I'd say they want to see how it performs in the US market...

I'd say they want to see how it performs in the US market first. However, whilst earlier acquisitions do often happen, I've seen far more examples of big pharma companies waiting for post-approval before acquiring them. This includes drugs for much more debilitating conditions with little to zero alternative treatment options. Big pharma don't need to take that sort of risk to get a high return, they get the return by taking advantage of their economies of scale for commercialisation etc. So it doesn't make much sense for them to acquire before approval