Evaluation of the systemic toxicity and pharmacokinetics of the immunoconjugate anti-B4-blocked ricin in non-human primates. Delivered by multiple bolus injections and by continuous infusion

Development of a highly sensitive in vitro endothelial cell toxicity assay for evaluating ricin toxin A chain-based vaccines or therapeutics

The ricin toxin A chain (RTA) is responsible for ricin intoxication due to inhibition of protein synthesis. RTA is also known to cause endothelial toxicity [via a 3 amino acid sequence (x)D(y) motif that acts as a natural disintegrin] resulting in vascular leak syndrome (VLS) in humans. An in vitro endothelial cell toxicity (ECT) assay was developed to evaluate if the ricin vaccine candidate (RVEc) exhibited endothelial toxicity, determined by altered transendothelial electrical resistance (TEER) across human umbilical vein endothelial cell (HUVEC) monolayers. Timepoints at 2 and 4 h were included to evaluate HUVEC monolayers before the effects of RTA ribotoxic activity are observed. Both the 3 μM and 6 μM RTA positive controls consistently demonstrated significantly reduced TEER values, compared to their corresponding vehicle control, in a time- and concentration-dependent manner at 2, 4, and 24 h. Fluorescent imaging of HUVECs exposed to 3 μM RTA showed cell rounding at 2 and 4 h and gap formation at 24 h. No changes in TEER or fluorescent imaging were observed after exposure to endothelial cell growth medium-2 (EGM-2) exchange (mock control). The negative controls, which included 2 mutant RTA vaccine derivatives [RVEc with an (x)D(y) VLS sequence modification to V76M or D75N] and bovine serum albumin (BSA), demonstrated no evidence of HUVEC toxicity at 3 μM and 6  μM concentrations. Overall, the performance of the ECT assay was consistent, allowing for the development of acceptance criteria that were related to time- and concentration-dependent decreases in TEER between 2 and 24 h.

In vivobehaviour of antibody–drug conjugates for the targeted treatment of cancer

It is a commonly held belief that most treatments for disseminated cancers are only moderately effective because the agents lack cell-killing mechanisms that act specifically on cancer cells. In antibody-drug conjugates, such nonspecific cytotoxic agents are combined with exquisitely specific monoclonal antibodies that bind to tumour-associated antigens and, thus, get endowed with new pharmacological characteristics. Not only is their activity newly targeted towards tumours and tumour cells, which hopefully renders them more tumour-specific, but they also acquire much of the pharmacokinetic behaviour of the monoclonal antibody component. With the structural composition of a macromolecular protein (the antibody), a small chemical cytotoxic agent and a linker to chemically connect these two molecules, antibody-drug conjugates are some of the most complex pharmacological agents ever developed. Their development over the last 20 years or so owes much to sophisticated in vitro and in vivo preclinical testing. This review attempts to summarise and exemplify many of the factors that had to be considered during the development, with special emphasis on the in vivo pharmacology of these agents.

Pharmacokinetics and biodistribution of the antitumor immunoconjugate, cantuzumab mertansine (huC242-DM1), and its two components in mice

The humanized monoclonal antibody maytansinoid conjugate, cantuzumab mertansine (huC242-DM1) that contains on average three to four linked drug molecules per antibody molecule was evaluated in CD-1 mice for its pharmacokinetic behavior and tissue distribution, and the results were compared with those of the free antibody huC242. The pharmacokinetics in blood were similar for (125)I-labeled conjugate and antibody with terminal half-lives of 154 and 156 h, respectively. Pharmacokinetic analysis using an enzyme-linked immunosorbent assay (ELISA) method, which measures intact conjugate in plasma samples revealed a faster clearance for the conjugate corresponding to a half-life of 42.2 h. This faster clearance is explained as the result of clearance from circulation and concomitant clearance of drug from circulating conjugate through linker cleavage. An antibody-specific ELISA allowed the determination of the clearance rate of the antibody component from circulation. The drug clearance rate from circulating conjugate was then calculated as the difference between the clearance of the conjugate and the clearance of the antibody component and found to be about three times that of the antibody component. The above results were confirmed with a conjugate, huC242-[(3)H]DM1, where the linked DM1 drugs carried a stable tritium label. Tissue distribution studies with (125)I-labeled conjugate and antibody showed antibody-like behavior for the conjugate; the antibody of the conjugate did not distribute or bind significantly to any solid tissue.

Immunotoxin combined with chemotherapy for patients with AIDS-related non-Hodgkin's lymphoma

BACKGROUND

The objective of this study was to develop and test a combined therapeutic approach for patients with AIDS-related lymphoma (ARL), employing agents with independent mechanisms of action and nonoverlapping toxicity. This study was designed to test the feasibility and tolerance of combining low dose chemotherapy with infusional immunotoxin in the treatment of ARL patients.

METHODS

Previously untreated patients received low dose methotrexate, bleomycin, doxorubicin, cyclophosphamide, and vincristine (m-BACOD) on a 21- to 28-day schedule. Patients who did not have progressive disease by Cycle 3 received anti-B4-blocked ricin (anti-B4bR), a murine monoclonal antibody linked to modified ricin, 20 microg/kg/day for 7 days administered by continuous infusion on an outpatient basis. A repeat cycle of anti-B4-bR was administered during Cycle 4 of chemotherapy based on tolerance. Patients received two cycles of chemotherapy beyond complete remission up to eight cycles. Study endpoints were toxicity, development of human antimurine antibody (HAMA) and human antiricin (HARA), tumor response, and survival.

RESULTS

Twenty-six of 44 patients received the immunotoxin therapy. Anti-B4-bR infusion was associated with transaminase elevation (Grade 3) in 14 of 26 patients (58%), and flulike symptoms were common. HAMA or HARA was observed in 8 patients (31%). The overall response rate was 57% (13 complete responses and 12 partial responses). The median survival for all patients was 8.9 months.

CONCLUSIONS

This study demonstrates the safety and feasibility of using chemotherapy and immunotoxin therapies in combination and supports their further evaluation to improve the outcomes of patients with ARL.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

An In Vitro Model for Toxin-Mediated Vascular Leak Syndrome: Ricin Toxin A Chain Increases the Permeability of Human Endothelial Cell Monolayers

Vascular leak syndrome (VLS) is the dose-limiting toxicity observed in clinical trials of immunotoxins containing ricin toxin A chain (RTA). RTA itself is thought to cause VLS by damaging vascular endothelial cells, but the exact mechanism remains unclear. This is partially due to the paucity of appropriate models. To study VLS, we developed an in vitro model in which human umbilical vein-derived endothelial cells were first grown to confluence on microporous supports and then cultured under low pressure in the presence or absence of RTA. Endothelial cell barrier function was assessed by measuring the volume of fluid that passed through each monolayer per unit time. We found that RTA significantly increased monolayer permeability at times and concentrations consistent with the onset of VLS in patients treated with RTA-based immunotoxins. Scanning electron microscopy showed that intercellular gaps formed in endothelial monolayers exposed to RTA. Intercellular gap formation followed endothelial cell death caused by the enzymatic activity of RTA. We conclude that RTA is directly toxic to endothelial cells in vitro and speculate that this contributes to VLS in vivo.

An In Vitro Model for Toxin-Mediated Vascular Leak Syndrome: Ricin Toxin A Chain Increases the Permeability of Human Endothelial Cell Monolayers

Vascular leak syndrome (VLS) is the dose-limiting toxicity observed in clinical trials of immunotoxins containing ricin toxin A chain (RTA). RTA itself is thought to cause VLS by damaging vascular endothelial cells, but the exact mechanism remains unclear. This is partially due to the paucity of appropriate models. To study VLS, we developed an in vitro model in which human umbilical vein-derived endothelial cells were first grown to confluence on microporous supports and then cultured under low pressure in the presence or absence of RTA. Endothelial cell barrier function was assessed by measuring the volume of fluid that passed through each monolayer per unit time. We found that RTA significantly increased monolayer permeability at times and concentrations consistent with the onset of VLS in patients treated with RTA-based immunotoxins. Scanning electron microscopy showed that intercellular gaps formed in endothelial monolayers exposed to RTA. Intercellular gap formation followed endothelial cell death caused by the enzymatic activity of RTA. We conclude that RTA is directly toxic to endothelial cells in vitro and speculate that this contributes to VLS in vivo.