A prototype transduction tag system (Delta LNGFR/NGF) for noninvasive clinical gene therapy monitoring

The dramatic expansion of clinical gene therapy trials requires the develop ment of noninvasive clinical monitoring procedures, which provide informati on about expression levels, expression kinetics, and spatial distribution o f transduced therapeutic genes, With the development of such procedures, in vasive sampling of tissue probes from patients potentially could be reduced significantly. In this study, an experimental platform for the rational de sign and in vitro testing of suitable receptor-ligand couples as components of future transduction tag systems for noninvasive gene therapy monitoring applications was developed. Initially, the feasibility of the Delta LNGFR/ nerve growth factor (NGF) transduction tag system was investigated; this sy stem employs a mutated version of the low-affinity nerve growth factor rece ptor (p75mut or Delta LNGFR) lacking the entire cytoplasmic domain. Specifi c binding of I-125-radiolabeled NGF was demonstrated for two stable Delta L NGFR-transduced cell lines, but not for Delta LNGFR-negative parental contr ol cell lines. An additional binding analysis performed in a MicroImager di rectly confirmed binding of radiolabeled ligands (I-125-NGF, I-125-anti-p75 monoclonal antibody) to the p75mut expressed on intact target cells, but n ot on control cells. Subsequent binding studies employing NGF radiolabeled with the positron-emitting isotope I-124 demonstrated a specific binding fo r LNGFR(+) PC12 cells. Consequently, the first in vitro proof of a transduc tion tag approach based on the specificity of the I-124-NGF/LNGFR interacti on was provided, which opens up the possibility for future noninvasive posi tron emission tomography monitoring in clinical gene therapy trials.