Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Clovis Oncology (NASDAQ:CLVS), a clinical-stage developer of therapies designed to treat cancer, tumbled as much as 18%, its second straight day of double-digit decline, following troubling ASCO data and analyst commentary.

So what: The big news continues to be the hangover effect from the company's presentation at the American Society of Clinical Oncology's annual meeting on CO-1686, its investigational non-small cell lung cancer drug designed to target T790M-positive mutation. The initial data looked very promising, with estimated median progression-free survival in excess of 12 months, according to its press release. However, it was also noted by CEO Pat Mahaffay in discussions with analysts over the weekend that a couple of CO-1686 patients had to be placed on insulin after bouts of hyperglycemia. There are now obvious safety concerns surrounding CO-1686, which investors don't believe Mahaffay addressed last night in an interview with CNBC.

The other news item affecting shares today was negative commentary from Citigroup. According to covering analyst Yaron Weber, AstraZeneca's (NYSE:AZN) AZN9291 appears to be superior to CO-1686 since prior concerns of cardiotoxicity for AZN92921 proved to be unfounded. With a better-than-expected safety profile, and Clovis' therapy producing high blood sugar in some patients -- which can lead to cardiovascular issues -- Weber noted that AZN9291 was the preferable candidate of the two. Weber also cut his rating on Clovis to neutral from buy and reduced his price target by more than 50% to just $53 from $109.

Now what: Even with Clovis shares down more than 50% from their 52-week high, I still feel the company could be grossly overvalued. It explored strategic options when it was worth more than $2 billion and found no takers, and now it's still worth more than $1.3 billion, yet can't seem to get a drug past mid-stage studies without some safety or efficacy problem arising. Clovis is the type of biotech company that you have to make prove its worth. Following the failure of CO-101 in late 2012, and with CO-1686 presenting potentially troubling side effects, there's just no viable reason to own Clovis in my opinion until we have late-stage, broad-based data to chew on. Until such time as we get that data Clovis will continue to burn through its cash on hand. Do yourself a favor and tread cautiously around Clovis.