The way enacarbil works, provides the sustained quality.
It is a prodrug and absorbed better than gabapentin, but has to be activated into gabapentin and that takes time--hence the sustained quality. This makes this form of gabapentin a NEW DRUG...hence MORE FDA attention and more approval issues.
more on this here:
http://chembl.blogspot.com/2011/04/n...1-part-xi.html

When I went to FDA today only one enacarbil product was listed=Horizant.

Depomed makes sustained products based on older drugs.
This makes FDA approval much easier, because it is only gabapentin, not a new drug form of it. They only have to provide bioavailability studies, and not efficacy studies like with the enacarbil form.
It is listed on the gabapentin page of the Orange Book. It does not state there ER either--which is confusing. Gralise is the name of it.
The delivery system is explained here:
http://www.rxlist.com/gralise-drug.htm

I agree it is very confusing, and a potential problem at the pharmacy level, most decidedly. Also knowing how doctors are, there is a good potential for wrong prescribing.