Trastuzumab labeled to high specific activity with ¹¹¹In by conjugation to G4 PAMAM dendrimers derivatized with multiple DTPA chelators exhibits increased cytotoxic potency on HER2-positive breast cancer cells.

PURPOSE: To conjugate trastuzumab with/without NLS peptides to G4 PAMAM dendrimers derivatized with DTPA and determine the specific radioactivity (SA) for (111)In labeling, HER2 immunoreactivity and cytotoxicity on breast cancer (BC) cells.

METHODS

G4 dendrimers were reacted with DTPA then conjugated through a thiol to maleimide-derivatized trastuzumab. The SA achievable was determined by incubating 2 to 20 μg with 60 MBq of (111)In. HER2 immunoreactivity, internalization and nuclear importation were measured. The effect of (111)In-DTPA-G4-trastuzumab (5.9 MBq/μg) on the clonogenic survival (CS) of SK-Br-3 or MDA-MB-231 cells with high or low HER2 density, respectively was compared to (111)In-DTPA-NLS-trastuzumab (0.5 MBq/μg). DNA double-strand breaks (DSBs) were measured.

RESULTS

DTPA-G4-trastuzumab was labeled with (111)In to a SA (23.6 MBq/μg) which was 100-fold higher than (111)In-DTPA-NLS-trastuzumab. (111)In-DTPA-G4-trastuzumab and (111)In-DTPA-G4-NLS-trastuzumab retained HER2 immunoreactivity and were internalized and imported into the nucleus of BC cells. G4-radioimmunoconjugates were 2-4 fold and 9-fold more cytotoxic to SK-Br-3 and MDA-MB-231 cells, respectively than (111)In-DTPA-NLS-trastuzumab which was associated with an increase in DNA DSBs.

CONCLUSIONS

Conjugation of trastuzumab to G4 PAMAM dendrimers modified with 30 DTPA permitted high SA (111)In labeling which increased their cytotoxic potency for BC cells with high or low HER2 density.