Development 68Ga trastuzumab Fab and bioevaluation by PET imaging in HER2/neu expressing breast cancer patients.

INTRODUCTION

Receptors on breast cancer cells play a crucial role in the management of patients. Trastuzumab is a widely used drug for the treatment of HER2/neu expressing tumors. ImmunoPET with trastuzumab is not feasible due to slow pharmacokinetics. Fragment of antigen-binding (Fab) radiolabeled with positron emitters can be used for immunoPET.

METHODS

Fab has been generated by papain digestion and conjugated with the bifunctional chelating agent NOTA. The SDS-PAGE and MALDI-TOF were used to see the integrity of Fab and conjugated Fab. In-vitro stability and target specificity for HER2/neu receptors were performed in plasma and receptor binding with bio-layer interferometry (BLI) techniques. Radiolabeling was standardized with 68GaCl3 and PET imaging was performed in seven patients showing 18F fluorodeoxyglucose (18F-FDG) uptake and correlated with HER2/neu expression by immunohistochemistry.

RESULTS

Fab production was optimized at molar ratio 23:1 of trastuzumab and papain at 37 °C with a constant stirrer at 850 rpm for 22-24 h, at pH 8. Conjugation with NOTA was standardized at molar ratio 1:25 of trastuzumab Fab and NOTA. Molecular mass of trastuzumab Fab-NOTA was found approximately 46.3 kDa (~1/3 of intact antibody). Trastuzumab Fab-NOTA showed radiolabelling efficiency of 48-70% with incubation time 15 min at 37-40 °C and pH 4.5-5.0. BLI demonstrated the affinity of trastuzumab, trastuzumab Fab and trastuzumab Fab-NOTA towards HER2/neu receptor with KD of <1pM, ~0.5nM and ~20nM, respectively. All immunohistochemistry proven patients showed uptake in primary breast lesion and lymph nodes.

CONCLUSION

Trastuzumab Fab-NOTA is suitable for radiolabelling with 68Ga and ImmunoPET imaging of HER2/neu receptor.