The present invention provides a composition for selectively delivering an active agent to a portion of an organism. The composition comprises first and second polymer portions, having first and second functional groups attached as a side-chain thereto, respectively. The first and second functional groups form cross-links between the first and second polymer portions. The cross-links are capable of being broken by a substance of the organism, thereby resulting in release of the active agent. The composition provides a novel means for controlling the selective release of the active agent in the organism.
