The kushans overran the empire

The Yuezhi invaded the Greco-Bactrian kingdom around 135 BCE, displacing the Greek dynasties. Kushan control fragmented into semi-independent kingdoms as the Sassanians attacked from the west and the Guptas attacked from the east, collapsing the Kushan Empire around 375 CE.

Explanation:

The Kushans spread from the Kabul River Valley to defeat other Central Asian tribes. These conquests included parts of the northern central Iranian Plateau, once ruled by the Parthian Empire—a major political and cultural power in ancient Iran and Iraq.

Hope it helps