Radioimmunotherapy using a La-specific monoclonal antibody as a novel apoptotic cancer cell ligand

Most cancer patients still die from metastatic disease. Conventional cytotoxic chemotherapy usually fails to eradicate metastases probably because cancer stem cells survive to repopulate secondary tumors. We have discovered and validated a unique target for cancer therapy1, 2 that may enable a non-crossreactive treatment modality such as ionizing radiation to target viable cancer cells after a first step of cytotoxic chemotherapy. The La/SS-B ribonucleoprotein chaperones transfer and ribosomal RNA products of RNA polymerase III, and is over expressed in malignant cells1. Other characteristics favor La antigen as a cancer therapy target and include the translocation of La during apoptosis from nucleus to cytoplasm where it is detected using the La-specific 3B9 monoclonal antibody (mAb), which enters permeable late apoptotic cells in vitro. Apoptotic tumor cell binding of 3B9 is specific, rapid, saturable, and of high avidity, and is induced by DNA damage, and then fixed by transglutaminase 2 activity1. Tumors often contain dead cells, particularly after antineoplastic treatment. Consequently, the use of cyclophosphamide and etoposide (CE) chemotherapy in mice bearing EL4 lymphomas augmented 3B9 binding to apoptotic tumor cells in vivo2.
 Now, we demonstrate a bystander killing effect of La-directed radioimmunotherapy (RIT) itself because levels of endogenous tumor cell death were sufficient to allow antigen-specific binding of the 90Y-DOTA-3B9 mAb radioimmunoconjugate (RIC). At radiation doses of ≥ 1.85 Gy per mouse, the RIC eradicated subcutaneous EL4 lymphoma implants presumably via a radiation crossfire effect.
 Next, we treated EL4 tumor-bearing mice with non-radiosensitizing but DNA-damaging CE chemotherapy, or La-directed RIT, or both. An improved therapeutic ratio was observed when chemotherapy was given 24 hours before RIT rather than concurrently. While neither CE chemotherapy nor RIT alone cured EL4 tumor-bearing mice, the combination cured virtually all mice, effectively lowering the dose of RIT required for cure by four-fold. We argue that this synergistic interaction depended on both levels of tumor cell death, which increased from 20% in control mice to >50% in treated mice, and increased expression levels of La antigen per 7-AAD+ dead tumor cell after chemotherapy.
 In contrast, despite levels of tumor cell death increasing by only ~10-15% after treatment of LL2, or LNCaP or PANC-1 carcinoma-bearing mice with radiosensitizing cisplatin and gemcitabine chemotherapy, this chemotherapy synergized with RIT to both inhibit carcinoma growth and prolong the survival of carcinoma-bearing animals. We hypothesize that radiosensitizers lowered the threshold for radiation-induced tumor cell death thus promoting tumor lethality via a ‘genotoxic chain reaction’.