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

To conjugate trastuzumab with/without NLS peptides to G4 PAMAM dendrimers derivatized with DTPA and determine the specific radioactivity (SA) for In-111 labeling, HER2 immunoreactivity and cytotoxicity on breast cancer (BC) cells. 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 mu g with 60 MBq of In-111. HER2 immunoreactivity, internalization and nuclear importation were measured. The effect of In-111-DTPA-G4-trastuzumab (5.9 MBq/mu g) on the clonogenic survival (CS) of SK-Br-3 or MDA-MB-231 cells with high or low HER2 density, respectively was compared to In-111-DTPA-NLS-trastuzumab (0.5 MBq/mu g). DNA double-strand breaks (DSBs) were measured. DTPA-G4-trastuzumab was labeled with In-111 to a SA (23.6 MBq/mu g) which was 100-fold higher than In-111-DTPA-NLS-trastuzumab. In-111-DTPA-G4-trastuzumab and In-111-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 In-111-DTPA-NLS-trastuzumab which was associated with an increase in DNA DSBs. Conjugation of trastuzumab to G4 PAMAM dendrimers modified with 30 DTPA permitted high SA In-111 labeling which increased their cytotoxic potency for BC cells with high or low HER2 density.