Preparation and characterization of immunoconjugates for antibody-targeted photolysis

Features of third generation photosensitizers used in anticancer photodynamic therapy: Review

Cancer remains a main public health issue and the second cause of mortality worldwide. Photodynamic therapy is a clinically approved therapeutic option. Effective photodynamic therapy induces cancer damage and death through a multifactorial manner including reactive oxygen species-mediated damage and killing, vasculature damage, and immune defense activation. Anticancer efficiency depends on the improvement of photosensitizers drugs used in photodynamic therapy, their selectivity, enhanced photoproduction of reactive species, absorption at near-infrared spectrum, and drug-delivery strategies. Both experimental and clinical studies using first- and second-generation photosensitizers had pointed out the need for developing improved photosensitizers for photodynamic applications and achieving better therapeutic outcomes. Bioconjugation and encapsulation with targeting moieties appear as a main strategies for the development of photosensitizers from their precursors. Factors influencing cellular biodistribution and uptake are briefly discussed, as well as their roles as cancer diagnostic and therapeutic (theranostics) agents. The two-photon photodynamic approach using third-generation photosensitizers is present as an attempt in treating deeper tumors. Although significant advances had been made over the last decade, the development of next-generation photosensitizers is still mainly in the developmental stage.

Site-specific antibody-drug conjugation through an engineered glycotransferase and a chemically reactive sugar

Conjugation of small molecule drugs to specific sites on the antibody molecule has been increasingly used for the generation of relatively homogenous preparations of antibody-drug conjugates (ADCs) with physicochemical properties similar or identical to those of the naked antibody. Previously a method for conjugation of small molecules to glycoproteins through existing glycans by using an engineered glycotransferase and a chemically reactive sugar as a handle was developed. Here, for the first time, we report the use of this method with some modifications to generate an ADC from a monoclonal antibody, m860, which we identified from a human naïve phage display Fab library by panning against the extracellular domain of human HER2. M860 bound to cell surface-associated HER2 with affinity comparable to that of Trastuzumab (Herceptin), but to a different epitope. The m860ADC was generated by enzymatically adding a reactive keto-galactose to m860 using an engineered glycotransferase and conjugating the reactive m860 to aminooxy auristatin F. It exhibited potent and specific cell-killing activity against HER2 positive cancer cells, including trastuzumab-resistant breast cancer cells. This unique ADC may have utility as a potential therapeutic for HER2 positive cancers alone or in combination with other drugs. Our results also validate the keto-galactose/engineered glycotransferase method for generation of functional ADCs, which could potentially also be used for preparation of ADCs targeting other disease markers.

Immunoconjugates of soybean Bowman-Birk protease inhibitor as targeted antitumor polymeric agents

To enhance the antitumor potential of soybean Bowman-Birk inhibitor (BBI), the conjugate of BBI with an antibody via a macromolecular carrier was prepared. Clinical dextran (D) was used as a biocompatible biodegradable carrier for co-immobilization of BBI and antibody. A model immunoglobulin isolated from sheep serum (sIgG), raised against human IgM was utilized to develop the procedure of immunoconjugate synthesis. The molar ratio of the ingredients in the conjugate was the following BBI:D:sIgG=9:1:1. Comparison of the dose response curves for the native sIgG and the BBI-D-sIgG conjugate indicated that sIgG completely retained its specific activity (>90%) after modification with dextran. The determination of the Ki values for chymotrypsin interaction with the native BBI and the BBI-D-sIgG conjugate indicated high anti-chymotrypsin activity. In the next step, the monoclonal antibody (ICO 25 MAb) against the mucin-like human epithelial membrane antigen was used for conjugation as it is the most universal vector for targeting different agents to human tumors of epithelial origin. The influence of conjugation on the specificity of the Mab reaction with its antigen was studied. The conjugated MAb reacted with tumor cells of different epithelial genesis (breast, lung, gastric, ovarian and uterus tumors), but did not react with tumor cells of non-epithelial origin. It was shown that BBI-D-ICO 25 MAb conjugate has almost the same immunohistochemical activity as non-conjugated MAb. These results demonstrated the feasibility of exploiting the activities of covalently bound BBI and ICO 25 MAb for anticarcinogenic agent targeting.

Control of fibroblast populated collagen lattice contraction by antibody targeted photolysis of fibroblasts

BACKGROUND AND OBJECTIVE

Hypertrophic scarring and rigid scar contracture are disorders of wound healing for which there is presently no effective therapy. The dermal fibroblast plays a major role in scar fibrillogenesis and contracture. The objective of this study was to establish a selective and effective method to destroy fibroblasts.

STUDY DESIGN/MATERIALS AND METHODS

An antifibroblast conjugate was synthesized by covalent attachment of the antifibroblast antibody PR2D3 to the photosensitizer Sn-chlorin e6. Fibroblasts were cultured in fibroblast-populated collagen lattices (FPCLs), incubated with the conjugate and exposed to light. The effect of the treatment on cell viability and the rate of contraction of the FPCL were assessed.

RESULTS

The toxicity of antifibroblast conjugates increased with increasing conjugate concentration, light dose, and number of photosensitizers per antibody molecule, until nearly complete killing was achieved. The rate of lattice contraction after irradiation linearly correlated with the remaining viable fraction of fibroblasts. These conjugates were not cytotoxic to keratinocytes cultured on collagen lattices, and nonspecific conjugates could not cause significant fibroblast killing. Spatial selectivity was demonstrated using a light mask.

CONCLUSIONS

Antibody-targeted photolysis is an effective and selective technique for controlling FPCL contraction in vitro and may have potential in vivo applications to modulate extracellular matrix remodeling by connective tissue cells.

In vivo fluorescence imaging of the transport of charged chlorin e6 conjugates in a rat orthotopic prostate tumour

Polymeric drug conjugates are used in cancer therapy and, varying their molecular size and charge, will affect their in vivo transport and extravasation in tumours. Partitioning between tumour vasculature and tumour tissue will be of particular significance in the case of photosensitizer conjugates used in photodynamic therapy, where this partitioning can lead to different therapeutic effects. Poly-l-lysine chlorin e6conjugates (derived from polymers of average Mr 5000 and 25000) were prepared both in a cationic state and by poly-succinylation in an anionic state. A fluorescence scanning laser microscope was used to follow the pharmacokinetics of these conjugates in vivo in an orthotopic rat prostate cancer model obtained with MatLyLu cells. Fluorescence was excited with the 454-528 nm group of lines of an argon laser and a 570 nm long pass filter used to isolate the emission. Results showed that the conjugates initially bound to the walls of the vasculature, before extravasating into the tissue, and eventually increasing in fluorescence. The anionic conjugates produced tissue fluorescence faster than the cationic ones, and surprisingly, the larger Mr conjugates produced tissue fluorescence faster than the smaller ones with the same charge. These results are consistent with differences in aggregation state between conjugates.

HPMA copolymer-anticancer drug-OV-TL16 antibody conjugates. 1. influence of the method of synthesis on the binding affinity to OVCAR-3 ovarian carcinoma cells in vitro

The influence of different methods of binding the OV-TL16 antibody and its Fab' fragment to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer--drug (adriamycin [ADR] or meso chlorin e6 mono(N-2-aminoethylamide) (Mce6)) conjugates on the affinity of conjugates to an ovarian carcinoma (OVCAR-3) cell associated antigen was investigated. The binding of the antibody to HPMA copolymer--drug (ADR or Mce6) conjugates via amino groups resulted in conjugates which were heterogeneous in their antigen binding. Coupling, the HPMA copolymer--Mce6 conjugate to the carbohydrate region of the antibody resulted in conjugates with a more homogeneous distribution of affinity constants than conjugates prepared by linking the antibody to the polymer via amino groups. However, both methods resulted in a decrease in the affinity constant compared to the native antibody. Conjugates prepared with the Fab' frgment of the OV-TL16 antibody demonstrated a more homogenous affinity than either conjugate prepared with the whole antibody. To verify the hypothesis that the changes in the binding affinity and homogeneity are a consequence of conformational changes in the antibody structure, a series of physiocochemical methods were employed to characterize the conjugates. The excitation energy transfer between OV-TL16 antibody and drugs (ADR and Mce6) and the spectral properties of Mce6 were used to monitor the interactions between the antibody and drugs. The quenching of the intrinsic fluorescence of the antibody was also employed to study its conformational changes. An attempt has been made to correlate the biorecognition at the cellular surface with the interactions of drug with the antibody molecule and with the changes in antibody conformation.

Antibody-targeted photolysis of bacteria in vivo

We have evaluated the efficacy of antibody-targeted photolysis to kill bacteria in vivo using specific antibacterial photosensitizer (PS) immunconjugates. After infecting the dorsal skin in mice with Pseudomonas aeruginosa, both specific and nonspecific tin (IV) chlorin e6-monoclonal antibody conjugates were injected at the infection site. After a 15 min incubation period, the site was exposed to 630 nm light with a power density of 100 mW/cm2 for 1600 seconds. Irradiation resulted in a greater then 75% decrease in the number of viable bacteria at sites treated with a specific conjugate, whereas normal bacterial growth was observed in animals that were untreated or treated with a nonspecific conjugate. Antibody-targeted photolysis may be a selective and versatile tool for treating a variety of infections.

Sn-chlorin e6 antibacterial immunoconjugates. An in vitro and in vivo analysis

Monoclonal antibody-Sn-chlorin e6 immunoconjugates were prepared by the site-selective covalent modification of the monoclonal oligosaccharide moiety. By carefully controlling the reaction conditions and introducing triethanolamine groups as axial ligands of the Sn moiety, conjugates with in vivo biodistribution properties similar to underivatized IgG were prepared. By varying the reaction conditions, conjugates were reproducibly prepared with a range of photosensitizer to mAb molar ratios from 1.6 to 10. Based on a competitive inhibition radioimmunoassay, conjugates prepared by this method showed selectivity and binding affinity comparable to the unmodified antibody. The immunoconjugates had only slightly lower singlet oxygen yields than that observed with the Sn-chlorin e6 precursor indicating that negligible aggregation or structural modification of the chromophores occurred during the synthesis process. In vitro cell killing experiments demonstrated that all conjugates possessed significant cytotoxic activity. Biodistribution studies in mice showed that conjugates prepared with axial ligands had significant serum retention 24 h after injection while conjugates prepared without the triethanolamine ligand were much more rapidly cleared. In vivo specificity was demonstrated using rats infected with Fisher immunotype I P. aeruginosa at a site in the left posterior thigh muscle. Target to background ratios exceeded 60 at 120 h after conjugate injection of the specific immunoconjugate, compared to a ratio of only 6 for a non-specific mouse IgG conjugate. Biodistribution patterns at 120 h post injection indicate that the conjugates were both biologically active and structurally intact.

The influence of periodate oxidation on monoclonal antibody avidity and immunoreactivity

Eight monoclonal antibodies of different classes and isotypes and rabbit IgG were oxidized under a variety of conditions with 5-50 mM periodate. The number of aldehyde groups generated per immunoglobulin were measured by reduction with tritiated sodium borohydride or coupling of fluoresceinthiosemicarbazide. There were up to 25.5 aldehyde groups detected on the periodate-oxidized antibody 96.5, measured by borohydride reduction whereas the same conditions led to only 9.6 aldehydes per IgG on the antibody L6 of the same IgG2A isotype. Fluoresceinthiosemicarbazide bound to oxidized antibodies but not to the same extent as tritium. On mildly oxidized IgMs it was possible to generate more than 200 aldehyde groups per antibody molecule. Depending on the conditions and the antibody used periodate oxidation could lead to antibody crosslinking. The avidities of the modified antibodies were determined by Scatchard analyses and inhibition assays. A new mathematical method to evaluate the immunoreactivities of modified antibodies relative to the unlabeled native antibody from inhibition binding data was established. Periodate concentrations higher than 50 mM decreased the avidities and immunoreactivities of all IgGs tested. This effect is more pronounced if the oxidation is performed at pH 5.6 and 25 degrees C instead of pH 4.6 and 0 degree C. The BR96 antibody is inactivated even under mild oxidation conditions.

Antibody-targeted photolysis. Bacteriocidal effects of Sn (IV) chlorin e6-dextran-monoclonal antibody conjugates

Antibody-targeted photolysis is a technique for damaging or killing cells using light and an antibody-bound photosensitizer. In the present study, immunoconjugates were constructed to selectively kill Pseudomonas aeruginosa bacteria using tin (IV) chlorin e6 which was linked to dextran and then bound to the carbohydrate moiety of a monoclonal antibody specific for Pseudomonas aeruginosa Fisher type I polysaccharide antigen. Killing of Pseudomonas during mid-log phase growth was shown to be dependent upon light dose with complete bacterial cell killing observed at an irradiation dose of 80 J/cm2. Individual components of the immunoconjugates (e.g., antibody or chlorin alone) showed no bacterial cytotoxicity and immunoconjugates constructed with nonbinding antibodies were also ineffective as cytotoxic agents. These studies demonstrate that killing of gram negative bacteria using photoradiation is feasible and suggest that this methodology may be applicable in treatment of infections in man.