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Experimental Therapeutics |
Departments of Experimental Radiation Oncology [L. M., T. F., N. H., M. E., K. M.], Clinical Investigation [W. P., W. H.], and Radiation Oncology [K. K. A.], The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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30% of cases, yet many of these patients suffer recurrence outside the irradiated port (1)
. For these reasons, increased attention is being paid to the combined use of radiotherapy and chemotherapy (2)
. A variety of approaches have been investigated, including alternating chemotherapy and radiotherapy and concomitant treatment. In some cases, the results have been encouraging, with trends toward decreased rates of distant failure, increased local control, and increased organ preservation rates (3, 4, 5)
. At the same time, combined treatments are frequently associated with increased normal tissue toxicities, and there is considerable room for improvement of the combined treatment strategies. Most agents have been chosen for combination with radiotherapy based on their known clinical activity in particular disease sites. For example, agents such as cisplatin (6 , 7) , 5-fluorouracil (8 , 9) , bleomycin (10) , methotrexate (11) , and mitomycin C (12) , which have been shown previously to have activity in the treatment of head and neck cancer on their own, have more recently been used in combination with radiotherapy. Alternatively, agents that might serve to overcome resistance mechanisms associated with radiotherapy could be chosen. Our group and others previously explored the use of nucleoside analogues such as fludarabine phosphate in combination with radiation (13 , 14) . The rationale for such a choice was that fludarabine is an inhibitor of DNA replication and a DNA chain terminator (15, 16, 17, 18) and thus might poison DNA repair in radioresistant tumor cells and also slow tumor regrowth during a fractionated schedule. Indeed, in vitro and preclinical mouse tumor model studies demonstrated that fludarabine could offer a radioenhancement ratio of 1.24–2.14, depending on the timing of fludarabine administration relative to radiation, the dose of fludarabine, tumor type, and schedule of radiation (19, 20, 21) .
Interestingly, whereas fludarabine enhanced radioresponse when given just before irradiation (which would be expected for a repair inhibitor), the greatest tumor radioresponse was observed when fludarabine was administered at least 24 h before radiation. Subsequent mechanistic studies demonstrated that radioenhancement was associated with fludarabine-induced apoptosis and preferential cell loss of cells in S-phase through an apoptotic pathway, delayed cell cycle progression, and subsequent parasynchronization of tumor cells into the radiosensitive G2-M phases of the cell cycle (22) . In addition, on an optimized schedule, fludarabine did not significantly modify the radioresponse of a number of normal tissues (20 , 21 , 23) . Thus, it appeared that nucleoside analogues could improve the therapeutic ratio of radiotherapy when used on an optimized schedule. A clinical Phase I trial is presently under way exploring this combination in patients with locally advanced head and neck cancer.
Because fludarabine has shown little single-agent activity in solid tumors at the schedules used (24) , other nucleoside analogues with similar mechanisms of action may have more favorable characteristics for use in solid tumors. One such possibility is gemcitabine,5 a fluorine-substituted 1-ß-D-arabinofuranosylcytosine that requires intracellular phosphorylation by deoxycytidine kinase to its active metabolites, gemcitabine diphosphate (dFdCDP) and gemcitabine triphosphate (dFdCTP; Ref. 25 ). Gemcitabine has better membrane permeability than 1-ß-D-arabinofuranosylcytosine, and dFdCTP has a longer half-life in the cell than 1-ß-D-arabinofuranosylcytosine or fludarabine triphosphate (26) .
Gemcitabine has been found to interfere with DNA synthesis through several mechanisms (27, 28, 29) : (a) dFdCTP inhibits ribonucleotide reductase and hence reduces deoxynucleotide pools; (b) dFdCTP competes with dCTP for incorporation into elongating DNA strands; and (c) in vitro studies have suggested that once dFdCTP is incorporated into elongating DNA, only one additional deoxynucleotide can be subsequently added into the strand, thus halting DNA polymerization. Moreover, gemcitabine exerts antitumor activity in a number of murine solid tumors and human xenografts (30, 31, 32) , and the frequently increased levels of deoxycytidine kinase in tumor cells may enhance the ability of gemcitabine to increase the therapeutic ratio (33) . Finally, gemcitabine has shown significant activity in several solid tumor types traditionally treated with radiotherapy, including pancreatic, head and neck, and lung carcinomas (34) .
The studies described above made gemcitabine an attractive candidate for combining with radiation. Previous studies suggested that gemcitabine radiosensitizes tumors cells in vitro, with EFs of up to 1.8, especially at higher concentrations and longer durations of gemcitabine exposure (35, 36, 37, 38) . Studies from our own laboratory demonstrated that gemcitabine effectively inhibits chromosome repair after irradiation, thus increasing the frequency of residual chromosome breaks (39) . Similarly, a recent report showed that gemcitabine, when given once or twice weekly, significantly enhanced the radioresponse of a human squamous cell carcinoma grown in nude mice treated with fractionated local tumor irradiation (40) . The challenge now is to determine the optimal dose and schedule with which to combine gemcitabine and radiation and to better understand the specific cellular determinants and mechanisms of interaction that determine the degree of radiosensitization. To this end, we examined the nature of response of a murine sarcoma SA-NH model system to gemcitabine alone and in combination with radiation to identify a combination schedule that optimized growth delay, enhanced tumor cure, and decreased metastasis.
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MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Gemcitabine.
Gemcitabine was generously provided by Lilly Research Laboratories (Indianapolis, IN). It was reconstituted in physiological saline and stored at 4°C. The drug was injected i.v. into mice at room temperature using single doses ranging from 2.5 to 600 mg/kg in a volume of 0.01 ml/g of mouse body weight.
Tumor Irradiation.
Unanesthetized mice were immobilized in a jig, and tumors were centered in a 3-cm diameter circular field. For tumor growth delay experiments, a single dose of 25 Gy radiation was locally delivered using a dual-source 137Cs unit at a dose rate of 6.5 Gy/min. The tumor-bearing mice were either treated with gemcitabine or local tumor irradiation alone or treated with gemcitabine at different times before or after local tumor irradiation. Untreated tumor-bearing mice served as controls.
Measures of Tumor Response.
The effect of radiation alone, gemcitabine alone, or the combination on tumor response was assessed by three endpoints. Tumor growth delay was assessed in serial measurements of three orthogonal tumor diameters using Vernier calipers; tumors were measured at 2–3-day intervals until the tumors grew to at least 12 mm in mean diameter. The degree of growth delay was expressed either as: (a) the AGD (defined as the time in days for tumors in the combined treatment arm to grow from 8 to 12 mm in diameter minus the time in days for the tumors in the untreated control group to reach the same size); or (b) the normalized growth delay (defined as the time for tumors in groups treated with a combined regimen to grow from 8 to 12 mm minus the time to reach the same size in mice treated with gemcitabine alone). In the tumor control dose (TCD50) assay, mice were irradiated with single radiation doses ranging from 27 to 63 Gy in combination with or in the absence of gemcitabine and subsequently observed for tumor cure until 100 days after irradiation. The antimetastatic activity of treatment was assessed by examining the lungs of mice with local tumor control that died during the observation period or that survived 100 days.
Flow Cytometric Analysis.
To determine the effect of gemcitabine administration on DNA synthesis in tumor cells in vivo and on tumor kinetics, tumor-bearing mice were injected i.p. with 60 mg/kg BrdUrd (Sigma Chemical Co., St. Louis, MO) dissolved in PBS 30 min before the mice were killed by CO2 inhalation and the tumor removed. The tumors were cut in small pieces and fixed in cold 70% ethanol (AAPER Alcohol and Chemical Co., Shelbyville, KY) at 4°C for at least 24 h. Tumor nuclei were then extracted by pepsin digestion and prepared for dual label flow cytometry as described previously (22)
. Briefly, BrdUrd uptake was detected using the BR3 antibody (MD5300; Caltag Laboratories, Inc. San Francisco, CA) and a fluorescein-conjugated second antibody, and DNA content was determined after counterstaining the nuclei with 5 µg/ml propidium iodide (Aldrich Chemical Co., Milwaukee, WI). Flow cytometry was performed using a FAC-SCAN (Becton Dickinson) as described previously. For each sample, 10,000–15,000 events were collected in list mode, and cell debris and doublets were excluded from the data acquisition using a doublet discriminator (Becton Dickinson). Data acquired on a HP computer were converted to a PC format (HP Reader; Verity Software) and analyzed using PCLYSIS II software (Becton Dickinson). Effects of treatment on DNA synthesis and cell kinetics were determined on scattergrams of relative DNA content versus relative BrdUrd content.
Analysis of Apoptosis.
Mice were killed by CO2 inhalation at different times after treatment, and the tumors were immediately excised and placed in neutral buffered formalin. The tissues were embedded in paraffin blocks, and 4-µm sections were cut and stained with H&E. The apoptotic cells were scored on coded slides at x400. The morphological features used to identify apoptosis in murine tumors have been described previously, illustrated, and associated with positive terminal deoxynucleotidyl transferase-mediated nick end labeling staining (22)
. Five fields of nonnecrotic areas were selected randomly across each tumor section, and in each field apoptotic bodies were expressed as a percentage based on the scoring of 1500 nuclei (2000 nuclei for untreated controls) at each time interval after treatment.
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RESULTS |
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for three example doses (10, 50, and 400 mg/kg), gemcitabine treatment resulted in a dose-dependent delay in growth. At low doses, the growth rate appeared delayed without significant decreases in tumor size, whereas at higher doses a transient reduction in tumor size was detected. This suggested that gemcitabine had cytotoxic as well as cytostatic activity. Taking the time for the tumor to reach a 12-mm diameter as an endpoint, growth-inhibitory effects were observed with bolus doses of gemcitabine as low as 5 mg/kg, and a steep dose-response curve was observed until about 200 mg/kg, after which the growth-inhibitory effect appeared to plateau (Fig. 1B)
. Perhaps high gemcitabine doses saturated the deoxycytidine kinase activation pathway or only a subfraction of the tumor cell population was sensitive to the cytotoxic mechanism of gemcitabine. None of these gemcitabine doses killed mice or achieved local tumor control when given as a single bolus.
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As reported previously (22)
, the SA-NH tumor contains two tumor cell subpopulations with DNA indices of 1.2 and 2.2. As shown in Fig. 2
, all three doses of gemcitabine inhibited DNA synthesis in the tumors within 1 h of treatment. This inhibitory effect was confirmed by immunohistochemical analysis of tumor tissue sections using an antibody to incorporated BrdUrd (Fig. 3)
. The duration of DNA synthesis inhibition in the tumor was dose dependent; DNA synthesis began to recover 12 h after 10 mg/kg (Fig. 2A)
, 18 h after 50 mg/kg (Fig. 2B)
, and not until 24–36 h after 400 mg/kg (Fig. 2C)
.
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). Cells in S phase at the initiation of gemcitabine exposure appeared exquisitely sensitive and were not able to reinitiate DNA synthesis. As a result of selective elimination of S-phase cells and a G1-S blockade during DNA synthesis inhibition, the tumor population exhibited a parasynchronous movement through the cell cycle after resumption of DNA synthesis. After 50 mg/kg, this parasynchronous population reached the more radiosensitive phase of the cell cycle (G2-M), starting around 36 h after treatment.
There was no evidence of a G2 block in these tumor populations either during the initial hours after gemcitabine administration or after the resumption of DNA synthesis. In fact, assessments of histological specimens showed no mitotic activity at 24 h, and a subsequent wave of mitoses occurred 36–60 h after 50 mg/kg gemcitabine (Fig. 4A)
. A similar trend was observed after the 400 mg/kg dose, but the peak of mitotic activity resumption was delayed until 48 h. After 10 mg/kg, the parasynchronous wave of kinetic movement was less pronounced, and mitotic activity gradually returned starting 24 h after gemcitabine treatment.
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, there was a dose- and time-dependent wave of apoptotic activity. For example, in the absence of treatment, <1% of the tumor cells appeared apoptotic. After a 50-mg/kg dose, the frequency of apoptotic cells increased within 3 h, reaching a peak of 6% at 18 h; apoptotic activity returned to normal by 60 h. After 400 mg/kg, an apoptotic peak approaching 10% was apparent between 24 and 36 h and gradually disappeared by 96 h after treatment.
Enhancement of Tumor Radioresponse.
To determine whether gemcitabine could augment radioresponse, mice bearing 8-mm tumors were treated with a single 25-Gy dose with and without 50 mg/kg gemcitabine. To determine the optimal time of administration of gemcitabine with regard to the time of irradiation, a single gemcitabine dose was given at 1, 3, 6, 12, 24, 36, 48, 60, 72, or 96 h before or 1, 3, 6, 12, or 24 h after irradiation. The antitumor effect, as assessed by tumor growth delay, was found to be either additive or supraadditive, depending on the interval between gemcitabine administration and irradiation (Fig. 5A)
. A sample calculation for the degree of antitumor interaction between gemcitabine and radiation is shown in Table 1
, where gemcitabine was administered 36 h before irradiation. Both gemcitabine and radiation were effective on their own in slowing tumor growth. However, when they were combined, the slowed tumor growth was more than the sum of the delays of individual treatments, suggesting that gemcitabine enhanced tumor radioresponse. In this case, the EF was calculated to be 2.03, obtained by dividing the normalized growth delay in the group treated by both agents by the AGD produced by radiation alone.
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, enhancement of radiation-induced tumor growth delay by gemcitabine was highly dependent on the interval between gemcitabine administration and tumor irradiation. Although enhancement was observed at all but one time interval (3 h before irradiation) when gemcitabine administration preceded tumor irradiation, the highest EFs were reached with the 24–60-h intervals (EFs of 1.68 to 2.03). Little modification in tumor radioresponse was observed when gemcitabine was given within 6 h of radiation, although parallel experiments had demonstrated profound inhibition of DNA synthesis in this time period. Under conditions where gemcitabine followed irradiation, there was some evidence of enhancement of tumor radioresponse, particularly when the drug was given 12 h after irradiation (EF, 1.36).
The original rationale for combining gemcitabine and radiation was based on the notion that gemcitabine, an inhibitor of DNA synthesis, would interfere with the repair of radiation-induced DNA damage, especially those more complex components requiring DNA resynthesis. Another rationale was based on the idea that gemcitabine induces an apoptotic signal in S-phase cells through creation of chain-terminated DNA strands. We hypothesized that radiation sensitizes cells in all phases of the cell cycle to gemcitabine by creating DNA regions that required repair synthesis and thus might generate apoptotic signals when chain terminated by gemcitabine incorporation. To explore the effect of the combination on apoptotic rates, apoptotic cells in tumors were quantified morphologically at 1, 2, 4, 6, 12, or 24 h after tumor irradiation, with or without gemcitabine pretreatment (36 h before irradiation). As shown in Fig. 6
, radiation alone induced a rapid rise in apoptotic activity that peaked at <4% within 6 h of treatment. If radiation was preceded 36 h by gemcitabine treatment, a peak of apoptotic activity appeared on top of the gemcitabine-induced apoptotic curve, with similar time-dependent characteristics as that observed after radiation alone. However, the combined effect on apoptosis was only additive, suggesting that different cell populations were being affected. Thus, under these conditions, the enhancement of gemcitabine tumor radioresponse could not be explained by increases in the sensitivity of cells for radiation-induced apoptosis.
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-rays ranging from 27 to 59 Gy to determine whether gemcitabine could alter the radiation dose necessary for tumor cure. Control tumor-bearing mice received local tumor radiation only. The frequency of mice with locally controlled tumors was determined at 100 days after irradiation, and the results were expressed as the radiation dose necessary to (locally) cure 50% of the tumors (TCD50). As shown in Table 2
, the TCD50 of radiation alone was 51.9 Gy (95% confidence limits, 50.4–53.8 Gy). Gemcitabine reduced the TCD50 at all time intervals, the largest effect occurring when gemcitabine was administered 1 day before irradiation, where a TCD50 of 33.8 Gy (32.0–35.7 Gy) was observed. This effect translated into a DMF of 1.54 (i.e., 51.9/33.8 Gy). The lowest DMF was observed when gemcitabine was administered 1 h before irradiation [TCD50, 46.0 Gy (44.4–47.8 Gy); DMF = 1.13].
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DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. The importance of the time interval and exposure duration of gemcitabine in relation to radiation exposure has been reported previously by others when studied in the in vitro cell culture setting (36, 37, 38) . For example, longer exposures (24 h) to nontoxic gemcitabine concentrations (10 nM) before irradiation were more effective in radiosensitizing human colon carcinoma and pancreatic carcinoma cell lines than shorter exposures (4 h; EFs of 1.7–1.8 versus 1.4; Refs. 36 and 37 ). In these experiments, irradiation was delivered immediately after gemcitabine exposure. On the other hand, a short exposure (2 h) of colon carcinoma cells to high gemcitabine concentrations (100 nM to 3 µM) also produced large radioenhancement ratios (1.8–3.0), but only when radiation was delivered 24–48 h after gemcitabine was removed from the culture medium (38) . No significant radioenhancement was observed when the cells were irradiated within 4 h after drug removal (38) , a finding similar to that reported here for tumors treated in vivo.
The mechanisms involved in the radioenhancing activity of gemcitabine in vivo are not well understood. However, the studies reported here may provide some insight. Our original working hypothesis was that gemcitabine would act by inhibiting repair in vivo and so enhance the radiation effect. This hypothesis was based on the observations that gemcitabine is an effective inhibitor of DNA synthesis (27, 28, 29) and inhibits the repair of radiation-induced chromosome damage in vitro (39) . If this were the only radiosensitizing mechanism, one would expect that the highest radioenhancement would occur when irradiation is administered when DNA synthesis was most inhibited in the tumor. However, we show here that, although there was a small radioenhancing effect in tumors irradiated within 6 h of gemcitabine exposure, greater enhancement occurred when gemcitabine was administered 24–96 h before radiation. It is unlikely that the small size of the enhancement observed within 6 h after gemcitabine administration was due to inadequate concentrations of the active intracellular metabolites of gemcitabine (i.e., dFdCDP and dFdCTP) because DNA synthesis was shown to be inhibited in these tumors for 12–24 h, depending upon gemcitabine dose.
A close examination of events within the tumor after gemcitabine administration may shed light on the basis for variation in radioenhancement with the timing of gemcitabine. Gemcitabine inhibited DNA synthesis in the tumor within 1 h of administration at doses between 10 and 400 mg/kg. A wave of apoptosis within the tumors then occurred, the timing and extent of which was also dependent on gemcitabine dose. Associated with histological evidence of apoptosis was an apparent preferential loss of cells from the S-phase component of the tumor, as detected by flow cytometric analysis of DNA content and BrdUrd uptake in vivo. It is unlikely that tumor cells moved through S phase because DNA synthesis remained inhibited during the period that S-phase cells disappeared from the tumor. When DNA synthesis resumed, tumor cells appeared to initiate DNA synthesis at the G1-S border, producing a parasynchronous movement of cells through S phase and through mitosis. An increasing enhancement ratio was observed as the parasynchronous cell population approached the G2 phase and mitosis. These combined results suggest that the radioenhancement involved the elimination of more radioresistant S-phase cells from the tumor population by gemcitabine and the redistribution of surviving cells into a more radiosensitive compartment of the cell cycle are largely consistent with our original hypothesis.
That the radioenhancing effect of gemcitabine persisted for up to 96 h after drug administration suggests that additional sensitization mechanisms must occur in the in vivo tumor setting. The magnitude and timing of the potentiation of gemcitabine of tumor radiation response was somewhat different when alternative response endpoints were used. For example, the magnitude of the potentiation of gemcitabine of local tumor cure by radiation was, in general, lower than that measured by tumor growth delay assays (compare Table 2
and Fig. 5
, respectively). The reasons for this discrepancy may shed light on the mechanisms of potentiation because it is possible that these assays are most sensitive to different populations within the tumor. Tumor growth delay is most likely a manifestation of cytotoxicity in the more radiosensitive, well-oxygenated tumor cell compartment. On the other hand, the TCD50 assay measures the response of the more radioresistant, hypoxic tumor cell compartment, where tumor cells proliferate poorly. In addition, hypoxic areas of the tumor are farther from tumor blood vessels, which also makes them less accessible to drug. Thus, the radioresistant hypoxic fraction would be less influenced by gemcitabine both in terms of the effect of gemcitabine on cells in S phase and by its ability to achieve sufficient intracellular concentrations of active gemcitabine metabolites.
In the TCD50 assay, the highest DMF (1.54) was observed at 24 h after gemcitabine administration; the DMF decreased to 1.32 by 36 h. One possible explanation for this time course of sensitization is that tumor reoxygenation occurred because of the large numbers of tumor cells being lost from the tumor in a wave of apoptosis after gemcitabine treatment. Some of this increased effect at 24 h associated with reoxygenation might be compromised by 36 h because of tumor regeneration prior to irradiation. A similar radiosensitizing effect was observed after paclitaxel treatment of tumors before irradiation (42 , 43) . If tumor reoxygenation truly plays a role in the potentiation by gemcitabine of tumor radiocurability, gemcitabine might then be particularly effective in the setting of fractionated irradiation schedules because reoxygenation would take place between fractions. In addition, gemcitabine might also slow tumor repopulation between fractions. Our present experiments have addressed a possible role of tumor reoxygenation in gemcitabine-induced enhancement of tumor radioresponse. The results of initial experiments show that the magnitude of induced radioenhancement was reduced when tumors were irradiated under hypoxic conditions.6 This implies that reoxygenation of hypoxic tumor cells is an additional mechanism by which gemcitabine enhances tumor radioresponse.
One rationale for the combined use of chemotherapeutic agents and radiation is that systemic control of micrometastases may improve. We report here that gemcitabine significantly reduced the incidence of lung metastasis in mice whose primary tumor was locally controlled by radiation. Under the conditions used for these experiments, the incidence of lung metastases decreased from 73% in mice treated by radiation alone to 40% in mice treated by combined therapy. Thus, this study demonstrated that gemcitabine treatment served to reduce primary tumor cell burden, especially in combination with radiation, and this was associated with decreased systemic spread of tumor cells in a high percentage of mice. Whether gemcitabine acted on its own on systemic disease or enhanced the effect of radiation on the primary tumor and thus decreased tumor dissemination was difficult to determine because gemcitabine alone did not induce local control after single-dose administration.
The findings reported here have some similarities and some dissimilarities to those reported previously for the combination of fludarabine and radiation (19, 20, 21, 22) . Fludarabine was also found to enhance radiation effect in a dose- and schedule-related manner (19, 20, 21) , and this was similarly associated with inhibited DNA synthesis in vivo, induction of a wave of apoptosis, and the generation of a parasynchronous wave of rebounding tumor cells (22) . However, unlike gemcitabine, fludarabine had a stronger radioenhancing effect compared with gemcitabine when given within 6 hours of irradiation (19) . This difference might reflect differences in the nature of the DNA chain-terminating event. On the other hand, the duration of the radioenhancing effect of fludarabine (i.e., around 48 h) was shorter than that observed here for gemcitabine (i.e., around 96 h). This difference may be attributable to the longer intracellular retention of active metabolites.
This longer-lasting radioenhancing effect of gemcitabine may have important clinical implications, especially with regard to the schedule of drug administration. If gemcitabine has a longer-lasting radioenhancing effect, it might be possible to administer the drug only once or perhaps twice a week to derive its full benefit in conventional radiotherapy fractionation schedules. However, it is also important to determine whether these schedules will still yield a positive therapeutic ratio. Studies are now under way to determine the effects of gemcitabine and radiation intervals on normal tissue radioresponse. Preliminary data suggest that tumor and normal tissues differ in their kinetic response to gemcitabine, and these differences may lead to the development of treatment schedules that improve the therapeutic ratio.
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FOOTNOTES |
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1 Supported by NIH National Cancer Institute Grants CA-06294, CA-27931, and Core Grant CA-16672 (for flow cytometry). 
2 To whom requests for reprints should be addressed, at Department of Experimental Radiation Oncology, Box 066, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030.
3 Present address: Ehime University School of Medicine, Department of Radiology, Ehime 791-02, Japan.
4 Present address: Ain Shams School of Medicine, Abbassia, 11566 Cairo, Egypt.
5 The abbreviations used are: gemcitabine, dFdC, 2',2'-difluoro-2'-deoxycytidine; EF, enhancement factor; AGD, absolute growth delay; BrdUrd, bromodeoxyuridine; DMF, dose modification factor; fludarabine, 9-ß-D-arabinofuranosyladenine-5'-monophosphate; TCD, tumor control dose.
6 K. Mason, L. Milas, N. Hunter, M. Elshaikh, L. Buchmiller, K. Kishi, W. Hittelman, and K. Ang, unpublished observations.
Received 7/17/98. Accepted 10/30/98.
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J. W. G. van Putten, H. J. M. Groen, K. Smid, G. J. Peters, and H. H. Kampinga End-joining Deficiency and Radiosensitization Induced by Gemcitabine Cancer Res., February 1, 2001; 61(4): 1585 - 1591. [Abstract] [Full Text]
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B. A. Teicher, V. Chen, C. Shih, K. Menon, P. A. Forler, V. G. Phares, and T. Amsrud Treatment Regimens Including the Multitargeted Antifolate LY231514 in Human Tumor Xenografts Clin. Cancer Res., March 1, 2000; 6(3): 1016 - 1023. [Abstract] [Full Text]
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) and mice treated with different doses (2.5–600 mg/kg) of gemcitabine (•). Bars, SE.
), 50 mg/kg (
), or 400 mg/kg (
) gemcitabine when tumors were 8 mm in diameter. Bars, SE.
), or both (
), where radiation was given 36 h after gemcitabine. The treatments were given when tumors were 8 mm in diameter. Bars, SE.