~30 year old article on how red blood cells metabolize many classes of drugs: https://onlinelibrary.wiley.com/doi/pdf/10.1002/bod.25100904...

Basically, RBCs have lots of enzyme systems that serve (in part) to protect the important parts (e.g. haemoglobin) from oxidation. Put a bunch of lisdexamfetamine in the blood, and the RBCs will gradually hydrolyze it to cleave off the l-lysine and leave d-amph.