The purpose of this study is to evaluate the investigational oral drug AMXT 1501 in combination with oral eflornithine (DFMO). An investigational drug is one that has not been approved by the U.S. Food \& Drug Administration (FDA), or any other regulatory authorities around the world for use alone or in combination with any drug, for the condition or illness it is being used to treat.

The goals of this part of the study are:

* Establish a recommended dose of AMXT 1501 in combination with DFMO
* Test the safety and tolerability of AMXT 1501 in combination with DFMO
* To determine the activity of study treatments chosen based on:
* How each subject responds to the study treatment
* How long a subject lives without their disease returning/progressing

Protocol JDI2007-01 is an Expanded Access Protocol with therapeutic 131I-MIBG for patients with neuroblastoma or pheochromocytoma / paraganglioma, who otherwise do not qualify for available treatments, or where approved treatment is not commercially available.