Immunoconjugates and immunotoxins for therapy of carcinomas

Enhanced delivery improves the efficacy of a tumor-specific doxorubicin immunoconjugate in a human brain tumor xenograft model

OBJECTIVE

To evaluate dose intensification with osmotic blood-brain barrier disruption (BBBD) and the potential use of drug targeting with monoclonal antibody (MAb) BR96 conjugated to doxorubicin (BR96-DOX, now called SGN15) for treatment of intracerebral and subcutaneous human LX-1 small cell lung carcinoma xenografts in rats.

METHODS

LX-1 tumors with high, low, or heterogeneous levels of the Lewis(y) antigen for BR96 were evaluated. Rats were treated with intracarotid or intravenous BR96-DOX, with or without osmotic BBBD.

RESULTS

Both BR96-DOX and MAb BR96 treatment resulted in significant regression of subcutaneous tumors, in contrast to control groups including doxorubicin alone, saline, or nonbinding doxorubicin immunoconjugate. BR96-DOX delivered with BBBD to brain tumors with low antigen expression resulted in significantly (P < 0.001) increased rat survival time compared with animals that received intravenous or intra-arterial BR96-DOX.

CONCLUSION

The combination of an effective drug such as doxorubicin with a MAb to facilitate tumor-selective localization and osmotic BBBD to increase tumor delivery may have practical application in the clinic, because an increased delivery of drug to tumor can be obtained without increasing the dose of systemic drug.

Monoclonal antibody drug conjugates in the treatment of cancer

Monoclonal antibodies directed to tumor-associated antigens have been chemically conjugated to drugs with different mechanisms of action and different levels of potency. Monoclonal-antibody-directed drug delivery has the potential to both improve efficacy and reduce systemic toxicity. Several immunoconjugates have demonstrated impressive antigen-specific antitumor activity in preclinical models. Phase I trials of a calicheamicin immunoconjugate for treatment of acute myeloid leukemia and a doxorubicin immunoconjugate for treatment of carcinoma have recently been completed.

Antiparasite activity of sea-anemone cytolysins on Giardia duodenalis and specific targeting with anti-Giardia antibodies

The killing activity of sea-anemone cytolysins on Giardia duodenalis was investigated. Three different toxins, sticholysin I and II from Stichodactyla helianthus (St I and St II) and equinatoxin II from Actinia equina (EqtII) were all found to be active in an acute test, with a C50 in the nanomolar range (St I, 0.5 nM; St II, 1.6 nM; and EqtII, 0.8 nM). A method to target the cytolysin activity more specifically towards the parasite cells by using anti-Giardia antibodies was then investigated. Parasite cells were sensitised with a primary murine monoclonal or polyclonal antibody followed by a biotinylated secondary anti-mouse-IgG monoclonal antibody. Subsequently, avidin and a biotinylated EqtII mutant were added, either in two separate steps or as a pre-formed conjugate. When the monoclonal antibody was used, the C50 of biotinylated EqtII was 1.3 nM with sensitised cells and 5 nM with non-sensitised cells, indicating a four-fold enhancement of activity with the cell treatment. Treatment with the polyclonal antibody was somehow more effective than with the monoclonal antibody in an acute test. This indicates that sea-anemone cytolysins can efficiently kill Giardia cells, and that it is possible to improve, to a certain extent, the anti-parasite specificity of these toxins with anti-Giardia antibodies. However, the feasibility of this approach "in vivo" remains to be demonstrated.

Novel therapeutic strategies to selectively kill cancer cells

Tumor invasion, metastasis, and resistance to chemotherapeutic drugs or radiation are major obstacles for the successful treatment of cancer. To overcome some of these limitations, therapeutic strategies that increase the specificity and efficacy and reduce the toxicity of the anti-cancer drugs or toxins are being explored. Cancer cells overexpress specific protein antigens and carbohydrate structures that may function as cell surface receptors. These cancer cell specific markers can be exploited while designing new cancer therapies. Monoclonal antibodies that have been humanized to reduce immunogenicity and targeted to specific antigens on cancer cells, enzyme-monoclonal antibody/prodrug conjugates that will selectively kill the target cells following drug activation, and recombinant toxins are some of the novel classes of agents in development. Another novel approach being investigated to treat cancers is the use of inactive pore-forming toxins with built-in biological "triggers" that will activate the toxin following a biological stimulus. These pore-forming cytolytic toxins can be rendered active by tumor-specific proteases, that are often overexpressed in cancer cells, thereby targeting the toxic effects. Such pore-forming or membrane-acting toxins may serve as novel cytolytic agents against solid tumors, which, to date, have proved to be more resistant to conventional toxins.

Homodimerization of tumor-reactive monoclonal antibodies markedly increases their ability to induce growth arrest or apoptosis of tumor cells

Monoclonal antibodies (mAbs) that exert antitumor activity can do so by virtue of their effector function and/or their ability to signal growth arrest or cell death. In this study, we demonstrate that mAbs which have little or no signaling activity-i.e., anti-CD19, CD20, CD21, CD22 and Her-2-can become potent antitumor agents when they are converted into IgG-IgG homodimers. The homodimers exert antigrowth activity by signaling G0/G1 arrest or apoptosis, depending upon which cell surface molecule they bind. This activity is specific and, in the case of the anti-CD19 mAb, did not require an Fc portion. These results offer the possibility that homodimers of other tumor-reactive mAbs which have little antitumor activity as monomers might be potent, antitumor agents.