Complex sentence: It is believe that azathioprine acts as a pro-drug for mercaptopurine, inhibits an enzyme that is required for the synthesis of DNA, thus most strongly affects proliferating cells, such as the T cells and B cells of the immune system.
Split into simpler sentences: It is believed that azathioprine acts as a pro-drug for mercaptopurine, inhibiting an enzyme that is required for the synthesis of DNA. Thus it most strongly affects proliferating cells, such as the T cells and B cells of the immune system.