This randomized dose-ranging study will evaluate the safety, tolerability, and preliminary efficacy (reduction in intraocular pressure) of multiple dose levels of ATS907, vehicle, or latanoprost in subjects with primary open angle glaucoma or ocular hypertension. In the first portion, approximately 75 subjects will be randomized to receive either ATS907 or vehicle eye drops for up to 28 days, administered both once and twice daily. In the second portion, up to 180 subjects will be randomized to receive either ATS907 or latanoprost for up to 28 days. Plasma pharmacokinetics will also be evaluated during the first portion of the study.