Species differences in levonorgestrel binding to serum proteins of monkey, rabbit and rat blood

Levonorgestrel binding to serum proteins of monkey, rabbit and rat was studied. It showed high affinity binding to monkey and rabbit serum proteins but not to rat serum. Characterization by polyacrylamide gel electrophoresis revealed that the binding of levonorgestrel occurs to sex hormone binding globulin (SHBG) and albumin, but in rat it is confined to albumin only. These results suggest species differences in levonorgestrel binding to serum proteins and extrapolation of data from one animal model to another may not be directly feasible.