JEFFREY GREENSTEIN, MD: The issue, of course, of neutralizing antibodies is a rather complicated one. It takes between nine and 18 months for neutralizing antibodies to develop, and it takes somewhere between three and six months in all probability for clinical effectiveness to be lost, so that if you're looking at a two-year study, it's hardly likely that you're going to see differences, or if you do see them, they're only beginning to become apparent, and in all probability we need to go for longer than two years to really see the significance of an impact of neutralizing antibodies.

FREDERICK MUNSCHAUER, MD: In the four year Avonex dose comparison trial reported just this year, the incidence -- or rather the prevalence of neutralizing antibodies in the conventional Avonex dose, 30 micrograms, was 2.3%. In the higher dose, the 60 microgram dose once-a-week, the prevalence of neutralizing antibodies was 5.8%. Both of those figures are quite low. They are lower than other preparations of interferon.

Why is that important? It's important simply because the emergence of neutralizing antibodies has been associated with a loss of therapeutic effect of interferons. So you want a drug that over the long haul -- in this case four years -- produces a low rate of neutralizing antibodies. I think that was a major result of this four-year Avonex dose comparison trial