The illustration provides a schematic representation of the anti-proliferative effect produced by an ester-linked conjugate between methotrexate, an antifolate cancer chemotherapeutic, and motexafin gadolinium, an experimental cancer agent demonstrating selective tumor localization.  The ester linkage undergoes cleavage within the A549 human lung cancer cells used for analysis and, presumably as a consequence, this conjugate displays greater activity at short incubation times than does methotrexate alone.  Neither the amide conjugate, which is stable under these in vitro conditions, nor motexafin gadolinium on its own, shows any appreciable activity under these conditions.