Chemical Analysis of Cancer Therapy Photosensitizers by Two-Step Laser Mass Spectrometry

We have applied two-step laser mass spectrometry (L2MS) to the qualitative chemical analysis of porphyrin-derived photosensitizer dyes used for detection and photodynamic therapy of cancer. The only ionization technique applied so far to these low-volatility compounds had been fast atom bombardment (FAB), which suffers from problems associated with cluster formation of the porphyrins upon FAB ionization. From FAB and chromatography data, the existence of porphyrin dimers, trimers, and oligomers in commercial photosensitizers was concluded to be crucial for tumor localization, but it is possible that clustering or preformed van der Waals aggregates gave rise to these results. L2MS indicates that commercially available hematoporphyrin derivative and Photofrin II contain no detectable amounts of covalently bound porphyrin oligomers. Synthetic porphyrin dimers and trimers could be unambiguously detected by L2MS.