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Institut National de la Santé et de la Recherche Médicale Research Unit 463, CEA L. R. C. 18V and Regional Cancer Center, 44093 Nantes, Cedex 01, France
| Abstract |
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| Introduction |
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10% of all thyroid cancers. As MTC tumor expresses and secretes CEA, it constitutes a potential application for RIT with anti-CEA antibodies (1, 2). RIT efficacy in the treatment of MTC has been demonstrated in preclinical studies performed in mice grafted with human MTC cell lines and in Phase I/II clinical trials performed in patients with recurrences of MTC (25). Tumor and/or biological responses showing decreased TCT levels were observed, particularly in small lesions and after repeated courses of RIT. The AES, a pretargeting technique using a BsMAb and a bivalent hapten, increases tumor:normal tissue ratios by reducing activity levels in normal tissues 35-fold as compared with directly labeled MAb fragments (6). Preclinical studies showed that the toxicity of AES RIT using a 131I-di-DTPA-In was significantly lower than with directly labeled MAb fragments and that repeated injections of AES reagents did not increase toxicity (3, 7). Thus, AES is a promising RIT modality, and its moderate myelotoxicity is favorable for combinations with myelotoxic chemotherapeutic drugs.
The purpose of the present study was to assess the toxicity and efficacy of combined chemotherapy and AES RIT using an anti-CEA x anti-DTPA-indium BsMAb and 131I-di-DTPA-In in nude mice grafted s.c. with a human MTC line. In particular, a combination of RIT (65% of the MTD) with the MTD of paclitaxel was studied and compared with the MTD of RIT, 80% of the MTD of RIT, the MTD of paclitaxel, and 65% of the MTD of RIT plus the MTD of doxorubicin.
| Materials and Methods |
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Animal Model.
Nude mice > 10 weeks of age were grafted s.c. in the right flank with 106 TT cells in 0.3 ml of sterile physiological serum. The animals were housed under aseptic conditions and used once tumors were
200 mm3
6 weeks after injection. Lugols solution 0.1% was added to drinking water (1/100 ml) the week before and then 2 weeks after injection of 131I-labeled hapten.
Antibody, Hapten, and Radiolabeling.
F6-734 BsMAb, obtained by chemical coupling of the Fab' fragment of F6 antibody (anti-CEA IgG1) to the Fab' fragment of 734 antibody (anti-DTPA-indium IgG1), was kindly provided by Immunotech (Marseille, France), together with the bivalent DTPA hapten di-DTPA.
Di-DTPA-In was provided as a solution in 10 mm citrate and 100 mm acetate buffer (pH 5). The following were added sequentially in a sterile 2-ml plastic tube: 25 µl of di-DTPA-In (25 nmol), 25 µl of 0.3 m phosphate buffer (pH 6), 50 µl of chloramine-T [1 mg/ml in 0.3 m phosphate buffer (pH 6)], and 100 µl of a 131I solution at 1418 GBq/ml in 0.1 m sodium bicarbonate (pH 8) [131I-S3B; CIS Bio International, Gif sur Yvette, France].
After 10 min of incubation at room temperature, the reaction was stopped by the addition of 50 µl of sodium disulfite [1 mg/ml in 0.3 m phosphate buffer (pH 6)]. The pH of the solution was brought to between 5 and 6 by the addition of 750 µl of N(2-hydroxyethyl) piperazine-N' 2-ethane sulfonic acid (1 m).
The resulting solution was purified on a C18-grafted silica cartridge (Sepack-C18; Millipore). Free iodine was eluted with 5 ml of 0.1 m phosphate buffer (pH 7) and radiolabeled hapten with 5 ml of a 0.1 m phosphate buffer (pH 7)-ethanol mixture (3-2).
Specific activity was measured in an ionization chamber. To determine the radiochemical purity of 131I-di-DTPA-In, 10 µl of 131I-di-DTPA-In solution diluted 1:1000 were deposited in tubes coated with 734 antibody (kindly provided by Immunotech) containing 250 µl of 0.1 m phosphate buffer (pH 7) supplemented with 0.5% BSA. Total activity was measured after 1 h of incubation at room temperature with slow stirring. The tubes were washed three times with 0.1 m phosphate buffer (pH 7) and 0.01% Tween 20, and the bound activity was measured.
Chemotherapeutic Agents.
Doxorubicin (Adriblastine; Pharmacia & Upjohn S.A., St. Quentin, Yvelines, France), a product of the anthracycline group, is a topoisomerase II inhibitor and an intercalating agent. Doxorubicin was administered i.p. in a 1000-µl sterile isotonic NaCl solution. Paclitaxel (Taxol; Bristol-Myers Squibb Co., Princeton, NJ), a natural product from the taxane group, has novel antimicrotubule properties. Paclitaxel was administered i.p. in 1000 µl of sterile physiological serum.
Experimental RIT and Chemotherapy.
Eight groups of 612 mice each were studied. Initial tumor volumes were not significantly different between the groups. The different treatment procedures are summarized in Fig. 1. BsMAb and hapten diluted in 0.2 ml of sterile physiological serum were injected i.v. into the lateral tail vein. Two groups were injected, respectively, with 3 and 4 nmol of BsMAb F6-734 and then 48 h later with 74 and 92.5 MBq of 131I-di-DTPA-In. Preliminary studies determined that 4 nmol of BsMAb represent an amount less than that required to saturate tumor
3 and that a hapten:BsMAb ratio of 0.5 at injection was most favorable for targeting purposes (8). A preliminary study also determined that the MTD of RIT performed with this scheme was 92.5 MBq.3 The MTD levels of a single dose of doxorubicin and paclitaxel were established in nude mice, as described previously (9). Three groups were injected i.p. with the MTD of paclitaxel (1000 µg), the MTD/2 of paclitaxel (500 µg), or the MTD of doxorubicin (200 µg). Two groups were treated with RIT (3 nmol of BsMAb and 60 MBq of 131I-di-DTPA-TL hapten injected 48 h apart) in association with the MTD of paclitaxel or doxorubicin delivered 24 h after hapten. Finally, a control group of 12 mice received no injection.
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/6 x L x W x T. All animals were weighed on the day of injection and then twice a week for 90 days. Biological monitoring was performed on a blood sample drawn from the inner border of the eye. The parameters used to evaluate the toxicity of each type of treatment were maximal weight loss and variation in the number of leukocytes and platelets measured on days 0, 15, 30, and 60. The parameters used to evaluate the efficacy of each type of treatment were relative tumor volume, mean tumor volume doubling time, and the variation in serum TCT concentration measured by RIA on days 0, 15, 30, 45, 60, and 90 (calcitonin immunoradiometric assay, CIS Bio International).
Analysis of Antitumor Efficacy.
The Loewe additivity model was used to determine whether the combination of RIT and paclitaxel had an additive or supra-additive (synergistic) antitumor effect (10, 11). Five groups of mice were used for this study: control group, groups treated with 74 and 92.5 MBq of 131I-di-DTPA-In, and groups treated with 500 and 1000 µg of paclitaxel. Mean relative tumor volumes 20, 40, and 60 days after treatment were used as end points. The dose-response relationships of both RIT and chemotherapy were considered to be exponential and were plotted with a logarithmic scale as relative tumor volume versus activities of radiolabeled hapten or doses of paclitaxel (11, 12). The intercepts of the dose-efficacy curves of the single-treatment modalities with the mean relative volumes of tumors treated with combination therapy were determined. These values represented the activities of hapten or the doses of chemotherapy alone that could be expected to be isoeffective, as compared with combination therapy. Combination therapy was considered to be synergistic if dRITDRIT-1 + dchemoDchemo-1 < 1, additive if dRITDRIT-1 + dchemoDchemo-1
1, or sub-additive if dRITDRIT-1 + dchemoDchemo-1 > 1; dRIT and dchemo were the activities of radiolabeled hapten and doses of chemotherapy actually given in the association, and DRIT and Dchemo were the calculated activities and doses expected to be isoeffective when given alone.
Statistical Analysis.
Because of the limited number of animals, the means of the quantitative variables of the different groups were compared using nonparametric tests. The Mann-Whitney rank-sum test was used for comparison of two groups. For comparison of more than two groups, the Kruskall-Wallis one-way ANOVA test was used; when P was <0.05 in the first step, multiple comparisons were computed as described for the Mann-Whitney test, with correction in accordance with the number of compared groups (13). Ps
0.05 were considered significant. BMDP Statistical Software (Cork, Ireland), version 7.0, was used for the analysis.
| Results |
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Toxicity.
Maximal weight losses observed after the different treatments are summarized in Table 1. The values were not significantly different after RIT alone, paclitaxel, and doxorubicin, and weight losses were not significantly higher after RIT + doxorubicin or paclitaxel than after RIT alone.
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Efficacy of the Combination of Doxorubicin and F6-734 BsMAb/131I-di-DTPA-In.
Doxorubicin and F6-734 BsMAb/131I-di-DTPA-In were evaluated as single modality therapeutic agents and in combination. Fig. 2 shows the growth curves of TT tumors after various treatment regimens and in the control group. Minimal relative tumor volumes and tumor volume doubling times are summarized in Table 1. In the control group, mean doubling time was 13 ± 4 days. After RIT, all tumors decreased in size, and tumor volume doubling times (respectively, 56 ± 10 and 86 ± 22 days with 74 and 92.5 MBq) were significantly longer than in the control group (P < 0.005). After treatment with doxorubicin at the MTD (200 µg), mean doubling time was not significantly different (15 ± 8 days) from that of the control group. The combination of RIT (60 MBq) with doxorubicin (200 µg) did not appear to improve efficacy as compared with RIT alone, giving a tumor volume doubling time of 60 ± 16 days. Changes in TCT concentrations were parallel to those in tumor volume (Fig. 3).
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Synergy between RIT and Paclitaxel.
The dose-response curves of both F6-734 BsMAb/131I-di-DTPA-In and paclitaxel alone showed an exponential pattern (Fig. 4). Dose-response curves based on relative tumor volumes measured at day 20, 40, or 60 were used to calculate the hypothetical doses of the respective single agent treatments (DRIT and Dchemo) expected to be isoeffective with the combination. The additivity factor dRITDRIT-1 + dchemoDchemo-1 was then calculated according to Loewe (Table 3). At days 20 and 40, factors were close to 1, showing simple additivity, whereas at day 60, the factor was 0.80, suggesting a synergistic effect between RIT and paclitaxel.
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| Discussion |
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MTC is characterized by good vascularization and strong membrane expression of CEA, allowing RIT to deliver significant radiation doses to tumor (3, 5, 16). However, studies using different targeting systems and radionuclides in animal xenograft models of MTC have shown that isolated courses of RIT induce prolonged tumor responses but that late relapses always occur (3, 4). Heterogeneous perfusion and the presence of hypoxic cells or of cells in nonresponsive cell cycle phases can limit the efficacy of RIT in solid tumors. Autoradiography of TT xenografts has shown a heterogeneous distribution of anti-CEA MAb (17). A histological proliferation study of TT xenografts, using an antibody (MiB1) directed against a nuclear protein expressed in all active cell cycle phases, but absent in G0 and early G1, showed that the tumor nodules remaining after RIT were essentially composed of cells in G0 and early G1 (18). Moreover, RIT delivers an irradiation dose that decreases with the distance from blood vessels so that radioresistant hypoxic tumors are less irradiated. Chemotherapy, because of a very different biodistribution of the active drug and a different mode of action, could be used as a means of killing these radioresistant cells.
The present study tested two drugs in association with RIT: (a) doxorubicin, which has induced the best clinical responses in metastatic MTC as a single agent and enhanced radiation effects in vivo (19, 20); (b) and paclitaxel, which has shown radiosensitizing effects in vitro (21) and given promising results in combination with RIT in human breast carcinoma and MTC xenograft models (2224).
It is clear that toxicity and antitumor activity depend on the timing between RIT and chemotherapy (23, 24). Doxorubicin, a vasoactive agent that reduces tumor blood flow, may decrease tumor uptake of a subsequently injected antibody (25). Paclitaxel is rapidly distributed to tumor and normal tissues and clears quickly (26). Thus, maximum tissue concentrations are reached shortly after administration. If chemotherapy is to act as a radiosensitizer, tumor cells must be irradiated when the chemotherapeutic agent is administered. A study involving a breast cancer animal model showed that the cure rate was much better when paclitaxel was delivered 624 h after the antimembrane glycoprotein 90Y-ChL6 MAb rather than 2472 h before (23). Another study evaluating anti-CEA 90Y-MN-14 MAb in an MTC animal model showed that administration of doxorubicin 24 h after radiolabeled MAb achieved greater antitumor efficacy than when doxorubicin was given 48 h after RIT (24). A biodistribution study performed with AES reagents (F6-734/125I-di-DTPA) in nude mice bearing MTC tumor showed that tumor uptake was maximal 5 h after hapten injection and then remained high at 24 h, with elevated tumor:nontumor tissue contrast ratios, before decreasing at 48 h (17). These results, as well as the relatively long half-life of iodine 131, led us to administer chemotherapy 24 h after injection of radiolabeled hapten to maximize possible synergistic effects on tumor cells.
Under these conditions, hematotoxicity was not increased when RIT was combined with doxorubicin or paclitaxel as compared with RIT alone. The 131I-di-DTPA-In hapten cleared rapidly from the bloodstream, and only a small fraction of the radiation dose was delivered to marrow after injection of the chemotherapeutic agent. Similarly, in a preclinical study evaluating paclitaxel combined with 90Y-ChL6 MAb, hematotoxicity was not increased when the drug was given after RIT, whereas the combination was more toxic when administered before (22, 23). A study evaluating 90Y-MN-14 combined with different doses of doxorubicin or paclitaxel injected 1 or 2 days after radiolabeled antibody also showed that
75% of the MTD of the chemotherapeutic agent combined with the MTD of RIT was equitoxic to the MTD of RIT alone (24). It is noteworthy that hematotoxicity with this one-step RIT approach (7080% reduction of leukocytes) was markedly greater than in our study.
Doxorubicin is an intercalating agent that stabilizes the formation of complexes between topoisomerase II and DNA by altering the three-dimensional structure of DNA and inhibiting enzyme repair of radiation-induced single- and double-strand breaks (20). Thus, doxorubicin may act as a radiosensitizer. This would be especially important because RIT delivers irradiation with a low-dose rate, potentially allowing recovery of radiation-induced damage of DNA. Moreover, doxorubicin improves tumor oxygenation, which is an important factor for radiosensitivity. However, in the present study, doxorubicin alone showed no significant antitumor effect and did not improve the antitumor effect of RIT on MTC xenografts. This is clearly inconsistent with the results obtained by Behr et al. (12) in the same MTC xenograft model. These authors found that doxorubicin administered at its MTD (200 µg) induced a significant delay in tumor growth as compared with untreated controls (though with considerable variability in response) and that a combination of doxorubicin with 131I-MN-14 IgG produced a synergistic antitumor effect. However, the i.v. route used in this study for chemotherapeutic injections probably allowed greater delivery of the agent to the entire organism, as well as to the tumor, as suggested by the effects of the high toxicity involved (2030% weight loss and gastrointestinal side effects). It is also possible that i.p. administration led to slower distribution of the drug to tumor, precluding the expected radiosensitizing effect. Finally, unlike external radiotherapy, RIT delivers low-dose rate irradiation, allowing potential recovery of radiation-induced damage to DNA. The main radiosensitizing effect of doxorubicin appears to be the inhibition of enzyme repair of radiation-induced single- and double-strand breaks. After a single injection, the agent is briefly present in tumor during irradiation, so that damage repair is inhibited for only a short period. Thus, it might be useful to repeat injections, using suitable doses and time intervals.
Paclitaxel is a microtubule stabilizer with significant activity against a variety of solid tumors when used alone or in combination with other drugs (21, 27). In the present study, paclitaxel showed a significant antitumor effect against MTC xenografts and a significant increase in the antitumor efficacy of RIT. The latter was of the same order as that observed by Stein et al. (24), who used a different therapeutic scheme (the MTD of 90Y-MN-14 combined with 78% of the MTD of paclitaxel). The moderate toxicity found in our study suggests that a higher hapten activity could be administered or that the interval between BsMAb and hapten administration could be shorter, which would probably provide greater efficacy for chemoradiotherapy with AES reagents and paclitaxel (8, 18). Analysis of the response curves showed that this combination was additive and possibly synergistic when the effect was measured over a longer time interval. This is consistent with the fact that RIT produces long-term effects that have not yet been fully elucidated. In the animal, control of tumor growth was very long after a single administration of AES RIT, even when all tumor cells were not killed (8). In a Phase I clinical study of F6-734 BsMAb/131I-di-DTPA-In in relapsing MTC patients, tumor and biological responses occurred 36 months after RIT (5), and this is not an isolated observation. With respect to MTC, this slow response is probably related in part to the slow proliferation rate. The high antitumor potential of the RIT/paclitaxel combination may be because of several properties of the respective antitumor activities of these agents. In mammary or cervical tumor cell lines, paclitaxel has induced arrest in G2-M, the most radiosensitive phase of the cell cycle (28). In the MTC model of slowly proliferating tumor cells, histopathology has shown that a majority of cells are in G0 and early G1 phases of low radiosensitivity (18). Thus, paclitaxel may recruit more tumor cells in the radiosensitive phases of the cell cycle. Moreover, paclitaxel induced apoptosis in an animal model of mammary carcinoma, and low-dose rate irradiation also triggered apoptosis (28, 29). Potentiation could be expected if the pathways of apoptosis induction are different for radiation and the drug. Finally, paclitaxel may increase tumor oxygen pressure and enhance the effect of radioactivity (30).
In summary, this study shows that paclitaxel increased the effect of RIT in inhibiting tumor growth, without producing any significant increase of toxicity. Evaluation of the antitumor effect over a longer time period suggested that the combination is synergistic. It is noteworthy that chemoradiotherapy using AES reagents appeared to be less toxic (for the same efficacy) than that using directly radiolabeled antibodies. Other administration schedules, as well as repeated courses of RIT and paclitaxel, may achieve much better antitumor effects and with limited toxicity. Clinical trials, sponsored by IBC Pharmaceuticals (Morris Plains, NJ), are in progress to optimize the dose and administration schedule of a new AES RIT product. This study clearly suggests that combinations of AES RIT and paclitaxel or other antitumor taxanes should be tested in clinical conditions as soon as possible.
| Acknowledgments |
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| Footnotes |
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2 The abbreviations used are: MTC, medullary thyroid carcinoma; CEA, carcinoembryonic antigen; RIT, radioimmunotherapy; TCT, calcitonin; AES, affinity-enhancement system; MAb, monoclonal antibody; BsMAb, bispecific antibody; DTPA, diethylenetriaminepentaacetic acid; 131I-di-diethylenetriaminepentaacetic acid-In, bivalent diethylenetriaminepentaacetic acid-indium hapten labeled with iodine-131; MTD, maximum tolerated dose; di-DTPA, N
-(diethylenetriamine-N,N,N',N''-tetraacetic acid-N''-acetyl)-tyrosyl-N
-(diethylenetriamine-N,N,N',N''-tetraacetic acid-N''-acetyl)-lysine. ![]()
Received 10/ 5/01; revised 12/20/01; accepted 1/ 4/02.
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