change line found by pattern using sed

I want to change a line like

CPM_THRESHOLD    0.8            //

to a new value using sed
I am trying

sed -i "s/CPM_THRESHOLD/CPM_THRESHOLD\t$COH\t\t\/\//" $INPUT_4

but how can i substitute the whole line begining with CPM_THRESHOLD and substitute it?

cat input
CPM_THRESHOLD is this

sed 's/^CPM_THRESHOLD.*$/change/' input
change

perfect, thanks