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

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.

The development and characterisation of porphyrin isothiocyanate–monoclonal antibody conjugates for photoimmunotherapy

“…This was thought to be a consequence of the photosensitiser diminishing the solubility of the MAb in PBS at higher loading ratios. This also corresponded to a reduced recovery of the conjugate from the Sephadex G25 column, an effect that has been reported elsewhere (Vrouenraets et al, 2000). The conjugates synthesised using PS 2 and MAb 35A7 were recovered quantitatively.…”

Targeting cancer cells by using an antireceptor antibody-photosensitizer fusion protein

“…In particular, focusing on the suppression of p185 HER-2-ECD -overexpressing cancer cells, different anti-p185 HER-2-ECD antibodies have been used for the targeting of photosensitizers (7,8). The main drawback of the chemical approach is the limited reproducibility of conjugate synthesis (9), which often results in loss of activity of either the monoclonal antibody (mAb) (reduced affinity) or the photosensitizer (altered photophysical properties leading to poor PDT efficiency) (10,11). Moreover, the main disadvantage of chemical antibody-photosensitizer chemical conjugates may be the impossibility of intracellular production of the molecule with subsequent secretion into the bloodstream [as presented by Wang et al (12)], thus generating an intraorganism photosensitizer-producing ''factory.'…”

Photochemical Targeting of Epidermal Growth Factor Receptor: A Mechanistic Study

“…In more recent photoimmunotherapy studies, investigators attempted to avert PIC aggregation and increase coupling efficiency by converting the photosensitizer to a water-soluble derivative (13,14). Similarly, in studies by Levy et al, the first group to make PICs with the potent second-generation photosensitizer, benzoporphyrin derivative (BPD, Verteporfin; refs.…”