In vitro melanin binding of NS-49, a phenethylamine class alpha(1A)-adrenoceptor agonist

Characteristics of the in vitro binding of NS-49 ((R)-(-)-3'- (2-amino-1-hp droxpethyl)-4'-fluoromethanesulfananilide hydrochloride, CAS 137431-04-0) t o synthetic melanin were determined and compared with those of other drugs. At all the drug concentrations tested (12.5-200 mu mol/l), chloroquine (CAS 54-05-7) had the highest melanin binding (%), followed by ofloxacin (CAS 8 2419-36-1), NS-49, norephedrine (CAS 154-41-6) and diazepam (CAS 439-14-5). Binding constants (Kb), calculated with the Langmuir adsorption isotherm w ere, in the order (largest to smallest) chloroquine, ofloxacin, NS-49, nore phedrine, the Kb value for NS-49 being one-thriteenth that for chloroquine, one-seventh that for ofloxacin and ten times higher than that for norephed rine. 53 %, of NS-49 that was bound to synthetic melanin dissociated during 24 h of incubation after the binding experiment. The percent of dissociation for NS-19 was higher than the values for chloroquine (5 %) and ofloxacin (39 % ). After 1 day incubation in the dissociation experiment, the amount of NS- 49 bound per mg of melanin was 23 nmol, being less than the amounts of chlo roquine (210 nmol) and ofloxacin (78 nmol). These findings indicate that NS-49 has a lower affinity for synthetic melan in than chloroquine and ofloxacin, which have high affinities and therefore accumulate in melanin containing ocular tissues.