I'd suggest warm water and dish soap. Don't use hot as this can ruin the bristles and also weaken the glue that holds them in the end of brush handle.
A small drop of soap in one palm, swirl the brush gently around a few times, then while the tap is running, rinse it off. Also don't scrub forward in the direction of the bristles are pointing, as that can ruin the brush. When done return the bristles to their original pointed look. Some people do this by sticking the brush in their mouth, puckering their lips and pulling out the brush - not very hygienic. Passing it between the thumb and first two fingers is good enough.
regards,
Jack