Fascinating! Have never heard of this drug - as a treatment for glaucoma or for this new use either. Sometimes the evolution of drug indications is quite amazing; for example Viagra was first intended as an anti-hypertensive because it causes vasodilation, but when people started noticing an interesting side effect its use changed completely!!
I was curious to know why it causes the changed pigmentation, so I did some digging around. This is from MIMS (one of the main Australian drug guidebooks) in case anyone else is interested:
"During treatment with Lumigan, darkening of the eyelid skin and gradual increased eyelash growth (lengthening, darkening and thickening) with no consequent untoward ocular effects have been observed. Increased iris pigmentation has also been reported. The change in iris pigmentation occurs slowly and may not be noticeable for several months. The effect has been seen in up to 2% of patients treated with Lumigan for up to six months. Before treatment is initiated, patients should be informed of the possibility of eyelash growth, darkening of the eyelid skin and increased iris pigmentation. Some of these changes may be permanent and may lead to differences in appearance between the eyes when only one eye is treated.
Preclinical findings. Ocular administration of bimatoprost in monkeys at concentrations of 0.03 or 0.1% once or twice daily for one year caused an increase in iris pigmentation and reversible dose related periocular effects characterised by a prominent upper and/or lower sulcus and widening of the palpebral fissure. No associated increase in melanocyte number was observed with the pigmentation. It appears that the mechanism of increased iris pigmentation is due to increased stimulation of melanin production in melanocytes and not to an increase in melanocyte number.
Periocular effects were also observed in an intravenous toxicity study at systemic exposures at least 235-fold higher than that observed in humans after ocular administration. No functional or microscopic changes related to the periocular effects were observed. The mechanism of action for the observed periocular changes is unknown."
