Is the 'take the useful fragment' design idea any good in general?

It recurs across this whole site. HGH Frag 176-191 from growth hormone. KPV from alpha-MSH. Semax from ACTH with the hormonal activity removed. TB-500 from thymosin beta-4.

The idea is elegant: isolate the region responsible for the effect you want, discard the rest, get the benefit without the baggage.

How often does it actually work?

Sometimes, and the failure mode is instructive.

It works when the activity really is localised to a contiguous region that folds the same way on its own. Semax is the best case here — a genuine registered medicine came out of it.

It fails when the activity depends on the whole molecule's shape, which for proteins is more often than not. A fragment can be the right sequence and the wrong conformation, and then it binds poorly or not at all. It also usually loses whatever stability the parent had.

AOD-9604 is the honest test case: the design worked in the sense that the fragment did separate the fat activity from the glucose problems, and it did not work in the sense that the trial did not show enough effect to continue. Both halves are real.

"Right sequence, wrong conformation" is the crisp version of what I was circling. Thank you.

And note the pattern in which fragments get sold: the ones whose parent had a good story, regardless of whether the fragment inherited it. The story travels better than the conformation does.