MicroPET imaging of gene transfer with a somatostatin receptor-based reporter gene and (94m)Tc-Demotate 1.

Abstract

Gene therapy trials would benefit greatly from the use of noninvasive imaging to determine the location, magnitude, and time course of gene transfer. Somatostatin receptor subtype 2 (SSTR2) has been used as a reporter probe for gamma-camera imaging of gene transfer in animal models. PET has greater sensitivity than gamma-camera imaging and therefore would have an advantage for the imaging of SSTR2 gene transfer. An adenovirus (AdHASSTR2) carrying sstr2, which encodes an N-terminal hemagglutinin epitope, was used for evaluating SSTR2 gene transfer. The somatostatin analog Demotate 1 (Tyr(3)-octreotate conjugated to the 1,4,8,11-tetraazaundecane chelator) was used for chelation of the positron emitter (94m)Tc (half-life, 52 min) and targeting to SSTR2. Gene transfer was evaluated in vitro with A-427 non-small cell lung cancer cells after infection with AdHASSTR2 by (94m)Tc-Demotate 1 binding and internalization assays. In vivo biodistribution and microPET studies were conducted with mice bearing A-427 tumor xenografts directly injected with AdHASSTR2 to determine the tumor localization of (94m)Tc-Demotate 1. (94m)Tc-Demotate 1 bound with high affinity and was internalized rapidly into AdHASSTR2-infected A-427 cells. Biodistribution studies showed uptake of (94m)Tc-Demotate 1 in tumors infected with AdHASSTR2 (4.0 percentage injected dose per gram [%ID/g] at 2 h) and background uptake in tumors infected with a control adenovirus (0.8 %ID/g at 2 h). The uptake of (94m)Tc-Demotate 1 in AdHASSTR2-infected tumors was greater than the uptake in all other tissues, except for the kidneys and the SSTR2-positive pancreas. MicroPET imaging showed similar results, with clear uptake of (94m)Tc-Demotate 1 in AdHASSTR2-infected tumors, background uptake in control tumors, and clearance through the kidneys. These studies show that the positron-emitting somatostatin analog (94m)Tc-Demotate 1 could be used to determine SSTR2 gene transfer by microPET imaging, a result that will improve the sensitivity of the SSTR2 reporter gene system.