An international collaborative study has identified several novel derivatives of anticancer proteasome inhibitors (PIs) that had potential as antimalarials, the authors reported in the September 28, 2021, online edition of Proceedings of the National Academy of Sciences (PNAS).

These agents were demonstrated to possess rapid, potent and selective activity against the Plasmodium falciparum (Pf) malarial parasite, including against drug-resistant strains, and were well tolerated with oral efficacy in a mouse model.

The study findings should aid development of new antimalarials, which are urgently needed to treat a disease that kills nearly half a million people annually, but is becoming increasingly resistant to artemisinin-based regimens.

"Artemisinin resistance is a major concern, especially in Southeast Asia, with this problem being significantly exacerbated by the recent clinical confirmation of resistance in Africa," said study co-leader Alexandra Gould, director of discovery strategy and operations at Takeda Pharmaceuticals International.

"Takeda scientists provided the study's medicinal chemistry design and synthesis support along with biochemical, cellular and drug metabolism and pharmacokinetic (DMPK) assessments of the new compounds," said Gould.

"We then collaborated with the team at The University of Melbourne and colleagues at the Medicines for Malaria Venture (MMV) to progress this project," she told BioWorld Science.

Malarial parasites are particularly susceptible to drugs compromising proteostasis, with the 20S Plasmodium proteasome enzymatic complex being a validated drug target.

"Both cancer cells and malaria grow very rapidly and are highly dependent on their proteasome, so the concept of repurposing cancer PIs as antimalarials has been known for some time," noted Gould.

"However, the proteasome is evolutionarily highly conserved, so generating specific Pf 20S inhibitors has been challenging. Moreover, most anticancer PIs are injected, so the difficulty has been to generate compounds with sufficient oral bioavailability and stability against malaria."

Previously, PIs have been demonstrated to rapidly kill the parasites at different developmental stages and strongly synergize with artemisinin-mediated killing of Pf in vitro and in mice.

The MMV recommends that new antimalarials show activity against all parasite stages and drug-resistant parasites, preferably with a different mechanism of action (MoA) to current treatments.

"Ideally, antimalarial treatments could be used for prophylaxis, treatment and prevention of transmission, which requires activity against the hepatic, and asexual and sexual blood stages of Pf development," said Gould.

"New antimalarials should also have different MoAs to existing compounds. Given the emerging or widespread resistance to all currently used agents, new compounds aimed at the same targets might be compromised by existing parasite resistance mechanisms."

The MMV further recommends that antimalarials should act rapidly and have pharmacokinetics enabling complete parasite clearance, preferably with a single oral dose.

Screening a library of peptidyl boronate inhibitors of the human proteasome has identified compounds chemically similar to bortezomib, which is used to treat multiple myeloma, with varying selectivity for inhibiting Pf growth.

"Bortezomib has activity against Pf growth, because proteasomes are highly conserved and both myeloma cells and Pf require high levels of protein synthesis to survive,"explained Gould. "High levels of protein synthesis require enhanced ability to eliminate misfolded or errant proteins, which is the primary function of the proteasome."

However, while bortezomib and related PIs have favorable safety and efficacy profiles in a cancer setting, malaria treatment requires selective inhibition of parasite growth, with minimal effects on human cells, as well as oral bioavailability.

In the new PNAS study, researchers co-led by Gould and Leann Tilley, a professor of biochemistry and pharmacology at The University of Melbourne, described a drug discovery program investigating the potential to optimize peptidyl boronate inhibitors as antimalarials.

Notably, they reported having identified potent and Pf-specific PIs showing fast-acting antimalarial activity, with increased potency against the Pf proteasome but reduced activity against the human enzyme.

"We used biochemical and cell-based assays to show that the new PIs have increased potency and selectivity for the Pf proteasome, which is important as anticancer PIs have toxic side effects in humans," Tilley told BioWorld Science.

The new compounds had a low propensity for developing resistance, and were able to block liver stage and transmission.

Furthermore, they selectively inhibited the growth of Pf compared with a human cell line and exhibited high potency against both Pf and Plasmodium vivax isolates, meaning "the same treatment could be used for both parasitic infections, even if the identity of the infecting species is unknown when starting treatment," said Tilley.

Two particularly promising compounds, MPI-5 and MPI-13, showed potent activity against Pf infections in a severe combined immunodeficiency (SCID) mouse model using a well-tolerated oral regimen.

The researchers further demonstrated that MPI-5 bound more strongly to Pf20S than to human constitutive 20S (Hs20Sc), "which underpins its selectivity," noted Tilley.

Moreover, cryoelectron microscopy revealed that the PIs bind to a hydrophobic pocket that is structurally different in the human proteasome.

Collectively, these findings provide valuable insights into inhibition of the 20S proteasome in Pf that will facilitate development of potent and selective antimalarial PIs, with MPI-5 and MPI-13 showing promise in this regard.

However, "for analogues to enter clinical trials, they need to be safe and effective, with the demonstrated ability of MPI-13 to cure malaria in the mouse model with no evidence of toxicity being very encouraging in this respect," said Tilley.

In fact, "our study is the first showing curative efficacy of orally delivered PIs in a mouse model of malaria," she noted.

"Our next step will be to improve the pharmacokinetics of these PIs, in order to generate compounds suitable for curing malaria in humans using a single oral dose."