The weak intratumoral fluorescence at early time points could be explained by concentration quenching within the liposomes as evidenced from fluorescence polarization studies. Progressive mTHPC redistribution from liposomes and its further incorporation into tumor tissue resulted in fluorescence build-up over time with a maximum at 24 hours post-injection. This correlates perfectly with the best therapeutic effect at this time point. The absence of total cure can be attributed to inhomogeneous photosensitizer distribution. mTHPC is reabsorbed into the blood stream but the total administered amount is much reduced as opposed to systemic administration so that repeated PDT sessions might be favorable in terms of side effects and tumor response.
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