Ferric pyrophosphate Ferric pyrophosphate (FPP) is a water-soluble and stable compound that is known to trigger the direct transformation of transferrin to ferritin, thus boosting iron exchange between transferrin molecules [126]. Vaka and colleagues investigated the influence of ETP on the transcutaneous absorption of FPP [126]. Ferric pyrophosphate (FPP) is a water-soluble and stable compound that is known to trigger the direct transformation of transferrin to ferritin, thus boosting iron exchange between transferrin molecules [126]. Vaka and colleagues investigated the influence of ETP on the transcutaneous absorption of FPP [126].