Targeting of a hydrophilic photosensitizer by use of internalizing monoclonal antibodies: A new possibility for use in photodynamic therapy

Coupling of photosensitizers to tumor-selective monoclonal antibodies (MAbs ) is an attractive option for improving the selectivity of photodynamic the rapy (PDT). For this purpose, hydrophilic sensitizers would be most suitabl e because of their solubility in water. However, such sensitizers are known to be ineffective in PDT, probably because they cannot readily pass the ce ll membrane and reach the critical intracellular target. We used the model compound TrisMPyP-Phi CO2H, a hydrophilic porphyrin derivative, to test the hypothesis that hydrophilic photosensitizers might become of therapeutic v alue when directed into the tumor cell by use of internalizing MAbs, TrisMP yP-Phi CO2H was conjugated using a labile ester, Conjugates showed no impai rment of integrity on SDS-PACE, full stability in serum in vitro, and optim al immunoreactivity when the sensitizer: MAb ratio was less than or equal t o 3. At higher molar ratios, the solubility of the conjugates decreased. In vitro internalization experiments showed that TyisMPyP-Phi CONH-I-125-cMAb U36 and TrisMPyPc Phi CONH-I-125-mMAb 425 conjugates were internalized by A431 cells, in contrast to TrisMPyP-Phi CONH-I-125-mMAb E48 conjugates, Dat a on the in vitro efficacy of PDT with MAb-conjugated TrisMPyP-Phi CO2H sho wed that the internalizing cMAb U36 and mMAb 425 conjugates were phototoxic to A431 cells, while the non-internalizing E48 conjugate and the unconjuga ted sensitizer were not. Biodistribution data of conjugates with sensitizer : I-125-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 I-125 -cMAb U36, resulting in lower tumor uptake but similar tumor-to-blood ratio s. Our data suggest that: hydrophilic photosensitizers might have therapeut ic value when targeted to tumors by internalizing MAbs. (C) 2000 Wiley-Liss . Inc.