Therapeutic effects of monoclonal antibody-beta-lactamase conjugates in combination with a nitrogen mustard anticancer prodrug in models of human renal cell carcinoma

Antibody-Drug Conjugates for the Treatment of Renal Cancer: A Scoping Review on Current Evidence and Clinical Perspectives

Antibody-drug conjugates (ADCs) are complex chemical structures composed of a monoclonal antibody, serving as a link to target cells, which is conjugated with a potent cytotoxic drug (i.e., payload) through a chemical linker. Inspired by Paul Ehrlich's concept of the ideal anticancer drug as a "magic bullet", ADCs are also highly specific anticancer agents, as they have been demonstrated to recognize, bind, and neutralize cancer cells, limiting injuries to normal cells. ADCs are among the newest pharmacologic breakthroughs in treating solid and hematologic malignancies. Indeed, in recent years, various ADCs have been approved by the Food and Drug Administration and European Medicines Agency for the treatment of several cancers, resulting in a "practice-changing" approach. However, despite these successes, no ADC is approved for treating patients affected by renal cell carcinoma (RCC). In the present paper, we thoroughly reviewed the current literature and summarized preclinical studies and clinical trials that evaluated the activity and toxicity profile of ADCs in RCC patients. Moreover, we scrutinized the potential causes that, until now, hampered the therapeutical success of ADCs in those patients. Finally, we discussed novel strategies that would improve the development of ADCs and their efficacy in treating RCC patients.

Three Members of Transmembrane-4-Superfamily, TM4SF1, TM4SF4, and TM4SF5, as Emerging Anticancer Molecular Targets against Cancer Phenotypes and Chemoresistance

There are six members of the transmembrane 4 superfamily (TM4SF) that have similar topology and sequence homology. Physiologically, they regulate tissue differentiation, signal transduction pathways, cellular activation, proliferation, motility, adhesion, and angiogenesis. Accumulating evidence has demonstrated, among six TM4SF members, the regulatory roles of transmembrane 4 L6 domain family members, particularly TM4SF1, TM4SF4, and TM4SF5, in cancer angiogenesis, progression, and chemoresistance. Hence, targeting derailed TM4SF for cancer therapy has become an emerging research area. As compared to others, this review aimed to present a focused insight and update on the biological roles of TM4SF1, TM4SF4, and TM4SF5 in the progression, metastasis, and chemoresistance of various cancers. Additionally, the mechanistic pathways, diagnostic and prognostic values, and the potential and efficacy of current anti-TM4SF antibody treatment were also deciphered. It also recommended the exploration of other interactive molecules to be implicated in cancer progression and chemoresistance, as well as potential therapeutic agents targeting TM4SF as future perspectives. Generally, these three TM4SF members interact with different integrins and receptors to significantly induce intracellular signaling and regulate the proliferation, migration, and invasion of cancer cells. Intriguingly, gene silencing or anti-TM4SF antibody could reverse their regulatory roles deciphered in different preclinical models. They also have prognostic and diagnostic value as their high expression was detected in clinical tissues and cells of various cancers. Hence, TM4SF1, TM4SF4, and TM4SF5 are promising therapeutic targets for different cancer types preclinically and deserve further investigation.

β-Lactams as promising anticancer agents: Molecular hybrids, structure activity relationships and potential targets

β-Lactam, commonly referred as azetidin-2-one, is a multifunctional building block for synthesizing β-amino ketones, γ-amino alcohols, and other compounds. Besides its well known antibiotic activity, this ring system exhibits a wide range of activities, attracting the attention of researchers. However, the structurally diverse β-lactam analogues as anticancer agents and their different molecular targets are poorly discussed. The purpose of this review is 3-fold: (1) to explore the molecular hybridization approach to design β-lactams hybrids as anticancer agents; (2) the structure activity relationship of the most active anticancer β-lactams and (3) to summarize their antitumor mechanisms.

Chimeric peptide containing both B and T cells epitope of tumor-associated antigen L6 enhances anti-tumor effects in HLA-A2 transgenic mice

Synthetic peptides are attractive for cancer immunotherapy because of their safety and flexibility. In this report, we identified a new B cell epitope of tumor-associated antigen L6 (TAL6) that could induce antibody-dependent cellular cytotoxicity (ADCC) in vivo. We incorporated the B cell epitope with a cytotoxic T lymphocyte (CTL) and a helper T (Th) epitope to form a chimeric long peptide. We formulated the chimeric peptide with different adjuvants to immunize HLA-A2 transgenic mice and evaluate their immunogenicity. The chimeric peptide formulated with an emulsion type nanoparticle (PELC) adjuvant and a toll-like receptor 9 agonist (CpG ODN) (PELC/CpG) induced the greatest ADCC and CTL responses. The induced anti-tumor immunity inhibited the growth of TAL6-positive cancer cells. Moreover, we observed that immunization with the chimeric peptide inhibited cancer cell migration in vitro and metastasis in vivo. These data suggest that a chimeric peptide containing both B and T cell epitopes of TAL6 formulated with PELC/CpG adjuvant is feasible for cancer immunotherapy.

TM4SF1 as a prognostic marker of pancreatic ductal adenocarcinoma is involved in migration and invasion of cancer cells

The cell surface protein Transmembrane 4 L6 family member 1 (TM4SF1) has been detected in various tumors, and its expression on tumor cells is implicated in cancer cell metastasis and patient prognosis. The role of TM4SF1 in malignant tumors remains poorly understood, particularly in pancreatic cancer. We performed immunohistochemical staining to analyze the expression of TM4SF1 in resected pancreatic tissues and investigated the correlation between TM4SF1 expression and prognosis. The function of TM4SF1 in the invasion and migration of pancreatic cancer cells was analyzed in vitro using an RNA interference technique. In pancreatic cancer tissues, TM4SF1 expression was detected in cancer cells, and patients with high tumor levels of TM4SF1 showed longer survival times than those with low TM4SF1 levels (P=0.0332). In vitro, reduced TM4SF1 expression enhanced the migration (P<0.05) and invasion (P<0.05) of pancreatic cancer cells partially via decreased E-cadherin expression. TM4SF1 protein levels were also reduced after TGF-β1-induced epithelial-mesenchymal transition (EMT).TM4SF1 expression is associated with better prognosis in pancreatic cancer. Loss of TM4SF1 contributes to the invasion and migration of pancreatic cancer cells.

Design and synthesis of a beta-lactamase activated 5-fluorouracil prodrug

An efficient synthesis of a 5-fluorouracil-cephalosporin prodrug is described for use against colorectal and other cancers in antibody and gene-directed therapies. The compound shows stability in aqueous media until specifically activated by beta-lactamase (betaL). The kinetic parameters of the 5-fluorouracil-cephalosporin conjugate were determined in the presence of Enterobacter cloacae P99 betaL (ECl betaL) revealing a K(m)=95.4 microM and V(max)=3.21 microMol min(-1) mg(-1). The data compare favorably to related systems that have been reported and enable testing of this prodrug against cancer cell lines in vitro and in vivo.

Towards a ligand targeted enzyme prodrug therapy: Single round panning of a β-lactamase scaffold library on human cancer cells

A novel beta-lactamase scaffold library in which the target-binding moiety is built into the enzyme was generated using phage display technology. The binding element is composed of a fully randomized 8 amino acid loop inserted at position between Y34 and K37 on the outer surface of Enterobacter cloacae P99 cephalosporinase (beta-lactamase, E.C. 3.5.2.6) with all library members retaining catalytic activity. The frequency and diversity of amino acids distributions in peptide inserts from library clones were analyzed. The complexity of the randomized loop appears consistent with standards of other types of phage display library systems. The library was panned against SKBR3 human breast cancer cells in 1 round using rolling circle amplification of phage DNA to recover bound phage. Individual beta-lactamase clones, independent of phage, were rapidly assessed for their binding to SKBR3 cells using a simple high throughput screen based on cell-bound beta-lactamase activity. SKBR3 cell-binding beta-lactamase enzymes were also shown to bind specifically using an immunochemical method. Selected beta-lactamase clones were further studied for their protein expression, enzyme activity and binding to nontumor cell-lines. Overall, the approach outlined here offers the opportunity of rapidly selecting targeted beta-lactamase ligands that may have a potential for their use in enzyme prodrug therapy with cephalosporin-based prodrugs. It is expected that a similar approach will be useful in developing tumor-targeting molecules of several other enzyme candidates of cancer prodrug therapy.

Alkylating benzamides with melanoma cytotoxicity: experimental chemotherapy in a mouse melanoma model

The in-vivo antineoplastic potential of the alkylating N-(2-dialkylaminoethyl)benzamides BZA1 and BZA2, novel melanoma targeted anticancer drugs, was evaluated in a mouse melanoma model with nude mice bearing subcutaneous SkMel28, B16 or WM266-4. The maximal tolerated dose (MTD) for the intraperitoneal application of both agents was found to be 24 mg/kg. Treatment was initiated with an intraperitoneal injection of 8 mg/kg of BZA1 or BZA2 on days 0, 2 and 4 in the case of B16 melanoma on days 0, 1 and 2 after the onset of the experiment, when the mean tumor diameter ranged within 4-6 mm. The experiment was terminated when the mean tumor diameter in the control group had reached a value of 12 mm. Tumor growth delay of these agents was compared with dacarbazine (3x250 mg/kg), chlorambucil (3x5 mg/kg) and an untreated control group. Significant tumor growth delay was observed under BZA1, BZA2 and dacarbazine treatment compared with the untreated control group in all three evaluated melanomas with insignificant differences among BZA1, BZA2 and dacarbazine. The insignificant effect of chlorambucil and the strong improvement on growth delay achieved with BZA1 and BZA2 demonstrated melanoma targeting characteristics of the N-(2-dialkylaminoethyl)benzamide structure element. Dacarbazine was more effective in the in-vivo antineoplastic assay compared with the in-vitro cytotoxicity studies, probably due to hepatic bioactivation. Similar side effect intensity of BZA2 and dacarbazine was observed, whereas BZA1 was more toxic. BZA2 might represent an alternative antimelanoma drug, especially in patients not responding to dacarbazine.

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.

Alkylating benzamides with melanoma cytotoxicity: role of melanin, tyrosinase, intracellular pH and DNA interaction

N-(2-Dialkylaminoethyl)benzamides have been shown to selectively accumulate in melanoma metastases with high uptake capacity. Therefore, this class of compound has previously been evaluated as a transporter for cytostatic drugs. It has been demonstrated that this significant targeting effect improves the cytotoxicity against melanoma cells. Although these agents are not accumulated by non-melanoma cells, they have been found to be toxic. In order to identify mechanistic reasons for this effect, we investigated the DNA and melanin binding affinities of a selection of four benzamide-drug conjugates, together with their parental cytostatics. An investigation of the influence of the melanin content on the cytotoxicity of these substances in B16 melanoma and Morris hepatoma (MH3924A) cells was performed, together with their influence on melanosomal pH and tyrosinase activity. The suppression of melanin formation with phenylisothiourea and the saturation of melanin binding sites with chloroquine were also investigated. These experiments demonstrated high DNA binding and low melanin affinity, in accordance with the toxicity against tumour cells. Melanin has a concentration-dependent scavenging effect, thereby reducing cytotoxicity. These compounds lead to an increase in the acidic pH of melanosomes, resulting in an increase in tyrosinase activity. The consequence of this reaction chain is an amplification of the scavenging effect for the benzamide-drug conjugates. These effects may be considered as limiting factors for the targeting characteristics of this class of compound, necessitating further modifications to the carrier system.

CD13 (aminopeptidase N) can associate with tumor-associated antigen L6 and enhance the motility of human lung cancer cells

Cancer metastasis is a multiple-step process that involves the regulated interaction of diverse cellular proteins. We recently reported that the expression of tumor-associated antigen L6 (TAL6) promoted the invasiveness of lung cancer cells and was inversely correlated with disease-free survival of squamous lung carcinoma patients. We now report that CD13 (aminopeptidase N) can associate with TAL6 and can enhance cancer cell migration. CD13 was shown by coimmunoprecipitation to associate in vitro with TAL6 on several cancer cell lines and to associate in vivo by antibody-mediated copatching immunofluorescence. CD13 was selectively expressed on highly invasive CL1-5 lung cancer cells as compared to poorly invasive CL1-0 lung cancer cells. The role of CD13 aminopeptidase activity in regulating cell motility was investigated with chemical inhibitors, specific antibodies and a catalytically inactive CD13 protein. Inhibition of CD13 aminopeptidase activity by nontoxic concentrations of leuhistin modestly decreased the migration of CL1-5 cells. In contrast, binding of CD13 by specific antibodies significantly reduced both the migration and the invasion of CL1-5 cells. Poorly invasive CL1-0 cells that stably expressed CD13 displayed significantly (p < or = 0.0005) enhanced cell migration (300% of control). Expression of an enzymatically inactive CD13 mutant on CL1-0 cells also significantly (p < or = 0.0005) enhanced cell migration (200% of control). Our results show that TAL6 and CD13 can form a complex on lung cancer cells, that these molecules can modulate cell migration and invasion and that the influence of CD13 on cell motility did not strictly depend on its aminopeptidase activity.

Toward the Development of a Cephalosporin-Based Dual-Release Prodrug for Use in ADEPT

In previous work we have shown that a cephalosporin structure bearing an S-aminosulfenimine at the 7-position behaved as a beta-lactamase-dependent dual-release prodrug. Scission of the beta-lactam ring of such a structure led to the rapid loss of the sulfur-attached side chain moiety via an intramolecular displacement, while the 3'-group was lost via the well-established elimination process at that position. In the present work we report on an evaluation of the scope and limitations of exploiting the S-aminosulfenimine functionality to generate a cephalosporin-based prodrug incorporating two biologically active components. Starting from 7-ACA, a viable synthetic cycle was put in place that avoided formation of the Delta(2) isomer throughout and that allowed incorporation of aminoglutethimide at the 3'-position and of a tosyl S-aminosulfenimine at the 7-position. The direct incorporation of a biologically active sulfonamide (ethoxzolamide) or a sulfamate (coumate) at this latter position was not achieved as a result of the difficulty of generating the corresponding sulfur diimides. An indirect route for the formation of an S-aminosulfenimine was put in place, as was a general method of alkylation (Mitsunobu reaction) of the tosyl S-aminosulfenimine following its incorporation.

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.

Alkylating benzamides with melanoma cytotoxicity

Radioiodinated N-(2-(diethylamino)ethyl)benzamides have recently been discovered as selective agents for melanotic melanoma and are used for scintigraphic imaging in nuclear medicine. Owing to the high binding capacity, benzamide derivatives conjugated with alkylating cytostatics were synthesized and tested for their potential for targeted drug delivery. Conjugates of chlorambucil with procainamide (1), diethylaminoethylamine (2) and 2-pyrrolidin-1-yl-ethylamine (3), as well as 4-(bis(2-chloroethyl)amino)- (6,7) and 4-(N,N-diethyltriazeno)-substituted (8-10) benzamides, were synthesized. Cell uptake studies with B16 melanoma cells revealed high uptake of radioiodinated 1 and 2, while radiolabelled chlorambucil was found to lack this characteristic. These results were confirmed by biodistribution studies in a mouse melanoma model. Viability measurements revealed that all chlorambucil-benzamide derivatives showed higher toxicity against B16 melanoma and SK-MEL-28 cells than did the parent chlorambucil itself, and that the triazene derivatives were more potent than dacarbazine, which is currently used as a standard cytostatic drug in melanoma therapy. Of all the compounds tested in this series, the triazenes 9 and 10 showed the most promising targeting effect. The toxicity of these compounds against hepatoma cells (MH3924A) and, to a lesser extent, against mouse fibroblast (NIH 3T3) and cervix carcinoma (HeLa) cells was also enhanced, but they were not as toxic as dacarbazine (HeLa). These findings support the concept of a selective, benzamide-mediated in vivo delivery of cytostatics in melanoma cells, leading to enhanced efficacy.

Penicillins as beta-lactamase-dependent prodrugs: enabling role of a vinyl ester exocyclic to the lactam ring

Incorporation of a vinyl ester exocyclic to the beta-lactam ring of a penicillin nucleus enables this to act as a beta-lactamase-dependent prodrug - rapid release of the (unactivated) alkoxy component of the vinyl ester is triggered by enzyme-catalysed hydrolysis of the beta-lactam ring, whilst buffer-catalysed hydrolysis of the structure at neutral pH is particularly slow.

Synthesis of beta-lactamase activated nitric oxide donors

In order to achieve site specific delivery of NO, we designed conjugates of cephalosporin with NO donors. NO donors such as cupferron and SIN-1 were evaluated as potential choices for conjugates. Cephalosporin conjugated with SIN-1 demonstrated promising beta-lactamase dependent NO releasing ability.

Design, synthesis, and biological evaluation of a cephalosporin-monohydroguaiaretic acid prodrug activated by a monoclonal antibody-beta-lactamase conjugate

A novel cephalosporin derivative of monohydroguaiaretic acid (cephem-M(3)N, 7) was synthesized and found to possess anticancer activity against human leukemia (K562), breast carcinoma (MCF7), human lung cancer (A549), human colon cancer (Colo205) and pancreatic cancer cells (Capan2 and MiaPaCa2). A tumor targeting fusion protein (dsFv3-beta-lactamase) was also used in conjunction with cephem-based M(3)N 7 and its potency toward K562, MCF7, A549, Colo205, Capan2, and MiaPaCa2 was found to approach that of the free M(3)N (4). In the presence of dsFv3-beta-lactamase, tumor cells were found to be much more susceptible to conjugate 7 than normal human embryonic lung (HEL) cells and normal fibroblasts (Hef522). These notions provide a new approach to the use of nordihydroguaiaretic acid (NDGA) and its derivatives for antitumor therapy.

Prodrug and antedrug: two diametrical approaches in designing safer drugs

The prodrug and antedrug concepts, which were developed to overcome the physical and pharmacological shortcomings of various therapeutic classes of agents, employ diametrically different metabolic transformations. The prodrug undergoes a predictable metabolic activation prior to exhibiting its pharmacological effects in a target tissue while the antedrug undergoes metabolic deactivation in the systemic circulation upon leaving a target tissue. An increased therapeutic index is the aspiration for both approaches in designing as well as evaluation criteria. The recent research endeavors of prodrugs include the gene-directed and antibody-directed enzymatic activation of a molecule in a targeted tissue, organ specific delivery, improved bioavailabilities of nucleosides and cellular penetration of nucleotides. As for antedrugs, emphasis in research has been based upon the design and synthesis of systemically inactive molecule by incorporating a metabolically labile functional group into an active molecule.

Efficient tumor targeting by single-domain antibody fragments of camels

The variable domain of functional heavy chain antibodies (VHH) devoid of light chains, found in camels, constitute the smallest intact antigen-binding domain fragment. Two camel single-domain fragments, cAb-Lys2 and cAb-Lys3, recognizing an overlapping epitope of lysozyme with a dissociation constant of 2 nM and 65 nM, respectively, and a bivalent cAb-Lys3 were investigated for their ability to target transgenic tumors expressing lysozyme on their membrane. Biodistribution studies revealed that these non-immunogenic monomeric and bivalent camel single-domain antigen binders specifically target lysozyme-expressing tumors and metastatic lesions. The excess of antibody is rapidly eliminated from the blood circulation and no cAb retention was observed in normal organs. The tumor to organ cAb-ratios at 2 and 8 hr were in the (2.1-10.8):1 and (6.2-23.7):1 range, respectively. The degree and specificity of tumor retention is independent of the affinity of the recombinant camel single-domain fragments for their antigen and from their univalent monomeric (15 kDa) or bivalent format (33 kDa). This study demonstrates the successful and specific in vivo targeting of tumors by camel single-domain fragments. It may open perspectives for their future use as tumor-targeting vehicle, due to their small size, soluble behaviour and because they are non-immunogenic and interact with epitopes that are less antigenic for conventional antibodies.

Design and synthesis of a cephalosporin-retinoic acid prodrug activated by a monoclonal antibody-beta-lactamase conjugate

Two novel series of all-trans-beta-retinoic acid derivatives were synthesized and found to possess anticancer activity. The first series, cephalosporin 3'-retinoic esters 6 and 7 were, respectively, obtained by the condensation of all-trans-beta-retinoic acid (2) with cephalosporins 4 and 5. The second series, 7-(retinamido)cephalosporins 11 and 12, were synthesized, respectively, by the condensation of 2 with cephalosporins 9 and 10. These four heretofore undescribed compounds 6, 7, 11, and 12 showed inhibitory activity against murine leukemias (L1210 and P388), sarcoma 180, breast carcinoma (MCF7), and human T-lymphocytes (Molt4/C8 and CEM/0). They also inhibited squamous metaplasia and keratinization in tracheal organ cultures derived from vitamin-A-deficient hamsters. Moreover, cephalosporin 3'-retinoic ester 7 exhibited enhanced activity against keratinization with ED(50)=3.91 x 10(-11) M in the presence of a beta-lactamase from Staphylococcus aureus 95. A tumor targeting fusion protein (dsFv3-beta-lactamase) was also used in conjunction with cephem-based retinoid 7 and the potency of 7 toward L1210, P388, and MCF7 was found to approach that of the free retinoic acid (2). In the presence of dsFv3-beta-lactamase, tumor cells were found to be much more susceptible to retinoid 7 than normal human embryonic lung cells. These notions provide a new approach to the use of beta-retinoic acid for antitumor therapy.

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.

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.

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.

Extending the beta-Lactamase-Dependent Prodrug Armory: S-Aminosulfeniminocephalosporins as Dual-Release Prodrugs

Cephalosporins bearing an S-aminosulfenimine (R'(R' ')NSN=) side chain at the 7-position are prototypic examples of a novel class of beta-lactamase-dependent prodrug. Enzyme-catalyzed hydrolysis of the beta-lactam ring in these structures triggers release of both the 3'-acetoxy group and the side chain sulfur-attached S-amino moiety as R'(R' ')NH. This reactivity pattern should allow site-specific corelease of two distinct drug components from a cephalosporin, thereby providing a singular enhancement to the capacity of a cephalosporin as a prodrug nucleus; a key advantage of a dual-release prodrug is the potential to establish synergy between the coreleased structures. Areas for exploitation of this new structure type are antibody-directed enzyme prodrug therapy (ADEPT), which is a key emerging anticancer therapy, and the further development of site-specific-release prodrugs to combat the problem of beta-lactamase-based resistance to antibiotics.