Minoxidil sulfate is a sulfateester of minoxidil, not a sulfate salt of the compound.[3] However, minoxidil sulfate forms an inner salt, which makes it more hydrophobic than minoxidil.[3] This is in contrast to most sulfate esters, which are usually more hydrophilic than their non-ester forms.[3] The bioactivation of minoxidil into minoxidil sulfate is very unusual and is among the only known instances of sulfation producing a more active drug form.[3][4] Normally, sulfation tends to inactivate drugs by reducing their biological activity and increasing their excretion.[3][4]
Minoxidil sulfate is highly unstable in aqueous solutions and alcohol-containing solvents, with a half-life of 6hours in aqueous solutions and a further much lower half-life in alcohol-containing solvents.[3] This has served as a limiting factor in its potential pharmaceutical use and therapeutic effectiveness.[5] Moreover, minoxidil sulfate has a 40% higher molecular weight than minoxidil, and this may reduce its absorption into the scalp.[5] In any case, a minoxidil sulfate-based topical formulation has been investigated for the treatment of scalp hair loss.[6][5] Additionally, minoxidil-sulfate-based topical formulations appear to be available for medical use in some parts of the world, for instance in Brazil.[5][7]
^ abVillani A, Fabbrocini G, Ocampo-Candiani J, Ruggiero A, Ocampo-Garza SS (July 2021). "Review of oral minoxidil as treatment of hair disorders: in search of the perfect dose". Journal of the European Academy of Dermatology and Venereology. 35 (7): 1485–1492. doi:10.1111/jdv.17216. PMID33660357. S2CID232115094.
^ abcdefgMeisheri KD, Johnson GA, Puddington L (January 1993). "Enzymatic and non-enzymatic sulfation mechanisms in the biological actions of minoxidil". Biochemical Pharmacology. 45 (2): 271–279. doi:10.1016/0006-2952(93)90061-z. PMID8435087.