The present invention relates to imidazopyridazine derivatives. More particularly, it relates to 4-(biphenyl-3-yl)-7H-imidazo[4,5-c]pyridazine derivatives of formula (I): and pharmaceutically acceptable salts thereof, wherein R1, R2, R3, R4 and R5 are as defined in the description. The imidazopyridazine derivatives of the present invention modulate the activity of the GABAA receptor. They are useful in the treatment of a number of conditions, including pain.