PURPOSE

Photoimmunotherapy may allow target-specific photodynamic destruction of malignancies and may also potentiate anticancer antibody therapies. However, clinical use of either of the two modalities is limited for different reasons. Antibody therapies suffer from being primarily cytostatic and the need for prolonged administration with consequent side effects. In the case of photoimmunotherapy, a major impediment has been the absence of well-characterized photosensitizer immunoconjugates (PIC). In this investigation, we suggest a strategy to overcome these limitations and present the successful targeting of epidermal growth factor receptor (EGFR) using a well-characterized PIC.

EXPERIMENTAL DESIGN

The PIC consisted of the EGFR-recognizing chimeric monoclonal antibody, C225, conjugated with a two-branched polyethylene glycol and benzoporphyrin derivative (BPD, Verteporfin). Mechanistic studies included photophysics, phototoxicity, cellular uptake, and catabolism experiments to yield dosimetric parameters. Target cells included two EGFR-overexpressing human cancer cell lines, OVCAR-5 and A-431. Nontarget cells included an EGFR-negative fibroblast cell line, 3T3-NR6, and a monocyte-macrophage cell line, J774.

RESULTS

BPD-C225 PICs targeted and photodynamically killed EGFR-overexpressing cells, whereas free BPD exhibited no specificity. On a per mole basis, PICs were less phototoxic than free BPD, but PICs were very selective for target cells, whereas free BPD was not. Phototoxicity of the PICs increased at prolonged incubations. Photodynamic dose calculations indicated that PIC photophysics, photochemistry, catabolism, and subcellular localization were important determinants of PIC phototoxic potency.

CONCLUSIONS

This study shows the efficacy of EGFR targeting with PIC constructs and suggests approaches to improve PIC designs and targeting strategies for in vivo photoimmunotherapy. The approach offers the possibility of dual effects via antibody-mediated cytostasis and photoimmunotherapy-based cytotoxicity.