Elsevier

Nuclear Medicine and Biology

Volume 33, Issue 8, November 2006, Pages 1065-1072
Nuclear Medicine and Biology

Fractionated radioimmunotherapy of intraperitoneally growing ovarian cancer in nude mice with 211At-MX35 F(ab′)2: therapeutic efficacy and myelotoxicity

https://doi.org/10.1016/j.nucmedbio.2006.07.009Get rights and content

Abstract

Objective

The aim of this study was to investigate the therapeutic efficacy and myelotoxicity during fractionated radioimmunotherapy of ovarian cancer in mice. The study was performed using the monoclonal antibody MX35 F(ab′)2 labeled with the α-particle emitter 211At.

Methods

Animals were intraperitoneally inoculated with ∼1×107 cells of the cell line NIH:OVCAR-3. Four weeks later, the mice were given the first treatment. Six groups of animals were intraperitoneally injected with ∼800, 3× ∼267, ∼400, 3× ∼133, ∼50 or 3× ∼17 kBq 211At-MX35 F(ab′)2 (n=18 in each group). The second and third injections for Groups 2, 4 and 6 were given 4 and 8 days after the first injection, respectively. As controls, animals were treated with unlabeled MX35 F(ab′)2 (n=12). Eight weeks after the last injection, the animals were sacrificed and the presence of macro- and microscopic tumors and ascites was determined. Blood counts were determined for each mouse in Groups 1 and 2 before the first injection and 3, 7, 11, 15 and 23 days after the first injection. The calculation of the mean absorbed dose to the bone marrow was based on the ratio between the 211At-activity concentration in bone and blood [i.e., the bone-to-blood ratio (BBLR)] as well as that between the 211At-activity concentration in bone marrow and blood [i.e., the bone-marrow-to-blood ratio (BMBLR)] and the cumulated activity and absorbed fraction of the α-particles emitted by 211At in the bone marrow.

Results

The tumor-free fractions of animals were 56% and 41% when treated with ∼800 kBq and 3× ∼267 kBq 211At-MX35 F(ab′)2, respectively; 39% and 28% when treated with ∼400 kBq and 3× ∼133 kBq 211At-MX35 F(ab′)2, respectively; and 17% and 22% when treated with ∼50 kBq or 3× ∼17 kBq 211At-MX35 F(ab′)2, respectively. The nadir of the white blood cell (WBC) counts was decreased (from 46% to 19%, compared with the baseline WBC counts) and delayed (from Day 4 to Day 11 after the first injection) during the fractionated treatment compared with the single-dose treatment. The percentage of injected activity per gram (%IA/g) for blood, bone and bone marrow all peaked 6 h after injection at 13.80±1.34%IA/g, 4.00±0.69%IA/g and 8.28±1.38%IA/g, respectively. The BBLR and BMBLR were 0.20±0.04 and 0.58±0.01, respectively. The mean absorbed dose to bone marrow was ∼0.4 Gy after intraperitoneally injecting ∼800 kBq 211At-MX35 F(ab′)2.

Conclusion

No advantage was observed in the therapeutic efficacy of using a fractionated regimen compared with a single administration, with the same total amount of administered activity. Alleviation of the myelotoxicity was observed during the fractionated regimen in terms of decreased suppression and delayed nadir of the WBC counts. No thrombocytopenia was observed during either regimen.

Introduction

Ovarian cancer frequently recurs through micrometastatic growth on the peritoneal surface, and patients succumb due to advanced peritoneal carcinomatosis including ascites, in spite of debulking surgery and systemic chemotherapy. External abdominal radiotherapy proved to be unsuccessful due to limitations on the absorbed dose to normal tissue. Therefore, adjuvant locoregional treatment with intraperitoneal injections of specific antibodies could be decisive in the treatment of remaining micrometastatic disease. In this study, we used the monoclonal antibody (mAb) MX35 F(ab′)2, which recognizes a cell surface glycoprotein of ∼95 kDa on ovarian tumor cells. The cytotoxicity was mediated by labeling the mAbs with an α-particle-emitting radionuclide, 211At. We used an animal model relevant in clinical oncology, which allows intraperitoneal radioimmunotherapy (RIT). The intraperitoneal approach increases the absorbed dose to the tumor and reduces myelotoxicity as the clearance rate from the peritoneal cavity to the systemic circulation is delayed compared with intravenous injection. Fractionated RIT (FRIT) could possibly alleviate the myelotoxicity and increase tumor nodule sterilization by a gradual peeling away of the outermost cells of larger tumors. Alleviation of myelotoxicity could also provide the possibility of an increase in the therapeutic efficacy, by allowing a larger total amount of activity to be administered.

Some studies have been performed on RIT of ovarian cancer, mostly mAbs labeled with 90Y and 131I, in both animals and humans [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Studies of FRIT of cancer in general have also been performed [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], to some extent indicating a possibility of increasing the therapeutic index. However, due to the fact that the β-particles emitted have too long a range for the treatment of microscopic tumors, we believe that it is important to investigate the therapeutic efficacy of mAbs labeled with α-particle emitters when treating microscopic disease on the peritoneum. In this study, we used the α-particle emitter 211At, with a half-life of 7.21 h, a mean range in tissue of ∼62 μm and a mean linear energy transfer (LET) of ∼111 keV/μm. The short half-life of this radionuclide makes it ideal for local treatment as the target cells are easily reached and as the transfer of the radioimmunocomplex to the systemic circulation is slow. The short range ensures a significant absorbed dose to very small tumors or even single cells [31]. The high LET, together with a high relative biological effectiveness (RBE), of the α-particles indicates that only a small number of 211At atoms have to be targeted to sterilize the cell. We have previously investigated the therapeutic efficacy of the intact specific IgG1 mAbs MOv18 and MX35 as well as fragmented mAbs [MX35 F(ab′)2 and nonspecific rituximab F(ab′)2] in the treatment of nude mice with ovarian cancer [32], [33], [34], [35], [36], [37]. Those studies showed good therapeutic efficacy with a tumor-free fraction (TFF; i.e., percentage of animals with no macro- and microscopic tumor growth and no ascites) of ∼60% when injecting ∼400 kBq 211At-MX35 3 weeks after cell inoculation, as well as an increasing importance of using a specific mAb compared with a nonspecific mAb when treating larger tumors. An activity of 400 kBq was chosen in the earlier studies based on indications that it should be within the toxic limit [38], [39]. Later studies have shown that the toxic limit is higher, that is, ∼1300 kBq [40]. However, an increase in the injected activity above ∼400 kBq does not improve the therapeutic outcome, probably because an increase in the absorbed dose to the tumors is prevented by restricted diffusion and saturation effects [35].

The aim of the present study was to investigate the therapeutic efficacy and myelotoxicity during FRIT of ovarian cancer in nude mice. The study was performed using the mAb MX35 F(ab′)2 labeled with the α-particle emitter 211At.

Section snippets

Radionuclide

211At was produced by the 209Bi(α,2n)211At reaction in a cyclotron (Scanditronix MC32 at the Positron Emission Tomography and Cyclotron Unit, Rigshospitalet, Copenhagen, Denmark) by irradiating a 209Bi target with 28-MeV α-particles. The 211At was isolated using a dry-distillation procedure [41].

Monoclonal antibodies

MX35 is a murine IgG1-class mAb, developed and characterized at the Memorial Sloan-Kettering Cancer Center (MSKCC), New York, USA. MX35 is directed toward a cell surface glycoprotein of ∼95 kDa on

Labeling and immunoreactivity of antibodies

The radiochemical yields were 30–40%, and the radiochemical purity was more than 95% as determined by methanol precipitation and gel-permeability chromatography. The immunoreactivity measurements of the 211At-MX35 F(ab′)2 and OVCAR-3 cells gave an immunoreactive fraction, r, of 0.98.

Therapeutic efficacy and myelotoxicity

The TFFs were 56% and 41% (P>.5) when treatment was given with a total of ∼800 kBq MX35 F(ab′)2 as a single dose (Group 1) and via fractionated administration (Group 2: 3× ∼267 kBq), respectively (Table 1). The TFFs

Discussion

Some reasons why FRIT appears promising are the possibility of (a) reducing the systemic toxicity and, hence, increasing the maximum tolerated activity (MTA); (b) achieving a more uniform absorbed dose distribution in the tumor; and (c) increasing the therapeutic index [13].

Regarding systemic toxicity in the clinical case, large, single administrations during RIT sometimes necessitate progenitor cell rescuing bone marrow transplantation (BMT). FRIT could decrease or even eliminate the need for

Conclusion

No advantage was observed in the therapeutic efficacy of a fractionated regimen compared with a single administration, with the same total amount of administered 211At-MX35 F(ab′)2. However, alleviation of myelotoxicity was observed following the fractionated regimen and delayed nadir of the WBC counts. No thrombocytopenia was observed during either the fractionated or the single-administration regimens. The results indicate the possibility of increasing the MTA when a fractionated regimen is

Acknowledgments

This work was supported by grants from the Swedish Cancer Society (No. 3548) and the King Gustaf V Jubilee Clinic Research Foundation in Göteborg, Sweden. We would like to thank Dr. Chaitanya R. Divgi, Director of Targeted Radiotherapy at the MSKCC, for making the mAb MX35 F(ab′)2 available.

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