A single dose of ketamine was discovered a couple of years back to provide a complete remission for weeks, for some varieties of depression including some that did not respond to any approved meds. But unfortunately ketamine is a "schedule 3" drug in the US, for reasons (strongly addictive, tends to persistent hallucination; cf. John Lilly).

Then somebody observed that DM operated on some of the same receptors, or something, and tried it, and then initiated a full RCT on a very limited dose. Approval for that coupled with a low-ish dose of bupropion just came out a few weeks ago. Presumably higher-dose trials are in the works now.

Depression is what is often called a "wastebasket diagnosis", analog to a wastebasket, or paraphyletic, taxon in biology, where unrelated species have been lumped together because their actual relationships were too hard to tease out before DNA analysis became practical. (Falcons ended up having to be split from hawks, on DNA evidence they are sister to e.g. pigeons, over birders' outrage.)

Numerous unrelated pathologies get called "depression", so the only way to distinguish them is by what med they respond to, if any. It is why we keep seeing spurious media reports that depression meds "don't work": RCTs depend utterly for validity on accurate diagnosis, which for depression does not exist.

So, search anywhere for "ketamine depression" for the original discovery, and "dextromethorphan bupropion" for the recent FDA announcement.