There's a reason that most CEOs and M.D.s are hard to get to and why their gatekeepers won't let you through...
They WANT it that way.
They are constantly being bombarded by people trying to sell them something!
My best advice is to discover ways to put the target on YOUR back.
Position your business/product/service in a way that compels them (or more likely someone from their office) to contact YOU.
Tip: Make sure you develop and maintain an very high level of integrity - because even if you get through to one and make a sale - if your product/service is of poor quality then word will spread and your reputation will get destroyed.
Best of luck!