• Warburg effect
• hypoxia
• glycolytic phenotype
• NHI
• LDH-5

Cells can produce energy through two different mechanisms: oxidative phosphorylation and glycolysis.
The oxidative phosphorylation (OXPHOS) is an oxygen-dependent pathway and it couples the oxidation of NADH e FADH2 with the phosphorylation of ADP to ATP, the “molecular unit of currency” of intracellular energy transfer. In this process oxygen acts as the final acceptor of the electrons resulting from the oxidation of NADH e FADH2.
The glycolysis, instead, provides the breaking-down of one molecule of glucose into two molecules of pyruvate, with the consumption of NAD+ to obtain ATP. Nevertheless, this process not require O2 and, in its absence, the regeneration of NAD+, which is necessary for the glycolysis to take place, occurs by the reduction of pyruvate to lactate. This last step is catalyzed by the enzyme lactate dehydrogenase (LDH).
OXPHOS is more efficient in generating ATP than glycolysis, since the oxidation of one molecule of glucose gives respectively a yield of 30-36 ATPs via OXPHOS and of only 2 ATPs via glycolysis. Therefore, it is demonstrated that cells usually produce ATP by OXPHOS in the presence of sufficient O2 levels, while, when the O2 levels decrease, the production of ATP shifts to glycolysis and the consumption of glucose decidedly increases (Pasteur effect).
However, it well-know that cancer cells have increased rates of glycolysis despite the presence of proper O2 levels. This phenomenon, called aerobic glycolysis or Warburg effect, plays a crucial role in cancer growth, development and invasion capacity, since the glycolysis has the advantages of quickly producing ATP and of providing carbon skeletons for biosyntheses, both of which are essential characteristics for rapid cell proliferation. Furthermore, this metabolic switch from oxidative phosphorylation to increased rate of glycolysis becomes indispensable in the hypoxic regions of the tumour tissue, which originate from an imbalance between the metabolic requirements of a rapidly growing tissue and the vascular ability to provide oxygen and nutrients. Tumour hypoxia could be due to an obstruction of microvessels, caused by an abnormal growing of the tumour (acute hypoxia), or to an increased diffusion distances caused by irregular and fast tumour growth (chronic hypoxia). The low O2 levels makes hypoxic tumour tissues highly resistant to the most common therapies, such as radiotherapy and chemotherapy and also increases their aggressiveness and invasion capacity. Therefore, hypoxic cancer represents an arduous challenge for new anticancer therapies.
It has been demonstrated that the constitutive promotion of glycolysis in tumour cells results in the activation of hypoxia-inducible factor 1 (HIF-1), which in turn increases the transcription of many genes coding for proteins involved in glucose metabolism, invasion and metastasis. Therefore, cancer cells acquire a glycolytic phenotype, characterized by the overexpression of important enzymes that ensure the tumour survival. In fact, HIF-1 furthers the overexpression of Vascular Endothelial Growth Factor (VEGF), which is responsible for the high angiogenic activity of the tumour. Furthermore, the glycolytic phenotype of tumour cells results in the up-regulation of the Glucose Transporter-1 (GLUT1) and of the Hexokinases 1 and 2 (HK-1 and HK-2), that highlights the strong dependence on the glucose metabolism. This also results in overexpression of the isoform 5 of lactate dehydrogenase (LDH5).
Nevertheless, this aggressive glycolytic phenotype may be used as Achilles’ heel to develop new anticancer strategies.
LDH5, catalyzing the reversible conversion of pyruvate to lactate, represents one of the major responsible promoters of the metabolic switch from oxidative phosphorylation to increased glycolysis, that characterizes the most invasive tumours and then, it could be considered a promising tumour target. In fact, with the repression of its expression by shRNA, it has been demonstrated that the inhibition of its activity causes a decrease of the energy production in hypoxic tumours that determines a reductions of invasiveness in metastatic cells. Furthermore, it is known that hereditary LDH5 deficiency causes myoglobinuria only after intense aerobic exercise, while it does not cause any relevant symptoms under normal circumstances. For these reasons, inhibition of LDH5 could be represents a valid action mechanism of new anticancer agents.
In this work a series of N-hydroxy-2-carboxyindole (NHI) derivatives was synthesized as isoform selective inhibitors of LDH5. These compounds presents the pharmacophore features required for the selective and competitive inhibitory activity against LDH5, which is also present in other studied inhibitors: a hydrophobic aromatic scaffold with the presence, in adjacent positions, of an acidic group and a hydroxyl function.