What is offered at your school?
My medicinal chemistry elective was a great precursor to my pharmaceutical chemistry classes.
Biochem is ok
Our 1st year is pretty biochem and anatomy/physiology intensive, so that helps. Neurochemistry comes in handy as well if you find a course that delves into it. Neurotransmitters are affected by many different classes of drugs.
Those would be my initial suggestions.
Genetics would come in handy if your school of choice is into pharmacogenomics.