NorthWest Biotherapeutics Inc (NWBO)

[longfellow95]

It was more of a rhetorical question than one born out of ignorance!
Of course I know why it is employed.
There are reasons why response rate is also employed in late-stage trials; it saves a good few years. And a ton of money!
Particularly handy if you seek to get an approval from a single-arm trial.
And it's a subjective assessment. There has even been studies on how two different assessors come up with significantly different tumor measurements from the same patient.

Those reasons for its use do not make for sound assessments of true efficacy.
It's nothing more than a very imperfect surrogate. More imperfect than PFS. It is not a good predictor of overall survival.
As such, it's a pet hate of mine.
But if regulators are going to accept it, of course the industry will use it!

Prasad puts it clearly:-

"The FDA has used surrogate end points approximately 194 times to approve cancer drugs since 1992, and about 1 in 3 times, a surrogate was used for the first time in a particular type of cancer. When this is the case, the strength of association between the surrogate end point and OS is often absent or weak. This means that the FDA’s use of these surrogate measures is justified neither by strength of association (ie, ability to predict gains in OS) nor previous first use, since 1 in 3 approvals constitute the use of a surrogate end point for the first time in the treament of a specific cancer type. Our study is limited by some missing FDA label updates before 2006 that are not publicly accessible.

Surrogate end points can expedite trial completion compared with OS,4 but they add substantial uncertainty regarding whether the drugs involved improve quantity or quality of life.6 Moreover, the FDA rarely demands stringent confirmation of clinical benefit following market approval.3 We find that the FDA is steadily accepting more surrogate measures over time, which are not justified by scientific validity or adherence to regulatory precedent. This reflects a greater tolerance of risk, and if postmarketing studies are slow, incomplete, or demonstrate negative results, then patients experience harm and cost without the intended benefit."

https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2764287

If one holds a purely investing standpoint, then an approval based on RR might be perfectly acceptable.
But as far as proper evidence-based medicine goes, it's not good.