INTRATUMORAL CONVERSION OF 5-FLUOROCYTOSINE TO 5-FLUOROURACIL BY MONOCLONAL-ANTIBODY CYTOSINE DEAMINASE CONJUGATES - NONINVASIVE DETECTION OF PRODRUG ACTIVATION BY MAGNETIC-RESONANCE SPECTROSCOPY AND SPECTROSCOPIC IMAGING

The monitoring of antibody-directed enzyme-prodrug therapies requires evaluation of drug activation within the tissues of interest, We have demonstrated the feasibility of noninvasive magnetic resonance spectro scopy and spectroscopic imaging (chemical shift imaging) to detect act ivation of the prodrug 5-fluorocytosine (5-FCyt) to the cytotoxic spec ies 5-fluorouracil (5-FU) by monoclonal antibody-cytosine deaminase (C D) conjugates, In vitro, L6-CD but not 1F5-CD selectively metabolized 5-FCyt to 5-FU on H2981 human lung adenocarcinoma cells because of the presence and absence of cell surface L6 and CD20 antigens, respective ly. After pretreatment of H2981 tumor-bearing mice with L6-CD, in vivo metabolism of 5-FCyt to 5-FU within the tumors was detected by F-19 m agnetic resonance spectroscopy; the chemical shift separation between 5-FCyt and 5-FU resonances was similar to 1.2 ppm. 5-FU levels were 50 -100 % of 5-FCyt levels in tumors 10-60 min after 5-FCyt administratio n. Whole body F-19 chemical shift imaging (6 x 6 mm in-plane resolutio n) of tumor-bearing mice demonstrated the highest signal intensity of 5-FU within the tumor region. This study supports further development of noninvasive magnetic resonance methods for preclinical and clinical monitoring of CD enzyme-prodrug therapies.