Isatuximab plus carfilzomib and dexamethasone in patients with early versus late relapsed multiple myeloma: IKEMA subgroup analysis.

Patients with multiple myeloma (MM) who experience early relapse within 12 months of therapy initiation are considered functional high-risk and represent an unmet need, needing better therapies to improve outcomes. The final IKEMA (clinicaltrials gov. identifier: NCT03275285) progression-free survival (PFS) analysis confirmed the significant PFS improvement reported at interim analysis with isatuximab (Isa) plus carfilzomib and dexamethasone (Kd; Isa-Kd) versus Kd in patients with relapsed MM (updated median PFS: 35.7 vs. 19.2 months; hazard ratio [HR] =0.58, 95% confidence interval [CI]: 0.42- 0.79). This IKEMA subgroup analysis examined efficacy and safety of Isa-Kd versus Kd in patients who experienced early (n=61 [Isa-Kd], n=46 [Kd]) vs. late relapse (n=104 [Isa-Kd], n=72 [Kd]). As expected, more aggressive features in baseline characteristics were observed in early relapse patients. Consistent with IKEMA overall population results, median PFS (early relapse: 24.7 vs. 17.2 months, HR=0.662, 95% CI: 0.407-1.077; late relapse: 42.7 vs. 21.9 months, HR=0.542, 95% CI: 0.355- 0.826), minimal residual disease negativity (MRD-) (early relapse: 24.6% vs. 15.2%; late relapse: 37.5% vs. 16.7%), and MRD- complete response (≥CR) rates (early relapse: 18.0% vs. 10.9%; late relapse: 30.8% vs. 13.9%) were higher with Isa-Kd versus Kd, respectively, in both early and late relapse patients. Grade ≥3, serious treatment-emergent adverse events, and death rates were higher in the late relapse Isa-Kd arm. However, the numbers of deaths were low and treatment exposure was significantly longer in Isa-Kd versus Kd late relapse patients. These results support the addition of Isa to Kd as standardof- care therapy for relapsed and/or refractory MM regardless of relapse timing.

Isatuximab plus pomalidomide and dexamethasone in patients with relapsed/refractory multiple myeloma in real-world: The retrospective IMAGE study.

BACKGROUND: IMAGE is a retrospective cohort study of patients enrolled in early access programs (EAPs) in France with relapsed/refractory multiple myeloma (RRMM) receiving isatuximab with pomalidomide and dexamethasone (Isa-Pd). METHODS: Patients aged ≥18 years with RRMM who received ≥1 dose of Isa under the EAPs between July 29, 2019 and August 30, 2020 were included. Effectiveness endpoints included progression-free survival (PFS) and response rates. Verbatim terms for adverse events (AEs) were coded using the Medical Dictionary for Regulatory Activities and not graded for severity. RESULTS: A total of 294 and 299 patients were included in the effectiveness and safety populations, respectively. IMAGE included patients who received one prior line of treatment (10.2%) and were daratumumab-refractory (19.1%). At median follow-up of 14.2 months, median PFS in the effectiveness population was 12.4 months (95% CI 9.0-15.0). Overall response and very good partial response rates were 46.3% and 27.9%, respectively. Subgroup analyses reflected similar results. In the safety population, 26.4% of patients reported at least one AE; the most common any-grade AE was neutropenia (9.4%). CONCLUSION: IMAGE demonstrated Isa-Pd had meaningful effectiveness in median PFS and depth of response and no new safety signals in a real-world context, consistent with clinical trial results.

Isatuximab Plus Carfilzomib and Dexamethasone Versus Carfilzomib and Dexamethasone in Patients with Relapsed Multiple Myeloma: IKEMA Subgroup Analysis by Prior Transplantation.

In the era of highly active novel agents for multiple myeloma (MM), the role, ideal timing, and impact of transplantation on further therapy after relapse remains a matter of debate. The impact of prior transplantation on treatment benefit from monoclonal antibodies in patients with relapsed/refractory MM (RRMM) is largely unknown. Few Phase 3 studies of monoclonal antibody combinations with proteasome inhibitors or immunomodulatory agents have reported outcomes according to transplantation status. This subgroup analysis examined efficacy and safety in patients from the Phase 3 IKEMA study with and without previous transplantation. IKEMA (NCT03275285) was a randomized, open-label, multinational, parallel-group Phase 3 study that investigated isatuximab (Isa), an anti-CD38 monoclonal antibody, combined with carfilzomib and dexamethasone (Isa-Kd; experimental group) versus Kd (control group) in 302 patients with RRMM and 1 to 3 prior lines of therapy. Patients were randomized in a 3:2 ratio to either Isa-Kd or Kd, with stratification by number of prior lines (1 versus more than 1) and Revised International Staging System (R-ISS) stage (I or II versus III versus not classified). Treatment was given until progressive disease, unacceptable adverse events, or patient choice. Of the 302 randomized patients in IKEMA, 185 (61.3%) had received a prior transplant, comprising 116 of 179 (64.8%) patients in the Isa-Kd arm and 69 of 123 (56.1%) patients in the Kd arm. After a median follow-up of 20.6 months, median progression-free survival (PFS) in patients with prior transplant was not reached with Isa-Kd versus 19.15 months with Kd (hazard ratio [HR] = 0.60; 99% confidence interval [CI], 0.31-1.16). After a median follow-up of 20.8 months, median PFS in patients without prior transplant was not reached with Isa-Kd versus 18.99 months with Kd (HR = 0.44; 99% CI, 0.18-1.05). The overall response rate in patients with prior transplant was 87.9% (Isa-Kd) versus 85.5% (Kd). More patients in the Isa-Kd arm achieved a complete response or better compared with the Kd arm (43.1% versus 29.0%). The overall response rate in patients without prior transplant was 84.1% (Isa-Kd) versus 79.6% (Kd). More patients in the Isa-Kd arm achieved a complete response or better compared with the Kd arm (33.3% versus 25.9%). The minimal residual disease negativity rate was higher with Isa-Kd versus Kd in patients with (31.9% versus 13.0%) and without prior transplantation (25.4% versus 13.0%). In patients with prior transplant, Grade 3 or higher treatment-emergent adverse events (TEAEs) were more common with Isa-Kd; however, no increases in serious TEAEs or definitive treatment discontinuations were seen versus Kd. Among patients without prior transplant, serious treatment-related TEAEs were similar, and there were fewer TEAEs leading to definitive discontinuation with Isa-Kd. The most common Grade 3 or higher TEAEs in patients with and without prior transplant were hypertension and pneumonia. For patients who underwent prior transplantation, Isa-Kd is an effective treatment option. Overall, these data demonstrate that Isa-Kd represents a standard of care for patients with RRMM, regardless of prior transplant status.

B7-H3 expression in colorectal cancer: nuclear localization strongly predicts poor outcome in colon cancer.

In colorectal cancer there is a need for molecular markers that can complement the histopathological staging in predicting the likelihood of disease recurrence following curatively intended surgery. B7-H3 is an immunoregulatory protein shown to be overexpressed in several cancer forms, often associated with more advanced disease and poor prognosis. We wanted to examine whether B7-H3 could be a potential prognostic marker in colorectal cancer. Paraffin-embedded samples from 277 colorectal cancer patients were immunostained with anti-B7-H3 antibody. B7-H3 was expressed in the tumor cell cytoplasm and cell membrane in 62% and 46% of the samples, respectively. Unexpectedly, B7-H3 was expressed in the nucleus in 30% of the tumors. The nuclear localization was confirmed by Western immunoblotting of subcellular fractions. Importantly, in colon cancer, nuclear B7-H3 expression was independently and significantly associated with reduced metastasis-free, disease-specific and overall survival. B7-H3 expression in tumor-associated vasculature and fibroblasts was observed in the majority of samples, and endothelial B7-H3 expression was also significantly associated with poor outcome in colon cancer. In rectal cancer patients, the only significant association was between fibroblast B7-H3 expression and shorter metastasis-free survival. Few significant associations to clinicopathological parameters were seen. The results indicate that nuclear B7-H3 might be involved in colon cancer progression and metastasis, and suggest that nuclear B7-H3 could become a useful prognostic marker in colon cancer.

B7-H3 contributes to the metastatic capacity of melanoma cells by modulation of known metastasis-associated genes.

B7-H3, an immunoregulatory protein, is known to play a role in tumor progression. In many cancer types, observed correlations between high B7-H3 expression and poor prognosis have been attributed to involvement in antitumor immunity. However, here we demonstrate a nonimmunological alternative function of B7-H3 in cancer metastasis. Since advanced malignant melanoma is a disease with a poor survival rate and a broad pattern of metastasis, we used this disease as a model in our studies. We found that shRNA silencing of B7-H3 reduced the in vitro migratory potential and matrigel invasiveness of MDA-MB-435 and FEMX-I melanoma cells. In an experimental metastasis model in vivo, B7-H3 silencing of MDA-MB-435 cells resulted in reduced metastatic capacity and significantly increased the median symptom-free survival of nude mice (147 vs. 65 days, p < 0.001) and rats (53 vs. 42 days, p = 0.025) injected with MDA-MB-435 cells. Furthermore, a smaller fraction of mice had microscopically detectable metastases compared to control animals, and the pattern of metastases was slightly different between the two groups but with the brain as the predominant organ. Immunohistochemistry on samples from two melanoma patients showed strong B7-H3 staining in both a primary tumor and metastases. Notably, the metastasis-associated proteins, matrix metalloproteinase (MMP)-2, signal transducer and activator of transcription 3 (Stat3), and the level of secreted interleukin-8 (IL-8) were reduced in the B7-H3 knock-down cell variants, whereas tissue inhibitor of metalloproteinase (TIMP)-1 and-2 levels were increased. Taken together, our findings indicate a novel role for B7-H3 in the regulation of the metastatic capacity of melanoma cells and it might be a potential therapeutic target for anti-metastasis therapy.

Efficacy and safety of carfilzomib-based regimens in frail patients with relapsed and/or refractory multiple myeloma.

Frailty is most prevalent among elderly multiple myeloma (MM) patients, and frail patients have a higher risk of poor outcomes due to reduced performance status or comorbidities. This post hoc analysis assessed efficacy and safety of carfilzomib combinations in frail patients with relapsed and/or refractory MM from the phase 3 ASPIRE (carfilzomib [27 mg/m2]-lenalidomide-dexamethasone [KRd27] vs lenalidomide-dexamethasone [Rd]), ENDEAVOR (carfilzomib [56 mg/m2]-dexamethasone [Kd56] vs bortezomib-dexamethasone [Vd]), and ARROW (once-weekly carfilzomib [70 mg/m2]-dexamethasone [Kd70] vs carfilzomib [27 mg/m2]-dexamethasone [Kd27]) studies. A frailty algorithm incorporating age, Charlson comorbidity index, and performance status classified patients as fit, intermediate, or frail. Results are presented for frail patients (ASPIRE, n = 196; ENDEAVOR, n = 330; ARROW, n = 141). In ASPIRE, median progression-free survival (PFS) (hazard ratio; 95% confidence interval) was 24.1 (KRd27) vs 15.9 months (Rd) (0.78; 0.54-1.12); median overall survival (OS) was 36.4 vs 26.2 months (0.79; 0.57-1.08). In ENDEAVOR, median PFS was 18.7 (Kd56) vs 6.6 months (Vd) (0.50; 0.36-0.68); median OS was 33.6 vs 21.8 months (0.75; 0.56-1.00). In ARROW, median PFS was 10.3 (once-weekly Kd70) vs 6.6 months (twice-weekly Kd27) (0.76; 0.49-1.16). In all 3 studies, rates of grade ≥3 treatment-emergent adverse events were consistent with those observed in the primary studies. The ASPIRE, ENDEAVOR, and ARROW primary analyses demonstrated favorable benefit-risk profiles with carfilzomib-containing regimens compared with controls. Across clinically relevant subgroups, including those by frailty status, consistent efficacy and safety were observed with KRd27, Kd56, and weekly Kd70, and treatment with these regimens should not be restricted by frailty status.

Letter--Value-Driven Treatment Decisions: Need of the Hour for Relapsed/Refractory Multiple Myeloma.

DISCLOSURES: No funding was involved in the writing of this letter. Medhekar, Sapra, Majer, Harrison, and Tekle are employees of and stockholders in Amgen.

p38 MAPK activation through B7-H3-mediated DUSP10 repression promotes chemoresistance.

Immunoregulatory protein B7-H3 is involved in the oncogenic and metastatic potential of cancer cells, as well as in drug resistance. Resistance to conventional chemotherapy is an important aspect of melanoma treatment, and a better understanding of how B7-H3 enhances drug resistance may lead to the development of more effective therapies. We investigated the in vitro and in vivo sensitivity of chemotherapeutic agents dacarbazine (DTIC) and cisplatin in sensitive and drug resistant melanoma cells with knockdown expression of B7-H3. We found that knockdown of B7-H3 increased in vitro and in vivo sensitivity of melanoma cells to the chemotherapeutic agents dacarbazine (DTIC) and cisplatin, in parallel with a decrease in p38 MAPK phosphorylation. Importantly, in B7-H3 knockdown cells we observed an increase in the expression of dual-specific MAP kinase phosphatase (MKP) DUSP10, a MKP known to dephosphorylate and inactivate p38 MAPK. DUSP10 knockdown by siRNA resulted in a reversion of the increased DTIC-sensitivity seen in B7-H3 knockdown cells. Our findings highlight the potential therapeutic benefit of combining chemotherapy with B7-H3 inhibition, and indicate that B7-H3 mediated chemoresistance in melanoma cells is driven through a mechanism involving DUSP10-mediated inactivation of p38 MAPK.

Molecular mechanisms underlying the synergistic interaction of erlotinib, an epidermal growth factor receptor tyrosine kinase inhibitor, with the multitargeted antifolate pemetrexed in non-small-cell lung cancer cells.

Because the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor erlotinib and the multitargeted antifolate pemetrexed are registered in the treatment of second-line non-small-cell lung cancer (NSCLC), empirical combinations of these drugs are being tested. This study investigated molecular mechanisms underlying their combination in six NSCLC cell lines. Cells were characterized by heterogeneous expression of pemetrexed determinants, including thymidylate synthase (TS) and dihydrofolate reductase (DHFR), and mutations potentially affecting chemosensitivity. Pharmacological interaction was studied using the combination index (CI) method, whereas cell cycle, apoptosis induction, and EGFR, extracellular signal-regulated kinases 1 and 2, and Akt phosphorylation were studied by flow cytometry, fluorescence microscopy, and enzyme-linked immunosorbent assays. Reverse-transcriptase polymerase chain reaction (RT-PCR), Western blot, and activity assays were performed to assess whether erlotinib influenced TS. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assays demonstrated that EGFR and k-Ras mutations were related to erlotinib sensitivity, whereas TS and DHFR expression were related to pemetrexed sensitivity. Synergistic cytotoxicity was found in all cells, most pronounced with pemetrexed + erlotinib (24 h) --> erlotinib (48 h) sequence (CI, 0.09-0.40), which was associated with a significant induction of apoptosis. Pemetrexed increased EGFR phosphorylation and reduced Akt phosphorylation, which was additionally reduced by drug combination (-70.6% in H1650). Erlotinib significantly reduced TS expression and activity, possibly via E2F-1 reduction, as detected by RT-PCR and Western blot, and the combination decreased TS in situ activity in all cells. Erlotinib and pemetrexed showed a strong synergism in NSCLC cells, regardless of their genetic characteristics. Induction of apoptosis, modulation of EGFR and Akt phosphorylation, and changes in the expression of critical genes involved in pemetrexed activity contribute to this synergistic interaction and support the clinical investigation of these markers.

Immunoregulatory protein B7-H3 promotes growth and decreases sensitivity to therapy in metastatic melanoma cells.

B7-H3 (CD276) belongs to the B7 family of immunoregulatory proteins and has been implicated in cancer progression and metastasis. In this study, we found that metastatic melanoma cells with knockdown expression of B7-H3 showed modest decrease in proliferation and glycolytic capacity and were more sensitive to dacarbazine (DTIC) chemotherapy and small-molecule inhibitors targeting MAP kinase (MAPK) and AKT/mTOR pathways: vemurafenib (PLX4032; BRAF inhibitor), binimetinib (MEK-162; MEK inhibitor), everolimus (RAD001; mTOR inhibitor), and triciribidine (API-2; AKT inhibitor). Similar effects were observed in melanoma cells in the presence of an inhibitory B7-H3 monoclonal antibody, while the opposite was seen in B7-H3-overexpressing cells. Further, combining B7-H3 inhibition with small-molecule inhibitors resulted in significantly increased antiproliferative effect in melanoma cells, as well as in BRAFV600E mutated cell lines derived from patient biopsies. Our findings indicate that targeting B7-H3 may be a novel alternative to improve current therapy of metastatic melanoma.

Decreased expression of B7-H3 reduces the glycolytic capacity and sensitizes breast cancer cells to AKT/mTOR inhibitors.

B7 family proteins are important immune response regulators, and can mediate oncogenic signaling and cancer development. We have used human triple-negative breast cancer cell lines with different expression levels of B7-H3 to evaluate its effects on the sensitivity to 22 different anticancer compounds in a drug screen. API-2 (triciribidine) and everolimus (RAD-001), two inhibitors that target the PI3K/AKT/mTOR pathway, showed enhanced inhibition of cell viability and proliferation in B7-H3 knockdown tumor cells compared to their B7-H3 expressing counterparts. Similar inhibition was seen in control cells treated with an anti-B7-H3 monoclonal antibody. In B7-H3 overexpressing cells, the effects of the two drugs were reduced, supported also by in vivo experiments in which B7-H3 overexpressing xenografts were less sensitive to everolimus than control tumors. In API-2 and everolimus-treated B7-H3 overexpressing cells, phospho-mTOR levels were decreased. However, phosphorylation of p70S6K was differentially regulated in B7-H3 cells treated with API-2 or everolimus, suggesting a different B7-H3-mediated mechanism downstream of mTOR. Both API-2 and everolimus decreased the glycolysis of the cells, whereas knockdown of B7-H3 decreased and B7-H3 overexpression increased the glycolytic capacity. In conclusion, we have unveiled a previously unknown relationship between B7-H3 expression and glycolytic capacity in tumor cells, and found that B7-H3 confers resistance to API-2 and everolimus. The results provide novel insights into the function of B7-H3 in cancer, and suggest that targeting of B7-H3 may be a novel alternative to improve current anticancer therapies.

B7-H3 and its relevance in cancer; immunological and non-immunological perspectives.

B7-H3 is a transmembrane glycoprotein and a member of the B7 family of proteins. It was previously known as an immunoregulatory molecule, shown in recent years to be of clinical significance in different types of cancer. In some tumor types high expression of B7-H3 has been linked to a poor prognosis, whereas in other cancers the opposite effect has been observed. Taken together, the precise role of B7-H3 in tumor immunity is unclear and further investigations are needed. Another aspect of B7-H3 that so far has received little interest is its role in non-immunological systems. We have demonstrated that knockdown of B7-H3 in melanoma and breast cancer cells results in both increased chemosensitivity and decreased metastatic potential. This has been observed in both in vitro and in vivo experiments. Several different signaling pathways seems to be involved, as B7-H3 knockdown can be linked to both higher expression of apoptotic markers and increased phosphorylation of Stat3. Increased knowledge of also the non-immunological role of B7-H3 protein is therefore of great biological and putative therapeutic interest.

B7-H3 silencing increases paclitaxel sensitivity by abrogating Jak2/Stat3 phosphorylation.

In many types of cancer, the expression of the immunoregulatory protein B7-H3 has been associated with poor prognosis. Previously, we observed a link between B7-H3 and tumor cell migration and invasion, and in present study, we have investigated the role of B7-H3 in chemoresistance in breast cancer. We observed that silencing of B7-H3, via stable short hairpin RNA or transient short interfering RNA transfection, increased the sensitivity of multiple human breast cancer cell lines to paclitaxel as a result of enhanced drug-induced apoptosis. Overexpression of B7-H3 made the cancer cells more resistant to the drug. Next, we investigated the mechanisms behind B7-H3-mediated paclitaxel resistance and found that the level of Stat3 Tyr705 phosphorylation was decreased in B7-H3 knockdown cells along with the expression of its direct downstream targets Mcl-1 and survivin. The phosphorylation of Janus kinase 2 (Jak2), an upstream molecule of Stat3, was also significantly decreased. In contrast, reexpression of B7-H3 in B7-H3 knockdown and low B7-H3 expressing cells increased the phosphorylation of Jak2 and Stat3. In vivo animal experiments showed that B7-H3 knockdown tumors displayed a slower growth rate than the control xenografts. Importantly, paclitaxel treatment showed a strong antitumor activity in the mice with B7-H3 knockdown tumors, but only a marginal effect in the control group. Taken together, our data show that in breast cancer cells, B7-H3 induces paclitaxel resistance, at least partially by interfering with Jak2/Stat3 pathway. These results provide novel insight into the function of B7-H3 and encourage the design and testing of approaches targeting this protein and its partners.

Pharmacological aspects of the enzastaurin-pemetrexed combination in non-small cell lung cancer (NSCLC).

Conventional regimens have limited impact against NSCLC. Current research is focusing on multiple pathways as potential targets, and this review describes pharmacological aspects underlying the combination of the PKCbeta-inhibitor enzastaurin with the multitargeted antifolate pemetrexed. Pemetrexed is commonly used, alone or combined with platinum compounds, in NSCLC treatment, and ongoing studies are evaluating its target, thymidylate synthase (TS), as predictor of drug activity. Enzastaurin is a biological targeted agent being actively investigated against different tumors as single agent or in combination. All the downstream events following PKCbeta inhibition by enzastaurin are not completely known, and assays to evaluate possible biomarkers, such as expression of PKC, VEGF and GSK3beta, in tissues and/or in blood samples, are being developed. Enzastaurin-pemetrexed combination was synergistic in preclinical models, including NSCLC cells, where enzastaurin reduced phosphoCdc25C, resulting in G2/M-checkpoint abrogation, and Akt and GSK3beta; phosphorylation, favoring apoptosis induction in pemetrexed-damaged cells. Enzastaurin also significantly reduced VEGF secretion and pemetrexed-induced upregulation of TS expression, possibly via E2F-1 reduction, while the combination decreased TS activity. Similarly, the accumulation of deoxyuridine (a marker of TS inhibition) and the reduction of GSK3beta phosphorylation were detectable in clinical samples from a phase-Ib trial of pemetrexed-enzastaurin combination. In conclusion, the favorable toxicity profile and the multiple effects of enzastaurin on signaling pathways involved in cell cycle control, apoptosis and angiogenesis, as well as on proteins involved in pemetrexed activity, provide experimental basis for future studies on enzastaurin-pemetrexed combination and their possible pharmacodynamic markers in NSCLC patients.

The immunoregulatory protein human B7H3 is a tumor-associated antigen that regulates tumor cell migration and invasion.

The monoclonal antibody (mAb) 376.96 has been used for detection of micrometastatic tumor cells due to its high binding specificity for a wide range of tumor cells, but the identity and function of its target antigen have not been known. Here, using immunoprecipitation and siRNA technology, we demonstrate that the antigen is the human 4Ig-B7H3 (4Ig-hB7H3) protein, previously known as an immunoregulatory protein in immune cells. Immunoblots of whole cell lysates, subcellular fractionation and tunicamycin treatment of human tumor cells indicated that 4Ig-hB7H3 is a approximately 100-kDa N-linked glycosylated membrane protein. The tumor promoter phorbol 12-myristate 13-acetate (PMA) enhanced the expression of 4Ig-hB7H3 in FEMX-I (melanoma), MA11 (breast cancer), and OHS (osteosarcoma) cells, suggesting that 4Ig-hB7H3 may be implicated in tumorigenesis. Most importantly, siRNA-downregulation of hB7H3 reduced cell adhesion to fibronectin of melanoma and breast cancer cells by up to 50 %, and migration and matrigel-invasion by more than 70 %, but surprisingly had no apparent impact on cell proliferation. In conclusion, our data present 4Ig-hB7H3 as a tumor-associated antigen and suggests a novel biological role of 4Ig-hB7H3 in tumor progression and metastasis.

Cellular trafficking of quantum dot-ligand bioconjugates and their induction of changes in normal routing of unconjugated ligands.

Can quantum dots (Qdots) act as relevant intracellular probes to investigate routing of ligands in live cells? The intracellular trafficking of Qdots that were coupled to the plant toxin ricin, Shiga toxin, or the ligand transferrin (Tf) was studied by confocal fluorescence microscopy. The Tf:Qdots were internalized by clathrin-dependent endocytosis as fast as Tf, but their recycling was blocked. Unlike Shiga toxin, the Shiga:Qdot bioconjugate was not routed to the Golgi apparatus. The internalized ricin:Qdot bioconjugates localized to the same endosomes as ricin itself but could not be visualized in the Golgi apparatus. Importantly, we find that the endosomal accumulation of ricin:Qdots affects endosome-to-Golgi transport of both ricin and Shiga toxin: Transport of ricin was reduced whereas transport of Shiga toxin was increased. In conclusion, the data reveal that, although coupling of Qdots to a ligand does not necessarily change the endocytic pathway normally used by the ligands studied, it appears that the ligand-coupled Qdot nanoparticles can be arrested within endosomes and somehow perturb the normal endosomal sorting in cells. Thus, the results demonstrate that Qdots may have severe consequences on cell physiology.