoxycytidine (dCyd) analogue with activity against several solid cancers. Gemcitabine is activated by dCyd kinase (dCK) and interferes, as its triphosphate dFdCTP, with tumor growth through incorporation into DNA. Alternatively, the metabolite gemcitabine diphosphate (dFdCDP) can interfere with DNA synthesis and thus tumor growth through inhibition of ribonucleotide reductase. Gemcitabine can be inactivated by the enzyme dCyd deaminase (dCDA). In most in vitro models, resistance to gemcitabine was associated with a decreased dCK activity. In all these models, resistance was established using continuous exposure to gemcitabine with increasing concentrations; however, these in vitro models have limited clinical relevance. To develop in vivo resistance to gemcitabine, we treated mice bearing a moderately sensitive tumor Colon 26-A (T/C = 0.25) with a clinically relevant schedule (120 mg/kg every 3 days). By repeated transplant of the most resistant tumor and continuation of gemcitabine treatment for >1 year, the completely resistant tumor Colon 26-G (T/C = 0.96) was created. Initial studies focused on resistance mechanisms known from in vitro studies. In Colon 26-G, dCK activity was 1.7-fold decreased; dCDA and DNA polymerase were not changed; and Colon 26-G accumulated 1.5-fold less dFdCTP, 6 hours after a gemcitabine injection, than the parental tumor. Based on in vitro studies, these relative minor changes were considered insufficient to explain the completely resistant phenotype. Therefore, an expression microarray was done with Colon 26-A versus Colon 26-G. Using independently grown nonresistant and resistant tumors, a striking increase in expression of the RRM1 subunit gene was found in Colon 26-G. The expression of RRM1 mRNA was 25-fold increased in the resistant tumor, as measured by real-time PCR, which was confirmed by Western blotting. In contrast, RRM2 mRNA was 2-fold decreased. However, ribonucleotide reductase enzyme activity was only moderately increased in Colon 26-G. In conclusion, this is the first model with in vivo induced resistance to gemcitabine. In contrast to most in vitro studies, dCK activity was not the most important determinant of gemcitabine resistance. Expression microarray identified RRM1 as the gene with the highest increase in expression in the Colon 26-G, which might clarify its complete gemcitabine-resistant phenotype. This study is the first in vivo evidence for a key role for RRM1 in acquired gemcitabine resistance.

Antitumor activity of prolonged as compared with bolus administration of 2@,2@-difluorodeoxycytidine in vivo against murine colon tumors

Abstract 2′,2′-Difluorodeoxycytidine (gemcitabine) is a cytidine analogue with established antitumor activity against several experimental tumor types and against human ovarian and non-small-cell lung cancer. Both preclinical studies and most clinical trials involving patients with solid tumors have focused on short-term administration schedules; however, mechanistic studies indicate that a continuous-infusion schedule may be more effective. We determined the maximal tolerated dose (MTD) of gemcitabine in mice using various schedules. At these MTDs we observed considerably better antitumor activity of gemcitabine in two of three murine colon carcinoma lines using a prolonged administration as compared with a standard bolus protocol (i.p. 120 mg/kg q3d×4). On the latter schedule, Colon 26–10 grown in BALB/c mice was the most sensitive tumor line, showing a growth-delay factor (GDF, number of doubling times gained by the treatment) of 6.7, whereas Colon 38 (grown in C57/B16 mice) was the least sensitive tumor, displaying a GDF of 0.9. Prolonged treatment (q3d×6) of Colon 26–10 at a lower dose (100 mg/kg) enhanced the antitumor activity (GDF 9.6) while producing similar toxicity. A similar weight loss was found following the continuous infusion (c.i.) of gemcitabine using Alzet osmotic pumps s.c. for 3 or 7 days (2 mg/kg), but the GDF increased to 2.4 in Colon 38 (C57/B16) as compared with that provided by the bolus injections. Continuous infusion of gemcitabine at 15 mg/kg per 24 h q7d×2 i.v. via the tail vein was more effective than bolus injection against Colon 26–10, with the GDF being >17.7 and 73% of the tumors regressing completely. However, against Colon 38 tumors this schedule was not effective (GDF 0.4), even with a 25% higher dose. The plasma pharmacokinetics of gemcitabine was determined after one bolus dose (120 mg/kg). The peak concentration of gemcitabine was 225 μM and that of the deaminated catabolite 2′,2′-difluorodeoxyuridine (dFdU) was 79 μM. The elimination of gemcitabine was much faster than that of dFdU, with the t1/2ß values being 15 min and 8 h, respectively. For the c.i. schedules, plasma concentrations were below the detection limit of the assay (<0.5 μM). Our results suggest that prolonged infusion of gemcitabine can give a better antitumor activity than bolus injections and shows promise of being active in clinical trials.

Increased sensitivity to gemcitabine of P-glycoprotein and multidrug resistance-associated protein-overexpressing human cancer cell lines

Gemcitabine (2′,2′-difluorodeoxycytidine) is a deoxycytidine analogue that is activated by deoxycytidine kinase (dCK) to its monophosphate and subsequently to its triphosphate dFdCTP, which is incorporated into both RNA and DNA, leading to DNA damage. Multidrug resistance (MDR) is characterised by an overexpression of the membrane efflux pumps P-glycoprotein (P-gP) or multidrug resistance-associated protein (MRP). Gemcitabine was tested against human melanoma, non-small-cell lung cancer, small-cell lung cancer, epidermoid carcinoma and ovarian cancer cells with an MDR phenotype as a result of selection by drug exposure or by transfection with the mdr1 gene. These cell lines were nine- to 72-fold more sensitive to gemcitabine than their parental cell lines. The doxorubicin-resistant cells 2R120 (MRP1) and 2R160 (P-gP) were nine- and 28-fold more sensitive to gemcitabine than their parental SW1573 cells, respectively (P<0.01), which was completely reverted by 25 μM verapamil. In 2R120 and 2R160 cells, dCK activities were seven- and four-fold higher than in SW1573, respectively, which was associated with an increased dCK mRNA and dCK protein. Inactivation by deoxycytidine deaminase was 2.9- and 2.2-fold decreased in 2R120 and 2R160, respectively. dFdCTP accumulation was similar in SW1573 and its MDR variants after 24 h exposure to 0.1 μM gemcitabine, but dFdCTP was retained longer in 2R120 (P<0.001) and 2R160 (P<0.003) cells. 2R120 and 2R160 cells also incorporated four- and six-fold more [3H]gemcitabine into DNA (P<0.05), respectively. P-glycoprotein and MRP1 overexpression possibly caused a cellular stress resulting in increased gemcitabine metabolism and sensitivity, while reversal of collateral gemcitabine sensitivity by verapamil also suggests a direct relation between the presence of membrane efflux pumps and gemcitabine sensitivity.

Scheduling of gemcitabine and cisplatin in Lewis Lung tumour bearing mice

We used the gemcitabine (dFdC) and cisplatin (cis-diamine dichloroplatinum CDDP) resistant murine NSCLC tumour Lewis Lung (LL) in C57/B16 mice to optimise scheduling of both drugs, since in previous in vivo studies no effective combination schedule of both compounds was found to overcome resistance to either drug. dFdC could not be combined at the previously determined maximum tolerated dose (MTD) (120 mg/kg, q3dx4) with CDDP at its MTD (9 mg/kg, q6dx2) (mean weight loss < 15% and < 15% toxic deaths), because of additive toxicity. Therefore, we lowered the dose of dFdC to 60 mg/kg (q3dx4) and of CDDP to 3 mg/kg (q6dx2), which caused an increase in antitumour effect compared with the activity of each compound alone at its MTD (growth delay factor (GDF) = 0.55, 0.13 and 2.56 for dFdC and CDDP alone and the combination, respectively). Changing the CDDP treatment schedule giving the total dose (6 mg/kg) only at day 0 caused unacceptable toxicity. This effect was not seen when mice were treated with the total dose of CDDP on day 9, but, the anti-tumour effect was not enhanced. To decrease toxicity, the dosage of dFdC was lowered to 50 mg/kg and combined with the total dose of CDDP on day 0, which caused a better antitumour effect than the combination of 60 mg/kg dFdC and 3 mg/kg CDDP (q6dx2) with acceptable toxicity. Schedule dependency was found for the combination: dFdC preceding CDDP by 4 h was the best treatment schedule in the LL tumours (GDF: 2.1) with acceptable toxicity. However, when the interval was increased to 24 h, toxicity became unacceptable (> 30% weight loss). The reverse schedule, in which CDDP preceded dFdC, did not lead to an increased antitumour effect or to increased toxicity. Adding amifostine, a selective chemoprotector, to the treatment decreased toxicity of the combination without affecting the antitumour effect. Increasing the CDDP dose to 9 mg/kg (day 0) under amifostine protection led to an improved therapeutic index.

Postconfluent multilayered cell line cultures for selective screening of gemcitabine

The in vitro cytotoxicity of gemcitabine (dFdC) was tested in ovarian and colon cancer cell lines grown as monolayers and three-dimensional multilayered cell cultures. In our model, dFdC showed slight selectivity in cytotoxicity against ovarian over colon cancer cells, when cell lines were grown as monolayers. However, when cell lines were grown as multilayers, this selectivity was accentuated: A2780 multilayers were 14 times less sensitive than monolayers, but the colon cancer cell lines were more than 1000 times more resistant than their corresponding monolayers. The accumulation of the active metabolite, dFdCTP, after 24 h exposure to 1 microM dFdC varied between 1100 and 1900 pmol/10(6) cells in monolayers. This was 5 times lower in multilayers compared with monolayers of all four cell lines, which can, in part, explain the lower sensitivity of the multilayers. In addition, it appears that the amount of the active metabolite retained is more important than the amount accumulated initially, since the differences between the ovarian and the colon cancer cell lines were more evident in retention experiments. Exposure to dFdC caused a 2-3-fold increase in the levels of several nucleotides, except for the CTP pools in the colon cancer lines, which were reduced by 3-fold at the highest dFdC concentration (10 microM). The findings with the multilayer model are in better agreement with in vivo activity in ovarian cancer and colon cancer than those with the monolayer system. This indicates the potential of the multilayer system to be a better predictive model than the conventionally used monolayer cultures.

Role of deoxycytidine kinase (dCK), thymidine kinase 2 (TK2), and deoxycytidine deaminase (dCDA) in the antitumor activity of gemcitabine (dFdC).

Deoxycytidine kinase (dCK) and deaminase (dCDA) are as activating and inactivating enzymes, respectively, in the metabolism of several chemotherapeutically important deoxynucleoside analogues [1]. 2′2′-Difluorodeoxycytidine (dFdC; gemcitabine) has considerable antitumor activity against solid tumors, such as against the chemoresistant non-small cell lung cancer (NSCLC) and pancreatic cancer [2]. dCK catalyses the rate-limiting phosphorylation of CdR and its analogues to their corresponding monophos-phates [1,3]. To avoid an overestimation of the dCK activity by thymidine kinase 2 (TK2), which can also efficiently phosphorylate CdR [3], dCK activity was measured in the presence of thymidine (TdR) to inhibit TK2 [4]. dCDA inactivates cytidine (CR), CdR and its analogues to their deaminated products [5,6]. Previously we could not establish a relationship between antitumor activity and the dCK and dCDA activities [6], while in a cell line study more precise measurements of dCK showed a relation between sensitivity to dFdC and efficiency of dCK [7]. We now reevaluated the role of dCK, TK2 and dCDA in the antitumor effect of dFdC against different solid tumors.

Synergistic interaction between cisplatin and gemcitabine in ovarian and colon cancer cell lines.

Cisplatin (cis-diammine dichloroplatinum, CDDP) is an established square planar coordination compound, which is effective against ovarian cancer (1). The antitumour activity of CDDP is the result of formation of adducts within the DNA (2). Resistance against CDDP treatment may be related to an increased DNA repair. Like CDDP, Gemcitabine (2’,2’-difluorodeoxycytidine, dFdC) is active against human ovarian carcinoma (3, 4). After entering the cell, dFdC requires activation catalyzed by deoxycytidine kinase (dCK), and can be inactivated by the action of deoxycytidine deaminase (dCDA)(5). The active metabolite dFdCTP can be incorporated into both DNA and RNA (6). For both 1-s-D-arabinofuranosylcytosine (ara-C) and 2’-deoxy-5-azacytidine (DAC) two other deoxycytidine analogues, synergy with CDDP has already been described (7, 8). The next logical step was to combine dFdC with CDDP. Both agents have a different mechanism of action. A combination is also attractive from a clinical point of view, since both drugs have different side effects. We tested this combination in different concentrations and schedules in the human ovarian cancer cell line A2780, its 50 fold CDDP resistant variant ADDP (9), its 150,000 fold dFdC resistant variant AG6000 (10), and in the murine colon cancer cell line C26-10, which has an inherent lower sensitivity to both drugs compared to A2780 (11). These results were related to both dCK and dCDA activities and the effect of CDDP on dFdCTP accumulation.

Mechanisms of synergism between gemcitabine and cisplatin.

2′,2′-difluorodeoxycytidine (Gemcitabine, dFdC) is an antineoplastic agent with clinical activity against several cancer types.1 cis-Diamminedichloroplatinum (cisplatin, CDDP) is a drug with long established anticancer activity, which acts by Platinum (Pt)-DNA adduct formation.2,3 Because of the low toxicity profile of dFdC and the differences in mechanism of cytotoxicity, preclinical studies were performed that demonstrated synergism between dFdC and CDDP in several cancer cell lines and in vivo,4–8 which is likely to be related to increased formation of Pt-DNA adducts.8 Pre-treatment with dFdC gave the best results both in vitro and in vivo.6,7,8 Several potential mechanisms underlying the synergism were studied in vitro, based on these results several schedules were studied in patients.