Characterization of Novel Fluorescent Bile Salt Derivatives for Studying Human Bile Salt and Organic Anion Transporters

Bile salts, such as cholate, glycocholate, taurocholate, and glycochenodeoxycholate, are taken up from the portal blood
into hepatocytes via transporters, such as the Na+-taurocholate– cotransporting polypeptide (NTCP) and organic anion–transporting
polypeptides (OATPs). These bile salts are later secreted into bile across the canalicular membrane, which is facilitated by the bile
salt export pump (BSEP). Apart from bile salt transport, some of these proteins (e.g., OATPs) are also key transporters for drug
uptake into hepatocytes. In vivo studies of transporter function in patients by using tracer compounds have emerged as an
important diagnostic tool to complement classic liver parameter measurements by determining dynamic liver function both for
diagnosis and monitoring progression or improvement of liver diseases. Such approaches include use of radioactively labeled
bile salts (e.g., for positron emission tomography) and fluorescent bile salt derivatives or dyes (e.g., indocyanine green). To
expand the list of liver function markers, we synthesized fluorescent derivatives of cholic and chenodeoxycholic acid by
conjugating small organic dyes to the bile acid side chain. These novel fluorescent probes were able to block substrate transport
in a concentration-dependent manner of NTCP, OATP1B1, OATP1B3, OATP2B1, BSEP, and intestinal apical sodiumdependent
bile salt transporter (ASBT). Whereas the fluorescent bile acid derivatives themselves were transported across the
membrane by OATP1B1, OATP1B3, and OATP2B1, they were not transport substrates for NTCP, ASBT, BSEP, and multidrug
resistance-related protein 2. Accordingly, these novel fluorescent bile acid probes can potentially be used as imaging agents
to monitor the function of OATPs.