Additional Study Information: This is a two-part research study of an investigational drug called ABBV-321 in subjects with glioblastoma (brain tumor). This study will be conducted in two-parts: 1) Dose Escalation Phase: The purpose of the first part, called a Dose Escalation Phase, is where we will increase the dose of ABBV-321 until we find the highest dose before unmanageable side effects are experienced, which is called the Maximum Tolerated Dose (MTD). 2) Expanded Safety Phase: The purpose of the second part, called the Expanded Safety Phase, is where we will administer the maximum tolerated dose (MTD) from the Dose Escalation Phase in subjects with glioblastoma multiform (GBM) in order to gain a better understanding of the safety of the drug at this dose in a larger number of people.