How can I move from chemical engineering into drug discovery? My situation is as follows:

I have already obtained a master in chemical engineering and my thesis is about drug delivery. I would like to continue my graduate study for a PhD, but as time goes by, I find myself begin interested in the field of drug discovery especially drug design. Since my high school I am actually more interested in chemistry and I think I should do something to try to engage in drug design in my PhD before it is too late. I have thought of some options: 


Try to approach some research groups to see if they allow me to work as a research assistant first, then after a year or so with some drug design experiences, apply PhD about drug design.
Purse another master program for drug design, work crazily hard to get good grades and apply PhD about drug design. (But some of them is quite expensive, this may be a financial issue but just want to make sure it is worth)
Directly apply some PhD about drug design. I am not sure anybody will accept someone with no background.


Something more about me:


My grade average in UG is really bad. Quite rebellious in my UG.
I do have some publications and patents but they are totally not related to drug design.
I actually did not take any course in my UG and PG related to drug, or pharmaceutical science.
My current professional connection is limited to my master supervisor. Here I mean really close communication and will support me. 


I am sure I have the determination to start my research in drug design. It is not an impulse. Is there any advice on my situation? Any comment is welcome particular harsh comments.
As I mentioned above, drug design is not really a "major" field, in the sense that it's rare to be able to get a diploma which lists "drug design" as the field of study. It's a dissertation topic or a focus area. 

The relative advantage of this is that very few people come in with experience in the field, because it's so specialized. That means it's harder to be "picky" in selecting students—if everybody has to have lots of experience, you may not end up with enough qualified candidates.

Given your past experience with drug delivery, you probably have a fairly good idea of some of what is involved in drug design, so it's not as if you're coming from a completely unrelated discipline.

So, I don't think you have much to lose by applying to PhD positions while you look for alternate opportunities to work in the field. (Sometimes you need to apply more than once before you get accepted!)