Abstract

(R)α-Methylhistamine [(R)α-MeHA], a potent and selective histamine H₃ receptor agonistin vitro and in vivo in rodents, was found to display comparatively low plasma level in healthy human volunteers, attributable to an extensive methylation of the drug’s imidazole ring by histamine-N-methyltransferase. To limit this inactivation process, BP 2-94, i.e., (R)-(-)-2-[[N-[1-(1H-imidazol-4-yl)-2-propyl]imino]phenylmethyl] phenol, was selected as a prodrug. A sensitive radioimmunoassay was developed to study the generation of (R)α-MeHA slowly released from BP 2-94 in vitro and in vivo by chemical hydrolysis. In mice after oral administration of BP 2-94 high levels of both prodrug and (R)α-MeHA were detected in plasma and various tissues except in the brain. In humans receiving 0.1 mmol BP 2-94 orally, plasma levels of(R)α-MeHA-like immunoreactivity decayed with at_1/2 more than 24 hr, the area under the curve being two orders of magnitude higher than after oral administration of (R)α-MeHA. BP 2-94 displayed antiinflammatory and antinociceptive properties in rodents, related to the H₃receptor stimulation. It dose-dependently inhibited capsaicin-induced plasma protein extravasation in many rat tissues with ED₅₀s of 0.6 to 14 μmol/kg p.o., and maximal reductions by 35 to 87%. BP 2-94 also reduced zymosan-induced paw swelling in mice with an ED₅₀ of 1 μmol/kg p.o. and showed marked activity in the phenylbenzoquinone-induced writhing (ED₅₀ = 0.03 μmol/kg, p.o.) or formalin tests in mice, but not in the hot plate jump test. From its pharmacokinetics and pharmacological profile BP 2-94 appears to be a promising novel therapeutic agent in disorders such as asthma, migraine or a variety of inflammatory diseases and pain associated with these disorders.