Preclinical evaluation of a series of 99mTc-labeled carbonic anhydrase IX (CA IX) inhibitors for the detection of solid tumors

109 Objectives Clear cell renal carcinomas constitutively express CA IX at high levels, and CA IX is upregulated in many hypoxic solid tumors, making it an attractive target for the development of radiopharmaceuticals for molecular imaging of cancer. The affinity, selectivity, tumor uptake and tissue distribution of a series of 99mTc-labeled benzenesulfonamide derived inhibitors of CA IX containing novel polar single amino acid chelators were tested both in vitro and in vivo, in cell lines engineered to express each of the extracellular carbonic anhydrase isoforms. Methods HEK-293 cells were stably transfected with human CA IV, IX, XII or XIV. Clones with similar growth rates, and expression levels of the four isoforms, were selected. Competitive and saturation binding assays were conducted in all four cell lines to determine affinity and selectivity of the CA IX inhibitors. 99mTc-labeled compounds were also evaluated in HEK-293/CA IX xenografts, +/- cold compete, to evaluate uptake and target specificity. Results IC50 values of the compounds tested ranged from 40 - 400 nM by competitive binding, and tissue distribution studies confirmed a strong positive correlation between affinity, and tumor uptake and retention. One of the most promising compounds, 99mTc-MIP-1505, bound to HEK-293/CA IX cells with an IC50 of 120 nM, and showed an approximate 3-fold selectivity for CA IX over CA XII, and XIV by competitive binding. 99mTc-MIP-1505 exhibited low accumulation in non-target tissues and high tumor uptake (15% ID/g) at 1hr in HEK-293/CA IX xenograft tumors. CA IX specific tumor localization was demonstrated in vivo via dose-dependent reduction with co-injection of the rhenium analogue (0.01 - 10 mg/kg). Conclusions 99mTc-MIP-1505 binds specifically and with good affinity to CA IX expressing cells, and exhibits high tumor uptake in vivo. The non-target organ kinetics and high tumor uptake make 99mTc-MIP-1505 an attractive candidate for clinical evaluation in patients with CA IX expressing tumors