[¹³¹I]FIAU labeling of genetically transduced, tumor-reactive lymphocytes: cell-level dosimetry and dose-dependent toxicity

Donor T cells have been shown to be reactive against and effective in adoptive immunotherapy of Epstein-Barr virus (EBV) lymphomas which develop in some leukemia patients post marrow transplantation. These T cells may be genetically modified by incorporation of a replication-incompetent viral vector (NIT) encoding both an inactive mutant nerve growth factor receptor (LNGFR), as an immunoselectable surface marker, and a herpes simplex virus thymidine kinase (HSV-TK), rendering the cells sensitive to ganciclovir. The current studies are based on the selective HSV-TK-catalyzed trapping (phosphorylation) of the thymidine analog [¹³¹I]-2′-fluoro-2′-deoxy-1-β-D-arabinofuransyl-5-iodo-uracil (FIAU) as a means of stably labeling such T cells for in vivo trafficking (including tumor targeting) studies. Because of the radiosensitivity of lymphocytes and the potentially high absorbed dose to the nucleus from intracellular ¹³¹I (even at tracer levels), the nucleus absorbed dose (D _n ) and dose-dependent immune functionality were evaluated for NIT⁺ T cells labeled ex vivo in [¹³¹I]FIAU-containing medium.

Based on in vitro kinetic studies of [¹³¹I]FIAU uptake by NIT⁺ T cells, D _n was calculated using an adaptation of the MIRD formalism and the recently published MIRD cellular S factors. Immune cytotoxicity of [¹³¹I]FIAU-labeled cells was assayed against ⁵¹Cr-labeled target cells [B-lymphoblastoid cells (BLCLs)] in a standard 4-h release assay.

At median nuclear absorbed doses up to 830 cGy, a ⁵¹Cr-release assay against BLCLs showed no loss of immune cytotoxicity, thus demonstrating the functional integrity of genetically transduced, tumor-reactive T cells labeled at this dose level for in vivo cell trafficking and tumor targeting studies.