Imaging of K(Ca)3.1 Channels in Tumor Cells with PET and Small-Molecule Fluorescent Probes

The Ca2+ activated K+ channel K(Ca)3.1 is overexpressed in several human tumor cell lines, e. g. clear cell renal carcinoma, prostate cancer, non-small cell lung cancer. Highly aggressive cancer cells use this ion channel for key processes of the metastatic cascade such as migration, extravasation and invasion. Therefore, small molecules, which are able to image this K(Ca)3.1 channel in vitro and in vivo represent valuable diagnostic and prognostic tool compounds. The [F-18]fluoroethyltriazolyl substituted senicapoc was used as positron emission tomography (PET) tracer and showed promising properties for imaging of K(Ca)3.1 channels in lung adenocarcinoma cells in mice. The novel senicapoc BODIPY conjugates with two F-atoms (9 a) and with a F-atom and a methoxy moiety (9 b) at the B-atom led to the characteristic punctate staining pattern resulting from labeling of single K(Ca)3.1 channels in A549-3R cells. This punctate pattern was completely removed by preincubation with an excess of senicapoc confirming the high specificity of K(Ca)3.1 labeling. Due to the methoxy moiety at the B-atom and the additional oxyethylene unit in the spacer, 9 b exhibits higher polarity, which improves solubility and handling without reduction of fluorescence quantum yield. Docking studies using a cryo-electron microscopy (EM) structure of the K(Ca)3.1 channel confirmed the interaction of 9 a and 9 b with a binding pocket in the channel pore.