Pacific Blue Derivatives of Paclitaxel as Fluorescent Probes of OATP1B‐Type Transporters

Paclitaxel (Taxol) is a widely used anticancer agent that undergoes extensive hepatic metabolism and that causes a debilitating, dose-limiting peripheral neurotoxicity. We previously reported that the uptake of paclitaxel in hepatocytes and dorsal root ganglion neurons, the site of injury within the nervous system, is mediated by the organic anion transporting polypeptides OATP1B1 and OATP1B3 (Oatp1b2 in rodents), transporters that are highly sensitive to pharmacological inhibition. To facilitate future screens of chemical libraries to identify modulators and imaging-based drug distribution studies, we explored the utility of PB-Gly-Taxol and PB-GABA-Taxol, derivatives of paclitaxel linked to the coumarin-derived fluorophore Pacific Blue, as in vitro and in vivo substitute biomarker probes of paclitaxel. Transport studies in transfected HEK293 cells revealed efficient uptake of these PB-taxoids by human and murine OATP1B/Oatp1b-type transporters, with up to 100-fold increases in uptake relative to values observed in vector control cells, and inhibition of this transport by known inhibitors. Although cell viability assays demonstrated lower cytotoxicity of both PB-taxoids (IC₅₀: 13~80 nM) against a panel of breast cancer cell lines, ensuing investigations confirmed their ability to induce peripheral neurotoxicity phenotypes in mice (p < 0.05), in an Oatp1b2-dependent manner, to the same extent as paclitaxel. These findings imply that PB-taxoids mimic the transport and toxicokinetic features of paclitaxel, and these agents thus offer potential as fluorescent imaging tools for exploring drug-drug interaction liabilities and paclitaxel-related toxicity profiles that involve OATP1B/Oatp1b-type transporters.