Aminopeptidase N as a potential drug target

Aminopeptidase N (APN), EC 3.4.11.2, also known as membrane alanyl aminopeptidase or CD13 antigen was first isolated from porcine kidney cells (Wachschmuth et al., 1966). It belongs to the MA class of zinc metallopeptidases. In cancer cells, inhibition of these aminopeptidases causes the phenomenon termed the amino acid deprivation response (AADR), a stress response characterized by an upregulation of amino acid transporters and synthetic enzymes, and activation of stress-related pathways such as nuclear factor κB (NFκB) and other pro-apoptotic regulators, which leads to a cancer cell death by apoptosis (DiNardo&Cortes, 2013). APN also serves as a receptor involved in endocytosis during viral infection, among others with the human coronavirus HCoV-229E (Li et al., 2019). APN inhibitors, thus, could serve for the treatment of cancer and viral diseases. The most known dipeptide APN inhibitor bestatin, first isolated from Streptomyces olivoreticuli in 1976, was tested for the treatment of lung cancer in the past in Japan (Ichinose et. al., 2003), and its synthetic analog tosedostat underwent more than ten clinical trials of phases 1 or 2 for the treatment of myelodysplastic syndrome (Lee et al. 2021), myeloid leukemias (Dennis et al., 2021), and solid tumors (Raid et al., 2009).

In the past two decades, we have been concerned with the study and synthesis of novel structure types of APN inhibitors. Among others, we published a series of 2-{[2-(2-oxo-1-azacycloalkyl)acetamido]phenoxyacetic acids exhibiting APN inhibitory activity (Farsa et al., 2008), a set of divalent metal salts of 2,5-dihydroxybenzenesulfonic acids (Farsa et al., 2014), and a small series of 4-(nitrophenoxymethyl)-[1,3,2] dioxaborolan-2-ols (Farsa et al., 2017) with the same type of activity. We also performed some QSAR studies with different types of APN inhibitors (Farsa & Haluska, 2016). Our results together with those of other authors (Amin et al., 2018) led us to hypothesize that the most promising mechanism of APN inhibition is the chelation of zinc cation. We, thus, prepared a series of basically substituted aromatic Schiff bases, particularly semicarbazones and thiosemicarbazones, capable of such chelation that exhibited APN inhibiting and even antiproliferative activity on cancer cell lines. Our most active Schiff bases exhibited an antiproliferative activity in the cell lines expressing APN but not in cell lines without APN expression (Farsa et al., 2022). These results encouraged us to our current work on compounds with better activity due to their improved chelation ability and on confirmation of the proposed mechanism of enzyme inhibition.

Aminopeptidase N seems to be a promising drug target mainly in groups of anticancer and antivirus drugs. During the past two decades, our small group working at the Department of Chemical Drugs of the Faculty of Pharmacy in Brno has been trying to contribute to the field of potentially therapeutically useful APN inhibitors by the synthesis of some novel structure types of the inhibitors as well the study of their both qualitative and quantitative structure-activity relationships. We continue in this research currently by structure improvements of our last Schiff base type of APN inhibitors.