Selective Estrogen Reuptake Modulators can control which parts of the body are affected by estrogen, they’re sometimes used for non-binary feminizing HRT regimens to prevent breast growth while getting the other effects. AFAIK there isn’t really much in the way of real medical research on them with regards to transgender HRT specifically (the medications typically used are intended for breast cancer treatment) so they’re mostly limited to a few enbys doing DIY who don’t mind living on the edge.