Targeting Cells That Overexpress the Epidermal Growth Factor Receptor with Polyethylene Glycolated BPD Verteporfin Photosensitizer Immunoconjugates¶

Abstract

Photoimmunotherapy was introduced two decades ago but has been studied infrequently in vivo and is virtually untested clinically. Progress has been limited because high-quality, well-characterized photosensitizer immunoconjugates (PICs) have been difficult to make. Here, we describe the development of an innovative conjugation method for producing water-soluble PICs that are free of insoluble aggregates and free of unacceptable amounts of noncovalently associated photosensitizer impurities. The method exploits two procedures previously untried in this research area. First, a small number of antibody lysines (<3 per antibody) are polyethylene glycolated (PEGylated) using a 10 kDa branched polyethylene glycol (PEG), which dramatically enhances PIC solubility and reduces PIC aggregation. Second, a 50% dimethyl sulfoxide-50% aqueous two-solvent system is used to prevent photosen-sitizer aggregation and noncovalent interactions. These measures allow efficient covalent linkage of the photosensitizer BPD Verteporfin (BPD) to antibody lysines, thorough purification of the resulting PICs (verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis), maintenance of PIC antigen-binding activity (verified by cellular binding-uptake assays) and reduction of nonspecific cellular uptake (e.g. macrophage capture) of the PICs. Loading levels could be varied controllably in the range <11 BPD/antibody. PICs of the C225 anti-epidermal growth factor receptor (EGFR) chimeric monoclonal antibody killed EGFR-overexpressing A-431 cells photodynamically but did not significantly affect EGFR-negative NR6 cells. Although fluorescence measurements demonstrated that the PICs were quenched by as much as an order of magnitude compared with free BPD, an impressive 90% reduction in A-431 cell viability was achieved using 20 J/cm 2 of 690 nm light after a 40 h incubation with the C225 PICs. The results suggest that PEGylated BPD-C225 PICs merit further investigation in animal models of EGFR-overexpressing cancers.

Selective treatment and monitoring of disseminated cancer micrometastases in vivo using dual-function, activatable immunoconjugates

“…S4). Furthermore, previous in vitro studies have shown that Cet-BPD is highly selective to cancer cells overexpressing EGFR surface molecules with 20-fold less accumulation in low EGFR-expressing cells (35). Hence, the tumor selectivity achieved by Cet-BPD is due to tumor-targeted activation.…”

“…This quenching phenomenon increases the tumor specificity when optimally designed; however, excess BPD loading results in the loss of cancer cell-specific delivery. In a series of chemical synthesis and in vitro cell culture studies, we previously identified an optimal conjugation ratio of approximately seven BPD molecules per mAb molecule, Cet-BPD(1:7), as a strongly quenched (sevenfold) and cancer cell-specific construct (35) that retains the biological activity of cetuximab (36). These in vitro studies suggest that, like cetuximab, Cet-BPD is trafficked to lysosomes as part of the EGFR internalization and degradation pathway where more than one-half of the Cet-BPD immunoconjugates release BPD resulting in dequenching and activation of both BPD phototoxicity and fluorescence emission in cell culture (37).…”

“…Uniquely, Cet-BPD incorporates two synergistic therapies into a single agentwith the photocytotoxic component remaining quenched in nonspecific tissues-to realize a molecular-targeted, activatable therapy. Because cancer cells overexpressing the target surface molecules take up immunoconjugates efficiently (28,35,37) (Fig. S4), this activation occurs predominantly within tumors while unbound conjugates remain quenched.…”

Targeted photodynamic therapy with multiply‐loaded recombinant antibody fragments

“…Previously verteporfin PICs consisting of an EGFR (epidermal growth factor recpetor-1) recognizing MAb C225 were found on a per mole basis to be significantly less phototoxic than free visudyne. 16 It is known that in the presence of aqueous buffers such as PBS, hydrophobic photosensitizers have a tendency to aggregate with consequent photophysical quenching. 38 The singlet oxygen data therefore strongly suggest that the PPa molecules are spatially arranged on the scFv such that there is less adverse photophysical interaction between PS molecules.…”

“…14 Photosensitizers conjugated to whole antibodies can target a range of cancers 12 or pathogens 15 ; however, experiences so far has shown that many technical problems exist in coupling high numbers of PS to whole antibodies. These include impaired antibody binding, reduced solubility [16][17][18][19] and unfavorable pharmacokinetics due to the resulting high molecular weight of such photoimmunoconjugates (PICs). Some researchers have tried using carrier moieties such as branched carbohydrate or polylysine chains, which are in turn linked to the antibody.…”