A phase 1 trial utilizing a pharmacokinetic endpoint to determine the optimal dose of ramucirumab in children and adolescents with relapsed or refractory solid tumors, including central nervous system tumors

BACKGROUND

Ramucirumab is a monoclonal antibody that binds the extracellular domain of vascular endothelial growth factor receptor (VEGFR-2) and prevents binding of VEGF ligands. Based on population pharmacokinetic (PK) analysis and correlation with efficacy in adults, a target steady state trough concentration (Css,min ) ≥ 50 µg/mL was established.

PROCEDURES

This phase 1 trial (ADVL1416) used a rolling six design and a PK primary endpoint to define the recommended phase 2 dose (RP2D) of ramucirumab in children with recurrent/refractory solid tumors. Two dose levels (DL) were planned (DL1: 8 mg/kg, DL2: 12 mg/kg administered intravenously [IV] every 2 weeks). Toxicity during the initial 6 weeks was used to assess maximum tolerated dose (MTD). Cycle 1 Day 42 trough (Cmin ) ≥ 50 µg/mL was the target concentration for the PK endpoint. At the RP2D, cohorts for PK expansion and children with central nervous tumors were planned.

RESULTS

Twenty-nine patients were enrolled; 28 were eligible; median age [range] = 13.5 [1-21] years; 22 were evaluable for the PK endpoint. Dose-limiting proteinuria occurred at both DLs; however, the MTD was not exceeded. At DL2 (12 mg/kg), the median Day 42 Cmin (n = 16) was 87.8 µg/mL; 15 of 16 patients achieved a Cmin  ≥ 50 µg/mL.

CONCLUSION

Ramucirumab was well tolerated in children and adolescents with solid tumors. The RP2D for ramucirumab was 12 mg/kg IV every 2 weeks. This trial demonstrates the feasibility of incorporating a primary PK endpoint to determine dose escalation and the RP2D in children. Studies of ramucirumab in children with selected solid tumors are ongoing.

A phase I/II study of lenvatinib (LEN) plus everolimus (EVE) in recurrent and refractory pediatric solid tumors, including CNS tumors

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Background: Proangiogenic signaling pathways cooperate with mTOR-mediated regulation of cell growth and maintenance to drive development of many pediatric cancers. We report results of the phase 1 dose escalation for LEN + EVE in pediatric patients (pts) with recurrent solid and CNS tumors conducted by Children’s Oncology Group. Methods: Dose escalation was conducted using a rolling-6 design. Pts received LEN + EVE orally once daily in continuous 28-day cycles. Dose determination was based on toxicity (CTCAE v4.03) during cycle 1. Pharmacokinetics (PK) of plasma LEN and EVE were monitored. Results: 17 pts were enrolled (9 male; 8 female). Median (range) age was 10 (3–21) years; 8 pts had CNS tumors. 17 were evaluable for dose-limiting toxicity (DLT). Enrollment started at dose level 1 (DL 1; LEN 11 mg/m2 + EVE 3 mg/m2) and, after treatment of 3 pts, was initially de-escalated to DL –1 (LEN 8 mg/m2 + EVE 3 mg/m2) due to DLT of proteinuria in 1 pt and self-resolving headache in another who, on review, did not meet the definition of DLT. No pts enrolled at DL –1 (n = 5) experienced DLT. Overall, DLTs were observed in 2 of the first 6 patients enrolled at DL 1: the initial pt with proteinuria and 1 more pt with hypertriglyceridemia and hypercholesteremia. Because 2 pts had reversible DLT of different categories not related to Cmax or AUC, the DL 1 cohort was expanded to enroll an additional 6 pts, none of whom had DLT. Thus, 2/12 pts experienced DLT at DL 1. Overall, most common treatment-emergent adverse events (TEAEs; ≥ 50% of pts) were diarrhea, hypertension, hypertriglyceridemia, vomiting, abdominal pain, headache, and hypothyroidism. 47% of pts had ≥ 1 treatment-related TEAE grade ≥ 3; the most frequent was proteinuria (n = 2). On cycle 1 day 15, mean (SD) Cmax (ng/mL) for LEN at DL –1 and DL 1, respectively, was 314 (150) and 359 (270), and mean (SD) AUC0-8h (hr•ng/mL) for LEN was 1570 (935) and 1780 (1100). Taking all toxicities and PK into account, no further dose escalation was recommended. Best overall response in pts with measurable disease was 2/11 stable disease, 7/11 progressive disease, and 2/11 not evaluable. Conclusions: The recommended phase 2 dose of LEN + EVE in children with solid and CNS tumors was LEN 11 mg/m2 + EVE 3 mg/m2, with maximum daily doses capped at 18 mg and 5 mg, respectively. The toxicity profile was no more than additive to single-agent therapy. PK exposure was comparable with children on single-agent LEN and to adults receiving LEN + EVE. Enrollment to the phase 2 portion (Ewing sarcoma, high-grade glioma, and rhabdomyosarcoma strata) is ongoing. Clinical trial information: NCT03245151.

Parental insights into phase I pediatric oncology clinical trials: Findings from a phenomenological study

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Background: Phase I clinical trials (P1Ts) are an essential step in the validation of novel therapies to treat childhood cancer. Children with cancer participate in P1Ts when no known curative therapy remains for their cancer. It is important to understand the experiences of children with advanced cancer and their families in P1Ts to ensure that participation supports the participants’ well-being. Methods: An empirical phenomenology study, using an adapted Colaizzi method, was conducted of 11 parents’ lived experiences of their child’s participation in pediatric oncology P1Ts. Parents were recruited from two pediatric academic medical centers in the Midwest United States and from national childhood cancer groups not affiliated with either medical center. Parents participated in individual interviews (mean length 59.1 minutes), with one broad data-generating question. Demographics and the child’s clinical trial records provided additional context for understanding the experiences. This presentation describes the study themes identified that were specifically related to pediatric oncology P1T processes and logistics. Results: Parents’ experiences during the P1Ts were primarily positive. However, data analysis revealed five aspects of P1T participation that were problematic for parents to varying degrees. These aspects were: (1) Learning About Clinical Trials, (2) Being Referred to Another Institution, (3) Undergoing Research-Only Procedures, (4) Complying with the Trial Requirements, and (5) Taking Oral Medications. Conclusions: While overall experiences during P1T participation were positive, improvements can be made that could potentially enhance participants’ experiences. In particular, enhancing the education provided to participants during on-trial processes and minimizing the logistical burdens associated with P1T requirements may help to address problematic aspects of P1Ts.

Abstract CT054: A dose escalation and toxicity study using pre-determined target concentration of Ramucirumab, a novel anti-angiogenic agent in children with recurrent / refractory solid tumors: A report from the children's oncology group phase I/ pilot consortium (ADVL1416)

Ramucirumab (RAM) is a high affinity, humanized, monoclonal antibody that binds to the extra-cellular domain of the vascular endothelial growth factor receptor (VEGFR). RAM prevents the binding of VEGF ligands with VEGFR-2, the most potent receptor of the VEGF tumor angiogenesis pathway. RAM is FDA approved for adults with NSCLC, as well as for gastric and colorectal adenocarcinoma where higher exposure was associated with better outcomes. Based on population pharmacokinetic (PK) analysis of the adult data, a minimum steady state concentration (Cmin ss) of ≥ 50 µg/ml of RAM was considered the target concentration for pediatrics. The Children's Oncology Group (COG) conducted a phase 1 study using a rolling six design to define the maximum tolerated dose (MTD) and /or a recommended phase 2 dose (RP2D), evaluate toxicities and explore circulating angiogenic cells as a biomarker for RAM monotherapy in children with recurrent/refractory solid tumors. The starting dose of 8 mg/kg IV q2 weeks (DL1) was chosen based on the adult safety profile and effective Cmin ss. Since adult data suggested that lower body weight was associated with lower Cmin ss, dose escalation to 12 mg/kg IV q2 weeks (DL2) was planned provided the MTD was not exceeded in DL1 and up to 16 mg/kg IV q2 weeks (DL3) if Cmin ss was < 50 µg/ml in 1 of 6 subjects on DL2 without exceeding the MTD. An expansion cohort was planned at the lowest dose where at least 5 of 6 subjects exceeded the target in order to generate PK data in at least 6 additional subjects < 12 years of age. In the absence of an MTD, the RP2D was defined as the dose achieving Cmin ss ≥ 50 µg/ml on day 42 ± 2, after 3 doses of RAM administered IV every 2 weeks, in at least 10 of 12 subjects. Sixteen subjects (15 eligible; 7 females), median age of 14 years (3-21 yrs.) have been enrolled. Of the 7 subjects on DL1, two had insufficient PK sampling to establish Cmin ss, including 1 who experienced grade 2 proteinuria (DLT). In the remaining 5, the median (range) Cmin ss was 66 (40-136) µg/ml; 2/5 did not achieve the target of ≥ 50 µg/ml. Of the 8 subjects on DL2, two had insufficient PK sampling, including 1 with grade 2 proteinuria (DLT). In the remaining 6, the median (range) Cmin ss was 64 (51-101) µg/ml, all ≥ 50 µg/ml; thus 12mg/kg IV q2 weeks was identified for the expansion cohort. The most common RAM related non-DLT toxicities were grade 1/2 headache in 6 subjects, grade 1/2 elevation in aspartate aminotransferase in 7 subjects and grade 2/3 hypertension in 2 subjects. Due to low participation, circulating angiogenic cells could not be analyzed. This is the first COG study in which dose escalation and RP2D for a targeted agent incorporated a primary PK endpoint (Cmin ss) based upon efficacy data from adult studies and paves the way for similar study design for future phase-1 studies.

Citation Format: Kamnesh Pradhan, Stacey L. Berg, Xiaowei Liu, Charles G. Minard, James Croop, Joel M. Reid, Elizabeth Fox, Brenda J. Weigel. A dose escalation and toxicity study using pre-determined target concentration of Ramucirumab, a novel anti-angiogenic agent in children with recurrent / refractory solid tumors: A report from the children's oncology group phase I/ pilot consortium (ADVL1416) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT054.

Fever and neutropenia hospital discharges in children with cancer: A 2012 update

Fever and neutropenia (FN) is a common precipitant for hospitalization among children with cancer, but hospital utilization trends are not well described. This study describes national trends for hospital discharges for FN among children with cancer for the year 2012, compared with the authors' previous analysis from 2009. Data were analyzed from the Kids' Inpatient Database (KID), an all-payer US hospital database, for 2012. Pediatric patients with cancer who had a discharge for FN were identified using age ≤19 years, urgent or emergent admit type, nontransferred, and a combination of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes for fever and neutropenia. The authors evaluated factors associated with a "short length of stay" (SLOS). Sampling weights were used to permit national inferences. In 2012, children with cancer accounted for 1.8% of pediatric hospital discharges (n = 120,675), with 12.2% (n = 13,456) of cancer-related discharges meeting FN criteria. Two fifths of FN discharges had a SLOS, which accounted for $91 million (2015 US$) in hospital charges. The majority had no serious infections; most common infections were viral infection (9.6%) or upper respiratory infection (9.6%). Factors significantly associated with SLOS included having a diagnosis of ear infection (odds ratio [OR] = 1.54, 95% confidence interval [CI]: 1.16-2.03), soft tissue sarcoma (OR = 1.47, CI: 1.10-1.95), and Hodgkin lymphoma (OR = 1.51, CI: 1.09-2.10), as compared with not having those diagnoses. SLOS admissions continue to be rarely associated with serious infections, but contribute substantially to the burden of hospitalization for pediatric FN. Implementation of risk stratification schemas to identify patients who meet low-risk criteria may decrease financial burden.

A pilot study of dose-intensified procarbazine, CCNU, vincristine for poor prognosis brain tumors utilizing fibronectin-assisted, retroviral-mediated modification of CD34+ peripheral blood cells with O6-methylguanine DNA methyltransferase

Administration of chemotherapy is often limited by myelosuppression. Expression of drug-resistance genes in hematopoietic cells has been proposed as a means to decrease the toxicity of cytotoxic agents. In this pilot study, we utilized a retroviral vector expressing methylguanine DNA methyltransferase (MGMT) to transduce hematopoietic progenitors, which were subsequently used in the setting of alkylator therapy (procarbazine, CCNU, vincristine (PCV)) for poor prognosis brain tumors. Granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood progenitor cells were collected by apheresis and enriched for CD34+ expression. Nine subjects were infused with CD34+-enriched cells treated in a transduction procedure involving a 4-day exposure to cytokines with vector exposure on days 3 and 4. No major adverse event was related to the gene therapy procedure. Importantly, the engraftment kinetics of the treated product was similar to unmanipulated peripheral blood stem cells, suggesting that the ex vivo manipulation did not significantly reduce engrafting progenitor cell function. Gene-transduced cells were detected in all subjects. Although the level and duration was limited, patients receiving cells transduced using fibronectin 'preloaded' with virus supernatant appeared to show improved in vivo marking frequency. These findings demonstrate the feasibility and safety of utilizing MGMT-transduced CD34+ peripheral blood progenitor cells in the setting of chemotherapy.

Gene therapy in the treatment of Fanconi anemia, a progressive bone marrow failure syndrome

Fanconi anemia (FA) is a genetic disease characterized by progressive, fatal bone marrow failure, congenital anomalies and predisposition to cancer. Although stem cell transplantation is therapeutic, human leukocyte antigen-identical sibling donors are available to a minority of patients. In murine models and human cells in vitro, gene transfer corrects the FA cellular phenotype of chromosomal breakage in response to DNA-damaging agents, suggesting therapeutic use of gene transfer is possible. However, disease-specific characteristics make application of viral vector technology difficult. Multiple studies are currently underway to develop a gene therapy approach for treating this disease, including phase I trials.

Packaging cell line DNA contamination of vector supernatants: implication for laboratory and clinical research

Investigators conducting retroviral gene therapy trials are required to monitor for the presence of replication-competent retrovirus (RCR). The required testing utilizes a combination of biologic assays and molecular tests using PCR. In the course of a human clinical gene therapy trial, we detected 4070A viral envelope sequences in CD34(+) peripheral blood stem cells 2 days after transduction using a PCR-based assay, suggesting the presence of RCR. The supernatant and producer cells used for vector generation had been negative in extensive screening using the extended S(+)/L(-) assay. The presence of a replication-competent virus was subsequently excluded by a combination of biologic and PCR analyses. The source of the 4070A viral envelope sequences was determined to be packaging cell line DNA in the vector supernatant. The analysis of a variety of vector supernatants by quantitative real-time PCR revealed 4070A envelope DNA sequences from the packaging cell line in concentrations equivalent to approximately 50-500 focus-forming units per milliliter of wild-type 4070A virus. When PCR was performed after reverse transcriptase treatment of supernatant (i.e., assessing both RNA and DNA content), 4070A envelope sequence concentrations ranged from 10(2) to 3.5 x 10(3) focus-forming units per milliliter of wild-type 4070A virus. Our data indicate that PCR should not be used to analyze transduced cells for RCR within the first 2 weeks of vector exposure. Furthermore, investigators using PCR to analyze transduction efficiency shortly after vector exposure may experience false-positive findings.

Women's sense of well-being before and after hysterectomy

OBJECTIVE

To describe women's perceived sense of well-being before and after hysterectomy by examining a broad array of outcomes experienced by women undergoing hysterectomies for benign conditions.

DESIGN

Prospective, descriptive.

SETTING

A regional tertiary care facility in central Texas.

PARTICIPANTS

One hundred seventy-eight women presenting for hysterectomies for nononcologic reasons who completed all three periods of data collection.

MAIN OUTCOME MEASURES

Subjects completed a questionnaire assessing information pertinent to their current gynecologic health and the SF-36 Health Survey before surgery and of 4 and 11 months after surgery. The women also completed the Zung Self-Rating Depression Scale preoperatively and at 4 months postoperatively. Additional demographic and medical information was extracted from the medical record.

RESULTS

In the initial period after surgery, the patients experienced an improved health status. In addition, the women reported on improvement in their psychologic well-being, including less depression and improved sexual functioning. Relationships with others also improved after the surgery.

CONCLUSIONS

Outcomes for these women undergoing hysterectomy for nononcologic reasons were generally positive. This information is vital for preoperative counseling by nurses of women contemplating or about to undergo this surgery.

Phosphatidylcholine and phosphatidylethanolamine behave as substrates of the human MDR1 P-glycoprotein

The multidrug resistant cell line CEM/VBL300 and the parental CEM T-lymphoblastic cell line from which it was derived were used to study the accumulation of fluorescent phospholipid analogs of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS). The fluorescent analogs NBD-PC, NBD-PE, and NBD-PS and [3H]PC were delivered in liposomes prepared by ethanol injection. Fluorescence microscopy demonstrated decreased accumulation of the NBD-PC analog in the multidrug resistant cell line compared to the parental cell line. Verapamil enhanced NBD-PC accumulation in the resistant cells. Similar results were obtained with insect cells expressing high levels of recombinant human MDR1. Elimination of NBD fluorescence on the outer leaflet of the plasma membrane with dithionite permitted quantification of the internal cellular fluorescence by FACS analysis. The drug resistant CEM/VBL300 cells accumulated approximately 10% the amount of NBD-PE and 20% the amount of NBD-PC compared to CEM drug sensitive cells. No difference in internal accumulation of NBD-PS was found between the drug resistant and drug sensitive cell lines. The internal accumulation of NBD-PE and NBD-PC was enhanced by the MDR reversal agents verapamil, cyclosporin A, and SDZ PSC 833 in the CEM/VBL300 cells but not in the CEM cells. The increased accumulation was dose dependent, and the relative potency of the reversal agents paralleled their ability to circumvent multidrug resistance. In addition, the monoclonal antibody UIC2 directed against the P-glycoprotein produced similar results. The evidence presented here suggests that PC and PE but not PS behave as substrates for human MDR1 P-glycoprotein.

Role of P-glycoprotein in dolastatin 10 resistance

Dolastatin 10, a cytotoxic pentapeptide isolated from the mollusk Dolabella auricularia, exhibits potent antitumor activity. The present studies demonstrated that sublines of murine PC4 and human U-937 leukemia cells expressing a multidrug resistance (MDR) phenotype are cross-resistant to this agent. We also demonstrated that such resistance was reversed by verapamil. While these findings suggested the involvement of the P-glycoprotein (P-gp) in dolastatin 10 resistance, we performed similar studies in a CHO cell line transfected with the human mdr1 cDNA. Expression of P-gp in the transfected cells was associated with resistance to dolastatin 10 by a verapamil-sensitive mechanism. The demonstration that photoaffinity labeling of P-gp was decreased in the presence of dolastatin 10 further supports the interaction of this cytotoxic peptide with P-gp. Taken together, these findings suggest that resistance to dolastatin 10 is conferred, at least in part, by P-gp and that this cytotoxic peptide is a novel member of the MDR phenotype.

Monoclonal antibody to an external epitope of the human mdr1 P-glycoprotein

A membrane glycoprotein, termed P-glycoprotein, has been shown to be responsible for cross-resistance to a broad range of structurally and functionally distinct cytotoxic agents. P-glycoprotein, encoded in humans by the mdr1 gene, functions as an energy-dependent efflux pump to exclude these cytotoxic agents from the resistant cell. In order to study the phenomenon of multidrug resistance in both normal and neoplastic cells, we have generated a mouse monoclonal antibody directed to an external epitope of the human P-glycoprotein. This monoclonal antibody, 4E3, is an IgG2a class antibody which specifically recognizes the human mdr1 P-glycoprotein but not the mdr3 gene product. The 4E3 monoclonal antibody immunoprecipitates both the glycosylated and nonglycosylated forms of P-glycoprotein under mild denaturation conditions. In addition, 4E3 can detect P-glycoprotein in immunocytochemical analysis of fixed tissue-cultured cells and in analysis of frozen sections of human tissue. Binding of the monoclonal antibody to multidrug-resistant cells does not significantly affect the intracellular accumulation or potentiate the cytotoxicity of daunomycin in multidrug-resistant cells. However, at high concentrations of antibody, 4E3 produces a mild potentiation of vinblastine and actinomycin cytotoxicity in multidrug-resistant cells. This monoclonal antibody will be useful both for analyzing P-glycoprotein expression in normal and neoplastic cells and for isolating live cells expressing the P-glycoprotein without significantly affecting the efflux functions of the transporter.

A single amino acid substitution strongly modulates the activity and substrate specificity of the mouse mdr1 and mdr3 drug efflux pumps

Specific protein domains and amino acids responsible for the apparent capacity of P-glycoprotein (mdr) to recognize and transport a large group of structurally unrelated drugs have not been identified. We have introduced a single Ser----Phe substitution within the predicted TM11 domain of mdr1 (position 941) and mdr3 (position 939) and analyzed the effect of these substitutions on the drug-resistance profiles of these two proteins. Mutations at this residue drastically altered the overall degree of drug resistance conveyed by mdr1 and mdr3. The modulating effect of this mutation on mdr1 and mdr3 varied for the drugs tested: it was very strong for colchicine and adriamycin and moderate for vinblastine. For mdr1, the Ser941----Phe941 substitution produced a unique mutant protein that retained the capacity to confer vinblastine resistance but lost the ability to confer adriamycin and colchicine resistance. These results strongly suggest that the predicted TM11 domain of proteins encoded by mdr and mdr-like genes plays an important role in the recognition and transport of their specific substrates.

Mammalian multidrug resistance gene: complete cDNA sequence indicates strong homology to bacterial transport proteins

The complete nucleotide and primary structure (1276 amino acids) of a full length mdr cDNA capable of conferring a complete multidrug-resistant phenotype is presented. The deduced amino acid sequence suggests that mdr is a membrane glycoprotein which includes six pairs of transmembrane domains and a cluster of potentially N-linked glycosylation sites near the amino terminus. A striking feature of the protein is an internal duplication that includes approximately 500 amino acids. Each duplicated segment includes a consensus ATP-binding site. Amino acid homology is observed between the mdr gene and a series of bacterial transport genes. This strong homology suggests that a highly conserved functional unit involved in membrane transport is present in the mdr polypeptide. We propose that an energy-dependent transport mechanism is responsible for the multidrug-resistant phenotype.

Isolation and characterization of DNA sequences amplified in multidrug-resistant hamster cells

The mechanism by which mammalian cells acquire resistance to chemotherapeutic agents has been investigated by using molecular genetic techniques. LZ and C5, two independently derived multidrug-resistant Chinese hamster cell lines, share specific amplified DNA sequences. We demonstrate that commonly amplified DNA sequences reside in a contiguous domain of approximately equal to 120 kilobases (kb). We report the isolation of this DNA domain in cosmid clones and show that the level of amplification of the domain is correlated with the level of resistance in multidrug-resistant cell lines. The organization of the amplified domain was deduced by a unique approach utilizing in-gel hybridization of cloned DNA with amplified genomic DNA. We show that the entire cloned region is amplified in adriamycin-resistant LZ cells and independently derived, colchicine-resistant C5 cells. A mRNA species of approximately equal to 5 kb is encoded by a gene located within the boundaries of this region. Genomic sequences homologous to the 5-kb mRNA span over 75 kb of the amplified DNA segment. The level of expression of this mRNA in multidrug-resistant cells is correlated with the degree of gene amplification and the degree of drug resistance. Our results strongly suggest that the 5-kb mRNA species plays a role in the mechanism of multidrug resistance common to the LZ and C5 cell lines.

Pseudocyesis in an adolescent patient. Case report and radiologic analysis

The physical and psychological changes of pseudocyesis have bewildered physicians since the time of Hippocrates. We present the case of a 14-year-old girl as a reminder that pseudocyesis needs to be considered in adolescents and that the diagnosis may not be obvious, even with current technologies. We document that gaseous distension and abdominal fat deposits can lead to dramatic abdominal fullness and can mimic a term pregnancy.

The response of chicken embryo dermal fibroblasts to cytochalasin B is altered by Rous sarcoma virus-induced cell transformation

The drug cytochalasin B (CB), which disrupts the cellular microfilament network, allows the identification of as yet unclassified structural differences between normal and Rous sarcoma virus-transformed chicken embryo fibroblasts. When exposed to CB, normal chick fibroblasts attain an arborized or dendritic morphology. This results as the cytoplasm collapses upon the remaining structural and adhesive components of the cell. Rous sarcoma virus-transformed cells did not form or maintain these dendritic-like processes in the presence of CB and, as a result, rounded up but still remained attached to the substrate. With a temperature-sensitive mutant of Rous sarcoma virus, LA24A, it was possible to show that these effects are completely reversible and dependent on the expression of pp60src. The cytoskeleton in these CB-treated cells was examined by both immunofluorescence and electron microscopy. After exposure to CB, the microfilaments were found to be disrupted similarly throughout both the transformed and the nontransformed cells. In the nontransformed cells arborized by exposure to CB, the extended processes were found to contain intermediate filaments in an unusually high concentration and degree of organization. The distribution of these filaments in the central body of the arborized cells was random. This lower concentration and random distribution was similar to that seen throughout the transformed cells rounded up by exposure to CB. The failure of these transformed cells to arborize in CB indicates that the structural component(s) which is necessary for the formation or maintenance or both of the arborized state is altered by the expression of pp60src.

Selective effects of phorbol 12-myristate 13-acetate on myofibrils and 10-nm filaments

Phorbol 12-myristate 13-acetate (PMA) has a prompt and selective catabolic effect on striated myofibrils in postmitotic myotubes. Fluorescein-labeled antibodies against light meromyosin were used to follow the effects of PMA on the muscle-specific myosin in myofibrils. The response of actin filaments was monitored by decoration with heavy meromyosin. The response of the two types of 10-nm filaments in myotubes was followed by fluorescein- and rhodamine-labeled antibodies to the fibroblastic and muscle-specific filament proteins, respectively. Within 2-3 days, PMA induced dismantling of virtually every striated myofibril in every myotube in the culture. These myotubes bound little or no anti-light meromyosin, and tests to detect the alpha-actin filaments of the myofibrils with heavy meromyosin were negative. In contrast, the nonmuscle actin in the subsarcolemmal microfilaments persisted in PMA-treated myotubes and was decorated with heavy meromyosin. The sarcoplasmic reticulum, mitochondria, and Golgi bodies appeared normal. Myotubes depleted of myofibrils by PMA displayed large numbers of muscle-specific 10-nm filaments. This preferential degradation of the myosin and actin of the myofibrils were reversible. These myotubes formed a normal complement of myofibrils 24-48 hr after removal of PMA. When, after 3 days in PMA, the cultures were treated for an additional 3-8 days, a transitory subpopulation of PMA-resistant myotubes appeared.

Differential location of different types of intermediate-sized filaments in various tissues of the chicken embryo

The location of constitutive proteins of different types of intermediate-sized (about 10 mm) filaments (cytokeratin, vimentin, desmin, brain filament protein) was examined in various tissues of 11--20 day chick embryos, using specific antibodies against the isolated proteins and immunofluorescence microscopy on frozen sections and on isolated serous membrane. The tissues studied which contained epithelia were small intestine, gizzard, esophagus, crop, liver, kidney, thymus, mesenteries, and epidermis. The results show that the different intermediate filament proteins, as seen in the same organ, are characteristic of specific lines of differentiation: Cytokeratin filaments are restricted to--and specific for--epithelial cells; vimentin filaments are seen--at this stage of embryogenesis--only in mesenchymal cells, including connective tissue, endothelial and blood cells, and chondrocytes; filaments containing protein(s) related to the subunit protein prepared from gizzard 10 nm filaments (i.e., desmin) are significant only in muscle cells; and intermediate filament protein of brain, most probably neurofilament protein, is present only in nerve cells. We conclude that for most tissues the expression of filaments of cytokeratin, vimentin, desmin, and neurofilament protein is mutually exclusive, and that these protein structurees provide useful markers for histochemical and cytochemical differentiation of cells of epithelial, mesenchymal, myogenic, and neurogenic differentiation.

Response of myogenic and fibrogenic cells to cytochalasin B and to colcemid. I. Light microscope observations

Cytochalasin B (CB) induces a biphasic retraction is some cell types. The rapid response that peaks in 30 min leads to the "dendritic" condition. Replicating myogenic and fibrogenic cells, as well as postmitotic myoblasts and myotubes, participate in this reaction. This is followed by a slower phase that requires 40 h for stabilization and leads to the fully "absorized" state. Only replicating myogenic and fibrogenic cells participate in this reaction. Postmitotic myoblasts and myotubes do not arborize but round up and float off into the medium. Pretreatment with Colcemid does not block the rapid response to CB, but does block arborization. CB-arborized cells exposed to Colcemid while in the presence of CB develop sufficient tension to pull themselves apart. If CB depolymerizes actin-like filaments, and if such filaments constitute the only contractile system in the cell, then it is difficult to visualize how cells in CB develop such tension. Colcemid induces twisting, birefringent bands in interphase- and metaphase-arrested myogenic and fibrogenic cells, and in postmitotic myotubes. Such bands are more evident when CB-arborized cells are removed from CB and allowed to relax in Colcemid. These birefringent bands assemble in the prescence of cycloheximide, and may constitute 20% of the volume of the cell.

Effects of cytochaslasin B and colcemide on myogenic cultures

Muscle cultures treated with cytochalasin B yield mono- and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arborized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mono-nucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonally-stacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous calbes are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.