| Enzymatic properties play a pivotal role in prodrug activation therapy and efficacy of reporter gene imaging. The enzymatic properties of wild type herpes simplex virus type 1 (HSV1-TK) and a mutant enzyme with similar properties, HSV1-sr39TK, with pyrimidine analogs as substrates have been studied.;The primary use of the HSV1-TK enzymes has been in gene therapy protocols where the function has been to activate a prodrug to induce a therapeutic response.;In conjunction with HSV1-TK as a prodrug enzyme, much effort has been made in utilizing the same gene-enzyme system to monitor gene expression, using the HSV1-tk gene as a reporter gene together with a selective reporter probe. The possibility of non-invasive monitoring of gene expression can be important when assessing therapeutic efficacy, site of drug action or duration of genetic activity.;The use of positron emission tomography (PET) in molecular imaging has been utilized for the past 30 years, and with increasing interest ever since. PET utilizes the properties of positron emitting isotopes to measure concentration, duration and localization of molecular events. This technique has successfully been employed with genetic imaging through radiolabeling of reporter probes. The imaging of HSV1-tk gene expression have focused on acycloguanosines and other purine based nucleosides. Recent research has, however, suggested that pyrimidine analog substrates are superior in both cellular uptake and selectivity towards the HSV1-TK enzyme.;This work has focused on determining the properties of HSV1-TK and HSV1-sr39TK towards a selection of pyrimidine analogs for the purpose of reporter gene PET-imaging. It was especially interesting to see if the enzyme specifically chosen on the basis of acycloguanosine activity, HSV1-sr39TK, also had advantageous properties towards pyrimidines.;The results obtained from the experiments indicated that the benefits obtained with the mutant HSV1-sr39TK enzyme and acycloguanosine analogs also can be translated to the pyrimidine analogs. FFAU showed the most dramatic difference with almost a 10-fold increase in efficiency towards the mutant. The reduced specificity for native thymidine should make FFAU and HSV1-sr39TK a promising combination in reporter gene imaging. |
