A search in the Pubmed database for "soursop cancer" (which is automatically expanded to
("annona"[MeSH Terms] OR "annona"[All Fields] OR "soursop"[All Fields]) AND ("neoplasms"[MeSH Terms] OR "neoplasms"[All Fields] OR "cancer"[All Fields]), so it will not only show exact matches) shows 43 results, though none of those are trials in humans.
One of those results is an article titled "Complementary therapies, herbs, and other OTC agents.", in the abstract it states
Graviola demonstrated anticancer effects in vitro, but has not been studied in humans. Despite the lack of human data, many websites promote graviola to cancer patients based on traditional use and on the in vitro studies. Caution is required as there is no evidence of safety or efficacy.
There are several in vitro studies, but no human studies seem to have been performed. The result of such studies can't just be transferred to humans, you have to actually perform some clinical studies. The in vitro studies show that it has some potential as an anti-cancer drug, but that is a far shot from being proven effective in actually being safe and effective against cancer in humans.
There is also some data that the annonacin in graviola can cause an atypical Parkinson disease (1, 2,3, 4, 5).
With the possible danger of causing atypical Parkinson and no evidence that soursop is actually effective against cancer in humans, I would strongly recommend to avoid it.