Purpose Drug-resistance is a major obstacle for the effective treatment of patients with high grade serous ovarian cancer (HGSOC). Currently, there is no satisfactory way to identify HGSOC patients that are refractive to the standard of care. Here, we propose the system xc- radiotracer (4S)-4-(3-[18F]fluoropropyl)-L-glutamate ([18F]FSPG) as a non-invasive method to measure upregulated antioxidant pathways present in drug resistant HGSOC. Experimental Design Using matched chemotherapy sensitive and resistant ovarian cancer cell lines, we assessed their antioxidant capacity and its relation to [18F]FSPG uptake, both in cells and in animal models of human ovarian cancer. We identified the mechanisms driving differential [18F]FSPG cell accumulation and evaluated [18F]FSPG tumor uptake as predictive marker of treatment response in drug-resistant tumors. Results High intracellular glutathione (GSH) and low reactive oxygen species corresponded to decreased [18F]FSPG cell accumulation in drug-resistant versus drug-sensitive cells. Decreased [18F]FSPG uptake in drug-resistant cells was a consequence of changes in intracellular cystine, a key precursor in GSH biosynthesis. In vivo, [18F]FSPG uptake was decreased ~80% in chemotherapy-resistant A2780 tumors compared to parental drug-sensitive tumors, with non-responding tumors displaying high levels of oxidised-to-reduced GSH. Treatment of drug-resistant A2780 tumors with doxorubicin resulted in no detectable change in tumor volume, GSH or [18F]FSPG uptake. Conclusions This study demonstrates the ability of [18F]FSPG to detect upregulated antioxidant pathways present in drug-resistant cancer. [18F]FSPG may therefore enable the identification of HGSOC patients that are refractory to standard-of-care, allowing the transferral of drug-resistant patients to alternative therapies, thereby improving outcomes in this disease.

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of aldehydes to carboxylic acids. Elevated ALDH expression in human cancers is linked to metastases and poor overall survival. Despite ALDH being a poor prognostic factor, the non‐invasive assessment of ALDH activity in vivo has not been possible due to a lack of sensitive and translational imaging agents. Presented in this report are the synthesis and biological evaluation of ALDH1A1‐selective chemical probes composed of an aromatic aldehyde derived from N , N ‐diethylamino benzaldehyde (DEAB) linked to a fluorinated pyridine ring either via an amide or amine linkage. Of the focused library of compounds evaluated, N ‐ethyl‐6‐(fluoro)‐ N‐ (4‐formylbenzyl)nicotinamide 4 b was found to have excellent affinity and isozyme selectivity for ALDH1A1 in vitro. Following 18 F‐fluorination, [ 18 F] 4 b was taken up by colorectal tumor cells and trapped through the conversion to its 18 F‐labeled carboxylate product under the action of ALDH. In vivo positron emission tomography revealed high uptake of [ 18 F] 4 b in the lungs and liver, with radioactivity cleared through the urinary tract. Oxidation of [ 18 F] 4 b , however, was observed in vivo, which may limit the tissue penetration of this first‐in‐class radiotracer.

Herein, we report that iron­(II)/ammonium persulfate in aqueous acetonitrile mediates the Newman–Kwart rearrangement of O-aryl carbamothioates. Electron-rich substrates react rapidly under moderate heating to afford the rearranged products in excellent yields. The mild conditions, rapid reaction rates, and suitability for scale up offers immediate practical benefits to access functionalized thiophenols.