Intratympanic injection of dexamethasone: time course of inner ear distribution and conversion to its active form.
OBJECTIVE
Intratympanically injected dexamethasone 21-phosphate is converted to its active form dexamethasone in the inner ear and follows the distribution of the glucocorticoid receptor.


BACKGROUND
Although dexamethasone is routinely delivered intratympanically for hearing loss, we know little of its inner ear pharmacokinetics. Dexamethasone 21-phosphate is the pharmaceutical compound available for injection, but it must be converted to its biologically active form (dexamethasone) to bind to the glucocorticoid receptor. Therefore, the current study was conducted to determine the time course of dexamethasone 21-phosphate movement from the middle ear into the inner ear, its conversion to dexamethasone, and the distribution of both forms relative to the glucocorticoid receptor.


METHODS
BALB/c mice were injected intratympanically with the prodrug dexamethasone 21-phosphate and inner ears collected at postinjection times ranging from 5 minutes to 7 days. Ears were immunohistochemically stained for dexamethasone 21-phosphate, dexamethasone, and the glucocorticoid receptor.


RESULTS
Both forms of dexamethasone were seen in the inner ear within 15 minutes, reaching their highest staining intensity at 1 hour. Neither drug was seen after 24 hours. The strongest staining occurred in the spiral ligament, organ of Corti, spiral ganglion, and vestibular sensory epithelia. Distribution of the drug paralleled locations of the glucocorticoid receptor except in the stria vascularis marginal cells, which stained heavily for the receptor but not the drug.


CONCLUSIONS
Dexamethasone rapidly travels from the middle ear into the inner ear and converts to its active form. The drug distribution follows that of the glucocorticoid receptor. However, it probably has little impact on ear tissues after 24 hours.
Hargunani
Christopher A
CA
Oregon Hearing Research Center, Department of Otolaryngology-Head & Neck Surgery, Oregon Health & Science University, Portland, Oregon 97201-3998, USA.
Kempton
J Beth
JB
DeGagne
Jacqueline M
JM
Trune
Dennis R
DR
D000818:Animals; D003907:Dexamethasone / Q000008:administration & dosage* / Q000493:pharmacokinetics*; D007758:Ear, Inner / Q000378:metabolism*; D004432:Ear, Middle / Q000378:metabolism; D005938:Glucocorticoids / Q000008:administration & dosage* / Q000493:pharmacokinetics*; D007150:Immunohistochemistry; D015552:Injections, Intralesional; D051379:Mice; D008807:Mice, Inbred BALB C; D009412:Nerve Fibers / Q000378:metabolism; D009925:Organ of Corti / Q000737:chemistry / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000032:analysis; D013136:Spiral Ganglion / Q000378:metabolism; D013316:Stria Vascularis / Q000378:metabolism; D014722:Vestibule, Labyrinth / Q000737:chemistry / Q000378:metabolism
D005938:Glucocorticoids; D011965:Receptors, Glucocorticoid; D003907:Dexamethasone