The manufacturer of
co-artemether notes that
in vitro lumefantrine significantly inhibits CYP2D6. They therefore contraindicate any drug that is metabolised by CYP2D6, and they name metoprolol. Additive QT-prolonging effects are likely with
sotalol, see drugs that prolong the QT interval.
This seems very restrictive as metoprolol is not contraindicated with proven CYP2D6 inhibitors. Until more is known, it would be prudent to at least monitor the effects of concurrent use.