Introduction: Our objective was to compare the cell penetration and nuclear importation properties of 111In-labeled and 123I-labeled immunoconjugates (ICs) composed of 16-mer peptides (GRKKRRQRRRPPQGYG) derived from HIV-1 transactivator of transcription (tat) protein and anti-mouse IgG (mIgG) in BT-474 breast cancer (BC) cells.
Methods: [111In]tat ICs were constructed by site-specific conjugation of tat peptides to NaIO4(-)-oxidized carbohydrates in the Fc domain of diethylenetriaminepentaacetic-acid-modified anti-mIgG antibodies. Immunoreactivity against mIgG was assessed in a competition assay. The kinetics of the accumulation of [111In]anti-mIgG-tat IC and [123I]anti-mIgG-tat ICs in BT-474 cells and the elimination of radioactivity from cells, cytoplasm or nuclei were determined. The effects of excess tat peptides or NH4Cl (an inhibitor of endosomal acidification) on cellular uptake and nuclear importation of [111In]anti-mIgG-tat were measured.
Results: [111In]anti-mIgG-tat was >97% radiochemically pure and exhibited preserved immunoreactivity with mIgG epitopes. [123I]Anti-mIgG-tat penetrated BT-474 cells more rapidly than [111In]anti-mIgG-tat ICs and achieved a 1.5-fold to a 2-fold higher uptake in cells and nuclei. Cell penetration and nuclear uptake of [111In]anti-mIgG-tat were inhibited by excess tat peptides and NH4Cl. Elimination of radioactivity from BT-474 cells and nuclei was more rapid and complete for 123I-labeled than for 111In-labeled anti-mIgG-tat ICs.
Conclusion: Tat peptides derived from HIV-1 tat protein promoted the penetration and nuclear uptake of radioactivity following the incubation of 111In-labeled and 123I-labeled anti-mIgG antibodies with BT-474 human BC cells. 111In-labeled tat ICs are feasible for inserting radionuclides into cancer cells with potential for targeting intracellular and, particularly, nuclear epitopes for imaging and/or radiotherapeutic applications.