Pronounced antitumor efficacy of doxorubicin when given as the prodrug DOX-GA3 in combination with a monoclonal antibody beta-glucuronidase conjugate

A glucuronide doxorubicin prodrug N-[4-doxorubicin-N-carbonyl (oxymethyl) phenyl] O-beta-glucuronyl carbamate (DOX-GA3) has been developed to improve the antitumor effects of doxorubicin (DOX). The prodrug was originally designed to be activated into drug by human beta-glucuronidase (GUS) released from tumor cells in necrotic areas of tumor lesions. The aim of this study was to further improve the antitumor effects of DOX-GA3 by means of antibody-directed enzyme prodrug therapy (ADEPT). We thus investigated if the administration of an enzyme-immunoconjugate prepared from the pancarcinoma Ep-CAM specific monoclonal antibody (MAb) 323/A3 and beta-glucuronidase would result in improved antitumor effects because of additional enzyme localization in tumor tissue. In vitro, the prodrug DOX-GA3 was found to be 12-times less toxic than the parent drug DOX in a human ovarian cancer cell line. Immunospecific and complete activation of the prodrug took place when the cells were pretreated with 323/A3-beta-glucuronidase conjugate. In nude mice bearing s.c. human ovarian cancer xenografts (FMa) the maximum tolerated dose (MTD) of DOX-GA3 (500 mg/kg weekly x 2) was much higher when compared with that of DOX (8 mg/kg weekly x 2). In mice bearing well-established FMa xenografts, the standard treatment of DOX at the MTD (8 mg/kg weekly x 2) resulted in a tumor growth inhibition of 67%. Treatment with DOX-GA3 at a single dose of 500 mg/kg resulted in a better tumor growth inhibition of 87%. The combination of DOX-GA3 (500 mg/kg) with 323/A3-mGUS conjugate and anti-GUS MAb 105, to clear circulating conjugate, improved the antitumor effect even further to 98%. At the lower dose of 250 mg/kg DOX-GA3 tumor growth inhibition (34%) was not better than that of DOX. The combination, however, of DOX-GA3 at 250 mg/kg and 323/A3-mGUS conjugate plus MAb 105 again greatly improved the antitumor effect (growth inhibition of 93%). DOX given at 8 mg/kg weekly x 2 did not result in tumor regressions. As a result of ADEPT, the number of regressions of tumors improved from 0 out of 12 to 9 out of 11 at a dose of 250 mg/kg DOX-GA3. At the higher prodrug dose (500 mg/kg) the number of regressions improved from 2 out of 12 to 9 out of 10 as a result from the addition of enzyme-immunoconjugate. Our studies show that the efficacy of the widely used anti-cancer agent DOX may be improved by using the prodrug DOX-GA3, in combination with the tumor-specific enzyme-immunoconjugate 323/A3-mGUS and a conjugate clearing antibody.

A novel doxorubicin-glucuronide prodrug DOX-GA3 for tumour-selective chemotherapy: distribution and efficacy in experimental human ovarian cancer

The doxorubicin (DOX) prodrug N-[4-doxorubicin-N-carbonyl (oxymethyl) phenyl] O-beta-glucuronyl carbamate (DOX-GA3) was synthesised for specific activation by human beta-glucuronidase, which is released in necrotic areas of tumour lesions. This novel prodrug was completely activated to the parent drug by human beta-glucuronidase with V(max)= 25.0 micromol x min(-1) x mg(-1) and K(m) = 1100 microM. The pharmacokinetics and distribution of DOX-GA3 in nude mice bearing human ovarian cancer xenografts (OVCAR-3) were determined and compared with DOX. Administration of DOX at 8 mg x kg(-1) i.v. (maximum tolerated dose, MTD) to OVCAR-3-bearing mice resulted in a peak plasma concentration of the drug of 16.4 microM (t = 1 min). A 7.6-times lower peak plasma concentration of DOX was measured after injection of DOX-GA3 at 250 mg x kg(-1) i.v. (50% of MTD). In normal tissues the prodrug showed peak DOX concentrations that were up to 5-fold (heart) lower than those found after DOX administration. DOX-GA3 activation by beta-glucuronidase in the tumour yielded an almost 5-fold higher DOX peak concentration of 9.57 nmol x g(-1) (P< 0.05) than the peak concentration of only 2.14 nmol x g(-1) observed after DOX. As a consequence, the area under the curve of DOX calculated in tumour tissue after DOX-GA3 (13.1 micromol x min(-1) x g(-1)) was 10-fold higher than after DOX (1.31 micromol x min(-1) x g(-1)). The anti-tumour effects of DOX-GA3 and DOX were compared at equitoxic doses in OVCAR-3 xenografts at a mean tumour size of 125 mm(3). The prodrug given i.v. at 500 mg x kg(-1) weekly x 2 resulted in a maximum tumour growth inhibition of 87%, while the standard treatment with DOX at a dose of 8 mg x kg(-1) i.v. weekly x 2 resulted in a maximum tumour growth inhibition of only 56%. Treatment with DOX-GA3 was also given to mice with larger tumours containing more necrosis. For tumours with a mean size of 400 mm(3) the specific growth delay by DOX-GA3 increased from 2.7 to 3.9. Our data indicate that DOX-GA3 is more effective than DOX and suggest that the prodrug will be specifically advantageous for treatment of advanced disease.

Recombinant adenovirus vectors with knobless fibers for targeted gene transfer

Adenoviral vector systems for gene therapy can be much improved by targeting vectors to specific cell types. This requires both the complete ablation of native adenovirus tropism and the introduction of a novel binding affinity in the viral capsid. We reasoned that these requirements could be fulfilled by deleting the entire knob domain of the adenovirus fiber protein and replacing it with two distinct moieties that provide a trimerization function for the knobless fiber and specific binding to the target cell, respectively. To test this concept, we constructed adenoviral vectors carrying knobless fibers comprising the alpha-helix trimerization domain from MoMuLV envelope glycoprotein. Two mimic targeting ligands, a Myc-epitope and a 6His-tag, were attached via a flexible linker peptide. The targeted knobless fiber molecules were properly expressed and imported into the nucleus of adenovirus packaging cells, where they were incorporated as functional trimers into the adenovirus capsid. Both ligands were exposed on the surface of the virion and were available for specific binding to their target molecules. Moreover, the knobless fibers mediated gene delivery into cells displaying receptors for the coupled ligand. Hence, these knobless fibers are prototype substrates for versatile addition of targeting ligands to generate truly targeted adenoviruses.

Targeting of adenoviral vectors through a bispecific single-chain antibody

Recombinant adenoviral vectors are attractive in the context of cancer gene therapy because they are capable of delivering genes to a wide variety of tissues. The utility of adenoviruses is limited by their lack of specificity and by the absence of the receptor(s) for these viruses on many tumor cells. Redirecting adenoviral vectors to tissue- or tumor-specific targets can be achieved by using bispecific conjugates produced by chemical linkage of an anti-adenovirus antibody (Ab) and a ligand or Ab directed toward a specific target. To avoid the limitations of chemical conjugates, molecular conjugates of anti-fiber knob and ligand have been proposed. We present here a novel strategy that allows the production of recombinant bispecific single-chain Abs directed at cell surface molecules. A construct was made that encodes a neutralizing anti-adenovirus fiber single-chain Fv (scFv) Ab (S11) fused to a scFv Ab (425) directed against the epidermal growth factor receptor. The fusion protein markedly enhanced the infection efficiency of adenoviral vectors in epidermal growth factor receptor-expressing cell lines. The bispecific scFv could be purified and concentrated after binding of its 6His tag to a nickel column without significant loss of activity. This approach should permit the production of high quantities of active bispecific scFv for in vivo use. The universal design of the construct allows rapid screening for relevant specific scFv directed at cell surface antigens that can be incorporated into adenoviral targeting strategies.

Cationic polymeric gene delivery of beta-glucuronidase for doxorubicin prodrug therapy

BACKGROUND

An approach to improve current chemotherapy is the selective transduction of tumor cells with suicide genes to sensitize these cells to prodrugs of cytostatic agents.

METHODS

In this study, gene transfer was accomplished with the cationic polymer poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), able to condense plasmid-DNA by electrostatic interaction. OVCAR-3 cells were transfected with plasmids encoding E. coli-derived or human beta-glucuronidase and the transfection efficiency and inhibition by serum was determined. Next, we measured the sensitivity of OVCAR-3 cells transiently expressing beta-glucuronidase to the glucuronide prodrug of doxorubicin (DOX-GA3) or to doxorubicin.

RESULTS

OVCAR-3 cells were efficiently transfected with a plasmid encoding E. coli-derived beta-glucuronidase. The degree of transfection (30% of cells) was higher than that achieved with commercially available cationic lipids (DOTAP, Lipofectamine) without inhibition by serum. OVCAR-3 cells transiently expressing beta-glucuronidase were equally sensitive to the glucuronide prodrug of doxorubicin (DOX-GA3) or to doxorubicin itself, indicating complete conversion of prodrug to drug. Similar studies were performed with the plasmid encoding for human beta-glucuronidase, which is likely to be less immunogenic. Also in this case, OVCAR-3 cells showed an increased sensitivity to the prodrug DOX-GA3, although less pronounced than when the bacterial enzyme was used. A strong bystander effect was observed when OVCAR-3 cells transfected with beta-glucuronidase were mixed with non-transfected cells at different ratios. Complete tumor cell growth inhibition was already observed when only 15% of the cells expressed the activating enzyme.

CONCLUSION

These studies suggest that beta-glucuronidase gene therapy using PDMAEMA as a carrier system and DOX-GA3 as the prodrug has a potential application in cancer gene therapy.

A rapid and versatile method for harnessing scFv antibody fragments with various biological effector functions

A versatile expression vector is described for the rapid construction and evaluation of bispecific scFvs and scFv-based fusion proteins. An important feature of this vector is the presence of two multiple cloning sites (MCS) separated by an in frame linker sequence. The first MCS was specifically designed to contain unique SfiI and NotI restriction enzyme sites that can be used for directional and in frame insertion of scFvs (or potentially any molecule) selected from established phage-display systems. Using this new vector, a functional bs-(scFv)(2) (2C11-MOC31) was constructed for retargeted T-cell cytotoxicity towards EGP2 positive tumor cells. The vector was also used for grafting of a number of promising biological effector principles onto scFv MOC31, including the prodrug converting enzyme cytosine deaminase, the anti-angiogenic factor angiostatin, and the thrombogenic molecule tissue factor. We aimed at producing biologically active fusion proteins by directing them through the endoplasmic reticulum-based protein folding machinery of eukaryotic cells (COS-7) using a kappa light chain leader, thereby taking advantage of the associated quality control mechanisms that allow only fully folded and processed fusion proteins to be secreted into the medium. Supernatants derived from fusion protein transfected COS-7 cells, which were transiently transfected at low transfection rates, were directly assayed for the biological and/or targeting activity of the excreted fusion proteins without any prior purification steps. This procedure might help to identify those fusion proteins that have favourable characteristics like stability and biological activity in the presence of serum and at low protein concentrations. Targeted delivery of all effector principles was subsequently assessed in an in vitro model system. The method we devised is both rapid and versatile and can be useful to construct and identify series of new chimeric proteins with enhanced therapeutic potential in human cancer therapy.

Cationic polymeric gene delivery of beta-glucuronidase for doxorubicin prodrug therapy

BACKGROUND

An approach to improve current chemotherapy is the selective transduction of tumor cells with suicide genes to sensitize these cells to prodrugs of cytostatic agents.

METHODS

In this study, gene transfer was accomplished with the cationic polymer poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), able to condense plasmid-DNA by electrostatic interaction. OVCAR-3 cells were transfected with plasmids encoding E. coli-derived or human beta-glucuronidase and the transfection efficiency and inhibition by serum was determined. Next, we measured the sensitivity of OVCAR-3 cells transiently expressing beta-glucuronidase to the glucuronide prodrug of doxorubicin (DOX-GA3) or to doxorubicin.

RESULTS

OVCAR-3 cells were efficiently transfected with a plasmid encoding E. coli-derived beta-glucuronidase. The degree of transfection (30% of cells) was higher than that achieved with commercially available cationic lipids (DOTAP, Lipofectamine) without inhibition by serum. OVCAR-3 cells transiently expressing beta-glucuronidase were equally sensitive to the glucuronide prodrug of doxorubicin (DOX-GA3) or to doxorubicin itself, indicating complete conversion of prodrug to drug. Similar studies were performed with the plasmid encoding for human beta-glucuronidase, which is likely to be less immunogenic. Also in this case, OVCAR-3 cells showed an increased sensitivity to the prodrug DOX-GA3, although less pronounced than when the bacterial enzyme was used. A strong bystander effect was observed when OVCAR-3 cells transfected with beta-glucuronidase were mixed with non-transfected cells at different ratios. Complete tumor cell growth inhibition was already observed when only 15% of the cells expressed the activating enzyme.

CONCLUSION

These studies suggest that beta-glucuronidase gene therapy using PDMAEMA as a carrier system and DOX-GA3 as the prodrug has a potential application in cancer gene therapy.

Tumor-specific gene transfer via an adenoviral vector targeted to the pan-carcinoma antigen EpCAM

The utility of adenoviral vectors for cancer therapy is limited due to their lack of specificity for tumor cells. In order to target adenovirus to tumor, the natural tropism of the adenovirus should be ablated and replaced by a tumor-specific binding domain. To this end, a neutralizing anti-fiber antibody conjugated to an anti-EpCAM antibody was created that targets the adenovirus to the EpCAM antigen present on tumor cells. The EpCAM antigen was chosen as the target because this antigen is highly expressed on a variety of adenocarcinomas of different origin such as breast, ovary, colon and lung, whereas EpCAM expression is limited in normal tissues. In these studies, the EpCAM-targeted adenovirus was shown to infect specifically cancer cell lines of different origin expressing EpCAM such as ovary, colon and head and neck. Gene transfer was blocked by excess anti-EpCAM antibody and dramatically reduced in EpCAM negative cell lines, thus showing the specificity of the EpCAM-targeted adenovirus. Importantly, infection with targeted adenovirus was independent of CAR, which is the natural receptor for adenovirus binding, since blocking of CAR with recombinant fiber knob did not affect infection with targeted adenovirus. Apart from the cancer cell lines, the efficacy of targeted viral infection was studied in freshly isolated primary human colon cancer cells. As colon cancer predominantly metastasizes to liver, and adenovirus has a high tropism for hepatocytes, we also sought to determine if the EpCAM-targeted adenovirus showed reduced infectivity of human liver cells. The bispecific antibody could successfully mediate gene transfer to primary human colon cancer cells, whereas it almost completely abolished infection of liver cells. This work thus demonstrates that EpCAM-targeted adenoviral vectors can be specifically directed to a wide variety of adenocarcinomas. This approach may prove to be useful for selective gene therapy of cancer.

Novel anthracycline-spacer-beta-glucuronide,-beta-glucoside, and -beta-galactoside prodrugs for application in selective chemotherapy

A series of anthracycline prodrugs containing an immolative spacer was synthesized for application in selective chemotherapy. The prodrugs having the general structure anthracycline-spacer-beta-glycoside were designed to be activated by beta-glucuronidase or beta-galactosidase. Prodrugs with -chloro, -bromo or -n-hexyl substituents on the spacer were synthesized as well as prodrugs containing a -beta-glucuronyl, -beta-glucosyl or -beta-galactosyl carbamate specifier. The key step in the synthesis of all prodrugs is the highly beta-diastereoselective addition reaction of the anomeric hydroxyl of a glycosyl donor to a spacer isocyanate resulting in the respective beta-glycosyl carbamate pro-moieties. The resulting protected pro-moieties were coupled to an anthracycline. Prodrugs were evaluated with respect to activation rate by the appropriate enzyme and additionally, their IC50 values were determined. Optimal prodrugs in this study were at least 100- to 200-fold less toxic than their corresponding drug in vitro and were activated to the parent drug in a half-life time of approximately 2 h.

Design of immuno-enzymosomes with maximum enzyme targeting capability: effect of the enzyme density on the enzyme targeting capability and cell binding properties

Immuno-enzymosomes have been proposed for the targeting of enzymes to cancer cells to achieve site specific activation of anticancer prodrugs. Previously, we reported that the enzyme beta-glucuronidase (GUS), capable of activating anthracycline-glucuronide prodrugs, can be coupled to the surface of inmunoliposomes directed against human ovarian cancer cells (OVCAR-3). This study aimed at the design of an immuno-enzymosome formulation with maximum enzyme targeting capability. By purification of the commercially available enzyme beta-glucuronidase (GUS), a 2-fold increase in the enzyme specific activity and a 4-fold increase in the enzymatic activity of immuno-enzymosomes was achieved. As a result, upon incubation with human ovarian cancer cells (OVCAR-3), cell-associated enzymatic activity increased correspondingly. The optimized immuno-enzymosomes were shown to bind to the target cells in a specific fashion. Above a GUS/Fab' molar ratio of 0.5, impairment of the target cell binding ability of the immuno-enzymosomes was observed. This was likely due to a steric hindrance effect mediated by the presence of large amounts of bulky GUS molecules on the liposome surface. Nevertheless, increasing the GUS density on the surface of the immuno-enzymosomes to levels by far exceeding the GUS/Fab' molar ratio of 0.5, yielded a considerably improved enzyme targeting capability.

Distribution and pharmacokinetics of the prodrug daunorubicin-GA3 in nude mice bearing human ovarian cancer xenografts

N-[4-daunorubicin-N-carbonyl (oxymethyl)phenyl] O-beta-glucuronyl carbamate (DNR-GA3) is a glucuronide prodrug of daunorubicin (DNR) which induced a better tumor growth delay than DNR when studied at equitoxic doses in three human ovarian cancer xenografts. These results suggested that the prodrug DNR-GA3 was selectively activated by human beta-glucuronidase present in tumor tissue. We determined the pharmacokinetics and distribution of DNR-GA3 in nude mice bearing human ovarian cancer xenografts (OVCAR-3, FMa, A2780, and MRI-H-207). Administration of DNR at 10 mg/kg i.v. (maximum tolerated dose) to OVCAR-3-bearing mice resulted in a peak plasma concentration of the drug of 12.18 microM (t = 1 min). DNR-GA3 at 100 mg/kg i.v. (approximately 50% of the maximum tolerated dose [MTD]) resulted in a peak plasma concentration of DNR that was 28-fold lower than that after DNR itself; in normal tissues, prodrug injection resulted in 5- to 23-fold lower DNR concentrations. DNR showed a relatively poor uptake into OVCAR-3 tumors with a peak concentration of 2.05 nmol x g(-1) after injection. In the same xenograft, DNR-GA3 resulted in a significantly higher DNR peak concentration of 3.45 nmol x g(-1) (P < 0.05). The higher area under the curve of DNR in tumor tissue after DNR-GA3 than after DNR itself would be the result of prodrug activation by beta-glucuronidase. In this respect, a considerably higher beta-glucuronidase activity was found in tumor tissue when compared to plasma. The specific activation of DNR-GA3 by beta-glucuronidase at the tumor site relative to normal organs leads to a more tumor-selective therapy, resulting in greater efficacy without increased toxicity.

The efficacy of the anthracycline prodrug daunorubicin-GA3 in human ovarian cancer xenografts

The prodrug N-[4-(daunorubicin-N-carbonyl-oxymethyl)phenyl] O-beta-glucuronyl carbamate (DNR-GA3) was synthesized for specific activation by human beta-glucuronidase, released in necrotic areas of tumour lesions. In vitro, DNR-GA3 was 18 times less toxic than daunorubicin (DNR) and the prodrug was completely activated to the parent drug by human beta-glucuronidase. The maximum tolerated dose of DNR-GA3 in nude mice bearing s.c. human ovarian cancer xenografts was 6-10 times higher than that of DNR. The prodrug was cleared more rapidly from the circulation (elimination t1/2 = 20 min) than the parent drug (elimination t1/2 = 720 min). The anti-tumour effects of DNR-GA3 and DNR were investigated in four different human ovarian cancer xenografts OVCAR-3, FMa, A2780 and MRI-H-207 at a mean tumour size between 100 and 200 mm3. In three out of four of these tumour lines, the prodrug given i.v. at the maximum tolerated dose ranging from 150 to 250 mg kg(-1) resulted in a maximum tumour growth inhibition from 82% to 95%. The standard treatment with DNR at a dose of 8 mg kg(-1) given i.v. weekly x 2 resulted only in a maximum tumour growth inhibition from 40% to 47%. Tumour line FMa did not respond to DNR, nor to DNR-GA3. Treatment with DNR-GA3 was also given to mice with larger tumours that would contain more necrosis (mean size 300-950 mm3). The specific growth delay by DNR-GA3 was extended from 2.1 to 4.4 in OVCAR-3 xenografts and from 4.4 to 6.0 in MRI-H-207 xenografts. Our data indicate that DNR-GA3 is more effective than DNR and may be especially of use for treatment of tumours with areas of necrosis.

Construction and characterization of a fusion protein of single-chain anti-CD20 antibody and human beta-glucuronidase for antibody-directed enzyme prodrug therapy

The CD20 antigen is an attractive target for specific treatment of B-cell lymphoma. Antibody-directed enzyme prodrug therapy (ADEPT) aims at the specific activation of a nontoxic prodrug at the tumor site by an enzyme targeted by a tumor-specific antibody such as anti-CD20. We constructed a fusion protein of the single-chain Fv anti-CD20 mouse monoclonal antibody (MoAb) 1H4 and human beta-glucuronidase for the activation of the nontoxic prodrug N-[4-doxorubicin-N-carbonyl(-oxymethyl) phenyl] O-beta-glucuronyl carbamate to doxorubicin at the tumor site. The cDNAs encoding the light- and heavy-chain variable regions of 1H4 were cloned, joined by a synthetic sequence encoding a 15-amino acid linker and fused to human beta-glucuronidase by a synthetic sequence encoding a 6-amino acid linker. An antibody-enzyme fusion protein-producing cell line was established by transfection of the construct into human embryonic kidney 293/EBNA cells. The yield of active fusion protein was 100 ng/mL transfectoma supernatant. Antibody affinity, antibody specificity, and enzyme activity were fully retained by the fusion protein. Immunoprecipitation and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the fusion protein has a relative molecular weight (Mw) of 100 kD under denaturing conditions. Gel filtration analysis indicated that the enzymatically active form of the fusion protein is a tetramer with an Mw of approximately 400 kD. The nontoxic prodrug N-[4-doxorubicin-N-carbonyl(-oxymethyl) phenyl] O-beta-glucuronyl carbamate was hydrolyzed by the fusion protein at a hydrolysis rate similar to that of human beta-glucuronidase. When the fusion protein was specifically bound to Daudi lymphoma cells, the prodrug induced similar antiproliferative effects as doxorubicin. Thus, it is feasible to construct a eukaryotic fusion protein consisting of a single-chain anti-CD20 antibody and human beta-glucuronidase for future use in the activation of anticancer prodrugs in B-cell lymphoma.

d. Meulen-Muileman I, Pinedo HM, Boven E. Construction and characterization of a fusion protein of single-chain anti-carcinoma antibody 323/A3 and human beta-glucuronidase

We report the construction and expression of a fusion protein between a single-chain antibody specific for human carcinomas and human beta-glucuronidase by recombinant DNA technology. The sequences encoding the murine monoclonal antibody 323/A3 light- and heavy-chain variable genes were joined by a synthetic sequence encoding a 15-amino-acid linker and combined with human beta-glucuronidase by a synthetic sequence encoding a 6-amino-acid linker. The construct was placed under the control of the cytomegalovirus promotor and expressed in COS-7 cells. The yield of active fusion protein was 10 ng/ml transfectoma supernatant. Antibody affinity, antibody specificity and enzyme activity were fully retained by the fusion protein. Biochemical characterization of the fusion protein by sodium dodecyl sulfate/polyacrylamide gel electrophoresis showed a molecular mass of 100 kDa under denaturing conditions. Gel-filtration analysis indicated that the enzymatically active form is a tetramer of approximately 400 kDa. The non-toxic prodrug N-[4-doxorubicin-N-carbonyl(oxymethyl)phenyl]-O-beta-glucuronyl carbamate was activated to the cytotoxic drug doxorubicin by the fusion protein with a hydrolysis rate similar to that of human beta-glucuronidase. The growth inhibition of tumor cells coated with the fusion protein and exposed to prodrug was similar to that obtained with doxorubicin. This study shows the feasibility of constructing eukaryotic fusion proteins consisting of a single-chain antibody and human beta-glucuronidase for use in the specific activation of anticancer prodrugs.

Determination of tumor-related factors of influence on the uptake of the monoclonal antibody 323/A3 in experimental human ovarian cancer

The epithelial glycoprotein 40 (EGP40) is an important target in the clinic for radioimmunolocalization and monoclonal antibody (MAb)-mediated therapy of cancer. We determined which tumor-related factors (including antigen distribution and density, vascularization and perfusion) were involved in the uptake of the anti-EGP40 MAb 323/A3 in 4 different human ovarian cancer xenografts grown s.c. in nude mice. The reactivity pattern of 323/A3 in all xenografts in vitro was similar and showed a strong and homogeneous distribution of the EGP40 antigen. FMa xenografts, however, showed the highest uptake of 323/A3 in vivo, which was 5.5-, 6.2- and 10.0-fold higher than that in OVCAR-3, Ov.Pe and Ov.Sh xenografts, respectively. FMa xenografts contained 2.1- to 3.5-fold more antigen per gram protein when compared with the antigen content of the other xenografts. FMa and Ov.Sh xenografts demonstrated a better vascularization pattern, whereas Ov.Pe and OVCAR-3 xenografts were moderately to poorly vascularized. FMa xenografts were also better perfused, as was shown by a 1.6- to 1.8-fold higher uptake of the (99m)Tc-labeled blood flow marker hexamethylpropyleneamine oxime (HMPAO). The tumor uptake of the non-specific MAb E48 was 2.2- to 11.2-fold lower when compared with that of 323/A3, but the sequence of uptake was similar (FMa > OVCAR-3 = Ov.Pe > Ov.Sh), indicating the lowest extravasation of MAbs in Ov.Sh xenograft tissue. Since both the antigen content and the perfusion appeared to be important factors of influence on the tumor uptake of 323/A3, attempts were made to manipulate these determinants to improve the tumor uptake. Neither gamma-interferon nor 5-fluorouracil were able to increase EGP40 expression in human ovarian cancer cells in vitro. Treatment of tumor-bearing mice with the calcium-antagonist flunarizine did not result in an improved perfusion, although a slight increase in the initial tumor uptake of 323/A3 was observed in Ov.Sh-bearing mice. Our results illustrate the relative contribution of various tumor-related factors that determine the usefulness of a MAb for imaging and therapy of cancer.

Synthesis and biological activity of beta-glucuronyl carbamate-based prodrugs of paclitaxel as potential candidates for ADEPT

The syntheses of prodrugs of paclitaxel, which can be used in ADEPT in order to target paclitaxel towards tumor cells, are described. The prodrugs 1 and 2a, b consist of a spacer molecule connected via a carbamate linkage to a beta-glucuronic acid. The spacer molecule is also connected via an ester linkage to the 2'-OH of paclitaxel. Enzyme-catalyzed hydrolysis of the glucuronic acid moiety by human beta-glucuronidase results in the liberation of the parent drug paclitaxel via gamma or delta lactam formation with half-lives of 45 min and 2 h (1 and 2b). The prodrugs 1 and 2b are two orders of magnitude less cytotoxic than paclitaxel.

Zirconium-labeled monoclonal antibodies and their distribution in tumor-bearing nude mice

A method to label monoclonal antibodies (MAbs) with 88Zr and 89Zr has been developed and tested on the MAbs 323/A3 and E48.

METHODS

The bifunctional chelating agent desferal (Df) was linked through a thioether bond to the MAbs. Labeling was accomplished by addition of the premodified antibodies to isolated Zr. The retention of the in vivo behavior of the MAbs was determined by comparing the biodistribution of 88Zr-labeled MAbs with those of 123I and 99mTc in mice bearing tumor xenografts.

RESULTS

The labeling was simple and the yields were high (above 90%). The obtained conjugates retained their immunoreactivity (> 80%). The blood clearance and biodistribution of Zr-labeled MAbs resembled those of the reference conjugates. The Zr-Df-MAb conjugates showed a specific tumor accumulation. Zirconium-89-labeled 323/A3 could be visualized with a PET camera. The absence of large amounts of Zr present in the bone pointed to a good in vivo stability of the Zr-Df-MAb conjugates.

CONCLUSION

This method is well suited for labeling MAbs with Zr isotopes. Using 89Zr, the biodistribution of the radioimmunoconjugate can easily be visualized with a PET camera.

Improved characteristics of a human beta-glucuronidase-antibody conjugate after deglycosylation for use in antibody-directed enzyme prodrug therapy

Antibody-directed enzyme prodrug therapy (ADEPT) aims at the specific activation of relatively nontoxic prodrugs into active drugs at the tumor site. One of the enzymes described to be useful in ADEPT is human beta-glucuronidase (GUS), which is expected to have low immunogenicity in patients. A major obstacle for the use of GUS, however, is its rapid glycan-specific hepatic clearance. The carbohydrates of GUS have been modified by subsequent treatment with NaIO4 and NaBH4 to improve its retention in the circulation. The modification of GUS did not decrease the enzyme activity. In vitro it was demonstrated that a conjugate prepared with a pancarcinoma specific monoclonal antibody (mAb) 323/A3 and the modified enzyme (mGUS), when bound to tumor cells, was capable of complete prodrug activation. In vivo, the 323/A3-mGUS conjugate was cleared faster from the circulation of BALB/c mice (t1/2 = 9 h) than mAb 323/A3 (t1/2 = 32 h), but it was retained in the circulation much longer than an immunoconjugate prepared with native GUS (t1/2 = 24 min). In nude mice bearing subcutaneous OVCAR-3 tumors the distribution of 323/A3-mGUS was qualitatively comparable to that of mAb 323/A3. The 323/A3-mGUS conjugate showed specific localization in the tumor but to a lesser extent than mAb 323/A3 (2.7% vs 6.4% injected dose per gram at 1 day after iv injection). A favorable tumor-to-blood ratio of > 2 was observed for the conjugate at 7 days after administration, which is necessary for tumor-specific prodrug activation.

Characterization of novel anthracycline prodrugs activated by human beta-glucuronidase for use in antibody-directed enzyme prodrug therapy

Antibody-directed enzyme prodrug therapy (ADEPT) aims at the specific activation of a prodrug by an enzyme-immunoconjugate localized in tumor tissue. The use of an enzyme of human origin is preferable in ADEPT because it might not be immunogenic when administered to patients. In the case of human beta-glucuronidase, prodrugs should be designed that are rapidly and completely activated at a neutral pH. Four new daunorubicin glucuronides were synthesized by coupling a glucuronide group to daunorubicin via an aliphatic (GA1 and GB1) or an aromatic (GA3, GB6) carbamate spacer, to be released by electron shift (A-type) or by ring closure (B-type). These prodrugs were characterized in vitro for their usefulness in ADEPT and were compared with the previously described prodrugs epirubicin-glucuronide and doxorubicin-nitrophenyl-glucuronide. The four new prodrugs were stable in serum, hydrophilic when compared to the lipophilic daunorubicin, and at least 20-fold less toxic than the parent compound. The hydrolysis rate at clinically relevant enzyme and prodrug concentrations (1 microgram/mL human beta-glucuronidase, 100 microM prodrug) at pH 6.8 were similar for GA3 (T1/2 160 min) and higher for GB6 (T1/2 40 min) when compared to that of doxorubicin-nitrophenyl-glucuronide (T1/2 170 min). Epirubicin-glucuronide, GA1, and GB1 showed a low hydrolysis rate (T1/2 > 400 min). GA1 and GA3, but not GB1 or GB6, were activated to the parent compound. Complete activation was confirmed in OVCAR-3 cells pretreated with a specific antibody-human beta-glucuronidase conjugate, where GA3 had similar antiproliferative effects to those of daunorubicin.

A facile method for the labeling of proteins with zirconium isotopes

To label proteins with positron emitters with a half-life in the order of days, a method has been developed to label proteins with zirconium (Zr)-isotopes. Therefore, the bifunctional chelating agent desferal (Df) was coupled to albumins via a thioether bond. Labeling of the premodified proteins was easily performed by addition of these proteins to freeze-dried Zr-oxalate. This labeling was efficient (> 90%) and accomplished in several minutes. The conjugates showed a high in vitro stability. Biodistribution studies were performed with 88Zr-citrate, 88Zr-Df, and 88Zr-labeled mouse serum albumin (88Zr-Df-MSA), modified with different amounts of chelating groups. Whereas Zr-citrate was found to accumulate in bone, Zr-Df was cleared very fast by glomerular filtration. The 88Zr-Df-MSA showed similar blood clearance as did 123I-labeled MSA. The biodistribution pattern of 88Zr-Df-MSA differed only from 123I-MSA in that a higher accumulation of Zr in liver, kidney, and spleen was found. The absence of large amounts of 88Zr in bone indicated that in vivo the conjugates are also reasonably stable.

Immunoliposomes as enzyme-carriers (immuno-enzymosomes) for antibody-directed enzyme prodrug therapy (ADEPT): optimization of prodrug activating capacity

PURPOSE

Immuno-enzymosomes are tumor-specific immunoliposomes bearing enzymes on their surface. These enzymes are capable of converting relatively nontoxic prodrugs into active cytostatic agents. The enzyme beta-glucuronidase (GUS)4 was coupled to the external surface of immunoliposomes directed against ovarian carcinoma cells. This study aimed at optimization of the prodrug-activating capacity of these immuno-enzymosomes by increasing the enzyme density on the immunoliposomal surface.

METHODS

To achieve coupling of GUS to the liposomes, introduction of extra thiol groups was required. Two thiolating agents were examined: iminothiolane and SATA.

RESULTS

When iminothiolane was used, aggregation of enzymosomes was observed above enzyme densities of 10 micrograms GUS/mumol lipid (TL). An increased electrostatic repulsion of the enzymosomes, created by inclusion of additional negatively charged lipids and by lowering the ionic strength of the external aqueous medium resulted in enzyme densities > or = 20 micrograms GUS/mumol TL without aggregation. Utilizing SATA, > or = 30 micrograms GUS/mumol TL could be coupled without aggregation, even at physiological ionic strength. It was shown that the enzyme density on immuno-enzymosomes, and thus on the tumor cell surface, strongly influences the antitumor effect of the prodrug daunorubicin-glucuronide against in vitro cultured ovarian cancer cells. The antitumor effect of immuno-enzymosomes with enzyme densities of about 20 micrograms GUS/mumol TL was similar to that of the parent drug daunorubicin.

CONCLUSIONS

SATA-mediated thiolation of GUS-molecules enabled the preparation of immuno-enzymosomes with high enzyme densities while avoiding spontaneous aggregation. In vitro antitumor activity experiments showed that the improved immuno-enzymosome system is able to completely convert the prodrug daunorubicin-glucuronide into its parent compound.

Comparison of monoclonal antibodies 17-1A and 323/A3: the influence of the affinity on tumour uptake and efficacy of radioimmunotherapy in human ovarian cancer xenografts

The low-affinity monoclonal antibody (MAb) chimeric 17-1A(c-17-1A) and the high-affinity MAb mouse 323/A3 (m-323/A3) were used to study the effect of the MAb affinity on the tumour uptake and efficacy of radioimmunotherapy in nude mice bearing subcutaneously the human ovarian cancer xenografts FMa, OVCAR-3 and Ov.Pe. Both MAbs are directed against the same pancarcinoma glycoprotein. In vitro, the number of binding sites on tumour cells at 4 degrees C was similar for both MAbs, but m-323/A3 had an approximately 5-fold higher affinity (1.3-3.0x10(9) M-1) than c-17-1A (3.0-5.4x10(8) M-1). This difference in affinity was more extreme at 37 degrees C, when no binding of c-17-1A could be observed. MAb m-323/A3 completely blocked binding of c-17-1A to tumour cells, whereas the reverse was not observed. Immunohistochemistry showed a similar but more intense staining pattern of m-323/A3 in human ovarian cancer xenografts than of c-17-1A. In vivo, the blood clearance in non-tumour-bearing nude mice was similar for both MAbs with terminal half-lives of 71.4 h for m-323/A3 and 62.7 h for c-17-1A. MAb m-323/A3 targeted better to tumour tissue, but was more heterogeneously distributed than c-17-1A. The cumulative absorbed radiation dose delivered by m-323/A3 to tumour tissue was 2.5- to 4.7-fold higher than that delivered by c-17-1A. When mice were treated with equivalent radiation doses of 131(I)m-323/A3 and 131(I)c-17-1A, based on a correction for the immunoreactivity of the radiolabelled MAbs, m-323/A3 induced a better growth inhibition in two of the three xenografts. When the radiation doses were adjusted to obtain a similar amount of radiation in the tumour c-17-1A was more effective in tumour growth inhibition in all three xenografts.

A monoclonal antibody against human beta-glucuronidase for application in antibody-directed enzyme prodrug therapy

The selectivity of anticancer agents may be improved by antibody-directed enzyme prodrug therapy (ADEPT). The immunogenicity of antibody-enzyme conjugates and the low tumor to normal tissue ratio calls for the use of a human enzyme and the development of a monoclonal antibody (MAb) against that enzyme for rapid clearance of the conjugate from the circulation. We isolated beta-glucuronidase from human liver. BALB/c mice were immunized with the roughly purified human liver beta-glucuronidase and we obtained an MAb designated 105. Immunoblotting showed reactivity with native tetrameric human beta-glucuronidase. MAb 105 neither bound to enzyme from bovine liver, rat liver, or mouse liver nor reacted with other human lysosomal enzymes. The antibody appeared to be useful to further purify human beta-glucuronidase from human liver or human placenta to homogeneity by affinity chromatography. MAb 105 did not inhibit the activity of human beta-glucuronidase. When human beta-glucuronidase was injected i.v. into BALB/c mice, the newly generated MAb 105 could indeed accelerate the clearance of the enzyme with a 50% drop in its activity within 5 min.