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Clinical Investigations |
Departments of Nuclear Medicine [C-H. K.] and Surgery [T-J. L.], Taichung Veterans General Hospital, Taichung; Department of Nuclear Medicine, Show-Chwan Memorial Hospital, Chunghua [S-C. T.]; Department of Radiology, Jen-Ai Hospital, Taichung [Y-J. H.]; Department of Medical Research, Chi-Mei Medical Center, Tainan [J-J. W.]; School of Medicine, National Defense Medical Center, Taipei [S-T. H.]; and Department of Nuclear Medicine, Chung-Shan Medical and Dental College Hospital, Taichung [S-P. C.], Taiwan
| ABSTRACT |
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| INTRODUCTION |
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Tc-MIBI has been reported to be a transport substrate for the Pgp and MRP pump mechanism (5 , 6) . Not surprisingly, resistant cancer cells showing Pgp or MRP expression were found to transport Tc-MIBI outside the cells. Thus, information from Tc-MIBI scintimammography may be useful in the prediction of breast cancer response to chemotherapy and could guide the design of the most effective therapy protocols. However, widespread clinical use of Tc-MIBI scintimammography for noninvasive determination of Pgp and MRP expression is hampered by the lack of complete clinical studies. The need for extended clinical studies to supplement the existing data provided the basis for this study, which was an attempt to determine whether or not there is any association between Pgp or MRP expression and Tc-MIBI cancer uptake ratios calculated from Tc-MIBI scintimammography in patients with breast cancer.
| PATIENTS AND METHODS |
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Tc-MIBI Scintimammography.
Before biopsy or fine needle aspiration, all patients received Tc-MIBI
scintimammography. The imaging procedure was started 30 min after oral
intake of 500 mg of perchlorate to prevent abnormal uptake of free
Tc-99m pertechnetate. A commercial methoxyisobutylisonitrile
preparation [maximum 5.56 GBq (150 mCi) in approximately 13
ml] was obtained from Dupont Co. (Cardiolite). Labeling and
quality control procedures were carried out according to the
manufacturers instructions. Labeling efficiencies were always higher
than 95%. Each patient received an i.v. injection of 740 MBq (20 mCi)
of Tc-MIBI in the arm contralateral to the breast with the abnormality.
Three planar images were obtained starting 10 min after injection of
Tc-MIBI using a gamma camera equipped with a low-energy,
high-resolution collimator. The energy peak was centered at 140 keV
with a 10% window. The imaging sequence for each patient was as
follows: (a) a lateral planar image of the breast with the
tumor in a prone position; (b) another lateral planar image
of the other breast in the same prone position; and (c) an
anterior planar image of the bilateral breasts in a supine position
with the patients arms raised behind her head. Each planar image was
recorded on a 256 x 256 matrix for 5 min, and a total
count of at least 1500 kilocounts was collected for each planar image.
For the bilateral planar images, all of the patients were imaged in a
prone position using a plastic table overlay that allowed the breast
being imaged to be freely dependent from the imaging table. The prone
planar image of a single dependent breast provided maximal separation
of breast tissue from the myocardium and liver, as well as exclusion of
any activity present in the opposite breast. The findings of Tc-MIBI
scintimammography were evaluated as follows. Based on the prone lateral
planar image of the breast with the tumor, a ROI was carefully drawn
around the tumor, and then another ROI of the same size was drawn over
the surrounding normal breast tissues. The T:B ratio was calculated by
the following formula: (total counts in the ROI over the
tumor) ÷ (the total counts in the same size ROI over
the surrounding normal breast tissues). Because the T:B ratio was
calculated from the same prone lateral image, image decay correction
was not necessary.
IHA.
The surgical breast specimens were laid on ice, embedded in Tissue-Tek
OCT (Miles Scientific), and maintained at -70°C. The specimens were
kept in liquid nitrogen. For Pgp immunohistochemical staining (8
, 9)
, endogenous peroxidase was blocked by 3% hydrogen peroxide
for 15 min. Antigen retrieval was performed by treatment with enzyme
digestion in 0.1% trypsin in PBS for 5 min at room temperature and
inhibited with 10% skim milk in PBS for 5 min. The sections were
incubated for 2 h in a moist chamber at 37°C with primary
antibody JSB-1 (50 µg/ml; Boehringer Mannheim Biochemica,
Germany) at a 1:50 concentration. After three 5-min washes in
PBS buffer, detection of the primary antibody was performed with a link
antibody according to the manufacturers instructions (DAKO LSAB 2
System, Peroxidase; Dako Corp., Carpinteria, CA). For MRP
immunohistochemical staining (1
, 3
, 4)
, antigen retrieval
was performed by treatment in a microwave oven for 5 min in citrate
buffer at 700 W. Endogenous peroxidase was blocked by 3% hydrogen
peroxide for 15 min followed by 5 min in PBS buffer. The sections were
incubated overnight in a moist chamber at 4°C with primary antibody
MRP QCRL-1 (10 µg/ml; Signet Laboratories, Inc., Dedham, MA) at a
1:100 concentration. Pgp and MRP expression was interpreted as follows
by an experienced pathologist blinded to the clinical outcome:
negative, <10% stained tumor cells; and positive,
10% stained
tumor cells (Refs. 10, 11, 12, 13
; Figs. 1
and 2
).
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2 tests. If the P was < 0.05, the difference was considered to be significant. | RESULTS |
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| DISCUSSION |
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Nuclear medicine techniques that rely on the biochemical and physiological characteristics of a tumor have been used to evaluate breast cancer. Recently, Tc-MIBI has been suggested as a radiopharmaceutical with a great deal of promise in imaging breast cancer (14) . Pgp and MRP, which together function as ATP-dependent efflux pumps, recognize certain chemotherapeutic agents as substrates and prevent the accumulation of lipophilic and cationic radiopharmaceuticals such as Tc-MIBI (5 , 6) . There have been reported differences in the methods of data acquisition and analysis between evaluation of Tc-MIBI washout (efflux rate constants; Refs. 15, 16, 17 ) and calculation of Tc-MIBI uptake (T:B ratios; Refs. 10 and 11 ), as in this study; however, there is no definite conclusion as to which one is better for correlation to Pgp expression. Because calculation of T:B ratios is less time-consuming and more easily available than evaluation of efflux rate constants in clinical practice, we selected semiquantitative T:B ratios from Tc-MIBI scintimammography to compare Pgp and MRP expression in human breast cancer. In addition, it is very time-consuming and almost impossible to calculate the actual percentage of Pgp and MRP cells by visual interpretation on IHA. Therefore, we could not evaluate the correlation with the actual percentage of Pgp or MRP cells and the T:B ratio of Tc-MIBI scintimammography.
In the current study, we found that tumor size, nodal status, and
presence of metastatic disease (TNM stage); receptor status (estrogen
receptor content); and histological grade did not correlate with Pgp or
MRP expression at diagnosis (Table 1)
. Our findings were similar to and
supported by those of some previous studies (3
, 4)
.
However, in the study by Filipits et al. (1)
,
positive MRP staining was more frequent in T3 and
T4 tumors than in T1 and
T2 tumors. Although Pgp expression was reported
to be associated with a shorter duration of overall survival of breast
cancer patients (18)
, the prognostic and predictive
significance of Pgp and MRP expression in breast cancers is still
unknown. Thus, additional studies are required to determine whether Pgp
and MRP expression in breast cancers affected disease-free survival and
the response to chemotherapy. In addition, among the four groups, we
found that the T:B ratios calculated from Tc-MIBI scintimammography
were lowest in group 1 patients and highest in group 4 patients. The
T:B ratios of groups 2 and 3 were between those of groups 1 and 4 (Fig. 3)
. Therefore, our results indicate that there is both MRP and Pgp
expression in primary breast cancers. This finding was also reported in
a previous study (1)
.
In light of this consistent finding, Tc-MIBI scintimammography could be used as a noninvasive test for guiding treatment when Pgp or MRP expression is pertinent. However, some patients did not receive chemotherapy, and most of the cases were still alive when the study was completed; therefore, we did not calculate the correlation between disease-free survival or response to chemotherapy and Tc-MIBI scintimammography. We believe that the accuracy of our findings should be reproduced by additional studies with large numbers of patients.
| FOOTNOTES |
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1 Supported in part by grants from Taichung
Veterans General Hospital (TCVGH-896708D) and the National Science
Council (NSC 89-2320-B-075A-001, 88-2314-B-075A-006), Taiwan. ![]()
2 To whom requests for reprints should be
addressed, at Department of Nuclear Medicine, Taichung Veterans General
Hospital, 160 Taichung Harbor Road, Section 3, Taichung 407, Taiwan.
Phone: 886-4-23741349; Fax: 886-4-23741348; E-mail: kaoch{at}vghtc.vghtc.gov.tw ![]()
3 The abbreviations used are: Pgp, P-glycoprotein;
Tc-MIBI, technetium-99m methoxyisobutylisonitrile; MRP, multidrug
resistance-related protein; T:B, tumor:background; IHA,
immunohistochemical analysis; ROI, region of interest; TNM,
tumor-node-metastasis. ![]()
Received 6/ 5/00. Accepted 12/11/00.
| REFERENCES |
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