I wonder about the weight too. I remember I read something about weight and clearance from the body in the insert. I went searching and found what I was looking for from Elan.

http://www.elan.com/Images/EMEA%20SPC_tcm3-14693.pdf

"Only body weight and the presence of anti-natalizumab antibodies were found to influence natalizumab disposition. Body weight was found to influence clearance in a less-than-proportional manner, such that a 43% change in body weight resulted in a 31% to 34% change in clearance."

And this from NIH.

http://www.pubmedcentral.nih.gov/art...?artid=1936307

"Analysis of pharmacokinetic data from Phase 2 trials and population modeling indicated that natalizumab clearance was only weakly correlated with body weight (over the range of 40 kg to 100 kg), but that natalizumab exposure (area under the plasma concentration-time curve and maximum plasma concentration), increased in proportion to weight despite the use of weight-based dosing (Bennett et al 2002; Rudick and Sandrock 2004; Biogen Idec Data on File). However, the effect of patient weight on natalizumab clearance and exposure was within the typical inter-patient range and was not considered clinically relevant. The 300 mg fixed dose selected to achieve maximum α4-integrin saturation was expected to produce acceptable pharmacokinetics in Phase 3 trials (Table 1)."


Essentially it sounds as if weight does make a difference but it is not "clinically relevant." If that's the case I have to wonder how they know with certainty it is not relevant?