With loglines, it’s good to try to use the least amount of words possible; for-instance, instead of saying, while “Following the leads” “You could say while “Investigating” which means the same thing, but instead of three words you are using one.

Also, this is only half a logline. You have told us who the protagonist is but you haven’t told us what she must do and what’s at stake.

“While investigating her brothers disappearance, a tenuous young woman discovers a secret underground lab filled with horrific man-eating creatures, now she must…. (Then you add what she must do and what the stakes are)

Now she must… save her brother and escape before they are both killed

Now she must… destroy the lab before the beasts escape and terrorize her small town.

Now she must… discover creator of the monstrosities and stop his evil plan before his nightmares are unleash upon the world.

Hope that helped, good luck with this.