The drug, diltiazem, sold under such names as Cardizem and Tiazac, is approved for treating high blood pressure and angina. French scientists tested it in mice and presented their findings in the October issue of the journal Nature Medicine.
Scientists are now trying to arrange for studies in humans.
"If the treatment could postpone the occurrence of blindness by a few years, it would be a big achievement, but this remains to be demonstrated in humans," said one of the study's authors, Serge Picaud of Louis Pasteur University in Strasbourg, France, and INSERM, the French equivalent of the National Institutes of Health.
Picaud said RP patients should not start taking the prescription drug on their own in hopes of treating the eye disease. He also noted that the doses given to the mice were much higher than what people take for heart conditions.
"It's a milestone," Dr. Jerry Chader, chief scientific officer of the Foundation Fighting Blindness, said of the study. He said it shows that a drug known to be relatively safe might slow down at least some forms of RP.
But he, too, warned that RP patients should not flock to the drug because of the possibility of side effects. Interested patients could discuss the study with an ophthalmologist who specializes in RP, he said.
"What we don't know is whether this is going to be effective in humans," he said.
RP is actually a group of diseases that gradually destroy the eye's light-sensing cells, called rods and cones. No treatment is known to stop the progressive blindness, though doses of vitamin A can retard it slightly in some cases.
Defects in more than 100 genes cause the various types of RP, which strike at a variety of ages. Only a few percent of RP cases come from the defective gene duplicated in the mice, but the drug treatment might work in other cases as well, Chader said.
He noted that in people, the kind of RP mimicked in the mice can be a particularly virulent form that is detected in childhood or the early teens and leads to early vision loss. So a protective therapy might have to start in early childhood, raising issues of safety, he said.
Side effects of diltiazem can include headaches, dizziness and a slowed heartbeat.
Picaud said his study did not involve any money from the pharmaceutical industry.
It is not clear how diltiazem retarded blindness in the mice. The drug is one of a class called calcium channel blockers, so-named because they block charged particles of calcium from passing through pores on the surface of cells.
The mouse mutation would keep those pores open all the time on rods. So by plugging the pores, diltiazem might prevent an abnormal influx of calcium ions that would kill those cells, Picaud said. The protected rods might then help conesurvive, he said.
In the study, mice were injected daily with diltiazem, beginning nine days after birth. This prolonged survival of the rods and cones. By age 25 days, all seven treated mice showed evidence of functioning retinal cells, while none of the seven untreated mice did.