How to use use late binding to get excel instance?

I'm using

[DllImport("Oleacc.dll")]
static extern int AccessibleObjectFromWindow(
int hwnd, 
uint dwObjectID, 
byte[] riid,
ref Excel.Window ptr);

to get an Excel Instance using his handle, which I get from the process ID of the excel instance.

This is how it looks like when I use these function

const uint OBJID_NATIVEOM = 0xFFFFFFF0;
Guid IID_IDispatch = new Guid("{00020400-0000-0000-C000-000000000046}");
Excel.Window ptr = null;  
int hr = AccessibleObjectFromWindow(hwndChild, OBJID_NATIVEOM, 
          IID_IDispatch.ToByteArray(), ref ptr);

Object objApp = ptr.Application;

This peace of code works great but the only problem is that I had to add a reference to the Office 2003 Primary Interop Assemblies.

As you can see, the last param in the function is the reason why I needed to add the reference to the Pias, so my question is if there is a way of avoiding the use of Interop Assemblies, I have tried with late binding but perhaps I've been doing it wrong because I haven't been able to make it work.