Abstract 1820: N,N- diethylaminobenzaldehyde-based small molecules to probe aldehyde dehydrogenase as target in prostate cancer

The human aldehyde dehydrogenases (ALDHs) play a major role in the metabolism of endogenous compounds and xenobiotics. Deregulation of ALDHs have implications in a number of cancers and their elevated expression in cancer stem cells (CSCs) have been used as (1) markers for the identification and isolation of these rare cell types and (2) as potential targets for therapeutic intervention. The elevated ALDH expression in tumors is also known to lead to chemotherapy-resistance including taxanes resistance. Although the exact role of ALDHs is not fully understood, emerging information indicates that several isoforms including ALDH1A1, 1A3, 3A1 and 7A1 play a key role in prostate cancer. To unravel ALDH isoform significance, more selective chemical tool compounds are required to probe their role in tumor growth, drug-resistance and CSC protection. To further elucidate the role of ALDHs in prostate cancer, we here report on the perturbation of ALDH expression and function using new chemical probes. N,N-diethylaminobenzaldehyde (DEAB) is a well-known pan ALDH inhibitor, widely used to interrogate ALDH activity. However, its nonselective nature and reported ability to act as a substrate complicates its use for biological readouts, which we wanted to explore further. In this study, a new library of compounds were synthesized and evaluated in cell free and cell-based assays; particular focus was on understanding its interaction with the three key ALDH isoforms ALDH1A1, ALDH1A3 and ALDH3A1. Kinetic inhibitory studies using the human recombinant ALDH isoforms were performed for the whole library of DEAB analogues against ALDH1A1, 1A3 and 3A1. Among all results, compound 14 yielded selective results against ALDH1A3 (Ki of 0.46µM), while compound 18 showed promising results against ALDH3A1 (Ki of 0.30µM). Significantly, sixteen analogues displayed increased cytotoxicity (IC50 = 10-200 µM) compared with DEAB (> 200 µM) against three different prostate cancer cell lines (PC-3, LNCaP, DU145). In addition, 3 of the DEAB-derivatives ALDH-affinic probe compounds (9, 14 and 18) were tested against 5 patient samples of primary prostate epithelial cultures at two doses (50 and 200 µM) in order to investigate their effect on cell viability. All four compounds showed reduction in cell viability at the highest concentration, while 14 and 18 showed a synergistic effect in combination treatment with docetaxel. Similar observations were also evident in the PC-3 prostate cancer cell li