Here is a great article that discusses how the ASCO results relate to the selloff.
https://endpts.com/nektar-wants-everyone-to-stay-calm-as-the…
The article mentions the issue Saul noted, that response rate takes more time to appear than is indicated by the numbers taken without context. Adjusted for the new patients, the numbers look promising but really nothing will be known for certain until October (November?).
Aside from this, a few normal factors are probably driving the price down artificially: margin calls, trading algorithms, and day traders.