Researchers from the Ribeirão Preto Medical School at the University of São Paulo (FMRP-USP) made two important discoveries in the same study. First, they identified the probable cause of the heart problems that affect people treated with doxorubicin, an antibiotic produced naturally by bacteria and widely used to fight some of the more common types of cancer. In experiments on rats, they found that this compound destroys dystrophin, a protein that maintains the shape and enables the contraction of heart cells. In the same work, researcher Érica Carolina Campos, from the team of pathologist Marcos Rossi, found a promising way to reduce doxorubicin damage to the heart.
Doxorubicin is a compound derived from the anthracyclines that were isolated in the 1960s from the bacterium Streptomyces peucetius. Since then, it has been used as a chemotherapeutic agent because of its broad spectrum of activity. “The anthracyclines cause irreversible damage to the tumor cells because they become interspersed in the DNA inhibiting the synthesis of proteins and producing reactive types of oxygen, causing cell death,” Rossi explains. “Because of this, they were always considered one of the most effective medications with which to treat human tumors.”
Over time, however, it has been found that doxorubicin produces increasingly strong side effects. The main one is dilation of the heart, which causes cardiac insufficiency and may lead to death. The insufficiency may be acute, in which case it is observed right after administering the first dose to the patient and is easily treated, or it may be chronic, manifesting itself over months of treatment. Currently, the strategy for trying to reduce cardiac damage is to limit the medication dose to something between 500 and 550 milligrams per square meter (mg/m2) of body surface – a person who is 1.80 m tall and weighs 80 kg has about 2 sq. m of body surface and can receive a maximum cumulative dose of 1000 to 1100 mg. Even so, it is impossible to avoid the toxic effects completely. “It is estimated that between 5% and 35% of the patients who receive a dose greater than 400 mg/m2 of anthracyclines suffer from a reduction in their cardiac function ratios or even cardiac insufficiency,” says Rossi.
Some hypotheses have tried to explain the origin of doxorubicin damage to heart cells (cardiomyocytes); the most studied of these is oxidative stress. According to the advocates of this idea, anthracyclines generate free radicals, highly reactive molecules that might cause lesions in the membrane and in other cell components. “But this mechanism has been questioned more and more,” says Rossi. “This raised our interest in studying the cause of the cardiac lesion and enabled us to propose that it’s the consequence of damage to the structural proteins of the cardiomyocytes, principally dystrophin.”
To test this hypothesis, Érica treated three groups of rats over a three-week period with different doses of doxorubicin and analyzed what happened to the rodents’ hearts. “We evaluated the expression of proteins in the heart cells and also cardiac function 7 and 14 days after the last dose had been administered,” says Érica.
She found a significant loss of dystrophin in the cardiac cells. The bigger the dose of medication, the less dystrophin there was in the heart and the greater the mortality rate among the animals, the researcher reported in an article in the European Journal of Pharmacology. Screening examinations, such as echocardiography, also revealed that the loss of dystrophin compromised blood pumping capacity. “The loss of dystrophin was considered to be the structural cause responsible for the loss of cardiac function,” states Érica.
However, it was not enough only to confirm the harmful effect. The researchers also wanted to know how dystrophin is destroyed by the medication. “We discovered that the damage in the membrane of the cardiomyocytes enables the entry of more calcium ions,” explains Rossi. “This, in its turn, activates the proteases (enzymes) that damage the cells.”
Armed with this knowledge, the researchers started looking for ways to reduce the cardiac damage caused by the anthracyclins. One of the compounds that they tested was the muscle relaxant dantrolene, which reduces the contraction of cells by blocking the entry of calcium. Administered jointly with doxorubicin, the muscle relaxant reduced the loss of dystrophin and lesion points. “The rats treated with dantrolene and doxorubicin maintained a cardiac function similar to that of the control animals [which had received a placebo instead of a chemotherapeutic agent],” says Rossi. “Our findings are encouraging because they open up the possibility that in the future they may perhaps guide clinical practice.”
Dystrophin and its associated proteins in the pathogenesis of doxorubicin-induced cardiomyopathy (nº 2010/13199-1); Modality Regular Research Project Funding; Coordinator Marcos Antonio Rossi – FMRP/USP; Investment R$ 263,235.82 (FAPESP)
CAMPOS, E. C. et al. Calpain-mediated dystrophin disruption may be a potential structural culprit behind chronic doxorubicin-induced cardiomyopathy. European Journal of Pharmacology. v. 670(2-3), p. 541-53. 30 Nov. 2011.