Kenneth D. Bagshawe

Antibody-directed enzyme prodrug therapy (ADEPT) for cancer.

Antibody-directed enzyme prodrug therapy was conceived as a means of restricting the action of cytotoxic drugs to tumor sites. Since antigenic targets were a central component of the approach, colonic cancer, with its virtually universal expression of carcinoembryonic antigen at the cellular level, presented an obvious starting point. The principle of antibody-directed enzyme prodrug therapy is to use an antibody directed at a tumor-associated antigen to vector an enzyme to tumor sites. The enzyme should be retained at tumor sites after it has cleared from blood and normal tissues. A nontoxic prodrug, a substrate for the enzyme, is then given and, by cleaving an inactivating component from the prodrug, a potent cytotoxic agent is generated. One of the potential advantages of such a system is that a small cytotoxic agent, generated within a tumor site, is much more diffusible than a large antibody molecule. Moreover, failure to express the target antigen by cancer cells does not protect them from the bystander action of the cytotoxic agent. This review will primarily consider the studies of the London group since this is the only group that has so far reported clinical trials and it is only through clinical trials that the requirements of a successful antibody-directed enzyme prodrug therapy system can be identified.

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.

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.

A Radioimmunoassay for Human Chorionic Gonadotrophin

A NUMBER of immunological assay methods for human chorionic gonadotrophin (HCG) have been described recently. These include hæmagglutination1, latex particle agglutination2, and complement fixation3. These methods have been devised principally to detect or confirm pregnancy, and qualitative tests are usually adequate for this purpose. Patients with untreated choriocarcinoma excrete HCG in the urine. During the treatment of these patients with anti-metabolite drugs the concentration of HCG in the body fluids, which provides an index of the tumours behaviour, falls rapidly to low levels. In these circumstances an assay method which is both more precise and more sensitive is needed. With this objective a radioimmunoassay technique, similar to that described by Hales and Randle for insulin4, has been applied to HCG.

Antibody-directed enzyme prodrug therapy (ADEPT): A three-phase study in ovarian tumor xenografts

Antibody-directed enzyme prodrug therapy (ADEPT) has been studied in a human ovarian carcinoma xenograft grown subcutaneously in nude mice. Radioimmunoassay of supernatants obtained from tumor homogenates showed these to contain carcinoembryonic antigen (CEA). Biodistribution studies with 125I-labeled monoclonal anti-CEA antibody, A5B7, and its F(ab)2 fragment showed localization in these xenografts. The AB57-F(ab)2 fragment conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), and, radiolabeled with 125iodine, also localized in the xenografts. The radiolabeled conjugate cleared from blood faster than the antibody alone. The percentage of injected dose per gram in tumor at 24 h postinjection was about fivefold lower than antibody alone. Tumor-to-blood ratio at 72 h after injection of the radiolabeled conjugate was 7 and the tumor-to-normal tissue ratios at this time point ranged from 20 (liver) to 75 (colon). A three-phase ADEPT antitumor study was carried out in which A5B7-F(ab)2-CPG2 was allowed to localize and was followed by accelerated inactivation/clearance of blood CPG2 by a galactosylated anti-CPG2 antibody (SB43gal). A benzoic acid mustard-derived prodrug was injected 24 h after the conjugate, which led to growth delay in this tumor compared to the control untreated group. Further antitumor studies in this model are in progress.Antibody-directed enzyme prodrug therapy (ADEPT) has been studied in a human ovarian carcinoma xenograft grown subcutaneously in nude mice. Radioimmunoassay of supernatants obtained from tumor homogenates showed these to contain carcinoembryonic antigen (CEA). Biodistribution studies with125I-labeled monoclonal anti-CEA antibody, A5B7, and its F(ab′)2 fragment showed localization in these xenografts. The AB57-F(ab′)2 fragment conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), and, radiolabeled with125iodine, also localized in the xenografts. The radiolabeled conjugate cleared from blood faster than the antibody alone. The percentage of injected dose per gram in tumor at 24 h postinjection was about fivefold lower than antibody alone. Tumor-to-blood ratio at 72 h after injection of the radiolabeled conjugate was 7 and the tumor-to-normal tissue ratios at this time point ranged from 20 (liver) to 75 (colon).A three-phase ADEPT antitumor study was carried out in which A5B7-F(ab′)2-CPG2 was allowed to localize and was followed by accelerated inactivation/clearance of blood CPG2 by a galactosylated anti-CPG2 antibody (SB43gal). A benzoic acid mustard-derived prodrug was injected 24 h after the conjugate, which led to growth delay in this tumor compared to the control untreated group. Further antitumor studies in this model are in progress.

Identification of prodrug, active drug, and metabolites in an ADEPT clinical study

Antibody-directed enzyme prodrug therapy (ADEPT) involves two phases. The first is an antibody-enzyme conjugate that localizes to tumor. The second phase is a prodrug that is administered when the enzyme-conjugate has cleared from blood and other nontumor tissues. In the pilot-scale clinical trial, the prodrug has been measured—in the plasma of patients, by liquid chromatography (HPLC) and by liquid chromatography-mass spectrometry (LC-MS). Active drug has been detected and metabolites identified. An indirect measurement of enzyme-conjugate in the plasma of patients has also been developed.

Antibody Directed Enzyme Prodrug Therapy (ADEPT): Trials and tribulations

ABSTRACT Antibody directed enzyme prodrug therapy has the potential to be an effective therapy for most common solid cancers. Clinical studies with CPG2 system have shown the feasibility of this approach. The key limitation has been immunogenicity of the enzyme. Technologies now exist to eliminate this problem. Non‐immunogenic enzymes in combination with prodrugs that generate potent cytotoxic drugs can provide a powerful approach to cancer therapy. ADEPT has the potential to be non ‐toxic to normal tissue and can therefore be combined with other modalities including immunotherapy for greater clinical benefit.

Translating antibody directed enzyme prodrug therapy (ADEPT) and prospects for combination

ABSTRACT Introduction: The generation of cytotoxic drugs, selectively within tumours, from non-toxic prodrugs by targeted enzymes provides a powerful system for cancer therapy. In the form of Antibody directed enzyme prodrug therapy (ADEPT), this approach has shown feasibility in the clinic. Areas covered: Although numerous enzyme prodrug combinations have been reported over the last two decades, only the CPG2 ADEPT system has progressed to clinical trials. Using readily available components such as chemical antibody enzyme conjugate or recombinant multifunctional fusion protein, delivery of a specific enzyme to tumours, its elimination from non-tumour sites and prodrug activation has been achieved with therapeutic benefit in the clinic. The challenge here is to overcome immunogenicity of CPG2. Technology exists to overcome this limitation together with prospects for rational design of combined therapy. Expert opinion: ADEPT has the potential to be an effective treatment for solid cancer. However, the system necessitates a multi-disciplinary and iterative approach. Although xenograft studies provide a consistent guide it is only through clinical studies that the real challenges can be identified. The emerging preclinical data with other enzyme prodrug systems may provide the opportunity to develop the next generation ADEPT comprising non-immunogenic enzymes to generate potent cytotoxic drugs within tumours.

Limitations of tests for human chorionic gonadotropin

reproductive system that produce hCG ectopically. Different kits use antibodies directed at different epitopes, so the numerical values they give for hCG can differ widely. Some are specific for particular fragments of hCG that may be absent in the samples from oncology patients, so false-negative results can occur. Even in the UK, where a radioimmunoassay service for hCG has long been available through the National Health Service for oncology patients, many oncology centres use immunometric assays and the clinicians are largely unaware of their limitations. Kits for the assay of hCG have been licensed in the USA only as an aid to the detection of pregnancy, and the leaflets accompanying the kits generally make this point clear. In the UK and some other European countries, the kits are unregulated but they will come under a European Directive that is expected in June 2000. Thus responsibility for their use in oncology seems to lie with the laboratories using them for this purpose. In a litigation-conscious world, ensuring that clinicians making critical decisions based on immunometric assays for hCG are aware of the limitations could well merit more attention from laboratory managers.

Treatment of Advanced Trophoblastic Disease

Knowledge of the diverse behaviour of trophoblastic tumours has increased greatly over the 25 years since the introduction of methotrexate [1] and methods of treatment have developed accordingly. The best possible management of the individual patient now involves complex considerations leading to treatment chosen according to the particular characteristics of the patient and her tumour. This has been achieved by sustained work in a limited number of specialized centres but in spite of the advances some of these young women still die. Many of the deaths can be attributed to inappropriate treatment started in centres lacking experience in this group of diseases. There remains a strong case for the continued referral of patients to units where experience has accumulated and where old lessons do not have to be re-learned.