The main characteristic of tumor cells is their fast growth and proliferation and in order to sustain their metabolism requirements they need of a big amount of nutrients and oxygen.
When a tumor mass reaches a considerable dimension, the distance of cells from blood vessels increases and this situation can foster the establishment of hypoxia. From several evidence we can see how hypoxic tumor cells enhance adaptation strategies in order to survive.
The effects of hypoxic conditions are mediated by HIF, a transcription factor family among which HIF-1 results to be strongly activated in tumor disease: in normal conditions there is a specific mechanism that inhibits the encoding activity of HIF-1 but in those tumor cells subjected to hypoxia its activity is uncontrolled. Among the targets of HIF-1 there are genes encoding for specific isoform of glucose transporters (GLUT1) and for the LDH-A subunit of lactate dehydrogenase with the consequent overexpression of LDH-5 isoform in hypoxic tumor cells. The importance of these isoforms is related to the Warburg effect, a metabolic switch of tumor cells from oxidative phosphorylation (OXPHOS) to “aerobic glycolysis” where glucose is converted to pyruvate that is reduced to lactate. Several studies were performed to understand the causes of the Warburg effect and it was observed that this switch even occurs when oxygen is still present, hypothesizing that this change could be a strategy to prepare tumor cells to hypoxia safeguarding the cellular proliferation.
The expression of specific isoforms offers the opportunity to develop molecules that could show specific activity towards hypoxic tumor cells, which are correlated to the most aggressive phenotypes of tumor, trying to avoid important side effects on healthy cells. This work is focused on the synthesis of some molecules which show a significant activity towards some of the isoforms particularly overexpressed in tumors.
GLUT1 is found to be overexpressed in several tumors and its inhibition can reduce the availability of glucose for tumor cells. Several compounds have shown a satisfactory activity towards this isoform; among them there is a group of ketoximes, developed by the University of Pisa research team, derived from modifications made on previously developed as estrogen receptors (ER oxime ligands (Figure 1).
Another potential target is LDH-5, a homotetramer made by LDHA subunits generally expressed principally in skeletal muscle cells. LDH-5 is a suitable anticancer target because of its overexpression in hypoxic tumor cells and also to the observation that hereditary LDHA deficiency is not characterized by any symptoms in normal conditions so, in theory, its possible use should not cause relevant side effects. In particular, the inhibition of LDH5 can lead to cellular death for starvation. The group of inhibitors synthesized in this work includes N-hydroxyindole glucose-conjugate and N-hydroxybenzimidazole derivatives (Figure 2).
N-hydroxyindole glucose-conjugate contains aglycone moiety common to other compounds that showed affinity and inhibitory activity towards LDH5. Moreover, the glucose-conjugation takes advantage of the high glucose uptake of tumor cells to increase the entrance of the compound into the cells.
The N-hydroxybenzimidazole derivative is developed to test if it could be a LDH-5 inhibitor, due to the high structural similarity with the N-hydroxyindole class of compounds.
Another enzyme encoding by HIF-1 is lysyl oxidase (LOX). LOX is an enzyme normally involved in the re-modeling of extracellular matrix but in tumor mass it results overexpressed under hypoxia. In particular, lysyl oxidase is correlated with metastasis promotion and the resulting inhibition with β-aminoproprionitrile (BAPN) shows a decrease of the metastatic colonization. In order to avoid the side effects of BAPN, a series of prodrugs were developed which selectively release BAPN under hypoxia by the reductive processes; the masking moieties consist in nitroaromatic groups and in this work, in particular, a prodrug with a nitrotiophene group is synthesized as hydrochloride salt to increase its solubility in water (Figure 3).
Figure 2. Structures of synthesized LDH-5 inhibitors
