In a new study published (Jun 2014) in Molecular Cancer (an open-access, peer-reviewed journal) alpha-mangostin, a substance derived from mangosteen pericarp, was shown to kill up to 99% of human breast cancer cells in vitro.*
What was particularly interesting about this study was that the researchers gained a deeper understanding of the mechanism of action of the mangosteen-derived substance. Alpha-mangostin was reported to block Fatty Acid Synthase – which disrupts the cancer cells’ ability to make fatty acids – without which they die. 
It appears that numerous cancer cell types require Fatty Acid Synthase for their survival, which may be the reason why mangostin has received much attention recently; it has also been found deadly also to prostate, liver, colon, and pancreatic cancers as well as leukemia.
Mangosteen (Garcinia mangostana) is an amazing, delicious tropical fruit – one of my favorites in fact – and was originally native to Thailand. Don’t be fooled by the name, it does not look or taste like a mango. Mangosteen has a thick, pithy casing (pericarp) which is easily broken or cut open to reveal the white, fleshy part inside that is eaten. Mangosteen is also considered potentially valuable with weight loss, inflammation, heart disease and diabetes! However it is the pericarp that may have the strongest medicinal qualities. For centuries, people in Southeast Asia have used dried mangosteen pericarp as antiseptic, anti-inflammatory, anti-parasitic, antipyretic, analgesic, and as a treatment for rashes. 
Tests like the study mentioned are the first step in the development and study of new medicines. It’s a long way before we can say for certain that mangosteen will have a direct anti-cancer action in humans – and in this study the beneficial substance was in the pericarp (the pithy case). However there have now been several promising studies on mangosteen extracts – including a 2013 study in which alpha-mangostin significantly suppressed tumor growth and reduced lymph node metastasis in mammary cancer in mice; leading researchers to conclude that “Mangosteen extracts appear to, in fact, have chemopreventive qualities and might prove useful as adjunctive and/or complementary alternative treatments in human breast cancer.” 
* “In vitro” (literally “in glass”) is a scientific term used to denote that the test was done on isolated cell cultures in laboratory glassware, as opposed to in living creatures (“In vivo”).
As always, this page is not medical advice.
 Alpha-mangostin inhibits intracellular fatty acid synthase and induces apoptosis in breast cancer cells – (Molecular Cancer, Jun 2014) – http://www.molecular-cancer.com/content/13/1/138 (Full text). Retrieved July 25th 2014. Pubmed: http://www.ncbi.nlm.nih.gov/pubmed/24894151
 Effects of Mangosteen Pericarp Extracts Against Mammary Cancer – Alternative and Integrative Medicine, 2013 – http://esciencecentral.org/journals/effects-of-mangosteen-pericarp-extracts-against-mammary-cancer-2327-5162.1000139.pdf (Full text). Retrieved July 25th 2014.