No abstract

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‐ΦCO2H, 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‐ΦCO2H 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‐ΦCONH–125I‐cMAb U36 and TrisMPyPΦCONH–125I‐mMAb 425 conjugates were internalized by A431 cells, in contrast to TrisMPyP‐ΦCONH–125I‐mMAb E48 conjugates. Data on the in vitro efficacy of PDT with MAb‐conjugated TrisMPyP‐ΦCO2H 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:125I‐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 125I‐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. Int. J. Cancer 88:108–114, 2000. © 2000 Wiley‐Liss, Inc.

A challenge in photodynamic therapy (PDT) is to improve the tumour selectivity of the photosensitizers by using monoclonal antibodies (MAbs). With this aim, we developed MAbconjugates with the hydrophobic photosensitizer meta-tetrahydroxyphenylchlorin ( Key words: photodynamic therapy; meta-tetrahydroxyphenylchlorin; aluminium (III) phthalocyanine tetrasulfonate; monoclonal antibody; in vitro phototoxicity; squamous cell carcinomaA challenge in photodynamic therapy (PDT) is to improve the tumour selectivity of the photosensitizers. Photoimmunotherapy, which combines phototoxicity of the sensitizers with the selectivity of monoclonal antibodies (MAbs) directed against tumourassociated antigens, could lead to this improvement. This approach would be very suitable, especially for the treatment of large surface areas, where normal tissue toxicity becomes dose-limiting. Recently, interest in photoimmunotherapy has grown. Soukos et al. 1 conjugated the sensitizer chlorin e6 to the anti-EGFR MAb C225 for treatment of oral precancer, whereas by the same group chlorin e6 -MAb 17.1A conjugates (an anti-EpCAM MAb) were developed for intraperitoneal photoimmunotherapy. 2 Carcenac et al.

Introduction. The objective was to improve the selectivity of photodynamic therapy (PDT) by targeting photosensitizers to tumours by the use of monoclonal antibodies (mAbs). Two sensitizers were selected for this approach. Meta‐tetrahydroxyphenylchlorin (mTHPC) was selected because it is one of the most effective photosensitizers in free form. Aluminium (III) phthalocyanine tetrasulphonate [AlPc(SO3H)4] was selected because of its ideal photochemical properties. However, owing to its hydrophilicity, this latter sensitizer is not able to enter the tumour cell and, therefore, in free form is ineffective in PDT. We hypothesized that AlPc(SO3H)4 might become suitable for PDT when coupled to tumour‐selective mAbs. Methods. These were developed to couple the sensitizers to mAbs, including mAb 425 directed against the epidermal growth factor receptor. These conjugates were evaluated for efficacy in PDT in vitro and for tumour‐targeting capacity in vivo. Results.  In vitro PDT showed that the AlPc(SO3H)4–mAb 425 conjugate was ≈7500 times more toxic to A431 cells than the free sensitizer (IC50 values 0.12 nm versus 900 nm), and was also more toxic than the mTHPC–mAb 425 conjugate and free mTHPC (IC50 values 7.3 nm versus 2.0 nm). Biodistribution analysis of the conjugates in tumour‐bearing nude mice showed selective accumulation in the tumour. Conclusion. These data show that AlPc(SO3H)4, in particular, has high potential for use in PDT when coupled to tumour‐selective mAbs.