Of course, the occurrence of a case of PML in the context of a new drug is not what we wish to see. However, as has already been said before, I strongly doubt that PML will become to be a serious concern when using Jakafi. The mechanism of the drug does not support an important role in the development of active JVC infection. If there would be a link, an incidence of 1/10'000 is as low as you that of the "no risk" patient population taking natalizumab (Tysabri). The ultra high risk group of Tysabri has an incidence of 1/20, just to put things into perspective ( overall it is about 1:3000 with Tysabri). PML has also been reported with rituximab, evalizumab and other immnuomodulating drugs, and these drugs continue to be big sellers or even blockbusters. What's probably the most important feature of this sole PML/Jakafi patient (who suffered from an underlying disease with an increased risk of developping PML) is, that he took the drug for not quite 3 months. In Tysabri we start to worry, when patients are 18 months or longer on Tysabri, not after 3 months.
Of course we'll have to see if unexpectedly there are more PML cases reported in Jakafi patients. All in all, I do not think that PML will be a significant problem with Jakafi. Therefore the drop in INCY's share price is an overreaction. Things like this happen. All the time.
The reaction seen so far (and perhaps a little more) may be appropriate to the extent that investors have been counting on seeing a PV market for Jakafi by Summer 2014. I can't see a small risk of PML mattering to therapeutic decisions in MF or pancreatic CA, but in less severe MPNs I can see it mattering a lot. (Although a larger leukemogenic risk doesn't seem to hurt hydroxyurea much--the devil we know...). To the extent people are waiting on relatively pure JAK 1 inhibitors (like '110), this event raises the stakes.
You might be right. Still, I think (without knowing the exact incidence) that PV has a lower risk of PML than MF (pancytopenia in MF). Therefore, if the 1/10'000 PML in MF is the number, the incidence for PV will be even lower. IMO, the most likely explanation for the PML/MF patient is, that MF itself, not Jakafi is the culprit. BTW, Novartis did not even drop a cent; quite to the contrary, it has been on a steady rise during these past days. Novartis is expecting Jakafi to become a billion dollar drug...