5-(N-Ethyl-N-2-hydroxyethylamino)-2-penthlamine-d4 is labelled 5-(N-Ethyl-N-2-hydroxyethylamino)-2-penthlamine (E678855) which is a useful synthetic intermediate in the synthesis of Hydroxy Chloroquine (H905300/H905305); a major metabolite of Chloroquine (C379965, diphosphate salt) which is a standard anti-malarial drug.