Construction, expression, and activities of L49-sFv-beta-lactamase, a single-chain antibody fusion protein for anticancer prodrug activation

Advances in epidermal growth factor receptor specific immunotherapy: lessons to be learned from armed antibodies

The epidermal growth factor receptor (EGFR) has been recognized as an important therapeutic target in oncology. It is commonly overexpressed in a variety of solid tumors and is critically involved in cell survival, proliferation, metastasis, and angiogenesis. This multi-dimensional role of EGFR in the progression and aggressiveness of cancer, has evolved from conventional to more targeted therapeutic approaches. With the advent of hybridoma technology and phage display techniques, the first anti-EGFR monoclonal antibodies (mAbs) (Cetuximab and Panitumumab) were developed. Due to major limitations including host immune reactions and poor tumor penetration, these antibodies were modified and used as guiding mechanisms for the specific delivery of readily available chemotherapeutic agents or plants/bacterial toxins, giving rise to antibody-drug conjugates (ADCs) and immunotoxins (ITs), respectively. Continued refinement of ITs led to deimmunization strategies based on depletion of B and T-cell epitopes or substitution of non-human toxins leading to a growing repertoire of human enzymes capable of inducing cell death. Similarly, the modification of classical ADCs has resulted in the first, fully recombinant versions. In this review, we discuss significant advancements in EGFR-targeting immunoconjugates, including ITs and recombinant photoactivable ADCs, which serve as a blueprint for further developments in the evolving domain of cancer immunotherapy.

Antibody-Based Immunotherapy: Alternative Approaches for the Treatment of Metastatic Melanoma

Melanoma is the least common form of skin cancer and is associated with the highest mortality. Where melanoma is mostly unresponsive to conventional therapies (e.g., chemotherapy), BRAF inhibitor treatment has shown improved therapeutic outcomes. Photodynamic therapy (PDT) relies on a light-activated compound to produce death-inducing amounts of reactive oxygen species (ROS). Their capacity to selectively accumulate in tumor cells has been confirmed in melanoma treatment with some encouraging results. However, this treatment approach has not reached clinical fruition for melanoma due to major limitations associated with the development of resistance and subsequent side effects. These adverse effects might be bypassed by immunotherapy in the form of antibody–drug conjugates (ADCs) relying on the ability of monoclonal antibodies (mAbs) to target specific tumor-associated antigens (TAAs) and to be used as carriers to specifically deliver cytotoxic warheads into corresponding tumor cells. Of late, the continued refinement of ADC therapeutic efficacy has given rise to photoimmunotherapy (PIT) (a light-sensitive compound conjugated to mAbs), which by virtue of requiring light activation only exerts its toxic effect on light-irradiated cells. As such, this review aims to highlight the potential clinical benefits of various armed antibody-based immunotherapies, including PDT, as alternative approaches for the treatment of metastatic melanoma.

Antibody-enzyme fusion proteins for cancer therapy

Advances in biomolecular technology have allowed the development of genetically fused antibody-enzymes. Antibody-enzyme fusion proteins have been used to target tumors for cancer therapy in two ways. In one system, an antibody-enzyme is pretargeted to the tumor followed by administration of an inactive prodrug that is converted to its active form by the pretargeted enzyme. This system has been described as antibody-directed enzyme prodrug therapy. The other system uses antibody-enzyme fusion proteins as direct therapeutics, where the enzyme is toxic in its own right. The key feature in this approach is that the antibody is used to internalize the toxic enzyme into the tumor cell, which activates cell-death processes. This antibody-enzyme system has been largely applied to deliver ribonucleases. This article addresses these two antibody-enzyme targeting strategies for cancer therapy from concept to (pre)clinical trials.

Phage-displayed combinatorial peptide libraries in fusion to beta-lactamase as reporter for an accelerated clone screening: Potential uses of selected enzyme-linked affinity reagents in downstream applications

Phage-display selection of combinatorial libraries is a powerful technique for identifying binding ligands against desired targets. Evaluation of target binding capacity of multiple clones recovered from phage display selection to a specific target is laborious, time-consuming, and a rate-limiting step. We constructed phage-display combinatorial peptide libraries in fusion with a beta-lactamase enzyme, which acts as a reporter. Linear dodecapeptide and cysteine-constrained decapeptide libraries were created at the amino-terminus of the Enterobacter cloacae P99 cephalosporinase molecule (P99 beta-lactamase). The overall and positional diversity of amino acids in both libraries was similar to other phage-display systems. The libraries were selected against the extracellular domain of ErbB2 receptor (ErbB2(ECD)). The target-selected clones were already conjugated to an enzyme reporter, therefore, did not require subcloning or any other post-panning modifications. We used beta-lactamase enzyme activity-based assays for sample normalizations and clone binding evaluation. Clones were identified that bound to purified ErbB2(ECD) and ErbB2-overexpressing cell-lines. The peptide sequences of the selected binding clones shared significant motifs with several rationally designed peptide mimetics and phage-display derived peptides that have been reported to bind ErbB2(ECD). beta-Lactamase fusion to peptides saved time and resources otherwise required by the phage-ELISA of a typical phage display screening protocol. The beta-lactamase enzyme assay protocols is a one-step process that does not require secondary proteins, several steps of lengthy incubations, or washings and can be finished in a few minutes instead of hours. The clone screening protocol can be adopted for a high throughput platform. Target-specific beta-lactamase-linked affinity reagents selected by this procedure can be produced in bulk, purified, and used, without any modification, for a variety of downstream applications, including targeted prodrug therapy.

Cancer cell-specific internalizing ligands from phage displayed beta-lactamase-peptide fusion libraries

The present study was focused on identifying cancer cell-specific internalizing ligands using a new kind of phage display library in which the linear or cysteine-constrained random peptides were at amino-terminus fusion to catalytically active P99 beta-lactamase molecules. The size and quality of libraries were comparable to other reported phage display systems. Several cancer cell-specific binding and internalizing beta-lactamase-peptide fusion ligands were isolated by selecting these libraries on the live BT-474 human breast cancer cells. The identified ligands shared several significant motifs, which showed their selectivity and possible binding to some common cancer cell targets. The beta-lactamase fusion made the whole process of clone screening efficient and simple. The ligands selected from such libraries do not require peptide synthesis and modifications, and can be used directly for applications that require ligand tracking. In addition, the selected beta-lactamase-peptide ligands have a potential for their direct use in targeted enzyme prodrug therapy. The cancer-specific peptides can also be adopted for other kinds of targeted delivery protocols requiring cell-specific affinity reagents. This is first report on the selection of cell-internalized enzyme conjugates using phage display technology, which opens the possibility for new fusion libraries with other relevant enzymes.

Developing bifunctional beta-lactamase molecules with built-in target-recognizing module for prodrug therapy: identification of Enterobacter Cloacae P99 cephalosporinase loops suitable for randomization and phage-display selection

This study was focused on developing catalytically active beta-lactamase enzyme molecules that have target-recognizing sites built within their scaffold. Using phage-display approach, nine libraries were constructed by inserting the randomized linear or cysteine-constrained heptapeptides in the five different loops on the outer surface of P99 beta-lactamase molecule. The pIII signal peptide of Sec-pathway was employed for a periplasmic translocation of the beta-lactamase fusion protein, which we found more efficient than the DsbA signal peptide of SRP-pathway. The randomized heptapeptide loops replaced native amino acids between positions (34)Y-(37)K, (238)M-(246)A, (275)N-(280)A, (305)A-(311)S, or (329)I-(334)I of the P99 beta-lactamase molecules for generating the loop-1 to -5 libraries, respectively. The diversity of each loop library was judged by counting the primary and beta-lactamase-active clones. The linear peptide inserts in the loop-2 library showed the maximum number of the beta-lactamase-active clones, followed by the loop-5, loop-3, and loop-4. The insertion of the cysteine-constrained loops exhibited a dramatic loss of the enzyme-active beta-lactamase clones. The complexity of the loop-2 linear library, as determined by the frequency and diversity of amino acid distributions in the randomized region, appears consistent with the standards of other types of phage display library systems. The selection of the loop-2 linear library on streptavidin protein as a test target identified several beta-lactamase clones that specifically bound to streptavidin. In conclusion, this study identified the suitability of the loop-2 of P99 beta-lactamase for constructing a phage-display library of the beta-lactamase enzyme-active molecules that can be selected against a target. This is an enabling step in our long-term goal of developing bifunctional beta-lactamase molecules against cancer-specific targets for enzyme prodrug therapy of cancer.

Potent cytotoxicity of an auristatin-containing antibody-drug conjugate targeting melanoma cells expressing melanotransferrin/p97

Identifying factors that determine the sensitivity or resistance of cancer cells to cytotoxicity by antibody-drug conjugates is essential in the development of such conjugates for therapy. Here the monoclonal antibody L49 is used to target melanotransferrin, a glycosylphosphatidylinositol-anchored glycoprotein first identified as p97, a cell-surface marker in melanomas. L49 was conjugated via a proteolytically cleavable valine-citrulline linker to the antimitotic drug, monomethylauristatin F (vcMMAF). Effective drug release from L49-vcMMAF likely requires cellular proteases most commonly located in endosomes and lysosomes. Melanoma cell lines with the highest surface p97 expression (80,000-280,000 sites per cell) were sensitive to L49-vcMMAF whereas most other cancer cell lines with lower p97 expression were resistant, as were normal cells with low copy numbers (< or = 20,000 sites per cell). Cell line sensitivity to L49-vcMMAF was found by immunofluorescence microscopy to correlate with intracellular fate of the conjugate. Specifically, L49-vcMMAF colocalized with the lysosomal marker CD107a within sensitive cell lines such as SK-MEL-5 and A2058. In contrast, in resistant cells expressing lower p97 levels (H3677; 72,000 sites per cell), L49-vcMMAF colocalized with caveolin-1, a protein prominent in caveolae, but not with CD107a. Thus, for antibody-drug conjugates targeting p97, antigen level and trafficking to the lysosomes are important factors for achieving robust in vitro cytotoxicity against cancer cells. Immunohistochemical analysis with L49 revealed that 62% of metastatic melanoma tumors had strong staining for p97. Overexpression of p97 in melanoma as compared with normal tissue, in conjunction with the greater sensitivity of tumor cells to L49-vcMMAF, supports further evaluation of antibody-drug conjugates for targeting p97-overexpressing tumors.

A beta-lactamase with reduced immunogenicity for the targeted delivery of chemotherapeutics using antibody-directed enzyme prodrug therapy

Antibody-directed enzyme prodrug therapy (ADEPT) delivers chemotherapeutic agents in high concentration to tumor tissue while minimizing systemic drug exposure. beta-Lactamases are particularly useful enzymes for ADEPT systems due to their unique substrate specificity that allows the activation of a variety of lactam-based prodrugs with minimal interference from mammalian enzymes. We evaluated the amino acid sequence of beta-lactamase from Enterobacter cloacae for the presence of human T-cell epitopes using a cell-based proliferation assay using samples from 65 community donors. We observed a low background response that is consistent with a lack of preexposure to this enzyme. beta-Lactamase was found to contain four CD4+ T-cell epitopes. For two of these epitopes, we identified single amino acid changes that result in significantly reduced proliferative responses while retaining stability and activity of the enzyme. The beta-lactamase variant containing both changes induces significantly less proliferation in human and mouse cell assays, and 5-fold lower levels of IgG1 in mice were observed after repeat administration of beta-lactamase variant with adjuvant. The beta-lactamase variant should be very suitable for the construction of ADEPT fusion proteins, as it combines high activity toward lactam prodrugs, high plasma stability, a monomeric architecture, and a relatively low risk of eliciting an immune response in patients.

Construction and optimization of a CC49-based scFv-beta-lactamase fusion protein for ADEPT

CC49 is a clinically validated antibody with specificity for TAG-72, a carbohydrate epitope that is over-expressed and exposed on a large fraction of solid malignancies. We constructed a single chain fragment (scFv) based on CC49 and fused it to beta-lactamase. The first generation fusion protein, TAB2.4, was expressed at low levels in Escherichia coli and significant degradation was observed during production. We optimized the scFv domain of TAB2.4 by Combinatorial Consensus Mutagenesis (CCM). An improved variant TAB2.5 was identified that resulted in an almost 4-fold improved expression and 2.5 degrees higher thermostability relative to its parent molecule. Soluble TAB2.5 can be manufactured in low-density E.coli cultures at 120 mg/l. Our studies suggest that CCM is a rapid and efficient method to generate antibody fragments with improved stability and expression. The fusion protein TAB2.5 can be used for antibody directed enzyme prodrug therapy (ADEPT).

Reduction-alkylation strategies for the modification of specific monoclonal antibody disulfides

Site-specific conjugation of small molecules and enzymes to monoclonal antibodies has broad utility in the formation of conjugates for therapeutic, diagnostic, or structural applications. Precise control over the location of conjugation would yield highly homogeneous materials that could have improved biological properties. We describe for the first time chemical reduction and oxidation methods that lead to preferential cleavage of particular monoclonal antibody interchain disulfides using the anti-CD30 IgG1 monoclonal antibody cAC10. Alkylation of the resulting cAC10 cysteine thiols with the potent antimitotic agent monomethyl auristatin E (MMAE) enabled the assignment of drug conjugation location by purification with hydrophobic interaction chromatography followed by analysis using reversed-phase HPLC and capillary electrophoresis. These analytical methods demonstrated that treating cAC10 with reducing agents such as DTT caused preferential reduction of heavy-light chain disulfides, while reoxidation of fully reduced cAC10 interchain disulfides caused preferential reformation of heavy-light chain disulfides. Following MMAE conjugation, the resulting conjugates had isomeric homogeneity as high as 60-90%, allowing for control of the distribution of molecular species. The resulting conjugates are highly active both in vitro and in vivo and are well tolerated at efficacious doses.

Antibody-directed enzyme prodrug therapy (ADEPT) for cancer

Antibody-directed enzyme prodrug therapy (ADEPT) aims to restrict the cytotoxic action to tumour sites. The obstacles to achieve this were recognised at the outset, but time and experience have given these better definition. The development of fusion proteins has provided the means of making consistent antibody-enzyme constructs on an adequate scale, and glycosylation has provided the means to control the clearance of enzyme from non-tumour sites. Human enzymes have yet to be tested in a clinical setting, and there are pointers indicating that the immunological response to foreign enzymes can be overcome. The relatively small number of purpose-designed prodrugs tested so far leaves this an area ripe for further development. The ongoing iterative process between preclinical and clinical studies is critical to achieving the objective.

Arming antibodies: prospects and challenges for immunoconjugates

Immunoconjugates--monoclonal antibodies (mAbs) coupled to highly toxic agents, including radioisotopes and toxic drugs (ineffective when administered systemically alone)--are becoming a significant component of anticancer treatments. By combining the exquisite targeting specificity of mAbs with the enhanced tumor-killing power of toxic effector molecules, immunoconjugates permit sensitive discrimination between target and normal tissue, resulting in fewer toxic side effects than most conventional chemotherapeutic drugs. Two radioimmunoconjugates, ibritumomab tiuxetan (Zevalin) and tositumomab-131I (Bexxar), and one drug conjugate, gemtuzumab ozogamicin (Mylotarg), are now on the market. For the next generation of immunoconjugates, advances in protein engineering will permit greater control of mAb targeting, clearance and pharmacokinetics, resulting in significantly improved delivery to tumors of radioisotopes and potent anticancer drugs. Pre-targeting strategies, which separate the two functions of antibody-based localization and delivery or generation of the toxic agent into two steps, also promise to afford superior tumor targeting and therapeutic efficacy. Several challenges in optimizing immunoconjugates remain, however, including poor intratumoral mAb uptake, normal tissue conjugate exposure and issues surrounding drug potency and conditional release from mAb carriers. Nonetheless, highly promising results from preclinical models will continue to drive the clinical development of this therapeutic class.

Sustained Tumor Regression of Human Colorectal Cancer Xenografts Using a Multifunctional Mannosylated Fusion Protein in Antibody-Directed Enzyme Prodrug Therapy

Purpose: Antibody-directed enzyme prodrug therapy (ADEPT) requires highly selective antibody-mediated delivery of enzyme to tumor. MFE-CP, a multifunctional genetic fusion protein of antibody and enzyme, was designed to achieve this by two mechanisms. First by using a high affinity and high specificity single chain Fv antibody directed to carcinoembryonic antigen. Second by rapid removal of antibody-enzyme from normal tissues by virtue of post-translational mannosylation. The purpose of this paper is to investigate these dual functions in an animal model of pharmacokinetics, pharmacodynamics, toxicity, and efficacy.

Experimental Design: MFE-CP was expressed in the yeast Pichia pastoris and purified via an engineered hexahistidine tag. Biodistribution and therapeutic effect of a single ADEPT cycle (1,000 units/kg MFE-CP followed by 70 mg/kg ZD2767P prodrug at 6, 7, and 8 hours) and multiple ADEPT cycles (9-10 cycles within 21-24 days) was studied in established human colon carcinoma xenografts, LS174T, and SW1222.

Results: Selective localization of functional enzyme in tumors and rapid clearance from plasma was observed within 6 hours, resulting in tumor to plasma ratios of 1,400:1 and 339:1, respectively for the LS174T and SW1222 models. A single ADEPT cycle produced reproducible tumor growth delay in both models. Multiple ADEPT cycles significantly enhanced the therapeutic effect of a single cycle in the LS174T xenografts (P = 0.001) and produced regressions in the SW1222 xenografts (P = 0.0001), with minimal toxicity.

Conclusions: MFE-CP fusion protein, in combination with ZD2767P, provides a new and successful ADEPT system, which offers the potential for multiple cycles and antitumor efficacy. These results provide a basis for the next stage in clinical development of ADEPT.

Cephalosporin prodrugs of paclitaxel for immunologically specific activation by L-49-sFv-beta-lactamase fusion protein

Paclitaxel conjugates of 7-phenylacetamidocephalosporanic acid were prepared as prodrugs for site specific activation by targeted beta-lactamase. Immunologically specific activation of the prodrug 5 containing 3,3-dimethyl-4-amino-butyric acid as linker in combination with the fusion protein L-49-sFv-beta-lactamase was demonstrated in vitro on 3677 melanoma cells.

Intracellular accumulation and mechanism of action of doxorubicin in a spatio-temporal tumor model

A spatio-temporal model of tumor response to sequestered, intracellular doxorubicin is presented and simulated. An important feature of the model is the characterization of different mechanisms by which doxorubicin initiates the cell death cascade. The model predicts that the long-term response of the tumor to repeated rounds of therapy is very sensitive to changes in the threshold level of doxorubicin required to initiate apoptosis at the maximum rate. In fact, perturbations of this parameter mediate the difference between effective tumor regression and minimal growth delays. The model is also used to investigate which parameters are most influential in rendering the tumor drug resistant. Sensitivity analysis shows that decreasing cellular permeability, as opposed to decreasing sequestration rate or increasing cellular efflux, is the most effective way for tumor cells to overcome the growth control afforded by successive rounds of treatment.

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.

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.

New approaches to antibody therapy

Antibody-based therapy of human cancers has led to several remarkable outcomes, particularly in the therapy of breast cancer and lymphoma. Many solid tumors have proven less responsive, due in part to difficulties in the tumor-selective delivery of antibodies and potential cytolytic effectors. However, antibodies that directly perturb signaling mechanisms in cells derived from epithelial malignancies have shown benefit; examples include antibodies directed against the extracellular domains of HER2/neu and epidermal growth factor receptor. A long-term goal of immunotherapy has been to induce anti-tumor inflammatory responses that can directly cause tumor regression or induce adaptive responses against tumor-related antigens. This review focuses on the use of antibodies to provide a means for initiating anti-tumor immune responses, and on the use of antibodies as delivery vehicles of radionuclides.

A mathematical model to study the effects of drug resistance and vasculature on the response of solid tumors to chemotherapy

A mathematical model is developed that describes the reduction in volume of a vascular tumor in response to specific chemotherapeutic administration strategies. The model consists of a system of partial differential equations governing intratumoral drug concentration and cancer cell density. In the model the tumor is treated as a continuum of two types of cells which differ in their proliferation rates and their responses to the chemotherapeutic agent. The balance between cell proliferation and death within the tumor generates a velocity field which drives expansion or regression of the spheroid. Insight into the tumor's response to therapy is gained by applying a combination of analytical and numerical techniques to the model equations.

Advances in animal cell recombinant protein production: GS-NS0 expression system

The production of recombinant proteins using mammalian cell expression systems is of growing importance within biotechnology, largely due to the ability of specific mammalian cells to carry out post-translational modifications of the correct fidelity. The Glutamine Synthetase-NS0 system is now one such industrially important expression system.Glutamine synthetase catalyses the formation ofglutamine from glutamate and ammonia. NS0 cellscontain extremely low levels of endogenous glutaminesynthetase activity, therefore exogenous glutaminesynthetase can be used efficiently as a selectablemarker to identify successful transfectants in theabsence of glutamine in the media. In addition, theinclusion of methionine sulphoximine, an inhibitor ofglutamine synthetase activity, enables furtherselection of those clones producing relatively highlevels of transfected glutamine synthetase and henceany heterologous gene which is coupled to it. Theglutamine synthetase system technology has been usedfor research and development purposes during thisdecade and its importance is clearly demonstrated nowthat two therapeutic products produced using thissystem have reached the market place.

Antitransferrin receptor antibody-RNase fusion protein expressed in the mammary gland of transgenic mice

Antibodies fused to human enzymes offer an alternative to specifically targeting tumors with antibodies linked to plant or bacterial toxins. Since large amounts of these reagents can be administered without eliciting non-specific toxicities, efficient methods of production are needed. The goal of this work was to express a complex immunoenzyme fusion protein (immunotoxin) in the mammary gland of transgenic mice. A chimeric mouse/human antibody directed against the human transferrin receptor (E6) was fused at its CH2 domain to the gene for a human angiogenic ribonuclease, angiogenin (Ang). It was expressed in the mammary gland of mice and secreted into mouse milk. Expression levels in milk were approximately 0.8 g/l. The chimeric protein retained antibody binding activity and protein synthesis inhibitory activity equivalent to that of free Ang. It was specifically cytotoxic to human tumor cells in vitro.

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.

Mathematical and experimental analysis of localization of anti-tumour antibody-enzyme conjugates

Considerable research has been aimed at improving the efficacy of chemotherapeutic agents for cancer therapy. A promising two-step approach that is designed to minimize systemic drug toxicity while maximizing activity in tumours employs monoclonal antibody (mAb)-enzyme conjugates for the activation of anticancer prodrugs. We present, analyse and numerically simulate a mathematical model based on the biology of the system to study the biodistribution, pharmacokinetics and localization properties of mAb-enzyme conjugates in tumour tissue. The model predictions were compared with experimental observations and an excellent correlation was found to exist. In addition, the critical parameters affecting conjugate half-life were determined to be the inter-capillary half-distance and the antibody-antigen binding affinity. An approximation is presented relating the per cent injected dose per gram to inter-capillary half-distance and time. Finally, the model was used to examine various dosing strategies in an attempt to determine which regimen would provide the best biodistribution results. We compared the results of administering a uniform dose of fusion protein via bolus injection, multiple injections and continuous infusion. The model predicts that dosing strategy has little effect on the amount of conjugate that localizes in the tumour.

Recombinant antibody fragments

Recombinant antibodies and their fragments now represent over 30% of all biological proteins undergoing clinical trials, which recently culminated in FDA approval for the first engineered cancer therapeutic antibody. Other successful applications include both the effective enhancement of the human immune response for cancer therapy and the reduction of unwanted immune rejections following transplantation and antibody therapy. Important advances have been made in the methods for design, selection and production of these new types of engineered antibodies. Innovative selection methods have enabled the isolation of catalytic antibodies and high-affinity viral-specific antibodies, the latter capable of redirecting viruses for gene therapy applications. In numerous practical applications, recombinant antibody fragments have been fused to radioisotopes, drugs, toxins, enzymes and biosensor surfaces.

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

A panel of 13 renal cell carcinoma cell lines was evaluated for the expression of antigens recognized by the L6 and L49 monoclonal antibodies. All of the cell lines were strongly positive for the L6 antigen, and 9/13 bound 96.5, which, like the L49 monoclonal antibody, recognizes the p97 melanotransferrin antigen. The L6 and L49 antibodies were chemically conjugated to Enterobacter cloacae beta-lactamase (bL), and their abilities to effect site-selective anticancer prodrug activation on two of the renal cell carcinoma cell lines (SN12P and 1934J) were evaluated in vitro and in vivo. L49-bL was 10-90-fold more potent in vitro than L6-bL for the activation of 7-(4-carboxybutanamido)cephalosporin mustard (CCM), a cephalosporin prodrug of phenylenediamine mustard (PDM). In addition, L49-bL showed higher degrees of specific SN12P and 1934J intratumoral uptake than L6-bL, even though the expression of L6 antigen was 2-fold higher than that of p97. These differences might be due to the high-affinity antigen binding of L49-bL relative to L6-bL. In vivo studies utilizing nude mice with established subcutaneous SN12P and 1934J tumor xenografts demonstrated that L49-bL/CCM combinations led to regressions and cures at well-tolerated doses, while L6-bL/CCM and the nonbinding control conjugate P1.17-bL in combination with CCM were ineffective. Conjugate localization in 1934J tumors was much lower than that observed in SN12P tumors, a finding that might acount for the higher activities of L49-bL/CCM in the latter model. These data show that the p97 antigen on renal cell carcinomas can be exploited for selective prodrug activation, even on tumors that localize very small amounts of the L49-bL conjugate.