Our objective was to determine whether the internalization and nuclear tran
slocation of human epidermal growth factor (hEGF) after binding to its cell
surface receptor (EGFR) could be exploited to deliver the Auger electron e
mitter In-111 into EGFR-positive breast cancer cells for targeted radiother
apy. Methods: hEGF was derivatized with diethylenetriamine pentaacetic acid
(DTPA) and radiolabeled with In-111-acetate. The internalization of In-111
-DTPA-hEGF by MDA-MB-468 breast cancer cells (1.3 x 10(6) EGFRs/cell) was d
etermined by displacement of surface-bound radioactivity by an acid wash. T
he radioactivity in the cell nucleus and chromatin, isolated by differentia
l centrifugation, was measured. The effect on the growth rate of MDA-MB-468
or MCF-7 (1.5 x 10(4) EGFRs/cell) cells was determined after treatment in
vitro with In-111-DTPA-hEGF, unlabeled DTPA-hEGF, or (111)InDTPA. The survi
ving fraction of MDA-MB-468 or MCF-7 cells treated in vitro with In-111-DTP
A-hEGF was determined in a clonogenic assay. The radiotoxicity in vivo agai
nst normal hepatocytes or renal tubular cells was evaluated by measuring al
anine aminotransferase (ALT) or creatinine levels in mice administered high
amounts of In-111-DTPA-hEGF (equivalent to human doses up to 14,208 MBq) a
nd by light and electron microscopy of the tissues. Results: Approximately
70% of In-111-DTPA-hEGF was internalized by MDA-MB-468 cells within 15 min
at 37 degrees C and up to 15% was translocated to the nucleus within 24 h.
Chromatin contained 10% of internalized radioactivity. The growth rate of M
DA-MB-468 cells was decreased 3-fold by treatment with In-111-DTPA-hEGF (45
-60 mBq/cell). Treatment with unlabeled DTPA-hEGF caused a 1.5-fold decreas
e in growth rate, whereas treatment with In-111-DTPA had no effect. Targeti
ng of MDA-MB-468 cells with up to 130 mBq/cell of In-111-DTPA-hEGF resulted
in a 2-logarithm decrease in their surviving fraction. No decrease in the
growth rate or surviving fraction of MCF-7 cells was evident. There was no
evidence of hepatotoxicity or renal toxicity in mice administered high amou
nts of In-111-DTPA-hEGF. Radiation dosimetry estimates suggest that the rad
iation dose to an MDA-MB-468 cell targeted with In-111-DTPA-hEGF could be a
s high as 25 Gy with up to 19 Gy delivered to the cell nucleus. Conclusion:
In-111-DTPA-hEGF is a promising novel radiopharmaceutical for targeted Aug
er electron radiotherapy of advanced, hormone-resistant breast cancer.