Coupling of
photosensitizers to
tumor-selective
monoclonal antibodies (MAbs) is an attractive option for improving the selectivity of
photodynamic therapy (
PDT). For this purpose, hydrophilic sensitizers would be most suitable because of their solubility in water. However, such sensitizers are known to be ineffective in
PDT, probably because they cannot readily pass the cell membrane and reach the critical intracellular target. We used the model compound TrisMPyP-PhiCO(2)H, a hydrophilic
porphyrin derivative, to test the hypothesis that hydrophilic
photosensitizers might become of therapeutic value when directed into the
tumor cell by use of internalizing MAbs. TrisMPyP-PhiCO(2)H was conjugated using a labile
ester. Conjugates showed no impairment of integrity on SDS-PAGE, full stability in serum in vitro, and optimal immunoreactivity when the sensitizer:MAb ratio was </=3. At higher molar ratios, the solubility of the conjugates decreased. In vitro internalization experiments showed that TrisMPyP-PhiCONH-(125)I-cMAb U36 and TrisMPyPPhiCONH-(125)I-mMAb 425 conjugates were internalized by A431 cells, in contrast to TrisMPyP-PhiCONH-(125)I-mMAb E48 conjugates. Data on the in vitro efficacy of
PDT with MAb-conjugated TrisMPyP-PhiCO(2)H showed that the internalizing
cMAb U36 and mMAb 425 conjugates were phototoxic to A431 cells, while the non-internalizing E48 conjugate and the unconjugated sensitizer were not. Biodistribution data of conjugates with sensitizer:(125)I-
cMAb U36 ratios varying from 1:1 to 3:1 in
tumor-bearing nude mice revealed selective accumulation in the
tumor. Conjugates with higher molar ratios were cleared more rapidly from the blood than the unconjugated (125)I-cMAb U36, resulting in lower
tumor uptake but similar
tumor-to-blood ratios. Our data suggest that hydrophilic
photosensitizers might have therapeutic value when targeted to
tumors by internalizing MAbs.