A fluorouridine-anti-CEA immunoconjugate is therapeutically effective in a human colonic cancer xenograft model

Targeted drug conjugates: principles and progress

Reports of targeting drugs using antibodies have appeared in the literature since 1958, but exciting clinical results in this field have only been reported in the last few years. Progress in this field has occurred largely through an understanding how drug-immunoconjugates work. The objective of this review is to draw together the fundamental principles on which this field of work is based, to examine the evidence supporting those principles, and the effectiveness and selectivity of targeted drug conjugates. The activity of many drug-immunoconjugates can now largely be accounted for by the underlying principles. Excellent development work, both with conventional anti-cancer agents and very potent drugs have led to a number of interesting clinical trials. In the best Phase I and II trials, good evidence of effectiveness have been reported, which suggest that drug-immunoconjugates may now be heralding a new era for chemotherapy.

Carbohydrates engineered at antibody constant domains can be used for site-specific conjugation of drugs and chelates

To improve the efficiency of site-specific conjugation of chelates and drugs to antibodies, and to minimize the incidence of immunoreactivity perturbation to the resultant immunoconjugates, Asn-linked oligosaccharide moieties were designed and engineered into the constant domains of a humanized anti-CD22 monoclonal antibody, hLL2. From 10 potential glycosylation mutants, two CH1 domain glycosylation sites, HCN1 and HCN5, were identified that were positioned favorably for glycosylation. The carbohydrate (CHO) chains attached at these sites were differentially processed so that HCN5-CHOs were physically larger than HCN1-CHOs. Although both the CH1-appended CHOs, and the LL2 Vkappa-appended CHOs conjugated efficiently with small chelates, the HCN5-CHOs, due to the structural and positional superiority, appear to be a better conjugation site for large drug complexes, such as 18 kDa doxorubicin (DOX)-dextran.

Requirement of monocytes and T-helper cells during development of tumor cell cytotoxicity in targeted T cells

In cocultures of human placental alkaline phosphatase(PLAP)-positive MO4 tumor cells and human peripheral blood mononuclear cells (PBMC), also containing a heteroconjugate (7E8-OKT3) synthesized between the anti-PLAP monoclonal antibody 7E8 and the anti-CD3 antibody OKT3, and supplemented with low levels of recombinant interleukin-2 (rIL-2), T cells are progressively activated, resulting in tumor cell lysis. To unravel the contribution of PBMC subsets to the generation of this targetable cytotoxicity, PBMC subsets were studied after their isolation by cell sorting, either from fresh PBMC or from PBMC pre-activated with OKT3 and rIL-2. Whereas no targetable cytotoxicity was found in Fc-receptor-bearing CD3- cells, tumor cells were lysed by CD3+ T cells (mostly CD8+) isolated from pre-activated PBMC. When isolated from fresh PBMC, neither the CD8+ T cell subset, nor the total CD3+ T cell population developed significant targetable cytotoxicity, even in the presence of rIL-2. Thus, additional cell types are essential for the CD8+ T cell activation. Indeed, CD4+ T cells isolated from pre-activated but not from fresh PBMC were capable of eliciting cytotoxicity in fresh CD8+ T cells. The non-targeted monocytes were found to be the activators of the CD4+ T cells. In summary, targeting T cells to the surface of a tumor cell is not sufficient per se to achieve activation and lysis. The progressive tumor cell lysis by targeted T cells seems to be initiated by non-targeted monocytes activating CD4+ T cells, these cells in turn promoting CD8+ T cell activation, necessary for the development of cytotoxicity.

Antibody-targeted drugs for the therapy of cancer

The advent of monoclonal antibodies has revitalised the concept of magic bullets and various agents (eg. drugs, toxins and isotopes) have been conjugated to monoclonal antibodies for selective delivery to tumours. Preclinical studies in mouse tumour models have been impressive and have lead to several clinical trials. These phase I trials have been less impressive. However, keeping in mind the aim of Phase I trials, the safety of using these conjugates in humans have been established. Several, major problems still remain to be overcome before these agents may be useful for the treatment of cancer. These problems stem from the nature of tumour vasculature, cytotoxic activity of the moiety linked to antibody and the targeted tumour antigen expressed on the cell surface. This review will deal with these various aspects described above and possible approaches to overcome these obstacles with a definite bias towards drug-monoclonal antibody conjugates. However, these concepts are equally applicable for improved targeting of other agents.

Monoclonal antibodies in cancer detection and therapy

Anticancer antibodies have had a long history in the management of cancer, with major applications having been shown in the immunohistochemistry and immunoassay of tumor-associated antigen markers. With the advent of hybridoma-derived monoclonal antibodies, attempts to use these more reproducible reagents in vivo for cancer detection and therapy have intensified. Radiolabeled monoclonal antibodies appear to be gaining a role in the management of cancer by means of imaging methods to detect sites of increased radioactivity, and several products have been developed and tested clinically. In the area of radioimmunotherapy, a number of problems still need to be solved, including low tumor uptake of the radioimmunoconjugate, dose-limiting myelotoxicity, and the induction of an immune response to repeated doses of murine (foreign) immunoglobulins. Similar problems exist for toxin and drug immunoconjugates, but these also fail to benefit from the "bystander" effect of the ionizing radiation delivered with radioimmunoconjugates, and plant and bacterial toxin molecules appear to have additional immunogenicity that restricts repeated injections. Despite these limitations, recombinant engineering and other chemical approaches are making progress in developing second-generation immunoconjugates that may be more efficacious and less immunogenic as cancer-selective therapeutics. Although nonconjugated, "naked", murine monoclonal antibodies have shown limited success in the therapy of human neoplasms, human and "humanized" forms may be more effective, particularly in lymphatic tumors. Some evidence also suggests that anti-idiotype antibodies (antiantibodies) may serve as surrogate antigens in cancer vaccines. Thus, a number of promising immunologic approaches for cancer diagnosis, detection, and therapy have made important progress in recent years.