Lipoylated Proteins Inhibitors
Lipoylation is a rare but essential post-translational modification involving the covalent attachment of a lipoic acid moiety to specific lysine residues of mitochondrial multienzyme complexes. In humans, only four proteins are known to be lipoylated: dihydrolipoamide S-acetyltransferase (DLAT) of the pyruvate dehydrogenase complex, dihydrolipoamide S-succinyltransferase (DLST) of the alpha-ketoglutarate dehydrogenase complex, dihydrolipoamide S-aminotransferase (DBT) of the branched-chain alpha-keto acid dehydrogenase complex, and the H-protein of the glycine cleavage system (GCSH). These Lipoylated Proteins are central to mitochondrial energy metabolism, facilitating the transfer of acyl groups and electrons during the oxidative decarboxylation of alpha-keto acids. The lipoyl group acts as a 'swinging arm,' moving substrates between different active sites within these large complexes. Consequently, the functional status of Lipoylated Proteins dictates the flux through the tricarboxylic acid (TCA) cycle and the availability of biosynthetic intermediates. The dysregulation of lipoylated proteins is associated with several pathological conditions.
In oncology, many cancer cells exhibit a high dependency on mitochondrial metabolism and the TCA cycle to support their rapid proliferation and survival, making these proteins attractive therapeutic targets. Furthermore, lipoylated proteins have been identified as the primary targets of copper-induced toxicity in the recently described cuproptosis pathway. Excessive copper binding to the lipoyl moieties of DLAT and DLST leads to their aggregation and inactivation, triggering proteotoxic stress. In addition to cancer, lipoylated proteins are involved in primary biliary cholangitis (PBC), an autoimmune liver disease where the immune system mistakenly targets the lipoylated E2 subunit of the pyruvate dehydrogenase complex. Inhibitors of lipoylated proteins are currently being explored for their potential to disrupt cancer metabolism. The most notable compound in this category is CPI-613 (Devimistat), a lipoic acid analog that acts as a multi-target inhibitor of DLAT and DLST.
CPI-613 mimics the catalytic intermediates of the TCA cycle, leading to the inhibition of mitochondrial respiration and the induction of metabolic stress. It has been evaluated in numerous clinical trials, particularly for the treatment of pancreatic cancer and acute myeloid leukemia, often in combination with standard chemotherapy. Other inhibitors include small molecules that interfere with the lipoylation process itself, such as those targeting lipoyltransferase 1 (LIPT1) or lipoic acid synthase (LIAS), effectively reducing the pool of functional lipoylated proteins. Classification of these inhibitors can be based on their mechanism: direct analogs of lipoic acid, inhibitors of the enzymes responsible for lipoylation, or agents that induce the aggregation of lipoylated proteins (like copper ionophores). A significant challenge in targeting these proteins is the potential for metabolic toxicity in healthy tissues, given their fundamental role in energy production. However, the 'metabolic flexibility' of normal cells compared to the 'metabolic addiction' of cancer cells provides a therapeutic window.
Future research is focused on identifying biomarkers, such as FDX1 expression or specific metabolic signatures, to select patients who are most likely to respond to therapies targeting lipoylated proteins.
