Shareholders of Bayer and Regeneron Pharmaceuticals (NASDAQ:REGN) should adopt a new slogan: "An approval a month, that's always what we ask," because for the last three months that's what they've gotten for Eylea.

The duo announced Monday the most recent approval for treatment of myopic choroidal neovascularization (myopic CNV) in Japan . Last month, the EU approved Eylea to treat diabetic macular edema, or DME . And the July approval was by the US Food and Drug Administration, also for DME.

Source: Regeneron Pharmaceuticals.

Expanding possibilities
Eylea was originally approved in 2011 to treat wet age-related macular degeneration, or AMD, but the indication has become less important to the companies as they've racked up approvals for other eye diseases.

Eylea is now approved for three indications in the U.S. and EU: wet AMD, macular edema following central retinal vein occlusion, and DME.

Of the three, DME is likely to be the biggest growth story going forward in the U.S. Regeneron believes there are about 1.5 million patients with DME in the U.S. and potentially another 1 million that are undiagnosed. That's about the same size market as wet AMD, but DME provides more of a growth opportunity since it's a new indication .

Regeneron will see competition from Roche's Lucentis, which has been approved for DME for over two years, but the biotech has already proven it can compete against Lucentis in wet AMD. For DME, like wet AMD, Eylea can be dosed every other month after an initial monthly treatment regimen; for optimal efficacy, Lucentis is recommended as a monthly treatment in perpetuity .

Eylea is also more convenient for doctors to buy since it's dosed at the same strength for DME and wet AMD, so doctors only have to stock one strength of the drug, unlike Lucentis, which is available at 10mg/ml for wet AMD and 6 mg/ml for DME.

Beyond switching from Lucentis, Eylea should be able to capture some of the 60% of patients that aren't currently being treated by Lucentis. The less-frequent regimen might pull in patients that wouldn't otherwise want to have their eyeballs injected.

More to come
Regulators are currently reviewing applications to use Eylea for macular edema following branch retinal vein occlusion in the in the U.S., EU, and Japan . And it's up for approval in Japan, Asia Pacific, and Latin America for the treatment of DME .

Today's approval for myopic CNV in Japan might not be the biggest growth story, but it'll be additive to all the current and upcoming indications. Myopic CNV is the second most common cause of blindness in Japan, so it's meeting an unmet need even if it's only a small portion of the total sales.

Profiting from expansion
In the U.S., Regeneron is solely responsible for the marketing of Eylea and gets to keep all the profits. Outside the U.S., the biotech shares profits with Bayer, except in Japan where Bayer pays Regeneron a royalty on Eylea sales.

No matter where the drug is being sold, the nice thing about expanding into multiple eye diseases is that they're usually treated by the same doctors, so there's no need to increase the sales force, making new indications more profitable than the first one.

The only question now is whether the duo can keep their monthly approval record going.

Brian Orelli and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.