Intracellular drug delivery: Potential usefulness of engineered Shiga toxin subunit B for targeted cancer therapy

A treasure trove of intracellular cancer drug targets remains hidden behind cell membranes. However, engineered pathogen-derived toxins such as Shiga toxins can deliver small or macromolecular drugs to specific intracellular organelles. After binding to ganglioglobotriaosylceramide (Gb3, CD77), the non-toxic subunit B (StxB) of the Shiga-holotoxin is endocytosed and delivers its payload by a unique retrograde trafficking pathway via the endoplasmic reticulum to the cytosol. This review provides an overview of biomedical applications of StxB-based drug delivery systems in targeted cancer diagnosis and therapy. Biotechnological production of the Stx-material is discussed from the perspective of developing efficacious and safe therapeutics.

In vivo tumor targeting by the B-subunit of shiga toxin

Delivery of drugs to the appropriate target cells would improve efficacy and reduce potential side effects. The nontoxic B-subunit of the intestinal pathogen-produced Shiga toxin (STxB) binds specifically to the glycosphingolipid Gb3, overexpressed in membranes of certain tumor cells, and enters these cells through the retrograde pathway. Therefore, STxB binding to Gb3 receptors may be useful for cell-specific vectorization or imaging purposes. Here we labeled STxB with a fluorophore to evaluate its potential as an in vivo cell-specific targeting reagent in two different models of human colorectal carcinoma. Fluorescent STxB was administered systemically to xenografted nude mice, and its biodistribution was studied by optical imaging. The use of fluorescent STxB allowed the combination of the macroscopic observations with analyses at the cellular level using confocal microscopy. After administration, the fluorescent STxB was slowly eliminated by renal excretion. However, it accumulated in the tumor area. Furthermore, STxB was demonstrated to enter the Gb3-expressing tumoral cells, as well as the epithelial cells of the neovascularization and the monocytes and macrophages surrounding the xenografts.

Pathologic changes in mice induced by subtilase cytotoxin, a potent new Escherichia coli AB5 toxin that targets the endoplasmic reticulum

Subtilase cytotoxin (SubAB) is the prototype of a recently discovered AB(5) cytotoxin family produced by certain strains of Shiga toxigenic Escherichia coli (STEC). The catalytic A subunit is a highly specific subtilase-like serine protease that cleaves the endoplasmic reticulum chaperone BiP. The toxin is lethal for mice, but the pathology it induces is poorly understood. Here, we show that intraperitoneal injection of SubAB causes microangiopathic hemolytic anemia, thrombocytopenia, and renal impairment in mice--characteristics typical of Shiga toxin-induced hemolytic uremic syndrome. SubAB caused extensive microvascular thrombosis and other histologic damage in the brain, kidneys, and liver, as well as dramatic splenic atrophy. Peripheral blood leukocyte levels were increased at 24 h; there was also significant neutrophil infiltration in the liver, kidneys, and spleen and toxin-induced apoptosis at these sites. These findings raise the possibility that SubAB directly contributes to pathology in humans infected with strains of STEC that produce both Shiga toxin and SubAB.

Fabry Disease in Mice Protects against Lethal Disease Caused by Shiga Toxin–Expressing EnterohemorrhagicEscherichia coli

Fabry disease is an X-linked recessive disorder in which affected persons lack alpha-galactosidase A (alpha -GalA), which leads to excess glycosphingolipids in tissues, mainly globotriaosylceramide (Gb3). Gb3 is the cellular receptor for Shiga toxin (Stx), the primary virulence factor of enterohemorrhagic Escherichia coli. alpha-GalA-knockout mice were significantly protected against lethal intraperitoneal doses of Stx2 or oral doses of Stx2-expressing bacteria, compared with wild-type (wt) control mice. Kidneys of moribund wt mice revealed tubular necrosis, but no histopathologic changes were observed in Gb3-overexpressing mice. Reducing Gb3 levels in alpha-GalA-knockout mice by the intravenous injection of recombinant human alpha-GalA restored the susceptibility of knockout mice to lethal doses of Stx2. These results suggest that excess amounts of Gb3 in alpha-GalA-deficient mice may impair toxin delivery to susceptible tissues.

In vivo tumor targeting using a novel intestinal pathogen-based delivery approach

Efficient methods for tumor targeting are eagerly awaited and must satisfy several challenges: molecular specificity, transport through physiologic barriers, and capacity to withstand extracellular or intracellular degradation and inactivation by the immune system. Through interaction with its hosts, the intestinal pathogen-produced Shiga toxin has evolved molecular properties that are of interest in this context. Its nontoxic B-subunit binds to the cellular toxin receptor, glycosphingolipid Gb3, which is highly expressed on human cancers and has recently been reported to be involved in the formation of metastasis in colorectal cancers. Its function as a target for cancer therapy has already been addressed in xenograft experiments. We here show that after oral or i.v. injections in mice, the B-subunit targets spontaneous digestive Gb3-expressing adenocarcinomas. The nontumoral mucosa is devoid of labeling, with the exception of rare enteroendocrine and CD11b-positive cells. As opposed to other delivery tools that are often degraded or recycled on cancer cells, the B-subunit stably associates with these cells due to its trafficking via the retrograde transport route. This can be exploited for the in vivo delivery of contrast agents to tumors, as exemplified using fibered confocal fluorescence endoscopy and positron emission tomography (PET) imaging. In conclusion, the data presented in this manuscript lay the groundwork for a novel delivery technology that, in addition to its use for molecular imaging applications such as noninvasive PET, could also be exploited for targeted tumor therapies.

Facilitated production of secretory IgA against Shiga toxin B subunits by intranasal application of antigen-coated polystyrene microspheres

We examined the effects of microspheres as antigen carriers in mucosal immunization. Shiga toxin B subunits (Stx1B) were adsorbed on 6 mum polystyrene microspheres, which were then intranasally administered to mice together with cholera toxin (CT). Stx1B-specific serum IgG production and secretory IgA production at local mucosal sites were enhanced by the use of microspheres. When OVA was used as a model antigen, secretory IgA production but not serum IgG production was enhanced on the use of microspheres. These results indicated that microspheres provide a useful means of potentiating the immune response against Stx1B with weak immunogenicity.

Patterns of failure after the multimodality treatment of uterine papillary serous carcinoma

PURPOSE

Uterine papillary serous carcinoma (UPSC) is an aggressive variant of endometrial carcinoma. The majority of patients with clinical Stage I UPSC are found to have extrauterine disease at the time of surgery. Most authors report survival rates of 35-50% for Stage I-II and 0-15% for Stage III and IV UPSC. Surgical treatment as the sole therapy for patients with Stage I-IV UPSC is unacceptable because of high recurrence rates. Chemotherapy, radiotherapy, or both have been added after surgery in an attempt to improve survival. However, the survival benefit to patients from such multimodality therapy remains uncertain. This study analyzes the patterns of failure in patients with FIGO Stages I-IV UPSC treated by multimodality therapy.

METHODS AND MATERIALS

Forty-two women with FIGO Stages I-IV UPSC who were treated by multimodality therapy were analyzed retrospectively between 1988 and 1998. Data were obtained from tumor registry, hospital, and radiotherapy chart reviews, operative notes, pathology, and chemotherapy flow sheets. All the patients underwent staging laparotomy, peritoneal cytology, total abdominal hysterectomy and salpingo oophorectomy, pelvic and para-aortic lymph node sampling, omentectomy, and cytoreductive surgery, when indicated followed by radiotherapy and/or chemotherapy. Therapy consisted of external beam radiation therapy in 11 patients (26%), systemic chemotherapy in 20 (48%), and both radiotherapy and chemotherapy in 11 (26%). The treatments were not assigned in a randomized fashion. The dose of external beam radiation therapy ranged from 45-50.40 Gy (median 45). Of the 31 patients (74%) who received chemotherapy, 18 received single-agent (58%), whereas 13 received multiagent chemotherapy (42%).

RESULTS

Median follow-up for all patients was 19 months (range 4-72). Median follow-up for the surviving patients was 36 months (range 21-72). Their median age was 65 years. Six patients (14%) had Stage I, 8 patients (19%) had Stage II, 10 (24%) had Stage III, and 18 (43%) had Stage IV disease. Twenty-nine patients (69%) had suffered recurrence at the time of last follow-up. The actuarial failure rate at 2 and 5 years was 58% and 67%, respectively. The majority of the patients (19/29) recurred in the abdomen, vagina, or pelvis (66%). Metastases outside the abdomen were much less common as the first site of failure (17%). Twenty-five patients (60%) had died at the time of reporting; the observed survival rate at 2 years and 5 years was 52% and 43%, respectively.

CONCLUSIONS

Our data suggest that, after multimodality therapy of FIGO Stage I-IV UPSC, most patients developed abdominopelvic (locoregional) failure, and the great majority of the failures occurred in the abdomen, vagina, and pelvis (66%). Abdominopelvic failure as a component of distant failure occurred in an additional 5 patients (17%). Distant failure alone occurred in 17% of the patients.We propose that future studies should combine whole abdominal radiotherapy (WART) with pelvic and vaginal boosts, in addition to chemotherapy for FIGO Stage I-IV UPSC, especially in patients with minimal residual disease, to attempt to improve the dismal prognosis of patients with UPSC.