Table 1.

Pharmacological characteristics of HNMT inhibitors.

Inhibitor	IC50	Effects on Histamine Brain Levels	Reference
Metoprine	100 nM	Metoprine administered at 10 mg/kg (orally) can increase histamine levels, being the highest 5 h after administration and correlates with peak drug levels in the brain. Histamine levels are still elevated more than 2-fold 4 h after administration of the drug.	[144]
Amodiaquine	400 nM	Antimalarian agent, potent HNMT inhibitor by in vitro studies. However, amodiquine did not change the endogenous histamine level in the rat brain.	[148]
Quinacrine	160 nM	Quinacrine, a drug that inhibits HNMT in vitro, has little or no effect on the levels in vivo of histamine.	[149]
Etoprine	760 nM	Etoprine inhibits dihydrofolate reductase. Etoprine can cross the BBB; however, its effect on histamine brain level has not been explored.	[150]
Dimaprit	8 μM	Dimaprit, an H2R antagonist, is a potent HNMT inhibitor. Dimatiprit increases histamine brain levels after intracerebroventricularly administration in rats.	[151]
SKF91488	1.85 μM	Due to the low permeability of the BBB SKF91488, research for this compound has been limited.	[152]