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 Imaging1

The monitoring of antibody-directed enzyme-prodrug therapies re quires evaluation of drug activation within the tissues of interest. We have demonstrated the feasibility of noninvasive magnetic resonance spectroscopy and spectroscopic imaging (chemical shift imaging) to detect activa tion of the prodrug 5-fluorocytosine (5-FCyt) to the cytotoxic species 5-fluorouracil (5-FU) by monoclonal antibody-cytosine deaminase (CD) 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, respec tively. After pretreatment of H2981 tumor-bearing mice with L6-CD, in vivo metabolism of S-FCyt to 5-FU within the tumors was detected by 19F magnetic resonance spectroscopy; the chemical shift separation between 5-FCyt and 5-FU resonances was -1.2 ppm. 5-FU levels were 50-100% of 5-FCyt levels in tumors 10-60 min after 5-FCyt administration. Whole body ''')•' chemical shift imaging (6x6 mm in-plane resolution) 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 mon itoring of CD enzyme-prodrug therapies.