PURPOSEThe aim of this study was to show that novel photodynamic therapy (PDT) sensitizers can be activated by two-photon absorption in the near-IR region of the spectrum and to show, for the first time, that such activation can lead to tumor regressions at significant tissue depth. These experiments also evaluated effects of high-energy femtosecond pulsed laser irradiation on normal tissues and characterized the response of xenograft tumors to our PDT protocols.EXPERIMENTAL DESIGNHuman small cell lung cancer (NCI-H69), non-small cell lung cancer (A549), and breast cancer (MDA-MB-231) xenografts were induced in SCID mice. Irradiation of sensitized tumors was undertaken through the bodies of tumor-bearing mice to give a treatment depth of 2 cm. Posttreatment tumor regressions and histopathology were carried out to determine the nature of the response to these new PDT agents. Microarray expression profiles were conducted to assess the similarity of responses to single and two-photon activated PDT.RESULTSRegressions of all tumor types tested were seen. Histopathology was consistent with known PDT effects, and no, or minimal, changes were noted in irradiated normal tissues. Cluster analysis of microarray expression profiling showed reproducible changes in transcripts associated with apoptosis, stress, oxygen transport, and gene regulation.CONCLUSIONSThese new PDT sensitizers can be used at a depth of 2 cm to produce excellent xenograft regressions. The tumor response was consistent with known responses to single-photon activated PDT. Experiments in larger animals are warranted to determine the maximal achievable depth of treatment.