Beta-lapachone, a naturally occurring member of the cytotoxic ortho-quinone family of compounds, failed clinical trials back in the 1960s due to its systemic toxicity, but in 2017 a team led by Gonçalo Bernardes, group leader at the Instituto de Medicina Molecular João Lobo Antunes in Portugal and Professor at the University of Cambridge in the UK, started a research program looking for potential targets for the compound.
“At that time, we knew it had anti-proliferative activity but had no idea through which mechanism or mechanisms,” said Tiago Rodrigues, Assistant Professor in the Faculty of Pharmacy at University of Lisbon and co-author of the Nature Chemistry paper in which the findings are published.
Using machine learning, the researchers discovered that beta-lapachone strongly inhibits the enzyme 5-lipoxygenase. This enzyme is also a candidate therapeutic target for the stem-cell-like blasts in AML.
However, the team still had the toxicity problems to contend with. Ortho-quinones produce reactive oxygen species, which in turn disrupt protein function and lead to DNA oxidation, ultimately resulting in cell death. Although this is a good tool against cancer cells, side effects include anemia and methemoglobinemia, a disorder in which the hemoglobin molecule is functionally altered in a way that prevents it from carrying oxygen.
Expertise in protein modification and experience in bringing antibody–drug conjugates (ADC) from academic laboratories to clinical trials allowed the researchers to tackle this obstacle in two ways.
Firstly, the team modified the structure of beta-lapachone by adding a chemical group to the side of it. This created a prodrug in which the toxic activity of the compound is masked. Once inside the cell, the prodrug is cleaved by cellular enzymes and the active part of beta-lapachone is released.
The investigators then created an ADC in which the modified beta-lapachone was attached to an antibody that binds to CD33, a cellular marker specific to AML and not present on healthy cells. This meant the drug could be delivered to the cancer cells without damaging healthy cells.
The dual modification is an important feature. “If beta-lapachone was merely attached to the antibody, it could still lead to metahaemoglobinemia,” said Rodrigues. “Our construct completely abrogates this adverse reaction through new chemistry that we developed, which is fantastic.”
In vivo studies in mouse models of human AML showed that the ADC had “strong antitumor activity” after just two doses. ADC-treated mice had significantly decreased tumor volumes and significantly improved survival, without significant toxicity, relative to those given a control compound.
Rodrigues told Inside Precision Medicine that the team is now “paving the way to clinical studies of this quinone but also others, potentially.”
He added: “We have shown that the chemistry can be applicable to other bioactive natural products, which opens unprecedented avenues in chemotherapy.”
The researchers are now taking the necessary steps to translate their findings to clinical trials, in the same way that they did in 2018 for a different natural product that targeted glioblastoma.