Site-specific prodrug activation by antibody-beta-lactamase conjugates: preclinical investigation of the efficacy and toxicity of doxorubicin delivered by antibody directed catalysis

Anti-CEA tagged iron nanoparticles for targeting triple-negative breast cancer

Systemic therapy is generally required for breast cancer. However, treatment toxicity and side effects are a concern, especially for triple-negative breast cancer (TNBC), a subtype that usually develops resistance to chemotherapy. To overcome this issue, new nanoformulations capable of targeting cancer cells have been developed and alternative biomarkers have been explored as target molecules for TNBC management. In this study, we performed an in vivo assay in a murine orthotopic TNBC model to evaluate the targeting ability of anti-carcinoembryonic antigen (anti-CEA) loaded nanoparticles (labelled MFCEA), which had been previously synthetized by our research group. 4T1 cells were injected in the mammary gland of balb-c mice, and tumors were evaluated for CEA expression by immunohistochemistry. Tumor-bearing mice received targeted (MFCEA) and non-targeted (MF) nanoparticles intraperitoneally. Tumors were removed 1, 4, 15 and 24h after treatment, and Prussian blue iron staining was performed. Our results showed, as far as we know for the first time, that 4T1 induced tumors are CEA positive, and this opens up new prospects for treating TNBC. Furthermore, MFCEA nanoparticles were able to target malignant tissue and were retained in the tumor for longer than MF nanoparticles. The retention property of MFCEA, together with the absence of toxicity observed in the MTT assay, make these nanoparticles a promising device for management of CEA positive tumors and perhaps for TNBC. Nevertheless, further studies must be carried out to improve their performance and ensure safety for clinical studies.

Dual-Pharmacophore Pyrithione-Containing Cephalosporins Kill Both Replicating and Nonreplicating Mycobacterium tuberculosis

The historical view of β-lactams as ineffective antimycobacterials has given way to growing interest in the activity of this class against Mycobacterium tuberculosis (Mtb) in the presence of a β-lactamase inhibitor. However, most antimycobacterial β-lactams kill Mtb only or best when the bacilli are replicating. Here, a screen of 1904 β-lactams led to the identification of cephalosporins substituted with a pyrithione moiety at C3' that are active against Mtb under both replicating and nonreplicating conditions, neither activity requiring a β-lactamase inhibitor. Studies showed that activity against nonreplicating Mtb required the in situ release of the pyrithione, independent of the known class A β-lactamase, BlaC. In contrast, replicating Mtb could be killed both by released pyrithione and by the parent β-lactam. Thus, the antimycobacterial activity of pyrithione-containing cephalosporins arises from two mechanisms that kill mycobacteria in different metabolic states.

Selective delivery of therapeutic agents for the diagnosis and treatment of cancer

Research activity aimed towards achieving specific and targeted delivery of cancer therapeutics has expanded tremendously in the last decade, resulting in new ways of directing drugs to tumours, as well as new types of drugs. The available strategies exploit differences in the nature of normal and cancer cells and their microenvironment. The discovery and validation of cancer-associated markers, as well as corresponding ligands, is pivotal for developing selective delivery technology for cancer. Although most current clinical trials are either monoclonal antibody- or gene-based, methodological advances in combinatorial libraries of peptides, single chain variable fragments and small organic molecules are expected to change this scenario in the near future. Nanotechnology platforms today allow systematic and modular combinations of therapeutic agents and tumour-binding moieties that may generate novel, personalised agents for selective delivery in cancer. This paper discusses recent developments and future prospects of targeted delivery technologies in the management of cancer.

Combined Radioimmunotherapy and Chemotherapy of Breast Tumors with Y-90-Labeled Anti-Her2 and Anti-CEA Antibodies with Taxol

Because breast cancer cells often express either Her2/neu or carcinoembryonic antigen (CEA) or both, these tumor markers are good targets for radioimmunotherapy using Y-90-labeled antibodies. We performed studies on nude mice bearing xenografts from MCF7, a cell line that has low Her2 and CEA expression, to more accurately reflect the more usual situation in breast cancer. Although uptake of In-111 anti-CEA into tumors was lower than that for In-111-labeled anti-Her2, radioimmunotherapy (RIT) with Y-90 anti-CEA was equivalent to that of Y-90 anti-Her2. When either Y-90 antibody was combined with a split-dose treatment with Taxol, the antitumor effect was greater than with either agent alone. When Y-90 anti-CEA was combined with a single dose of Taxol, the results were equivalent to the split-dose regimen. RIT plus cold Herceptin had no additional effects on tumor size reduction over RIT alone. When animals were first treated with Y-90 anti-Her2 and imaged 1-2 weeks later with In-111 anti-CEA or anti-Her2, tumor uptake was higher for anti-CEA and improved over tumor uptake with no prior RIT. These results suggest that a split dose of RIT with anti-Her2 antibody followed by anti-CEA antibody would be more effective than a single dose of either. This prediction was partially confirmed in a controlled study comparing single- vs split-dose anti-Her2 RIT followed by either anti-Her2 or anti-CEA RIT. These studies suggest that combined RIT and Taxol therapy are suitable in breast cancers expressing either low amounts of Her2 or CEA, thus expanding the number of eligible patients for combined therapies. They further suggest that split-dose RIT using different combinations of Y-90-labeled antibodies is effective in antitumor therapy.

Targeting enzymes to cancers - new developments

Two methods of using tumour located enzymes have been described. These are antibody directed enzyme prodrug therapy (ADEPT) and macromolecule directed enzyme prodrug therapy (MDEPT), where the tumour located enzyme converts a non-toxic prodrug into a cytotoxic drug at tumour sites. The alternative use of tumour located enzymes is to inactivate rescue agents that protect cells from antimetabolite action, and is described as 'Antimetabolite with inactivation of rescue agent at cancer sites' (AMIRACS). The leakiness of tumour blood vessels and poor lymphatic drainage allows enzymes to be targeted to many cancers by attachment to polymeric macromolecules (MDEPT), as well as to antibodies and antibody fragments (ADEPT). To avoid systemic toxicity, enzyme activity in blood and normal tissues must be very low before giving a prodrug or rescue agent. Antibodies directed against the enzyme component of macromolecular conjugates have proved to be very efficient at clearing normal tissues. Human enzymes which are over expressed by cancer cells can be exploited particularly if they require co-factors or co-substrates, either in situ or targeted to extracellular sites. Bacterial enzymes have advantages in specificity but require some form of immunological control in view of their immunogenicity. Prodrugs which generate drugs with very short half lives are desirable, and have been developed, including one which has a differential toxicity between prodrug and the active drug of 1000 to 10,000 fold. The range of antimetabolites available for AMIRACS was initially restricted to inhibitors of dihydrofolate reductase but has been greatly extended by the introduction of inhibitors of other enzymes. The limitations of these systems are discussed.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Improved yield and stability of L49-sFv-beta-lactamase, a single-chain antibody fusion protein for anticancer prodrug activation, by protein engineering

The L49 single-chain Fv fused to beta-lactamase (L49-sFv-bL) combined with the prodrug C-Mel is an effective anticancer agent against tumor cells expressing the p97 antigen. However, large-scale production of L49-sFv-bL from refolded E. coli inclusion bodies has been problematic due to inefficient refolding and instability of the fusion protein. Sequence analysis of the L49-sFv framework regions revealed three residues in the framework regions at positions L2, H82B, and H91, which are not conserved for their position, occurring in <1% of sequences in Fv sequence databases. One further unusual residue, found in <3% of variable sequences, was observed at position H39. Each unusual residue was mutated to a conserved residue for its position and tested for refolding yield from inclusion bodies following expression in E. coli. The three V(H) single mutants showed improvement in the yield of active protein and were combined to form double and triple mutants resulting in a 7-8-fold increased yield compared to the parental protein. In an attempt to further improve yield, the orientation of the triple mutant was reversed to create a bL-L49-sFv fusion protein resulting in a 3-fold increase in expressed inclusion body protein and producing a 20-fold increase in the yield of purified protein compared to the parental protein. The triple mutants in both orientations displayed increased stability in murine plasma and binding affinity was not affected by the introduced mutations. Both triple mutants also displayed potent in vitro cytotoxicity and in vivo antitumor activity against p97 expressing melanoma cells and tumor xenografts, respectively. These results show that a rational protein-engineering approach improved the yield, stability, and refolding characteristics of L49-sFv-bL while maintaining binding affinity and therapeutic efficacy.

Selective activation of anticancer prodrugs by monoclonal antibody-enzyme conjugates

A great deal of interest has surrounded the activities of monoclonal antibodies (mAbs), and mAb-drug, toxin and radionuclide conjugates for the treatment of human cancers. In the last few years, a number of new mAb-based reagents have been clinically approved (Rituxan, Herceptin, and Panorex), and several others are now in advanced clinical trials. Successful therapeutic treatment of solid tumors with drug conjugates of such macromolecules must overcome the barriers to penetration within tumor masses, antigen heterogeneity, conjugated drug potency, and efficient drug release from the mAbs inside tumor cells. An alternative strategy for drug delivery involves a two-step approach to cancer therapy in which mAbs are used to localize enzymes to tumor cell surface antigens. Once the conjugate binds to the cancer cells and clears from the systemic circulation, antitumor prodrugs are administered that are catalytically converted to active drugs by the targeted enzyme. The drugs thus released are capable of penetrating within the tumor mass and eliminating both cells that have and have not bound the mAb-enzyme conjugate. Significant therapeutic effects have been obtained using a broad range of enzymes along with prodrugs that are derived from both approved anticancer drugs and highly potent experimental agents. This review focuses on the activities of several mAb-enzyme/prodrug combinations, with an emphasis on those that have provided mechanistic insight, clinical activity, novel protein constructs, and the potential for reduced immunogenicity.

Synthesis and HPLC analysis of enzymatically cleavable linker consisting of poly(ethylene glycol) and dipeptide for the development of immunoconjugate

A model compound of anti-tumor agent, segment B of duocarmycin derivative DU-86, was conjugated to tumor-specific antibody via a cleavable linker consisting of poly(ethylene glycol) (PEG) and dipeptide, L-alanyl-L-valine (Ala-Val), to confirm the feasibility of the linker for application to immunoconjugate. The release of segment B from the linker was evaluated by HPLC analysis. When segment B was derivatized to have an amino residue and then linked to PEG through a dipeptide, segment B was cleaved at the peptide bond by a particular enzyme, thermolysin (EC 3.4.24.4), but not by plasmin (EC 3.4.2 1.7.), indicating that certain protease specifically expressed at the tumor site would be capable of peptide-specific digestion and release of anti-tumor agent since a thermolysin-like enzyme has been reported to be expressed at many tumor cells. Furthermore, the results showing that cell extract from G361 human melanoma had an ability to digest the linker peptide while the linker was stable in normal human serum suggested the tumor-specific activation of the conjugated agent. Segment B was conjugated via the linker to murine monoclonal antibody KM641 reactive to GD3 ganglioside to form immunoconjugate and the quantitative release of segment B under the treatment with the enzyme was also confirmed. These results indicate the possibility of double targeting based on both the recognition ability of tumor specific antibody and tumor specific activation of the anti-tumor agents to enhance tumor treatment efficacy and to decrease unwanted side effects.

Efficient clearance of poly(ethylene glycol)-modified immunoenzyme with anti-PEG monoclonal antibody for prodrug cancer therapy

The F(ab')(2) fragment of the anti-TAG-72 antibody, B72.3, was covalently linked to Escherichia coli-derived beta-glucuronidase that was modified with methoxypoly(ethylene glycol). The conjugate (B72.3-betaG-PEG) localized to a peak concentration in LS174T xenografts within 48 h after injection, but enzyme activity persisted in plasma such that prodrug administration had to be delayed for at least 4 days to avoid systemic prodrug activation and associated toxicity. Conjugate levels in tumors decreased to 36% of peak levels at this time. Intravenous administration of AGP3, an IgM mAb against methoxypoly(ethylene glycol), accelerated clearance of conjugate from serum and increased the tumor/blood ratio of B72. 3-betaG-PEG from 3.9 to 29.6 without significantly decreasing the accumulation of conjugate in tumors. Treatment of nude mice bearing established human colon adenocarcinoma xenografts with B72. 3-betaG-PEG followed 48 h later with AGP3 and a glucuronide prodrug of p-hydroxyaniline mustard significantly (p< or =0.0005) delayed tumor growth with minimal toxicity compared to therapy with a control conjugate or conventional chemotherapy.

Folate-targeted enzyme prodrug cancer therapy utilizing penicillin-V amidase and a doxorubicin prodrug

In antibody-targeted enzyme prodrug therapy, a monoclonal antibody (mAb) covalently linked to an enzyme is commonly exploited to concentrate the enzyme on the tumor cell surface prior to administration of a relatively nontoxic prodrug. The tumor-localized enzyme then converts the prodrug into a cytotoxic agent, which in turn diffuses into the tumor causing localized cell death. In this paper, we have substituted folic acid for the mAb as a mean of delivering an attached enzyme, penicillin-V amidase (PVA), to folate receptor (FR)-positive tumor cells. The enzyme PVA is capable of converting a doxorubicin-N-p-hydroxyphenoxyacetamide prodrug (DPO) into its potent parent drug, doxorubicin. For PVA targeting, each PVA molecule was covalently labeled with three molecules of folic acid via the formation of amide bonds. In vitro binding assays showed that folate-PVA-125I conjugates bind specifically to KB cells (FR-positive tumor cells) but not to A549 cells (FR-negative tumor cells). Moreover, in a series of in vitro cytotoxicity tests, folate-PVA conjugates were found to kill folate receptor positive but not receptor negative cells, and when bound to FR-positive cells, folate-PVA conjugates rendered the DPO prodrug as toxic as free doxorubicin (IC50, approximately 0.6 microM). Finally, preliminary in vivo plasma clearance studies in normal mice revealed that i.v. administered folate-PVA-125I and PVA-125I are both cleared from the blood within a 24 h time period, removing concern that nonspecifically trapped folate-PVA might activate prodrug in nontargeted tissues. In view of the fact that only a small number of folate-PVA molecules are required to mediate killing of target cells in vitro, these data argue that folate-targeted enzyme prodrug therapy should be considered for tumor eradication in vivo.

Comparison of recombinant and synthetically formed monoclonal antibody-beta-lactamase conjugates for anticancer prodrug activation

Conjugates of the L49 monoclonal antibody (binds to the p97 antigen on melanomas and carcinomas) were formed by attaching Enterobacter cloacae beta-lactamase (bL) to the L49-Fab' fragment using a heterobifunctional cross-linking reagent or by linking the enzyme to L49-sFv using DNA recombinant technology. The conjugates thus formed, L49-Fab'-bL and L49-sFv-bL, were designed to activate cephalosporin containing anticancer prodrugs at the surfaces of antigen positive tumor cells. Results from in vitro experiments using two lung carcinoma cell lines demonstrated that the conjugates were equally active in effecting the release of phenylenediamine mustard from the cephalosporin nitrogen mustard prodrug CCM. While treatment with either of the conjugates combined with the maximum tolerated doses of CCM led to cures of established SN12P renal cell carcinoma tumors in nude mice, only the L49-sFv-bL conjugate maintained its ability to do so at 1/4 the maximum tolerated dose of CCM. L49-sFv-bL was also superior to L49-Fab'-bL in the 1934J renal cell carcinoma tumor model and was shown to be quite active in two in vivo models of human lung carcinoma. These results demonstrate that the recombinant fusion protein leads to more pronounced therapeutic windows than the chemical conjugate and is active in an array of human tumor models.

Developments with targeted enzymes in cancer therapy

Cancer therapy based on the delivery of enzymes to tumour sites has advanced in several directions since antibody-directed enzyme/prodrug therapy was first described. It has been shown that methoxypolyethylene glycol (MPEG) can be used to deliver enzyme to a variety of solid tumours. MPEG-enzyme conjugates show reduced immunogenicity and may allow repeated treatment with enzymes of bacterial origin. Enzyme delivery to tumours by polymers can be used to convert a low toxicity prodrug to a potent cytotoxic agent. An example of such a prodrug is CB1954, which can be activated by a human enzyme in the presence of a cosubstrate. Tumour-located enzymes can also be used in conjunction with a combination of antimetabolites and rescue agents. The rescue agent protects normal tissue but is degraded at cancer sites by the enzyme, thus deprotecting the tumour and allowing prolonged antimetabolite action.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Self-immolative anthracycline prodrugs for suicide gene therapy

Four novel potential prodrugs derived from daunorubicin (8, 10) and doxorubicin (12, 14) were designed and synthesized. They are self-immolative prodrugs for suicide gene therapy activation by the enzyme carboxypeptidase G2 (CPG2) subsequently releasing the corresponding anthracyclines, by a 1,6-elimination mechanism. A mammary carcinoma cell line (MDA MB 361) was engineered to express CPG2 intracellularly (CPG2) or extracellularly, tethered to the outer cell membrane (stCPG2(Q)3). The prodrugs derived from doxorubicin showed prodrug/drug cytotoxicity differentials of 21-fold (compound 12) and 23-fold (compound 14). Prodrug 12 underwent an 11-fold activation when assayed in the cell line expressing externally surface-tethered CPG2.

The antitumour activity of the prodrug N-L-leucyl-doxorubicin and its parent compound doxorubicin in human tumour xenografts

The antitumour activity of the investigational agent N-L-leucyl-doxorubicin (Leu-DOX) was compared with that of doxorubicin (DOX) in human tumour xenografts growing subcutaneously in athymic nude mice. Leu-DOX was developed as a prodrug of DOX, and may be converted into the clinically active parent compound by hydrolytic enzymes present in or on tumour cells. It has been suggested that a better therapeutic index with a reduced cardiac toxicity and higher efficacy might be obtained. Both compounds were administered intravenously weekly for 2 weeks, each at maximum tolerated doses of 8 mg/kg and 28 mg/kg for DOX and Leu-DOX, respectively. The panel of xenografts represented three different tumour types. Leu-DOX showed antitumour activity, defined as tumour growth inhibition > 50% and specific growth delay > 1.0, in 10 of the 16 tumours, including two of five breast, five of seven small cell and three of four non-small cell lung carcinomas. In comparison, DOX was active in one breast, four small cell lung and two lung adenocarcinoma xenografts. In all the DOX sensitive lung tumours, Leu-DOX showed higher efficacy than the parent compound. Based on the results of the present study, and since phase I clinical trials with Leu-DOX have already been performed, phase II clinical evaluation of Leu-DOX in patients with breast and lung cancer is recommended.

Immunologically specific activation of a cephalosporin derivative of mitomycin C by monoclonal antibody beta-lactamase conjugates

The syntheses of two cephalosporin derivatives 2 and 3 of mitomycin C (1) containing 7-phenylacetamido and 7-delta-carboxybutanamido side chains, respectively, are described. These compounds were prepared for evaluation as cephalosporin prodrugs capable of being activated by mAb-beta-lactamase conjugates. In vitro cytotoxicity assays performed on H2987 lung adenocarcinoma and clone 62 melanoma cell lines indicated that compound 2 was comparable in cytotoxicity to the parent drug. In an effort to improve upon the cytotoxic differential of 2, an alternative prodrug 3 containing a polar carboxyl group in the side chain of the cephalosporin moiety was prepared. Compound 3 consistently behaved as a prodrug and was approximately 40- and 10-fold less toxic than 1 toward H2987 and clone 62, respectively. Determination of kinetic constants for hydrolysis by beta-lactamase from Enterobacter cloacae P99 indicated kcat values of 476 +/- 170 and 248 +/- 15.1 s-1 for 2 and 3, respectively. The kcat/Km ratios for 2 and 3 were found to be approximately 9.7 and 2.1 microM/s, respectively. Comparison of these kcat/Km values with those obtained for similar cephalosporin derivatives of other antitumor agents demonstrated that compounds with delta-carboxybutanamido side chains generally have slightly diminished efficiency of enzymatic hydrolysis compared to the corresponding 7-phenylacetamido analog. It was also demonstrated that the less toxic prodrug 3 was activated in an immunologically specific manner by L6-F(ab')-beta-lactamase and 96.5-F(ab')-beta-lactamase conjugates, selective for H2987 and clone 62 cells, respectively.

Antibody-directed enzyme prodrug therapy (ADEPT): a review

Antibody-directed enzyme prodrug therapy (ADEPT) is a therapeutic strategy which aims to improve the selectivity of anticancer drugs. ADEPT is a two-step antibody targeting system that has benefits over a one-step chemo-, toxin- or radioimmunoconjugate. The basic principles of ADEPT are discussed alongside the requirements of the components: antibodies, enzymes and prodrugs. The design and syntheses of prodrugs are detailed particularly prodrug/drug systems of potential clinical use, the rationale behind their design and the in vitro and in vivo results obtained. The main features of ADEPT, such as targeting of cancer cells by the antibody-enzyme conjugates, enzymic activation of the prodrugs, selection of the prodrug/drug and enzyme/prodrug systems are reviewed.

Toward antibody-directed enzyme prodrug therapy with the T268G mutant of human carboxypeptidase A1 and novel in vivo stable prodrugs of methotrexate

Antibody-directed enzyme prodrug therapy (ADEPT) has the potential of greatly enhancing antitumor selectivity of cancer therapy by synthesizing chemotherapeutic agents selectively at tumor sites. This therapy is based upon targeting a prodrug-activating enzyme to a tumor by attaching the enzyme to a tumor-selective antibody and dosing the enzyme-antibody conjugate systemically. After the enzyme-antibody conjugate is localized to the tumor, the prodrug is then also dosed systemically, and the previously targeted enzyme converts it to the active drug selectively at the tumor. Unfortunately, most enzymes capable of this specific, tumor site generation of drugs are foreign to the human body and as such are expected to raise an immune response when injected, which will limit their repeated administration. We reasoned that with the power of crystallography, molecular modeling and site-directed mutagenesis, this problem could be addressed through the development of a human enzyme that is capable of catalyzing a reaction that is otherwise not carried out in the human body. This would then allow use of prodrugs that are otherwise stable in vivo but that are substrates for a tumor-targeted mutant human enzyme. We report here the first test of this concept using the human enzyme carboxypeptidase A1 (hCPA1) and prodrugs of methotrexate (MTX). Based upon a computer model of the human enzyme built from the well known crystal structure of bovine carboxypeptidase A, we have designed and synthesized novel bulky phenylalanine- and tyrosine-based prodrugs of MTX that are metabolically stable in vivo and are not substrates for wild type human carboxypeptidases A. Two of these analogs are MTX-alpha-3-cyclobutylphenylalanine and MTX-alpha-3-cyclopentyltyrosine. Also based upon the computer model, we have designed and produced a mutant of human carboxypeptidase A1, changed at position 268 from the wild type threonine to a glycine (hCPA1-T268G). This novel enzyme is capable of using the in vivo stable prodrugs, which are not substrates for the wild type hCPA1, as efficiently as the wild type hCPA1 uses its best substrates (i.e. MTX-alpha-phenylalanine). Thus, the kcat/Km value for the wild type hCPA1 with MTX-alpha-phenylalanine is 0.44 microM-1 s-1, and kcat/Km values for hCPA1-T268G with MTX-alpha-3-cyclobutylphenylalanine and MTX-alpha-3-cyclopentyltyrosine are 1.8 and 0.16 microM-1 s-1, respectively. The cytotoxic efficiency of hCPA1-268G was tested in an in vitro ADEPT model. For this experiment, hCPA1-T268G was chemically conjugated to ING-1, an antibody that binds to the tumor antigen Ep-Cam, or to Campath-1H, an antibody that binds to the T and B cell antigen CDw52. These conjugates were then incubated with HT-29 human colon adenocarcinoma cells (which express Ep-Cam but not the Campath 1H antigen) followed by incubation of the cells with the in vivo stable prodrugs. The results showed that the targeted ING-1:hCPA1-T268G conjugate produced excellent activation of the MTX prodrugs to kill HT-29 cells as efficiently as MTX itself. By contrast, the enzyme-Campath 1H conjugate was without effect. These data strongly support the feasibility of ADEPT using a mutated human enzyme with a single amino acid change.

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.