Mapping Aldehyde Dehydrogenase 1A1 Activity using an [¹⁸F]Substrate‐Based Approach

Abstract

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.

A Prodrug Strategy for the In Vivo Imaging of Aldehyde Dehydrogenase Activity

“…The procedure was adapted from that previously reported by our group. 21 (3 × 0.40 mL). The precursor 4 (10.0 mg, 28.8 µmol) was added to the reaction vessel as a solution in anhydrous DMSO (1 mL).…”

“…In order to circumvent these inherent limitations, us and others have proposed the use of positron emission tomography (PET) as an alternative to fluorescence-based imaging. 21,22 We have recently reported that [ 18 F]N-ethyl-6-(fluoro)-N-(4-formylbenzyl)nicotinamide -[ 18 F]1 -has excellent affinity and isozyme selectivity for ALDH1A1 in cancer cells but is rapidly oxidised to its corresponding carboxylic acid in the blood when administered at tracer doses. 21 Here, we employed an enzyme-cleavable pro-drug strategy to increase the blood stability of our ALDH1A1targeted radiotracer.…”

“…21,22 We have recently reported that [ 18 F]N-ethyl-6-(fluoro)-N-(4-formylbenzyl)nicotinamide -[ 18 F]1 -has excellent affinity and isozyme selectivity for ALDH1A1 in cancer cells but is rapidly oxidised to its corresponding carboxylic acid in the blood when administered at tracer doses. 21 Here, we employed an enzyme-cleavable pro-drug strategy to increase the blood stability of our ALDH1A1targeted radiotracer. Protecting the metabolically labile aldehyde from oxidation maintained ALDH1A1 specificity, producing positive tumour-to-background images in mouse models of ovarian cancer.…”

“…[ 18 F]2 therefore provides a sensitive measure of ALDH activity in tumour cells. Aldehyde protection and carrier-added formulation improve radiotracer blood stability Given that [ 18 F]1 had previously undergone complete oxidation when incubated for 2 min in blood 21 , likely due to high ALDH1 expression in erythrocytes 25 , we next examined whether geminal diacetate protection afforded improved blood stability. Whilst [ 18 F]2 was still present in murine blood at 2 min, rapid cleavage and oxidation to the carboxylate [ 18 F]3 was observed (80 ± 12 % activity; n = 6), increasing to 94 ± 1 % activity (n = 3) at 5 min and negligible amounts of the aldehyde [ 18 F]1 and alcohol [ 18 F]4 seen (Fig.…”

Latest Advances in Imaging Oxidative Stress in Cancer

“…Many cancer cells upregulate aldehyde dehydrogenases in response to this oxidative stress, which mediates aldehyde detoxification (22). The enzymatic activity of aldehyde dehydrogenase 1A1 has recently been quantified with a novel substrate-based radiotracer (23). Using a complementary strategy, Kirby et al developed 18 F-NA3BF3 for the imaging of total aldehydic load through radiotracer-aldehyde complex formation (24).…”