Malaria
puts at risk nearly half the world’s population
and causes high mortality in sub-Saharan Africa, while drug resistance
threatens current therapies. The pyrimidine biosynthetic enzyme dihydroorotate
dehydrogenase (DHODH) is a validated target for malaria treatment
based on our finding that triazolopyrimidine DSM265 (1) showed efficacy in clinical studies. Herein, we describe optimization
of a pyrrole-based series identified using a target-based DHODH screen.
Compounds with nanomolar potency versus Plasmodium DHODH and Plasmodium parasites were
identified with good pharmacological properties. X-ray studies showed
that the pyrroles bind an alternative enzyme conformation from 1 leading to improved species selectivity versus mammalian
enzymes and equivalent activity on Plasmodium falciparum and Plasmodium vivax DHODH. The best
lead DSM502 (37) showed in vivo efficacy
at similar levels of blood exposure to 1, although metabolic
stability was reduced. Overall, the pyrrole-based DHODH inhibitors
provide an attractive alternative scaffold for the development of
new antimalarial compounds.
The catechol derivative
RC-12 (WR 27653) (
1
) is one
of the few non-8-aminoquinolines with good activity against hypnozoites
in the gold-standard
Plasmodium cynomolgi
–rhesus monkey (
Macaca mulatta
) model, but in a small clinical trial, it had no efficacy against
Plasmodium vivax
hypnozoites. In an attempt to better
understand the pharmacokinetic and pharmacodynamic profile of
1
and to identify potential active metabolites, we now describe
the phase I metabolism, rat pharmacokinetics, and
in vitro
liver-stage activity of
1
and its metabolites. Compound
1
had a distinct metabolic profile in human vs monkey liver
microsomes, and the data suggested that the
O
-desmethyl,
combined
O
-desmethyl/
N
-desethyl,
and
N,N
-didesethyl metabolites (or a combination
thereof) could potentially account for the superior liver stage antimalarial
efficacy of
1
in rhesus monkeys vs that seen in humans.
Indeed, the rate of metabolism was considerably lower in human liver
microsomes in comparison to rhesus monkey microsomes, as was the formation
of the combined
O
-desmethyl/
N
-desethyl
metabolite, which was the only metabolite tested that had any activity
against liver-stage
P. vivax
; however,
it was not consistently active against liver-stage
P. cynomolgi
. As
1
and all but one of
its identified Phase I metabolites had no
in vitro
activity against
P. vivax
or
P. cynomolgi
liver-stage malaria parasites, we suggest
that there may be additional unidentified active metabolites of
1
or that the exposure of
1
achieved in the reported
unsuccessful clinical trial of this drug candidate was insufficient
to kill the
P. vivax
hypnozoites.
OZ439 is a potent
synthetic ozonide evaluated for the treatment
of uncomplicated malaria. The metabolite profile of OZ439 was characterized in vitro using human liver microsomes combined with LC/MS-MS,
chemical derivatization, and metabolite synthesis. The primary biotransformations
were monohydroxylation at the three distal carbon atoms of the spiroadamantane
substructure, with minor contributions from N-oxidation
of the morpholine nitrogen and deethylation cleavage of the morpholine
ring. Secondary transformations resulted in the formation of dihydroxylation
metabolites and metabolites containing both monohydroxylation and
morpholine N-oxidation. With the exception of two
minor metabolites, none of the other metabolites had appreciable antimalarial
activity. Reaction phenotyping indicated that CYP3A4 is the enzyme
responsible for the metabolism of OZ439, and it was found to inhibit
CYP3A via both direct and mechanism-based inhibition. Elucidation
of the metabolic pathways and kinetics will assist with efforts to
predict potential metabolic drug–drug interactions and support
physiologically based pharmacokinetic (PBPK) modeling.
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