Cobimetinib in Refractory Langerhans Cell Histiocytosis (LCH), and Other Histiocytic Disorders

This study is testing cobimetinib — a drug that blocks a specific protein involved in cancer cell growth — in patients with Langerhans cell histiocytosis (LCH) and other rare immune cell cancers that have not responded to prior treatment. LCH is a rare disease where abnormal immune cells accumulate and damage organs. **You may be eligible if...** - You have been diagnosed with LCH confirmed by biopsy, or another related histiocytic disorder - Your disease has come back or not responded to at least one prior line of treatment - You are at least 6 months old (age requirements vary by study group) - You are able to take oral medication (pill or liquid suspension, including through a feeding tube) - Adults 21 and older may qualify for Group 4 **You may NOT be eligible if...** - You have not had prior treatment that has failed - You are unable to take oral or enteral (tube-fed) medication - You do not meet the specific age criteria for the group you would be enrolled in Talk to your doctor to see if this trial is right for you.