Metal complexes demonstrate antitumoral and antiparasitic action

Compounds made by combining an organic molecule with copper or zinc ions were tested in vitro against different types of tumors.

By Karina Toledo

Agência FAPESP – A class of metal complexes developed by researchers at the University of São Paulo Chemistry Institute (IQ-USP) have demonstrated potential antitumoral and antiparasitic action in in vitro experiments.

The studies are being conducted by the National Institute for Science and Technology (INCT) Redox Processes in Biomedicine (Redoxoma) – one of the INCTs supported by the Ministry of Science, Technology and Innovation (MCTI) and a FAPESP Research, Innovation and Dissemination Center (RIDC).

“This is a class of compounds that contains a central metal ion and an organic molecule that acts as a ligand. We worked primarily with copper and zinc ions, and the precursor of our ligands is a metabolite of the amino acid tryptophan called isatin,” explained Ana Maria da Costa Ferreira, the coordinator of the FAPESP thematic project, “Development of compounds with pharmacological and medicinal interest and of systems for their transport, detection and recognition in biological medium.”

According to the researcher, there are other classes of metal-based drugs, such as cisplatin, that are already in use to treat neoplasias. The advantage of the compounds synthesized at the IQ-USP is that their metal ion composition is naturally present in the human body and, thus, is more easily metabolized.

“Both the copper and the zinc ions are considered essential elements for the body because they are part of several proteins and enzymes involved in important metabolic processes, such as respiration,” Ferreira said.

The initial studies with tumor cell lines were carried out during the doctoral studies of FAPESP fellow Giselle Cerchiaro, under the mentorship of Ferreira. The experiments showed that at certain concentrations, the metal complexes were capable of diminishing the viability of the cultures in vitro.

“We started with copper and then expanded the line of research to include zinc and vanadium as we modified the ligands,” explained Ferreira.

By studying the mechanism of action of the metal-based drugs, the researchers observed that they induced oxidative stress in the tumor cells; that is, they increased the release of reactive oxygen species, which in large numbers damage DNA. Incapable of reversing the damage, the cell enters the process of apoptosis, a type of physiological death that causes no inflammatory reaction in the body.

“This is the type of reaction we would like to see in an antitumoral drug. If it were to cause necrosis, the result would be inflammation in other tissues, but that does not happen in apoptosis,” Ferreira explained.

In addition, the metal complexes affect the mitochondria – the organelles responsible for converting oxygen into energy – she added. “In the presence of the metal-based drugs, the mitochondria continue to consume oxygen but are unable to synthesize ATP [adenosine triphosphate, the molecule that stores energy]. This could compromise the cell, the organ and the entire body. But, because tumor cells multiply very quickly, there ends up being a certain amount of selectivity in the drug action, as in the case of chemotherapy,” Ferreira said.

The metal complexes were tested on neuroblastoma (brain cancer), melanoma and uterine sarcoma cell lines. In a test known as IC50 determination, which establishes the concentration of drugs needed to cause the death of 50% of the cells in the culture, the metal complexes proved to be effective at nanomolar doses (10-9 nM), which, according to the scientific literature, is equivalent to the required dose of cisplatin.

In 2006, the IQ-USP group was awarded a patent from the Brazilian Industrial Property Institute (INPI) for this class of metal complexes as well as for their antitumoral action. In 2013, a patent was also filed for their antiparasitic activity. “It’s the same class of compounds, but it has different combinations and structural modifications. The best antiparasitic is not the best antitumoral,” Ferreira explained.

Antiparasitic action

Metal-based drugs have already been successfully tested in vitro against Trypanosoma cruzi, the protozoan that causes Chagas disease. Now, in a partnership with the group led by Professor Marcia Aparecida Silva Graminha of the School of Pharmaceutical Sciences of Universidade Estadual Paulista (Unesp) in Araraquara, they are being tested against Leishmania amazonensis, one of the parasites that causes leishmaniasis.

During recent studies, scientists discovered a third mechanism for the drugs’ action: inhibition of the protein topoisomerase, which is one of the enzymes responsible for maintaining the integrity of DNA, vital for human cells as well as for the growth of parasites.

“We compared the action of our compounds with that of benznidazole, the drug most commonly used for decades to treat parasitosis. In the case of T. cruzi, two types of tests were performed: one against the form of the parasite that is found in the insect vector and is injected by the insect into the human bloodstream [trypomastigote] and the other against the form the parasite assumes inside the macrophages [amastigote],” Ferreira explained.

In each of these cases, the IC50 – the concentration required to kill 50% of the parasites – was analyzed. In addition, the LC50 was also measured to determine the concentration required to kill 50% of the human macrophages.

“In the IC50 test, some of our compounds were shown to have the same effectiveness as the benznidazole at half the dose. In other words, they were twice as potent. In the LC50 test, however, some cases needed a dose 12 times higher of the metal complex to cause the same damage to the human macrophages. This means that the margin of safety of the metal-based drugs could be higher than that of benznidazole,” Ferreira explained.

According to the researcher, the findings were similar in experiments with L. amazonensis, but in each case, a different combination was proven to be more effective. In the future, the group is planning to test the action of the compounds against other parasites and to examine the pharmacological mechanisms of action.