Research has proven the effectiveness of nitric oxide in lowering intraocular pressure (IOP) in multiple animal and human studies.  Nitric oxide is a known vasodilator, which can help reduce IOP by decreasing episcleral venous pressure that creates hemodynamic resistance against the flow of aqueous out of the eye. Nitric oxide has an additional benefit of increasing outflow of the aqueous humor through the trabecular meshwork and Schlemm's canal, two of the more prominent drainage structures of the anterior chamber. Prostaglandin analogue category drugs also increase aqueous outflow and drainage, primarily through the uveoscleral pathway (which includes flow across the ciliary muscle and choroid). Theoretically a combination of nitric oxide and prostaglandin analogue medication would cause an additive effect of increased outflow.  Studies suggest that as much as 75% of the aqueous humor outflow occurs through the trabecular meshwork through Schlemm's canal, while the remaining 25% is from other tissue.  That means our most effective glaucoma drop therapy on the market currently (prostaglandin analogues) targets that 25% of outflow, while if we could target trabecular meshwork outflow (like nitric oxide appears to do in studies) we'd be influencing a much larger area of impact.  

No news yet on how much IOP lowering effect we should see from the addition of nitric oxide, but Valeant has targeted a summer 2016 date for FDA testing approval and results should be available at this time.