Carbonic Anhydrase Inhibitor (CAI) Eye Drops Medication for Glaucoma

Approved for use in the USA for the last 20 years, this class of medications is rarely prescribed on its own. 

How it works:

Works on the ciliary body to reduce production of aqueous fluid. 

Available in the USA:

Trusopt (dorzolamide)
Azopt (brinzolamide) 

Cap color:

Orange 

Note on Fixed Combinations:

• Cosopt (dorzolamide + timolol)
• Simbrinza (brimonidine + brinzolamide)
• Various combinations of dorzolamide plus other classes available through compounding pharmacies such as Imprimis Pharmacy
  • Some are even preservative-free

Note about Preservative-Free options:

Most prescription eye drops have a preservative which keeps bacteria from growing in the bottle. However, the most commonly used preservative, Benzalkonium chloride (BAK), can worsen dry eye and may even be toxic to the trabecular meshwork over time. Given that the trabecular meshwork is already not functioning well in most forms of glaucoma, I generally prefer to prescribe preservative-free versions of glaucoma eye drops when available and affordable. 

Cosopt PF is the branded preservative-free fixed combination dorzolamide + timolol 

Imprimis has a number of preservative-free fixed combinations of dorzolamide plus other classes

How it is taken:

These eye drops are often prescribed to be used twice daily (especially when in the fixed combination Cosopt). However, there is evidence that three times daily (about every eight hours) provides additional IOP lowering benefit. I generally tell my patients to use them twice daily (once in the morning and again in the evening) and (if they remember) to do so early afternoon as well. I’m well-aware of how difficult it is to remember and find the time to take a medication during what is often the busiest time of the day. 

How well does it work:

Just OK. Not quite as well as timolol^[1]. On average this class of topical medications will provide a 20% reduction in IOP^[2]. Given that most glaucoma specialists will attempt to reduce the IOP by 20-30% when concerned about worsening glaucoma, this class will miss the mark about 50% of the time. This is why this class is usually an “add-on” or prescribed as part of a fixed combination eye drop.

It should also be noted that the IOP lowering benefit of generic dorzolamide appears to be manufacturer dependent. In other words, not all generic dorzolamide eye drops work as well as the branded Trusopt. 

Cost?

All over the place. Generic dorzolamide tends to be covered whereas the branded Trusopt, Azopt, Cosopt, and Cosopt PF (preservative-free) are often associated with a higher copay. 

If you are prescribed this medication and it is not covered by your insurance or even with insurance coverage it’s quite expensive then I recommend you search out a good cash price through the website “GoodRx”. I frequently recommend this site to my patients and have been surprised at how much they can often save on certain classes of medications despite not using their insurance. 

When might these be prescribed?

Typically after another class of medication has failed to achieve adequate IOP lowering. 

Side effects:

Local^[3]:

• Stinging/irritation of the eye
• Bitter taste

Systemic (tend to be rare with topical therapy but still possible):

• Need to urinate more frequently
• Gastrointestinal upset
• Fatigue
• Rare but life-threatening:
  • Stevens-Johnson Syndrome
  • Aplastic anemia (low blood count)

How to minimize side effects:

Bitter taste can be limited by:

• Digital punctal occlusion
• Balled up tissue
• Eye drop applicator such as the Simply Touch (available on Amazon.com)

Summary:

As with most available glaucoma medications, this class works by reducing the production of fluid in the eye. It does not directly affect the problem area of glaucoma which is the reduced flow through the trabecular meshwork. It’s IOP-lowering benefit is less impressive than most other classes of glaucoma medications and the generic version of dorzolamide may not be as effective as the branded topical CAIs. As such, this class tends to be a second- or third-line agent reserved for those with moderate to advanced glaucoma or in whom the IOP has not been adequately controlled with first-line glaucoma eye drops.

References

[1] Strahlman E, Tipping R, Vogel R, et al. A double-masked, randomized 1-year study comparing dorzolamide (Trusopt), timolol, and betaxolol. Arch Ophthalmol. 1995;113:1009

[2] Sall K. The efficacy and safety of brinzolamide 1% ophthalmic suspension (Azopt) as a primary therapy in patients with open-angle glaucoma or ocular hypertension: Brinzolamide Primary Therapy Study Group. Surv Ophthalmol. 2000;44(suppl 2):S155

[3] Barnebey H, Kwok SY. Patients’ acceptance of a switch from dorzolamide to brinzolamide for the treatment of glaucoma in a clinical practice setting. Clin Ther. 2000;22(10):1204–1212.

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