MDD
Major depressive disorder, or MDD, is the clinical diagnosis of depression itself, the broad population from which treatment-resistant depression is drawn once two adequate courses of antidepressants have failed.
Why it matters here
In our opinion the single most useful thing to know about MDD is that it is the larger and less sick group, so a result in MDD and a result in TRD are two different tests of the same drug and cannot be netted against each other.
The European regulator's depression guideline draws the line between the two populations: treatment resistance is defined as a lack of clinically meaningful improvement despite at least two adequate antidepressant courses, and everything short of that is MDD (EMA/CHMP/185423/2010 Rev 2, guideline PDF). MDD is also one of the three Qualifying Indications named in the AtaiBeckley CVR agreement (Annex III to EX-2.1, Section 1.1).