Out of the five senses -- sight, sound, taste, touch, and smell -- what would you rate as the most important? They're all valuable, but I'd bet that most people would say that vision tops the list. Unless I'm in the midst of eating some Ben & Jerry's, I'd agree.

Unfortunately, of course, there are conditions that impair eyesight. One such condition is a disease called age-related macular degeneration (AMD), and it is the leading cause of vision loss in people 50 and older. AMD deteriorates the central vision that's used when a person looks straight ahead at an object. Instead of seeing a sharp and clear image, AMD patients instead have a blurry or dark area in the middle of the visual field. Vision loss gets progressively worse over time and affects normal daily activities like reading, driving, and watching TV. It cannot be corrected with lenses.

With hundreds of thousands of new cases occurring worldwide every year, AMD is quite common. Because this is a serious medical problem affecting a large number of people, the development of newer, more effective drugs is a high priority in the drug industry. AMD is a hot area of research in biotechnology -- and it is quickly becoming a crowded field.

From an investor's perspective, two important AMD drugs are on the market today. Visudyne, from QLT (NASDAQ:QLTI) and Novartis (NYSE:NVS), has been the standard of care. Because it was the first mover, Visudyne has a high market share, and its sales have reached nearly half a billion dollars annually.

Although Visudyne has long been a leading therapy in the treatment of AMD, the market is in flux. Eyetech Pharmaceuticals (NASDAQ:EYET) and Pfizer (NYSE:PFE) have recently launched Macugen, which has started to erode Visudyne's domestic market share.

Although Macugen was second on the scene, its position is not secure, either. Close on its heels are Genentech's (NYSE:DNA) Lucentis and Genaera's (NASDAQ:GENR) Evizon (formerly called Squalamine). Lucentis looks to be an especially noteworthy competitor in light of clinical trial data that was released yesterday.

In a phase 3 clinical trial, Lucentis met the primary endpoint of maintaining vision in patients with the "wet" type of AMD. In the study, 95% of patients taking Lucentis had their vision stabilize or improve over the course of a year of therapy. This compares favorably with 62% of patients who had this outcome in the control arm of the study, meaning that they were not using an active drug.

Genentech also reported that Lucentis improved the vision of patients over the course of the study. That is an interesting finding, considering that vision typically deteriorates over time in AMD patients. Details were not disclosed in the press release, but that might prove to be an important result. It will have to stand up to scrutiny later this summer at the annual meeting of the American Society of Retinal Specialists, during which the full data set will be unveiled.

While Lucentis is certainly shaping up as an important drug in the treatment of AMD, its impact on the competitive dynamics of the market remains uncertain. Even though its phase 3 data looks exciting, the fact remains that the control group in this study was a placebo, not an approved drug such as Visudyne or Macugen. It is not scientifically sound to draw conclusions about how good one drug is with respect to another unless a head-to-head trial is conducted.

Soon, though, a relative comparison will be able to be made: Genentech is conducting a comparative phase 3 study of Lucentis against Visudyne. The data from that trial should be released in the fourth quarter of this year. That study could very well be key in determining which drug is the standard of care in the treatment of wet AMD.

But this is not necessarily a winner-take-all situation. It could very well turn out that combinations of these drugs are more effective than any of them used alone. QLT, Eyetech, and Genentech are all interested in these combination studies. During the current quarter, Genentech is expected to release data from an early-stage clinical trial with Lucentis used in combination with Visudyne, compared to Visudyne alone.

Final thoughts
Lucentis will very likely give Genentech yet another drug in its rapidly growing portfolio. Given the timing of its clinical development, the drug could launch in 2007. Its entry into the AMD market will surely shake things up.

I view its emergence as potentially a big threat to Macugen because both drugs are designed to inhibit VEGF. (Apparently Mr. Market agrees. Eyetech is down more than 40% since Genentech's announcement.) VEGF is a growth factor that stimulates the formation of blood vessels, a process that is the underlying cause of wet AMD. Because of the similarity, the drugs are clearly direct competitors that will likely square off.

Lucentis' impact on Visudyne is not as clear. The two drugs might be complementary and so could be used together -- though that needs to be demonstrated in a clinical trial. Genentech will release the results of such a combination study in the near future. Aside from that outcome, the most important study is the head-to-head trial of Lucentis vs. Visudyne. That trial could influence which drug will have the largest market share in the treatment of AMD. This is a story that bears watching over the remainder of the year.

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Motley Fool Rule Breakers biotech analyst Charly Travers owns shares of QLT, which is a Motley Fool Hidden Gems recommendation. Pfizer is a Motley Fool Inside Value recommendation. The Fool has a disclosure policy.