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© 2000 American Society for Clinical Oncology Targeting Superficial Bladder Cancer by the Intravesical Administration of Copper-67Labeled Anti-MUC1 Mucin Monoclonal Antibody C595From the Departments of Urology and Pathology, City Hospital; Departments of Medical Physics and Radiology, University Hospital; and Cancer Research Laboratory, School of Pharmaceutical Sciences, University of Nottingham, Nottingham, United Kingdom; and Centre for Radiopharmaceutical Sciences, Paul Scherrer Institute, Villigen, Switzerland. Address reprint requests to Prof A.C. Perkins, Department of Medical Physics, University Hospital, Nottingham NG7 2UH, United Kingdom; email alan.perkins{at}nottingham.ac.uk
PURPOSE: More effective intravesical agents are required to limit the recurrence and progression of superficial bladder cancer. This study assessed the ability of copper-67 (67Cu)-C595 murine antimucin monoclonal antibody to bind selectively to superficial bladder tumors when administered intravesically, with a view to its development for therapy. PATIENTS AND METHODS: Approximately 20 MBq of 67Cu-C595 monoclonal antibody was administered intravesically to 16 patients with a clinical indication of superficial bladder cancer. After 1 hour, the bladder was drained and irrigated. Tissue uptake was assessed by imaging and by the assay of tumor and normal tissues obtained by endoscopic resection. RESULTS: Tumor was correctly identified in the images of 12 of 15 patients who were subsequently found to have tumors. Assay of biopsy samples at 2 hours showed a mean tumor uptake of 59.4% of the injected dose per kilogram (SD = 48.0), with a tumor-to-normal tissue ratio of 14.6:1 (SD = 20). After 24 hours (n = 5), this decreased to 4.3% of the injected dose per kilogram (SD = 2.9), with a tumor-to-normal tissue ratio of 1.8:1 (SD = 0.8). CONCLUSION: This study indicates a promising method for the treatment of superficial bladder cancer. Although the mean initial tumor uptake was high, effective therapy of bladder tumors will require an increased retention of the cytotoxic radionuclide in tumor tissue.
BLADDER CANCER is the fifth most common cancer in men and the fifth most common cause of death from cancer in men in the United Kingdom. The incidence in women is lower, but the disease still accounts for some 1,000 deaths per year among women in the United Kingdom. The majority of patients with bladder cancer present with superficial disease that is confined to the mucosa. However, 70% of these superficial bladder tumors will recur after endoscopic resection, and 20% progress to life-threatening invasive disease.1 Intravesical chemotherapy or immunotherapy is a promising approach, but studies to date have shown limited efficacy, particularly in preventing tumor progression, and significant side effects.2 Patients with poor prognostic factors or muscle-invasive disease need more radical treatment in the form of external-beam radiotherapy or cystectomy, both of which carry significant morbidity and mortality. More effective local treatments would reduce the recurrence rate of superficial tumors and decrease the number of patients for whom aggressive treatment for invasive disease is required. Radiation has been shown to be of value in treating patients with superficial tumors,3-6 and by using a suitable carrier for a radionuclide to target the therapeutic effect, damage to normal structures may be limited. MUC1 mucin is a glycoprotein present on normal urothelium that is aberrantly expressed in bladder cancer.7,8 Monoclonal antibody C595 (also known as NCRC48) is reactive with the protein core of MUC1 mucin.9 The target epitope of the C595 antibody is the tetrameric motif Arg-Pro-Ala-Pro that is repeated many times within the MUC1 protein core. The reactivity of the C595 antibody with synthetic peptides containing this motif permits efficient antibody purification using peptide-epitope affinity chromatography, which, unlike other methodologies, enables exclusion recovery of functionally active antibody. We have previously shown antibody localization of bladder tumors after intravesical administration of indium-111 (111In)labeled C595 with a mean tumor uptake to normal tissue ratio of 12:1.10 111In is suitable for imaging, but with no beta emission, it is poorly suited for therapy. We have sought an alternative radiolabel with both a gamma emission for external imaging and also a beta emission with suitable characteristics for therapy. Most clinical studies for radioimmunotherapy (RIT) have used yttrium-90 or iodine-131 (131I) as the therapeutic moiety. As a pure beta emitter, yttrium-90 is unsuitable for imaging. Although 131I is inexpensive, widely available, and well established as a radiolabel, it has several major drawbacks, including a predominant gamma emission of 364 keV (82% abundance), which accounts for two thirds of the absorbed dose equivalent of this nuclide, and a relatively long physical half-life (8 days).11 To achieve higher tumor doses without compromising patient or staff, radionuclides with better physical characteristics are required. Copper-67 (67Cu) has a gamma emission of 185 keV (42% abundance) that is suitable for imaging and, together with a physical half-life of 2.6 days, results in a lower systemic radiation burden to the patient and staff. Its beta particle emission with a maximum range in tissue of 2.2 mm is appropriate for the treatment of small tumor deposits.12 The present study was performed to assess the ability of 67Cu-labeled C595 monoclonal antibody to bind preferentially to superficial bladder tumors when administered intravesically, with a view to its development for therapy.
Monoclonal Antibody The monoclonal antibody C595 (murine IgG3 immunoglobulin) was labeled with 67Cu using the 14N4 macrocycle. The antibody production, purification, radiolabeling method, and preclinical testing have previously been described in detail.13 The immunoreactive fraction of the labeled antibody for each administration was assessed by its capacity to rebind to peptide-epitopeconjugated Sepharose beads (Pharmacia Biotech, Uppsala, Sweden).13
Patients, Imaging, and Surgery
Immunohistochemistry
Assessment of Systemic Absorption Monoclonal antibody C595 (50 µL at 5 µg/mL) was applied to one half of a 96-well plate and allowed to adsorb overnight at 4°C, with the other one half coated with phosphate-buffered saline (PBS; pH 7.3) as control. Plates were washed three times with PBS, 0.02% (weight-to-volume ratio) Tween-20 (Sigma Chemical Co, Poole, United Kingdom) and then blocked with 1% bovine serum albumin in PBS. Serum samples taken from patients before and 3 weeks after antibody administration were diluted 1/30 in 1% bovine serum albumin in PBS for assay and applied in triplicate to each side of the plate. Positive control serum was obtained from a patient who was known to have developed an antimouse response after previous systemic administration of the C595 antibody in a prior study.14 The sample obtained from each patient before antibody administration was used as the negative control for the respective postinjection sample. The plates were washed again three times before adding 50 µL of horseradish-peroxidase conjugated rabbit anti-IgM/G immunoglobulins (Dako), diluted 1/1,000 with PBS. After a further wash at 1 hour, the reaction was developed with 2,2 azinodi(3-ethylbenz-thiazoline-6-sulfonic acid) (ABTS; Sigma Chemical Co) in citrate phosphate buffer (pH 4.0) and 0.1% (volume-to-volume ratio) hydrogen peroxide. Plates were read on a spectrophotometer at 405 nm. HAMA levels between the pre- and postdose group as a whole were compared using the Wilcoxon signed-rank test. Individual pre- to postinjection samples were compared using an unpaired t test.
Radiolabeling The mean labeling efficiency of 67Cu incorporation to the antibody protein was 55%. The mean activity administered to patients was 15.8 MBq (SD = 3.6; n = 16), conjugated to 0.5 mg of C595 antibody. The mean immunoreactive fraction of the 67Cu labeled antibody was 87.6% (SD = 12.5; n = 10).
Immunoscintigraphy
Biopsy Samples The assay of radioactivity in the resected tumor and normal urothelial specimens from 14 of the 15 patients with bladder tumors was carried out. Nine patients underwent surgical tumor resection at approximately 2 hours, and the remaining five patients underwent surgery at 24 hours. Data are presented in Fig 3.
For patients who underwent tumor resection at 2 hours, the mean percentage of injected dose per kilogram of tumor was 59.4% (SD = 48) and the mean percentage of injected dose per kilogram of normal tissue was 9.1% (SD = 9.1). The mean tumor-to-normal ratio in this group was 14.6 (SD = 20). For patients who underwent resection at 24 hours, the mean percentage of administered dose per kilogram of tumor was 4.3% (SD = 2.9) and the mean percentage of administered dose per kilogram of normal tissue was 2.9% (SD = 1.51). In this group, the mean tumor-to-normal ratio was 1.8 (SD = 0.8). There was a significant difference between the mean counts of tumor and normal specimens at 2 hours after instillation (P = .016), but the difference was no longer significant at 24 hours (P = .30).
Correlation of Uptake With Tumor Stage and Grade
Immunohistochemistry A comparison of imaging results with MUC1 expression was obtained by immunohistochemistry. Tumor specimens were obtained at 2 and 24 hours after surgery. Paraffin blocks were prepared, sectioned, and processed for immunohistology. In control tests when antibody diluent alone was used, no staining with secondary immunoconjugate was observed, which indicates that possible residual tumor-bound C595 antibody remaining after intravesical administration at either 2 or 24 hours before resection was not detected by this procedure. Therefore, the actual distribution of instilled antibody was not revealed in these tests. Conversely, when C595 antibody was added to sections (at an antigen-saturating concentration to reveal the distribution of MUC1 mucin), it was noted that tumors that had imaged well by immunoscintigraphy also showed increased staining with C595 antibody. Thus the immunohistology results, which confirmed variability of MUC1 expression between samples, correlated with the results of external-gamma camera imaging. Examples of strong and weak tissue staining are illustrated in Fig 4.
Systemic Uptake and HAMA Response Assay of the level of radioactivity in serum samples taken from patients after instillation of the 67Cu-C595 into the bladder showed that there was no detectable increase above background. There was also no significant change in the measured level of HAMA when comparing individual preinjection samples with postinjection samples (.1 < P < .5) and all preinstallation samples with postinstillation samples (P = .69) after the intravesical administration of 67Cu-C595 monoclonal antibody.
Radioimmunotherapy, using the antibody-targeted approach, offers an attractive alternative to conventional chemotherapy. Although the results of therapeutic trials have been encouraging in the treatment of diffuse malignancies such as leukemia, there has been little progress toward the effective treatment of solid tumors. As a result, some researchers have adopted direct tumor injection or alternative routes of administration to optimize the treatment. In the case of superficial bladder cancer, the intravesical approach provides an attractive and well-tolerated route of administration. In the present study, the production of radiolabeled antibody with a reproducibly high immunoreactive fraction confirms the reliability of the labeling method and suitability of the 67Cu-C595 monoclonal antibody for radioimmunoscintigraphy. The mean tumor-to-normal ratio of almost 15:1 shown by this study is encouraging, although the SD of 20 is larger than would be hoped for, which indicates that this form of therapy may be more effective for some patients than for others. These results are similar to the 12:1 ratio previously shown by Kunkler et al10 using 111In-C595. Nevertheless, both of these studies demonstrate the preferential reaction of C595 antibody for tumor relative to normal urothelium. It is possible that the C595 antibody possesses a higher affinity for bladder tumorassociated MUC1 than other anti-MUC1 antibodies that have been administered intravesically, such as HMFG1 and HMFG2.15,16 Both the level of antibody uptake and the tumor-to-normal ratios were significantly higher in the present study in comparison with those reported in other intravesical studies using antimucin antibody15,16 and the monoclonal antibody AUA1.17 We demonstrated positive bladder tumor imaging in 80% of patients, with the three false-negative scans occurring in patients with low-grade, noninvasive (G1pTa) tumors. These tumors have the lowest biologic potential for progression and invasion and are therefore of less clinical significance. Enhanced tumor localization in terms of tissue counts and imaging was seen in the higher-grade tumors. These have the greatest risk of recurrence and progression to invasive disease and would be the main target tumors for a therapeutic conjugate. Although there is high variation in MUC1 expression by different tumors, it is possible to determine the relative levels of expression and select those patients who are likely to respond to this form of therapy. This could achieved via immunostaining of tumor biopsies taken at the time of the diagnostic cystoscopy and also via immunoscintigraphy with a diagnostic rather than therapeutic dose of radionuclide. One problem with the method of obtaining tumor biopsy material via an irrigating cystoscope in the bladder is that this procedure may wash off some of the bound antibody. As a result, the uptake shown in the images would appear to be higher than that shown by direct assay of radioactivity in tissues. To achieve a cytotoxic tumor dose, the residence time should be maximized. Further study is underway to identify the form of and reason for the loss of activity with time. We are also currently exploring a number of different strategies to deliver an effective therapeutic tumor dose, including increasing the specific activity of the radioimmunoconjugate, increasing the residence time to encourage internalization of radiolabeled antibody, and developing new antibody constructs. We have developed a single-chain fragment and diabody with the same epitope as the parent Mab, but the smaller molecular weight of these molecules should increase tissue penetration without causing systemic absorption.18,19 An important finding from this study, which has also been demonstrated by others who have studied intravesical administration of antibody conjugates, is that no detectable amounts of antibody or radionuclide were absorbed systemically from the bladder. This will, therefore, allow the safe administration of multiple doses to achieve a cumulative therapeutic effect, in a manner that is similar to current intravesical therapies. The findings of the immunohistochemistry confirm the heterogeneous MUC1 expression with areas of high and low staining and activity. With antibody-targeted chemotherapy, the whole tumor population needs to bind antibody for cytotoxic therapy to be successful, and for reasons of limited antibody penetration and low antigen expression in some areas of the tumor, this may not be likely. This supports the use of a therapeutic radionuclide as the "warhead" attached to the selective antibody carrier rather than a cytotoxic drug. By the use of an appropriate radionuclide with a suitable length of beta emission to produce a "bystander" effect, the problem of heterogeneous and low antigen expression may be reduced. Immunohistology demonstrated that the pattern of staining was more appropriate for targeting of a medium energy beta emitter than a cytotoxic drug. We selected 67Cu on the basis of its suitable beta emission, stable radiolabeling, and favorable biodistribution. Previous in vivo studies using 67Cu-labeled anti-CEA antibody in a nude mouse tumor xenograft system have indicated a distinct advantage to be gained from using this metallic nuclide, in that activity associated with 67Cu accumulation in target tumors was found to be up to two times that observed with 131I.20 In addition, antibody targeting of 67Cu with a maximum tissue penetration of 2.2 mm should eliminate the extravesical complications that are associated with external-beam radiotherapy. Assuming that the complex is stable at the tumor site, extrapolation from the data obtained suggests that it is theoretically possible to deliver a therapeutic dose as great as 25 Gy to a 5-g superficial tumor. We therefore anticipate that, in practice, a course of repeated administration would be necessary for effective tumor therapy. Radiolabeling with therapeutic amounts of 67Cu-C595 is in progress, and a dose-escalation study is planned to obtain additional patient data on tumor dosimetry. One unique advantage of the natural access to the bladder via the urethra is not only the ease of introducing the conjugate directly into the bladder, but also the easy means of assessing the response to therapy. A therapeutic trial of this will first be tested using the marker tumor concept as recommended by the European Organization for Research and Treatment of Cancer21 to study the efficacy and incidence of side effects. Studies will use the 67Cu-C595 radioimmunoconjugate as adjuvant treatment to transurethral resection of the tumor.
Funded by project grant no. 2168 from the Cancer Research Campaign United Kingdom and a grant from Trent Regional Health Authority United Kingdom. Radionuclide production and antibody labeling was performed by the Paul Scherrer Institute research facility in Villigen, Switzerland.
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Copyright © 2000 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
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